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Relay-based Communications in WBANs: A Comprehensive Survey AVANI VYAS and SUJATA PAL, Indian Institute of Technology Ropar, India BARUN KUMAR SAHA, Hitachi ABB Power Grids, India ACM Comput. Surv., Vol. 54, No. 1, Article 2, Publication date: December 2020. DOI: https://doi.org/10.1145/3423164 Wireless Body Area Networks (WBANs) constitute an emerging technology in the field of health care that makes health monitoring possible from one's home itself. WBANs open many challenges by placing sensors on/inside human bodies for collecting various health-related information. Unlike traditional Wireless Sensor Networks (WSNs), communication in WBANs suffers from high channel fading and attenuation due to human body fat. Therefore, relay-based communication with data forwarding techniques is used to handle link failures and poor network connectivity. Accordingly, in this survey article, we present a comprehensive study of relay-based communication mechanisms in WBANs. We begin with a brief look at the multi-tiered architecture of WBANs, how direct communication works, and how relay-based communication is different. Subsequently, we present a detailed review of relay node selection approaches, which, in turn, also affects how a WBAN performs. In this context, we also look at the unique quality of service (QoS) demands of WBANs and how they can be assured.
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1
Relay-based Communications in WBANs: A Comprehensive
Survey
AVANI VYAS, Indian Institute of Technology Ropar, India
SUJATA PAL, Indian Institute of Technology Ropar, India
BARUN KUMAR SAHA, Hitachi ABB Power Grids, India
Wireless Body Area Networks (WBANs) constitute an emerging technology in the eld of health care that
makes health monitoring possible from one’s home itself. WBANs open many challenges by placing sensors
on/inside human bodies for collecting various health-related information. Unlike traditional Wireless Sensor
Networks (WSNs), communication in WBANs suers from high channel fading and attenuation due to human
body fat. Therefore, relay-based communication with data forwarding techniques is used to handle link failures
and poor network connectivity. Accordingly, in this survey paper, we present a comprehensive study of relay-
based communication mechanisms in WBANs. We begin with a brief look at the multi-tiered architecture of
WBANs, how direct communication works, and how relay-based communication is dierent. Subsequently,
we present a detailed review of relay node selection approaches, which, in turn, also aects how a WBAN
performs. In this context, we also look at the unique quality of service (QoS) demands of WBANs and how
they can be assured.
CCS Concepts:
General and reference Surveys and overviews
;
Networks Wireless personal
area networks;Network performance analysis;Network protocol design.
Additional Key Words and Phrases: survey, wireless body area networks, relay selection, cooperative diversity,
cooperation.
ACM Reference Format:
Avani Vyas, Sujata Pal, and Barun Kumar Saha. 2019. Relay-based Communications in WBANs: A Compre-
hensive Survey. ACM Comput. Surv. 1, 1, Article 1 (January 2019), 35 pages. https://doi.org/10.1145/nnnnnnn.
nnnnnnn
1 INTRODUCTION
WBANs [1] are a specialized subclass of WSNs [2, 3] where a network of biomedical sensors are
implanted on or inside a human body to monitor various health parameters, such as heart rate
and blood glucose level. WBANs constitute an emerging technology that is expected to provide
personalized health care services at aordable prices [4]. WBANs, also known as Wearable Human
Monitoring Systems [5], can also be viewed as an application of cyber-physical systems in the
medical area [6, 7, 8].
Sensors used in WBANs sense and transfer human health readings, such as body temperature,
respiration rate and blood pressure, to a personal device acting as a coordinator node. Rashidi
This work was done when B. K. Saha was with ABB Corporate Research Center, India.
Authors’ addresses: Avani Vyas, 2017csz0007@iitrpr.ac.in, Indian Institute of Technology Ropar, India; Sujata Pal, sujata@
iitrpr.ac.in, Indian Institute of Technology Ropar, India; Barun Kumar Saha, barun.kumarsaha@hitachipowergrids.com,
Hitachi ABB Power Grids, Bangalore, India .
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https://doi.org/10.1145/nnnnnnn.nnnnnnn
ACM Comput. Surv., Vol. 1, No. 1, Article 1. Publication date: January 2019.
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1:2 Avani Vyas, Sujata Pal, and Barun Kumar Saha
et al. [9] listed various wearable sensor nodes used for collecting human health and mobility
measurements. The collected data are either analyzed at the coordinator node or sent to a medical
server (or cloud) by the coordinator node for analyses and storage. A mobile phone acting as
a coordinator node can provide any healthcare services to a patient anytime. Few models are
developed for monitoring health in the home environment. Bellos et al. [10] provided an intelligent
indoor and outdoor monitoring system for early diagnosis of chronic obstructive pulmonary disease
(COPD). They used readings from multiple wearable sensors which include electrocardiogram
(ECG), respiration, and acceleration sensors. The readings are given as input to a hybrid classier,
which determines a patient’s state by comparing it against clinical rules under various severity levels.
Triantafyllidis et al. [11], discussed various mobile-based applications to provide self-monitoring
facility for a patient based on sensed data. There exist many such wearable medical systems that
can provide real-time self-monitoring of COPD and diabetes [12].
The transmission power level of WBAN’s sensor nodes is low due to short-range communication
[13]. Low power reduces the chances of interference of signals, preserves energy, and minimizes the
heating of body tissues. However, it is challenging to meet the performance requirements with low
power, especially for sensors positioned in non-line-of-sight (NLOS) with the coordinator node. For
example, a node deployed on the lower back of a patient communicates with the coordinator node
positioned on the patient’s chest. Various relay-based routing techniques have been proposed to
improve the performance of WBANs in such situations. Unlike direct communication mechanisms
between two nodes, in relay-based methods, an intermediate node is selected between the source
and the coordinator node to make accurate delivery of messages using low transmission power [14].
Consequently, the selection of an appropriate relay node requires careful considerations. Selection
of relay nodes is a key challenge in designing relay-based intra-WBAN communication protocols for
WBANs. This survey paper compares the performance of relay-based communication with direct
communication and summarizes various methods for selecting a suitable intermediate relay node
for data transmission. An intelligent relay selection protocol ensures reliable and energy-ecient
communication in WBANs.
WBANs suer inter-WBAN interference when more than one network co-exists in one area. For
example, in crowded places like hospitals, there are high chances of inter-WBAN interference. As a
consequence, intra-WBAN communication gets eected by the interfering signals coming from
another network. In this survey, we will study how intra-WBAN relay-based communications are
resilient to non-cooperative inter-WBAN interference. Relay-based communication addresses and
helps improve the QoS by meeting data rate requirements, maximizing lifetime and reducing bit
error rate at the coordinator node during link failures. Moreover, relay nodes can be exploited to
minimize the redundant information generated by multiple sensors and create energy harvesting
opportunities at the intermediate nodes.
Table 1 summarizes some of the existing surveys [15, 16, 17, 18, 19, 20, 21] on WBANs, and points
out how their scope diers from this current survey. Among other related works, Chen et al. [23],
for example, discussed BAN communication across physical and data link layers. However, the
discussion did not include intra-WBAN communication and various relaying methods to support
reliable communication during link failures in intra-WBAN networks. A survey on WBANs by
Cavallari et al. [20] compared the packet loss rate and delay of various communication standards
(Zigbee, WBAN standard, and Bluetooth Low Energy) used in WBANs. Cavallari et al. also provided
a short discussion on energy consumption issues at the physical and MAC layer. On the other hand,
Yessad et al. [24] compared existing routing protocols in WBANs serving QoS. Several metrics, such
as multiple trac, priority of sensor node, transmission delay, network lifetime, and path loss, were
considered to measure QoS. The authors dened an analytical model to compare the transmission
probability of existing routing methods. Another study by Salayma et al. [22] discusses the fault
ACM Comput. Surv., Vol. 1, No. 1, Article 1. Publication date: January 2019.
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Table 1. Comparison with prior surveys on WBANs.
Survey Description Routing?
(Yes/No)
If yes, what is new in this sur-
vey?
Cao et al.
[15] and
Patel et
al. [16]
Sensor/actuator systems
in WBANs and wireless
technology used to make
communication possible in such
systems.
No
Lara et al.
[17]
Human activity recognition
from the data generated by
human wearable sensors.
No
Javaid et
al. [18]
Comparison of MAC layer chan-
nel access methods. No
Ullah et
al. [19]
Comprehensive survey on the
physical, MAC, and network lay-
ers of WBANs.
Yes
This survey discusses empirical
studies conducted to compare
the network performance under
direct and in-direct communica-
tion.
Cavallari
et al. [20]
WBAN applications and their re-
quirements. Yes
Cavallari et al. did not include de-
tailed discussion on performance
of direct vs relay-based commu-
nication in intra-WBANs.
Qu et al.
[21]
Overview of dierent types of
routing protocols in WBANs. Yes
Qu et al. did not discuss about
the use of diversity combining
techniques in intra-WBAN com-
munication.
Salayma
et al. [22]
Fault tolerance at node level,
channel level, and hybrid (node
and channel) level along with in-
terference mitigation methods.
No
tolerance at dierent levels in WBANs such as node level, channel level, and hybrid (node and
channel) level. Along with this, they discuss fault tolerance and the interference mitigation method,
as well. However, the scope of these works lacks in consideration of relay-based communication,
relaying strategies, and channel properties that aect intra-WBAN communication.
In contrast, this survey paper primarily focuses on relay-based communication in WBANs. To
this end, we discuss the aspects of intra-WBAN communication where two nodes communicate
directly or use another node as a relay. Subsequently, we address the problem of relay selection for
intra-WBAN communication, and also discuss proposed mechanisms for opportunistic routing in
this context. We also look at scopes for performance improvement in relay-based communication
in WBANs. In brief, the specic contributions of this paper are as follows:
Presenting a comprehensive review of contemporary relay-based communication mechanisms
for intra-WBANs.
Highlighting the key approaches and decision making behind intelligent selection of relay
nodes.
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Reviewing empirical studies conducted to compare the performance of relay-based commu-
nication with direct transmission in intra-WBANs.
Discussing various techniques to mitigate interference in non-cooperative inter-WBANs
using relay-based communication.
Addressing the QoS aspects of WBANs, redundancy handling, and examining the various
approaches for performance assurance.
Identifying the various design challenges and open research issues in WBANs.
The remainder of this paper is organized as follows: Section 2 discusses the architecture of
WBANs. In Section 3, intra-WBAN network, along with data forwarding strategies and diversity
combining techniques used in relay-based communication are discussed. Section 4 compares
direct communication with relay-based communication in intra-WBANs. The section thoroughly
discusses the empirical and non-empirical studies carried out to make the comparison. Relay
node selection strategies for intra-WBANs that did not exploit human mobility are discussed
in Section 5. In contrast, Section 6 discusses the relay selection approaches that exploit human
mobility. Section 7 presents intra-WBAN communication methods that use additional nodes (special
nodes) to support relay-based methodology. Section 8 discusses performance issues addressed by
relay-based communication in intra-WBANs. Section 9 discusses how relay-based communication
mitigates inter-WBAN interference occurring in an intra-WBAN. Section 10 discusses the strength,
weaknesses and best application of routing methods presented in this paper. Design challenges of
WBANs and open issues are addressed in Section 11. Section 12 concludes this paper.
2 WBANS: CHARACTERISTICS, ARCHITECTURE, AND STANDARDS
We begin this section by taking a detailed look at how WBANs dier from WSNs. Such a per-
spective would allow a reader to understand why protocols designed for WSNs, in general, are
unsuitable for use in WBANs. Moreover, these would also help a protocol designer to understand
the basic requirements while designing a protocol for WBANs. Once the key features of WBANs
are understood, we take a brief look at their typical architecture. Subsequently, we present a quick
review of the wireless communication standards used by WBANs.
2.1 Characteristics of WBANs
WBANs constitute a subcategory of WSNs. However, WBANs possess some network constraints
due to the limited network size and sensitivity issues of the human body. The characteristics of
WBANs with respect to WSNs are highlighted in Table 2 [21, 25, 23] and are discussed as follows:
Transmission power level: Sensor nodes in WBANs use low transmission power to prevent
specic absorption rate (SAR) and interference. Sensor nodes in WBANs communicate with
the coordinator node using radio frequency. The radio energy gets absorbed by the body
tissues. SAR measures the amount of energy absorbed per unit mass by the human tissues
when exposed to radio frequencies [26]. Power level of 0dBm and above, may increase packet
delivery rate but can cause interference in WBANs and rise in SAR.
Area of deployment: The deployment area of WBAN is limited to the size of the human
body. However, WSNs can spread over geographical regions where thousands of nodes can
communicate with each other.
Communication channel: The channel of communication in WSNs are either air or water.
However, in WBANs, communication happens on or inside the human body, which is lossy
than air and water.
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Propagation delay: The propagation delay in WSN is not tolerable. Health readings are very
critical and require high priority. Therefore, the propagation delay of WBAN’s readings
should be minimum and less as compared to WSN.
Node density: The number of nodes used in WBANs is limited compared to WSNs. Moreover,
redundant nodes are not allowed in WBANs, which are otherwise used in WSNs to improve
the QoS.
Table 2. Characteristics of WBANs with respect to WSNs.
Characteristics WBAN WSN
Sensor’s transmission power level
<0dBm [27] Can be greater than 0dBm.
Area of communication Size of Human body Geographical areas
(Human body) (Forests & water).
Channel of communication Human fat and muscles Air and Water.
Propagation delay Low Varies from low to high.
Network density Low High (No upper bound).
Data loss Intolerable Tolerable.
Bio-compatibility Compulsory
Not compulsory (application
dependent).
Sensitivity to SAR High Low.
The network density of WSNs is high and can tolerate data loss and propagation delays. The
communication protocols developed for WSNs are not suitable for WBANs due to energy-constraints
and stringent QoS requirements. This raises the need for specialized communication protocols
for WBANs. Existing literature suggests that relay-based communication protocols improve the
lifetime and quality of communication in WBANs when the communication channel is highly lossy.
2.2 WBAN Architecture
Movassaghi et al. [2] discussed a typical three-tier architecture of WBANs, as shown in Fig.
1. Tier 1 essentially constitutes the Personal Area Network (PAN) of a WBAN, where sensors
Person
Coordinator
S1
S2 S3
S4
Tier 1 (PAN)
Access point
Internet
Tier 2 (LAN)
Doctor
Medical server
Family
Tier 3 (WAN)
Fig. 1. A typical three tier architecture of WBANs [2].
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(
𝑆
1
, 𝑆
2
, 𝑆
3
and 𝑆
4) implanted on or inside a body communicate among themselves or with the con-
cerned coordinator node. The coordinator can be a smartphone, tablet, or any other personal device
in which sensed medical readings are recorded. Triantafyllidis et al. [11], for example, discussed
dierent smartphone-based applications to provide self-monitoring facility for a patient based on
sensed health data.
Tier 2 can be regarded as a Local Area Network (LAN), which helps in transporting medical
signals acquired from tier 1 of WBANs to further outwards, for example, the Internet and cloud. Tier
2 consists of various local access points, which can be infrastructure-based or ad hoc [2]. Finally,
tier 3 architecture constitutes the Wide Area Network (WAN), the outermost layer of WBANs,
where medical readings stored on the medical server are accessed by doctors and alerts are sent to
family members. Data stored at medical servers are called E-health records (EHR). These are used
for medical diagnosis and various research purposes, for example, disease prediction using deep
learning [28].
A typical WBAN, as discussed above, consists of biomedical sensor nodes with dierent function-
ality and a coordinator node (also called as hub/sink node of a WBAN network). The positioning of
the sink node/hub has a vital role in achieving ecient WBAN communication. Sipal et al. [29]
experimented the impact of hub location on the performance of WBANs. They considered three
possible locations—waist, foot, and head—for hub placement. These three locations, respectively,
lead to waist-centric, footwear-centric, and head-centric WBANs. Sipal et al. [29] simulated the t-
ness scenario by performing ten push-ups and ten squats on male and female subjects. The collected
data was then analyzed to nd the impact of body movement on path gain, outage probability, and
channel fading on all three networks. The experiments concluded that head-centric networks, with
the lowest path gain and minimum channel fading, to be the best among the three categories. The
authors also agreed with observations from the previous studies [30] that waist and hip may not be
the best locations for placing hub in WBANs.
2.3 Communication Standards for WBANs
The IEEE 802.15.4 and IEEE 802.15.6 standards are commonly used to support communication in
WBANs. Table 3 summarizes their characteristics. In the following, we take a brief look at these
two standards.
IEEE 802.15.4
: This standard is also known as Zigbee. It is designed for low battery devices
supporting short-range communication. Zigbee is well-suited for PANs, where devices operate
on low bandwidth in battery-saving modes. Since WBANs typically require low power and
short-range communication, many WBANs are implemented using Zigbee. However, Zigbee
does not provide high data rate, which is necessary for WBANs [31].
IEEE 802.15.6
: The IEEE 802.15.6, also known as WBAN standard, is designed specically for
WBANs [32] in order to provide high data rate communication for low battery devices. This
standard consists of three physical sub-layers, a single MAC layer, and a security layer [33].
The three physical layers—NarrowBand (NB), Ultra WideBand (UWB), and Human Body
Communication (HBC)—are used according to the application requirements. It may be noted
that HBC uses human body for transmitting communication signals with the use of galvanic
coupling and capacitive coupling methods. In contrast, NB and UWB use conventional
wireless communication [34].
3 INTRA-WBAN COMMUNICATION
In the previous section, we discussed about the general architecture and communication standards
of WBANs. In this section, we focus on tier 1 of the architecture, where the sensor nodes and the
ACM Comput. Surv., Vol. 1, No. 1, Article 1. Publication date: January 2019.
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Table 3. Summary of IEEE 802.15.4 and 802.15.6 standards [35].
Specications IEEE 802.15.4 IEEE 802.15.6
Data rate 250 Kbps 10 Mbps
Range 10 m 3 m
Power Consumption 1 to 100 mW 1 to 10 mW
Frequency bands
2.4 GHz, 784 MHz, 868 MHz,
915 MHz
2.4 Ghz, 400 MHz, 800 MHz, 900
MHz
Singlehop Relay-based
One-hopdistant
Two-hopdistant
Sinknode
Fig. 2. Intra-BAN Communication in WBANs.
coordinator form a PAN. We also refer to this as an intra-WBAN communication scenario. Nodes in
an intra-WBAN are either organized in a star topology or are two-hop away from the coordinator
node (also known as two-hop extended star topology). The IEEE 802.15.6 communication standard
for WBANs suggests the use of at most two-hop topology in intra-WBAN networks [36], as shown
in Fig. 2. In a star topology (Fig. 2 (left)), a node delivers any message directly to the sink node.
In contrast, in relay-based communication (Fig. 2 (right)), some of the nodes use other nodes as
relays or intermediates to transfer information to the sink node. Since the human body area is
small, most sensor nodes maintain a star topology with the sink node in an ideal situation. The
energy consumption of nodes in WBANs is calculated as [37]:
𝐸𝑛𝑒𝑟𝑔𝑦𝑇𝑥 (𝑘, 𝑑 , 𝑛)=𝑘(𝐸𝑡𝑟 𝑎𝑛𝑠 +𝐸𝑎𝑚𝑝 .𝑑𝑛)(1)
𝐸𝑛𝑒𝑟𝑔𝑦𝑅𝑥 (𝑘)=𝐸𝑟𝑒 𝑐𝑣 𝑘(2)
where,
𝐸𝑛𝑒𝑟𝑔𝑦𝑇𝑥 (𝑘, 𝑑 , 𝑛)
is the energy required for transmitting
𝑘
bits at distance
𝑑
and
𝐸𝑛𝑒𝑟𝑔𝑦𝑅𝑥 (𝑘)
is the energy consumed in receiving
𝑘
bits.
𝐸𝑡𝑟 𝑎𝑛𝑠
,
𝐸𝑎𝑚𝑝
, and
𝐸𝑟𝑒𝑐𝑣
are the energies consumed by
the transmission circuit, amplication circuit, and receiver circuit, respectively. Finally,
𝑛
is the
path loss exponent.
The star topology is suitable when nodes are in line-of-sight (LOS) with the coordinator node,
whereas the relay-based topology supports communication during NLOS situations. In relay-based
communication, either an existing sensor node is chosen as a relay, or additional nodes are deployed
for that purpose. Since the additional nodes are used to support relaying in the network, we term
them as “special relay nodes.” The special relay nodes may or may not sense health parameters. The
primary purpose of these nodes is to help in relaying data. The special nodes are usually placed
outside the human body, for example, nodes attached to clothes [38]. Section 7 discuss routing
methods that use special relay nodes in intra-WBAN. Fig. 3 summarizes the message delivery
techniques for intra-WBANs.
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Transmission of
sensed data
Direct delivery Multi-hop path
Same sensors for
sensing and relaying
Special relay
node
Fig. 3. Message delivery techniques used in WBANs.
Communication methods in relay-node selection techniques exploit the estimation of channel
quality. The following sub-sections discuss channel state information, data forwarding strategies,
and diversity combining techniques of intra-BAN communications. More specically, Section 3.1
discusses the measurement matrices used to estimate communication channel condition. It also
discusses the data forwarding techniques used at intermediate relay nodes. Section 3.2 discusses
the diversity combining techniques used at the coordinator node to combine the common signal
received from multiple paths.
3.1 Channel State Information and Data Forwarding Strategies in Intra-WBANs
The eciency of communication depends on the properties of the channel used for communication
like the fading model of a channel. The information about the channel characteristics is termed
as Channel State Information (CSI) [39]. The CSI is estimated using various parameters, such as
channel gain, path loss, outage probability, channel fading, and shadowing. We dene some of
these terms below.
(1) Channel gain: The gain of any wireless channel is expressed as [39]:
𝑌=ℎ𝑋 +𝑍(3)
where
𝑋
is the signal transmitted by source,
𝑌
is the signal received at destination, and
𝑍
is
Adaptive White Gaussian Noise (AWGN).
(2) Path loss: The loss in signal quality due to propagation over the space is dened as [40]:
𝑃𝐿(𝑑)=𝑃𝐿(𝑑0) + 10𝑛log10
𝑑
𝑑0(4)
where
𝑃𝐿(𝑑)
is path loss in dB at distance d,
𝑃𝐿(𝑑0)
is path loss at the reference distance
𝑑0
and 𝑛is path loss exponent.
(3) Outage probability
: The outage probability is the likelihood that transmitted information
will suer from quality and quantity loss in a particular time period. The loss occurs due to
various reasons, for example, signal fading and shadowing.
Researchers have proposed dynamic relay selection mechanisms based on CSI [41, 42]. In dynamic
relay selection, in general, the relaying role may shift from one node to another depending upon the
contemporary CSI, for example, channel gain threshold. Moreover, selecting an intermediate relay
node for each round of data transmission changes dynamically based on the routing parameters.
Any given relay node processes data received from other sensor nodes and forwards such data
to the sink node. In the following, we discuss two popular methods used for data forwarding in
intra-BAN relay-based communications [43]:
Amplify-and-Forward (AF) [43]
: In this scheme, when a relay node receives data from a
sensor node, the relay node amplies the signal strength and forwards it to the next hop
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(destination in case of two-hop communication). This scheme is helpful when a message
travels a long distance, and the sensor node does not have sucient energy for direct
transmission. In such cases, the source node relays its message to a nearby relay node. The
relay node amplies and forwards the message toward the sink node. However, AF also
amplies noise at the relay node.
Decode-and-Forward (DF) [43]
: In this scheme, the relay node accepts and decodes the
received packet. The decoded packet is then passed through acceptance tests, such as Signal
to Noise Ratio (SNR) value of the packet and Bit Error Rate (BER). If the packet passes
the acceptance test, it is re-encoded and forwarded to the coordinator node. DF helps in
improving the quality of communication by allowing only non-erroneous packets to travel
in the network. However, DF requires calculation capabilities at the relay node resulting into
complex hardware requirements.
3.2 Diversity Combining Techniques Used in Relay-based Intra-BAN Communication
In relay-based communication, intermediate nodes use their own resources to forward data from
source nodes toward the corresponding destination nodes. Such intermediate relays are known as
cooperative relays. In poor network conditions, a source node sends message to the hub/coordinator
using direct and relay-based communication. When a hub/coordinator node receives a particular
packet along dierent paths, it uses diversity combining techniques to extract the transmission
with the best signal quality. Fig. 4 illustrates this process. In the following, we discuss three popular
techniques:
Fig. 4. Relay-based communication using diversity combining techniques.
Selection Combining (SC)
: In the SC technique, the hub/coordinator selects the transmission
with the strongest signal among all the received signals [44]. The “strength” of a signal can be
decided by its SNR. Other parameters, such as channel gain, can also be used in place of SNR. For
example, when channel gain is used, the signal with the highest channel gain is selected at the
receiver side.
Maximum Ratio Combining (MRC)
: Under this scheme, received signals are assigned weight
as per their SNR value [44]. A received signal with the highest SNR value is given the highest
weight. The weighted signals are then summed up to merge them into a single signal.
Switched Combining (SwC)
: This method uses a predened threshold of channel quality.
Channel parameters, such as channel gain and SNR, are used to dene the threshold. When a
signal’s quality in a particular channel drops below the threshold, the communication is shifted to
another channel that satises the dened threshold [45]. The channel switching can be based on
dierent strategies, such as two-branch switching and three-branch switching. In three-branch
switching, when the rst channel fails the threshold test, the second channel is tested. If this new
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channel also fails, then the third channel is chosen for communication irrespective of the threshold.
The third channel is used for communicating until the next time slot.
The remainder of this paper compares the performance of direct delivery with relay-based
communication and discusses various methods used to make relay node choice in intra-WBAN
communication. Fig. 5 presents a pictorial summary of these topics.
15.2%
9.1%
3.0%
3.0%
3.0%
9.1%
6.1%
12.1%
9.1%
12.1%
3.0%
6.1%
9.1%
Direct vs relay-based communication empirical studies
Direct vs relay-based communication non-empirical
studies
AHP based relay selection
Compressed sensing and relay selection exploiting
heterogeneity
Priority based energy aware routing
Cooperation based relays
Dynamic relay node selection
Experiments conducted with opportunistic routing in
WBANs
Opportunistic routing exploiting mobility
Achieving QoS in WBANs
Handling redundancy
Energy harvesting at relay node
Interference mitigation
Fig. 5. Overview of topics covered in relay-based communication in intra-WBANs.
4 DIRECT VS RELAY-BASED COMMUNICATION IN INTRA-BANS
Nodes in intra-BAN are, in general, organized in a star topology with the coordinator node at the
center [19]. However, the position of nodes deployed on the human body changes with the change
in human postures. Characteristics of the channel between the sensor node and the coordinator
also vary with the movement. In such cases, it is challenging to meet the QoS requirements of a
network.
In the following paragraph, we study and compare the network performance of relay-based and
direct communications. We divide these works into two categories, empirical and non-empirical
experiments. At the end of this section, we discuss the shortcomings of the studies presented in
this section.
4.1 Empirical Studies on Direct vs Relay-based Communication
Table 4 presents a summary of dierent empirical studies performed to dierentiate the two
broad categories of intra-BAN communication. “Wearable radios” in Table 4 refer to the testbed
containing CC2500 RF front end, an antenna, a micro controller SD card, and a battery [46]. “YAGEO
CAN4311111002451K” Bluetooth antenna is used in the setup, which is claimed to mimic the chip
antenna used in sensors of WBANs.
Dong et al. [47] compared the performance of double relay-based communication with single
relay-based and direct communication in WBANs. In double relay-based communication, two
dierent relay nodes listen to the signal from a source node and forward it to the destination
(hub). In single relay-based communication, only a single relay node forwards the packet. A
message generated by a source node reaches via direct-link and one among the two relay-based
communications. The multiple signals at the destination are then combined by using spatial diversity
combining technique. The authors designed two experimental scenarios by changing the positions
of relay nodes and a hub. The rst scenario considered hub is placed on the chest of a person, and
two relay nodes are placed on the left and right hip, respectively. In the second scenario, the position
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Table 4. Empirical Studies on Relay-based Communication (Section 4.1).
Ref Objective Performance
metric Device used
Human
activity
Dong et al.
[47]
Determining the possible
performance improvement
by using relay-based com-
munication over direct com-
munication.
BER Wearable radios
Day-
to-day
activity.
Smith et al.
[48]
Comparing eect of coopera-
tive communication over di-
rect delivery on MRC and
SC.
Outage proba-
bility
Wearable radios
Day-
to-day
activity.
Smith et al.
[49]
Comparing cooperative SwC
with cooperative SC.
Outage prob-
ability and
switching rate
Channel sounder
with IEEE
802.15.6 carrier
frequency
Day-
to-day
activity.
Jamjareegulgarn
[50]
Comparing performance of
cooperation based STBC-PC
with direct delivery.
BER in LOS
and NLOS
A sensor with an
attached antenna
Standing
posture.
Shimly et al.
[51]
Comparing SC and SwC in
relay based communication
with direct communication
in case of sleeping posture.
Outage proba-
bility and out-
age duration
Wearable radios
Sleeping
posture.
of the hub is changed from chest to the left hip, and the other two nodes acted as relay nodes. In
both scenarios, the hub node records the RSSI value of the signal received from seven nodes placed
at dierent parts of the body. Results indicated that BER for two-relay based communication is
less than that in one-relay based communication, which, in turn, is less than the BER in direct
communication.
Smith et al. [48] investigated the potential benets of using well-dened cooperative communica-
tion over direct communication. The experimental set up collected data from ten radio nodes, which
is now available as testbed data [46]. Among these ten nodes, three were special nodes capable of
acting both as a relay node and a hub. Rest of the nodes in the network acted as normal sensor
nodes. The authors considered three networks with hub locations at the chest, left hip, and right
hip, respectively. Relay nodes used the DF data forwarding scheme. The experiments conducted
involve ve adults (three males and two females) wearing radio nodes for two hours and were
engaged in day-to-day activities. Conclusions driven by the experiment were similar to the results
of an investigation done by Dong et al. [47]. The experiment result says that the relative position of
relay nodes with respect to the hub is a critical factor in improving the network performance. The
theoretical and experimental analysis of the outage probability of SC and MRC (diversity combining
techniques used a receiver) proved that relay-based communication, as compared to direct, can
improve the performance of WBANs.
Smith et al. [49] conducted a comparative performance analysis of cooperative single-relay SC
with two-relay SC and three-branch SwC with two-branch SwC. Data collected from their previous
experimental setup [48] was used in this study as well. They used channel gain to make the selection
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and switching decisions in cooperative SC and SwC, respectively. The high branch switching rate
reduces the duration of communication on any channel, and hence frequent disconnections occur.
Therefore, reducing the branch switching rate would help in improving the reliability of the
communication. The switching decision in the proposed three-branch SwC not only considers the
current channel condition but channel gains in the previous time instant as well. In order to make
a fair comparison, they experimented with the dierent values of channel gain threshold (
𝑆𝑇
) for
SC and SwC, and decided
86 dB as the nal
𝑆𝑇
for both cooperative (SC and SwC) techniques.
The results state that at 1% outage probability, three-branch SwC has 3dB improvement over
two-branch SwC. Whereas, the switching rate at
𝑆𝑇 =
86 dB reduced by 60% for three-branch
SwC in comparison to two-branch SC.
Jamjareegulgarn [50] extended the work of An et al. [52] and studied the use of Space time
block coding-precoding (STBC-PC) on WBANs with cooperative diversity-based relay selection
procedures (RSPs). STBC is a wireless communication technology that uses multiple antennas to
improve the quality of signal received at the receiver [53]. In STBC, a device having
𝑛
antennas
encodes the message, splits, and transmits the message over
𝑛
streams. The receiver removes noise
components from every received message and combines them into a single message. An et al.
proposed STBC-PC for devices with single antennas. In STBC-PC, the destination receives messages
through two paths, i.e., direct transmission and via a relay node. This generates the eect of two
transmission antennas at the source node. Source node sends continuous sequences of information
through multiple streams in STBC. An et al. achieved this eect by using precoding at the source
node and named their scheme as STBC-PC.
Jamjareegulgarn used RSPs proposed by Woradit et al. [54] without any modication in their
experiment. RSPs used are Fixed Selective Decode and Forward (FSDF) with and without direct
link combining (under high and low SNR regime), respectively. The author considered six nodes
where each node consists of a sensor, an electronic module, and a single antenna. Four nodes
out of the six acted as relays; the other two nodes placed on head and stomach acted as source
and destination, respectively. Relay selection in all schemes was based on CSI and SNR value
of the channel between the source and relays, and source and destination. Experimental results
showed that FSDF outperformed direct transmission when the channel’s SNR value was below
some pre-dened threshold. Hence, it was concluded that in a high SNR region, the outage capacity
of FSDF was almost half of the outage capacity given by direct communication.
4.1.1 Direct vs Relay-based Communication in Sleeping Human Posture. Communication in sleeping
human posture is challenging. This is because the duration of link disconnections is long compared
to other human postures. Several experiments were performed considering the sleeping posture
of the human body. Shimly et al. [51] studied the performance of three-branch cooperative SC
(SC technique with a direct branch and two indirect, two-hop branches via two dierent relays)
and three-branch cooperative SwC combining techniques in connection with the sleeping posture.
They considered the IEEE 802.15.6 channel model with 2
.
4GHz ISM band. Intra-BAN relaying data
were collected from eight adults [55]. The authors used seven nodes in their experimental setup.
Four nodes out of seven are receivers (Rx) only. The remaining three nodes are transmitters as
well as receivers (Tx/Rx). Table 5 mentions the position of nodes and Fig. 6 shows the position of
dierent nodes. Nodes were placed as wearable radios, where each Tx was transmitting for 5ms in
robin round fashion. Since there is little movement in WBANs while sleeping hence, the blockage
of any channel can last for a long time. Taking this into account, the authors considered outage
probability and outage duration as the best parameters to evaluate the two relaying strategies.
Among direct link (DL), SwC, and SC, three-branch SC was found to provide better outage
probability and outage duration. In the case of on-body communication, SC and SwC experienced a
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continuous outage duration of more than 10 secs for 2% of the total time, whereas DL experienced
a similar outage duration for 16
.
3% of the complete time duration. The best-case outage probability
for SwC was 3.3% of the entire time, whereas SC experienced 1.15% outage probability.
Table 5. Position of nodes.
Symbols Node position
HbHip-back
HfHip-front
LaLeft ankle
LwLeft wrist
NTBfNear to bed-foot
NTBhNear to bed-head
RwRight wrist
NTBh
Lw
La
NTBf
Rw
Hf
Hb
Rx
Tx/Rx
Fig. 6. Top view of nodes deployed on a sleeping human.
4.2 Non-empirical Studies on Direct vs Relay-based Communication
In this section, we discuss the simulation-based studies conducted to compare direct transmis-
sion performance with relay-based communication. The non-empirical studies use tools, such as
MATLAB, OMNET++, Castalia, and NS3, to simulate the communication environment and test the
performance of newly proposed algorithms.
Haung et al. [56] analyzed the outage probability in WBANs for direct delivery, single-relay,
and multi-relay cooperative communication. The single-relay communication used proactive relay
selection, whereas the multi-hop relay used reactive relay selection strategy. Both the strategy used
two time slots. In the rst slot, a source sends a packet to the destination. In the single-relay strategy,
there exists a predened neighbor who overhears the ongoing communication. If the destination
replies with a negative acknowledgment (NACK), then in the second time slot, the overhearing
neighbor re-transmits the packet to the destination. In the case of multi-relay communication,
there is no predened relaying neighbor. Let us assume that
𝑘
potential relays exist.
𝑚
relays out
of
𝑘
overhear packet transmitted by the source node successfully. A node having the best channel
quality with the destination among
𝑚
nodes is elected as the relay node. The candidate relay node
having maximum SNR value for the channel between itself and the destination is considered as
the best node. However, the authors did not mention how message exchange would happen to
measure the SNR value. All types of communications observed channel outage depending upon
the human posture. Therefore, the authors collected results for two dierent path loss values— 50
dBm and 70 dBm. Direct communication provides low energy consumption when path loss is low
(50 dBm) because, in multi-relay communication, the receiver circuit of the relay node consumes
energy. However, in higher path loss scenarios (NLOS communication due to change in human
posture), multi-relay communication provides better results over the direct transmission.
Ismail et al. [57] conducted a study to decide an ecient mode of communication out of direct
delivery, single-relay, and two-relay based communication in WBANs. The position of intermediate
relay node
𝑟
in single-relay communication is xed at distance
𝑑𝑟𝐷
from the destination
𝐷
. In
two-relay based communication, the position of two relay nodes is distributed uniformly within
the distance
𝑑𝑟𝐷
. The authors used AF data forwarding scheme at relay nodes to forward data
towards the destination. Relay nodes in their proposed scheme split their power, and use frequency
division multiple access (FDMA) to support relaying from multiple sources. The authors derived an
energy minimization problem based upon power division and outage probability, where power
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division is adjusted as per the QoS requirement. Power used to transmit a message depends upon
the relaying scheme used. For example, in two-relay based communication, distance between the
source and the destination is partitioned into three small distances. Intermediate relays would
reduce the long-range communication into short-range communication. Hence, the sender in
two-relay based communication will use low power for transmission compared to the single-relay
communication. The AF scheme used at the intermediate node will help retain the quality of the
received low power signal. Two-relay based communication outperformed the single-relay based
communication because of reduced distance communication and AF forwarding scheme. They
minimized energy consumption per bit by using cooperation with two-relay based communication.
4.2.1 Relay Network Model for Implanted Wireless Capsule Endoscopy (WCE). Liu et al. [58] studied
Wireless Capsule Endoscopy (WCE) as a WBAN application. In WCE images of small and large
intestines are captured using a capsule endoscope and are transmitted to wireless nodes xed on
a pad worn by a patient around his/her waist. Liu et al. investigated the eects of using multiple
relay nodes in the pad placed on the capsule’s power consumption. The objective is to improve
the lifetime of the capsule by reducing the transmission power to achieve eective monitoring
of the Gastrointestinal (GI) tract. In their system model, they considered a belt (pad) as an ellipse
with relay nodes implanted on it. The even distribution of relay nodes is assured by the angle
placement, rather than the distance of the relay node from the center, because distance may vary
from fat to slim person. The power consumption decreased gradually with the increasing number
of relay nodes provided the topology had more than ve relay nodes. However, in the case of two
relay nodes with one relay xed in front and other at the back, the distance between capsule and
relays increased, which increased the power consumption. To further experiment with the position
of relay nodes, Liu et al. considered three sensor nodes on the belt and rotated them to a new
position. The results indicated that there was an optimal angle of placement of relay nodes for every
organ, which helped reduce power consumption of WCE. Considering both the experiments, it was
observed that the best monitoring of the intestine was obtained with 523 relay nodes. Outside this
range, no signicant improvement was observed, in general, irrespective of the angle of placement
of the relay nodes.
4.3 Discussion
To summarize, we see that various empirical and non-empirical studies have been conducted
to characterize intra-WBAN communication. We derived the following important observations
from this Section. First, the studies, in general, suggest that relay-based communication is more
energy-ecient than its direct counterpart during poor channel conditions. Moreover, relay-based
communication seems to fare better in outage probability, outage duration and BER, which helps in
improving the quality of wireless communications. Second, positioning the relay node with respect
to the hub is signicant in determining the network’s performance. Third, the performance of
relay-based communication increases with increasing branches in MRC and SC diversity combining
techniques. However, it is not feasible to use multiple branches in WBANs due to the size con-
straints of WBANs. This arises a need for more experiments on making use of diversity combining
techniques in WBANs.
5 RELAY NODE SELECTION METHODS IN INTRA-WBANS NOT EXPLOITING
HUMAN MOBILITY
In the previous section, we observed that relay-based intra-BAN communication could improve
network usage eciency, in general. In order to attain such eciency, it is important to select
appropriate nodes as relays. Further, in this section, we explore relay node selection methods
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of WBANs in detail under four categories: decision-making technique, heterogeneity based cost
function, game theory based, and opportunistic relay-based approaches. These methods focus on
relay node selection mechanisms without considering physical layer parameters, such as spatial
diversity combining techniques. Section 5.1 discusses the use of decision-making techniques, such
as the analytical hierarchy process for selecting a relay node. In 5.2, we discuss the relay selection
methods that consider the heterogeneity of nodes in a WBAN. Section 5.3 discusses the use of game
theory in making relay node choice and Section 5.4 discusses the example of opportunistic routing
in WBANs.
5.1 Using Decision-making Technique: Analytical Hierarchical Process
Analytical Hierarchical Process (AHP) is a comparison based decision-making process. AHP takes
multiple decision parameters as input and performs a pair-wise comparison to decide the weight of
dierent input parameters towards the nal decision. AHP is used to assign weights to multiple
network-related parameters under consideration and select the suitable relay node among various
candidates. The AHP technique was rst used by Wu et al. [59] to achieve ecient routing in body
sensor networks. They used four parameters of a sensor node—remaining battery level, distance
to the gateway, mobility, and queue size—for making relay node selection. The idea helped in
minimizing the energy consumption and packet loss rate of the network.
Kim et al. [60] addressed the issue of changing link quality arising due to mobility in WBANs.
Therefore, they focused on designing an energy-ecient relay selection mechanism for extended
two-hop star topology using IEEE 802.15.6. The authors noted that contemporary methods lack in
considering exibility in channel link conditions. Moreover, attempts to reuse previously-broken
but now-restored links are mainly lacking. Motivated by these, Kim et al. proposed a exible solution
based on AHP by considering dierent parameters, such as average SNR, trac load, and residual
energy level. The proposed design consists of three modules, where the rst module initiated
discovery for relay candidates. On receiving a relay request, the neighbor nodes send their AHP
parameter values to the requesting node. These values are used by the second module to select the
relay node using decision factors at each hierarchy of AHP. The third module is designed to recover
the lost direct communication link using the following mechanism: the coordinator generates
a beacon for the source node upon hearing a communication between relay and source nodes.
Based on the successful delivery of this beacon, the source node infers that the previously-broken
direct link has regained its quality. Consequently, the source node reverts to the direct link with
the coordinator node for subsequent communications. The relay node clears its table entry after
overhearing beacons from the coordinator. The proposed algorithm was simulated using OMNET++,
and results were compared against IEEE 802.15.6. Improvement in energy, packet reception rate,
latency, and network stability was noticed in comparison to the contemporary communication
approaches.
5.2 Node Heterogeneity Based Cost Function for Relay Node Selection
A WBAN is a network of sensor nodes monitoring dierent health parameters. Every health
parameter of a human needs a dierent type of sensing technology resulting in dierent designs
and features of biomedical sensors. One biomedical sensor could vary from the other in terms
of assigned initial energy and sampling rate. Researchers exploited heterogeneity in biomedical
sensors to make optimal relay node selection. Priority-based Energy Aware (PEA) and Energy-
ecient Data Forwarding Strategy (EDFS) are examples of such relay node selection methods in
intra-WBANs.
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5.2.1 Priority-based Energy Aware Routing. Talha et al. [61] proposed the Priority-based Energy
Aware (PEA) routing scheme for WBANs. The relay selection mechanism of PEA arranges the
nodes in a logical hierarchy. Nodes with higher residual energy level act as parents, whereas those
with lower energy levels as child nodes. Each child node is assigned a priority based on data it
senses. A parent node can directly communicate with the sink node, and acts as a relay for its
child nodes. A child node can use other child node having priority lower than itself, as its relay
node. The proposed parent-child relationship is exible; it alters with changes in posture, which
can help in handling dynamic topology. In general, any change in a user’s posture can alter the
WBAN’s topology. The authors investigated the performance of PEA by comparing the proposed
cost function,
𝑐𝑜𝑠 𝑡 (𝑖)=𝐸(𝑖)
𝐷(𝑖)+𝑃(𝑖)
, with existing base cost functions,
𝑐𝑜𝑠 𝑡 (𝑖)=𝐸(𝑖)
,
𝐷(𝑖)
and
𝐸(𝑖)
𝐷(𝑖)
. Here,
𝑃(𝑖)
and
𝐸(𝑖)
is the priority and residual energy of node
𝑖
, respectively, and
𝐷(𝑖)
is
the communication distance. The use of the proposed cost function indicated to provide better
performance in terms of residual energy and throughput.
5.2.2 Exploiting Heterogeneity and Sparsity in sensed data. Compressed Sensing (CS) [62] is a signal
processing method which processes the sparse signal to reduce the number of samples required
to reconstruct the signal at the receiver. The number of signal samples suggested by CS is lesser
than the Nyquist sampling rate. CS has been used widely by researchers in the area of WBANs.
Wu et al. [63] claim that the data generated by sensors attached to the human body is sparse and
could be compressed using CS method. Wu et al. used CS at sensor nodes for data generation.
They proposed an energy-ecient data forwarding strategy, named EDFS to deliver data to the
destination. EDFS considers the following heterogeneous parameters of sensor nodes to make a
suitable relay-node choice— 1) sampling frequency, 2) residual energy and 3) the critical data sensed
by a node. The overall communication process of EDFS is illustrated pictorially by Fig. 7.
Fig. 7. Pictorial representation of relay selection proposed by Wu et al. [63].
In the initialization phase, the sink node broadcasts sampling frequency, the impact of health
reading and initial energy to all nodes. The impact of health reading is decided based on health
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parameters. On receiving the initial message from the sink node, all other nodes update their
respective distance from the sink node. Next, the sensor nodes collect data samples at the sampling
frequency decided as per the compressed sensing method. Once the sensed data is ready to send,
the source node
𝑖
broadcasts a relay selection request as shown in Fig. 7. On receiving the relay
request, only the nodes having residual energy (
𝐸𝑟𝑒𝑠
) greater than the dened energy threshold
(
𝐸𝑡ℎ
) (let say
𝑗
nodes) prepare a response packet as shown in Fig. 7. After receiving responses from
candidate relays, the source node
𝑖
compares the distance of the candidate relays. Let say
𝑋𝑗
is
a set of nodes present between the source node
𝑖
and sink, i.e.,
𝑑𝑖 𝑗 <𝑑𝑗𝑠 , 𝑑 𝑗𝑠 <𝑑𝑖 𝑠
are qualied for
further relay node selection. A node, among the qualied nodes having a maximum value of the
response is selected as an optimal relay node.
Liu et al. [64] performed CS on electroencephalogram (EEG) signals. They multiplied EEG signals
with the binary permuted block diagonal matrix to compress the data. The simulation results
revealed that the original EEG signal is reconstructed at the receiver side. Likewise, there are many
contributions that use CS to make WBANs more ecient [65, 66, 67].
5.3 Game Theory Based Approaches
Moosavi et al. [68] designed a relay selection approach using Nash equilibrium to manage QoS in a
non-cooperative environment. In their system, they considered the GI/G/1 queuing model [69] with
a single packet capacity. The probability of successful delivery of a packet was calculated by dening
inter-arrival and service time distributions. Inter-arrival time distribution is the probability with
which a node will generate its packet. Service time distribution is the likelihood of the node getting
a free channel to transmit its packet. Channels are allocated to nodes using the slotted ALOHA
channel access mechanism. The authors designed a utility function addressing the power selection
of the sensor node and an intermediate relay selection while satisfying QoS constraints such as
end-to-end delay and jitter. They named their game as RSPCG (Relay Selection and Power Control
Game). The designed utility function satises the Nash equilibrium where a node tries to maximize
its energy eciency. Simulation results revealed that with the increasing tolerable packet error
rate, energy eciency also increased. They also compared the performance of direct transmission
with RSPCG in terms of end-to-end delay. Larger packet size suers more packet errors and hence
require more service time. In such a scenario, RSPCG outperformed direct transmission with the
increasing packet size.
5.4 Opportunistic Relays
Opportunistic routing (OR) uses the favourable circumstances such as good channel state (low path
loss) or encountered nodes (nodes responding to a broadcast request) to build the connection gap.
OR has been found useful in many wireless networks, such as Delay Tolerant Networks (DTNs)
[70, 71] and Vehicular Networks (VANETs) [72, 73].
Abbasi et al. [74, 75] investigated the performance of OR in WBANs. In order to simulate the
on-body communication scenario, the authors implemented OR using the lognormal path loss
model suggested by Smith et al. in [76] and a path loss model dened by IEEE 802.15.6 CM 3A for
body area networks [32]. The lognormal and IEEE 802.15.6 CM 3A path loss models are dened by
(5) and (6), respectively.
𝑃𝐿(𝑑)=𝑃𝐿(𝑑0) + 10(𝑑) + 𝑋𝜎(5)
Here,
𝑃𝐿(𝑑)
is path loss at distance
𝑑
and
𝑃𝐿(𝑑0)
is path loss at reference distance
𝑑𝑜
.
𝜂
is the path
loss exponent and
𝑋𝜎
is zero-mean Gaussian distributed random variable with standard deviation
𝜎.
𝑃𝐿(𝑑)=𝑎𝐿𝑜𝑔(𝑑) + 𝑏+𝑁𝜎(6)
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In (6), 𝑃𝐿(𝑑)is path loss at distance 𝑑;𝑎and 𝑏are the coecients of linear tting. 𝑁𝜎is normally
distributed random variable with standard deviation 𝜎.
The proposed OR communication works as follows—a source node broadcasts a request to send
(RTS) packet; the node whose clear to send (CTS) response reaches the source node rst is chosen
as a relay node. The authors implemented direct communication on the IEEE 802.15.6 CM 3A
path loss model. Results of the investigation concluded that OR (for both path loss models) has
better performance over direct communication. More precisely, IEEE 802.15.6 CM provided better
outcomes for delivery probability, network lifetime, and end-to-end delay in comparison to the
Lognormal shadowing model and direct transmission.
The OR method discussed by Abbasi et al. [74, 75] uses ooding. However, ooding may increase
energy consumption and lead to congestion, which is not suitable for resource-constraint WBANs.
Consequently, Abbasi et al. in [77] proposed the Cross-Layer Opportunistic MAC/Routing (COMR)
scheme to mitigate this issue. They used the four-way handshaking mechanism based on RTS/CTS
to prevent collisions. In particular, the relay selection process involves the following steps:
(1)
Initially, the sink broadcasts a beacon packet. Every other node
𝑖
in the WBAN calculates
RSSI of the received beacon, 𝑅𝑆𝑆 𝐼 𝑖
𝑏𝑒𝑎𝑐𝑜 𝑛 .
(2) The source node initiates a relay request by broadcasting an RTS packet on idle channel.
(3)
Every node
𝑖
that received RTS calculates
𝑅𝑆𝑆 𝐼 𝑖
𝑟𝑡 𝑠
, the RSSI of the received RTS, and compares
with
𝑅𝑆𝑆 𝐼 𝑖
𝑏𝑒𝑎𝑐𝑜 𝑛
. Nodes with
𝑅𝑆𝑆 𝐼 𝑖
𝑏𝑒𝑎𝑐𝑜 𝑛 >𝑅𝑆𝑆𝐼 𝑖
𝑟𝑡 𝑠
, i.e., those closer to the sink than the source,
are considered as the candidate relay nodes.
(4)
Subsequently, every candidate relay node
𝑗
replies to the received
𝑅𝑇𝑆
after a time delay
𝑡𝑑𝑒𝑙 𝑎𝑦 , which is calculated as follows:
𝑡𝑑𝑒𝑙 𝑎𝑦 =𝑤𝑒×𝑒𝑖𝑛𝑖𝑡𝑖 𝑎𝑙
𝑒𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙
+𝑤𝑟×𝑅𝑆𝑆 𝐼 𝑗
𝑟𝑡 𝑠
𝑃𝑝𝑟𝑡
(7)
Here,
𝑤𝑒, 𝑤𝑟∈ [
0
,
1
]
are constants, and
𝑒𝑖𝑛𝑖𝑡 𝑖𝑎𝑙
and
𝑒𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙
are initial and residual energies,
respectively. 𝑃𝑝𝑟𝑡 is the packet reception-power threshold in milliwatts.
(5)
The node
𝑗
having higher residual energy and high
𝑅𝑆𝑆 𝐼 𝑗
𝑟𝑡 𝑠
would have lower value of
𝑡𝑑𝑒𝑙 𝑎𝑦
and therefore, is selected as the suitable relay node.
The authors compared COMR with Simple Opportunistic Relays (SOR) [78]. SOR is an OR method
proposed for WSNs and selects relay node randomly. The results obtained were in favour of COMR.
Therefore, the said comparison of SOR with COMR also indicates that the protocols used for WSNs
are not necessarily suitable for WBANs.
In this , we discussed how decision-making techniques like AHP are used to select an optimal
relay node during link failures in the network. The other most popular method used in relay
selection is by designing a cost function. We discussed how dierent important parameters of the
network are combined into a single cost function to make an optimal relay node choice. We also
discussed how game theory could be integrated to achieve reliable communication in intra-WBANs.
However, the methods proposed are not limited to this discussion. Table 6 provides a quick overview
of methods discussed in this Section.
6 RELAY NODE SELECTION METHODS IN INTRA-WBANS EXPLOITING HUMAN
MOBILITY
Human mobility is a challenging task in achieving adequate functionality in intra-WBAN commu-
nication because the channel characteristics of the communication link change with the change
in human posture. For example, Fig. 8 shows the channel gain, sampled per millisecond when a
node placed on the left ankle of a human communicates with the coordinator node placed on the
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Table 6. Summary of relay selection methods (Section 5).
Ref Method Critics
Wu et al.
[59]
AHP based weight assignment for de-
cision parameters.
The routing decision is made at every in-
termediate node which could cause delay
in transmission.
Kim et al.
[60]
AHP based weight assignment for de-
cision parameters + channel recovery
module.
The model lacks evaluation in real mobil-
ity scenario.
Wu et al.
[63]
Compressed sensing by the source
and relay selection decision based
upon priority and distance of nodes.
There is a possibility of network conges-
tion on selected relay node because a sin-
gle node can satisfy the cost function for
more than one source.
Talha et al.
[61]
Nodes are assigned priority in dy-
namic topology. Node with the high-
est priority can use node with lowest
priority as the relay node.
Authors considered parent-child topology
however they did not consider channel
assessment at the parent node.
Moosavi
et al. [68]
Exploiting existence of Nash equi-
librium in proposed relay selection
method to achieve energy ecient
communication.
Authors did not consider the delay in ser-
vices introduced due to iterations con-
ducted until Nash equilibrium is achieved.
Abbasi et
al. [77]
Cross-Layer opportunistic
MAC/routing (COMR) using Four-
way handshaking to prevent
congestion.
Authors considered a static network with
all nodes positioned on the front body.
There is less scope of opportunistic selec-
tion in such scenarios.
chest [79]. Since human walking is a periodic event, the pattern of channel gain in Fig. 8 is also
0 2000 4000 6000 8000 10000 12000 14000 16000 18000
Data samples
-80
-70
-60
Channel gain (dB)
Fig. 8. Typical channel gain in walking human posture.
periodic. When the channel gain is low, the channel is considered to be good for transmissions.
The communication methods that exploit human mobility can be categorized into two sections.
First, methods directly consider the acceleration in the human body for relaying data and second,
which considers change in channel quality due to human mobility.
6.1 Using Human Body Acceleration for Communication
Nodes that are positioned on the human limbs suer frequent disconnections while walking
or running. Whereas, nodes positioned on the human torso are static for the coordinator node
positioned on the same side of the human body. Though the human body is highly dynamic, most
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of the highly dynamic motions of the body are periodic such as walking or running. Researchers
took advantage of periodic movement of the human body and used it to improve the network
performance. Maskooki et al. [80] observed that placing the sink node on either of the wrists helps
in handling non-line of sight (NLOS) communication as well. They placed a sink node on the wrist,
a sender node on the chest, and a relay node on the waist of a human subject, as shown in Fig. 9. In
Sink node
Signal from
sensor
LOS link
Relay node
Fig. 9. Using human body movement while running to build connectivity.
the proposed opportunistic routing, a sensor node broadcasts an RTS packet. The power level of the
RTS packet is adjusted so that it reaches only to the LOS nodes. As a human walks, the sink node is
expected to change its position from the front side to the back and vice versa depending on the wrist
(hand). If sink node is not in LOS of the source node, the source node sends wake up message to the
relay node. The subsequent communication is supported by relay node, which forwards messages
from the source node to the sink. The proposed method was found to be more energy-ecient than
multi-hop communication, in general. Similarly, Adhikary et al. [81] considered the velocity (
𝑣(𝑖)
)
of an intermediate node
𝑖
to make optimal relay node selection in terms of energy and latency. The
proposed cost function
𝑑(𝑖)−𝑣(𝑖)
𝑅𝐸 (𝑖)
selects node moving fast towards the coordinator and having high
residual energy (𝑅𝐸(𝑖)) as a relay node.
6.2 Using Channel Variation for Communication
Human mobility aects channel quality, which is shown in Fig. 8. Yang et al. [82] proposed a relay-
based communication method by considering variation in channel quality in walking and running
human postures. The proposed relay node selection procedure calculates a connectivity cost for
every candidate relay node. A node with the highest connectivity cost is selected for relaying. The
connectivity cost is a weighted sum of historical channel knowledge and instantaneous change
in the network topology. The authors claim that instantaneous postural knowledge plays an
important role in determining channel connectivity status. They used entropy to measure the
change in pairwise node connection at any instant of time. The value of entropy increases with the
increasing change in pairwise connectivity of nodes. This entropy value is used to assign weights
to historical and instantaneous topology knowledge. The results obtained provides high delivery
ratio in compared to the probabilistic method that considers historical knowledge only.
Hamida et al. [83] investigate the mobile links quality in WABNS and classied them into three
classes—long-term reliable, short-term reliable, and unreliable links. Long-term reliability indicates
that such links would continue to be available for a longer time than the short-term ones. On the
other hand, links with dicult propagation conditions are categorized as unreliable. The authors
proposed an opportunistic routing scheme for WBANs by exploiting link quality. Whenever a node
has to transmit a message, the direct link is tested for its quality. If the estimated link duration is
found to exceed the expected packet transmission time, then the message is forwarded along that
(direct) link. Otherwise, a multi-hop path is selected.
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As the study evolved in opportunistic routing in intra-WBANs, research emphasized transmitting
data directly to the coordinator in the case of periodic human movements [84]. These methods may
take support from additional sensors such as accelerometer and gyroscopes to nd the correlation
in varying human posture. The knowledge of human mobility is then used to predict the positions
and time points when the channel state would be good for data transmission [85]. A study on such
methods, however, are scoped out of this paper.
From the studies discussed so far in this section, we conclude that human mobility can be handled
eectively by understanding the channel behaviour. Table 7 summarizes the methods discussed in
this section.
Table 7. Summary of relay selection methods exploiting human mobility (Section 6).
Ref Method Critics
Maskooki
et al. [80]
Place sink on the wrist and exploit
hand movement to deliver message.
Placing sink at the wrist would cause fre-
quent link failures.
Adhikary
et al. [81]
Select a node moving fast towards the
coordinator as a relay node.
The authors assumed that direction of
movement of a node is known. However,
the relative position of nodes is challeng-
ing to achieve in WBANs. The method
could be improved further by consider-
ing relative positioning on nodes in intra-
WBANs.
Yang et al.
[82]
Use instantaneous topology knowl-
edge along with historical knowledge
in relay selection cost function.
The proposed method does not consider
connectivity of relay node with the coor-
dinator. Therefore, there is a possibility
that the selected relay node may not be
in connection with the coordinator node.
Hamida et
al. [83]
Test the link quality, if quality persists
for the expected packet transmission
time, then transmit the packet.
Authors considered only human walk-
ing model. Human walking has short-
duration links. Therefore, the method
lacks in comprehensive evaluation.
7 RELAY-BASED ROUTING USING SPECIAL NODES ALONG WITH BIOMEDICAL
SENSORS IN INTRA-WBANS
Relay-based communication provides a scope of using special nodes, i.e., nodes other than biomedi-
cal sensors, in an intra-WBAN. Such special nodes are usually equipped with extra functionality or
resources such as high computation, high power transmission antennas, or rechargeable batteries,
which help in improving the performance of the network. In this Section, we discuss relay-based
communication that uses special nodes to support intra-WBAN communication.
7.1 Special Nodes not Capable for Sensing
The intra-WBANs discussed in this Section use special relay nodes for supporting multi-hop
communication. These special relay nodes do not have sensing capabilities and help in data
forwarding only.
Direct communication transmits packet fast and is useful in emergency situations, whereas
cooperative communication helps increase the lifetime of the network by reducing transmission
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distance. Yousaf et al. [86] combined the best features of direct and cooperative communication and
proposed an incremental relay-based cooperation scheme, InCo-CEStat. In the proposed scheme,
each node has two potential relays, R1 and R2. The relay nodes are dierent from the sensor
nodes. In the rst phase of communication, the source node transmits its data packets to R1 and is
overheard by the relay R2. In the second phase, R1 veries the packet for BER, and if acceptable,
it is transmitted to the sink node. In case R1 fails to forward the packet due to distortion of the
packet, R2 forwards the same packet to the sink node. A sensor node sends messages directly to
handle emergency cases. At the receiver side, switched combining technique is used to select the
signal with the lowest BER from multiple received signals. The proposed scheme was implemented
using MATLAB. InCo-CEStat provided better delivery ratio, energy consumption, and network
stability in contrast to the benchmarks considered. However, high throughput is achieved at the
cost of high latency.
Rostampour et al. [38] assumed two types of nodes in a WBAN, biomedical sensor nodes
(positioned on the human body) and relay nodes (attached to the patient’s cloth). The relay nodes are
equipped with rechargeable batteries. Rostampour et al. stated that, in relay-based communication
methods, it is necessary to consider uniform and optimal energy consumption at relay nodes. The
increased energy consumption of relay nodes could result in frequent node recharging, aecting
the network’s performance. The authors highlighted that positioning of relay nodes in a WBAN
plays an essential role in achieving optimal energy consumption at relay nodes. They proposed a
utility function to maximize the uniform utilization of the energy of relay nodes and thus extending
the lifetime of the network. The utility function considered relay node installation cost, and the
energy consumption of biomedical sensors and relay nodes. Considering the installation cost helped
in restricting the maximum number of relay nodes required to achieve optimal performance. In
order to nd the position of relay nodes they divide human clothing into dierent ellipsoidal areas,
as shown in Fig. 10. The feasible places of placing relay nodes (candidate sites) are distributed
uniformly on the ellipsoidal areas. Sensor nodes communicate with the relay node placed in the
candidate site closer to the nodes. blueIn the simulation, authors considered multiple candidate sites
and tested how sensor nodes use many of them for relaying data. They assumed a WBAN consisting
of 13 biomedical sensors. It was observed that with the increasing sample space of candidate sites,
selected relay nodes decreased. Out of 40 candidate sites, 12 relay nodes are selected whereas, out
of 70 candidate sites 10 relay nodes are selected. This means that when the candidate sites are more,
the proposed model selects best relays only, which are providing full connectivity with uniform
energy consumption.
x
y
z
Fig. 10. Ellipsoidal areas on human body [38].
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7.2 Special Relay Nodes Capable for Sensing
The use of special relay nodes increases the hardware cost while providing improved network
performance. The other option is to provide extra resources to some of the biomedical sensors
belonging to the network. These nodes, equipped with additional resources could provide the
benets of using additional relay nodes while performing sensing also.
Ahmed et al. [87] proposed Co-LEEBA, a multi-hop data transmission scheme for intra-WBAN
communication. In their system model, they considered two types of nodes named as simple nodes
and advanced nodes. The advanced nodes have more initial energy than simple nodes and are
capable of performing sensing as well as relaying of data generated by simple nodes. A simple node
performs direct transmission if the data generated is urgent, the residual energy of a simple node is
greater than the residual energy of the advance node, and the sink is closer to the simple node than
any adance node. A simple node (
𝑆
) selects a new advanced node (
𝑅
) as its relay in every round of
data transmission using (8).
𝑡𝑟 𝑎𝑛𝑠𝑠𝑒𝑙𝑒 𝑐 =(Direct transmission, 𝑖 𝑓 𝐸𝑟𝑒 (𝑆)>𝐸𝑟 𝑒 (𝑅)
Multihop relay path, 𝑜𝑡ℎ𝑒 𝑟𝑤𝑖 𝑠𝑒 (8)
Here,
𝐸𝑟𝑒
is remaining energy of the node and
𝑡𝑟 𝑎𝑛𝑠𝑠𝑒𝑙𝑒 𝑐
is the transmission selection decision.
Node
𝑆
selects nearest
𝑅
for which
𝑡𝑟 𝑎𝑛𝑠𝑠𝑒𝑙𝑒 𝑐 =
1. The selected relay node use AF to boost the
received signal before forwarding it to the sink node. As a result, the network observed improved
stability period and throughput.
From Section 7, we conclude that relay-based communication can also be performed by using
additional nodes in the network. This opens new research areas to solve the problem of achieving
balanced energy consumption among relay nodes, nding the optimal number and position of
relay nodes in an intra-WBAN. Table 8 summarizes the Section 7 and highlights the lacuna of work
discussed in this Section.
Table 8. Summary of relay-based routing using special nodes (Section 7).
Ref Method Critics
Yousaf et
al. [86]
Consider two potential relays per sen-
sor node.
The method suggests static routing which
may not work eectively for dynamic hu-
man postures such as walking and run-
ning.
Rostampour
et al. [38]
Use special relay nodes attached to the
human cloths. Find optimal number
and position of relay nodes to achieve
improved network lifetime.
As per the simulation results, the opti-
mal solution suggests 10 relay nodes for a
WBAN of 13 nodes, which is almost equal
to the number of biomedical nodes in the
network. The number of relays can be
further optimized to a smaller number.
Ahmed et
al. [87]
Assign additional energy to some of
the biomedical sensors and use them
for sensing as well as relaying.
The relay selection cost function can be
improved further to consider the chan-
nel quality. Moreover, the authors did not
evaluate the proposed method on link fail-
ures.
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8 PERFORMANCE ISSUES ADDRESSED USING RELAY-BASED COMMUNICATION
IN INTRA-WBANS
The studies discussed till now suggest that the use of relay-based communication over direct
delivery can help to deal with various network crises, such as link failure and NLOS communication.
In this section, we extend the discussion by looking at how relay-based communication can
help in improving network performance, for example, in terms of achieving QoS levels, handling
redundancy, and harvesting energy at the intermediate nodes.
8.1 Achieving QoS in WBANs
QoS measures reect how well an operational network is performing. Moreover, networks are
designed in a way to support desired QoS levels. It is, therefore, essential to understand how QoS
can be achieved in WBANs, which are critical networks vis-a-vis a patient’s health. To this eect,
in this section, we look at some of the QoS aspects of WBANs, such as data rate, network lifetime,
stability period, and bit error rate (BER).
8.1.1 Meeting Data Rate Requirements. Meeting data rate requirements is a prime feature of any
advancing communication network. In multi-hop paths, a situation of bottleneck may arise in
intra-WBANs. Therefore, a routing protocol must satisfy the data rate requirements of a WBAN
application. Zuhra et al. in [88] proposed a relay-based communication method to handle data
trac such that situation of congestion could be prevented. The authors proposed the low latency
prioritization method (LLTP), which categorizes the trac at the network layer on a priority
basis and favors the transmission of sensitive data earlier than others. After categorizing data,
AODV based route selection algorithm selects an appropriate path that could satisfy the data rate
requirement of the application.
There exists minimal work in intra-WBANs achieving data rate requirements. However, data
rate requirements are often satised by appending then as constraints to a utility function designed
to make relay node selection. For example, a relay selection problem designed by Chai et al. [89]
considers data rate requirement as a constraint to be satised by the optimal route selection problem.
8.1.2 Maximizing Network Lifetime. Zhang et al. [90, 91] proposed a heuristic iterative algorithm
for the lifetime maximization of WBANs. Their solution also worked toward identifying and
improving the lifetime of the node with the minimum residual energy. They designed a matrix
considering all possible routes from the nodes to the sink via the relays. The matrix arrangement
of all possible paths helps in exploring all relay selection possibilities. They formulated a network
optimization problem subjected to energy consumption constraints. The heuristics-based relay
selection scheme chooses a suitable relay node, which maximizes the target node’s lifetime while
satisfying optimization constraints. The iterative selection process continues until there is no
further improvement in the lifetime of a target node.
In addition, there are several other methods, some of which are discussed in Section 5, that also
help in maximizing the lifetime of WBANs.
8.1.3 Maximizing Stability Period of the Network. Sandhu et al. [92] proposed the Reliable Energy
Ecient Critical (REEC) data routing scheme for WBANs. REEC aims to distribute the data for-
warding burden over all the nodes, which would improve the stability period of the network. In
each round of data forwarding, the relay nodes are dynamically chosen using a cost function, as
outlined below.
(1) All nodes in a WBAN are divided into two disjoint sets.
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(2)
From each set, a node with the minimum cost function is chosen as forwarding node. The
cost function uses distance between node
𝑖
and the coordinator node (
𝑑𝑖
) and residual energy
of the node (𝐸𝑟𝑒𝑠𝑖) as 𝑑𝑖
𝐸𝑟𝑒𝑠𝑖.
(3)
A non-forwarding node communicates directly with the sink if the distance between them is
less than the node and the forwarding node in the concerned set. Otherwise, nodes in each
set send data to their respective forwarding nodes.
(4)
A forwarding node aggregates data and forwards it to the sink node. Nodes having less
energy than a pre-dened threshold are relinquished from the next round of forwarding node
selection.
8.1.4 Reducing BER. The medical data is critical. A single bit error in the received message could
change the meaning of the information, which could result in incorrect medication. Various
researchers focused on improving the BER as well while designing energy-ecient WBANs [93,
94].
Nguyen et al. [93] proposed the Adaptive Relay Transmission Scheme (ARTS) in which the
role and quality of relay nodes change with requirements. In ARTS, the SNR threshold is decided
by the quality of BER required. The exibility with the BER helps ARTS in adjusting the QoS
requirement. The proposed approach works as follows: the source node communicates directly with
the coordinator by broadcasting its message. The message is overheard by intermediate nodes but
is not decoded unless required. The coordinator decodes the message and calculates the SNR value.
If the received SNR is below the dened threshold, then the coordinator asks for re-transmission.
The intermediate nodes decode the packet (received by overhearing from the source node) and
check it for the SNR threshold. The intermediate receivers that satisfy the SNR threshold condition
re-transmit the overheard original packet to the coordinator. Nguyen et al. claim that if the signal
from the source to the coordinator has suered an outage once, then it may suer again while
re-transmitting over the same channel from source to the coordinator. To mitigate such a scenario,
the duty of re-transmission is handed over to the intermediate relays, rather than the source node.
It may be noted that a source node can have multiple candidate relays satisfying the threshold
requirement for the rst phase and channel gain requirement (from the relay to coordinator) for
the second phase. If all of these relays start forwarding the same packet, it will cause collisions and
thereby degrade the communication quality and lifetime of the network. Consequently, only one
suitable relay is selected.
The correct modeling of channel impairments such as signal fading and shadowing is also
necessary to understand the BER possible over the channel. Many researchers use Rician fading
channel model [95, 96]. Boussaid et al. [94] used Rician and Rayleigh fading to model LOS and NLOS
communication, respectively. In the proposed communication model the information received at a
relay node from the source node and other relay nodes connected to this node, is XORed and then
transmitted. Every relay node is connected to other three relay nodes. The idea was to increase the
redundancy to decrease the BER. However, redundant packets in the network and multiple relay
node connectivity could consume maximum part of the network energy and hence is not in favour
of network lifetime.
8.1.5 Security and Privacy. The health status of a patient is condential information that can
be used by an antagonist to cause harm to the patient. Wireless sensor nodes positioned on a
human send their data to the coordinator node by broadcasting. An intruder, aware of the on-going
transmission between a biosensor and the coordinator node, can introduce noise or Hello ood to
degrade the service. Arfaoui et al. [97] suggested that the intermediate node in an intra-WBAN
network can be used to authenticate the source of the trac. However, security checks done at
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the relay node may consume more energy of the relay node. Therefore, the authors designed a
stochastic game to adapt whether to make a security check at the relay node or not. For example,
when interference exceeds the dened threshold, the relay node switches to the security mode and
when remaining energy is below the dened threshold, the node skips security check. Liu et al.
[98] claimed that the use of an intermediate node shortens the path of communication and thus,
the physical characteristics of the two-hop channel are dierent from direct communication. These
characteristics are utilized at the coordinator node to dierentiate between the signal generated by
an on-body node and an intruder transmitting packets from outside the network.
8.2 Handling Redundancy
Sensors implanted on the human body continuously generate data. However, such data can often
be redundant and may not be required. For example, a motion sensor will generate redundant infor-
mation when the concerned human subject is in sleeping and sitting posture. Sensing, transmitting,
and managing redundant data, consume network resources and decrease the lifetime of WBANs.
Such redundancy can be avoided, for example, by considering the dierence among consecutive
readings generated by the sensor and comparing them with a threshold.
Liao et al. [99] proposed a cooperative routing protocol for WBANs intending to reduce transmis-
sion of redundant data. The authors noted that mutual information is derivable from the consecutive
readings. If such mutual information drops below a pre-dened threshold, then a link is established
(direct or relay-based) and new reading is transmitted to the sink. Data aggregation at the interme-
diate relays is another popular method to handle redundancy in wireless networks. The relay node
selection by Liu et al. [100] favours node with high residual energy and longer queues. Every relay
node collects incoming data until the collective data storage limit at a relay exceeds or the waiting
time nishes. The waiting time is decided as the latency requirement of an application. Messages
present in the queue are aggregated and forwarded to the coordinator.
8.3 Energy Harvesting at Relay Nodes
The lifetime of sensor nodes used in WBANs is limited to the life battery used inside the sensors.
Energy harvesting (EH) is an opportunity which could provide a recharging feature to nodes
and could help in increasing the lifetime of the network. The idea of EH from the received radio
signal was rst introduced by Varshney [101]. Simultaneous Wireless Information and Power
Transfer (SWIPT) [102] is a recent trend of harvesting energy from radio frequency used in wireless
communication. Sui et al. [103] inherited the EH feature of SWIPT in WBANs and designed a relay
selection mechanism to achieve a long-lasting, reliable communication in WBANs.
The proposed relay selection problem consider the CSI of channel used for communication
between the nodes and the amount of energy harvested at the intermediate node. Consideration of
only CSI while making relay node choice could result in the overuse of optimal relay node. Hence,
Sui et al. [103] added EH condition to maximize the network stability period. Relay nodes are
assumed to be congured with the EH circuitry and a buer of innite size. The intermediate node
which crosses the EH threshold and satises CSI requirements is selected as a suitable relay node.
The energy harvested by a node is given as follows:
𝐸𝑘=𝜂𝑃𝑠|(𝑘) |2𝛼𝑇 (9)
In
(9)
,
𝐸𝑘
is energy harvested at
𝑘𝑡ℎ
block,
𝜂∈ [
0
,
1
]
is the energy conversion coecient,
𝛼∈ (
0
,
1
)
is the energy harvesting duration,
𝑇
is the transmission frame duration,
𝑃𝑠
is the transmission power
of the sender node
𝑠
and
is the channel gain while transmitting
𝑘𝑡ℎ
block. The data is transmitted
in the form of blocks of duration
𝑇
. A single block contains
𝑁
number of transmitted signals. Out
of total time
𝑇
,
𝛼𝑇
is used for energy harvesting in time switching (TS) scheme. Similarly, out of
ACM Comput. Surv., Vol. 1, No. 1, Article 1. Publication date: January 2019.
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Relay-based Communications in WBANs: A Comprehensive Survey 1:27
the total power
𝑃
,
𝛼𝑃
is used in energy harvesting in power switching (PS) scheme. The remaining
time (power),
(
1
𝛼)𝑇
(
(
1
𝛼)𝑃
) is used for transmitting information in TS (PS). The harvested
energy at nodes, is utilized completely for transmission. Hence, the energy of nodes is saved for
sensing purpose only.
Sui et al. observed that their method can mitigate the channel mismatch problem. The channel
mismatch occurs when the quality of the source-to-relay channel does not match the quality of
the relay-to-destination channel. Moreover, while selecting the best relay-to-destination channel,
there could be the case that a relay node may have an optimal path with the destination node, but
harvests minimum energy. Therefore, by considering the EH model into relay node selection Sui et
al. improved the stability of the network.
9 RELAY-BASED METHODS TO ACHIEVE RELIABLE INTRA-WBAN
COMMUNICATION IN NON-COOPERATIVE NETWORKS
Intra-WBAN communication experiences interference due to co-existing WBANs. As shown in
Fig. 11, signal transmitted by a node interferes with the signal from another node belonging to
a dierent WBAN. Such networks are called non-cooperative networks. According to Fig. 11,
studies discussed in this section assume relay-based intra-WBAN communication in WBAN
𝐴
(also
called ‘network-of-interest’) and WBAN
𝐵
is an interfering network communicating using direct
intra-WBAN transmission.
Interfering zone Sensor node
Coordinator
WBAN-of-inerest Co-existing WBAN
Fig. 11. Interference in co-existing WBANs.
Dong et al. [104] examined the performance of relay-based intra-WBAN communication in
the presence of a co-existing, interfering WBAN. The network
𝐴
consists of sensor nodes, two
relay nodes, and a hub. The relay node uses DF to forward packets to the hub, and hub combines
received signals using three-branch coherent SC. Three branch SC consists of a direct link and
two relay links. SC used SNR as the selection criteria at the destination. Both the intra-WBAN and
inter-WBAN communication is accomplished by using TDMA based transmission. To create a real
WBAN environment, Dong et al. analyzed the network performance by considering channel gain
obtained in their previous empirical study [47] (discussed in Section 4.1) and full shadowing eect.
Simulation results concluded that cooperative communication is adequate to achieve optimization
in a non-cooperative environment compared to direct communication.
Interference can also be handled by reducing the transmission power level used at a sensor
node. However, reducing the power level can compromise the reliability of the communication.
To address these issues, Dong et al. [105] proposed a cooperative communication scheme with
a prediction based transmission power control model. In the proposed scheme, the hub predicts
channel condition from the received packets and informs back to the sensor node through a beacon
ACM Comput. Surv., Vol. 1, No. 1, Article 1. Publication date: January 2019.
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packet. The sensor node then adjusts its power level varying from
30 dB to 0dB, according to the
received beacon information. The power adjustment is done at the intermediate node as well while
forwarding the message toward the hub. SC diversity technique is used at the hub for extracting
the best signal out of all the received signals. The proposed joint relay selection algorithm with ve
co-existing WBANs was observed to have better tolerance to interference and improved battery
life over the non-cooperative communication at a constant power level of 0dB.
In another experiment, Dong et al. [106] used MICAz motes with IEEE 802.15.4 communication
standard and created two WBANs by placing motes on the human body. Out of two WBANs, one
was the network-of-interest, named as network
𝐴
, and the other was an interfering network, named
as network
𝐵
. Network A used two-hop topology (three branch cooperative communication) and
network
𝐵
used star topology. The motive of the experiment is to study the eect of interference
on two-hop intra-WBAN communication. The cooperative communication at network
𝐴
used
beacon-enabled mode with little modication. In beacon-enabled mode of IEEE 802.15.4, nodes
not in their duty cycle remain asleep. However, the authors prevented relay nodes from moving
into the sleep state. This modication allowed relay nodes to overhear ongoing communication
and support forwarding of packets. In three branch cooperative communication, the coordinator
received a message via direct and two relay paths. The coordinator used SC to select the signal
with the highest channel gain among all the received signals. Dong et al. tested the performance of
the network
𝐴
using outage probability, level crossing rate, average non-fade, and fade duration,
and branch switching rate. The three branch cooperative communication performed remarkably
well against the star topology. However, the branch switching rate increased when the interference
occurred. The authors suggested the use of SwC designs to reduce branch switching rates.
Table 9. Interference mitigation using relay-based communication in WBANs (Section 9).
Ref Method Measurement matrix Dataset
Dong et al.
[104]
TDMA based transmission
slot selection for all interfer-
ing WBANs.
SINR
Collected
from wear-
able radios in
[47].
Dong et al.
[105]
Power adjustment at the
sender node and the relay
node based on channel con-
dition.
Outage probability and channel
gain
On-body and
inter-body
channel
dataset [55].
Dong et al.
[106]
Beacon-enabled relay-based
cooperation.
Outage probability, Level cross-
ing rate, Average non-fade du-
ration, Average fade duration,
Branch switching rate
MICAz motes
used directly
on human
body.
Table 9 summarizes the important features of interference mitigation discussed so far. A key
takeaway from the works reviewed in this Section is that the use of diversity combining tech-
niques with cooperative communication managed to achieve better results than non-cooperative
communication in WBANs.
10 DISCUSSION ON COMMUNICATION METHODS
Service requirements of WBANs change from application to application. Some applications are
of short-duration and require low-latency, such as live video-gaming. Whereas some applications
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can last from days to weeks, such as remote patient monitoring. The general idea of satisfying a
WBAN application requirement is to design a minimizing or maximizing optimization problem by
considering the essential parameters of an application such as SNR, energy, latency, etc. Then, one
of the biosensors, satisfying the optimization problem is selected as an optimal relay node. Relay
selection methods discussed in this paper diers in terms of their problem generation methods
or problem-solving approach. The AHP based solution uses a feedback approach in which a relay
node is selected after receiving the channel quality feedback from every candidate relay. However,
channel-quality varies frequently in highly dynamic scenarios. In those cases, feedback given by a
candidate relay node may be dierent from the current situation. The decision-making methods
can be adapted easily to dierent application requirements and are useful in supporting remote
patient monitoring applications. Whereas, using decision-making for highly dynamic scenarios
need more testing and analysis.
On the other hand, opportunistic relay node selection is done instantly. These methods are
suitable for dynamic environments such as virtual reality video games or a WBAN implanted
for self-health assessment. However, such opportunistic relay selection methods could lead to
the overuse of a single relay node. The durability of a WBAN implanted on the human body can
be improved further by considering node heterogeneity. Relay-based communications exploiting
node heterogeneity are found to be useful when multiple nodes are implanted on the human body.
For example, a patient under supervision in a hospital is tested for pulse rate, body temperature,
respiration rate, insulin level, simultaneously. In such cases, a node with less criticality or low
sampling rate could help in relaying messages. However, this could also result in the overuse of
low priority nodes.
The above-discussed methods may not handle the non-cooperation in intra-WBAN. Game-theory
based methods could be considered as indirect service level agreements which prevent nodes within
the network from misbehaving. However, game-theory solutions need more exploration of time
taken to adjust the changing parameters of nodes with varying network topologies.
In many sports, the body movement is fast and periodic, for example, swimming, gymnasium,
racing, etc. A moving WBAN has situations when connections between nodes are at their best, for
example, LOS and high SNR. The relay-selection methods that exploit human mobility to catch the
best network situations can be found suitable for sports applications. These communication methods
could be considered as hybrid methods as they consider direct communication under favorable
network situation and relay-based, otherwise. WBANs with additional relay nodes adds extra cost
for improving the network services. Such networks could be used under short duration critical
applications. The added relay node could improve the signal quality by using data forwarding
techniques.
11 DESIGN CHALLENGES AND OPEN RESEARCH ISSUES
The complexity of the human body and mobility posses various design challenges in the area of
WBANs. The primary design challenges are discussed in the following.
Dynamic topology
: WBANs exhibit complex mobility. The sensors implanted on the torso
of a human are static for the coordinator node (positioned on the human body). In contrast,
sensors placed on arms and legs give rise to a relatively mobile network. The versatility of
human limbs constrains the range of motion in WBAN. Moreover, the change in the position
of nodes aects the channel characteristics. Thus, human mobility challenges communication
methods to provide QoS in a highly lossy environment.
Power supply
: The biomedical sensor nodes are small in size so that they can be installed
and carried easily as per human convenience. The battery attached to the sensor also has a
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limited capacity due to size constraints. It is challenging to replace or recharge the batteries
of sensors implanted through surgery inside a human body. Thus the size and capacity of
batteries used in biomedical sensors challenge the prolonged lifetime of a WBAN.
Limited hardware support
: Security is always a concern in wireless networks. Medical
details of a person are condential information and require strict privacy and security
layers. However, security protocols are often time-consuming and require high processing
capabilities. A biomedical sensor node supports limited computation resources, and thus, it
is challenging to achieve secure communication with limited resources in intra-WBANs.
Node heterogeneity
: Dierent sensors are required to monitor dierent health parameters
of a patient. Sensors used in a WBAN vary in terms of their sensing technology and frequency.
The heterogeneity in human health parameters requires some sensors to operate frequently
than others. The non-uniformity in operating frequencies of sensors challenges the stability
period of a WBAN.
The characteristics of WBANs impose design challenges for communication methods. The open
research issues to improve the services provided by WBANs are discussed as below:
Designing sensor node
: WBANs need new type of bio-sensors which can sense and com-
municate data through wireless medium to the access points. Moreover, bio-sensors with
implanted antenna is not an existing technology till date. Therefore, designing WBAN sensors
largely remain an unexplored territory.
Dynamic and heterogeneous routing
: With the advent of smart watches and other wear-
able gadgets, we are increasingly equipping us with various types of extra-corporeal sensors
to keep track of our vitals. Such advances would lead to WBANs with dynamic topologies
and heterogeneous communication technologies, for example, electromagnetic and molecular
[107, 108]. It would be interesting to see how such issues can be addressed.
Addressing heating issues
: Circuits involved in radio communication cause heating up of
tissues due to SAR. The amount of SAR occurs and the degree of its harmfulness is still not
assured. Hence, thermal issues of WBANs need wide exploration.
Resilient to Security risks
: Security is a signicant concern in medical services provided
by WBANs. Methods to improve the security of the database and anonymity of the patient
are need to be addressed in WBANs.
Quality of service (QoS)
: The data latency and quality of information are of high importance
in WBANs. Along with quality, energy eciency is also necessary. Existing methods need to
be optimized to improve the quality of communication in WBANs.
Acceptance by the society
: Acceptance of WBANs is still a challenge as many people would
not like to t electronic circuits inside their bodies. The solution to this problem emphasizes
on user-friendly devices used for WBANs. However, as mentioned earlier, we are getting more
accustomed to wearable devices. However, a critical requirement for success is to extensively
verify and validate the sensors and networking devices in order to improve their reliability
and acceptability among the general public.
12 CONCLUSION AND FUTURE WORK
By facilitating remote health care, WBANs can potentially improve the quality of our life. In this
survey, we presented a comprehensive review of relay-based routing protocols of intra-WBANs.
Beginning with the typical architecture of WBANs, we highlighted their dierences with WSNs.
Subsequently, we reviewed energy conservation methods by preventing redundant transmissions,
utilizing the shortest distance for communication, and balancing power consumption. In reality,
multi-hop communication increases the delay in message delivery. However, existing research
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suggests that relay-based communication can improve the lifetime of WBANs during link failures.
The existing research explored in this paper suggests that selection diversity combining techniques
at the receiver side can improve the quality of communication in WBANs. This survey also observed
that channel state information plays an essential role in improving relay-based communications.
Limited research in WBANs considered the mobility and body fat of humans. Consideration of
human mobility is therefore a challenging area in WBANs. WBANs still need high-quality services
and security while transferring raw sensed data to the cloud from a patient’s body. Therefore, the
scope of WBANs could be extended to monitor a patient from any part of the globe. Consequently,
5G and related innovations can play a crucial role in augmenting the scope and capabilities of
future WBANs.
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... Moreover, our survey does not have selfish behavior as its singular focus but also considers such behavior as an aspect of user-provided relaying for which incentive mechanisms can be proposed, as shown in Section IV. The work in [28] limits its focus to relay-assisted wireless body area networks (WBANs). Therein, the authors discuss network architectures, relay node selection and point out the unique quality of service requirements of relay-based WBANs. ...
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