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WIMAX Basics From PHY Layer to Scheduling And Multicasting Approaches

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WiMAX (Worldwide Interoperability for Microwave Access) is an emerging broadband wirelesstechnology for providing Last mile solutions for supporting higher bandwidth and multiple serviceclasses with various quality of service requirement. The unique architecture of the WiMAX MAC andPHY layers that uses OFDMA to allocate multiple channels with different modulation schema andmultiple time slots for each channel allows better adaptation of heterogeneous user’s requirements. Themain architecture in WiMAX uses PMP (Point to Multipoint), Mesh mode or the new MMR (Mobile Multihop Mode) deployments where scheduling and multicasting have different approaches. In PMP SS(Subscriber Station) connects directly to BS (Base Station) in a single hop route so channel conditionsadaptations and supporting QoS for classes of services is the key points in scheduling, admission controlor multicasting, while in Mesh networks SS connects to other SS Stations or to the BS in a multi hoproutes, the MMR mode extends the PMP mode in which the SS connects to either a relay station (RS) orto Bs. Both MMR and Mesh uses centralized or distributed scheduling with multicasting schemas basedon scheduling trees for routing. In this paper a broad study is conducted About WiMAX technology PMPand Mesh deployments from main physical layers features with differentiation of MAC layer features toscheduling and multicasting approaches in both modes of operations.
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International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
DOI : 10.5121/ijcses.2011.2101 1
WIMAX
B
ASICS
F
ROM
PHY
L
AYER TO
S
CHEDULING
A
ND
M
ULTICASTING
A
PPROACHES
Manal Al-bzoor
1
and Khaled Elleithy
2
1
Department of Computer Science Engineering, University Of Connecticut, CT, USA
mbzoor@engr.uconn.edu
2
Department of Computer Science Engineering, University Of Bridgeport, CT, USA
elleithy@bridgeport.edu
A
BSTRACT
WiMAX (Worldwide Interoperability for Microwave Access) is an emerging broadband wireless
technology for providing Last mile solutions for supporting higher bandwidth and multiple service
classes with various quality of service requirement. The unique architecture of the WiMAX MAC and
PHY layers that uses OFDMA to allocate multiple channels with different modulation schema and
multiple time slots for each channel allows better adaptation of heterogeneous user’s requirements. The
main architecture in WiMAX uses PMP (Point to Multipoint), Mesh mode or the new MMR (Mobile Multi
hop Mode) deployments where scheduling and multicasting have different approaches. In PMP SS
(Subscriber Station) connects directly to BS (Base Station) in a single hop route so channel conditions
adaptations and supporting QoS for classes of services is the key points in scheduling, admission control
or multicasting, while in Mesh networks SS connects to other SS Stations or to the BS in a multi hop
routes, the MMR mode extends the PMP mode in which the SS connects to either a relay station (RS) or
to Bs. Both MMR and Mesh uses centralized or distributed scheduling with multicasting schemas based
on scheduling trees for routing. In this paper a broad study is conducted About WiMAX technology PMP
and Mesh deployments from main physical layers features with differentiation of MAC layer features to
scheduling and multicasting approaches in both modes of operations.
K
EYWORDS
WiMAX, PMP, Mesh, Scheduling WiMAX, and Multicasting WiMAX
1.
I
NTRODUCTION
WiMAX is the end to end technology that provides low cost applications and last mile solution
for broadband wireless access. WiMAX is based on the standard family defined by IEEE 802.16
which provides Coverage of up to 30 miles (last mile) compared to other technologies; DSL can
cover 3 miles; WiFi can only cover 30 meters. WiMAX has unique characteristics which allows
the base station to handle thousands of subscriber stations (SS) , provides a collision- free MAC
Uplink/downlink (UL/DL) channels, it also provides efficient handover procedure and Power
control mechanism by introducing the sleeping mode for mobile stations. WiMAX Supports
Data, Legacy voice systems, VoIP, TCP/IP, Applications with different QoS classes, and
different level of guarantees. The basic two layers in IEEE 802.16 are the MAC and the physical
(PHY) layers. The PHY layer Combines OFDM/OFDMA Orthogonal Frequency Division
Multiplexing/ Multiple Access and uses multiple inputs multiple output antenna technology
with an adaptive coding and Modulation schemas to support various bandwidth request
demands issued by the MAC layers services. The MAC layer in turn provides a medium
independent interface to the physical layer as the standard medium access functionality.
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
2
Even though WiMAX is a relatively new standard, there is a tremendous work conducted to
address hot issues. But a very little work was conducted to classify and survey the work and
mechanisms proposed for addressing design issues in WiMAX. Some Studies Focused in the
cross layer design capabilities of PHY Layer and MAC Layer for a better Quality of Service.
Other Studies proposed new Scheduling Techniques for WiMAX taking one or more of class of
service type as a key design factor. A survey in [12] made a good effort in describing and
classifying the scheduling algorithms proposed earlier for other technologies and the ones
proposed specifically for WiMAX and are based on their use of the channel conditions. Only
few studies was conducted to provide routing and multicasting schemas for WiMAX because in
point to multipoint mode multicasting is defined by a specific Multicast Broadcast service
while the mesh mode of WiMAX follows the same mechanisms and techniques proposed for
earlier wireless mesh networks.
In This survey we provide a study of basic key design and research issues for WiMAX. We
describe the WiMAX standard evolutions, Deployments, basic Protocol Layer Stack mainly the
Physical and MAC Layers by addressing the characteristics and features in both Point to Point
and Mesh deployments, we then give a description of the scheduling Mechanisms and studies
proposed for both deployments and finally we give a brief description and differentiation of
routing and multicasting schemas used with WiMAX.
2.
WiMAX Standards
In 2001, the WiMAX forum introduced the first fixed Standard 802.16 with line of sight
requirement using a single carrier frequency with 10-66GHz spectrum support. This basic
standard provides theoretical rates of up to 134Mbps. In January 2003, 802.16a standard was
approved with a non line of site support of 2-11 GHz frequency, where the first orthogonal
frequency division Multiplexing (OFDM) and the mesh mode were added to WiMAX. The
802.16 b&c were amendment to the 802.16a and all of them were theoretical standards which
later were grouped in the 2004 WiMAX standard 802.16d. The 802.16d known as the first fixed
working standard of WiMAX supports data rates of up to 70Mbps, uses 256 point fast Fourier
transform for Orthogonal Frequency Division Multiplexing (OFDM) and 2048 points transform
OFDMA (OFD Multiple Access)[3][4]. In 2005 the mobile WiMAX IEEE 802.16e [5] was
introduced with the following improvement over previous standards:
It supports mobility by Introducing a Mobile Stations (MS) instead of SS. MS in this
standard can stay connected during movement from one BS coverage area to another BS
coverage area through efficient handover procedures between Base stations.
It uses Scalable OFDMA (SOFDMA) technology to enhance spectrum efficiency and
reduce cost in wide and narrow band channels. It obtains this scalability by allowing
different FFT point values for each channel width to resulting in a constant carrier
spacing.
It adapts to the Advance antenna technology supporting the Multiple In Multiple Out
MIMO technology and uses hybrid automatic repeat-request (HARQ) to enhance
reliability
Introduces Turbo Coding and Low-Density Parity Check (LDPC)
It uses the downlink sub-channelization, allowing administrators to trade coverage for
capacity or vice versa
In 2009 the WiMAX forum approved the 802.16j called the MMR mode that is just an
extension to the original the PMP. MMR mode have a tree structure where intermediate
subscriber stations can work as relays to forward traffic and is fully backward compatible with
802.16e standard.
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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3.
WiMAX Deployments
WiMAX architectures use either Point to Multipoint (PMP), Mesh, or Mobile Multihop Relay
(MMR) mode. PMP architecture was introduced as the first standard of WiMAX in 2001. In
this mode subscriber stations connects to the base station in a single hop route. In Mesh mode
subscriber stations can communicate in an ad-hoc fashion. Mesh mode gained too much
attention by researchers but was not much deployed in the real world because it supports only
OFDM version and is not compatible with PMP with completely different frame structure and
network entry procedure. The mobile multi hop relay (MMR) mode in 802.16j was introduced
as an extension for PMP mode in IEEE 802.16e [6]. MMR outperforms PMP but its achieving
higher throughput and enhancing coverage [6] Unlike PMP mode, MMR Supports both OFDM
and OFDMA operations and is backward compatible wita h PMP mode. MMR differs from
PMP by introducing the Relay Stations in which mobile station can use as an intermediate route
forwarders to the BS in a like tree topology rather than a PMP or MESH topology. MMR
consist of three network entities BS, relay station (RS) and mobile station (MS). RSs have the
functionality of playing and intermediate forwarder to forwards traffic between any MS and the
BS. RS was classified to work in two modes transparent or non transparent [6]. In transparent
mode only data signals are forwarded and no control signals are allowed to pass. RS In non
transparent is being also divided according to the scheduling role they play and follow as
distributed or centralized. Distributed if they are allowed to share any scheduling decision and
bandwidth allocation with the BS or centralized if RS are just forwarder and scheduling
decisions are made by the BS. Radio links between entities in the MMR mode are named
Access links (AL) if it connects MSs with RS and are called relay links (RL) if the radio link is
between the RS and the BS [7]. Figure 1 shows an example of different WiMAX deployments
in which BS can work as a gateway for the internet; MS is a mobile SS; RS is a SS that works
as a relay agent to forward traffic flows to BSs or other RSs.
Figure 1.WiMAX Deployments
4.
Protocol Stack of WiMAX
The two basic layers in WiMAX are the physical (PHY) layer and the Medium access control
(MAC) layer and were designed mainly for the PMP mode. In This section we are introducing
the main features in both layers.
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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4.1 PHY LAYER
The WiMAX physical layer (PHY) is designed to work with different specifications for licensed
and unlicensed frequency bands. For example, one is based on a single carrier (SC) to support
line of site with high data rates , others use orthogonal frequency division multiplexing
(OFDM), and OFDMA to support both line of site(LOS) and none line of sight (NLOS) .
[3][4][8][9]
4.1.1 OFDM /OFDMA: OFDM is an efficient modulation schema used for transmitting
large amount of data over radio waves. OFDM is a multi carrier transmission method that
is based on dividing the frequency of a carrier into orthogonal frequency sub carriers each
carrying different stream of data and is can be modulated and coded separately. Since
OFDM selects the sub carriers such that they are orthogonal to each other over the time
duration, it limits or eliminates overlapping and the sub carrier interference. OFDMA is the
access method that is based on the OFDM modulation technique to divide the carriers
among the users to form sub channels. Each sub channel is allocated a separate coding and
modulation parameters to allow are adapted separately, allowing channel optimization on a
smaller scale (rather than using the same parameters for the whole channel). This technique
optimizes the use of spectrum resources and enhances indoor coverage by assigning a robust
scheme (yet, with low rates) to vulnerable links. OFDMA is an option in 802.16 (for fixed
access), but it’s not required for certifying 802.16 products. However, OFDMA is necessary
in 802.16e devices and is required for certification. SOFDMA is an enhancement of
OFDMA that scales the number of sub carriers in a channel using the four set of scaling
factors 128, 512, 1,024, and 2,048.
4.1.2. Modulation and Coding: WiMAX has the capability of changing its burst profile
values depending on channel conditions per link and per connection. The two main
parameters adjusted for the burst profile are the modulation and coding parameters used in
addition shown in Table I below. For the Downlink burst profile other parameters are also
specified like CINR threshold for entry and exit of this specific burst profile. In Uplink
burst profile we can see a ranging data ratio. A Base station gets an estimate about the
uplink channel conditions for each connection while it gets information about the downlink
channel quality from control information provided by the mobile stations.
Table 1. Modulation and Coding used with Uplink and Downlink
Downlink Uplink
Modulation
BPSK, QPSK, 16 QAM, 64
QAM; BPSK optional for
OFDMA-PHY
BPSK, QPSK, 16 QAM; 64
QAM optional
Coding
Mandatory: convolutional codes
at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo
codes at rate 1/2, 2/3, 3/4, 5/6;
repetition codes at rate 1/2, 1/3,
1/6, LDPC, RS-Codes for
OFDM-PHY
Mandatory: convolutional
codes at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo
codes at rate 1/2, 2/3, 3/4, 5/6;
repetition codes at rate 1/2,
1/3, 1/6, LDPC
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
4.1.3 PHY-
Layer Data Rates:
Table II provi
des an example of data rate obtained using different values for channel
Table 2. PHY Layer Data Rates
4.1.4 Duplexing
: PHY layers support duplexing, where a station’s concurrent transmission
4.2. MAC LAYER
The p
rimary task of the WiMAX MAC layer is to provide an inte
layers
and the physical layer. The 802.16 MAC was first designed for point
(PMP) applications and is based on collision sense multiple access with collision avoi
(CSMA/CA). Several features characterize the MAC layer in WiMAX which makes it an
efficient broadband technology.
4.2.1. Main Features of MAC Layer
The MAC layer in WiMAX has unique
almost all of the known broadband applications with different mob
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
Layer Data Rates:
The design of physical layer in WiMAX allows multiple
data rates that depends on parameters specified during the entry procedure and service
request. The parameters that
affects the physical layer data rate or what that is specified in
the burst profile is determined from the modulation and coding schema used in addition to
the communication channel bandwidth which makes the physical layer uniquely flexible.
des an example of data rate obtained using different values for channel
bandwidths, with various modulation and coding schemes.
Table 2. PHY Layer Data Rates
: PHY layers support duplexing, where a station’s concurrent transmission
and reception, is possible through time
division duplex and frequency division duplex. In
TDD, a station transmits then receives (or vice versa) but not at the same time. This optio
helps reduce subscriber station costs, because the radio is less complex. In FDD, a station
transmits and receives simultaneously on different channels. PMP supports both TDD and
FDD while Mesh mode only uses TDD duplexing [9].
rimary task of the WiMAX MAC layer is to provide an inte
rface between the higher
and the physical layer. The 802.16 MAC was first designed for point
-
to
(PMP) applications and is based on collision sense multiple access with collision avoi
(CSMA/CA). Several features characterize the MAC layer in WiMAX which makes it an
efficient broadband technology.
4.2.1. Main Features of MAC Layer
The MAC layer in WiMAX has unique
characteristics
and features that make it suitable for
almost all of the known broadband applications with different mob
ility rates.
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
5
The design of physical layer in WiMAX allows multiple
data rates that depends on parameters specified during the entry procedure and service
affects the physical layer data rate or what that is specified in
the burst profile is determined from the modulation and coding schema used in addition to
the communication channel bandwidth which makes the physical layer uniquely flexible.
des an example of data rate obtained using different values for channel
: PHY layers support duplexing, where a station’s concurrent transmission
division duplex and frequency division duplex. In
TDD, a station transmits then receives (or vice versa) but not at the same time. This optio
n
helps reduce subscriber station costs, because the radio is less complex. In FDD, a station
transmits and receives simultaneously on different channels. PMP supports both TDD and
rface between the higher
to
-multipoint
(PMP) applications and is based on collision sense multiple access with collision avoi
dance
(CSMA/CA). Several features characterize the MAC layer in WiMAX which makes it an
and features that make it suitable for
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
6
MAC layer incorporates higher security by introducing the Privacy key management.
PKM that uses extensible authentication protocol.
It has a special Multicast and broadcast support initially built as a separate
multicast/broadcast zone in PMP mode.
It introduces multiple Manageability primitives.
It is Connection Oriented, where all service flows are mapped into a unique connection
identifier CID for all services including services issued by connectionless applications at
higher layers.
It has an efficient handover and mobility management primitives through soft and hard
handover mechanisms
It allows sleep and idle power management modes in addition to normal mode to increase
the life time of mobile devices.
Mac layer also supports Header suppression and fragmentation which implies more
efficiency in utilizing Total bandwidths.
IT has five service classes to be described in more details later in this paper and they are,
unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling
service (nrtPS), best effort (BE) and extended real-time variable rate (ERT-VR) service.
The MAC layer features combined with the PHY layer features like the SOFDMA makes
WiMAX an appealing technology for applications that requires high data rate transmission such
as VOIP and multimedia applications.
4.2.2 MAC Frame Structure
The MAC layer receives service requests from the upper layer that is represented as MAC
service data units (MSDUs). The MAC layer then arrange them into MAC layer protocol data
units (MPDUs) and send these data units to lower PHY layers and it collects the MPDUs into
MSDUs when it receives them from PHY layer. MPDUs in the MAC layer could have different
lengths. The WiMAX MAC layer can combine multiple variable sized MPDUs in one burst to
reduce the PHY overhead. It also can combine more than small sized MSDU coming from same
upper layer service into one MPDU reducing overhead at MAC layer level. In the other hand
larger MSDUs can be divided to smaller MPDUs that makes MAC layer flexible and efficient.
Figure 2. shows an example of various MAC PDU frames used with different data packets.
Each MPDU frame contains a generic MAC header with connection identifier (CID), in
addition to the length, and bits used for cyclic redundancy check (CRC), and other control data
about encryption method and keys if any. The MAC payload contains communication message
or a management message. The communication message contains the MSDUs and in some
cases may contain bandwidth requests or retransmission requests. Automatic Repeat Request
(ARQ) mechanism is also supported by WiMAX MAC which allows retransmission of MSDUs
which achieves better reliability [8].
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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Figure 2. Examples of various MAC PDU frames where
CRC is Cyclic Redundancy Check, FSH
is theFragmentation Subheader, GMH is the Generic MAC Header, PSH is the Packet Subheader and the
SH stands for Subheader
4.2.3. MAC Sublayers
The IEEE standards for WiMAX in 2004 and 2005 have defined a convergence sub layer,
common part sublayers and the security sub layer as part of the MAC layer. The Convergence
sub layer maps the transport layer traffic to a MAC according to the type of traffic to be
scheduled and handled according to its quality of service requirement like ATM, TDM Voice,
Ethernet, IP, and any other. Convergence sub layer is also responsible for MSDU header
suppression that reduces upper layer overheads. Common Part Sub layer performs regular
MAC layer functions; it uses TDM multiplexing on the Downlink and allows sharing the uplink
between SSs in TDMA fashion. Common part sub layer also maps all services, including upper
layers connectionless services, to a unique connection identifier (CID). Common part sub layer
is the layer to provide, Grant/request mechanism, associates QoS parameters, and routing data
to the correct convergence sub layer and provides downlink scheduling services. The MAC
security sub layer is responsible for access authentication, connection setup and providing key
exchange during the network entry procedure and encryption for data privacy.
4.2.4 QoS Support in MAC Layer
The most important part of the MAC layer design that distinguishes WiMAX from all other
broadband wireless standards is its support for QoS classes. MAC architecture provides a
connection oriented architecture which helps achieving a good control of QoS, where the Base
station is responsible for controlling all downlink and uplink connections. A certain set of QoS
parameters could be associated with a service flow which is a unidirectional flow of packets
identified by a service flow identifier (SFID). WiMAX has five classes of flows, each having
different QoS requirements, and these are: Unsolicited Grant Service (UGS), Real-Time Polling
Service (rtPS), non-Real-Time Polling Service (nrtPS), Enhanced-Real-Time Polling Service
(ertPS), and best effort (BE). Table 3. [11] shows a brief description of each service class with it
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
8
QoS requirement. Also for each class’s uplink connections, the type of service specifies which
mechanisms to use in order to request bandwidth [12].
UGS support applications that require stringent delay requirements with fixed data packet on a
periodic manner such as VoIP. The UGS service doesn’t request bandwidth instead bandwidth
for uplink is granted regardless of the channel quality estimate hence, UGS connections use the
unsolicited granting bandwidth-request. A periodic bandwidth is granted for any UGS service
class without any polling or contention. The bandwidth granted to each UGS service is
determined by the BS as the average of lowest amount of data transmitted on the connection
when over time. SS may ask the BS to poll it to additional allocated bandwidth if it needs more
bandwidth.
Real-time polling service rtPS is designed to support applications with less delay requirements
and has variable size data frames at periodic intervals, such as video streaming of (MPEG)
video and streaming audio applications. In rtPS connection the QoS guarantees it supports are a
minimum reserved bandwidth with an upper threshold for waiting times for each packet at the
MAC layer and a minimum latency. Unlike the UGS, packet sizes in rtPS are of variable length
and a SS is required to inform the BS of its current bandwidth at the time of service request. For
each rtPS connection the BS grants unicast polls every polling interval specified by QoS
parameters. nrtPS and BE are used with applications which doesn’t have any strict delay
requirements. nrtPS is used with application that requires a minimum data rate, like File
Transfer Protocol (FTP) while BE doesn’t need minimum data rate requirement as an example
is HTTP and email applications . nrtPS and BE uses piggybacking to embed the bandwidth
request into an uplink PDU or by replying to a broadcast poll initiated by the base Station
which in a contention based basis
Table 3. QoS Classes Supported by WIMAX MAC layer
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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4.2.5. Network Entry Procedure and Grant Mechanisms in WiMAX
PMP mode:
When a subscriber station wishes to register itself with a BS in a PMP
deployment the first thing the subscriber station does is to tune up with a downlink channel
(DL) and an uplink channel (UL) to get the frame structure of the UL which is called UL-
MAP from the BS. Then the Ranging Procedure starts by aligning the subscriber station for
transmission to the correct mini slot boundary specified by the base stations. Subscriber
station establishes IP connectivity with the Base stations and as a final step for completing
registration base station exchanges time and security parameters with the Subscriber
stations. After successful registration stations request for transmission opportunities on the
uplink UL channel. Base station gathers these requests and determines the grant size and
number of time slots that each SS will be allowed to transmit in the UL Frame. This
information is broadcasted in the downlink (DL) channel by the BS using the UL-MAP
message at the beginning of each DL-Frame, The UL-MAP contains Information Elements
(IE) that describes the transmission opportunities in the UL channel, such as initial
maintenance, station maintenance, contention, and reservation access. A SS receiving UL-
MAP will transmit data in the predefined transmission opportunities indicated by IE.
Transmission opportunities are assigned by the BS using QoS agreements to support a
certain service class requested by SS’s.
Mesh mode:
Network entry procedure for mesh modes differs totally from that for PMP
mode. In mesh Mode current nodes connected to the mesh network periodically broadcast
Mesh Network Configuration messages. These messages provide information about the
current configuration of the Network through a field called Network Descriptor. A node
wishing to join this mesh listens first to any Mesh Network Configuration message on its
range. The new node selects closest neighbor as its host node and uses it to send its Mesh
Network Entry request with registration information to the BS. The Mesh BS registers the
new node and adds it as the child node of the host node and broadcasts an update
configuration messages to all the nodes in the network [18]. Unlike PMP mode the WiMAX
forum didn’t identify the request and grant mechanisms for this mode. All communication is
done through Distributed or Centralized scheduling with exchange of scheduling messages
MSH-DSCH and MSH-CSCH described later in this paper.
5.
S
CHEDULING IN
W
I
MAX
Although IEEE 802.16 standards specify several QoS schemes and related message
f
ormats, the problem of scheduling algorithms for both PMP and Mesh mode are left unsolved
and left as an open research issue.
5.1. Scheduling in PMP mode
[12][14][15]
In PMP mode, the BS is responsible for a scheduling decision for SS. Each frame is assigned a
subcarrier in a certain time slot with scheduling decision made on a frame by frame basis. The
time period and the sub channel assigned to frames is done in scheduling decision for each SS
by BS and broadcasted to all users in multicast group if any or sent to the user with unicast
request. A scheduling decision in WiMAX should satisfy the QoS requirements for each
requesting service. Scheduler to be defined in this mode is a downlink and uplink scheduler for
the base station and only an uplink scheduler for the subscriber station. In [9] and [11] the
scheduling algorithms for MAC layer were categorized into two main categories, channel
unaware and channel aware scheduling algorithms. Channel unaware are further divided into
homogeneous and hybrid algorithms. Channel aware are known also as opportunistic
algorithms.
Homogeneous Schedulers:
These are referred to the assigning one classical scheduling
algorithm for all classes of service not taken into consideration the varying condition of
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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subscriber channel. Examples of such algorithms are the
round robin, deficit round robin,
weighted round robin, and earliest deadline first. None of the homogeneous algorithms provided
the required fairness and QoS guarantees for all the services classes introduced in WiMAX. For
example if earliest deadline first suits the QoS requirement of rtPs service class, it will work
poorly with UGS.
Hybrid Schedulers:
The Second category algorithms were introduced to overcome the problem
incurred by using one homogeneous scheduling algorithm. Some algorithm that uses a hybrid
approach uses different scheduling strategies for each service class. One uses earliest deadline
first (EDF) for rtPS service class, uses Weighted Fair Queue (WFQ) for nrtPS and a First In
First Out Scheduling algorithm for the Best Effort Service class. In this hybrid algorithm
bandwidth for all classes is allocated using priority to allow fairness between all types of the
service classes. .Another Hyprid scheme uses combinations of EDF and WFQ with fair
bandwidth sharing between nrtPS and BE cthat both assigned WFQ, the authors did not describe
the mechanism for fair allocations. In[31] the authors proposes a hybrid scheduling algorithm
that assigns bandwidths or transmission opportunities based on a priority mechanism with
weighted round robin nrtPS and rtPS and round robin scheduling method for other classes. At
first opportunities are assigned to nrtPS and rtPS.s until their QoS guarantees are fulfilled. BE
requests from other SS are then served using with any remaining bandwidth using the round
robin approach. Hybrid schemas provided some fairness between the different qualities of
service classes but again didn’t take into consideration the variable channel conditions of each
subscriber station and the flexibility of WiMAX by providing different bandwidth grant size for
different quality of service classes. Some studies proposed other mechanisms that should be
used with scheduling algorithms such an admission control procedure and a traffic policing
mechanism.
Channel Aware / Opportunistic Scheduling:
Opportunistic Scheduling is the kind of
algorithms designed specifically for WiMAX to achieve the best quality of services for various
services classes within a variable channel condition. Such algorithms where also called cross
layer since their scheduling decision is based on channel condition estimation provided by the
PHY layer to the MAC layer. A Temporary Removal (TR) scheduler in [28] is an example of
opportunistic algorithms in which the scheduler uses information from lower level to identify
service packets power for each SS. Packets with low power are temporary removed from the
scheduling queue for a time specified by a TR timer. Only SS with packets that can be served
is left on the queue. SS that were removed from the queue are checked again after timer expires
and is returned to the queue if its packet power increases. For Each packet there is a limit for
number of times a packet is checked for better channel condition quality and is unconditionally
added to top of the queue. In [29] an Opportunistic Deficit Round Robin scheduler (O-DRR) A
BS uses periodic polls to identify the SS to be served. The decision to include any of the polled
SSs in the scheduled set is built upon the channel conditions and radio quality of each one. Any
SS to be given an opportunity to send during the scheduling frame should have data to be
transmitted and should have SIR above a certain threshold value. Both TR and O-DRR didn’t
consider the different service classes and made its scheduling decision based on the channel
quality only. In [30] Frame Registry Tree Scheduler (FRTS) the authors used a deadline
criterion for the scheduler to assign time frames for SS packets. .The Deadline is the same as
arrival and latency time For UGS and rtPS services, while for nrtPS and BE services there is
no packet deadline specified. Higher priorities are given to UGS and
rtPS services over other
classes. Scheduling
decision will be evaluated one any SS changes its modulation type or QoS
requirement. Some Opportunistic Scheduling algorithms was proposed to deal with only one
type of Services Classes such as The adaptive rtPS scheduler in [30]. This algorithm tried to
reduce the delay incurred between multiple grant/request operation between the SS and the BS
for multiple packets arriving at different times. The SS here will be assigned time slots for the
packets currently in the SS queue and for other packets that is expected to arrive from upper
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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layers. The basic idea of the adaptive rtPS scheduler is to propose an rtPS bandwidth request
process in which the subscriber requests time slots for the data present in the rtPS queue and
also for the data which will arrive. Authors have defined a prediction method to estimate packet
arrival times. In [31] authors introduced Cross-Layer scheduling algorithm in which Scheduling
opportunities and time slots are assigned based on a priority. Priority for each SS is given
according to its channel condition and its service request class.
5.2 Scheduling for Mesh Mode
[11][13]
Most researches for scheduling and routing done for wireless mesh network were based on the
IEEE 802.11 standard. These researches could be used with WiMAX mesh, but will not produce
efficient results since it wasn’t build based on the characteristics of WiMAX technology and
there was only few researches done specifically for WiMAX mesh networks. Mesh mode at the
PHY layer supports only Time Division Duplexing (TDD) and hence at the MAC layer the
subscriber stations compete for transmission opportunities using TDMA (Time Division
Multiple Access). A frame in WiMAX mesh frame consists of two main sub frames, one
dedicated for carrying control information and the other is for carrying data. The Control sub
frame carries information about the connections establishment/maintenance and scheduling of
data transmission between different SSs.
Only some nodes in the mesh network can be used to connect the mesh network to the backhaul
links in the same fashion as BS do. Scheduling in mesh mode is built over scheduling trees that
is rooted at the BS. Mesh can use centralized scheduling or distributed Scheduling schemas.
When using Mesh centralized scheduling, the BS nodes perform much of the same basic
functions as do the BS in PMP mode. Thus, the key difference is that in Mesh mode all the SSs
may communicate directly with other SSs eliminating the need for direct connections between
SSs and BS of the Mesh network. In Centralized Scheduling Algorithms Communication
between all Stations is controlled by a centralized algorithm provided by the BS. In distributed
Scheduling direct communication or links can be coordinated by all nodes periodically.
5.2.1 Distributed Scheduling
In Distributed Scheduling SS’s stations may contribute in the scheduling decisions and hence
each node in the mesh network is aware of its neighbours. A neighbor is the node with direct
connection or is separated by multiple node. Distributed scheduling is classified as either
coordinated or uncoordinated. Subscriber nodes in addition to the base stations coordinate’s
their communications with all other nodes in a two hop neighbourhood. Nodes Shares the
scheduling information through the control part of each frame it transmits. Neighbour stations
use the same channel for exchanging scheduling. Scheduling decision in coordinated distributed
scheduling doesn’t rely on the BS and can be directed to any other node in the neighbourhood.
In the other hand Uncoordinated distributed scheduling is based on direct request and grant
mechanism between two nodes. Uncoordinated is suitable most for schedules that is required to
be fast. Uncoordinated distributed schedules are established by a direct requests and grants
between two communicating nodes. In uncoordinated scheduling competing for the shared radio
channel is not governed by neighbourhood status as in coordinated, and may collide with other
scheduling messages from either coordinated or centralized scheduling. Distributed Scheduling
is done by the exchange of MSH-DSCH messages.
5.2.2. Centralized Scheduling
Centralized Scheduling refers to algorithms for mesh networks where communication and data
transmission taken place between BS and other nodes in the Mesh. In Centralized Scheduling
the Mesh Centralized Scheduling Configuration (MSH-CSCF) message is broadcasted by Bs to
inform all nodes of the current configuration and routing tree data of the mesh, each node then
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
12
forwards this message to its neighbours. The Bs then BS gathers resource request from SSs
using Mesh Centralized Scheduling (MSH-CSCH) messages. Flow schedule is assigned to the
requesting SS and broadcasted. SS Then uses the flow assignment to determine its actual
transmission opportunities using a predetermined algorithm. In this type a BS plays the same
role as in PMP mode but with no direct connection between BS and SSs. SS will also use MSH-
CSCH messages to report its request change to the BS. BS in response adjusts its flow
assignment and broadcast it again to all nodes. Frames scheduled to be transmitted in one
MSH- CSCH message is governed by how many frames are needed for the following Schedule
to be packed and distributed. The MSH-CSCH message carries data about the transmission
period for each node, the last node to receive the schedule and the time taken by the BS to send
the current schedule. The process of broadcasting the centralized messages starts at the BS
down the tree and the hop count is incremented until all nodes receives the grant or the
configuration message. Gathering the request messages starts from the farthest node in the tree
with the largest hop count and propagates up the tree until it reaches the BS.
5.2.3. Algorithms for WiMAX mesh
Centralized and distributed scheduling is two approaches used in general with any networks
with mesh topology and is also applicable to WiMAX mesh. Studies that proposed scheduling
schemas specifically for WiMAX mesh was also conducted. In [21] a Scheduling schema was
proposed in which the authors tried to exploit more of the capabilities of the WiMAX
technology for a better throughput. They make use of parallel transmission for this purpose. For
each scheduling round, the number of active connections is determined by the scheduler
through exchange of scheduling messages. The connection that has more traffic requests is
allocated the next traffic opportunity. In this schema links that has high interference is excluded.
This process is repeated until there is no unallocated traffic.
In [11], a centralized scheduling and routing tree construction algorithms was introduced. This
algorithms schedules each service flow individually to serve better each flow QoS requirements.
Even if this algorithms serves better the QoS guarantees for each service flow but it assumes
only one transmission link is active per time slot in the whole mesh network which is not
practical and will reduce the throughput of the network. An efficient scheduling schema should
exploit the capability of concurrent transmission on non interfering mesh network links.
A fair scheduling schema for WiMAX was proposed in In [16]. In this paper the authors
followed a fairness model for the assignment of scheduling opportunities for all traffic requests
of all types. They formulated a problem to get the best throughput while making scheduling
decisions that satisfies the fairness criteria. There algorithm was based on finding an optimal
fair schedule obtained by optimizing the formulated problem.
6.
Multicasting Approaches in WiMAX
Multicasting is an efficient and crucial scheme to be used in all broadband technologies to
reduce the bandwidth cost for applications that sends the same data to multiple recipients.
Multicasting in PMP mode takes a much simpler way than multicasting in Mesh mode. In PMP
mode the BS MAC layer has defined a special multicast broadcast service (MBS) while in Mesh
mode multicasting is build over routing algorithms introduced for Mesh networks.
6.1. Multicasting PMP mode
The WiMAX Multicast Broadcast Service (MBS) was defined for PMP mode specifically for
broadcasting or multicasting data or video over the WiMAX air interface. The MBS zone is
built using either one BS or multiple BSs in the same region such that all sends the same
broadcast or multicast message at the same rate at the same frequency channel. For each MBS
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
13
service a down link frame can be assigned totally for the MBS messages or can use a separate
MBS zone in the down link frame. BSs in this service should maintain time synchronization
when sending broadcast/multicast messages, use one connection identifier CID and use same
security association (SA) information for encryption of broadcast/multicast messages. Each BS
can be part of multiple MBS zones and each zone can have multiple BSs. SSs gets information
about MBS zone from The BS transmitted data. The SSs uses the Downlink map to confirm the
MBS zone. At the same time, there is an MBS-MAP in an MBS zone, which contains
information about the location of current MBS packets and the time when the next MBS packet
is transmitted.
A connection Id is assigned for all services at the Mac layer and for the MBS service a
Multicast connection ID (MCID) is assigned for each packet. MBS is defined for PMP and
hence it can use either TDD or FDD modes. MBS features make it an efficient way to
multicast/broadcast streaming data to multiple users using a shared radio channel.
Using MBS service at MAC layer for Multicasting implies that one MCID is assigned for the
service flow to satisfy the QoS requirements and will be assigned one channel with a specific
modulation and coding schemas. The problem arises when the users in multicast group has the
same quality of service requirement but has different channel conditions, so which channel and
at what modulation rate should the Multicast service be assigned. Several multicasting schemas
where proposed for handling this issue.
A Cooperative Multicasting Schemas in [17] tried to reduce the differences in channel
conditions between SSs in the same multicast group. Such Schemas has an assumption that SS’s
can communicate directly and can relay data to each other. In such schemas the BS sends
multicast data at a high rate for users with very good channel condition. User getting this
multicast cast can relay it to other users with bad channel conditions. To control this relay
operation and making each SS knows what data it gets and what data to relay, authors combined
this idea with concept of random network coding. A Cooperative Multicasting with network
coding then works well with the new WiMAX standard 802.16j where PMP mode is extended
to MMR mode and where nodes can works as relays.
In another multicasting approach [18], the multicast users in the multicast group are divided into
two sub-groups. Users in each group are selected based on their channel conditions. To better
adjust to the quality of channel conditions for each group two copies of multicast data are sent
in different time slots. Each copy is modulated and coded with different values with two
different data rates. This schema was shown to enhance the overall throughput. However, it has
limitations and will not be very efficient when the number of users in the poor condition group
is very small.
In [19], authors didn’t make any effort to calculate optimal rate for all multicast users and
instead they focused on finding an optimal rate for a certain subset of multicast group for each
transmission opportunity. They showed that it works well on the one hop shared channel
scenario, but it does not consider the cooperative diversity in the broadcasting channels.
Other research on multicasting for real time video in [20] makes use of video layering
techniques to divide video into multiple layers and send these layers through a multicast channel
where user with good channel condition can receive all the video layers at the full multicasting
rate and users with poor channels will watch a low quality video because they can receive only
the basic layer of the video.
6.2. Multicasting Mesh Mode.
The WiMAX standard defined the MBS service for PMP mode in a single hop route. In mesh
mode a mesh network is managed by a mesh base station that provides an interface to external
networks and acts as the central control node to the mesh network. Multicasting in mesh modes
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
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depends on routing approaches which is built over scheduling trees. Scheduling trees are built
depending on the actual location of SSs and are rooted at the BS. As any other broadband
technology, multicasting schemas for WiMAX mesh networks follow the multicasting
approaches and researches done for wireless ad hoc networks. These protocol runs on networks
with same topology as WiMAX, and called tree-based protocols as the Multicast operation od
Ad hoc On demand Vector (MAODV) protocol [24], and the adaptive Demand Driven
Multicast Routing (ADMR) [25]. In Both protocols a sender can have one path until for a
predefined receiver. Tree based multicast protocols could be either source-tree-based or share-
tree-based. In source-tree-based, the tree is rooted at the source and at each multicasting session
a new multicast tree is created multicast tree. In shared-tree multicast protocols, a shared
multicast tree is built once and is not necessarily rooted at the source. Some protocols for Ad
hoc networks tried to reduce the overhead and delay produced by frequent topology change due
to nodes joining and leaving the network or moving within the same network.
Few researches in literatures addressed multicasting in WiMAX mesh mode. All researches
done are for wireless mesh networks based on 802.11 families of standards. One study for
WiMAX in [22] proposes a multicast protocol based on building a high source efficient
multicast tree that uses messages defined for centralized and distributed scheduling in the
WiMAX standard. They introduce an easy to implement tree construction mechanism based on
the adjustment of the centralized routing tree. In [11], authors studied a cross layer routing and
centralized scheduling schema. They showed that a good performance can be obtained if
combining the shortest path routing algorithm with a scheduling algorithm that works on a per
time slot maximizing criteria as discussed in the scheduling part of this paper. Another joint
schema in [32] proposed for TDMA based mesh networks in general. In this work an integrated
QoS routing and scheduling schema was proposed. In this schema QoS guarantees for TDMA
mesh networks was shown to be fulfilled and the authors formulated a linear programming
optimization models to find the non collision bandwidth in a specific path.
7.
C
ONCLUSIONS
In this paper we conducted a broad study of WiMAX. We started by describing main features
and the evolution of the standards with focus on the current working standards IEEE 802.16d &
e. Like all 802.x families, the standards define only the MAC and the PHY layers to allow
interoperability with higher layers and other standards. PHY layer uses modulation and coding
schemas combined with OFDMA to produce variable data rates to support variable channel
conditions. MAC layer in WiMAX has many unique features from which supporting different
quality of service classes is the most important. By giving an overview of both MAC and
physical layers and differentiate between the two modes of operations helps understands the
research issues in terms of scheduling and Multicasting. WiMAX provided support for QoS
classes but didn’t define the scheduling mechanism for those classes in all modes. In this survey
we addressed and compared different scheduling approaches defined for WiMAX PMP and
Mesh modes. We also studied the multicasting schemas in PMP modes which is based on the
Multicast Broadcast service already defined in MAC layer for PMP mode, and showed how
multicasting schemas in Mesh mode follows the multicasting schema used for wireless mesh
networks defined for IEEE 802.11. It is worth to mention that the current working standard
approved in late 2009 is the 802.16j that supports MMR mode of operation, and withdraw the
Mesh Mode. MMR mode has a tree structure which is a special structure a mesh can follow, so
part of researches and schemas used for mesh mode can be alleviated to work with MMR mode.
This study gives a good reference or a good start for understanding WiMAX technology and the
main research issues.
International Journal of Computer Science & Engineering Survey (IJCSES) Vol.2, No.1, Feb 2011
15
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17
Authors
Manal AL-Bzoor is a Ph.D. student in
Computer Science and Engineering at the
University of Connecticut. She has received her
B.S. degree from Jordan University of Science
and Technology –Jordan and her MS degree in
Computer Engineering from University of
Michigan-Dearborn. She worked as an
Instructor at Computer Engineering Department
Yarmouk University Jordan (2006-2009)
where she supervised multiple projects in
wireless networks and in image processing.
Manal current research interests are in wireless
sensor networks and distributed/parallel
systems.
Prof. Khaled Elleithy is the Associate Dean for
Graduate Studies in the School of Engineering
at the University of Bridgeport. His research
interests are in the areas of network security,
mobile communications, and formal approaches
for design and verification. He has published
more than one hundred twenty research papers
in international journals and conferences in his
areas of expertise. Dr. Elleithy is the co-chair of
the International Joint Conferences on
Computer, Information, and Systems Sciences,
and Engineering (CISSE). He is also the editor
or co-editor of 10 books published by Springer
for advances on Innovations and Advanced
Techniques in Systems, Computing Sciences
and Software. Dr. Elleithy received his B.Sc.
degree in computer science and automatic
control from Alexandria University in 1983, his
MS Degree in computer networks from the
same university in 1986, and an MS and Ph.D.
degrees in computer science from The Center
for Advanced Computer Studies in the
University of Louisiana at Lafayette in 1988
and 1990, respectively.
... 16 standard has no specifications on any scheduling algorithm to be used. IEEE 802.16"s first standard was a single carrier WirelessMAN SC designed for frequencies beyond 11GHz, which require a line-of-sight (LoS) operation environment [5]. It was then upgraded to a single carrier WirelessMAN SCa designed to cover frequencies between 2GHz and 11GHz on Point-tomultipoint (PMP) none-line-of-sight (NLoS) conditions. ...
... RR, PF for all service classes without considering the subscriber"s varying channel conditions. Homogeneous algorithms do not always guarantee QoS and required fairness for all the service classes in WiMAX [5], [20]. Hybrid Schedulers counteract the problem faced when using one homogeneous scheduling algorithm by applying different scheduling algorithms for each service class. ...
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The IEEE standard 802.16x based broadband wireless access is a promising technology to provide wireless broadband connectivity. Besides point to multi-point mode, a mechanism to create multi-hop network through mesh mode is defined for the frequency spectrum band 2-11 GHz. The standard only defines unicast transmission under mesh mode, but does not specify multicast transmission scheme. This paper proposes a novel multicast mechanism in WiMax mesh network by building a high efficient multicast tree with utilization of scheduling messages defined in the standard. An easy-to-implement tree construction algorithm is presented based on the adjustment of the centralized routing tree. Simulation results show the proposed multicast mechanism improves transmission efficiency and reduces group latency over the traditional way to fulfil multicast by multiple unicast transmissions
Conference Paper
The IEEE 802.16 standard for broadband wireless metropolitan area network supports real time and non-real time services. It has a provision to design new packet-scheduling algorithms according to requirements to support quality of service (QoS) for real-time services. Till now published literature on WiMax states that, a service station (SS) requests for bandwidth to a base station (BS) for already arrived packets at SS from users. The BS then allocates the bandwidth to SS according to priority-based request. In this paper we propose a novel adaptive-bandwidth scheduling algorithm at SS, for real-time polling services (rtPS), wherein the SS predicts the arrival of rtPS packets prior to the arrival and requests the BS for bandwidth in advance. It has been observed by analytical model and simulation that, this adaptive algorithm provided better results with respect to the number of packets waiting at SS and average delay as compared to the widely accepted weighted scheduling algorithm
Conference Paper
WiMax broadband MAN based on the IEEE 802.16d/e standard offers high data rate scenarios for both mobile and fixed wireless access. Different power control and scheduling options are investigated within the context of interference limited WiMax deployments in tight frequency reuse assuming nomadic mobility. A novel, very promising scheduling approach based on a flexible temporary removal strategy is introduced and analyzed in detail. The temporary removal can be easily combined with conventional scheduling techniques providing considerable performance benefits. Detailed system level simulations have been performed over a wide range of system load scenarios demonstrating a significant throughput and capacity gain of approximately 50% compared to the well-known cyclic/round-robin and maximum CIR schedulers
Conference Paper
IEEE 802.16 standards define different service flows to better support the quality of service (QoS), but donpsilat provide related management strategies. Based on this, we propose a new management strategy which mainly includes an admission control algorithm named QBAC (QoS Based Admission Control) and scheduling algorithm named QBMS (QoS Based Multi-Level Scheduling). QBAC and QBMS are provided to grant the QoS request of different service flows. At the same time, we try to satisfy the request of real-time service flows and improve the performance of non-real-time service flows through this new strategy. Simulation results show that the performance of real-time service flows is good and the fairness of non-real-time services gets improvement compared with current strategies.
Conference Paper
Placed in the context of emergency communication situations, where all network infrastructures are totally down, the audio communication is required in the first hours. This paper presents a new WiMAX PHY layer model used to determine thresholds for modulation profile switching in order to enhance the communication robustness or throughput. The model used in this paper is our implemented WiMAX model using Adaptive Modulation and Coding (AMC) schemes. Two channel models and two WiMAX modulation profiles are studied using two different audio samples. The simulation results obtained and confirmed by listening experiments, have demonstrated that the audio quality is improved in terms of perceived noise. Thus, the new criterion using WiMAX radio is well suited for emergency communications.