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IEEE Wireless Communications • June 2012
96
1536-1284/12/$25.00 © 2012 IEEE
UE as
gateway
UE
ACCEPTED FROM OPEN CALL
INTRODUCTION
Device-to-device (D2D) communications com-
monly refer to the technologies that enable
devices to communicate directly without an
infrastructure of access points or base stations,
and the involvement of wireless operators. The
term “device” here refers to the user who uses
cell phones or other devices in Human-to-
Human (H2H) communications as well as
“machine” in Machine-to-Machine (M2M)
communications without the involvement of
human activities. The most widely known D2D
technologies are Bluetooth and WiFi working
at the 2.4GHz unlicensed band. Up to now,
wireless operators don’t include the D2D func-
tion in the universal cellular network standards,
e.g., Global System for Mobile Communica-
tions (GSM), Universal Mobile Telecommuni-
cations System (UMTS) and 3rd Generation
Partnership Project (3GPP) Long Term Evolu-
tion (LTE). This is largely because the D2D
function was only envisioned as a tool to reduce
the cost of local service provision, which is frac-
tional according to the operators’ current mar-
ket statistics. Recently, the wireless operators’
attitude towards the D2D function is changing
because of several new trends in the mobile
market. First, the context-aware applications
are emerging in smart phones which are envi-
sioned as an important value added service
since an wireless operator can provide a plurali-
ty of services to a user according to its location
information and its working status. For exam-
ple, a user may be informed of a nearby restau-
rant, and the user can reserve a seat and get a
coupon by making a call or sending a short
message. Since most of the context-aware appli-
cations involve discovering and communicating
with nearby devices, the D2D function can
facilitate the discovery of neighboring devices
and reduce the communication cost between
these devices. Secondly, M2M applications are
fast growing recently. Since the cellular equip-
ments are getting smaller and cheaper, the
wireless operators have great opportunities to
connecting consumer electronic devices to their
networks, e.g., washing machines and ovens.
Since most consumer devices work around their
owners, the cellular phone can be the hub for
these devices and used as the gateway to the
cellular networks. The D2D function enables
the communications between consumer devices
and cell phones.
The above emerging services and applications
are driving wireless operators to pursue the D2D
function in their networks. However, the tradi-
tional D2D technologies are inadequate. First,
there are more than 5 billion cellular users glob-
ally, who can only realize D2D function by WiFi
or Bluetooth, which is not an integral part of the
cellular networks and thus causes inconvenience
customer usage experience. For example, both
Bluetooth and WiFi require manual pairing
between two devices. The distance of WiFi-
direct is claimed to be 656 inches, which means
that dozens of devices within the range may be
on the list. This process will make the user quite
cumbersome compared to making a phone call.
Second, the traditional D2D technologies are
unable to meet the requirements of some users
or applications due to several technical limita-
tions. Since most of the traditional D2D tech-
nologies work on the crowded 2.4GHz
unlicensed band, the interference is uncontrol-
lable. In addition, traditional D2D technologies
cannot provide security and Quality-of-Service
(QoS) guarantee as the cellular networks. Last
but not the least, the wireless operators cannot
make profits using traditional D2D technologies
LEI LEI AND ZHANGDUI ZHONG, BEIJING JIAOTONG UNIVERSITY
CHUANG LIN, TSINGHUA UNIVERSITY
XUEMIN (SHERMAN) SHEN, UNIVERSITY OF WATERLOO
ABSTRACT
This article studies direct communications
between user equipments in the LTE-advanced
cellular networks. Different from traditional
device-to-device communication technologies
such as Bluetooth and WiFi-direct, the operator
controls the communication process to provide
better user experience and make profit accord-
ingly. The related usage cases and business mod-
els are analyzed. Some technical considerations
are discussed, and a resource allocation and data
transmission procedure is provided.
OPERATOR CONTROLLED DEVICE-TO-DEVICE
COMMUNICATIONS IN LTE-ADVANCED NETWORKS
The authors study
direct communica-
tions between user
equipments in the
LTE-advanced cellular
networks. Different
from traditional
device-to-device
communication
technologies such as
Bluetooth and
WiFi-direct, the
operator controls the
communication pro-
cess to provide bet-
ter user experience
and make profit
accordingly.
SHEN LAYOUT_Layout 1 6/13/12 12:49 PM Page 96
IEEE Wireless Communications • June 2012
97
since they work independently without the
involvement of the operators.
Unwilling to lose the emerging market that
requires the D2D function, the wireless opera-
tors and vendors are exploring the possibilities
of introducing the D2D function in the cellular
networks. In [1], the concept of D2D communi-
cations as an underlay to an LTE-Advanced cel-
lular network is introduced. A wireless
technology called as the FlashLinQ that enables
devices to directly sense their surroundings and
directly communicate with each other is pro-
posed in [2], which can be used in licensed band
and as a complementary to the Wide Area Net-
work (WAN). At the 3GPP meeting held in June
2011, a study item description on the radio
aspects of device-to-device discovery and com-
munication has been submitted by Qualcomm.
Meanwhile, a study item description on LTE-
direct is submitted to the 3GPP meeting held in
August 2011, which proposes the study of the
service requirement of direct over-the-air LTE
device-to-device discovery and communication.
Although interested in bringing the D2D func-
tion into cellular networks, the operators require
to control the D2D services. Furthermore, the
operator controlled D2D communications is fac-
ing a great dilemma in which if the users are
charged for their D2D services, they may turn to
traditional D2D technologies, which are free but
with lower speed and less security. Therefore,
the operators must answer the “pay for what”
question before they can push forward the oper-
ator controlled D2D technology, which requires
some analysis on the usage cases and business
models.
In this article, we first classify the operator
controlled D2D communication technologies
into two broad categories according to the level
of operator control over D2D communications.
The usage cases and business models are ana-
lyzed, followed by some technical considera-
tions on the radio aspects of operator
controlled D2D communications. Finally, the
article is concluded.
D2D CONTROLLED MODE
The operator controlled D2D (OC-D2D) com-
munications are defined as the technologies with
which the devices communicate directly with
each other under a cellular network or an opera-
tor control. The operator controls over normal
user communication process which mainly lies in
four aspects: access authentication, connection
control, resource allocation, and lawful intercep-
tion of communication information. The last
aspect is very difficult to achieve for D2D com-
munications, since information is directly
exchanged between users bypassing the operator
deployed base stations. According to the level of
operator control over D2D communications, two
categories of operator controlled D2D technolo-
gies can be classified.
FULLY CONTROLLED D2D MODE
The D2D link between two User Equipments
(UEs) is an integral part of the cellular net-
works, just like the common cellular downlink or
uplink connections. The cellular network has the
full control over the D2D connection, including
control plane functions, e.g., connection setup
and maintenance, and data plane functions, e.g.,
resource allocations. The D2D connections
share the cellular licensed band with the normal
cellular connections. The network can either
dynamically assign resources to each D2D con-
nection in the same way as a normal cellular
connection or semi-statically assign a dedicated
resource pool to all D2D connections. The oper-
ator can charge the users for using D2D service
based on how many minutes or how much band-
width they use.
LOOSELY CONTROLLED D2D MODE
The operators perform the access authentication
for the D2D enabled devices. Apart from this,
these D2D devices can setup D2D connections
and start D2D communication autonomously
with little or no intervening from the operators.
To avoid interference to the normal cellular
users, the D2D communications can make use of
either the unlicensed band with WiFi or Blue-
tooth for data transmission or a dedicated carri-
er on the licensed band. The operators can
charge a certain amount of fee per month for
providing the D2D service irrespective of the
actual D2D data flow in the network. However,
the operators must be able to disable the D2D
service if the users do not pay for it.
USAGE CASES AND BUSINESS MODELS
The D2D usage cases can also be classified into
two broad categories. The first category is
referred to as the peer to peer case, in which
the D2D devices are the source and destination
of the exchanged data. The second category is
the relay case, which means that one of the
communicating D2D devices has to relay the
exchanged information to the base station
which further forwards the data to the destina-
tion device.
PEER TO PEER
Local Voice Service
— OC-D2D communications
can be used to offload local voice traffic when
two geographically proximate users want to talk
on the phone, e.g., people in the same large
meeting room want to discuss privately, or com-
panions get lost in a supermarket, as shown in
Fig. 1a. However, this usage case is rare accord-
ing to the operators’ current market statistics.
Local Data Service
— OC-D2D communications
can also be used to provide local data service
when two geographically proximate users or
devices want to exchange data, as shown in Fig.
1b. Some scenarios of D2D communications are
illustrated below.
Content Sharing: Friends exchange photos or
videos through their smart phones, or people
attending a conference download materials from
a local server.
Multiplayer Gaming: The famous Japanese
game “Dragon Quest IX” has a co-op mode con-
sisting of up to four players using local wireless
connections to play together. The three guests
join the host system’s world and can go any-
where that the host has explored.
The operators can
charge a certain
amount of fee per
month for providing
the D2D service irre-
spective of the actu-
al D2D data flow in
the network. Howev-
er, the operators
must be able to dis-
able the D2D service
if the users do not
pay for it.
SHEN LAYOUT_Layout 1 6/13/12 12:49 PM Page 97
IEEE Wireless Communications • June 2012
98
Local Multicasting: The shops advertise the
sale promotion information to the customers.
Machine to Machine: A laptop connects to a
printer, or a smart phone connects to a televi-
sion for the photo or video display.
Context-aware Application: It is a driving fac-
tor for the D2D technologies and is based on the
people’s desire to discover their surroundings
and communicate with nearby devices (machines
or people). One example is the “Dragon Quest
IX” game, which has a tag mode allowing nearby
game devices to discover each other and
exchange messages automatically without the
players’ awareness. Therefore, a player can take
his game device to a mall or a coffee shop and
find he has “met” a lot of interesting people
when he comes back. Another example is loca-
tion aware social networking, such as
Foursquare, where users “check-in” at venues
using a mobile website, text messaging or run-
ning a device-specific application and selecting
from a list of venues that the application locates
nearby. Each check-in awards the user points
and sometimes “badges”. Therefore, context-
aware applications may be based on any of the
above four types of D2D communications, but
they should be able to notice/interact when
something nearby is interesting.
Although it is appealing, making consumers
adopt the OC-D2D technologies is tricky. The
toughest problem is the competition with the
traditional D2D technologies, which are current-
ly dominating the market and allow the users to
use local data service freely. Therefore, the OC-
D2D technologies have to be attractive enough
for consumers to switch and be willing to paying
for this service.
Some of the good reasons to attract users for
operator controlled D2D services are listed
below:
Pay for identity: Use loosely controlled D2D
technologies to link cellular phone number with
WiFi or Bluetooth identity, which facilitates
D2D connection setup and provides value-added
service through identity management. For exam-
ple, the users can start photo or video exchange
through WiFi by dialing phone numbers instead
of searching for the WiFi name;
Pay for QoS and security: Use fully con-
trolled D2D technologies for those services
which require high QoS and/or security;
Pay for context information: Operators have
deep contextual information about end users
and have an emerging opportunity to leverage
context to make it pay off competitively.
RELAY
UE as the Gateway to Sensor Networks
— Most M2M
devices are not “directly cellular”. In other
words, M2M devices usually first connect to an
M2M gateway using Wireless Personal Area
Network (WPAN), e.g., zigBee, and the M2M
Figure 1. D2D usage cases:a) local voice service; b) local data service; c) UE as gateway to sensor networks; and d) UE cooperative relay.
Multiplayer
gaming
Content sharing
Railway
station
Supermarket
Airport
UE as
gateway
UE
Sensors on
home devices
Sensors on car
UE
Computer
Smart phone
Mobile internet
devices
Sensors on
customers
Meeting room
(a)
(c) (d)
(b)
Machine to
machine
Context-aware
application
Local
multicasting
Internet
Camera
DV
SHEN LAYOUT_Layout 1 6/13/12 12:49 PM Page 98
IEEE Wireless Communications • June 2012
99
gateway connects to a cellular network. For
many consumer M2M devices, e.g., sensors on
home devices, cars, or even the onbody health
care devices, cell phones on the consumers are
the most suitable M2M gateways. The communi-
cations between these sensors and UEs can use
the OC-D2D technologies.
Similar to the local data service usage case,
the OC-D2D technologies also faces challenges
from the traditional D2D technologies. The fol-
lowing are some possible answers to the “pay for
what” question of the OC-D2D communications.
Pay for management: One possible business
model for the OC-D2D communications is given
in [3], where non-cellular devices are included
into the operator’s subscriber database and auto-
matically associated with the owner’s cellular
devices. An M2M profile is created for such
device to store its relevant information, such as
the owner and device specific access policies.
Authentication/key management can be provid-
ed for the sensor devices that require security.
An operator can also separately meter the data
from different devices behind a phone/gateway.
Pay for QoS and security: Use fully con-
trolled D2D technologies for those applications
that require high security and QoS, e.g., sensors
for life care or security.
UE Cooperative Relay
— In the wireless telecommu-
nications systems that have a large number of
subscribers, it is well known that one efficient
communication method is to break a long path
into a number of smaller hops so that the infor-
mation is relayed between a number of termi-
nals. The integration of cellular and ad-hoc
networks to provide the UE relay capability has
been well studied [4, 5]. 3GPP has even consid-
ered to apply this technique for the UMTS Time
Division Duplex (TDD) under the concept of
Opportunity Driven Multiple Access (ODMA)
[6]. However, the UE relay faces a number of
business model difficulties apart from the techni-
cal challenges. The biggest obstacle is the users’
concern on the information security, wireless
radiation and excessive consumption of their
battery power, all of which are due to opening
up their mobile devices to other users. There-
fore, 3GPP has finally decided to give up the
ODMA standardization. These problems still
exist today and a proper business model with
enough incentives for users needs to be designed
if the OC-D2D technologies is to be applied to
this scenario.
A further optimization for the UE relay is to
apply the cooperative techniques [7], i.e., the
D2D communications capability enables the
users’ cooperation to achieve the transmit diver-
sity, multi-antenna transmission and network
coding, etc.. These techniques are still mostly
under the academic research and not mature
enough for the standardization and implementa-
tion in the near future.
The benefits, marketing challenges and poten-
tial business models for different operator con-
trolled D2D usage cases are summarized in
Table 1. They should be taken into account in
the design of various OC-D2D technologies.
TECHNICAL CONSIDERATIONS IN
RADIO ACCESS NETWORKS
SPECTRUM FOR OPERATOR CONTROLLED
D2D COMMUNICATIONS
The categories of spectrum that the OC-D2D
communications can operate on are listed as fol-
lows.
Unlicensed band: The advantage is that oper-
ators do not need to sacrifice valuable licensed
spectrum for providing D2D services. However,
the uncontrolled interference condition makes
this option rather unattractive to the users. One
possible scenario of carrying D2D traffic on unli-
censed band is that operators provide automatic
device pairing, device authentication etc. using
loosely controlled D2D technologies for the
users.
Frequency Division Duplex (FDD) licensed
band: In order to support D2D function in FDD
band, UE has to add Rx chain in the uplink
spectrum or Tx chain in the downlink spectrum
or both, which will increase UE cost and com-
plexity. Therefore, for those operators who only
have FDD spectrum, providing D2D service may
be more difficult.
TDD licensed band: Since UE has both Rx
Table 1. Usage cases for operator controlled D2D communications.
Usage Cases
Peer to Peer Relay
Local voice service local data service UE as gateway to sensor networks UE cooperative relay
Benefits Enhance capacity Provide new services Enhance capacity
Marketing
Challenges
Rare occasion Competition from traditional free D2D techniques
Users’ concern on informa-
tion security etc.
Potential Business
Models
• Attract users to pay for identity, QoS and security,
context information, and management, etc.
• Charge the users based on how many minutes or how
much bandwidth they use in fully controlled D2D
communications; and charge a certain amount of fee per
month irrespective of the actual D2D data flow in loosely
controlled D2D communications
SHEN LAYOUT_Layout 1 6/13/12 12:49 PM Page 99
and Tx chains for data transmission and recep-
tion in TDD spectrum, no additional Rx/Tx
chains are needed to support D2D communica-
tions, which makes TDD spectrum more suitable
for carrying D2D traffic. For operators with both
TDD and FDD spectrum, TDD spectrum can be
dedicated to D2D communications. For opera-
tors with only TDD spectrum, D2D communica-
tions can share resources with normal cellular
communications, or occupy one or several dedi-
cated carriers.
Guard band between FDD and TDD: TDD
and FDD wireless systems that are deployed in
the same geographical area need frequency sep-
aration referred to as “guard band” to prevent
Radio Frequency (RF) interference with one
another. Normally, guard band cannot be used
for the sake of interference coexistence between
TDD and FDD. However, it is possible to trans-
mit data on the guard band with properly
designed techniques to increase resource utiliza-
tion. For example, one approach is to apply
half-duplex FDD in the guard band, where only
uplink (or downlink) transmission is performed
in the TDD uplink (or downlink) period on the
guard band between TDD and FDD uplink (or
downlink) carriers [8]. Using guard band for
D2D communications can be more cost-efficient
than using normal licensed band, which allows
operators to provide low cost D2D services in
competition with traditional D2D technologies.
One possible method for transmitting the D2D
service on the guard band between the TDD
and FDD uplink carriers is shown in Fig. 2,
where the carrier aggregation technique is used
to transmit the downlink control signaling from
the base station to the D2D UEs on the TDD
carrier.
POWER CONTROL, RESOURCE ALLOCATION AND
INTERFERENCE MANAGEMENT
Power control and resource allocation of D2D
connections can be either distributively deter-
mined by the UEs themselves or centrally per-
formed by the base station, which is referred to
as the Evolved Node B (eNodeB) in LTE sys-
tems. In the former case, dedicated resources
have to be allocated to all the D2D connections
statically or semi-statically so that no interfer-
ence should be caused to the cellular connec-
tions. In the latter case, the D2D connections
may either use dedicated resources or share
resources with cellular users, since the eNodeB
can make sure that the mutual interference
between cellular and D2D connections are
acceptable via scheduling and power control.
An example of distributed resource allocation
scheme is FlashLinQ, which is an Orthogonal
Frequency Division Multiplexing (OFDM)-based
synchronous MAC/PHY architecture for D2D
communications. Unlike traditional D2D tech-
nologies, FlashLinQ is designed to work on
licensed band, where interference is more con-
trollable. The goal is to schedule a channel-state
aware maximal independent set at any given
time slot based on the current traffic and chan-
nel condition, and the scheduling algorithm
leads to spatial throughput gains over an IEEE
802.11g system [9]. This resource allocation
approach can be used in loosely controlled D2D
communications on licensed band or fully con-
trolled D2D communications where the base sta-
tion semi-statically assigns a dedicated resource
pool for all D2D users.
Significant research has been done on the
centralized resource allocation and power con-
trol algorithms considering mutual interference
between D2D and cellular connections, where
D2D communication is considered as an under-
lay to LTE-advanced networks [10–14]. The
resource sharing mode selection problem, which
decides whether the network shall assign D2D
communication mode or not to a user pair and
whether a pair of D2D users shall share
resources with cellular users or use dedicated
resources instead, is considered in [10, 11]. In
[12, 13], resource allocation algorithms among
the cellular and D2D links are studied. Refer-
ence [14] investigates power control algorithms
for the D2D mode communications. Although
the emphases of these works are different, the
resource sharing mode selection, resource allo-
cation and power control algorithms are usually
considered jointly in order to achieve the opti-
mal performance.
IEEE Wireless Communications • June 2012
100
Figure 2. Use of guard band between TDD and FDD for D2D communications.
Cellular
UE
D2D UE
Data transmission for
D2D UE
FDD
downlink
TDD
carrier 1
...
TDD
carrier N
Guard
band
FDD
uplink
Guard
band
TDD
downlink
period
FDD
downlink
TDD
carrier 1
...
TDD
carrier N
Guard
band
FDD
uplink
Guard
band
TDD
uplink
period
Data transmission for
cellular UE
Downlink control
signaling for cellular and
D2D UEs
Uplink control signaling
for cellular and D2D UEs
Power control and
resource allocation of
D2D connections
can be either
distributively
determined by the
UEs themselves or
centrally performed
by the base station,
which is referred to
as the Evolved Node
B (eNodeB) in
LTE systems.
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IEEE Wireless Communications • June 2012
101
For the centralized resource allocation
approach, the eNodeB has full control over the
resources allocated to each D2D connection and
needs to inform the D2D UEs of the scheduled
resources for data transmission via L1/L2 control
signaling, e.g., Physical Downlink Control Chan-
nel (PDCCH). However, this problem has not
been adequately addressed.
In this article, we provide a resource alloca-
tion and data transmission procedure, as shown
in Fig. 4. This procedure gives an example of
how a centralized resource allocation approach
could be implemented in an LTE-advanced sys-
tem, identifying the possible control and data
channels between the eNodeB and the D2D
UEs for control signaling and data service trans-
mission. Assume UE1 and UE2 have established
an D2D connection and UE1 has data wait to be
transmitted to UE2. The eNodeB is responsible
for resource allocation. First, UE1 notifies the
eNodeB that it has data to be transmitted to
UE2. According to the LTE protocol, UE1 can
send a buffer status report (BSR) [15] to the
eNodeB through the Physical Uplink Shared
Channel (PUSCH) [16] for this purpose. If no
uplink resources are available for the BSR trans-
mission, UE1 can send a one bit scheduling
request (SR) [15] signaling through the Physical
Uplink Control Channel (PUCCH) [16]. Once
the eNodeB receives the SR from UE1, it will
allocate a small amount of uplink resources for
the BSR transmission.
After the eNodeB receives the notification
(e.g., BSR) from UE1, it will allocate resources
for the data transmission between UE1 and
UE2. The specific resource allocation algo-
rithms will not be discussed here. In an LTE
system, the eNodeB usually considers the
channel status when performing resource allo-
cation. For the D2D communications, the
eNodeB can obtain the channel status of D2D
links between UE1 and UE2 by the periodic
or aperiodic channel quality indication (CQI)
[17] reports from UE1 and UE2 through the
PUCCH. It is assumed that UE1/UE2 can per-
form the CQI estimation from the received
Sounding Reference Signal (SRS) [16] trans-
mitted by its D2D peer.
Once the eNodeB determines the allocated
resources, it notifies the result to both UE1
and UE2 through the PDCCH. In an LTE sys-
tem, a UE performs blind decoding using its
identity (i.e., Cell Radio Network Temporary
Identifier (C-RNTI) [18]) to locate the specific
PDCCH for it [17]. Therefore, in order to
simultaneously notify UE1 and UE2 the
resource allocation result, two possible meth-
ods are described below.
Method 1: The eNodeB sends two indepen-
dent PDCCHs to UE1 and UE2 with their own
C-RNTIs. In order for UE1 and UE2 to know
whether it should transmit or receive data on the
allocated resources, the PDCCHs that the
eNodeB sends to UE1 and UE2 can be in differ-
ent Downlink Control Information (DCI) for-
mats [19]. In this example, the eNodeB sends
Uplink (UL) grant to the sending UE1 and
Downlink (DL) grant to the receiving UE2.
Method 2: The eNodeB sends only one
PDCCH to UE1 and UE2 with the C-RNTI of
the sending UE (UE1 in this example). There-
fore, UE2 needs to know the C-RNTI of UE1 in
order to decode this PDCCH. This can be
obtained during the D2D connection establish-
ment phase. Compared with Method 1, this
approach can reduce the signaling overhead but
increase the blind decoding attempts.
After UE1 receives the PDCCH from the
eNodeB, it will transmit data to UE2 on the
allocated resources. In an LTE system, downlink
and uplink data transmissions are carried on the
Physical Downlink Shared Channel (PDSCH)
[16] and the PUSCH, respectively. In the D2D
communications, however, there is no differenti-
ation between downlink and uplink since the two
communicating devices are both UEs. Therefore,
it seems that both the PDSCH and the PUSCH
can be used to carry the D2D traffic. However,
there are several issues if the D2D links are con-
sidered as downlink. In an LTE system, a UE
needs to estimate the downlink channel by
detecting the Cell-specific Reference Signals
(CRS) [16] from the eNodeB to carry out down-
link coherent demodulation. Since a D2D UE
cannot transmit the CRS as an eNodeB does, or
mutual interference between the eNodeB and
the transmitting UE will arise and the signal
demodulation may not be performed correctly.
In addition, the PDCCH has to be transmitted
one or several subframes prior to the PDSCH in
the D2D case, which will cause the cross-sub-
frame scheduling problem. On the other hand, it
is more straightforward to consider the D2D
links as uplink. In the current LTE systems, the
uplink reference signals are UE-specific, so
there is no interference problem for the refer-
ence signals from a D2D UE and a cellular UE.
In addition, the eNodeB notifies the UE its
scheduled uplink resources one or several sub-
frames prior to the PUSCH transmission by the
UE, the timing relationship between the PDCCH
and PUSCH are suitable for the D2D case and
there shall be no cross-subframe scheduling
problem.
After UE2 receives the PDCCH from the
eNodeB, it will receive data on the allocated
resource from UE1, which are transmitted
through the PUSCH as discussed above. It is
assumed that UE2 has the Single-Carrier Fre-
quency Division Multiple Access (SC-FDMA)
baseband reception ability and uplink Demodu-
lation Reference Signal detection ability. UE2
then provides an ACK/NACK feedback to UE1
according to whether the data is correctly
received or not. In an LTE system, the eNodeB
transmits an ACK/NACK to a UE for its
PUSCH transmission by the Physical Hybrid
ARQ Indicator Channel (PHICH) [16], which is
mapped to the first three OFDM symbols. How-
ever, since UE2 may simultaneously transmit
data to UE1 by the PUSCH, which cannot be
multiplexed in the same subframe with the
PHICH, it is proposed to use the PUCCH for
the ACK/NACK transmission instead. In an
LTE system, the PUCCH is used to carry
ACK/NACK for downlink data transmission.
However, the physical resources that the
PUCCH is mapped to are related to the PDCCH
that schedules the corresponding PDSCH.
Unlike cellular connections, where the PDCCH
Significant research
has been done on
the centralized
resource allocation
and power control
algorithms
considering mutual
interference between
D2D and cellular
connections, where
D2D communication
is considered as an
underlay to
LTE-advanced
networks.
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and the related PDSCH are transmitted in the
same subframe, the PDCCH has to be transmit-
ted one or several subframes prior to the actual
data transmission for the D2D connections.
Therefore, the PUCCH resource for D2D
ACK/NACK may collide with a cellular
ACK/NACK, since their corresponding PDC-
CHs are transmitted in different subframes. In
order to solve this problem, two possible meth-
ods are provided.
Method 1: Reserve specific resources for the
D2D PUCCH.
Method 2: Use the cross-carrier scheduling
capability in the carrier aggregation technique,
where a dedicated carrier is used to carry the
D2D traffic and the related PDCCHs are carried
on the other carriers for cellular traffic. In this
way, the D2D ACK/NACK will be transmitted
on a different carrier from the cellular
ACK/NACK, which will avoid collision between
them. One example is to use the guard band of
TDD and FDD for D2D traffic.
Finally, when UE1 receives the ACK/NACK
from UE2, it will decide whether to perform
data retransmission or not. Since the LTE sys-
tem uses synchronous HARQ in the uplink, UE1
and UE2 both know on which subframes to send
and receive the retransmitted data. In addition,
if non-adaptive HARQ is adopted, UE1 can
retransmit the data on the same resource blocks
as the initial transmission, so that the eNodeB
does not need to send the PDCCH to UE1 and
UE2 again. However, the eNodeB has to listen
to the ACK/NACK from UE2 to determine
whether it can schedule new data on these
resources.
The control and data channels for the above
D2D communication procedure is shown in Fig.
3. Since this article only discusses about the
radio aspects of D2D communications, the
resource allocation issue in core networks is out
of our scope. However, it should be noted that
no core network resources are needed to carry
D2D traffic.
PEER DISCOVERY, PAGING AND
CONNECTION ESTABLISHMENT
Before the resource allocation and data trans-
mission phase, two devices need to find each
other, i.e., peer discovery and D2D connection
setup. The peer discovery phase is relatively
independent of the D2D communication phase.
Existing work can be classified into centralized
and distributed approaches.
Centralized approach: A certain entity in the
cellular network, e.g., Packet Data Network
(PDN) gateway or Mobility Management Entity
(MME), detects that it may be better for two
communicating UEs to set up a D2D connec-
tion. This entity then informs the eNodeB to
request measurements from the UE to check if
the D2D communications offers higher through-
put. If so, the eNodeB decides that the two UEs
can communicate in D2D mode [1].
Distributed approach: The UE broadcasts
identity periodically so that other UEs may be
aware of its existence and decides whether it shall
start a D2D communication with it. This approach
does not need the involvement of the base station
[2]. The distributed peer discovery approach is
more flexible and scalable than the centralized
one. However, the operator cannot forbid illegal
users to announce or listen information to/from
the D2D peers using the operators’ licensed band.
For fully controlled and loosely controlled
D2D communications, different paging and con-
nection establishment methods may be used:
Fully controlled D2D communications: The
paging and connection establishment procedure
is mostly the same with normal LTE procedure
[20]. However, since the D2D UEs shall
exchange data directly over the air after the con-
nections are established, it may be necessary to
inform one D2D UE of the configured informa-
tion of its peer regarding data transmission, e.g.,
C-RNTI, sounding reference signal configura-
tion, and ciphering key etc.
Loosely controlled D2D communications on
licensed band: When communication between
two UEs is desired, contact can be initiated via a
form of direct D2D paging to create a D2D con-
nection without the intervention of the base sta-
tion [2]. However, the operator should be able
to control whether the D2D connection is
allowed to be setup or not.
Loosely controlled D2D communications on
unlicensed band: The cellular network perform
authentication when two UEs want to start D2D
communication. After that, the data transmission
between these UEs takes place on unlicensed
band with traditional D2D technologies.
CONCLUSION
In this article, we have studied the potential
usage cases and technical design considerations
in the operator controlled device-to-device com-
munications. The potential usage cases have
been analyzed and classified into four categories.
Each usage case has its own marketing chal-
lenges and the design of the related techniques
should take these factors into consideration.
Furthermore, some technical considerations on
the radio aspects of the operator controlled
Figure 3. Resource allocation and data transmission procedure for D2D com-
munications.
PUSCH (retransmission)
PUCCH
(ACK/NACK)
PUSCH (initial transmission)
PDCCH (UL grant)
UE1 UE2
PUCCH (SR)
PDCCH (UL grant for BSR)
PUCCH (BSR)
PDCCH (UL grant)
eNB
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D2D communications are discussed, including
the usable spectrum, resource allocation and
connection establishment, etc. Specifically, a sig-
naling procedure of the resource allocation and
data transmission of operator controlled D2D
communications has been provided.
The operator controlled D2D communica-
tions should enable the operators to control
their networks in order to provide better user
experience and make profit accordingly. At the
same time, they should be flexible and low-cost
to compete with traditional free D2D communi-
cations. The operators still face several chal-
lenges in providing such a D2D solution that can
address the above two “contradicting” objectives
simultaneously.
ACKNOWLEDGEMENT
This work was supported by the National 973
Projects (No. 2010CB328105, No.
2009CB320504), the Fundamental Research
Funds for the Central Universities (No.
2012JBM003), and the Key Project of the
National Natural Science Foundation of China
(No. 60932003).
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[15] 3GPP TS 36.321 v10.3.0, “Evolved Universal Terrestrial
Radio Access (E-UTRA); Medium Access Control (MAC)
protocol specification,” Sept. 2011.
[16] 3GPP TS 36.211 v10.3.0, “Evolved Universal Terrestrial
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BIOGRAPHIES
LEI LEI (leil@bjtu.edu.cn) received a B.S. degree in 2001 and
a Ph.D. degree in 2006, respectively, from Beijing University
of Posts & Telecommunications, China, both in telecommu-
nications engineering. From July 2006 to March 2008, she
was a postdoctoral fellow at Computer Science Depart-
ment, Tsinghua University, Beijing, China. She worked for
the Wireless Communications Department, China Mobile
Research Institute from April 2008 to August 2011. She
has been an Associate Professor with the State Key Labora-
tory of Rail Traffic Control and Safety and the School of
Electronic and Information Engineering, Beijing Jiaotong
University, since Sept. 2011. Her current research interests
include performance evaluation, quality-of-service and
radio resource management in wireless communication
networks.
C
HUANG LIN [SM] (clin@csnet1.cs.tsinghua.edu.cn) is a pro-
fessor of the Department of Computer Science and Tech-
nology, Tsinghua University, Beijing, China. He is a
Honorary Visiting Professor, University of Bradford, UK. He
received the Ph.D. degree in Computer Science from the
Tsinghua University in 1994. His current research interests
include computer networks, performance evaluation, net-
work security analysis, and Petri net theory and its appli-
cations. He has published more than 400 papers in
research journals and IEEE conference proceedings in
these areas and has published four books. He is a senior
member of the IEEE. He serves as the Technical Program
Vice Chair, the 10th IEEE Workshop on Future Trends of
Distributed Computing Systems (FTDCS 2004); the General
Chair, ACM SIGCOMM Asia workshop 2005 and the 2010
IEEE International Workshop on Quality of Service (IWQoS
Figure 4. Data and control channels for D2D communications.
PDCCH (UL grant)
PDCCH (UL grant)
PUCCH (CQI)
PUCCH (SR, CQI)
PUSCH (BSR)
PUSCH (data)
PUCCH (ACK/NACK)
UE UE
eNodB
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IEEE Wireless Communications • June 2012
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2010); the Associate Editor, IEEE Transactions on Vehicular
Technology; and the Area Editor, Journal of Computer
Networks.
X
UEMIN (SHERMAN) SHEN [M’97, SM’02, F’09]
(xshen@bbcr.uwaterloo.ca) received the B.Sc.(1982) degree
from Dalian Maritime University (China) and the M.Sc.
(1987) and Ph.D. degrees (1990) from Rutgers University,
New Jersey (USA), all in electrical engineering. He is a Pro-
fessor and University Research Chair, Department of Electri-
cal and Computer Engineering, University of Waterloo,
Canada. He was the Associate Chair for Graduate Studies
from 2004 to 2008. His research focuses on resource man-
agement in interconnected wireless/wired networks, wire-
less network security, wireless body area networks,
vehicular ad hoc and sensor networks. He is a co-
author/editor of six books, and has published more than
600 papers and book chapters in wireless communications
and networks, control and filtering. He served as the Tech-
nical Program Committee Chair for IEEE VTC’10 Fall, the
Symposia Chair for IEEE ICC’10, the Tutorial Chair for IEEE
VTC’11 Spring and IEEE ICC’08, the Technical Program
Committee Chair for IEEE Globecom’07, the General Co-
Chair for Chinacom’07 and QShine’06, the Chair for IEEE
Communications Society Technical Committee on Wireless
Communications, and P2P Communications and Network-
ing. He also serves/served as the Editor-in-Chief for IEEE
Network, Peer-to-Peer Networking and Application, and IET
Communications; a Founding Area Editor for IEEE Transac-
tions on Wireless Communications; an Associate Editor for
IEEE Transactions on Vehicular Technology, Computer Net-
works, and ACM/Wireless Networks, etc.; and the Guest
Editor for IEEE JSAC, IEEE Wireless Communications, IEEE
Communications Magazine, and ACM Mobile Networks
and Applications, etc. He received the Excellent Graduate
Supervision Award in 2006, and the Outstanding Perfor-
mance Award in 2004, 2007 and 2010 from the University
of Waterloo, the Premier’s Research Excellence Award
(PREA) in 2003 from the Province of Ontario, Canada, and
the Distinguished Performance Award in 2002 and 2007
from the Faculty of Engineering, University of Waterloo. He
is a registered Professional Engineer of Ontario, Canada, an
Engineering Institute of Canada Fellow, and a Distin-
guished Lecturer of IEEE Vehicular Technology Society and
Communications Society.
Z
HANGDUI ZHONG (zhdzhong@bjtu.edu.cn) received the
B.Eng. and M.Sc. degrees from Northern Jiaotong Universi-
ty (currently Beijing Jiaotong University), Beijing, China, in
1983 and 1988, respectively. He has been a Professor with
the School of Electronic and Information Engineering, Bei-
jing Jiaotong University, since 2000. He has authored seven
books and over 150 technical papers in the field of wire-
less communication. His current research interests include
wireless communication theory for railway systems, wire-
less ad hoc networks, channel modeling, radio resource
management, intelligent transportation systems, and GSM-
R systems.
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