Communications and Network, 2013, 5, 367-373
http://dx.doi.org/10.4236/cn.2013.53B2067 Published Online September 2013 (http://www.scirp.org/journal/cn)
Resource Allocation and Beamforming Algorithm Based on
Interference Avoidance Approach for Device-to-Device
Communication Underlaying LTE Cellular Network
Huy-Dung Han, Chenxi Zhu, Dorin Viorel, Akira Ito
Fujitsu Laboratories of America Inc. Sunnyvale, CA, USA
Received July, 2013
ABSTRACT
In this work, we consider device-to-device (D2D) direct communication underlaying a 3GPP LTE-A network. D2D
communication enables new service opportunities, provides high throughput and reliable communication while reduc-
ing the base station load. For better total performance, D2D links and cellular links share the same radio resource and
the management of interference becomes a crucial task. We propose a radio resource allocation for D2D links based on
interference avoidance approach. For system with multiple transmit antennas, we apply beamforming technique based
on signal to leakage criterion to reduce the co-channel interference. The results show that, D2D transmission with the
resource allocation and beamforming technique provides significant gain compared to that of the regular cellular net-
work.
Keywords: Device-to-Device; LTE; Resource Allocation; Beamforming
1. Introduction
In recent years, 3GPP Long Term Evolution (LTE)
technology has become the key standard for IMT-Ad-
vanced. LTE and its extended version LTE-Advanced
(LTE-A) have already adopted many advanced technolo-
gies such as Multiple-Input Multiple-Output (MIMO)
antennas, Coordinated Multiple Points (CoMP), enhance
Inter-cell Interference Cancelation (eICIC) to meet the
increasing demand for high throughput and Quality of
Service (QoS) [1]. The next generation LTE network is
expected to adopt technologies that not only increase
system performance but also b uild up foundatio n for new
type of services. Short range direct communication or
device-to-device (D2D) communication is a candidate to
satisfy this requirement and has become one of the study
items under investigation in release 12 of 3GPP LTE.
Unlike the existing D2D communication solutions,
such as Bluetooth [2] or direct WIFI, D2D communica-
tion in LTE shares the licensed radio resource with regu-
lar LTE links. It has significant advantages on power
consumption and spectral efficiency. It allows devices to
transmit data without passing it through base stations (BS
or eNB) and the backbone. Because of the proximity, the
direct links often have good channel quality, i.e., low
pathloss. Thus, even with low transmit power, high
throughput transmission can be made. Furthermore, it is
possible to gain higher spectrum usage by letting the
D2D links utilize the spectrum resource of other links
without causing substantial performance degradation to
each other. Such way of communication requires a care-
ful design on radi o t raf fi c m anagement.
In D2D communication underlaying cellular network,
interference management is one of crucial task. As D2D
communication is an ad ditional feature besid e the regular
cellular communication, D2D mode UEs (DUE) should
not cause significant interference to cellular UEs (CUE)
receivers when they are sharing the same radio resource.
Similar to the cognitive radio context, the DUEs can be
considered as secondary users and access the licensed
channel in an opportunistic way [3]. However, there
would be no QoS guarantee for them. Therefore, it is
expected that the eNB controls and performs interference
management to improve the QoS of D2D link as well as
that of the other legacy devices sharing the same band-
width.
Given a group of devices sharing the same radio re-
source, interference management usually become the
problem of resource allocation in which each device
transmits using a time and frequency slot, aka, radio re-
source block (RB) with certain power. In [4], the maxi-
mum transmit power D2D links is set by eNB such that
the interference to the co-channel CUE link is limited.
Power adjustment for D2D links is also proposed in [5]
C
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H.-D. HAN ET AL.
368
to dynamically reduce the interference. In [6], the power
level is set to optimize the transmission sum rate of both
CUEs and DUEs. Nevertheless, the power control should
be jointly considered with reso urce allocation/scheduling
to minimize the potential critical interference [7]. Be-
sides, under LTE scenario, the spectrum resource alloca-
tion/scheduling rate is faster than that of the power ad-
justment; thereby, the instanta n eou s sign al to interference
and noise (SINR) for each UE depends more on sched-
uling. Therefore, channel/interference aware scheduling
algorithms can be more beneficial for the system per-
formance. In [8], the area of possible D2D transmission
is calculated by pathloss threshold. However, the area
modeling is over simplified leading to the sub-optimal
performance.
In this work, we investigate an interference avoidance
scheduling algorithm for D2D links. Given a schedule for
CUEs, each D2D link is paired up with a CUE link and
sharing the scheduled RBG by a pairing algorithm. The
D2D pairs are allocated such that the interference at each
involved device is below a threshold. For the D2D pairs
violating the requirement, a default spectrum resource is
provided so that they still can transmit with best effort
basis. For multiple transmit antennas system, we apply a
beamforming based on signal to interference and noise
(SLNR) metric [9,10] to further reduce the interference.
The results show that, with the pairing algorithm, the
co-channel interference between D2D pairs and the CUE
uplink is mitigated and the system throughput is opti-
mized.
2. System Model
We consider D2D links sharing uplink bandwidth with
the regular CUEs. The total uplink frequency resource
for data is assumed to be divided into N resource block
groups (RBG)1. At a given time slot t, the RBG can be
indexed as (, where )tf {1,.., }
f
N

. There are c
uplink CUEs and d D2D pairs sharing this radio re-
source. If D2D links and a CUE uplink sharing RBG
, co-channel interference degrades the performance
of both links. Figure 1 represents channel model where
D2D links j, k and an uplink CUE i transmit in the same
RBG. Here, D2D pair k consists of a DUE transmitter
(DUE-T) k and a DUE receiver (DUE-R) k. For the sake
of simplicity, the RBG index indicator is omitted. The
channel and interference include the following compo-
nents:
L
L
(, )tf
 ()
c
H
i: the channel matrix fro m CUE i to the eNB.
 ()
cd
H
k: the channel matrix from DUE-T k to the
eNB.
 (,)
dd
H
kj: the channel matrix from DUE-T j to
DUE-R k.
 ()
d
H
k: the channel matrix from DUE-T k to
DUE-R k.
 (,)
dc
H
ki: the channel matrix from the CUE i to
DUE-R k.
If multiple antennas are equipped at the transmitter,
beamforming technique can be used to improve the SINR.
We define the precoding matrices for CUE i and for
DUE-T k as Wand W respectively. In practice,
the precoding matrices belong to a codebook .
()
ci () dk,
The performance of the system depends on the co-
channel interference causing by the devices. The intra-
cell co-channel interference measured at eNB is
2
()( )( )
i
ccdd
kD
I
iHkWk

 (1)
where i is the index set of the D2D links sharing the
bandwidth with CUE i. The SINR for uplink CUE i can
be calculated as

2
0,
() ()
() () ()
cc
ccc
H
iW i
iNiIi   (2)
where 0, are the noise power plus the interference
from other cells for the link i. Similarly, the interference
measured at DUE-R k and its SINR are calculated as,
respectively:
()
c
Ni
22
,
()()()(,)() ,
i
ddcc ddd
jDj k
I
kHkWi HkjWj


 (3)
2
0,
() ()
() () ()
dd
ddd
H
kW k
kNkIk
  (4)
where 0, are the noise power plus the interference
from other cells for the D2D link .
()
d
Nk k
The problem of radio resource management at eNB is
to decide the sets i for all i together with precoding
matrices such that the optimum sum throughput is
achieved over a long time period as:

Figure 1. Channel model for D2D pairs sharing bandwidth
with an uplink CUE. The solid lines represent the direct
links. The dot lines represent co-channel interference.
1The term resource block group is taken from LTE specification [1]
since we are using LTE scenarios for simulation.

H.-D. HAN ET AL. 369
() () ()
() ()
11 ,(),()
Maximizing( )()
c
tt t
c i
d
d
LL
tt
cd
ti k
WiWk
Ti Tk





 





(5)
where , are the throughput of CUE i and
DUE k at time t. The throughputs can be defined using
Shannon capacity formula [11] as
()
()
t
c
Ti () ()
dt
Ti

2
log 1B

 with B
is the bandwidth of a RBG and

is the SINRs defined
in (2), (4). Th e time index t is added to
to represent their changes over time. ,(
ic
Wi),()
d
Wk
The problem in (5) can be classified as a NP-hard as it
can be reduced to a si mpler, but still NP-hard, scheduling
algorithm [12]. In [12], a proportional fair algorithm can
be performed to optimize the total throughput over time
while allowing all the UEs receive at least a minimal
QoS. Such algorithms usually depends on the assumption
that the interference is small and relative unchanged, i.e.,
()
c
I
 and ()
d
I
 are negligible. When there are multiple
devices scheduled on one RBG, this assumption no
longer holds. Therefore, new designs considering inter-
ference after resource allocation are required.
The instantaneous rate of a UE depends on its relative
location with eNB, the varying channel and the co-
channel interference. Compare to the transmission rate of
CUEs in traditional cellular system, the instantaneous
rate of CUEs in the presence of D2D communication
depends more on the intra-cell co-channel interference.
Hence, for a group of a CUE and DUEs sharing the same
RBG, the instantaneous interference depends on the re-
source assignment. Therefore, the task of resource allo-
cation for UEs sharing resource is very different from the
one for uplink only. In section 3, we propose a resource
allocation mechanism for D2D pair sharing bandwidth
with an uplink CUE, called pairing algorithm. Based on
interference avoidance approach, the pairing algorithm
allows many D2D links reuse the uplink radio resource
while maintaining the uplink performance.
The problem in (5) considers resource allocation and
precoding matrices to optimize the performance and,
thereby, hard to solve. Therefore, we separate the two
processes by applying the pairing algorithm first; then,
given the resource assignment, we determine the opti-
mum beamforming. In section 5, we introduce a beam-
forming technique to reduce the in-cell co-channel inter-
ference and improve throughput performance.
3. Pairing Algorithm Based on Interference
Threshold and Default RBG
As the general problem is NP-hard, we propose a heuris-
tic algorithm whose assu mption is that the CUE has been
scheduled without knowledge of DUE channels. The
assumption is based on two folds: CUEs requires better
QoS protection as the DUEs always have better channel
due to their proximity; CUEs should have higher pri-
orityas D2D communication is an additional feature in
cellular network. With this assumption, we can reduce
the general problem to the pairing algorithm whose task
is to assign a group of DUEs to pair with a CUE and
share its scheduled resource.
The pairing algorithm also assumes that the signal
strength of D2D links is often very high and eliminating
the co-channel interference is essential. Therefore, the
pairing process is based on the thresholds of interference.
A CUE link and a D2D link are paired to transmit on th e
same RBG only when the interference satisfies thresh-
olds conditions for both receivers. There are several ex-
isting interference avoidance algorithms [13] that work
well when the number of D2D pairs is not too large.
However, if the number of D2D pairs is large, only few
D2D pairs can share resource this way. In our pairing
algorithm, we propose that if the D2D pair cannot share
the resource with the CUE, they still can transmit on the
default resource with limited rate. With the default re-
source, all of D2D pairs can transmit at the same time
and the total throughput is increase. In the rest of the
paper, without loss of generality, we assume that the
RBG 1 is used as the default RBG. The candidates, in-
cluding CUE i and DUE k are paired up only the inter-
ference at all involved receivers are below the thresholds:
()
ci

for CUEs and ()
dk

for DUEs. Otherwise, the
DUE will be assigned to transmit on the default RBG.
The complete pairing algorithm including the interfer-
ence calculation is described in the following pseudo
code:
k
Inputs:
: set of CUEs index that already scheduled on
RBG. N
N
: set of DUEs need to be scheduled.
RBG 1: default RBG.
CUE 1: CUE that scheduled on RBG 1.
Outputs:
i
Begin:
: set of DUE indices paired with CUE i.
i


 for all i

.
For each k


For each i


Calculate interference levels )(
c
I
i and using
equations (1),(3). )(
d
Ik
If )(
cc
)(
I
ii


and )(
dd )(
I
ii

 then
Pair up DUE link i and CUE link i: {}
ii
k 
End if
End For
If D2D link k is not paired with any CUE in 
Pair up D2D link k and CUE 1:
11
{}k
End If
End For
End.
The pairing algorithm calculates the interference by
measuring the signals power from all other interferer.

H.-D. HAN ET AL.
370
Therefore, only the amplitude of signal is needed.
4. The Choice of Interference Thresholds
The interference thresholds can be calculated from the
target SINRs. As the expression derivations for CUEs
and DUEs are similar, we only describe the CUE case.
The pairing algorithm should pick the channel such that
the average SINR is larger than the target SINR 0, ()
ci

as
2
0,
0,
() ()().
() ()
cc c
cc
HiWi
Ei
Ni Ii








(6)
Considering the signal, interference and noise are in-
dependent, we arrive at


2
0,
0.
()()
() () .
()
cc
cc
c
EHiWi
IiN i
i 

(7)
We can set (i)
c

to be the right hand side of (7). If
the devices is equipped with single transmit antenna
can be omitted. Applying the pairing algorithm
with this threshold guarantees that the target SINR is
met.
()
c
Wi
For multiple transmit antennas case, the threshold
conditions may not be guaranteed as the desired signal
and the interference depend on beamforming. One can
set the interference threshold corresponding to the mini-
mum signal strength. However, this value may be nega-
tive. Therefore, we can set the threshold as the average
value over all possibilities o f the precoding matrix as:

|| 2
()
10,
0.
1()
||
() ()
()
n
c
n
c
c
EHiW
i
i






c
N i
(8)
where is the value of -th entry of the codebook
and is the cardinality of the codebook. Although,
there is no guarantee that the interference is always be-
low the threshold, we still can choose the precoding ma-
trix of the interferer so that the interference is below the
threshold most of the time. In the next section, we dis-
cuss several precoding matrix selection algorithms.
()n
W
||
n
5. Beamforming Strategy Based on SLNR
Criterion
Given the spectrum allocation, the optimum beamform-
ing should consider all beamformers at the same time to
maximize the sum throughput of the CUE and DUEs as
in (5).
However, the complexity of this app roach is high as it
is proportional to the exponential of number of devices.
Furthermore, because the SINRs of CUE and DUEs are
coupled, this optimization problem has no closed form
solution. Therefore, we consider an alternative beam-
forming technique based on SLNR criterion. Using this
criterion, the precoding matrix can be determined locally
and independent from other links. The principle is to find
the precoding matrix that achieving the balance of
maximizing the received signal strength at the intended
receiver and minimizing the interference at the others. As
the result, the throughput can be optimized. The SLNR
for CUE is defined as:
i
2
2
0,
() ()
() () ()
i
cc
c
ccdc
k
HiWi
iNHiWi 



(9)
and the SLNR for DUE i k

 is
2
22
0, ,
() ()
() .
() ()(,) ()
i
dd
d
ddcddd d
llk
HkWk
kNHkWk HlkWk 




(10)
The optimum solution can be calculated using
Rayleigh-Ritz quotient[14]. Under predefined codebook,
the optimum precoding matrix can be found by trying all
possibilities as
()argmax(),( )argmax( )
ccd
Wii Wkk
d



 (11)
For each transmitter, th e selection of precoding matrix
based on SLNR requires channel information between
the transmitter and the involved receivers. If the precod-
ing matrix is calculated at eNB, all the channel informa-
tion is transmitted to the eNB. On the other hand, if the
precoding matrix is calculated at the transmitter, for ex-
ample, DUE-T k, the corresponding channels, (,)
cd
H
ik ,
(,)
dd
H
jk , are reported to the DUE-T. Nevertheless,
under Time-Division Duplex (TDD) system, the recip-
rocity of channels can be exploited such that the channel
from a DUE-T to the corresponding DUE-R and eNB is
already available at the DUE-T.
Therefore, in TDD system, the SLNR approach re-
quires less channel information exchange and further
reduces the control overhead.
6. Numerical Results
In this section, we evaluate the performance of CUEs and
DUEs using aforementioned algorithms by a system level
simulator (SLS).
We consider uplink synchronized LTE cellular net-
work with Urban Microcell (UMi) scenario with UE
moving speed of 3km/h as defined in model 1 of [16].
There are 7 eNBs equal space located with distance of
500 m and each divides into 3 cells using 120 degree
vertical antennas. The network is assumed to operate on
a 10 MHz band with 2GHz carrier. The band is divided
into 9 RBGs. Each cell is assigned 20 CUEs and 10
DUEs. The location of CUEs and DUE-T are uniformly
random generated over the area of network. The DUE-Rs

H.-D. HAN ET AL. 371
are uniformly random located around the correspondent
DUE-Ts with maximum distancemax . The UE-
eNB pathloss model in dB is defined in [16] as
30dm
dkm
dkm
10
128.137.6log( [])
UE eNB
L
 (12)
and UE-UE pathloss model is defined in [17] as
10
14840log( []).
UE UE
L
 (13)
We assume open loop power control with complete
pathloss compensation. The average receive signal power
is dBm for CUEs and 10080

dBm for DUEs. As-
sumptions on the network includes full buffer traffic
model, proportional fair (PF) algorithm for scheduling
the CUEs and ideal channel estimation with MMSE re-
ceiver.
The throughput is measured by the number of bits
successfully transmitted using certain Modulation and
Coding Scheme (MCS) defined in [15] over the simu-
lated time. The success of transmission is signaled by
ACK/NAK assuming a Block Error Rate of 10%, the
HARQ is incremental redundancy (IR) with maximum 3
retransmissions.
We study the proposed algorithms under SIMO (12

)
MIMO () channel model. To see the benefits of the
algorithms, we compare the following setups for SLS:
22
 Only uplink CUEs transmit (marked as “CUE
only”).
 Each D2D pair is assigned with a uniformly random
RBG (marked as “Random RBG”).
 The D2D pairs use the default RBG only (marked
as “Default RBG”).
 The DUE-Ts is assigned using the aforementioned
pairing algorithm. There could be more than one D2D
pairs transmitting on the same RBG. The interference
threshold for CUEs is 100dBm and for DUEs is
90dBm (marked as “Pairing”).
We first examine the pairing algorithm for system with
one transmit antenna. Figure 2 and Figure 3 show the
CUE throughput and DUE throughput cumulative distri-
bution function (CDF), respectively. Compar ing “Default
RBG” and “CUE only” setups, we see that the use of the
default RBG for D2D transmission does not affect CUE
throughput significantly. Instead, enabling of D2D
transmission has improved the total cell throughput.
The transmission with random RBG allocation does
not take in to account the CUEs' QoS protection; th ereby,
the CUE throughput degrades compare to the “Default
RBG” case. On the other hand, the D2D throughput has
improve as the D2D links do not need to share only one
RBG.
The pairing algorithm takes into account the possible
interference to the other devices. Therefore, we can see
that, the CUE throughput does not degrade compared to
the case that D2D pairs transmittin g on the default RBG.
The DUE-Ts, whenever possible, transmit on the RBG
other than the default one. Hence, the DUE throughput
improves and is comparable with that of random RBG
case.
Table 1 shows the average cell throughput of each test
case. The cell throughpu t is measu r ed by summing all th e
CUEs and DUEs throughput of the cell and averaged
over cells. We can see that the D2D transmission im-
prove the cell throughput by 25% (Random RBG) and up
to 90% (using pairing algorithm).
02004006008001000 1200140016001800200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Throughput (k bps )
CDF
Default RB G
Random RB G
Pairing
CUE onl y
Figure 2. CUE throughput CDF (SIMO case).
05001000 15002000 2500 3000 35004000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Throughput (kbp s )
CDF
Default RBG
Random RB G
Pairing
Figure 3. DUE throughput CDF (SIMO case).
Table 1. The average throughput of cells and UEs.
Average cell throughput (Mbps) SIMO M IMO
CUE only 11.6 14.73
Default RBG 16.3 28.6
Random RBG 14.56 23.3
Pairing 20.7 35.9

H.-D. HAN ET AL.
372
In Figure 4 and Figure 5, we show the CUE through-
put and DUE throughput CDF, respectively, for multiple
antennas system. The pairing algorithm is used first to
allocate the resource; then the precoding matrix selection
solution. We can see that the pairing algorithm still ef-
fective in this test although the interference threshold
may be violated. It is because, the beamforming algo-
rithm with SLNR approach helps to reduce the number of
violation. From Table 1, we can see that the pairing al-
gorithm with SLNR beamforming method improves the
cell throughput by 145%, 55%, 25% compared to that of
CUE only, default RBG and random RBG case, respec-
tively.
7. Conclusions and Future Works
In this work, we have shown the advantage of enabling
D2D communication underlaying LTE cellular network.
We propose a pairing algorithm for appropriately assign
radio resource for D2D communication and causing only
0500 10001500 2000 25003000
0
0. 1
0. 2
0. 3
0. 4
0. 5
0. 6
0. 7
0. 8
0. 9
1
Throughput (k bps)
CDF
Default RB G
Random RB G
Pairing+ SLNR
CUE o nl y
Figure 4. CUE throughput CD F (MIMO case).
05001000 15002000 2500 3000 35004000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Throughput (k bps)
CDF
Default RB G
Random RBG
Pairing+SLNR
Figure 5. DUE throughput CD F (MIMO case).
minimal interference to the uplink. The results sh ow that
the D2D links performance is improved compared to the
random RBG assignment and default RBG assignment
while maintaining the CUEs’ throughput. For multiple
transmit antennas scenario, the pairing algorith m togeth er
with the beamforming technique using SLNR approach
also increase the cell throughp ut significantly. The use of
SLNR may reduce the channel information exchange,
especially in TDD scenario.
The future works include the examination of the pair-
ing algorithm in MIMO case, where the violation of in-
terference threshold may happen. Addressing the outage
problem can help to optimize the threshold parameters
and further improve the system performance. Dynamic
default RBG can also be studied to optimize the per-
formance.
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