Improving Relay Selection in Cellular Networks

Michel Nahas, Zouhair El-Bazzal

School of Engineering

Lebanese International University

Beirut, Lebanon

{michel.nahas, zouhair.bazzal}@liu.edu.lb

Nadine Akkari

Faculty of Computing and Information Technology

King Abdulaziz University

Jeddah, Saudi Arabia

nakkari@kau.edu.sa

Abstract—To extend and improve the cell coverage, an

efficient solution is to place several relay antennas in the cell.

Several protocols for relay selection were proposed in the past

years. However, the asynchronism between the received signals

due to the different locations of the relays was not addressed. In

this paper, we propose to select the relays based on several

important criteria. The SNR (Signal to Noise Ratio) between the

relays and the mobile is first taken into consideration. Then, the

delay differences between the different signals arriving to a mobile

are computed. This proposed protocol will allow the receiver to

choose the relays that verify a certain SNR threshold and delay

conditions. Thus, the best relays in terms of SNR and delay are

selected for the communication between the base station and a

mobile. Moreover, the overall enhancement of the cooperative

system performance, in terms of signaling time and error rates, is

shown by numerical calculations and simulations.

I. INTRODUCTION

Cooperative communication uses multiple relays to

retransmit the source signal to the destination, which improves

both area coverage and quality of services [1]. This

cooperation forms a virtual MIMO (Multiple Input Multiple

Output) system, where the relays are located in different

geographical places to achieve a spatial diversity [2].

In a cellular system, relay antennas are distributed inside the

cell to help the Base Station (BS) to communicate with the

mobile nodes [3]. This low-cost solution extends the coverage

and improves the throughput and reliability of cellular

networks [4]. However, the spatial distribution of relays would

generate an asynchronous system due to the difference in local

oscillators, and different propagation and/or processing delays

of the relays [5].

Therefore, to enhance the communication performance in

cooperative cellular networks, a good selection of relays

should be carried out. This selection has to take into

consideration many parameters such as channel coefficients,

the position of the receiver relatively to the relays and the time

delay between the received signals. An efficient relay

selection protocol would choose relays that will enhance the

cellular communication performance [6] [7].

Several relay selection protocols exist in the literature. In

[8] the concept of BRS (Basic Relay Selection) protocol was

introduced. The authors also proposed several modifications to

this basic protocol, such as RSOD (Relay Selection On

Demand). This latter protocol decreases the energy

consumption, by allowing the receiver to decide whether

diversity is needed or not. Further saving in energy

consumption was achieved by RSER (Relay Selection with

Early Retreat) [8]. RSER protocol proposed that relays having

low Signal to Noise Ratio (SNR) values will retreat and will

not continue the selection process.

However, all the protocols presented above ignored the

asynchronism issue to simplify the relay selection procedure.

Recent studies have studied the effect of asynchronism in

relay networks and its influence on the performance gain of

the system [9] [10]. These latter works have proposed relay

selection protocols for asynchronous cooperative networks at

the expense of the diversity. In fact, to eliminate the effect of

asynchronism between several signals forwarded by the

relays, only one relay is chosen at the destination. Therefore,

relay selection protocols in [9] and [10] solved the problem of

asynchronism in cooperative networks but they only achieved

a diversity order of one.

Recently, an asynchronous relay selection protocol based

on the RSER protocol was proposed in [11] for distributed

networks (Ad hoc, Wireless sensors networks…). This latter

protocol eliminated successfully the effect of asynchronism,

while ensuring a high diversity order.

In this paper, we are going to design a protocol that selects the

relays in an asynchronous centralized network like cooperative

cellular networks. This protocol will complete the selection

process in few steps by enforcing several constraints

concerning the SNR and the delay of the different received

signal at the destination. After the process is completed, only

the relays that respond to the SNR and delay criteria are

selected. Moreover, the centralized relay selection protocol

will reduce the energy consumption and signal and complexity

by implementing an early retreat strategy for some relays.

The paper is organized as follows. In Section II, we present

the system model of the cooperative cellular network. The

new relay selection protocol for asynchronous cellular

communication is described in Section III. The derivation of

the signaling complexity of the selection process is proposed

in Section IV along with some simulation results. We

conclude in Section V.

II. SYSTEM MODEL

Let us consider a circular cell with a base station at its

center and N fixed-position relay nodes (see Fig. 1). The

relays can enhance the communication between the BS and the

mobile M in the cell. To achieve a diversity order d, the

number of relays participating successfully in the

communication with the BS must be equal to (d – 1) [5].

However, these relays should be selected carefully.

978-1-4673-5820-0/13/$31.00 ©2013 IEEE

Fig. 1. Cell with some relay nodes.

Actually, due to the different location of the relays, and

difference in process and propagation delays, each BS-relay-

mobile path will have a different time delay. Therefore, the

selection process should choose the best (d – 1) relays for the

in terms of SNR and relative time delays between the different

arriving signals.

We assume that the BS, relay nodes and mobile has each

one antenna. Therefore, in order to have diversity in the cell, a

Space Time Code (STC) should be implemented across the

transmitter and the relays. Several optimal STCs were

designed for different antennas configuration like Alamouti

code [12], Golden code [13] and TAST codes (Threaded

Algebraic Space-Time)… But due to asynchronism between

received signals, diversity will be lost for some values of

delays. It has been shown in [5] that a delay of one symbol

period � would destruct the STC structure and leads to a drop

in the diversity order. Thus, a critical delay difference between

any two received signals �������� � should be avoided.

III. CENTRALIZED RELAY SELECTION PROTOCOL

In this section, we will describe the relay selection protocol

designed to select relays having good SNR (higher

than �������), while avoiding critical delays ��������.

This protocol, that we will name Centralized Relay Selection

(CRS) protocol is conceived to eliminate the signals arriving

at the destination after a certain delay �������.

Without loss of generality, let us assume a downlink

communication system where the BS wants to communicate

with a mobile M inside the cell boundaries. Each of the N

relay available within the cell, has a different identifier ID.

First, the BS broadcasts a RTS (request-to-send) in the cell.

The RTS signal is captured by the mobile M and all the relays,

as shown in Fig. 2.

Then, the mobile will reply with a CTS (clear-to-send). This

message will be received by the BS and the neighboring relays

(Fig. 3). CTS signal may not reach the distant relays which

will no longer participate in this process.

Fig. 2. RTS transmitted by the Base Station.

The remaining relays calculate their respective SNR values by

using both RTS and CTS messages as in RSER protocol [8].

Then, each relay compares its calculated SNR to the SNR

threshold value. The relays with �������, which

means that they have bad channel gains, will retreat from the

relay selection procedure. Thus, the relays with bad channel

conditions are going to retreat as can be seen in Fig. 3.

Fig. 3. Mobile node sending CTS to BS and relays.

Subsequently, the BS sends another pilot message DTS

(delay– time symbol) received by all non-retreated relays and

the mobile (Fig. 4). The relays then forward the DTS message

to M. The mobile computes the delay between the direct signal

from the BS and the signals forwarded by the relays. The

relays with delay difference bigger than ������� or equal to

�������� , will be discarded by the mobile.

Finally, M will reply with SFR (select-for-relay) message

containing the ID list of the selected relay or relays (Fig. 5).

This will conclude the relay selection phase and the

communication between the BS and mobile can start through

the selected relays.

Fig. 4. DTS transmitted by BS and forwarded by non-retreated relays to the

mobile.

Besides, if more than (d – 1) relays remain after the DTS

phase, the mobile can choose the best relays in terms of SNR.

The CRS protocol will be repeated every T seconds depending

on the channel state variations of the relay-mobile links. The

selection procedure can also be triggered if the mobile’s

position changes significantly.

Fig. 5. SFR transmitted by the mobile to notify the selected relay(s).

Other selection protocols presented above (BRS, RSOD

and RSER) did not take into consideration the delay difference

between the signals received from the selected relays, which

may reduce the diversity to one for a relative delay of

one �.

Another important characteristic of the proposed CRS protocol

is that the relays do not need to be synchronized with each

other and with the BS. This will reduce considerably the

signaling overhead between the different nodes in the

cooperative network.

In the following section, we are going to compare the

proposed CRS protocol to the RSER protocol in terms of

needed time to accomplish the selection process and the error

rate performance in the communication phase.

IV. NUMERICAL RESULTS

The signaling complexity during the CRS selection process

is shown in Table I. Let us differentiate the transmitted

messages and the received messages . For instance, the

first column reflects the fact that the BS sends the RTS signal

and the N relays with the mobile receive this signal. However,

the CTS, sent by the mobile, will reach only � relays,

and � non-retreated relays receive the DTS from the BS and

forward it to M. At last, relays are selected by the mobile.

It is easily noticed that: � � .

Table I. Signaling messages of CRS protocol

RTS CTS DTS SFR

� � �

� 1 1 � 1

One important characteristic of the CRS protocol is the time

efficiency through the way of exchanging pilot messages and

the definition of a �������between the signals coming from

the base stations and the relays. This will speed up the

selection procedure, since some relay nodes may experience

processing delays while executing multiple tasks. Therefore a

well-defined value of ������� can solve this problem.

Next, we illustrate the approximate time needed by each

protocol to complete its selection process and start a

communication session.

In CRS protocol, we confine ������� to six symbol periods.

It is assumed that the maximum delay for the RSER

protocol may change from 0 to 10 �, since there is no

limitation concerning this issue. We assume a GSM system

with a cell radius of 1000 meters and the symbol period is

equal to � .

The propagation delays are in the order of 5 and a random

processing delay is considered at the relays, BS and mobile.

Fig. 6 shows the average time needed by CRS and RSER

protocols to finish the relays selection process for different

maximum relative delay � . As can be seen from the

simulation results, our proposed CRS protocol chooses the

appropriate relays faster than the RSER does. Moreover, the

needed time for signaling becomes the same for CRS for

delays bigger than 6 � because CRS does not take into

account signals arriving after �������. However, the

signaling time of RSER increase with high relative delays

Hereafter, we compare the error rate performance of the

CRS protocol with the RSER protocol. A diversity order of 2

is required, therefore only the best relay is chosen in addition

to the BS. The Alamouti code [12] is used in the

communication session and 4-QAM symbols are sent between

the BS and the mobile. All the channels are considered to be

Rayleigh fading with unit variance (σ2 = 1).

In Fig. 7, the Symbol Error Rate (SER) and Bit Error Rate

(BER) are plotted for both CRS and RSER protocols.

It can be noticed from the figure that CRS selection protocol

outperforms the RSER protocol. This is due to the fact that

CRS eliminates the signals arriving at a delay difference of

�������� � that has a destructive effect on the diversity,

but the RSER does not eliminate the relays with critical

delays.

Fig. 6. Signaling time comparison between CRS and RSER protocols.

Fig. 7. Error rates of CRS and RSER protocols.

V. CONCLUSION

Relay selection is a critical operation to ensure better

performance in cooperative networks. In this paper, we have

designed an asynchronous relay selection protocol that takes

into consideration both SNR and delay while choosing the

most appropriate relays for a given cellular communication.

The different steps of the centralized relay selection protocol

were shown. The relays with low SNR may retreat in the early

stages of the selection process. Moreover, the receiver will

choose the relays with signals arriving within a certain delay

threshold and a relative delay not equal to some critical values.

The number of needed signaling messages was also computed

for the proposed protocol. We have computed the average time

spent in the selection process and compared it with an existing

protocol. Not only our protocol has lower signaling time, but it

was also shown that it has better error rate performance in the

communication phase where the selected relays are involved.

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