Energy Efficient Cooperative Wireless Communication and Networks

CHAPTER 9

Joint Beamforming and

Power Allocation

Chee Yen Leow, Universiti Teknologi Malaysia, Malaysia

Zhiguo Ding, Newcastle University

Kin K. Leung, Imperial College

9.1 INTRODUCTION

In real-life communication, data flows in both forward and backward link

directions between source and destination. As an example, in a cellular net-

work, downlink and uplink channels are used to support data flows in both

link directions. This communication scenario is known as the information

exchange channel. This chapter focuses on the study of the information ex-

change channel where both link directions are considered simultaneously in

the system modeling.

In certain channel conditions, the direct link between the source and des-

tination is unavailable to support two-way information exchange. This hap-

pens in situations such as when the source-to-destination channel is in a deep

fade or undergoing severe shadowing, where the link quality is too weak to

support any communication. In cellular systems, this also commonly occurs

when the mobile user is located at cell edge, where the coverage of the base

station is weak. In the absence of a direct communication link, the informa-

tion exchange between a pair of users has to rely on the relay. Relay is a

transceiver node placed in between the user pairs to help forward the data

between users.

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118  Energy-Efficient Cooperative Wireless Communication and Networks

User 2RelayUser 1

1

st

time slot 2

nd

time slot

4

th

time slot 3

rd

time slot

FIGURE 9.1 Information exchange using one-way relaying.

Using a conventional one-way relaying technique designed for uni-

directional communication, the information exchange can only be completed

in four channel uses due to half duplex constraint. Figure 9.1 shows the con-

ventional one-way relaying scheme used for the information exchange. The

information flows from user 1 to the relay, then from the relay to user 2 and

vice versa, where a total of four time slots are used. This doubles the num-

ber of time slots used in direct point-to-point communication without a relay

(when direct link between source and destination exists). In such an infor-

mation exchange scenario, one-way relaying is spectrally inefficient because

the achievable data rate is at most half of the data rate achievable by direct

point-to-point communication.

Coincidentally, in the wired networking community, similar two-way in-

formation exchange scenarios have been addressed. An efficient technique

known as network coding is first proposed in Ahlswede et al. [56]. The unique

feature of network coding is that it allows intermediate nodes or relays to

combine the information packets from multiple sources before forwarding to

the destinations. This technique is shown to significantly enhance the overall

network throughput [56].

Attracted by the benefit of network coding, two-way relaying has been

proposed in wireless networks. Two-way relaying utilizes the broadcast na-

ture of wireless transmission to enable data mixing between the user pair.

Based on the original idea of network coding, two-way relaying is adapted in

wireless networks to enhance the overall network throughput by reducing the

channel resources used in the information exchange between users.

Two-way relaying schemes such as the DF-based scheme [57], analogue

network coding [58], physical network coding [59], and so on, are able to

complete the two-way information exchange in only two channel uses. Fig-

ure 9.2 explains the generic two-way relaying protocol in two time slots. In

the first time slot, two users transmit simultaneously in the same channel to

the relay. In the second time slot, the relay forwards the processed mixture