Relay-Assisted Wireless Networks

Abstract

The growth of wireless networks in recent decades is motivated by their ability to provide communications anywhere and anytime, encompassing a high proliferation of wireless services and devices such as mobile communications, WiFi (Wireless Fidelity), or cordless phones. However, wireless networks present two important drawbacks: scarcity of radio spectrum and channel impairments. In this respect, wireless networks must be designed to exhibit a high spectral efficiency and must combat channel impairments (i.e. multipath fading, shadowing, interference and pathloss).Wireless networks are built around a number of nodes communicating one to each other over a wireless channel, some having a wired backbone with only the last hop being wireless, such as cellular voice and data networks. But the provision of high capacity and reliable wireless multimedia communications to carry bursty packet traffic as well as voice and delay constrained traffic continues to be a challenging aspect of modern and future wireless communication networks. Recent advances in radio transceiver techniques such as Multiple Input Multiple Output (MIMO) architectures have shown an enhancement in the capacity of the current systems by alleviating the effect of channel multipath fading. Similarly, cooperative communication is based on collaboration amongst several distributed terminals so as to transmit/receive their intended signals (Laneman, Tse, Wornell, Erkip, Sendonaris, Aazhang, Host-Madsen). This type of communication is based on the seminal work published in the 1970s by van der Meulen, Cover and El Gamal, where a new element is introduced in conventional point-to-point communication, the relay. The new network architecture exhibits some of the properties of MIMO systems, but in contrast to those systems, relay-assisted transmission is able to combat channel impairments due to the shadowing and path-loss provided by the source-destination and relay-destination links.The application of relay-assisted transmission for practical systems is constrained by current radio technology, which cannot transmit and receive simultaneously in the same band because of the dynamic range of the incoming and outgoing signals through the same antenna element. As a result, the relays must operate in half-duplex mode. In spite of this drawback, relay-assisted multi-hop networks are expected to play a significant role in 4G wireless communications systems, because they can extend coverage cost-effectively, increase spectral efficiency and drive the cost of deploying 3G+ and 4G systems lower.This chapter aims to unearth the properties of relay-assisted communications and provide some examples of how this kind of transmission can be incorporated in wireless cellular networks. Amplify-and-Forward (AF)This is the simplest strategy that can be used at the relay because it acts as a dummy with a constraint on maximum power. The relay amplifies the received signal from the source and transmits it to the destination without making any decision. The main drawback of this ...