Spectral Efficient Opportunistic Relay Selection Policies for Next Generation Mobile Systems

Spectral Efficient Opportunistic Relay Selection Policies for Next Generation Mobile Systems

Nikolaos Nomikos (University of the Aegean, Greece) and Demosthenes Vouyioukas (University of the Aegean, Greece)
DOI: 10.4018/978-1-4666-8732-5.ch005
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This book chapter presents various relay selection policies based on spectral efficient techniques for the next generation mobile systems. Successive opportunistic relaying (SOR) that leverages the half-duplex constraint of conventional relays through concurrent transmissions is described, while proposing techniques to reduce the effect of inter-relay interference (IRI). An extension of SOR is defined for the case of out-of-band relaying, when additional spectrum bands are available and in networks with buffer-aided relays. Moreover, the use of full-duplex (FD) relays is outlined, in view of the effect of loop-interference (LI) from the relay's output to its input and, also, the power reduction is presented. As networks with multiple relays suffer from increased coordination overhead, a reduced Channel State Information (CSI) policy is proposed. For every policy, performance evaluation is provided in terms of outage probability, average throughput, power reduction and switching rate. Finally, open problems in spectral-efficient opportunistic relay selection policies are discussed.
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In recent years, wireless networks have seen a tremendous increase in terms of the achieved Quality of Service (QoS). Cooperative techniques have significantly contributed towards improving the capacity of the fourth generation 4G and beyond networks and are considered a basic element of the imminent 5G networks. Among cooperative techniques, cooperative relaying has received significant attention from researchers due to the gains it offers to the network. More specifically, relay transmissions reduce the pathloss, as the transmitter is located closer to the receiver, compared to traditional single-hop transmissions. Also, the use of more than one relays allow the formation of distributed antenna arrays that increase the diversity of the network, as the signal propagates through independent paths (Laneman, Tse, & Wornell, 2004). In addition, shadowing can be mitigated through optimal positioning of relays in order to avoid blockages from obstacles and clutter.

Regarding the relay transmissions, a technique that has received a large amount of contributions is the opportunistic relay selection. Opportunistic Relay (OR) selection has been proved to achieve the same diversity order as multiple relay transmissions, while requiring only one orthogonal resource in time or frequency (Bletsas et al., 2006). This additional resource is needed due to the half-duplex relay operation, which dictates that a relay cannot transmit and receive simultaneously. Recently, several selection policies using relays with buffering capabilities have been presented and so, the constraint of using the same relay for reception and transmission in the next time-slot has been broken. An additional degree of freedom has been given to relay selection through the adaptive link selection that activated the best link among the source-relay (SR) and relay-destination (RD) links (Krikidis, Charalambous, & Thompson, 2012a).

The purpose of this book chapter is to present a family of spectral efficient OR selection policies. First, we provide the state-of-the art of conventional cooperative relaying and we outline its shortcomings. One major drawback of conventional relaying protocols is that a transmission spans in two time-slots and thus the end-to-end rate is halved. Thus, to leverage the half-duplex constraint, spectral-efficient protocols, such as successive relaying, have been investigated and a corresponding survey is provided. As multiple relays can be considered to facilitate the communication between a source and a destination, OR selection can enhance significantly the spectral efficiency of the network and this is evident through the relevant works that we discuss.

The first policy employs the Successive Opportunistic Relaying (SOR) technique that allows two transmissions to be performed concurrently in networks with at least two available relays. As one relay forwards a previous packet to the destination, while the source transmits a new packet to another relay with the exception of the first time-slot, the destination receives one packet in each slot and, in the long-term, full-duplex operation is achieved. However, this overlap of transmissions introduces IRI that must be mitigated. In continuity we present SOR for networks with buffer-aided relays that offer additional degrees of freedom in IRI mitigation.

As 5G techniques are starting to be developed, the use of additional spectrum bands and multiple interface (or multi-mode) terminals are considered as measures to avoid interference in relay transmissions. So, we will include in the chapter an extension to SOR with multi-mode relays that are Long Term Evolution (LTE) and Wi-Fi capable and switch interfaces in order to nullify interference. In this particular relay selection policy, power adaptive transmissions lead to power reduction, as well as IRI mitigation, due to the lower level of interference from the transmitting relay.

Key Terms in this Chapter

Successive Relaying: Protocols where at least two relays take turns in each time-slot by having one relay to receive the source’s signal while the other forwards a previously received signal to the destination.

Spectral Efficient Relaying: Protocols where the main target is to leverage the half-duplex loss of conventional relaying through techniques that mimic FD operation.

Cooperative Relaying: Protocols where relays help to establish the communication between a source and a destination or improve the characteristics of an established source-destination communication through increased diversity.

Inter-Relay Interference: Interference that is introduced between a relay that is receiving the source’s signal while another relay is transmitting towards the destination.

Out-Band Relaying: Protocols where different frequency channels are used in the link between the source and the relay the link between the relay and the destination.

Power Efficient Relaying: Protocols where power adaptation is employed in order to match the required transmission levels of the source and the relay to a specific transmission rate.

Conventional Relaying: Protocols where multiple half-duplex relays are employed and all of them are activated to forward the source’s signal to the destination.

Full-Duplex Relaying: Protocols where one or more relays with the capability to receive and transmit on the same frequency channel at the same time, are employed.

Loop-Interference: Interference that is introduced between the output and input antennas of a relay that is transmitting to the destination, while it is receiving the source’s signal.

Opportunistic Relaying: Protocols where multiple relays are employed and only a selected subset of relays is activated to forward the source’s signal to the destination.

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