Relaying methods for cooperative communication networks

Abstract

A cooperative communication system is generally referred to as a wireless communication system in which the transmission between a source node and a destination node is improved by the participation of one or more relay(s). The main interest of the academia and the industry on cooperative communications is to optimally harness the available relays to enhance the robustness of the desired transmission to fading and interference. For a single-relay system, the simplest relaying protocols are the decode-and-forward (DF) and amplify-and-forward (AF) protocols. These well-known relaying schemes are usually fulfilled in the time-division-duplexing platform. Specifically, in the first time slot, the source node broadcasts a frame of information signals to the relay and the destination; then the relay processes the received signal by amplifying it in the AF scheme or re-encoding the decoded result of the received signal in the DF scheme, and goes on to transmit the processed signal frame to the destination in the second time slot. Finally the destination decodes the information frame based on the combination of the received signals from the source node and the relay node. In this thesis, we will explore the relaying schemes for single-relay networks, multi-relay networks, and multi-relay two-way networks, and further analyze the performance of those schemes. Firstly, two new relaying protocols will be proposed for single-relay systems, namely incremental selection amplify-and-forward (ISAF) and jointly incremental selection relaying (JISR) protocols. It will be shown that both protocols improve the reliability of the desired transmission compared with existing relaying protocols. Secondly, we will systematically study a multi-relay network in which only one of the relays that satisfies a selection criterion will be allowed to re-transmit the signal in the second time-slot. If the AF, DF, selection decode-and-forward (SDF), IAF, ISAF or JISR protocol is further used, the relaying scheme is called opportunistic AF, (OAF), opportunistic DF (ODF), opportunistic SDF (OSDF), opportunistic IAF (OIAF), opportunistic ISAF (OISAF), and opportunistic JISR (OJISR), respectively. We will derive closed-form expressions of the asymptotic outage performances for all these opportunistic relaying schemes and compare their capabilities. We will also show that the OIAF, OISAF and OJISR schemes outperform the OAF, ODF and OSDF schemes in terms of diversity-multiplexing tradeoff (DMT). Moreover, the OISAF and OJISR schemes have accomplished better outage performance than the OIAF scheme. Thirdly, we will analyze the performance of the channel-state-information-assisted OAF (CSI-assisted OAF) scheme in terms of outage performance, ergodic achievable rate and average symbol error rate. Previous analysis of the CSI-assisted AF scheme has approximated (¯γ²xy)/(¯γx +¯γy + 1) by (¯γ²xy)/(¯γx +¯γy) , where ¯γ represents the signal-to-noise ratio (SNR); x and y are the squares of the magnitudes of the channel fading coefficients. However, this approximation is invalid for the CSI-assisted OAF system. In this part, we will derive more accurate upper- and lower-bounds of the performance of the CSI-assisted OAF system. Finally, we will extend the opportunistic relaying notion to multi-relay two-way networks. In a two-way relaying system, the channel accommodates the transmissions from the source to the destination and vice versa simultaneously. In this kind of system, the source and the destination are usually called Source One and Source Two. Suppose that there is no direct link between the sources, and that multiple relays are located between the sources. To reach a high overall throughput in the two-way system, a pair of information frames is exchanged during two time slots by using physical-layer network coding. During the first time slot, both the sources broadcast their information frames to the relay nodes. Then during the second time slot, the relays broadcast some network-coded and temporally aligned information-bearing frames back to the sources. Subsequently, the sources decode the information frame from their counterparts based on the received frames from the relays. Here, we will propose a new relaying protocol, namely opportunistic two-way relaying (O-TR), in which only the best/opportunistic relay is active in performing the relaying duty. It will be proved that the proposed O-TR outperforms the case when all the available relays are used to construct the distributed space-time code.