Performance Evaluation of Cooperative Relaying Systems with Channel Estimation Error and RF Impairments

Abstract

Over the past decade, cooperative communication has emerged as an attractive technique for overcoming the shortcomings of point-to-point wireless communication systems.
Cooperative relaying improves the performance of wireless networks by creating an array of multiple independent virtual sources transmitting the same information as the source node. In addition, when relays are deployed near the edge of the network, they can provide additional coverage in network blind spots. Interference in the network can also be reduced in cooperative communication systems as the nodes can transmit at lower power levels as compared to equivalent point-to-point communication systems. In practice, the actual benefit of multi-node cooperation is affected by a variety of factors, including the quality of Channel State Information (CSI), the constraints on the feedback and backhaul links, hardware impairments, resource allocation and data processing schemes. This thesis investigates the
combined impact of imperfect CSI and Radio Frequency Impairments (RFIs) on distributed relay networks and cellular relaying systems. First, we study the impact of CSI imperfections on single-antenna based dual-hop relaying systems in the presence of RFI over Nakagami-m fading environment. Herein, we consider two most common relaying protocols viz., Amplify-and-Forward (AF) and Decode-and-Forward (DF) to investigate the performance of One-way Relaying (OWR) system. To fully reap the benefits of relaying protocols, CSI of the network is essential.
In practice, CSI must be estimated using the pilot symbols prior to the transmission of data symbols. A unified analysis is conducted for both the relaying protocols by deriving the generalized closed-form outage probability expressions with channel estimation error and RFI. Moreover, we also analyze the outage performance with respect to the severity of channel conditions. The analytical results are used to obtain several crucial design insights such as the transmission rate and the combined level of imperfections that the system can handle without affecting its performance. Next, we extend our analysis to the cellular scenario where the wireless network comprises of multi-antenna Base Station (BS), single-antenna multiple users and multiple relays. Herein, we analyze the outage and Expected Spectral Efficiency (ESE) performance of downlink cellular relaying system in a Rayleigh fading environment. In particular, by employing joint antenna and relay selection with multi-user scheduling, we have proposed and analyzed two opportunistic transmission strategies for both AF and DF relaying protocols vii with different implementation complexities in practical scenario when the direct links
are in the deep fade. We also deduce the asymptotic outage expressions in the high signal-to-noise ratio regime, from which the impacts of CEE and RFI on the achievable
diversity order are highlighted. Moreover, to provide spectrally efficient transmission over traditional unidirectional flow of information, we consider another approach to improve the efficient transmission i.e., Two-Way Relaying (TWR) network. Specifically,
we analyze the performance of TWR-AF system with RFI at sources and the relay node. CEE is also considered for TWR network which creates imperfect self-interference
cancellation at both sources that exchange the information through a single relay. The expressions for effective end-to-end signal-to-noise plus-distortion-and-error ratio (SNDER) and closed-form expression for the exact analysis of OP are derived by taking a Rayleigh block fading channel. In addition, the combined impacts of RFI and CEE on outage
performance of TWR and OWR systems for low and high transmission rates are also evaluated. For further insights, we perform asymptotic analysis of OP and observe irreducible
outage floors which act as fundamental performance limits.
The last part of the thesis focuses on the comparative study of multi-user multi-relay TWR and OWR cooperative networks. Due to less implementation complexity involved in AF relaying protocol, we focus on AF relay assisted TWR and OWR systems with two groups of users exchanging information opportunistically over the best relay (out of available relay candidates) in the presence of CEE and RFI over Rayleigh fading channels. Considering such practical set-up, we first deduce the pertinent instantaneous end-to-end
SNDERs. Specifically, we derive approximate as well as exact closed-form expressions for the outage probability and expected spectral efficiency for both the systems. In addition, simple asymptotic expressions at the high SNR regime are obtained, which facilitate the characterization of the achievable diversity order of the systems. To validate the derived analytical expressions, we have presented simulation results which are sufficiently tight
across the entire range of SNRs. We have also optimized power to reduce system outages and emphasize that optimum power allocation can provide noticeable outage performance
improvement. Findings of the chapters suggest that full diversity order can be achieved only when the RF front-end hardware is assumed to be perfect, while in practice the imperfections in hardware are inevitable and reduce the diversity order of the system. Moreover, the influence of key parameters such as the number of users and relays on the system performance has been presented with the dependency on the level of RFI. Different design guidelines and possible power allocation between relay and end-user terminals are proposed to improve the system reliability under a total transmit power constraint.