Ağ kodlamalı işbirlikli OFDMA sistemleri için çeşitleme-çoğullama ödünleşimi

Cooperative communication (CC) exploits the broadcasting nature of wireless transmission and creates a distributed multiple-input multiple-output (MIMO) system among the wireless nodes which are physically separated from each other. Earlier works on cooperative communication typically build on time division multiple access (TDMA). However, it is well known that conventional CC systems experience throughput loss due to additional time slots required for relaying phase which particularly become prohibitive with the increasing number of relay nodes. Network coded cooperative (NCC) systems have been proposed as a powerful alternative with significantly higher throughput efficiency. In an NCC system, the relay nodes perform either XOR operation or form a linear combination of the messages received from distinct sources. Through proper combining, network coding improves the spectral efficiency over conventional unidirectional cooperative communication systems. In the first chapter of this thesis, we provide an overview of NCC systems and discuss their advantages over conventional CC systems in terms of diversity multiplexing tradeoff (DMT). All existing works on the DMT analysis of NCC systems assume perfect channel state information (CSI) at relay and destination nodes and focus on asymptotical analysis. It is known that asymptotical DMT analysis overestimates the performance since most communication systems operate in low and moderate signal-to-noise ratio (SNR) regime. To characterize DMT within more practical SNR regime, finite-SNR DMT is a more meaningful performance measure. In the second chapter, we investigate the outage probability and finite-SNR DMT performance for NCC wireless networks in the presence of imperfect CSI. We show that our results are generalized versions of those earlier presented in the literature and coincide with them in high SNR regime. Our analysis further reveals that the DMT is highly dependent of channel estimation quality particularly at finite-SNR values. In an effort to have further gains over the initial works, which build upon the assumption of TDMA, the combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been proposed in the literature. In the third chapter, we consider orthogonal frequency division multiple access (OFDMA), an extension of the OFDM to a multiuser system where subsets of carriers are assigned to different users. We derive a closed-form expression for the outage probability of the system over Rayleigh fading channels and present the DMT analysis. Our results demonstrate that NCC-OFDMA system is able to fully exploit both frequency and spatial diversity. In the third chapter, we investigate only asymptotic DMT analysis of NCC-OFDMA systems over Rayleigh fading channels. In the fourth chapter, we investigate both finite-SNR DMT and asymptotic DMT performance over Rician fading channels. Such an analysis helps us see the difference of performances over Rayleigh and Rician fading channels. Otherwise, DMT results for Rayleigh and Rician channels coincide each other for asypmtotical SNR values.