Whenever size, power, or other constraints preclude the use of multiple-input multiple-output (MIMO) systems, wireless systems cannot benefit from the well-known advantages of space-time coding (STC) methods. Also the complexity (multiple radio-frequency (RF) front ends at both the transmitter and the receiver), channel estimation, and spatial correlation in centralized MIMO systems degrade the performance. In situations like these, the alternative would be to resort to cooperative communications via multiple relay nodes. When these nodes work cooperatively, they form a virtual MIMO system. The destination receives multiple versions of the same message from the source and one or more relays, and combines these to create diversity. There are two main cooperative diversity techniques for transmission between a pair of nodes through a multiple relay nodes: decode-and-forward (DF) and amplify-and-forward (AF) modes. In the DF mode, the signal received from the source node is demodulated and decoded before retransmission. In the AF mode, the relay node simply amplifies and retransmits the signal received from the source node. No demodulation or decoding of the received signal is performed in this case.;In this thesis, we present a coding scheme suitable for cooperative networks where the source and relays share their antennas to create a virtual transmit array to transmit towards their destination. We focus on the problem of coding for the relay channels. While the relays may use several forwarding strategies, including AF and DF, we focus on coded DF relaying. We derive upper bounded expressions for the bit error rate (BER) assuming M--ary phase shift keying (M--PSK) transmission and show that the proposed scheme achieves large coding gains and frill diversity relative to the coded non-cooperative case for a wide range of signal-to-noise ratio (SNR) of interest.;To improve the detection reliability further, we consider antenna/relay selection on the performance of cooperative networks in conjunction with the distributed coding scheme proposed. For simplicity, we assume that there is one relay that is equipped with nR antennas and only the best antenna is selected. For this scenario, assuming DF and AF relaying, we derive upper bounds on the BER for M--PSK transmission. Our analytical results show that the proposed scheme achieves full diversity for the entire range of BER of interest, unlike the case without antenna selection.;In the last part of the thesis, we consider the same system considered in the ideal case but now with system imperfections. In particular, we consider the case when the channel state information is estimated at all nodes involved in the transmission process. We derive upper bounds on the performance with imperfect channel estimation. Our results show that there is a performance degradation due to the presence of channel estimation error. However, the observations made in the case of ideal channel state information still hold for the non-ideal case.;In encoded cooperative communication networks, the diversity of the system degrades significantly. This diversity degradation is attributed to the errors made at the relay nodes. Consequently, if better reliability is achieved at the relay nodes, the diversity may improve, or even may be preserved, as compared to the error-free case. In light of this, the objective of this thesis is to devise coding schemes suitable for relay channels that aim at improving the end-to-end performance of such systems.
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