Unlike wire-line networks, one primary challenge in wireless communication is the existence of fading, which may significantly attenuate the signal power seen at the receiver. This adverse effect of fading can be combated by using diversity techniques to transmit signals over multiple independent channels and coherently combining them at the receiver. In fading relay channels, cooperative communication, which is capable of creating diversity, has been an active research area over the past few decades. Whilst numerous results have been published, many fundamental questions still remain open, such as the characterization of the best possible outage performance of fading relay channels, as well as the exact outage performance of some popular relaying protocols. This thesis aims to...[ Read more ]

Unlike wire-line networks, one primary challenge in wireless communication is the existence of fading, which may significantly attenuate the signal power seen at the receiver. This adverse effect of fading can be combated by using diversity techniques to transmit signals over multiple independent channels and coherently combining them at the receiver. In fading relay channels, cooperative communication, which is capable of creating diversity, has been an active research area over the past few decades. Whilst numerous results have been published, many fundamental questions still remain open, such as the characterization of the best possible outage performance of fading relay channels, as well as the exact outage performance of some popular relaying protocols. This thesis aims to address these fundamental problems using derived analytical expressions.

The max-flow min-cut (MFMC) bound serves as an upper bound on the instantaneous capacity of relay channels. Using the MFMC bound, a lower bound on the outage probability of fading relay channels can be obtained. The first major contribution of this thesis is to address the achievability of the MFMC bound on a multiple source antenna half-duplex fading relay channel. Specifically, at general signal-to-noise ratios (SNRs), a decode-and-forward (DF) protocol is shown to asymptotically achieve the MFMC bound when the relay is near the source, and a compress-and-forward (CF) protocol asymptotically achieves the MFMC bound when the relay is either near the source or near the destination. Moreover, we observe that the CF protocol achieves the MFMC bound to within 1/2 bits/s/Hz irrespective of the relay position. Further, at low SNR and low outage, when the source has multiple antennas, the MFMC bound can be achieved by a DF protocol up to the first order approximation. Finally, it is shown that if bursty transmission is adopted when the SNR is low, the CF protocol is able to achieve the MFMC bound up to the first order approximation for an arbitrary number of source antennas. These observations reveal that the MFMC bound can effectively be used to characterize the best possible outage performance of some fading relay channels.

Despite the effectiveness of the bound, it typically does not admit closed-form expressions to allow for efficient calculation. Using the MFMC bound, the second major contribution of this thesis is thus the derivation of new closed-form lower bound expressions for the outage probability of the fading relay channel. To the best of our knowledge, these results are the first closed-form expressions for the MFMC outage probability bounds for dual-hop relaying channels, applying for arbitrary SNRs. Based on these results, we investigate upper bounds on the ε-outage capacity and characterize the effects of power allocation and relay position.

In addition to the fundamental limit to the outage performance of fading relay channels, it is also important to characterize the outage performance of some popular relaying protocols, such as the amplify-and-forward (AF) and DF protocols. The characterization constitutes the third major contribution of this thesis. For this part, we consider a multiple-relay channel, where each terminal has a single antenna. For the AF protocol, we derive accurate approximations for the outage probability when the relays are clustered between the source and destination. For the case where all the relays are clustered near the destination, an exact closed-form expression for the outage probability is obtained. For the DF protocol, under general multiple relaying configurations, we derive an exact closed-form expression for the outage probability. The obtained outage results are then used subsequently to yield expressions for the finite-SNR diversity-multiplexing trade-off (DMT).