Efficient methods for improving the achievable rate in cooperative multi-relay networks

Abstract

This dissertation consists of two research topics. The first topic considers a hybrid relay network consisting of the source, the~amplify-and-forward (AF) relay, the~decode-and-forward (DF) relay, and the destination. In hybrid three-hop relay systems, the~transmitted signal from source can be received at the destination after processing the signals through two relays. If the first relay amplifies and forwards the received signal, and the second relay decodes and forwards the received signal, the system model is considered to be an AF-DF relay system. The reverse case is considered for the DF-AF relay system. The AF-DF and DF-AF relay systems have different error rates and achievable throughput with respect to the channel conditions between two nodes. The optimal power allocation schemes for two different relays in order to maximize the achievable rate under a~sum relay power constraint for given channel gains and transmit power from the source are proposed. By solving the optimization problem to maximize the achievable rate for each relay network, the transmit power values in closed form are derived. When~the channel gains are the same, the optimal power allocation scheme for the AF-DF relay network proves that greater power should be allocated at the first relay to maximize the achievable rate. In~the case of the DF-AF relay network, the optimal power allocation schemes for the four possible cases are derived. Under the same signal-to-noise ratio (SNR) condition at the first hop, it is showed that the achievable rate of the AF-DF relay network is greater than that of the DF-AF relay network when the channel gain between two relays is greater than that between the second relay and destination. Simulation results show that the proposed power allocation schemes provide a higher achievable rate than the equal power allocation scheme and the grid search schemes. In the second topic, a new cooperative scheme using orthogonal frequency division multiple access (OFDMA) and space-time block code (STBC) scheme is proposed for increasing transmission rate. The cooperative communication has a problem for the decreased throughput because it is necessary that users send and receive the information to improve reliability. By modifying transmission structure, the proposed cooperative scheme can improve reliability in the fading channel and solve the problem for bit error rate (BER) performance that is dependent on the errors in the first phase that exchanges the information between both users. Because the proposed structure guarantees the channel diversity effects, the reliability of transmitted data is improved. Furthermore, the received signals can be well-decoded because of the channel diversity effects.