The performance of cooperative-diversity wireless networks using adaptive modulation

This thesis analyzes the throughput performance of the cooperative diversity wireless network using adaptive modulation over Rayleigh fading channels. Cooperative diversity is achieved by utilizing neighboring terminals as relays. These relays can generate copies of the same signal, which can provide spatial diversity gain and high Signal-to-Noise Ratio (SNR). The main drawback of cooperative diversity is the throughput decrease due to the extra resources needed for relaying. Therefore, throughput is greatly reduced. In this thesis, the adaptive modulation is used to convert the obtained SNR gain to throughput gain to compensate for the throughput reduction. Results show that the cooperative diversity networks with adaptive modulation not only compensate for the throughput loss but also achieve considerable throughput gain compared with the classical system with direct transmission when the Channel State Information (CSI) is perfectly known in advance. Moreover, this thesis investigates the throughput gain of the cooperative diversity networks with adaptive modulation when the CSI is imperfect. Results show that a great throughput loss is caused by the imperfect CSI, but considerable throughput gain still exists. More importantly, this thesis sets up the mathematical models for the simulated systems. Hence, the analytical evaluation of the system performance is presented in both perfect CSI and imperfect CSI case to verify the simulation results. In addition, this thesis also analyzes the relationship between the throughput gain and the number of relays. The optimal number of relays that maximizes the throughput is found to be one, when simultaneous relaying is not considered. It is also found that the throughput increases linearly with the number of relays, if simultaneous relaying is considered. Besides, the BER performance of the cooperative diversity network using fixed modulation is also investigated in simulation and analytical methods based on the analytical results from the cooperative diversity network using adaptive modulation. Results show that the BER performance of the cooperative diversity network under perfect CSI using fixed modulation is much better than that of the classical direct transmission system. In addition, there is significant BER performance degradation of the cooperative diversity network under imperfect CSI compared with that from the cooperative diversity network under perfect CSI. However, the cooperative diversity network under imperfect CSI is still better than the classical direct transmission system in BER performance.