Research On Cooperative Transmission And Resource Allocation In Wireless Relay Systems

Multiple input multiple output (MIMO) technique can overcome the limitations of multi-path fading in radio channels with the help of receiving multi-signal copys from different fading channels, and do not need more time or frequency resources. Therefore MIMO technique holds the potential to significantly improve the capacity and reliability in future wireless communication system. However, due to size, cost or hardware complexity limitations, it may be difficult for a wireless terminal to support multiple antennas. Recently, cooperative diversity technique, which is based on user cooperation and relay cooperation etc, has emerged as a promising technique and received considerable interests. The essence of this technique lies in that single-antenna terminals in a multi-user scenario are allowed to share their antennas and other resources through distributed transmission and processing. Hence, cooperative diversity technique is a virtual multi-antenna transmitter technique, and can reap the spatial diversity benefits of the virtual MIMO system. Focusing on the design of cooperative schemes and resource optimization allocation strategies in the cooperative diversity fields, we have introduced the solutions for particular scenarios and further analyzed in theoretical and simulation aspects. The main contribution of this work can be summarized as follows:1. Two schemes are designed to overcome the impact of synchronization with the methods of designing the structure of space-time code and using the orthogonal frequency division multiplexing (OFDM) technology.a) An asynchronous differential space-time cooperation scheme has been presented. Binary phase-shift keying (BPSK) information symbols are divided into two parts and differential modulated on in-phase axis (I axis) and quadrature-phase axis (Q axis) respectively, then coded with time-reversal space-time block code (TR-STBC). Receiver decodes the signal with the similar method of differential space-time block code (DSTBC) , and the decoded signal on I axis and Q axis are two separate parts of transmit signal without the asynchronous effect. Through the analysis and simulation of the BER, we demonstrate that the scheme can still achieve cooperative diversity in an asynchronous condition.b) Based on the correlation of subcarriers in orthogonal frequency division multiplexing (OFDM) and the characteristic of transmission in cooperative diversity system, a simple space-time cooperative scheme has been proposed. In different phases of cooperative transmission, the same signals are transmitted on the least correlated subcarriers. This scheme can obtain one-order multi-path diversity without reducing the transmission rate. Through the research on the power allocation, an optimal power allocation (OPA) strategy is given. Finally, simulation results demonstrate that the proposed scheme can provide significant gains in symbol error rate (SER) performance.2. Combining the wireless network coding and the cooperative diversity, the schemes has been given as followsa) A relaying scheme based on network coding has been proposed, and an optimal power allocation (OPA) strategy has been given. According to the phase characteristic of MPSK modulation, network coding and decoding with PSK signal can be directly implemented at the relay node and base station. Through the analysis of outage probability, an upper bound has been derived in high signal-to-noise ratio (SNR). Minimizing the upper bound, an OPA strategy has been obtained. The simulation results show that the performance of the proposed scheme is better than that of the traditional one. The performance of our OPA strategy can provide significant gains in symbol error rate (SER) performance.b) An OPA algorithm for a space-time cooperative network coding scheme employing decode-and-forward (DF) and amplify-and-forward (AF) protocols has been proposed. The system performance in terms of outage probability and its approximation in high signal-to-noise (SNR) regimes have been derived, and by minimizing the larger outage probability of the two sources with respect to the characteristic of network coding, optimal power factors have been obtained through iterative computation. Furthermore, the closed-form solutions for some special cases also have been presented. The proposed algorithm only needs the average channel gain information and has low computation complexity, which facilitates the practical implementation. Simulation results show that the proposed OPA algorithm outperforms the equal power allocation (EPA) algorithm in terms both of outage probability and symbol error rate (SER).3. For relay selection systems, the schemes are given by two points. For one point is that the relay node whether forward signals or not in one-relay systems, for another is that chose one relay node in multi-relays systems.a) An incremental coded bi-directional relaying (ICBR) scheme based on network coding and space-time cooperation has been proposed, which improves resource efficiency and network throughput. The outage probabilities and the network-coding gain of different bi-directional relaying schemes have been developed and analyzed. The numerical results show that compared with two other schemes, ICBR can obtain three-order diversity gains in fast fading channel and higher coding gain in slow fading channel. Finally, it has been showed that the theoretical results coincide with the simulation results.b) Optimal relay selection schemes with OPA strategies in multi-destinations cooperative networks have been proposed. First, the OPA strategies with statistical channel-state information (SCSI) and OPA strategies with instantaneous channel-state information (ICSI) have been given. The outage probability shows that optimal relay selection schemes with SCSI-OPA strategy and ICSI-OPA strategy can achieve full diversity. The simulation results show the performance of optimal relay selection schemes with OPA strategies is better than that with EPA strategy . The performance of optimal relay selection schemes with ICSI-OPA strategy is better than that with SCSI-OPA strategy.