Research On Key Techniques Of Wireless Cooperative Communication

Some key techniques of cooperative communication are studied in this dissertation, where four topics are emphasized: a cooperative multiple packet reception (MPR) method based on network-assisted diversity multiple access (NDMA) and its relay selection, cooperative MPR scheme with collision detection and its throughput and packet loss rate (PLR) analysis, cooperative packet retransmission with joint diversity and its bit error ratio (BER) and symbol error ratio (SER) analysis, energy efficiency and time efficiency in cooperative communication.Communications over wireless channels suffer from fading induced by multipath propagation, which causes a random fluctuation at the received signal level. Cooperation diversity occurs as a new form of spatial diversity and soon becomes a popular approach to combate the fading induced by multipath propagation, which allows single-antenna terminals in a multiuser environment to share their physical resources to help one another to transmit information. Due to the combination of spatially independent copies of the same signal, the advantage of space diversity can be well utilized to combat the impact of channel fading and instability, improve the reliability and transmission rate and even expand the coverage of wireless networks. Cooperative communication will play an important role in next generation wireless network, such as multi-hop cellular networks, multi-hop wireless local area network, wireless Ad Hoc networks and wireless sensor networks.This dissertation begins with an overall introduction of cooperative diversity and cooperative communication. Then the origin of the research topic and the significance of the investigation are provided.Chapter 2 first study the cooperative MPR method based on NDMA, i.e., ALLIANCES scheme, which can improve original NDMA through cooperative diversity and overcome the adverse effects aroused by multipath fading in cellular networks and wireless LAN. In the slots following the collision, a set of nodes designated as nonregenerative relays will cooperate with source node and destination node to form virtual antenna array and retransmit the signal that they received during the collision slot. By processing the originally collided packets and the signals forwarded by the relays, the destination node can recover the original packets. But when it is applied to wireless sensor networks, some problems occur. First, due to the difference of communication abilities among nodes or the limitation of system sources, not every relay nodes can hear all the collided packets. So the random relay selection in ALLIANCES may be not proper. We design a new relay selection method, which considering both the full rank requirement of channel matrix in physical layer and retransmission in MAC layer. This method can limit the retransmission times effectively and achieve higher throughput than the existing access techniques.To accurately assess the performance of cooperative MPR based on NDMA, collision detection and its effects on the system's performance are taken into consideration in Chapter 3. We break the ideal assumption in NDMA and ALLIANCES that collision order can always be correctly detected and formulate the cooperative MPR problem with incorrect decision on collision order. And then a new MPR model is established, in which the length of cooperative transmission epoch is fixed to be the detected collision order. The mathematical analysis of potential throughput (PTP) and potential PLR (PPLR) are given under the pessimistic assumption and the optimistic assumption. To guarantee PTP to be maximum or PPLR to be minimum, the selection of detector's threshold is optimized. The thresholds are selected according to PTP under the pessimistic assumption in our simulations. Simulation results show that the proposed cooperative MPR scheme can achieve higher throughput and lower PLR than original NDMA and slotted ALOHA.The effects of fading can be combated by transmitting and processing independent copies of the same signal, i.e., diversity. Different diversity strategies or the combination of them can be selected to improve system performances. As a form of spatial diversity, cooperative diversity can be easily combined with other diversity. To improve the effect of diversity, a joint cooperation diversity and symbol mapping diversity strategy is proposed for cooperative packet retransmission system with high-order modulation such as 16QAM. Substantial BER gains and SER gains result from multiple packet transmissions over independent paths and distinct bit-to-symbol mappings for each packet transmission. The SER and BER performance of relay assisted retransmission system is analyzed. Simulation results show that the joint diversity strategy can provide more BER gains than other relaying strategies when no relay makes a decision error; but if some relays make decision errors, the joint diversity strategy outperforms other relaying strategies only when the relays are closer to the source than to the destination.In Chapter 5, we study the application of cooperative multiple input multiple output (MIMO) technique in wireless sensor networks and want to solve the problem that"whether to cooperate","when to cooperate"and"how to cooperate". Energy efficiency and time efficiency of cooperative communication are taken into account. First, we establish a basic analysis model with two clusters of sensor nodes, analyze its statistical BER under Rayleigh fading channel and then calculate energy consumption under cooperative MIMO, cooperative multiple input single output (MISO) and traditional single input single output (SISO) model. Secondly, we induce multi-rate modulation in cooperative communication system. Through adaptive modulation, more energy and time delay can be saved. Thirdly, we consider the tradeoff of energy consumption and time delay in network environment and propose a strategy to optimize above performance with limited transmission power. Cross-layer design is used to achieve this strategy by considering routing architecture.In the end, a brief summary of all discussed topics in this dissertation is given. The main contributions of the dissertation and several further studies or worth studies are pointed out at this part.