Cross-Layer Design Of Cooperative Diversity In Wireless Communication System

In recent years, cooperative diversity technologies in wireless communication system have been paid much attention to. Cooperative diversity technologies could improve system performance in physical layer under fast fading environments. Meanwhile, cooperation in Medium Access Control (MAC) layer could increase system throughput and also decrease system delay.Cooperative diversity technologies in wireless communication system, with related keywords like virtual multi-input multi-output (VMIMO), cooperation, etc., have been done lots of work by many researchers. Advantages of this issue are reflected in the way of throughput improving and link stabilities. The idea of cooperation was proposed early in 2003, but has not been taken into consideration for quite a long time. There are several restrictions for such phenomenon.Firstly, there are too many assumptions while data transmitting between cooperative users. For example, assume cooperation is taken place under flat fading channels, and nodes are well synchronized. In fact, in a real wireless communication system, especially with high data rates or high mobility, the channels are frequency selective fading. These assumptions are difficult to be solved in reality.Secondly, the new mechanism, which use cooperation between different user to improve system performance, have too many differences compared with original ones, especially in the key technologies’way. For example, the compatible problems with other protocols, or improvements in the base stations and mobile terminals. Besides, the introduction of cooperative diversity would raise base station complication.There are several key problems waiting to be solved in the cooperative diversity of the wireless communication system.PHY key technologies include channel coding, synchronization between nodes, communication security, and so on. For example, if communication nodes could decode all of the information they got while operating on cooperative communication mechanism? In other words, which level could be reached in a cooperative communication?MAC key technologies include efficiency routing, network optimization and distribution, etc. How to pick up an optimal cooperative node to participate in the communication? What effect would happen on the system performance with the change of network topology?Problems that been faced in the implementary are appeared as followed: base stations and mobile terminals should be enhanced to fit cooperation, which will increase complication, also the cost. Besides, the compatible with other systems should also be paid attention to.This thesis is based on cooperative diversity in both PHY and MAC layer. The author proposed several enhancement and optimization in the key technologies, and simulated respectively.In the PHY layer cooperative diversity mode, compared with traditional communication ones, cooperation could improve system performance in the manner of increasing Signal-to-Noise Ratio (SNR). Such improvement and gain is carried out by adding spatial and temporal diversity. PHY cooperation in this thesis is based on Additive White Gaussian Noise (AWGN) channels, as well as Rayleigh channels. Simulation results show space time code cooperation can improve system BER in the fast fading environment.In the MAC layer, thesis optimized system performance analytical model based on Wireless Local Area Network (WLAN). The analytical model considered both retry limits and non-saturated transmit probability on the basis of bi-dimensional Markov Chain, and gave a uniform solution. Meanwhile, thesis researches the mathematic methods of system normalized throughput and delay with cooperation. Results show this enhanced analytical model can describe system performance more precisely.This thesis also improved MAC layer cooperation simulation model. The enhancement is embodimented in three aspects. Change the physical position of CoopTable from MAC_Header to the nodes’memory. In this way system only need to update at the initialization of communication and saved time spending. Meanwhile, thesis adds more information in the CoopTable. The second enhancement is improved Network Allocation Vector (NAV). New NAV scheme supports compatible access mechanisms, dispenses with judgement of picking up or not cooperation mode. Besides, thesis enhanced control frame formats, adds information storaging cooperative nodes’ID to support compatibility of new NAV scheme. Simulation results indicate such model can support MAC cooperation perfectly, and also validate the system performance analytical model proposed above.