Research On MAC Protocol In Wireless Cooperative Communications

Currently, cooperative communication which is an effective technology to obtain multi-diversity gains becomes a hot point of research. In virtue of the nature of wireless broadcast, neighbors can retransmit the overheard the signals transmitted by the source to the destination, which not only minigates the the impacts of fading and varieties of the wireless links, but also improve the reliability, data rate and the coverage. However, cooperative communication brings complexities to design the MAC protocol of the wireless network.The existing cooperative MAC protocols are analysed and compared. Moreover, the cooperative MAC based on instant channel state is especially studied. Several key techniques of the cooperative protocol, such as the transmit mode selection, the cooperative nodes selection, and the methods of cooperation and the combination are anaylized in detail. Then a cooperative strategy where the transmit mode selection in the source node is proposed based on the instantaneous SNR.A single relay cooperative MAC protocol (SCN-CoopMAC), in which only a single cooperative relay is selected to help source node according to instantaneous channel information, is proposed. The main characteristics of this protocol are : (1) The source node selectes the transmit mode according to the channel information feedbacked by the destination node. (2) Each neighbor node comprehensively considers the channel status informations in the links of source-relay pair, source-destination pair and the relay-destination pair. If the constrains are met, the relay node will be the active relay to participate in the competition of being the cooperative node. The method of time window, which is initialized in proportion to the residual energy, is used during the process of cooperative node selection. One active relay which has the most residual energy will become the cooperative relay. (3) The destination node combines the data signal from the source node and copy from the cooperative relay using maximum ratio combination (MRC). The protocol is described in detail and theoretical deduced. Meanwhile, an OPNET simulation model is established to validate the protocol. With individual power constraint, the simulation result shows that this protocol is better than the traditional IEEE802.11 DCF about 15% in terms of the packet delivery ratio or delay.A two nodes cooperative MAC protocol (TCN-CoopMAC), which is the expandation of SCN-CoopMAC, is proposed to form the cooperative MIMO transmission. In this protocol, two cooperative relays are selected based on instantaneous channel information. This protocol focuses on: (1) During the process of nodes selection, the active node whose residual energy is higher than the lowest threshold will participate the competition. Also the time window is initialized in reverse proportion to the instantaneous channel gain of each relay-destination link. The selected cooperative relays transmit the data signal with Alamouti's space and time coding. The destination node decodes the received the data signal from the cooperative nodes to obtain diversity gains. Ihe performance of this protocol is analyzed in detail, and is validated through an OPNET simulation model. With individual power constraint, the simulation results show that this protocol is better than SCN-CoopMAC about 10% in terms of the packet delivery ratio or delay.