Research On The Cooperation Mode And Optimization Of MAC Layer In Wireless Communication Network

With the exploiting of spatial diversity gain,multiple input multiple output(MIMO)technology can significantly improve the system capacity,transmission rate,spectrum efficiency and link reliability without any increase in power and spectrum.But limited by the volume of small mobile terminals,MIMO technology is difficult to be implemented because of the space requirement between the antennas.To reduce the multi-path fading and obtain spatial diversity gain,cooperative communication exploits the distributed antennas in the network to form a virtual antenna array.It is a new efficient method in network coverage enhancement and performance improvement and has drawn much research attention recently.Due to the addition of the relay node,the traditional transmission model and the media access control(MAC)timing are not suitable.New transmission and access patterns are needed to be designed.Focusing on this topic,two transmission protocols and two cross-layer cooperative MAC protocols are proposed,which are based on the energy restricted wireless network.The power allocation,the relay selection and the cost of the protocol frame overhead are jointly considered to reduce energy consumption,enhance spectral efficiency and spatial frequency reuse.The main contributions are as follows:(1)In order to prolong the lifetime of an energy limited network,a total transmission power minimized cooperative MAC protocol is proposed.Based on the bit error rate(BER)constraint,the transmission power of the direct link and the cooperative link are analyzed.The optimization method of the derivative is used to minimize the total transmission power and allocate the power between the source and the relay nodes.From the perspective of the network,the best relay is selected under the criterion of minimum total transmission power.By the control frame expansion and addition based on the IEEE 802.11 distributed coordination function(DCF),the cooperative MAC timing and processing algorithm are given.The simulation results show that the proposed cooperative MAC protocol can effectively reduce the energy consumption and extend the network lifetime.Compared with the similar enengy efficient MAC,it can achieve higher performance especially in a high BER requirement network.(2)Control frames of a MAC protocol cause the increasing energy consumption of a network.And being frequently selected causes the early death of a best relay.Aiming at these two problems,an adaptive double cooperative MAC protocol for energy consumption is proposed.According to the message queue of a relay node,the data transmission mode is divided into direct transmission mode,cooperative mode,double cooperative mode I and double cooperative mode II.Considering the energy consumption of control frames,the minimum energy consumptions of these four transmission modes for successfully transmiting one data frame are analyzed.Based on it,the transmission mode is adaptively selected.Considering the residual energy and the minimum transmission energy consumption of the node,a measurement method of a best relay was designed.It can avoid the high quality relay consuming up energy too quickly and balance the energy between the nodes in the network.Simulation results show that the proposed protocol can effectively reduce the network energy consumption and prolong the network lifetime.In addition,among the four modes,double-cooperation mode I has the best energy saving effect.(3)In order to improve the achievable rate of cooperative transmission,a superposition coding cooperative transmission protocol is proposed.Based on the traditional three-node model,the relay signal is superposed with the signal of the source node by means of superposition coding.The superposition factor at the relay is optimized to maximize the transmission rate,and its optimal expression is given.Under thr constraint of SNR,the single-link transmission rate and outage probability of the new transmission protocol,the winning probability of a best relay competition under the constraint of relay candidate region,and the average rate of the region are analyzed.The simulation results show that the proposed protocol can effectively improve the transmission rate,and the closer the relay node to the destination node,the higher the achievable ransmission rate and the higher the spectral efficiency.(4)In order to solve the interference to neighbor links caused by the cooperative communication between a source node and a destination node,improve the spatial frequency reuse,an interferencefree cooperative transmission protocol is proposed.By adjusting the transmission power of relay node,the communication area of cooperative transmission is limited to the direct transmission link,and the interference-free relay selection region is given.In order to compensate for the loss of outage performance caused by transmission power limitation,the best relay selection method is improved,and the source node is included in the candidate relay set to participate in the best relay competition.Based on the joint optimization of relay selection region and transmission power,the outage probability of a single transmission link and the outage probability under the constraint of the relay selection region are analyzed.The spatial frequency multiplexing gain of the system is given and the number of effective concurrent links in the network is calculated.Simulation results show that the protocol can effectively increase the number of concurrent links in the cooperative network,improve the spatial frequency reuse and network throughput,and reduce the outage probability of transmission.