Research On The Capacity And Transmission Strategy Of Relay Channel

As a key technology to improve network coverage and spectrum efficiency,relay cooperative communication has been widely used in many practical communication scenarios.The capacity and transmission strategy of the relay system has always been a hot topic in academia and industry,because it has an irreplaceable guiding role in the actual communication system design.Although the research of relay system capacity and transmission strategy has made fruitful achievements in recent decades,there are still some basic key problems that have not been solved well.Even the capacity of the simplest three-node relay cooperative channel is still unknown.There are still a lot of valuable problems worth further study in the relay system.Firstly,this paper studies a special type of relay channel-wireless butterfly network from the perspective of information theory,and then studies the beamforming design of full-duplex MIMO relay multicast system.The main works and innovations of this paper include the following aspects:1)Wireless butterfly network capacity is studied.In this part,the capacity of two modes of wireless butterfly network,i.e.discontinuous mode and continuous mode,is studied.The only difference between these two modes is that these source nodes remain silent when the relay transmits data to these destination nodes in discontinuous mode,while these source nodes don't remain silent in continuous mode.For the two modes of symmetric butterfly networks,the capacity regions of symmetric butterfly networks under the linear shift deterministic model are given from the perspective of information theory in this paper.The results show that the overhearing link in butterfly network provides considerable gain for system capacity.The gain provided by discontinuous mode is half of the capacity of the overhearing link.The gain provided by continuous mode is the capacity of the overhearing link,but the capacity gain provided by the overhearing link has an upper limit,that is,it will not exceed the capacity of source-relay-destination two hop link.Furthermore,for the two modes of symmetric butterfly networks,the upper bounds of their capacity region and the constant gap achievable rates under the Gauss channel model are given.The upper bounds is obtained by using information theory tools such as cut-set theory and genie-aided approach.In order to obtain the constant gap achievable rates,this paper proposes a transmission strategy that can effectively utilize the overhearing link.That is,the source node uses the superposition coding of lattice coding and random Gaussian coding,the receiving node uses successive interference cancellation decoding,and the relay node uses decoding and forwarding.The results show that for any channel gain in discontinuous mode,the gap between the achievable sum rate of this transmission strategy and the sum rate upper bound is within 1.15 bits/s/Hz per user;for any channel gain in in continuous mode,the gap between the achievable sum rate of this transmission strategy and the sum rate upper bound is within 1.65 bits/s/Hz per user.2)The degrees of freedom(DoF)of a general MIMO butterfly network is studied.In this paper,the optimal DoF region of a general MIMO butterfly network is given from the perspective of information theory.The characterization of the DOF region is carried out in two directions: one is to obtain the upper bound of the DOF region by means of information theory tools such as cut-set theory and genie-aided approach;the other is to prove that the upper bound of the DOF region is achievable by eliminating interference with the overhearing signal.The optimal DoF regions of the MIMO butterfly network and the two-pair MIMO relay channels are analyzed and compared.The results show that,compared with two pairs of MIMO relay channel,the overhearing links can obtain the gain of DOF in most cases.Especially when each node is symmetrically equipped with the same number of antennas,the DOF of MIMO butterfly netwok can be two times as that of two pairs of MIMO relay channel.The result corresponds to the well-known double gain result in the wirline butterfly network scenario.3)Beamforming design of the full-duplex MIMO relay multicast system is studied.In order to optimize the multicast rate of this system,the beamforming schemes of full-duplex MIMO relay multicast system with Amplify-and-Forward(AF)and Decode-and-Forward(DF)modes are designed respectively.Since the problem is highly non-convex and difficult to solve directly,three-step iteration algorithms for AF and DF are proposed in this paper.For the three-step iterative algorithm,both the source beamforming vector and the relay receiving beamforming vector can find the closed-form solution,but the relay transmitting beamforming vector is coupled with the performance of the two communication links,and the optimization problem is highly non-convex.In order to solve the beamforming vector of the relay,high SNR approximation,rank 1 approximation and semi-definite programming are used in AF mode,and continuous convex approximation,rank 1 approximation and semi-definite programming are used in DF mode.Numerical results show that full-duplex MIMO relay multicast system can achieve higher spectrum efficiency than half-duplex MIMO relay multicast system regardless of AF mode or DF mode.