Experimental Study Of Wireless Multi-relay Transmission Based On USRP Platform

In the “Internet+” era,people's demand for wireless communication services is increasing,especially the demand for the quality of transmission services for multimedia services.Wireless communication has problems in system capacity and coverage,especially in the coverage of indoor areas,tunnels,remote mountain areas,and other areas covered by high-rise buildings and coverage distances.The signal strength of the base station received by the mobile terminal is too weak and sometimes the base station even does not receive signals,which seriously affects the reliability of data transmission and the quality of communications.However,the research related to relay communication is mostly based on theory and simulation software for performance evaluation,simulation can not simulate the real wireless channel environment and has limitations.In order to solve the above problems,this paper introduces software-defined radio platform(USRP)to implement communication signal processing through software programming,which has the advantages of low cost and short development cycle.Wireless multi-relay cooperation technology has the advantages of providing space diversity to overcome multi-path fading,increasing coverage,increasing system capacity and low cost.This paper focuses on multi-relay system combined with Raptor code,adaptive modulation and optimal relay selection algorithm,and it validates the above technologies based on USRP platform for multimedia applications.The main achievements are as follows:(1)To ensure the quality of data transmission on the channel,the receiver needs to send feedback information to the source to retransmit the information.The amount of feedback information of common code is large,which will cause serious data loss.In order to solve the problem,this paper proposes a solution that apply the Raptor code to the wireless multi-relay transmission,which can reduce the amount of feedback information.Raptor code has low complexity,does not need to know the channel state information in advance,has no fixed rate and low feedback information,and can choose the number of participating relays adaptively according to the channel state.Experimental results show that the Raptor code reduces the amount of feedback information on the one hand,and on the other hand,it greatly reduces the amount of information and transmission power used for forward transmission.(2)In the prior art,the single type of adaptive modulation and multi-relay selection algorithm has the problems of low spectrum utilization and high system complexity.This paper proposes a relay system combined with adaptive modulation and optimal relay selection algorithm.The system selects different modulation methods and relay numbers according to the channel state setting threshold and adapts to different channel changes.Experimental results show that compared with the single modulation system,the system has lower bit error rate(10%)and high spectrum utilization.Besides,it can reduce system complexity and improve system transmission efficiency.(3)Aiming at the problems of small coverage of single relay and direct-transmission systems,difficulty in deployment of MIMO systems on small communication terminals,and low system capacity,this paper implements a scheme for building a wireless multi-relay transmission system for multimedia applications based on GNU Radio software and USRP platform.Then,Raptor code and adaptive modulation technology are used to optimize the system.This system mainly applies the time synchronization algorithm in the GNU Radio environment,and uses the flow label technology to perform signal and time synchronization processing.The use of cooperative relays to replace multi-hop networks enables resource and power sharing among multiple nodes,while effectively implementing Space diversity and overcoming multipath fading phenomena.A frequency-locked loop and a Costa ring are used to eliminate frequency offset and phase offset.A decoding and forwarding scheme is adopted and an optimal relay location algorithm is proposed.Experimental results show that compared with single relay and direct transmission,this system has the advantages of large coverage,low bit error rate(correct rate of 86%),increased diversity gain and low cost.