Research On Energy-efficient Transmission Schemes For New Wireless Cooperative Communications

Cooperative communication technology can effectively alleviate the problems of multi-path fading and path loss of wireless channels and improve the coverage,the channel capacity and the transmission reliability of wireless networks,which plays a very important role in the current wireless communication system.Traditional cooperative wireless relays mostly use the half-duplex(HD)mode,i.e.,the signal reception and transmission at the relay node occupy different time or frequency resources,which leads to a waste of communication resources and degradation in the spectral efficiency.With the continuous development of radio frequency technologies,especially the self-interference(SI)cancellation technology,the full-duplex(FD)relay that simultaneously transmits and receives signals in the same frequency band has attracted a number of attentions.Theoretically,the FD relay can at most save half of the time-frequency resources of the HD relay and double the system spectrum efficiency.In addition,ultra dense network(UDN)is also a new cooperative communication technique,which deploys a large number of low-cost small base stations and establishes a heterogeneous ultra dense network architecture to satisfy the communication requirements of the user terminals in hotspot areas and significantly enhance the system performance.This thesis mainly conducts in-depth research on the energy-efficient transmission for the above two modern cooperative communication scenarios,i.e.,FD relay and UDN.Firstly,concerning a three-node relay system,the thesis compares the maximum achievable energy efficiency of HD relay and FD relay through theoretical analysis.For the scenario with limited relay transmit power,the performance advantage of the HD relay compared to the FD relay in terms of energy efficiency is strictly proved.Furthermore,a minimum spectrum efficiency constraint is introduced in addition to the relay transmit power constraint.In this context,it is found that the FD relay can achieve higher energy efficiency than the HD relay under certain conditions.Finally,simulations verify the correctness of the above theoretical analysis results.Secondly,the thesis studies the energy efficiency maximization problem of the FD relay,and proposes an efficient joint power optimization algorithm.By constraining the maximum transmission power of the source and relay nodes and the minimum spectrum efficiency of the system,we formulate an energy efficiency maximization problem to jointly optimize the transmit powers of the source and relay.Regarding this fractional programming problem,the thesis first proposes a concave-convex procedure(CCCP)based iterative optimization algorithm to obtain its local optimal solution.In addition,in order to reduce the complexity of the iterative algorithm,the paper proposes a improved low-complexity algorithm,which avoids the iterative process and only needs to solve one convex optimization problem,while approaching the optimal performance.Numerical simulations verify the effectiveness of the proposed energy efficiency optimization algorithms.Finally,focusing on a wireless self-backhaul based UDN,we formulate an energy efficiency maximization problem with respect to the transmit powers of all base stations,by imposing constraints on the maximum power of each base station in the network,the minimum rate of each user,and the minimum rate of the wireless backhaul link.After that,we introduce auxiliary variables and employ the convex approximation technique to transform the original problem,and obtain a local optimal solution to the transmit power of each base station by iteratively solving a convex problem.Finally,the excellent performance of the proposed algorithm is verified by simulations.