Research On Transceiver Designs For Mm Wave Systems With Lens Antenna Arrays

Recently,due to the rapid consumer adoption of mobile devices such as tablets and smart-phones,the tele-traffic has experienced a tremendous growth,resulting in a dramatic spectrum shortage at the centimeter-wave wireless frequencies.As a remedy,communication over millime-ter wave(mmWave)frequencies is expected to find its way into next generation cellular networks.Nevertheless,mm Wave signals suffer much higher free-space path loss than centimeter-wave for a given propagation distance and antenna gain.Fortunately,the significantly reduced wavelength makes it possible to realize massive MIMO systems that use a large number of antennas within a small physical size,and yet achieve high array gain for directional communications by exploiting precoding techniques.However,in the case of massive MIMO,the conventional fully digital precoding techniques lead to unaffordable costs in terms of RF chains and power consumption.To address this limita-tion,a number of studies have proposed the concept of beamspace MIMO based on discrete lens arrays(DLA)with an angle-dependent energy focusing property.Furthermore,the phase shifters needed in the hybrid structure are replaced by a switching network,which decreases the cost and complexity of the RF hardware as well as improving the power budget.Within this context,a crit-ical problem of mm Wave lens array systems is the design of effective beam selection and digital precoding schemes.Aiming to improve the power budget of wireless communication systems,wireless powered communication networks(WPCN)is proposed as a potential technology which provides sufficient power supply to energy constrained wireless systems.However,in WPCNs,the users located far away from the hybrid access point(HAP)harvest much less energy than those close to the HAP,but have to consume more energy to transmit data back to the HAP,which is named as a "doubly near far" problem.User cooperation(UC)is an effective approach for this problem,but the overhead consumed on information exchange between users is large.So we consider an ambient backscatter(AB)based UC scheme in mm Wave WPCN with lens antenna arrays.Besides,to realize simultaneous wireless information and power transfer in WPCNs,we need to jointly design the transceivers of the downlink wireless energy transfer and the uplink wireless information transmission.To solve these above problems,in this thesis,we firstly consider the joint design of the beam selection and precoding matrices of a downlink lens MU-MIMO mmWave system.Then we focus on an AB based UC scheme for mmWave WPCNs relying on lens antenna arrays.Finally,we consider the joint design of power allocation,beam selection and receiving matrices for decou-pled WPCN with lens antenna arrays.The specific contributions of this thesis are summarized as follows.First,we consider the joint design-of the beam selection and precoding matrices of a downlink lens MU-MIMO mmWave system.The system model contains K single-antenna users and a base station(BS)equipped with DLA.In order to maximize the sum-rate of all users,we formulate the optimization problem with transmit power constraint and binary variables.The optimization problem is nonconvex with coupled variables,hence we transform the optimization problem into a tractable form using the weighted minimum mean squared error(WMMSE)approach.To solve this problem,we propose an efficient joint beam selection and precoding design algorithm based on the penalty dual decomposition(PDD)method.To reduce the design complexity,we also propose a simplified algorithm by combining the interference-aware beam selection(IA-BS)scheme with the WMMSE approach.After that,we compare the computational complexity and performance of the proposed algorithms with existing schemes.Simulation results demonstrate that our proposed algorithms can converge in a few iterations and achieve near-optimal performance when compared to the fully digital precoding scheme,thus enabling them to outperform the competing methods.Besides,the PDD-based algorithm can take full advantage of available RF chains,and have more potential for applications with large number of users.Second,for the "doubly near far" problem of a mmWave WPCN with lens antenna arrays,we consider an ambient backscatter based user cooperation scheme.The system model contains one HAP and two single-antenna users.To analyze the throughput of each user,we introduce the transmission protocol of the ambient backscatter based user cooperation.To maximize the mini-mum throughput of two users,we formulate an optimization problem by jointly designing power and time allocation.In order to solve this nonconvex problem,we introduce auxiliary variables and transform the original problem into a convex form.After that,we propose an efficient algorithm to solve the transformed problem,and compare the performance of different schemes versus the power of HAP,the number of antennas and distances between users and the HAP.Simulation re-sults demonstrate that the AB based UC scheme outperforms the competing schemes,thus improve the throughput fairness performance in WPCN.Last,we consider a decoupled WPCN with lens antenna arrays.The system model contains a power beacon(PB),an information receiving station(IRS)and K users,both the PB and the IRS are equipped with DLA.To maximize the minimum rate of all users,we formulate the op-timization problem by joint design of power allocation,beam selection and receiving matrices.The formulated problem contains complicated discrete binary constraints,as well as the highly coupled nonconvex objective function.Hence,we introduce auxiliary variables and transform the original problem into a more tractable form.For the nonconvex constraints,we transform them into difference-of-convex(DC)forms and apply linearization to approximate them by convex con-straints.To solve the transformed problem,we propose an efficient algorithm based on the PDD method.Simulation results demonstrate that our proposed algorithm approaches the optimal per-formance of the fully digital transceiver scheme,and outperforms the competing scheme.Since the decoupled WPCN with lens antenna arrays decreases the cost of the RF hardware as well as improving the power budget,it is a very attractive scenario.