Multi-resolution Codebook Design And Low-overhead Beam Training For Millimeter-wave Communications

The surging traffic and ever-growing services are defining a new generation of wireless communications,where the conflict between limited spectrum resource and in-satiable demand becomes more and more serious.Exploring new spectrum resource in higher frequency bands(such as millimeter-wave,mmWave)has received tremendous interest from academia and industry.To compensate the severe path loss over mmWave channel,it is inevitable to deploy largescale antenna arrays.It is commonly hard to adopt conventional channel estimation techniques used in traditional MIMO(Multiple-Input Multiple-Output)systems owing to heavy overhead for channel estimation.To reduce the overhead,this thesis will provide a new guide on multiresolution codebook designs and low-complexity beam training solutions for mmWave communications.Chapter 2 investigates a multi-resolution codebook design with a single RF(Radio Frequency)chain and a hierarchical beam alignment for single-user mmWave links.The codebook design is formulated as a optimization problem with PAPC(Per-Antenna-Power-Constraint),which can be addressed by jointly exploiting virtual-subarray and multi-subbeam-synthesis techniques.The derivation of full beam gain reveals that spatial-frequency selective fading is inevitable due to the phase difference between adjacent virtual subarrays.A criterion is then established to pursue flat beam pattern,which combats the spatial-frequency selective fading by using the phase compensate factor(PCF)method.The multi-resolution codebook design and the hierarchical beam alignment are summarized as algorithms,respectively.The proposed codebook design is given in a closed form,which generates codebooks efficiently and enables adaptive beam alignment.Each antenna is enforced by equal power constraint,yielding sufficient array gain as well as low-complexity hardware.Chapter 3 focuses on designing multi-resolution codebooks with multiple RF chains and adaptive beam alignments for hybrid-structure mmWave links,which can be logically categorized as followsFor fully-connected structures,section 3.2 firstly analyzes the phase difference between adjacent DFT vectors' beam gain,which reveals the phase difference is fractionally constant and the controllability of resultant beams.Then,a criterion for pursuing ideal beam patterns is established.With the criterion,the closed-form expression of combining coefficients is obtained,which enables to design phase-shifted DFT(PS-DFT)codebooks analytically and predict achievable beamforming gain for different training stages under line-of sight(LoS)mmWave channel.With the predictable transceiver gain,we then design an adaptive beam alignment sequence to balance beamforming gain and training overhead and maximize spectrum efficiency.Simulation results demonstrate that angle-detection performance and achievable rate linearly increase with transmission signal-to-noise ratio(SNR).For partially-connected(array-of-subarray)structures,since the phase difference between adjacent DFT vectors is linear,an PCF criterion is established to pursue flat beam patterns.With the criterion,baseband beamforming vectors are constructed under total power constraint,which enables analytically designing multi-resolution codebooks and adaptively selecting the layer of codebook for initial beam alignments.Numerical results show that the received SNR at the initial stage can be guaranteed,avoiding the severe 'mismatch' in beam alignment under low SNRs.To avoid sequential beam training,chapter 4 studies simultaneous beam training for multiuser mmWave communications.For fully-connected base station,section 4.2 firstly presents the strategy of simultaneous beam training,which is based on multi-resolution codebook.Under the simultaneous beam training background,a signal transmission is mathematically formulated,in which baseband precoding matrix and RF beamformers are required to be designed.The designs are finally summarized as algorithm.For partially-connected base station,to reduce the impact of interference among concurrent beams,section 4.3 firstly proposes a radix-4 codebook and the corresponding beam training algorithm,where interference upper bound is obtained.Simulation results show that these simultaneous beam training schemes can achieve better performance than counterparts.Additionally,radix-4 scheme is necessary for subarray structures when the scale of subarray is limited.