Low Complexity Beamforming In Millimeter Wave Communication And Its Applications

The fifth generation(5G)communication system requires higher data rate,larger bandwidth and higher spectrum efficiency.Millimeter wave communication has huge available spectrum resources,which is considered as a promising solution to solve the spectrum shortage of the current communication system.However,the path loss in millimeter wave frequency band limits the coverage of millimeter wave communication system.Fortunately,because the wavelength of millimeter wave is shorter than that of the traditional sub-6 GHz band,more antennas can be integrated into small form factors,which makes it possible to use large-scale antenna array to combat path loss and improve spectrum efficiency.How to use large-scale antenna technology to improve the performance of millimeter wave communication system has become a core issue in recent years.The implementation of large-scale antenna array is facing two main challenges:1)Using large-scale antenna array,traditional full digital beamforming requires a large number of radio frequency(RF)chains,which will lead to huge power consumption and hardware cost.With the introduction of analog beamforming technology,large-scale antenna array can be driven by much fewer RF chains.In multi-user millimeter wave communication system,the hybrid beamforming method combining analog beam-forming and digital beamforming is widely used to eliminate inter user interference.However,the implementation of large-scale antenna array is still the key to restrict the millimeter wave communication system because of the heavy usage of analog devices including phase shifters,combiners,and splitters.2)Due to the constraints of technol-ogy,cost,and volume,the dispersion error of components in millimeter wave band is more severe than that in sub-6 GHz band.The ability of inter-user interference can-cellation in hybrid beamforming methods relies heavily on the accurate channel state information(CSI),so the device dispersion error will have a critical impact on the system performance.Based on the above two challenges,the following problems are studied in this dissertion:1)Performance analysis and improvement methods with phase-shifting net-work error in hybrid beamforming.The dispersion error model of phase shifter is es-tablished,and the influence of random and non-reciprocal characteristics of dispersion error on system performance is analyzed.Theoretical and numerical results show that the hybrid beamforming with imperfect phase shifters suffers from the severe inter-user interference,and the achievable sum rate is limited by a platform in the high SNR region.To improve the performance,we give up the traditional digital beamforming design based on channel reciprocity,and we estimate and feedback the downlink equiv-alent channel instead.We also propose a discrete Fourier transform(DFT)interpolation algorithm for beam training,which greatly reduces the training overhead.The closed form theoretical analysis and numerical results show that the proposed method has a good balance between performance and system cost.For the lens antenna array where the phase shifter network is replaced with a low-cost switch selection network,we pro-pose a group antanna selection method to lower the hardware cost.2)The common amplitude weighted analog beamforming(CAW-ABF)technol-ogy.Hybrid beamforming methods are constrained by the performance of devices since the digital beamforming requires accurate acquisition of device characteristics and chan-nel information to eliminate the interference between beams.The random distribution of device imperfections leads to the gap between the actual performance and the theo-retical performance.Thus,we propose a novel pure analog beamforming in two stages,namely,standard beamforming in specific direction and spatial angle modulation.The former uses the amplitude weighting technology shared by all links to suppress the beam sidelobe,and the latter is realized by phase shifter network.Furthermore,we present op-timization methods of maximizing the achievable sum rate to design both the adjustable and fixed CAW array.More simply,we can design the fixed CAW array by using win-dow function for spatial filtering.The CAW-ABF method greatly reduces the system overhead without the digital beamforming,and reduces the implementation complexity and power consumption.The performance of this method is very close to that of the hybrid beamforming method with perfect device performance and channel state infor-mation,and is superior to that of the hybrid beamforming method with imperfect device performance and channel estimation error.3)A suitable communication system framework for the CAW-ABF technology in order to give full play to its advantages.Based on the CAW-ABF technology,we propose an analog amplify-and-forward(AAF)relay.Compared with the relay based on hybrid beamforming technology,the AAF relay can use cheaper analog amplifying links to replace the RF links.In addition,in order to improve the transmission effi-ciency in the local service scenario,the direct communication between devices(D2D)system has been proposed.However,due to limitation of the hardware cost and power consumption,devices can only deploy few antennas and can not use beamforming to ensure sufficient link gain.Therefore,it is necessary to enhance the link performance using relay.Using the AAF relay designed based on the CAW-ABF method,the uplink and downlink do not need the participation of the digital processing,thus greatly reduc-ing the hardware implementation cost.We also propose a resource scheduling scheme.Considering the coexistence of D2D and cellular communication,under the premise of ensuring the quality of service of cellular users,we schedule the D2D devices by direct-connecting or relay-forwarding to maximize the total throughput of the system.