Research On Optimization Of The Deployment Of UAV-Base Stations For Mm Wave Communications

With the demand of the high speed and dense connection,unmanned aerial vehicles employed as base stations(UAV-BSs)have drawn a lot of attention in academic and industry due to its flexible deployment.By properly designing the deployment or trajectory of the UAV-BS,the performance could have significant improvement.However,a long-distance transmission may result in low signal to noise ratio(SNR),since the UAV-BS with single antenna working in the microwave band,especially when the UAV-BS has limited on-board energy.The combination of the UAV-BS and millimeter-wave(mmWave)makes the UAV-BS have abundant bandwidths and large scale antenna arrays to improve the coverage and capacity of the communication system.Moreover,due to the mobility of the UAV-BS,the shortcomings of the mm Wave can be compensated by establishing high-quality line of sight(LoS)path.However,since the UAV-BS has limited on-board energy,the power consumption of the radio frequency(RF)is significant.Furthermore,due to the mobility of the UAV-BS,it is difficult to obtain instantaneous channel state information(CSI)to achieve precoding for multiuser inference mitigation.It is challenge for UAV-BS to cover random users with limited RF chains.Motivated by these backgrounds,this dissertation focuses on the deployment and trajectory optimization of the mm Wave UAV-BS combined with multiple antennas.The main contributions of this dissertation are summarized as following:1.Notice that the adjustment of the deployment can affect the establishment of the high-quality channel.Taking the abundant antenna resources and the consumption of the RF chain into account,a novel grouped spatial modulation(GrSM)scheme is proposed,where the antennas are grouped with the aid of the channel correlation information.Within each of the groups,the spatial modulation(SM)is independently utilized by activating one transmit antenna per time slot,and the multiplexing gain is achieved by the different antenna groups.The GrSM scheme is capable of reducing the consumption of the RF chain and improving the sum rate by conveying information in spatial domain.An optimization problem is proposed to maximize the sum rate of the GrSM assisted UAV-BS deployment.The achievable sum rate,the upper bound of the average bit error ratio,and the spectral efficiency is analyzed.Simulation results demonstrate that the GrSM assisted UAV-BS deployment outperforms the random UAV-BS deployment and the conventional SM and Generalized SM.2.When the mm Wave UAV-BS serves multiusers,considering the high-quality users and the mutual effect between the precoding and UAV-BS trajectory,a joint optimization of UAV-BS trajectory,user scheduling,and statistical precoding is proposed to maximize the sum rate of all users.Due to the mobility of the UAV-BS,there exists many potential locations of the UAV-BS.Thus,the amount of pilot or feedback overhead for acquiring the instantaneous CSI is tremendous.Compared to the instantaneous CSI,it is easy to obtain the statistical CSI(SCSI)via long time feedback or statistical channel reciprocity.Thus,in this dissertation,the SCSI is utilized to mitigate the multiuser interference.The joint optimization problem is under the UAVBS flying speed constraint,the initial and final location constraints,the mitigation interference among users,the minimum communication rate,and the maximum number of users served by the UAV-BS.To tackle the joint optimization problem with nonconvex objective function,non-con vex constraints and integer constraints,the joint optimization problem is decomposed into two subproblems.With given UAV-BS trajectory,the joint user scheduling and statistical precoding is optimized to maximize the sum rate of all users.The SCSI is utilized to mitigate multiuser interference.With given user scheduling and statistical precoding,the UAV-BS traj ectory is optimized by converting non-convex problem to the convex problem.An iterative algorithm is proposed to alternatively optimize two subproblems.The simulation results demonstrate the effectiveness of the iterative algorithm,and the sum rate is analyzed under different communication scenarios.3.When focus on the mm Wave UAV-BS multiuser system,the edge users communication quality,the consumption of the RF chain and the deployment and trajectory should be considered.A beam selection optimization for beamspace and UAV-BS deployment is proposed to maximize the sum rate of all users.By employing mm Wave antenna array and discrete lens array(DLA),the signals could be pointed to different directions to form beamspace channel.Due to the sparse nature of the beamspace channel in beam division multiple access(BDMA),the users have dominant beams.By adjusting the locations of the UAV-BS,not only the better channel gain can be achieved,but also the interfering channel distance can be enlarged to alleviate the multiuser interference.The non-convex optimization problem is decomposed into two subproblems.First,a signal to interference plus noise(SINR)based greedy beam selection scheme is proposed.If the served users have unique dominant beams,the UAV-BS directly selects the corresponding dominant beams.If two or more users share same dominant beams,the UAV-BS reselects the beams for the users according to the SINR beam selection scheme.Zero forcing is utilized to further mitigate the multiuser interference.With given SINR based greedy beam selection scheme,the continuous genetic algorithm(CGA)is designed to optimize the beam pattern and UAV-BS deployment.Similarly,considering the mobility of the UAV-BS,given the SINR based greedy beam selection scheme,the CGA is designed to optimize the trajectory of the mmWave UAV-BS.