HF Skywave Massive MIMO Communication

The continuing increase of the application demand on mobile internet and internet of things,brings the technique demand of increasing the rate and user terminal(UT)capacity of wireless communications by an order of magnitude and also the global coverage.Massive multiple-input multiple-output(MIMO)wireless communication technology,which has the potential to deeply exploit space resource,can significantly improve the system spectrum and energy efficiency,as well as the system rate and user capacity.Massive MIMO is one of the key technology in the 5th generation(5G)mobile communications and is expected to be further developed and applied in the evolution of 5G and the sixth generation mobile communication.With the continuous evolution of future mobile communication systems,massive MIMO will continue to be researched,applied and developed in various frequency bands and scenarios.High frequency(HF)skywave communications over 3 to 30 MHz frequency band can achieve a long-range information transmission up to thousands of kilometers via ionopheric reflection for global converage.Due to very limited spectrum resource and challenging ionospheric channel condition,HF communications usually have a low system rate and can only serve few UTs.In this dissertation,we apply massive MIMO for HF skywave communications for the first time.We carry out research on the theory and method of HF skywave massive MIMO communications,and confirm that massive MIMO can significantly improve the spectrum and energy efficiency,transmission bandwidth and distance,system rate and user capacity of HF skywave communications.Specifically,the major results and contributions of this dissertation are listed as follows.Firstly,we establish a wideband HF skywave massive MIMO channel model and analyze the sum-rate performance for the minimum mean-squared error(MMSE)based receiver and precoder.We first introduce a model for HF skywave massive MIMO channels within the orthogonal frequency division multiplexing transmission framework by using the matrix of sampled steering vectors.Considering the large antenna array aperture and increased signal bandwidth,the effect of the propagation delay across the large-scale antenna array cannot be ignored,and thus the steering vectors vary across different subcarriers.Specifically,we derive a wideband beam based channel model and show that the beam domain statistical channel state information(CSI)is subcarrier-independent.Then,we consider MMSE based uplink receiver and downlink(DL)precoder with perfect CSI at the base station(BS)and analyze the asymptotic sum-rate performance of the HF skywave massive MIMO systems.The results show that with a large number of antennas at the BS,the sum-rate can be asymptotically increased proportionally to the number of UTs while the transmit power per UT is scaled down inverse-proportionally to the number of antennas.Simulation results demonstrate very significant performance advantages of the proposed HF skywave massive MIMO system.Secondly,we propose the DL transmitter design for HF skywave massive MIMO systems with only statistical CSI available at the BS.Due to the spatial sparsity of HF skywave massive MIMO channels,we first extract the non-zero elements of the beam domain channel and provide the reduced-dimensional representation of the beam based channel.We prove that the ergodic sum-rate maximization problem under total power constraint can be solved in the beam domain without any loss of optimality.We prove that the beam domain transmission only involving power allocation for the beams that each UT occupies is asymptotic optimal if there are a sufficiently large number of antennas at the BS.Then,we derive an iterative beam domain power allocation algorithm using majorization-minimization(MM).We obtain an upper bound of the ergodic sum-rate by using Jensen’s inequality,which does not include computationally complex expectation operations.Furthermore,we investigate a low-complexity DL transmitter design with ergodic sum-rate upper bound under total power constraint.We show that the beam domain design is still optimal and the asymptotic optimal solution can also be obtained by the beam domain power allocation.Specifically,we prove that the power allocation for different UTs should be performed in non-overlapped beams and simplify the MM based iterative power allocation algorithm.Simulation results confirm the effectiveness of proposed DL transmitter designs for HF skywave massive MIMO systems.Finally,we propose the robust precoding with imperfect CSI for HF skywave massive MIMO systems.We establish a beam-based a posteriori channel model to describe the available imperfect CSI at the BS,which includes the channel mean and variance information.Utilizing the beam domain sparsity in the HF skywave massive MIMO channels,the reduced-dimensional representation of the beam based a posteriori channel is provided.Then,we prove that the robust precoder for ergodic sum-rate maximization under total power constraint can be designed by optimizing the beam domain robust precoder(BDRP)without any loss of optimality.Furthermore,the asymptotic optimal precoder is beam structured for a sufficiently large number of antennas at the BS,involving a low-dimensional BDRP.As a result,the beam structured robust precoding is asymptotic optimal and can be efficiently implemented based on chirp ztransform.We then derive an iterative algorithm to design the BDRP using MM.Replacing the ergodic sum-rate with its compact upper bound,we develop a low-complexity BDRP design and simplify the MM based design algorithm.Based on our simulation results,the proposed beam structured robust precoding can achieve a near-optimal performance in various scenarios for HF skywave massive MIMO systems.