Beam Division Multiple Access Transmission For Massive MIMO Communications

With the development of the information society,there is an exponential increase in the demand of wireless communication data rate.However,spectrum resources are limited,and thus,there are still enormous challenges for wireless physical layer transmissions.To substantially increase the communication rate,one of the solutions is to develop massive multiple-input multiple-output(MIMO)communications,which has the potential to significantly improve the spectral efficiency and power efficiency,and is a promising candidate for 5G wireless communication networks.An alternative solution is to exploit higher spectrum resources including the optical spectral.Optical communication employs the optical spectral to transmit signals.As it can mitigate the radio frequency(RF)spectrum resources and improve the system throughput,optical communication is a promising wireless communication technology.In massive MIMO systems,the instantaneous channel state information(CSI)acquisition limits the application in the user mobility scenario,while in the optical communication system,the highly correlated channel decreases the system performance and becomes the system bottleneck.This dissertation studies beam division multiple access(BDMA)transmission for massive MIMO systems in both RF and optical communications.First,in the RF massive MIMO communications,to cope with the instantaneous CSI acquisition,high processing complexities at transceivers,and the adaptability of user mobility,we propose a BDMA transmission scheme.Further,we investigate power allocation strategies for BDMA transmission in multi-cell massive MIMO communications.We prove the optimality of BDMA transmission and propose efficient power allocation algorithms.In the optical communications,highly correlated channels lead to low multiplexing gains.To solve this problem,we present beam domain massive MIMO transmission for optical wireless communications with a transmit lens at the base station(BS).Our proposed optical communication scheme can significantly improve the spatial multiplexing gains and the sum rate.Finally,we propose network optical BDMA transmission with transmit and receive lenses.The degree of freedom increases linearly with the number of BSs and user terminals(UTs).Specifically,the major results and contributions of this dissertation are listed as follows.1.We propose BDMA transmission for massive MIMO communications with only statistical CSI.In the existing massive MIMO systems,it is not practical that the BS employs instantaneous CSI for precoding in the high mobility scenarios,frequency division duplexing(FDD)systems,and high frequency communications.Focusing on the physical channel model,we analyze the spatial characteristics of massive MIMO channel,and provide a beam domain channel model,where the channel gains are independent of OFDM sub-carriers.For this model,we derive a closed-form upper bound on the achievable ergodic sum rate,based on which,we develop asymptotically necessary and sufficient conditions for optimal downlink transmission,that require only statistical channel state information at the transmitter.Furthermore,we propose a BDMA transmission scheme that simultaneously serves multiple users via different beams.By selecting users within non-overlapping beams,the multi-user MIMO channels can be equivalently decomposed into multiple single-user MIMO channels;this scheme significantly reduces the overhead of channel estimation,as well as,the processing complexity at transceivers.For BDMA transmission,we work out an optimal pilot design criterion to minimize the mean square error(MSE),and provide optimal pilot sequences by utilizing the Zadoff-Chu sequences.Simulations demonstrate the near-optimal performance of BDMA transmission and that the proposed pilot can significantly reduce the BER performance.2.We extend BDMA transmission to multi-cell scenarios,and propose optimal power allocation strategies.In multi-cell massive MIMO systems,BSs serve multiple UTs with only the statistical CSI.In the beam domain,the eigenmatrices of channel transmit covariance matrices become identical and independent of terminals.Utilizing these eigenmatrices as precoding matrices,we consider power allocation of beam domain transmissions,maximizing the sum rate.By treating co-channel user interferences as noises,the objective function of achievable ergodic sum rate of the multi-cell massive MIMO downlink is reduced to a difference of concave functions.To maximize the sum rate,we identify the orthogonality conditions for optimal power allocation.The results show that the transmit power allocated to the users should be non-overlapping across beams defined by the channel transmit covariance matrices,which indicates that BDMA transmission can achieve the optimal performance.Then,we present an efficient power allocation algorithm,which converge to the point satisfying the orthogonality conditions.Further,we calculate the deterministic equivalent(DE)of the sum rate and propose a DE based power allocation to reduce the complexity.Numerical results illustrate that within a few iteration steps,the algorithms can approach the optimal solutions.3.We present beam domain massive MIMO transmission for optical wireless communications.A BS equipped with massive transmitters communicates with a number of UTs through a transmit lens.Focusing on LED transmitters,we analyze the light refraction of the lens and establish the channel model for optical massive MIMO transmissions.Physically narrow beams can be generated by using the transmit lens.More interestingly,for a large number of LEDs,the channel vectors of different UTs become asymptotically orthogonal.Then,we investigate the maximum ratio transmission and regularized zero-forcing precoding in the optical massive MIMO system,and propose a linear precoding design to maximize the sum rate.We further consider the precoding design when the number of transmitters goes asymptotically large,and show that BDMA transmission achieves the asymptotically optimal performance for sum rate maximization.Compared with the design without a transmit lens,BDMA can increase the sum rate proportionally to 2K and K under the total and per transmitter power constraints,respectively,where K is the number of UTs.In the non-asymptotic case,we prove the orthogonality conditions of the optimal power allocation in the beam domain and propose efficient beam allocation algorithms.Numerical results confirm the significantly improved performance of our proposed beam domain optical massive MIMO communication approaches.When the number of served UTs is 484,the sum rate achieves 2000 bps/Hz.4.We extend beam domain massive MIMO optical communications to network MIMO systems,and propose network massive MIMO optical wireless communications with transmit and receive lenses.Multiple BSs equipped with massive LEDs and transmit lenses,simultaneously communicate with a number of UTs,each UT equipped with massive receiver and receive lens.We establish an optical channel model with transmit and receive lenses.With a transmit lens at the BS,lights emitted from different LEDs are refracted to different directions.When the number of LEDs tends to infinity,the channel matrices from one BS to different UTs become row orthogonal.Moreover,with a receive lens at the UT,lights from different directions are refracted to and received by different photodetectors.As the number of photodetectors increases,the channel matrices from different BSs to one UT become column orthogonal.Then,we design the transmit covariance matrix maximizing the sum-rate under total power and per LED power constraints.Under both power constraints,the optimal transmit strategy is that different LEDs transmit independent signals and that the transmit beams for different UTs are orthogonal(nonoverlapping),which indicates that BDMA transmission can achieve the asymptotically optimal performance.In addition,we analyze the degree of freedom of network massive MIMO optical communications.The degrees of freedom under both power constraints scale with the number of BSs and UTs.Numerical results illustrate that the spectral efficiency of 4 BSs serving 500 UTs reaches 6500 bps/Hz,and the single UT rate is 13 bps/Hz.