Beam Domain Massive MIMO Optical Wireless Communications

In the modern society where all kinds of intelligent terminals are widely used,the demand for wireless communication business has shown an explosive growth trend with the emergence and development of new technologies such as cloud virtual reality,Io T industrial automation,remote holographic unmanned systems and digital twin body area network.However,the increasing shortage of traditional RF spectrum resources has been unable to meet the rapid growth of data transmission rate requirements.At this time,optical wireless communication is emerging with its unique advantages.It can provide rich spectrum resources,support ultra-high data rates,and meet the needs of high-speed business scenarios of future mobile communications.Nevertheless,the high correlation of optical wireless channels and the limitation of intensity modulation and direct detection hinder the further improvement of optical wireless communication systems.In order to solve these problems,this dissertation studies beam domain massive multiple-input multiple-output(MIMO)optical wireless communications,proposes the architecture of optical wireless communications system using the fiber microlens and proposes the beam domain massive MIMO orthogonal frequency division multiplexing(OFDM)transmission for optical wireless communications.Firstly,we design the optical antenna based on fiber ports and establish the channel model.We derive the ray equation and ray transformation matrix of the fiber microlens based on geometrical optics and wave optics.In order to adjust the far-field divergence angle of the beam emitted from the fiber port,we propose the design method of the fiber microlens.Then,we design the optical antenna by using the fiber port array and lens to generate multiple optical beams in different directions so as to realize the full-beam coverage of the communication area.In this system architecture,we analyze the optical transmission channel gain from the base station to user terminals and establish the channel model.The optical simulation results show that the designed fiber microlens can significantly expand the far-field divergence angle of the beam.The base station can effectively improve the spatial resolution of different beams by using the optical antenna and realize the full-beam coverage of the communication area.Then,we propose the beam domain massive MIMO-OFDM downlink transmission.For the downlink of the optical wireless communication system,we present the massive MIMO-OFDM optical wireless transmission system architecture.In this system architecture,we proposed a linear precoding design to maximize the sum rate and propose the beam scheduling and power allocation(BSPA)algorithm with low implementation complexity.As the OFDM system will cause high peak-to-average power ratio(PAPR)of transmit signal,we utilize the sequential parametric convex approximation method to solve the nonconvex optimization problem and propose the linear precoding design to maximize the sum rate under the PAPR constraint.At the same time,we design the BSPA algorithm with low PAPR.Numerical results show that both precoding schemes have excellent performance.The precoding performance based on the BSPA algorithm is better than that of maximum ratio transmission precoding and regularized zero forcing precoding under PAPR constraints.Finally,we proposed the beam domain massive MIMO single carrier frequency division multiple access(SC-FDMA)uplink transmission.Aiming at the problem that the uplink is sensitive to PAPR performance,we modify the discrete Fourier transform spread OFDM(DFTS-OFDM)in RF communication to apply to optical wireless communications and present the optical SC-FDMA transmission by using DFTS-OFDM.In order to further improve the PAPR performance of the system,we modify offset quadrature amplitude modulation OFDM(OQAM-OFDM)to meet the requirements of the optical wireless communication system.By adding conjugate symmetric subcarriers and cyclic prefixes,we design the optical SC-FDMA transmission by using CP-OQAM-OFDM.Numerical results show that the PAPR performance of the two transmissions is better than that of OFDMA,and the PAPR performance of the optical SC-FDMA transmission by using CP-OQAM-OFDM is the best.