Research On Key Techniques Of Ultraviolet MIMO Communication System

The increasing bandwidth requirement,scale of accessing users and connected equipments,multi-service data traffic bring serious challenges for wirless spectral resources and infrastructure enengy consumption.Accordingly,optical wireless communication(OWC)provides a new perspective for us to resovle these bittlenecks.In particular,among these OWC techniques,compared with free space optics(FSO)and visible light communicat(VLC),two major unique features and advantages make ultraviolet communication(UVC)able to meet special needs by future communication scenarios.Firstly,non-line-of-sight(NLOS)links are feasible by atmospheric scattered propagation.Secondly,solar-blind feature in UV spectral region makes wide receiver(Rx)FOV possible with low background noise.Furthermore,the expirical testbed and devices technologies of UVC systems have become mature recently.Consequently,great potential of UVC systems can be found in many military and commercial applications.Due to the path loss induced by scattered propagation and limited effective bandwidth restricted by impulse response broadening,the communication range and data rate are the main facters constraining the current UVC systems.Thus,constructing high data rate and reliable technical schemes to improve the system performance based on the UV channel characteristics is vital for future researches on UVC.Multiple-input-multiple-output(MIMO)techniques could lift up the information rate and transmission performance of communication systems by spatial multiplexing gains,which provide a new way for overcome the abovementoined problems.However,studies on UV MIMO systems are still on the starting status and many problems need to be dicussed.The available works about UV MIMO communication focus on diversity reception at the Rx side.Discussons on sptial multiplexing UV MIMO communication with high spectral efficiency,MIMO coded systems with reliable transmission and impacts of channel mobility are absent.Two key challenges are faced by reseacrches on high rate scattered UV MIMO wireless transmission theory and systems:1)Channel modeling in UV scattered environments variable with space and time.2)UV MIMO system design suitable for mobile channel.Therefore,this article investigates the NLOS UV MIMO channel characteristics and key system techniques to profoundly understand and build the theory architecture for NLOS MIMO UVC.The main contexts of this article are organized as follows:1.At first,we develope the UV MIMO channel model based on the UV scattered propagation.We analytically derive the UV MIMO channel model using the Monte Carlo method.In chapter 2,we discuss the UV MIMO channel model of UV MIMO systems configured by conventional array distributed antennas.As MIMO channels are close related to the MIMO channel spatial correlation,we propose a geometrically structured method to calculate the spatial correlation of UV MIMO channels.Compared with radio communication systems,UV MIMO systems show completely different behaviors.In addition,we build the UV MIMO channel matrix model under the narrowband assumption and then explore the relationship between MIMO channel capacity and spatial correlation.2.In chapter 3,through slight modification on typical photon multiplier tube(PMT)receiver output statistics,a generalized received response model considering both scattered propagation and random detection is presented to investigate the impact of inter-symbol interference(ISI)on link data rate of short-range non-line-of-sight(NLOS)ultraviolet communication.Based on the received response characteristics,a heuristic check matrix construction algorithm of low-density-parity-check(LDPC)code is further proposed to approach the data rate bound and complement the disadvantaged brought by scattered channel propagation.Moreover,based on this heuristic LDPC code and the abovementioned UV MIMO channel model,we firsly investigate the LDPC MIMO system structure on NLOS UV links.3.In chapter 4,we discuss the mobile UV MIMO system design and signal detection algorithms.In existing studies,the Tx/Rx nodes maintain a fixed position for both cases.Since node positions may move in realistic environments,the mobile support becomes crucial for practical UV systems.Firstly,we analyze the link bandwidth of mobile UV channel and propose an analytical model to characterize it.A square array reception is designed by this model to improve the link bandwidth.Then we take a first step towards the mobile support for non-line-of-sight(NLOS)ultraviolet(UV)links by introducing a mesh UV network.Spatial multiplexing multiple-input-multiple-output(MIMO)system is designed geometrically for the mobile mesh network to overcome the data rate bottleneck induced by scattered UV channels.Based on the symmetric UV MIMO channel characteristics,we further modify the sphere decoding(SD)method to accomplish MIMO signal detection at a linear computational complexity.Numerical simulations demonstrate that the noncoplanar UV MIMO system structure can achieve near fully multiplexing gains with proper geometric parameters and the modifeied SD successfully reduce the computational complexity with only a bit of BER performance loss.