Optical Scattering Communication Channel Characterization And System Performance Analysis

Optical wireless communication(OWC)has many advantages when compared with the conventional radio frequency(RF)communication,such as license free,ex-tremely high bandwidth,lower power consumption and high data transmission rate,etc,which has attracted extensive attentions from the whole world.For outdoor sce-nario,the optical scattering communication provides a good solution when the direct link is blocked or the transmitter and receiver are difficult to be perfect aligned.How-ever,the performance of optical scattering communication channel varies with the at-mosphere condition,more specifically,the scattering and absorption effects have sig-nificantly impacts on the information transmission over scattering channel.Moreover,due to the weak nature of scattering communication link,the received signal becomes extremely weak,which is different from the traditional RF communication and light-of-sight(LOS)free-space optical(FSO)communication.Hence,it is important to charac-terize the optical scattering communication channel,analyze the performance of trans-mission link,and optimize the communication system architecture.This dissertation provides several innovative studies on the optical scattering char-acterization and system optimization,where the main contributions are given in the following.· Consider different atmospheric conditions,such as different particle densities,radiuses,and different altitudes.We analyze the path loss for different optical wavelengthes via Monte Carlo simulation,and evaluate the bit error rate(BER)performance when adopting on-off keying(OOK)modulation.Moreover,we modify the traditional Monte Carlo method so as to obtain the precise path loss in inhomogeneous atmosphere.· We provide the relationship between channel path loss and transmission system geometric parameters(elevation angle at both transmitter and receiver sides,di-vergence angle of emitted light beam,optical detector's field-of-view(FOV)).The simulation results show that the link gain of scattering channel is able to be increased via beam reshaping.Moreover,we first analyze the two-dimensional(2D)scattering intensity distribution with respect to the receiver's 2D position,and find whose contour could be well modeled as ellipse,and the relationship be-tween the characteristics of fitting ellipses and the system geometric parameters are also provided.· Since the scattering communication has promising potential in military commu-nication,the security analysis on the scattering channel is necessary,where two scenarios are presented in the following.-The transmitter sends information to legitimate receiver through LOS laser link,while there exists potential eavesdropper who can capture information from NLOS scattering link.The sensitive area where the required secrecy rate cannot be guaranteed is discussed,and the secrecy outage probability is also investigated considering different turbulent intensities.-Consider multiple transmitters and two types of receiver:legitimate receiver and eavesdropper,who both receive optical signal from scattering channel with only one photon-counting detector.Based on the studies on secrecy achievable rate of Poisson channel,we investigate the secrecy rate of multi-input-single-output(MISO)communication system,and optimize the power allocation and duty cycle of transmitted OOK signal to maximize the secrecy achievable rate.· Consider the receiver architecture in scattering communication system,the fol-lowing two types of receiver are presented.-The first type of receiver consider continuous waveform signal.Due to the finite small processing interval and existing electrical noises in PMT,the output signal could be modeled as a mixed Gaussian distribution(MGD).The bounds on the achievable rate of this system are studied based on the data processing theorem.Then two piecewise approximations are presented to decrease the computational complexity in maximum posterior probabil-ity(MAP)detection.Moreover,based on the sampled value of continuous waveform signal,a non-ideal photon counting receiver is considered,where the quantizing threshold is optimized in order to maximize the system per-formance.-The second type of receiver is based on the rising-edge detection for pulses generated by PMT.This type of receiver.consists of a PMT,a pulse-holding circuit,an finite sampling rate ADC and a rising-edge detector.Upon one photon is received,the PMT detector generates a short continuous pulse;the pulse-holding circuit detects each short pulse and then outputs a square pulse with certain width.The output signal of pulse-holding circuit is sam-pled by the ADC and then quantized according to a certain threshold.We adopt a rising-edge pulse detector,where one pulse is recorded upon detect-ing a rising edge from zero to one.We examine that the recorded number of photons in one symbol duration of these two types of photon counting receiver could be both well approximated by binomial distribution.Based on which we optimize the decision threshold and holding time to maximize the KL distance between two binomial distribution for transmitted symbols 0 and 1 as a substituted approach to minimize the total error probability.