Free-space optical communication systems over fading channels
Creators: name: Yang, Fan
Publisher: name: University of British Columbia
Year: 2016

description: Free-space optical (FSO) communication systems can provide larger bandwidth and rapid deployment for communication links. Such systems do not interfere with existing radio frequency (RF) systems and can make communication more secure. However, the performance of FSO communication systems is highly dependent on its channel conditions. The atmospheric channels can impose attenuation and scintillation effects on the communication link, and these effects can hinder the correct detection of information on receiver side. In this thesis, we focus on the performance analysis of terrestrial FSO systems over atmospheric fading channels. One successful channel model to fit the experiment data is the lognormal-Rician model, but its widely adoption is impeded by its analytically intractable probability density function (PDF). Therefore we use Padé approximants method to obtain accurate approximations of the PDF, cumulative density function, and moment generating function of lognormal-Rician distribution. Simple closed-form bit-error rate (BER) expression are obtained for binary phase-shift keying (BPSK) modulation with maximum ratio combining (MRC) reception and for binary differential phase-shift keying (DPSK) with selection combing (SC) reception. Asymptotic error rate analysis for BPSK and DPSK is also presented to reveal the performance behavior in large signal-to-noise ratio regimes. The pointing error effects in FSO systems can also contribute to channel impairments. In order to study the influence of pointing error on system performance, we propose a statistical model for pointing error with nonzero boresight error, which takes into account of laser beamwidth, detector aperture size, and jitter variance. A novel closed-form PDF is derived for this pointing error model. Furthermore, we obtain closed-form PDF and series PDF, respectively, for the composite lognormal and Gamma-Gamma turbulence channels with nonzero boresight pointing errors. We conduct error rate analysis of on-off keying signaling with intensity modulation and direct detection over the lognormal and Gamma-Gamma fading channels. The BER results are presented in highly accurate converging series. Asymptotic error rate analysis and outage probability of such a system are also presented based on the derived composite PDFs. It is shown that the boresight error can only affect the coding gain, while the diversity order is determined by either the atmospheric fading effect or the pointing error effect, depending on which effect is more dominant. For subcarrier intensity modulated FSO systems, the carrier phase estimation error (CPE) would degrade the system performance. We study the BER performance of subcarrier M-ary phase-shift keying systems with carrier phase errors (CPE) in lognormal turbulence channels. The CPE is modeled as a Tikhonov random variable. The CPE induced asymptotic noise reference losses for the studied systems are quantified analytically by introducing the lognormal-Nakagami fading as an auxiliary channel model. One effective counter fading technique is spatial diversity, which requires multiple apertures at transmitter or receiver side. We first conduct a diversity analysis on single-branch FSO systems over atmospheric fading channels. We find that the diversity order of an FSO system is usually determined by small scale effects in its fading channels when the irradiance fluctuation can be modeled as a modulation process (K, lognormal-Rician, Gamma-Gamma and M distribution). Based on this observation and the fact that lognormal channel does not have valid diversity order, we propose a lognormal-Nakagami model to facilitate asymptotic analysis on lognormal channels. Using such an approach, we study different multi-branch FSO systems over correlated lognormal fading channels that may have nonidentical variance. We discover that the correlation among the lognormal channels can impose large signal-to-noise ratio (SNR) penalty to system bit-error rate performance, compared to that of a similar system with independent lognormal channels. This property is not shared with the other commonly used fading channels. In addition, we also derive a compact expression for the asymptotic relative diversity order (ARDO) between an L-branch combining system over correlated lognormal channels and a single-branch system. It is found that the ARDO is related to the number of diversity branches as well as entry-wise norm of the covariance matrix of the logarithm of the lognormal channel states. While maximal ratio combing (MRC), equal gain combining (EGC) and selection combining (SC) result in the same ARDO, we find that the coding gain difference between MRC and EGC is negligible, but SC suffers a 10log(L) dB loss.