Prior performance analyses of diversity receptions over arbitrarily correlated Nakagami-m fading channels use power correlation matrix to define correlation among branches. However, the power correlation matrix cannot uniquely determine the joint probability density function of the branch amplitudes as well as the system performance. In this paper, we derive asymptotically tight and closed-form upper and lower bounds for the error rates and the outage probabilities of maximum-ratio combining, equal-gain combining, and selection combining over arbitrarily correlated Nakagami-m fading channels. The correlation between the Nakagami-m random variables is defined through the correlation matrix of the accompanying Gaussian random variables, which can uniquely determine the joint probability density function of the Nakagami-m variables. Using the analytical results and Monte Carlo simulation, we show that the performance of diversity reception systems over Nakagami-m fading channels cannot be uniquely determined by the power correlation matrix of the branches. Furthermore, we also study the factors that determine the asymptotic performance of the diversity receptions.

Bit-error rate (BER) performance of subcarrier M-ary phase-shift keying (MPSK) and M-ary differential phase-shift keying (MDPSK) is analyzed for optical wireless communications over the lognormal turbulence channels. Both exact BER and approximate BER expressions are presented. We demonstrate that the approximate BER, which is obtained by dividing the symbol error rate by the number of bits per symbol, can be used to estimate the BER performance with acceptable accuracy. Through our asymptotic analysis, we derive closed-form asymptotic BER performance loss expression for MDPSK with respect to MPSK in the lognormal turbulence channels.

The performance of free-space optical (FSO) communication systems is compromised by atmospheric fading and pointing errors. The pointing errors are widely considered as a combination of two components: boresight and jitter. A statistical model is investigated for pointing errors with nonzero boresight by taking into account the laser beamwidth, detector aperture size, and jitter variance. A novel closed-form probability density function (PDF) is derived for this new nonzero boresight pointing error model. Furthermore, we obtain closed-form PDF for the composite lognormal turbulence channels and finite series approximate PDF for the composite Gamma-Gamma turbulence channels, which is suitable for terrestrial FSO applications impaired by building sway. We conduct error rate analysis of on-off keying signaling with intensity modulation and direct detection over the lognormal and Gamma-Gamma fading channels. 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 can only affect the coding gain, while the diversity order is determined by the atmospheric fading effect as well as the pointing error effect.

Free-space optical links are vulnerable to atmospheric fading and pointing errors. When a laser beam wanders, most previous works model the random radial displacement distance of the pointing error as a Rayleigh distributed random variable. However, for a non-ideally fast-tracked laser beam, the radial displacement distance at the receiver aperture may not obey the Rayleigh distribution, since the displacements in vertical and horizontal directions may not always follow a zero-mean Gaussian distribution. In this paper, we generalize a widely used pointing error model by assuming that the radial displacement distance follows a Rician distribution and derive a closed-form probability density function (PDF) of the generalized pointing error model. Furthermore, we derive closed-form PDFs for the composite lognormal and Gamma-Gamma turbulence channels using this generalized pointing error model. Error rate performance is also studied for an on-off keying signaling system with intensity modulation and direct detection in the presence of this generalized pointing error.

A free-space optical communication system using subcarrier binary phase-shift keying modulation is analyzed for Gamma-Gamma turbulence channels with pointing errors. We study the bit-error rate performance of such system and obtain highly accurate bit-error rate approximations using a series expansion approach. Asymptotic error rate analysis is also presented. Our asymptotic analysis reveals some unique transmission characteristics of such system.