Xuegui Song

Error Rate of Subcarrier Intensity Modulations for Wireless Optical Communications

Subcarrier intensity modulations are analyzed for the Gamma-Gamma turbulence channels. We study the error rate of a subcarrier intensity modulated wireless optical communication system employing phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying. Closed-form error rate expressions are derived using a series expansion approach. Asymptotic error rate analysis and truncation error analysis are also presented. Our asymptotic analysis shows that the diversity order of the system and the signal-to-noise ratio penalty factors for noncoherent modulations depend only on the smaller channel parameter.

Subcarrier intensity modulated wireless optical communications with rectangular QAM

The average symbol error rate is studied for subcarrier intensity modulated wireless optical communication systems employing general order rectangular quadrature amplitude modulation. We consider three different turbulence channel models, i.e., the Gamma-Gamma channel, the K-distributed channel, and the negative exponential channel with different levels of turbulence. Closed-form error rate expressions are derived using a series expansion of the modified Bessel function. In addition, detailed truncation error analysis and asymptotic error rate analysis are also presented. Numerical results demonstrate that our series solutions are highly accurate and efficient.

Performance analysis of coherent wireless optical communications with atmospheric turbulence

Coherent wireless optical communication systems with heterodyne detection are analyzed for binary phase-shift keying (BPSK), differential PSK (DPSK), and M-ary PSK over Gamma-Gamma turbulence channels. Closed-form error rate expressions are derived using a series expansion approach. It is shown that, in the special case of K-distributed turbulence channel, the DPSK incurs a 3 dB signal-to-noise ratio (SNR) penalty compared to BPSK in the large SNR regime. The outage probability is also obtained, and a detailed outage truncation error analysis is presented and used to assess the accuracy in system performance estimation. It is shown that our series error rate expressions are simple to use and highly accurate for practical system performance estimation.

Subcarrier intensity modulated optical wireless communications in atmospheric turbulence with pointing errors

An optical wireless communication system using subcarrier intensity modulation is analyzed for gamma-gamma turbulence channels with pointing errors. We study the error rate performance of such a system employing M-ary phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying. Highly accurate error rate approximations are derived using a series expansion approach. Furthermore, outage probability expressions are obtained for such a system. Asymptotic error rate and outage probability analyses are also presented. Our asymptotic analysis reveals some unique transmission characteristics of such a system. Our findings suggest that pointing error compensation is necessary as pointing errors can severely degrade the error rate and outage probability performance of an uncompensated system.

Optical Communication Using Subcarrier Intensity Modulation in Strong Atmospheric Turbulence

Error rate performance of subcarrier intensity modulations is analyzed for optical wireless communications over strong atmospheric turbulence channels. We study the error rate of a subcarrier intensity modulated optical wireless communication system employing M -ary phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying. Both K -distributed turbulence channel (strong) and negative exponential turbulence channel (saturated) are considered. Closed-form error rate expressions are derived using a series expansion of the modified Bessel function. Furthermore, the outage probability expressions are obtained for subcarrier intensity modulated optical wireless communication systems over the K-distributed turbulence and the negative exponential channels. Asymptotic error rate analysis and truncation error analysis are also presented. Our asymptotic analysis shows that differential phase-shift keying suffers a constant signal-to-noise ratio performance loss of 3.92 dB with respect to binary phase-shift keying under strong atmospheric turbulence conditions. The numerical results demonstrate that our series solutions are efficient and highly accurate.

Subcarrier intensity modulated MIMO optical communications in atmospheric turbulence

We investigate spatial diversity techniques for subcarrier phase-shift keying (PSK)-modulated optical wireless communication links over the Gamma-Gamma channels. Both repetition code and the Alamouti-type orthogonal space-time block code (OSTBC) are considered. Highly accurate series error rate expressions are derived by using a moment generating function approach with a series expansion of the modified Bessel function. Truncation error analyses and asymptotic error rate analyses are also presented. Our asymptotic analyses show that the diversity order of the studied system depends only on the effective number of small-scale cells of the scattering process in the atmosphere. Our performance analyses confirm that the repetition code outperforms OSTBC for subcarrier PSK-based systems over the Gamma-Gamma channels. The asymptotic performance loss of the Alamouti-coded system with respect to the repetition-coded system is also quantified analytically.

Subcarrier Intensity Modulated Optical Wireless Communications Using Noncoherent and Differentially Coherent Modulations

Error rate performance is analyzed for subcarrier noncoherent and differentially coherent modulated optical wireless communication systems over atmospheric turbulence channels. Both Gamma-Gamma and negative exponential channels are considered. Using a moment generating function approach, we obtain a unified bit-error rate expression for a family of noncoherent and differentially coherent modulations. We also analyze the symbol-error rate performance of subcarrier M-ary differential phase-shift keying and M -ary noncoherent frequency-shift keying modulated systems using the same frequency domain approach. Our asymptotic analysis reveals relative error rate performance of M -ary differential phase-shift keying and M-ary noncoherent frequency-shift keying in different channel conditions.

Maximum Likelihood Based Channel Estimation for Macrocellular OFDM Uplinks in Dispersive Time-Varying Channels

Coherent modulation is more effective than differential modulation for orthogonal frequency division multiplexing (OFDM) systems requiring high data rate and spectral efficiency. Channel estimation is therefore an integral part of the receiver design. Two iterative maximum likelihood (ML) based channel estimation algorithms are proposed for OFDM uplinks in dispersive time-varying channels. The uplink multipath fading channel is modeled such that the channel state can be determined by estimating the unknown channel parameters. A second-order Taylor series expansion is adopted to simplify the channel estimation problem. Based on the system model, an iterative ML-based algorithm is first proposed to estimate the discrete-time channel parameters. The mean square error performance of the proposed algorithm is analyzed using a small perturbation technique. Based on a convergence rate analysis, an improved iterative ML channel estimation algorithm is presented using a successive overrelaxation method. Numerical experiments are performed to confirm the theoretical analyses and show the improvement in convergence rate of the improved algorithm.

Enhancing the security of free-space optical communications with secret sharing and key agreement

Security issues of free-space optical (FSO) communications are discussed. Based on a subcarrier intensity-modulated air-to-ground FSO system model, we first analyze the coherence time of the air-to-ground FSO link and show that under practical assumptions, scintillation reciprocity holds in the FSO communication system. A private secret-key-based cryptosystem with key management is introduced to enhance FSO security, and a key agreement approach is proposed based on statistics of the random atmospheric-turbulence-induced fading channel measurements. The secret key rate of the key agreement scheme is investigated for the gamma-gamma turbulence model. Practical key agreement protocols based on bidirectional channel identification are designed for different FSO communication scenarios. Our numerical results reveal that the key rate is not sensitive to the strength of the atmospheric turbulence; the per-symbol signal-to-noise ratio and the training sequence length are the dominating factors.

Generalized Method of Moments Estimation of the Nakagami-m Fading Parameter

The generalized method of moments (GMM) is introduced in the framework of estimating the Nakagami-m fading parameter. This GMM approach provides a systematic procedure for finding the moment-based m parameter estimators. Using the multivariate delta method, we present a derivation for the asymptotic variance of the GMM Nakagami m parameter estimators. Monte Carlo simulation results show that the GMM approach can lead to estimators outperforming existing moment-based m parameter estimators over a wide range of channel conditions. It is shown that the asymptotic performance of these GMM estimators are close to that of the maximum-likelihood based estimator. The proposed method can be easily applied to both noiseless and noisy environments.