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Estimators motivated by the minimum mean-square estimation (MMSE) criterion are presented for the estimation of the optical signal intensity in free-space optical (FSO) communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected using an APD array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a log-normal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform the maximum-likelihood estimators for small scintillation indices while providing a performance similar to the maximum a posteriori estimator. Index Terms--Estimation, free-space optical (FSO), optical, scintillation, turbulence.

Closed-form solutions for the bit-error rate of a free-space, heterodyne optical communication system is derived when the optical beam is subjected to K-distributed optical turbulence. It is assumed that the scintillation index is confined to the range (2,3) or that the number of scatterers in the propagation path is a random variable. Index Terms--Coherent, free space, optical, turbulence.

In this paper, we characterize the performance of a direct-detection, avalanche photodiode-based free-space optical (FSO) communication system in terms of the overall bit-error rate. The system of interest uses pulse-position modulation (PPM) and is subjected to scintillation due to optical turbulence. Two scenarios are considered. In one case, a weak turbulence (clear-air) scenario is considered, for which the received signal intensity may be modeled as a log-normal random process. In the other case, we consider a negative exponentially distributed received signal intensity. To arrive at the desired results, it is assumed that the system uses a binary PPM (BPPM) modulation scheme. Furthermore, it is assumed that the receiver thermal noise is nonnegligible, and that the average signal intensity is large enough to justify a Gaussian approximation at the receiver. Union bound is used to assess the performance of M-ary PPM systems using the results of the BPPM scenario. Numerical results are presented for the BPPM case to shed light on the impact of turbulence on the overall performance. Index Terms--Avalanche photodiode (APD), scintillation, optical, pulse-position modulation (PPM), turbulence.

In this work, a receiver architecture for the detention of on-off-keying (OOK) signals using adaptive signal estimation and threshold setting for free-space optical communications is investigated. It is assumed that the received signal is impaired by channel fading, modeled using Málaga (M) distribution, and an imperfect pointing acquisition. In OOK communications, knowledge of the received signal is required to set the optimum receiver threshold. To that end, a linear estimation technique is utilized to estimate the combined impact of the residual pointing error and channel fading. The performance of the resulting receiver is evaluated and is compared with those of a receiver in which the channel and pointing impairments are ignored and with a receiver in which the channel and pointing impacts are replaced by the statistical averages of those impairments. It is shown that, for reasonable system parameters, the proposed receiver offers a significant improvement in performance in terms of the overall bit error rate (BER) when compared with its alternatives. For a scenario in which a significant pointing error is present, it is observed that channel estimation does not offer a benefit in terms of the overall BER. [ABSTRACT FROM AUTHOR]

Two novel Viterbi decoders, known as joint Viterbi decoder (JVD) and enhanced Viterbi decoder (EVD), for synchronization and data detection are introduced and analyzed for convolutionally-encoded (CE) optical pulse-position modulation (PPM) and overlapping PPM (OPPM) channels. For the JVD algorithm, a count metric is employed to perform the Viterbi decoding in the absence of synchronization. For the EVD algorithm, a memory array is utilized to reduce the computation time of the JVD algorithm. With the aid of a recently proposed upper-bound and taking advantage of the minimal asymptotic normalized timing error (MANTE) concept, approximate performance measures for the EVD and JVD receivers are obtained. Index Terms - Convolutionally-encoded, OPPM, PPM, Viterbi.

The performance of a binary differential phase-shift-keying (DPSK) communications system impaired by slow fading, additive noise, timing error, and mixer imbalance is investigated. Mixer imbalance is shown to cause substantial power penalty at error rates less than [10.sup.-6] in the absence of fading. In the presence of Rayleigh fading, the performance is degraded substantially, resulting in a less appreciable deterioration in the error rate due to mixer imbalance. Index Terms - DPSK, mixer imbalance, timing error, Rayleigh fading.

In this paper, we developed a 3-dimensional (3D) ray-tracing simulator using MATLAB for establishing the viability of heating, ventilation, and air conditioning (HVAC) ducts as a reliable communication channel for indoor communication at millimeter-wave (mm-wave) frequencies. We present theoretical analysis of image theory ray-tracing and provide the equations for total electric field due to different rays undergoing reflections at the duct walls. We also computed the received signal strength indicator (RSSI) for the dry and moisture-laden air flowing through the HVAC ducts. The ray-tracing results are compared with the experimental and theoretical results we obtained for the HVAC ducts. With transmitter effective isotropic radiated power (EIRP) of 7 dBm, we obtain RSSI which varies between -34 dBm and -53 dBm for dry atmospheric pressure and temperature of 1013.25 hPa and 294.26K, respectively, and duct lengths of up to 8m at 60 GHz. [ABSTRACT FROM AUTHOR]

In this letter, the power spectral density (PSD) of a direct-detected optical signal with overlapping pulse-position modulation (OPPM) scheme is derived. It is demonstrated that this spectrum contains a strong periodic component at the signal symbol frequency, thereby enabling a simple PLL device to provide joint symbol and slot synchronization. Variance of timing error for a linearized PLL synchronizer is shown to be mainly dominated by, and is inversely proportional to, the product of PLL closed-loop bandwidth and symbol duration of the OPPM signal. Performance of a shot-noise limited PLL-based optical OPPM communication system is then assessed in terms of the overall system symbol error rate.

A novel optical phase-locked loop (OPLL) system for the self-homodyne detection of digitally phase modulated optical signals is introduced. A Mach-Zehnder type interferometer is used to self-homodyne binary phase-modulated optical signals with an external phase modulator inserted in the control arm of the interferometer. In the absence of thermal noise, this arrangement is 3 dB less sensitive than a homodyne receiver for a background noise limited operation. For a negligible background noise level and when the receiver thermal noise is ignored, a power penalty of 6 dB is incurred.

A clock recovery scheme for direct-detection optical on-off keying (OOK) communication systems with nonreturn-to-zero (NRZ) pulse shaping is proposed and investigated. In the suggested model, the optical field is detected with the aid of an avalanche photodiode (APD) photodetector, which is followed by a clock regeneration subsystem. The proposed clock recovery subsystem here consists of a delay-and-multiply (DM) nonlinearity followed by a conventional phase-locked loop (PLL), tuned to the slot frequency of the desired optical OOK signal. Performance of the proposed system is obtained in terms of the signal-to-noise ratio (SNRL) of the linearized PLL device (or, equivalently, the inverse of phase, or timing, error variance) when background noise and receiver thermal noise are present. Numerical results are presented in order to explain the influence of noise processes, such as receiver thermal noise, APD excess noise, and background noise, on the performance of the proposed clock recovery subsystem. The performance of this system is also compared to that of an early-late gate (ELG) and square-law symbol synchronizers.