Kamran Kiasaleh

Performance of APD-based, PPM free-space optical communication systems in atmospheric 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.

Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence

Closed-form solutions for the bit-error rate of a freespace, 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.

On the probability density function of signal intensity in free-space optical communications systems impaired by pointing jitter and turbulence

Expressions for the probability density function (pdf) of optical signal intensity in an optical communication channel impaired by motion-induced beam jitter and turbulence are derived. It is assumed that the optical beam possesses a Gaussian profile, generated by a pulsed laser, and that the beam scintillation is governed by either log-normal distribution for weak turbulence or K distribution for moderate to strong turbulence in the saturation region. For extreme propagation distances or very strong turbulence, a negative exponential pdf is used to model turbulence. For the aforementioned beam scintillation statistics, approximate pdfs for the signal intensity are also obtained and the conditions for which these approximations seem to be valid are also discussed.

Turbo-coded optical PPM communication systems

A number of parallel concatenated convolutionally coded (PCCC) photon communication systems is introduced and investigated. It is assumed that the optical channel is an intensity modulated (IM) channel and that the received optical signal is detected using a direct-detection (DD) scheme. Two modes of operation are considered. In one scenario, it is assumed that the receiver is limited by shot noise (i.e., negligible receiver thermal noise, or Poisson channel). In the other case, we consider a nonnegligible receiver thermal noise where an avalanche photodetector (APD) is employed to detect the received optical signal. It is also considered that the modulation scheme is the binary pulse-position modulation (PPM). With the aid of the best available upper bounds, the performance of the rate 1/n PCCC encoded optical PPM systems is assessed in terms of the upper bound on the system bit error rate (BER) for the shot-noise-limited IM/DD systems with nonnegligible background noise and for the thermal-noise-limited systems with APD detectors when a uniform interleaver is used. Numerical results for the rate 1/3 PCCC encoded PPM channels are presented. The numerical results demonstrate the enormous potential of this novel coding scheme in enhancing the performance of the aforementioned optical channels by a sizeable margin across the board.

Modification of Real-Number and Binary PSO Algorithms for Accelerated Convergence

Modifications in the velocity calculation of the particle swarm optimization (PSO) algorithm are proposed. The suggested modifications aim to arrive at a faster, more straightforward and still robust search procedure as compared to the conventional method. Two main factors, i.e., personal best influence and initial velocity values, are evaluated. It is shown that in problems with wide-range parameters, the effect of personal best locations is intrinsically encompassed by that of global best locations, thereby allowing for further simplification of the PSO algorithm by eliminating the factor which accounts for the personal best solutions in the velocity calculation. This simplification expedites the convergence procedure in real PSO. It is also shown that the initial velocity values can be modified to enhance the performance in terms of achieving better solution when compared with the existing algorithms, particularly in binary PSO. In order to validate the viability of the proposed procedure, the performances of the real-number and binary PSO algorithms with different velocity calculations are assessed in 1000-run sets, and pros and cons are studied. In particular, the performance of the proposed algorithm, when used to design software defined thinned array antennas, is shown to be superior to those of the existing algorithms.

Performance analysis of free-space on-off-keying optical communication systems impaired by turbulence

The performance of a free-space optical (FSO) communication system is investigated when communication is established via a short-range, turbulent optical channel. The system under investigation utilizes on-off-keying (OOK) modulation combined with direct-detection to establish a duplex communication link. It is further assumed that the optical beam obeys a Gaussian profile. The received signal is detected using a p-i-n diode which is followed by a trans-impedance amplifier (TIA), limiting amplifier, and a clock/data recovery subsystem. Furthermore, it is assumed that optical front-end provides a relatively large aperture so that the impact of turbulence is somewhat mitigated and that the channel/system parameters result in a weak turbulent condition. The performance of the proposed system for a bit error rate of 10-9 in the absence of forward error correction (FEC) is assessed in terms of probability of fade (PF), average number of fades per second (FPS), mean fade duration (MFD), mean-guard-to-mean-burst (MGMB) ratio, and mean time between fades (MTBF).

Hybrid ARQ for FSO Communications Through Turbulent Atmosphere

Hybrid automatic-repeat request (HARQ) technique is investigated for binary pulse-position modulation (BPPM), free-space optical (FSO) communications through turbulent optical channels. It is assume that the received optical signal is detected using a direct-detection (DD) mechanism and that shot-noise, background noise, and thermal noise are present at the receiver. Performance of the HARQ-FSO system is assessed in terms of packet error rate, and is compared with that of a conventional ARQ-FSO system to draw conclusions about the effectiveness of HARQ mechanism in circumventing the impact of turbulence.

Signal intensity estimators for free-space optical communications through turbulent atmosphere

Estimators which are motivated by the minimum mean-square error estimation criterion are proposed for the estimation of optical signal intensity in free-space optical communication systems impaired by atmospheric-induced scintillation. It is assumed that the received signal is detected using a p-i-n photodiode and that the receiver operates under a shot-noise-limited condition. The optical channel is modeled using a log-normal distribution for the received signal intensity for low (<0.75) scintillation indexes, whereas for the fully developed speckle case, a negative exponential probability density function is used to model the received signal intensity. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood estimators for small scintillation indexes.

Fiber optic frequency hopping multiple access communication system

A network architecture is presented for a fiber optic frequency hopping multiple access (FO-FHMA) communication system. It is shown that the network, in addition to providing an independent (from other users) and asynchronous access to the optical fiber channel for the network subscribers, effectively utilizes the available bandwidth when the optical channel is sparsely utilized. In this arrangement the network users employ pseudorandom codes to frequency modulate the optical signal on the fiber bus using an optical pulse modulator. A balanced optical phase discriminator followed by a standard frequency dehopping receiver are used to decode the user provided information at a desired point on the fiber bus. It is demonstrated that the system may easily be implemented with the aid of a number of available optical components and standard frequency hopping transmitter and receive.<<ETX>>

On the scintillation index of a multiwavelength Gaussian beam in a turbulent free-space optical communications channel

The scintillation statistics of a multiwavelength Gaussian optical beam are characterized when the beam is subjected to a turbulent optical channel. It is assumed that the level of turbulence in the atmosphere ensures a weak-turbulence scenario and that fluctuations in the signal intensity are due to variations in the refractive index of the medium, which in turn are caused by regional temperature variations due to atmospheric turbulence. Furthermore, it is assumed that the propagation path is nearly horizontal and that the heights of the transmitter and receiver justify a near-ground propagation assumption. The Rytov approximation is used to arrive at the desired results. Furthermore, it is assumed that the first- as well as second-order perturbation terms are present in modeling the impact of atmosphere-induced scintillation. Numerical results are presented to shed light on the performance of multiwavelength optical radiation in weak turbulence and to underscore the benefits of the proposed approach as compared with its single-wavelength counterpart in combating the effect of turbulence. Furthermore, it is shown that if the separation of wavelengths used is sufficiently large, wavelength separation affects the scintillation index in a measurable way.