Haitham S. Khallaf

Performance Analysis of Both Shot- and Thermal-Noise Limited MultiPulse PPM Receivers in Gamma–Gamma Atmospheric Channels

The performance of free-space optical (FSO) communication systems adopting multipulse PPM (MPPM) techniques is investigated taking into account the effects of both the atmospheric turbulence and receiver noise. The atmospheric turbulence is modeled by a gamma-gamma distribution, which is suitable for both weak and strong turbulence. As for the receiver noise, both shot- and thermal-noise limited scenarios are considered. For the shot-noise limited system, both exact and approximate expressions of the average symbol-error rate (SER) of the system are obtained. For the thermal-noise limited system, a closed form for the upper bound of the average system SER, based on the Meijer G function, is obtained. Then, we validate it using Monte Carlo simulation results. Furthermore, we study the effects of changing the atmospheric conditions, operational wavelengths, and number of time slots on the average system performance. In addition, we compare the performance of the aforementioned system with that of the traditional PPM technique, in a gamma-gamma channel, under same constraints on the average energy per bit, transmission data rate, and bandwidth.

Proposal of a Hybrid QAM-MPPM Technique for Optical Communications Systems

M-ary quadrature amplitude modulation (QAM) is a class of non-constant envelope scheme that can achieve high bandwidth efficiency under an average signal power constraint. In this paper, a hybrid M-ary quadrature amplitude modulation multi-pulse pulse-position modulation (hybrid QAM-MPPM) technique is proposed for optical communications systems. Both transmitter and receiver block diagrams, which utilize intensity modulation/direct detection (IM/DD) scheme, are introduced. The proposed system is analyzed theoretically and its bit-error rate (BER) performance is derived and investigated. Our results reveal that, under the conditions of fixed data rates, bandwidth, and energy per bit, the proposed technique outperforms both traditional QAM and MPPM sub-carrier modulation techniques.

Performance analysis of a hybrid QAM-MPPM technique over turbulence-free and gamma-gamma free-space optical channels

The performance of a hybrid M-ary quadrature amplitude modulation multi-pulse pulse-position modulation (hybrid QAM-MPPM) technique is investigated in both turbulence-free and gamma-gamma freespace optical (FSO) channels. Both the spectral efficiency and asymptotic power efficiency of the hybrid QAM-MPPM are estimated and compared to traditional QAM and MPPM techniques. The bit error rate (BER) of intensity-modulation direct-detection (IM-DD) systems adopting the hybrid technique is investigated over turbulence-free FSO channels. Upper-bound expressions for the average BER and outage probability are derived for FSO systems by adopting a hybrid QAM-MPPM scheme over gamma-gamma turbulent channels. In addition, the performance of a Reed-Solomon coded hybrid QAM-MPPM is considered. The obtained expressions are used to numerically investigate the performance of the hybrid technique. Our results reveal that, under the conditions of comparable data rates, the same bandwidth, and the same energy per bit, FSO systems adopting the hybrid technique outperform those adopting traditional MPPM, QAM, and on-off keying (OOK) techniques by 1.5, 0.4, and 3 dB, respectively, in the case of turbulence-free channels. Moreover, the new technique shows a better BER performance under different turbulence levels when compared with traditional MPPM and QAM techniques in turbulent FSO communication channels. Also, it shows an improvement in outage probability compared to MPPM, QAM, and OOK over gamma-gamma FSO channels.

Performance analysis of free-space optics systems adopting multi-pulse PPM techniques in gamma-gamma channels for thermal noise limited systems

Atmospheric turbulence has a strong effect on the performance of the terrestrial free space optics system and there are many statistical models that are used to describe the atmospheric turbulence but gamma-gamma is shown to be the most suitable model in both weak and strong turbulence. In this paper, we adopt gamma-gamma model and use the characteristics of Meijer G function to get a closed form expression for the average symbol error rate of multi-pulse PPM under gamma-gamma fading channel. Next, we use our derived form to study the effect of atmospheric conditions, operating wavelength and modulation level on the system performance.

Performance analysis and comparison of optical IDMA and optical CDMA techniques using unipolar transmission scheme

Interleave-division multiple-access (IDMA) has been recently proposed as a new spread spectrum multiple-access technique in optical communication systems. However, IDMA has not been studied for intensity modulation/direct detection (IM/DD) optical networks. In this paper, we introduce modifications to traditional IDMA decoding algorithm so as to make it suitable for IM/DD optical communication systems. In addition, we compare the performance, in terms of the bit-error rate (BER), of optical IDMA systems to the corresponding systems adopting optical CDMA. Our results reveal that under the above modifications, optical IDMA systems show significant improvement in the BER performance when compared to that of optical CDMA systems under same conditions.

Hybrid BPSK-modified MPPM: A scheme for enhancing optical MPPM in optical fiber communications

A hybrid binary phase shift keying-modified multipulse pulse-position modulation (hybrid BPSK-modified MPPM) scheme is proposed as a new modulation technique to improve both the symbol-error rate performance and bandwidth utilization efficiency of conventional optical multi-pulse pulse position modulation (MPPM) scheme in optical fiber communication systems. Whereas in conventional MPPM scheme, unmodulated pulses are transmitted in every signal block, BPSK modulated pulses is transmitted in the proposed hybrid scheme. That is, the information is encoded in both the positions and phases of the transmitted pulses. The transmission characteristics, transmitter and receiver structures, bandwidth utilization, and optimum decoding for the proposed scheme are presented in this paper. Several performance measures are derived and compared to those of conventional MPPM schemes, adopting both coherent and direct detection receivers, under the same data transmission rates. Our results reveal that, at same average power levels, the proposed hybrid BPSK-modified MPPM scheme achieves much lower levels of symbol-error rates compared to those of ordinary MPPM schemes. Furthermore, in terms of bandwidth-utilization efficiency, the proposed hybrid modulation scheme achieves much higher efficiencies than that of ordinary MPPM schemes.

Two approaches for the modified asymmetrically clipped optical orthogonal frequency division multiplexing system

Two approaches for Modifying the performance of asymmetrically-clipped optical orthogonal frequency-division multiplexing (ACO-OFDM) technique are proposed. These two approaches involve the use of unipolar encoding which facilitates transmission through optical channels. Monte Carlo simulations have been carried out for the proposed system to evaluate its bit error rate (BER) performance. Our simulation results are then compared to that of both conventional ACO-OFDM and bipolar OFDM systems. It turned out that the proposed modifications improve the BER performance when compared to that of traditional systems. Specifically, in case of the first approach the performance of ACO-OFDM scheme can be improved by more than 3 dB when adopting the proposed modifications. In addition these modifications would eliminate the reduction in the BER performance between ACO-OFDM and bipolar OFDM and surpass its performance.

Performance analysis of visible light communication systems over fading channels

The performance of visible light communication (VLC) systems, adopting hybrid modulation techniques, is investigated under dynamic fading channels. Hybrid M-ary quadrature amplitude modulation and multi-pulse pulse-position modulation (hybrid QAM-MPPM) scheme is considered because of its high-power efficiency under constrained spectral efficiency. Expressions for bit-error rate (BER) and outage probability are derived for VLC systems adopting hybrid QAM-MPPM scheme under dynamic channels modeled by Rayleigh distribution. The derived expressions are used to investigate the performance of proposed VLC and compare it to corresponding ordinary QAM and MPPM schemes. Our results reveal that the BER of the proposed hybrid scheme outperforms that of ordinary MPPM by more than 4 dB. In addition, the outage probability of VLC systems is improved when using the hybrid scheme compared to that of ordinary QAM scheme.

Average SER of MPPM technique over exponentiated Weibull fading FSO channels considering fog and beam divergence

The performance of free-space optical (FSO) transmission systems adopting multi-pulse pulse-position modulation (MPPM) scheme is investigated taking into account the effects of the atmospheric turbulence, fog, and beam divergence. The atmospheric turbulence is modeled by the exponentiated Weibull (EW) distribution. A closed-form expression for the symbol-error rate ( SER) of our MPPM-FSO system is obtained based on MeijerG function. The results show that the turbulence effect can be mitigated by aperture averaging.

Efficiency of opportunistic cellular/LiFi traffic offloading

Data offloading efficiency in a cellular/light fidelity network is investigated. This offloading efficiency is a measure of the ratio of traffic carried by light fidelity (LiFi) network to total traffic carried by both LiFi and cellular networks. The effect of LiFi-signal blocking and channel characteristics are considered. Different LiFi offloading timing cases are discussed. Simulation is performed in order to explore the performance of opportunistic cellular/LiFi traffic offloading scenario. Simulation results reveal that LiFi-signal blocking can reduce the offloading efficiency by more than 30%.