I.M.I. Habbab

ALOHA with capture over slow and fast fading radio channels with coding and diversity

The effects of capture on the average system throughput and delay performance of slotted ALOHA were analyzed for slow and fast Rayleigh fading radio channels. A short-range multipoint-to-base station packet radio network is considered. It is shown that larger capture effects and thus improved network performance can be achieved with proper choice of modulation. It is also shown that the use of simple error-correcting codes improves capture. The use of selection diversity also improves the capture effect both for fast and slow fading. It is concluded that the inverse distance variability of the received signal is the main reason for the capture effect. The Rayleigh fading alone yields a very small contribution in terms of throughput; nonetheless, it helps to stabilize the system. Numerical results are presented for a slotted ALOHA system with 50 users. It is found that the maximum average throughput can be increased from about 36% to almost 60% by using channel coding and space diversity. >

Effects of semiconductor-optical-amplifier nonlinearity on the performance of high-speed intensity-modulation lightwave systems

It is shown that when the amplifier is driven near saturation, its inherent nonlinearity causes significant bit-pattern-dependent pulse distortion, particularly in the bit-rate range between about 2 and 32 GB/s. Without proper countermeasures, this distortion can degrade system performance appreciably due to two basic mechanisms. The first, which can result in a system power penalty of as much as 10 dB, occurs in a standard decision circuit that automatically sets the threshold voltage to the average signal level, rather than in the middle of the eye opening. The second mechanism, which occurs even with the threshold set properly, is due to the nonlinear enhancement of the simple linear intersymbol interference (ISI) within the receiver filter. For example, computations of system performance at 8 Gb/s using an RC filter that gives a quite acceptable 10% of eye closure under linear conditions show that when the amplifier is driven to its saturation output power level, this mechanism causes a system power penalty of about 1 dB, which increases to about 4.5 dB when the power is doubled. Interestingly, with the proper threshold setting, an ideal integrate-and-dump receiver, which introduces no ISI, is shown to suffer no power penalty due to amplifier nonlinearity. >

Polarization-switching techniques for coherent optical communications

Several techniques which rely on forcing the polarization state of either the transmitted signal or local oscillator to vary with time in a nonadaptive manner so that polarization-sensitive performance is obtained are discussed. The proposed schemes adopt a novel approach which uses a birefringent medium (for example, high-birefringence single-mode fibers) to implement polarization switching. These techniques require only a single photodetector and give a fixed level of detection performance, with a power penalty of 3 dB relative to heterodyne detection with perfectly matched polarizations. >

Optimized performance of erbium-doped fiber amplifiers in subcarrier multiplexed lightwave AM-VSB CATV systems

The authors study the effects of noise when erbium-doped fiber amplifiers are used as signal boosters for signal distribution in amplitude modulation, vestigial-sideband (AM-VSB) subcarrier multiplexed lightwave cable television (CATV) systems. A procedure for finding the amplifier length required to achieve a desired carrier-to-noise ratio at the receiver, while maximizing the allowed post-amplifier splitting ratios and transmission losses has been developed. A simple and easy-to-use approach that circumvents the computationally intensive optimization procedure and allows the design of systems whose performance is very close to optimal is discussed. >

The side-mode-suppression ratio of a tunable DBR laser

The authors investigate, both experimentally and theoretically, how the side-mode-suppression ratio (SMSR) of a multisection multiple-quantum-well distributed-Bragg-reflector (MQW-DBR) laser varies with tuning current. It is found that as the tuning current is adjusted the wavelength and the SMSR vary, with the peak SMSR=43 dB. The output power also varies with tuning current, and one can find when the laser is near a mode hop by examination of the light versus tuning current curve. This implies that the tuning current can be adjusted so that the laser frequency is near the center of the Bragg band without measuring the SMSR. The measurement are in good agreement with theory.<<ETX>>

Noise reduction in long-haul lightwave all-amplifier systems

The overall gain of a chain of erbium-doped fiber amplifiers in a long-haul all-amplifier system would be automatically stabilized if each amplifier were operated slightly into saturation. However, with the required low level of amplifier output power which is imposed by nonlinearity in the transmission fiber, the resulting pump power becomes too low to effectively invert the gain medium of the amplifiers. Consequently, the amplifier output noise level becomes too high for proper system operation. This problem is solved by pumping the amplifiers harder so that a higher gain and higher output power are achieved. The excess gain is then counteracted by an appropriate value of post-amplifier loss. Because of the higher pump power in this case, the amplifier noise is reduced significantly. This technique is investigated theoretically, and experimental work that verifies it is reported. >

Asymmetric channel gain and crosstalk in traveling wave optical amplifiers

Previous studies have shown that in multichannel optical amplification using a traveling wave amplifier, nonlinear frequency mixing effects can arise due to the modulation of the carrier density. It was shown that in the small-signal case, the carrier density modulation leads to a weak asymmetry in the gains experienced by individual channels. The authors extend the theoretical results by including the effects of gain saturation. They find, in the two-channel case, that the gain asymmetry can be as large as 10 dB depending on the channel separation, carrier lifetime, and input powers. The authors discuss the implications of this on the choice of modulation scheme. >

Capacity of broadcast channels in the near-future CATV architecture

Reports fundamental information theoretic results for near-future CATV downstream digital distribution networks. The CATV industry is evolving to this network architecture to quickly replace plant that would otherwise be left with serious reliability problems owing to deployment of cascades of electronic amplifiers. This new architecture is composed of a passive fiber trunk feeding COAX branches altogether serving about 200 homes. Frequency-division-multiplexed digital signals subcarrier intensity modulate a laser illuminating a single-mode fiber. Only one (electronic) amplifier is used and it is located at the COAX feed. Aside from additive white Gaussian noise (AWGN) from shot noise exacerbated by the amplifier, a significant impairment is the clipping stemming from the intensity modulation. A dimensionless parameter identified as key to describing channel capacity is I/sub 0//(qFB) where I/sub 0/ (0.01 to 1 mA) is the photocurrent delivered, q is electron charge, F (10 log F=2 to 10 dB) is the electronic amplifier effective noise figure and B (200 MHz to 1 GHz) is the overall system bandwidth. The following are reported: the optimal design of the head end where signals are launched, availability of extraordinarily high capacities, and how a hefty fraction of capacity might be achieved. >

Erbium-doped fiber amplifiers: linear approximations

In this paper, we show that there is a simple linear model for an erbium-doped fiber amplifier. Specifically, we show that the amplifier gain, raised to a wavelength-dependent power, is strongly linear with respect to the amplifier-induced increase in signal power. >

Fundamental limitations in EDFA-based subcarrier-multiplexed AM-VSB CATV systems

This paper presents and discusses fundamental limitations for EDFA-based subcarrier-multiplexed AM-VSB CATV distribution systems. The impairments we consider are fundamental, namely, shot noise, amplifier spontaneous emission noise, and laser, clipping-induced, nonlinear distortion. Treating the amplifier input signal power and pump power as resources, we obtain limits on the number of receivers to which we can deliver, say, 60 (or 80) channels at a carrier-to-noise ratio (CNR) of 55 dB (or 48 dB). Moreover, we present a simple approach that can be used to recalculate the results presented here for different system and amplifier fiber parameters.