Pulse Position Modulation Signals Research Articles

Word timing recovery in direct detection optical PPM communication systems with avalanche photodiodes using a phase lock loop

A technique for word timing recovery in a direct detection optical pulse position modulation (PPM) communication system is described. It tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase locked loop. The experimental system consisted of an AlGaAs laser diode transmitter ( lambda =833 nm) and a silicon avalanche photodiode photodetector, and its used Q=4 PPM signaling at a source data rate of 25 Mb/s. The mathematical model developed to characterize system performance is shown to be in good agreement with the experimental measurements. Use of this recovered PPM word clock, along with a slot clock recovery system described previously, caused no measurable penalty in receiver sensitivity when compared to a receiver which used common transmitter/receiver clocks. The completely self-synchronized receiver was capable of acquiring and maintaining both slot and word synchronizations for input optical signal levels as low as 20 average detected photons per information bit. The receiver achieved a bit error probability of 10/sup -6/ at less than 60 average detected photons per information bit. >

Interleaved convolutional coding for the turbulent atmospheric optical communication channel

The coding gain of a constraint-length-three, rate one-half convolutional code over a long clear-air atmospheric direct-detection optical communication channel using binary pulse-position modulation signalling is directly measured as a function of interleaving delay for both hard- and soft-decision Viterbi decoding. Maximum coding gains theoretically possible for this code with perfect interleaving and physically unrealizable perfect-measurement decoding were about 7 dB under conditions of weak clear-air turbulence, and 11 dB at moderate turbulence levels. The time scale of the fading (memory) of the channel was directly measured to be tens to hundreds of milliseconds, depending on turbulence levels. Interleaving delays of 5 ms between transmission of the first and second channel bits output by the encoder yield coding gains within 1.5 dB of theoretical limits with soft-decision Viterbi decoding. Coding gains of 4-5 dB were observed with only 100 mu s of interleaving delay. Soft-decision Viterbi decoding always yielded 1-2 dB more coding gain than hard-decision Viterbi decoding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Split analog-to-digital converter for intensity-modulated optical fiber sensors

A technique is described which provides a linear pulse-position modulated signal from an intrinsically analog remote optical sensor having a nonlinear response to the sensed parameter. The technique is applied to a differential absorption optical liquid level sensor. Sensor and system performance are presented.

Duty Cycle Effects On Intensity And Wavelength Fluctuations In A Pulsed Variable Spacing Phase-Locked Laser Diode Array

Wavelength and intensity fluctuations in the output of a variable spacing phase-locked laser diode array were investigated. The diode was pulsed at duty cycles appropriate for low order optical pulse position modulation signaling. Both the spectral width and intensity fluctuation levels were observed to increase as the duty cycle was increased.

Direct-Detection Optical Communication with Color Coded Pulse Position Modulation Signaling

The performance characteristics of a direct-detection optical communication system, in which a laser transmitter sends short optical pulses of selected nonoverlapping center frequencies, are analyzed. It is shown that this signaling scheme can be used in conjunction with ReedSolomon coding to obtain high data rate, reliable communication under conditions of optimal energy efficiency.

Optical data storage using PPM and PWM

A method for analyzing the binary signal produced by pulse position modulation (PPM) and pulse width modulation (PWM) is presented. The results are used to study the conversion of analog signals into binary waveforms. The Fourier transform is a complex analog signal. A method of using a combination of PPM and PWM to make binary Fourier transform holograms of digital data is presented. Two optical methods for reading out the optically recorded PPM signals are discussed. Experimental results demonstrate the different aspects of these modulation techniques for optical storage.

The Design of a Pulse-Position Modulated Optical Communication System

In recent literature the advantages of an idealized narrow-width pulse-position modulated (PPM) optical communication system, using coherent sources and direct photodetection, have been shown. In this paper the practical design of such an operating PPM link is considered. System performance in terms of error probabilities and information rates is derived in terms of key parameters, such as power levels, number of PPM signals, pulse width, and bandwidths. Both background radiation and receiver thermal noise are included. Design procedures utilizing this data are outlined. Whenever possible, optimal design values and parameter tradeoffs, in terms of maximizing information rate or minimizing transmitter power, are shown. The effect on performance of photomultipliers and their inherent statistics is also presented. Although the basic analysis is derived in terms of photon counts, the necessary system optics equations are introduced to allow for overall optical hardware design. The primary underlying assumption is that synchronization is maintained at all times between the transmitter and receiver.

A PPM/PM Hybrid Modulation System

The objective of a pulse-modulation (PM) system is to yield as high as possible an output signal-to-noise ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0}</tex> for a given channel signal-to-noise ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0}</tex> and bandwidth expansion factor β. In particular, one would like to design a modulation system in which the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0}</tex> approaches that of the Shannon bound <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0} \leq [1 + (S/N)_{c}\beta^{-1}]^{\beta}</tex> . For a fixed system such as the pulse-position modulation (PPM), the relationship between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{c}</tex> , must be linear above threshold. Thus to approach the β-power behavior of the Shannon bound, we must consider a family of PM systems. A class of systems is analyzed in which phase modulation of varying modulation indices is amplitude modulated by a PPM signal of fixed bandwidth. I t i s shown that when <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{c}</tex> , is large, the performance is proportional to the square of the modulation index which can be increased without changing the signal bandwidth. However, as the modulation index is increased, more sidelobes appear in the output of likelihood receiver which lead to large so-called anomolous errors. The effect of the sidelobes can be decreased significantly by redesigning the PPM signal since its autocorrelation function amplitude modulates the sidelobes. A new upper bound on the mean-squared error is derived which takes into account the effects of the noise at these sidelobe peaks. The bound depends functionally on the waveform which is used in the PPM modulator, and this raises the question as to which signal is the best one to use. An optimization problem is formulated using state-space techniques and solved numerically using a known algorithm. It is shown that it is possible to reduce the threshold significantly using optimally designed waveforms. The Barankin bounds are applied to the PPM/PM waveform to give lower bounds on the mean-squared error. It is therefore possible to bracket the actual performance region of the PPM/PM modulation system. The lower bounds lead to upper bounds on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S/N)_{0}</tex> and a sequence of bounds, each representing the best performance possible for a given modulation index, is obtained. The envelope of these bounds is then compared with the Shannon bound, and it is shown that although the PPM/PM modulation does not have performance closely approaching the Shannon bound, it does represent a substantial improvement over straight PPM.