Optimization of level spacing in quaternary optical communication systems

Abstract

In this work, the optimization of level spacing in terms of the extinction ratio variation in quaternary optical communication systems is investigated, for signal-dependent noise dominance. It is shown that the performance of optically preamplified quaternary communication systems can be optimized in terms of levels spacing using very simple formulas, also derived in this work. For the null extinction ratio, sensitivity improvements of about 6 dB can be achieved by proper location of signal levels in comparison with the sensitivity of equally spaced levels. At high extinction ratio, this improvement is not so significant.

Spectral Efficiency Limits and Modulation/Detection Techniques for DWDM Systems

“…While an amplified signal has Poisson statistics, ASE noise obeys Bose-Einstein statistics. For a signal having signal photons and ASE photons, the ASE noise is equivalent to Poisson-distributed light where the mean number of photons has the distribution (14) and the output photon-number distribution is given by [22], [53] (15) where is the Laguerre polynomial…”

“…Setting the 1 decision thresholds at the geometric means of pairs of adjacent levels approximately equalizes the downward and upward error probabilities at each threshold. In order to equalize the error probabilities at the 1 different thresholds, the intensity levels should form a quadratic series [14], i.e., 0, 1, 4, 9, etc. Under these assumptions, and assuming Gray coding, it is easily shown that the BER is approximately given by (27) For , (27) indicates that is required for , which is lower by 0.2 dB than the requirement .…”

“…In this paper, we compare the spectral efficiencies and power efficiencies of several techniques, assuming in all cases that ASE is the dominant noise source. Techniques considered include pulse-amplitude modulation (PAM) with direct detection [13], [14], differential phase-shift keying (DPSK) Fig. 1.…”

Quaternary amplitude optical packets generated by four-wave mixing: Power level optimization

“…Other papers [17][18][19] presented a BER minimization of quaternary amplitude-shift keying (4-ASK) signals that were obtained by using an electronic quaternary amplitude signal to modulate an optical carrier. This differs from the situation analyzed here for two reasons.…”

“…First, in the 4-ASK case it is assumed that the power level distribution may be arbitrarily positioned whereas (4) indicates that this is not possible for the QAOP generated by the FWM. Second, in [17][18][19] a single BER is associated with the 4-ASK signals; however, when we are dealing with QAOPs we should evaluate two BERs, one concerning to the label and the other regarding to the payload. This will be approached in sub-section 3.2.…”