To achieve the maximum information transfer and face a possible eavesdropper, the samples transmitted in continuous-variable quantum key distribution (CV-QKD) protocols are to be drawn from a continuous Gaussian distribution. As a matter of fact, in practical implementations the transmitter has a finite (power) dynamics and the Gaussian sampling can be only approximated. This requires the quantum protocols to operate at small powers. In this paper, we show that a suitable probabilistic amplitude shaping of a finite set of symbols allows to approximate at will the optimal channel capacity also for increasing average powers. We investigate the feasibility of this approach in the framework of CV-QKD, propose a protocol employing discrete quadrature amplitude modulation assisted with probabilistic amplitude shaping, and we perform the key generation rate analysis assuming a wiretap channel and lossless homodyne detection.

In coherent optical systems, the phase noise due to transmit and receive lasers has a significant impact on the receiver performance - the sensitivity to phase noise and the capability of electronic processing to compensate for its effects are reduced for increasing lasers'' linewidths. However, a detailed analysis of the effects of phase noise on the system performance cannot leave other aspects out of consideration. In fact, the picture is completely different for optical systems using single-carrier or orthogonal frequency division multiplexing, in the presence or absence of inline dispersion compensation, or when an unmodulated carrier is also transmitted for phase noise compensation purposes. In addition, transmit and receive phase noise components may have a different impact. All these aspects are analyzed and discussed in this paper. A novel digital coherence enhancement technique, able to significantly reduce the phase noise of transmit or receive lasers by using an interferometric device plus a very simple electronic processing, is also proposed.

Orthogonal frequency division multiplexing (OFDM) and single-carrier (SC) data transmission are compared as potential candidates for the next generation coherent long-haul 100 Gbps optical communication systems. The comparison takes into account impairments either general or mainly related to coherent detection, e.g., group velocity dispersion (GVD), polarization mode dispersion (PMD), non-linear effects, phase noise due to transmit and receive lasers, uncompensated frequency offset, other synchronization aspects, overall complexity, power and spectral efficiency, technological constraints.

We propose a novel simulation-based method to evaluate the performance of Viterbi decoders. In particular, we address scenarios where the error probability is very low, that is, scenarios where classical Monte Carlo simulations would require impractical execution times before producing reliable results. As other recent fast-simulation approaches, the proposed method relies on the multicanonical Monte Carlo technique, but, unlike the existing general-purpose methods, it is specifically designed for Viterbi decoders, the algorithm being driven by a control variable that depends on the state metrics of the various survivors over the trellis. In simple scenarios for which analytical tools are available, the simulation results agree with them, while, in the most common scenarios, no analytical tool is available and the proposed method gives the fastest way for the estimation of low error probabilities.

The optimal receiver processing based on maximum-likelihood sequence detection (MLSD) is employed with high-order phase and amplitude modulations in optical transmission systems affected by typical channel impairments such as group velocity dispersion (GVD), polarization mode dispersion (PMD), and phase uncertainties due to phase noise. A couple of widely known front ends, with proper modifications, can be used to extract the required sufficient statistics from the received signal. Receiver adaptivity and complexity reduction are also discussed. It is demonstrated that, as long as a sufficient receiver complexity is employed, GVD and PMD entail no performance degradation with respect to the case of no channel distortions (the back-to-back case).

We present a theoretical analysis of the performance of coherent optical FSK systems when the driving laser signal is AMI or Manchester (biphase) line-coded to counteract the nonideal FM characteristic of the transmit laser diode, and the received signal is heterodyned and detected by a delay-and-multiply demodulator. The analysis takes into account IF filtering by assuming linear filtering of the noisy signal phase, accounting, for small linewidths, for laser phase noise in a straightforward and accurate manner. A simple equivalent baseband model of the system is derived for performance evaluation, applicable to both cases of large demodulator delay and when the delay tends to zero (ideal discriminator). Noise statistics include the clicks due to both signal and phase noise. The problem of performance evaluation is reduced to a classical intersymbol interference problem which is solved by mead of the method of Gauss quadrature rules. The analysis accurately predicts the effectiveness of AMI and Manchester line coding, depending on several system parameters such as linewidth, modulation index, IF bandwidth and laser FM response, and can be easily extended to other line coding techniques.>

The probability distribution function of the FM clicks is derived for the case of a carrier impaired by Wiener-Levy phase noise and detected by a limiter-discriminator receiver with pre- and post-detection filtering. The analysis is performed accounting for the effects of predetection (RF) filtering on signal and phase noise and holds for /spl Delta//spl nu//B/spl Lt/1, /spl Delta//spl nu/ being the 3-dB linewidth of the unmodulated carrier and B the bandwidth of the RF filter. How small /spl Delta//spl nu//B is to be will depend on the kind of RF filter. For the raised cosine filter with small rolloff /spl epsiv/ considered here the analysis holds for /spl Delta//spl nu//B>

A detailed theoretical analysis, based upon Rice's (1963) theory of clicks, for evaluating the error performance of heterodyne optical FSK receivers with limiter-discriminator detection and pre- and integrate-and-dump (I&D) post-detection filtering is developed, accounting for laser phase noise, intersymbol interference (ISI) and intermediate-frequency (IF) filtered carrier envelope fluctuation effects. From the analysis it turns out that the envelope fluctuations induced by phase noise may produce, for operating IF bandwidths, a much smoother transition to the bit error rate floor than expected when considering IF infinite bandwidth, thereby generating performance degradation right in the operating signal-to-noise ratio (SNR) region. The contribution to this degradation due to a change in the click statistics generated by the envelope fluctuations is found to be negligible. From the analysis, it is also found that the floor gets lower and approached more rapidly as the modulation index and the bandwidth get larger, until the known floor for infinite IF bandwidth is reached. Finally, general curves are presented for system performance relating bit error probability to optimum IF bandwidth, optimum modulation index, laser linewidth and receiver sensitivity. These curves represent the ultimate limit to performance of a heterodyne optical FSK receiver with limiter-discriminator detection and pre- and I&D post-detection filtering.>

Several strategies for combining convolutional codes (CCs) and pulse position modulation (PPM) in a direct-detection optical channel are considered, including binary and 2/sup n/-ary PPM with rate-1/n CCs, 2/sup m/-ary PPM with a dual-m CC, and interleaved 2/sup L/-ary PPM with a rate-1/n CC. In the latter case, J.L. Massey's (1981) concept of coding of L separate component channels constituting a 2/sup L/-ary PPM erasure channel is carried out for both the ideal photon counting and the avalanche photodetection (APD) cases, providing a comparative evaluation of the various strategies in terms of bit error probability and achievable coding gain for different code structures and decoding schemes.>