Abstract Efficient quantizer design is vital in high‐speed optical receivers. In this paper, the authors propose a three‐level (1.5‐bit) reliability‐based quantizer for low‐complexity soft decoding of low‐density parity‐check (LDPC) codes, and show that it outperforms a conventional quantizer employing eight levels (3 bits). Different approaches for obtaining the quantization thresholds are discussed, and it is shown that the achievable channel capacity with the proposed quantizer is within 0.4 dB of the Shannon limit of the unquantized channel. Rate‐compatible LDPC codes are also designed tailored to the asymmetric optical channel terminated with the proposed reliability‐based quantizer, and it is shown that the resulting quantizer‐aware LDPC codes significantly outperform other existing codes of similar rates.

In this study, the authors address the spreading sequence estimation in direct‐sequence spread‐spectrum signals. At first, the maximum‐likelihood (ML) estimator is derived. In order to alleviate the greater computational complexity of the ML estimator, an innovative algorithm based on the ML method is proposed. The authors’ proposed algorithm uses an initial estimation with low complexity and low estimation accuracy as a result. In the second step, the estimation accuracy increases using the ML decision rule. They analyse their proposed algorithm and they derive an analytical approximation for the error probability of the proposed suboptimal algorithm. Simulation and analytical results show great performance and acceptable complexity of the proposed method.

In the above titled paper (ibid., vol. 58, no. 5, pp. 1589-1598, May 10), Fig. 4 was reproduced incorrectly. The correct Fig. 4 is presented here.

Autocorrelation receiver (AcR) is usually used as the general receiver for transmitted reference (TR) communication systems on a quite intuitive basis. In this paper, we apply the principle of statistical invariance to obtain the optimum TR receiver on a decision-theoretic basis, and it is shown that the autocorrelation receiver is optimal only in a special case. Performance of the optimum receiver and the suboptimum AcR for some important modulation schemes is also evaluated. The results imply that allocating a part of the transmitted energy and bandwidth to reference pulses would not help improve the performance of the system and the optimal performance might be achieved via much simpler non-TR transceiver structures. However, it is shown that differential TR and some other generalizations might be used to boost the performance, and numerical results are provided to assess the degree of performance improvement achieved in each case.

Detection of ultrashort light pulses with conventional bandlimited photodetectors can lead to substantial performance degradation in digital lightwave communication systems. All-optical nonlinear processing of the received field prior to the optical to electrical conversion can greatly overcome this problem. In this letter, through a decision-theoretic framework and using a basic invariance property of the photodetector, we obtain the optimal zero-memory nonlinear element and evaluate its performance. Possible suboptimum nonlinearities are also investigated and their performance is examined. The results in this letter strongly justify the intuitive use of power-law devices, such as those implemented via two photon absorption (TPA) or second harmonic generation (SHG), in the detection of ultrashort light pulses and show how by using them almost optimal performance can be achieved.

In this paper we study the performance of spectrally-encoded spread-time (SE/ST) technique when applied to ultrawideband (UWB) signals in the context of wireless code-division multiple-access (CDMA) communication. In this technique UWB pulses are spectrally encoded with pseudorandom codes that are assigned uniquely to each user. We shall show that the SE/ST ultrawideband system provides an appropriate multi-user capacity in realistic UWB channels, along with the ability of narrowband interference (NBI) suppression. To this end, after obtaining a suitable statistical model for spectrally encoded spread-time signals, we investigate the performance of the system in the presence of multiple-access and narrowband interferers, and simple and accurate approximations will also be developed which are in good agreement with simulation results. Furthermore, the effect of variations of the main system parameters and some of the relevant trade-offs will be numerically demonstrated.