Demodulation

Spectrally-efficient optical communications systems employ polarization division multiplexing (PDM) as a practical solution, in order to double the capacity of a fiber link. Polarization demultiplexing can be performed electronically, using polarization-diversity coherent optical receivers. The primary goal of this paper is the optimal design, using the maximum-likelihood criterion, of polarization-diversity coherent optical receivers for polarization-multiplexed optical signals, in the absence of polarization mode dispersion (PMD). It is shown that simultaneous joint estimation of the symbols, over the two received states of polarization, yields optimal performance, in the absence of phase noise and intermediate frequency offset. In contrast, the commonly used zero-forcing polarization demultiplexer, followed by individual demodulation of the polarization-multiplexed tributaries, exhibits inferior performance, and becomes optimal only if the channel transfer matrix is unitary, e.g., in the absence of polarization dependent loss (PDL), and if the noise components at the polarization diversity branches have equal variances. In this special case, the zero-forcing polarization demultiplexer can be implemented by a 2 2 lattice adaptive filter, which is controlled by only two independent real parameters. These parameters can be computed recursively using the constant modulus algorithm (CMA). We evaluate, by simulation, the performance of the aforementioned zero-forcing polarization demultiplexer in coherent optical communication systems using PDM quadrature phase shift keying (QPSK) signals. We show that it is, by far, superior, in terms of convergence accuracy and speed, compared to conventional CMA-based polarization demultiplexers. Finally, we experimentally test the robustness of the proposed constrained CMA polarization demultiplexer to realistic imperfections of polarization-diversity coherent optical receivers. The PMD and PDL tolerance of the proposed demultiplexer can be used as a benchmark in order to compare the performance of more sophisticated adaptive electronic PMD/PDL equalizers.

In this paper, a single element receiver for short-range shallow water communication is proposed as an alternative to costly beamformer structures by utilizing state-of-the-art iterative demodulation techniques. The proposed receiver structure employs an adaptive decision feedback equalizer (DFE) and bit interleaved coded modulation with iterative decoding (BICM-ID) in conjunction with adaptive Doppler compensation. The performance of the proposed receiver is compared with an iterative receiver employing turbo-coded BICM (turbo-BICM) and a noniterative receiver employing a Reed-Solomon (RS) code. Experimental results show that the performance of a coding scheme strongly depends on the ability of the adaptive DFE to equalize the time-varying channel. It is also shown that under severe channel conditions the RS code, which is capable of correcting burst errors, fails to give acceptable performance even in the case where perfect decisions were fed back into the DFE. It is, therefore, established that the BICM-based receivers, i.e., turbo-BICM and convolutional BICM-ID, outperform the non-BICM-based receiver. Furthermore, it has been established that there exists a fundamental tradeoff between the Doppler correction factor and the receiver performance. In practice, a large Doppler correction factor is required to compensate for severe Doppler effects, however, it leads to noise enhancement in the system. The critical result is that although the proposed convolutional BICM-ID receiver matches approximately the performance of turbo-BICM, it exhibits lower implementation complexity and reduced memory requirements, attributes that are attractive for robust high data-rate underwater acoustic modems.