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In this paper, we describe a novel update algorithm for the filter coefficients of the adaptive digital equalizer in coherent receivers, which is based on error signals evaluated in Stokes space and is insensitive to both phase-noise and frequency-offset. We also introduce an optimized decision rule in the Stokes space, which takes into account the exact statistics of noise and yields a performance improvement with respect to the minimum distance decision criterion. We compare the performance of the new algorithm to the standard constant-modulus algorithm (CMA) for polarization-multiplexed 16QAM modulation, achieving similar performance in the absence of phase noise, with comparable complexity. Differently from CMA, the proposed Stokes-space algorithm allows us to remove the phase offset between polarizations, thus enabling the use of a joint carrier-phase estimation algorithm on both polarizations, which in turns yields a nearly doubled phase noise tolerance.

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Absrrucr -In this paper we discuss multidimensional signaling for transmission over thesatellite channel, Le. , a bandiimited radio channel with nonlinearities. We derive a set of basis signals that are orthogonal and whcse bandwidth occupancy is minimal, in the sense that they maximize the power Inside a fixed frequency interval (-B, B). Since we remove the assumption that the signals be confined In one symbol interval, the concept of orthogonality must be revisited. We design basis signals that satisfy the following requirements. (a) When passed through their matched filter, the sample sequence has no intersymbol interference; (b) when passed through the filter matched to another hasis signal, the sample sequence has no interchannel interference, and (c) their bandwidth occupancy is minimum.The performance of four-dimensional signals designed according to this criterion is evaluatedin terms of bandwidth occupancy and error probability over nonlinear satellite channels. Comparisons with two-dimensional modulation schemes (in particular, QPSK) are also made.