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Feedforward Carrier Recovery for Coherent Optical Communications

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2 Author(s)
Ip, E. ; Stanford Univ., Stanford ; Kahn, J.M.

We study a carrier-synchronization scheme for coherent optical communications that uses a feedforward architecture that can be implemented in digital hardware without a phase-locked loop. We derive the equations for maximum a posteriori joint detection of the transmitted symbols and the carrier phase. The result is a multidimensional optimization problem that we approximate with a two-stage iterative algorithm: The first stage is a symbol-by-symbol soft detector of the carrier phase, and the second stage is a hard-decision phase estimator that uses prior and subsequent soft-phase decisions to obtain a minimum mean-square-error phase estimate by exploiting the temporal correlation in the phase-noise process. The received symbols are then derotated by the hard-decision phase estimates, and maximum- likelihood sequence detection of the symbols follows. As each component in the carrier-recovery unit can be separately optimized, the resulting system is highly flexible. We show that the optimum hard-decision phase estimator is a linear filter whose impulse response consists of a causal and an anticausal exponential sequence, which we can truncate and implement as an finite-impulse- response filter. We derive equations for the phase-error variance and the system bit-error ratio (BER). Our results show that 4, 8, and 16 quadrature-amplitude-modulation (QAM) transmissions at 1 dB above sensitivity for BER = 10-3 is possible with laser beat linewidths of DeltanuTb = 1.3 X 10-4, 1.3 X 10-4, and 1.5 x 105 when a decision-directed soft-decision phase estimator is employed.

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Lightwave Technology, Journal of  (Volume:25 ,  Issue: 9 )