Near-optimal frame synchronization for free-space optical packet communications

In this paper, we present optimal and near-optimal frame synchronization algorithms for a packet-based free-space optical communications system. Optimal preamble detection in the presence of random data was first studied by Massey for radio frequency phase-shift keying modulation over an additive white Gaussian noise channel with statistically independent noise. That analysis demonstrated, perhaps somewhat counterintuitively, that correlation against the known preamble is suboptimal and that the degradation from optimal performance can be severe. Subsequent researchers following Massey's approach have developed algorithms that apply to optical communication systems with pulse-position modulation over a Poisson channel, with similar results. In this paper, we derive the optimal maximum likelihood metric for an FSO system having an on-off keying transmitter and an optically amplified p-i-n photodiode receiver, for which the link can be well modeled by signal-dependent additive white Gaussian noise. We then develop approximations suitable for very high-speed implementation at typical FSO line rates. In addition, we discuss the problem of preamble design and present sequences that enable efficient frame synchronization. Simulation trade studies using link budget parameters representative of typical optical transceivers demonstrate the excellent performance of the proposed preambles and synchronization algorithms.