16-DAPSK innovations-based sequence detection for satellite communications with imperfect carrier synchronization

The adoption of spectrally-efficient modulation in future satellite standards has to take into account the increased sensitivity to carrier synchronization, the nonlinear distortion caused by high-power amplifiers and the need for a precise decision threshold adjustment. These issues are considered in the proposed receiver. We use a 16-DAPSK (differential amplitude phase shift-keying) scheme with one bit absolutely encoded in amplitude and the other two bits differentially encoded in phase by an 8-state Ungerboeck trellis-code. Sequence detection is performed by the Viterbi algorithm with the branch metrics being computed from the innovative processes of a bank of stochastic nonlinear filters. These filters estimate the fading and the phase drift induced by the Doppler effect and/or oscillator instabilities, thus avoiding the need for a pilot carrier (or pilot symbols) and an autonomous decision threshold mechanism. Simulations show that the receiver is robust to fair amounts of fading and phase drift within the symbol interval, being suitable for nongeostationary-satellite communications with nonlinear amplifiers.