Syndrome-based Viterbi decoder node synchronization and out-of-lock detection

A method for joint Viterbi decoder node synchronization and out-of-lock detection which applies to trellis codes with a linear encoding rule (such as convolutional codes, Ungerboeck codes, etc.) is presented. Quadrature phase shift keying (QPSK) transmission combined with a transparent rate 1/2 convolutional code is studied; in this case node synchronization corresponds to 90 carrier phase ambiguity resolution. Usually, node synchronization is derived from the metric growth of the most likely path at the Viterbi decoder. These conventional methods are not optimal and become unreliable at low E_b/N_o. Moreover, they are not well suited for out-of-lock detection. The proposed method provides both node synchronization and out-of-lock detection by processing, in an optimum way, the receiver´s hard decisions: this involves syndrome (parity check) sequence computations. Computer simulations show good performance, even at low E_b/N_o values (0 dB -2.3 dB). This has also been verified experimentally by measurements on a 300 Mb/s QPSK modem for satellite communication