On the Derivation of the Ungerboeck Observation Model for Offset QPSK

This report derives maximum likelihood sequence estimator for Offset QPSK (OQPSK) operating over frequency selective channel. The maximum likelihood sequence estimator takes the form of the Viterbi Algorithm operating on a trellis defined intersymbol interference caused by the frequency selective channel. Because the distorted pulse shape does not satisfy the Nyquist no-ISI criterion, the matched filter output samples contain correleted noise. The derivation uses Ungerboeck’s method to create a recursive causal metric suitable for use with the Viterbi Algorithm.