A reduced complexity equalizer for OQPSK

In this paper a reduced complexity equalizer for Offset-QPSK (OQPSK), GMSK, or any other staggered modulation formats is presented. It is known that for OQPSK, due to the relative time offset of the in-phase and quadrature components, the received signal must be sampled at twice the baud rate of its corresponding non-staggered signal. This doubles the number of equalizer coefficients required, relative to QPSK, where using a symbol-spaced equalizer is an option. In this work it is shown that it is possible to implement an equalizer, for OQPSK, with about half the complexity normally required for the linear equalizer or for the feed forward filter in Decision Feedback Equalizer. By this, an option for a symbol-spaced equalizer for OQPSK is actually introduced for the first time. Both MMSE and DFE are treated in this work. We test the sensitivity to timing and propose a novel matched filter approximation that avoids the need of timing compensation.