Channel equalization with staggered modulation formats

The channel equalization problem for staggered (or offset) modulation formats is considered. It is shown that, 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 the corresponding non-staggered signal. This suggests that perfect zero-forcing equalization may be achievable as in standard fractionally sampled systems. To explore this feature, the MMSE linear equalizer (LE), which turns out to be a periodically time-varying filter, is obtained. It is shown that the MMSE LE can be implemented with an LTI (linear time invariant) filter and a set of periodic switches and modulators. Using this, necessary conditions for zero-forcing equalization with FIR filters are derived.