On phase-space equalization of CPM in severe intersymbol interference

Future wireless personal and sensor networks will be established by a huge number of mobile nodes with transceiver capabilities. In this context, cost aware and power efficient technologies have attracted considerable attention. A proven technology for physical layer cost reduction is the use of continuous phase modulation (CPM) and hard (amplitude) limiting receiver structures that substantially reduce the cost for the analogue domain of the receivers. The major penalty of those systems is their failure in severe intersymbol-interference (ISI) environments even in combination with maximum likelihood sequence estimation (MLSE) decoders. In this paper we present a family of new MLSE based decoders that improve the ISI performance to such an extent that receiver structures with hard limiter become an attractive alternative to the expensive linear receivers. The most powerful equalizers for such nonlinear receivers are phase-space equalizers: MLSE decoders for phase detection generally use a look-up-table (LUT) consisting of the phase values of the desired signal. The key idea of the new decoder is to use a LUT that additionally includes amplitude values associated with the phase values. Based on this information metrics with different and feasible complexity are derived. By means of simulations the performance of a GMSK transmission system is examined for a set of representative channels. Resulting bit error ratios (BER) are compared with BER for state-of-the-art decoders.