CPM Equalization of ISI due to Band-Limiting

A. Moctezuma, T. Ikuma, and A.A.L. Beex (USA)


CPM, MLSE, Equalization, Group delay, Channel truncation


We examine and compare three approaches to the problem of equalization of band-limited CPM signals, with a view towards practical implementation. The performance benchmark approach is to use a max likelihood sequence estimator for equalization (MLSE). The first alternative approach – used in conjunction with MLSE – is to shorten the channel impulse response so that a low complexity MLSE equalizer can be utilized. Thereto the channel truncation structure by Falconer and Magee is adapted to CPM signals, and uses a pre-filter to shape the overall response of the channel to a desired impulse response of short duration. Simulation results show that near MLSE performance can be obtained while reducing the complexity of the MLSE equalizer. In the second alternative approach, we focus on eliminating the group delay variations inside the channel passband using an FIR pre-filter, before MLSE processing. Assuming the channel to be time-invariant, an FIR group delay compensator is designed so that the resulting group delay is more constant over the filter passband. Simulation results show that reducing the channel passband group delay variation reduces the amount of ISI and improves the resulting bit error rate (BER) performance. The group delay compensation approach is simpler than channel shortening, while approaching it in terms of BER performance.

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