LTV equalization of CPM signals over doubly-selective aeronautical channels

In this paper, transmission of continuous phase modulated (CPM) signals over time- and frequency-selective (i.e., doubly-selective) channels is considered. Leveraging on the well-known Laurent representation for a CPM signal, we design a two-stage receiver, composed of a linear time-varying (LTV) equalizer, followed by a detector: the former one aims at mitigating the channel dispersion and it is synthesized under the zero-forcing (ZF) or minimum mean-square error (MMSE) criterion; the latter one recovers the transmitted symbols from the pseudo-symbols of the Laurent representation in a simple recursive manner. Relying on a Basis Expansion Model (BEM) of the doubly-selective channel, we derive frequency-shift (FRESH) versions of the proposed LTV equalizers, discussing also their complexity issues and proposing simplified implementations. Monte Carlo simulation results, carried out in two typical aeronautical scenarios (arrival/takeoff and en-route), show that the proposed approach is able to work satisfactorily also over rapidly time-varying channels.

I. Introduction

Continuous phase modulated (CPM) [1] signals are widely employed for telemetry data trasmission in aeronautical applications, due to its many advantages, such as constant envelope properties, spectral efficiency, and noise robustness. Indeed, the IRIG-I06 standard [2] adopts different CPM modulation techniques, starting from legacy PCM/FM and SOQPSK, to the most advanced multi- ARTM one.