Pierre Vila

MMSE antenna diversity equalization of a jammed frequency-selective fading channel

For digital radiocommunications systems operating on a jammed frequency-selective fading channel, the receiver performance can be improved by using the joint antenna diversity and equalization techniques to combat both time- and frequency-selective fades and jammers effects. The optimum, in the sense of minimum mean-squared error (MMSE), structure of the linear equalizer (LE) and the decision feedback equalizer (DFE) for coherent receiver antenna diversity has been derived for an unjammed environment. The structure of the MMSE antenna diversity equalizer is determined for a jammed multipath channel. We show that is composed of the multidimensional matched filter (MMF) followed by a symbol-rate sampler and the MMSE LE or DFE equalizer for single antenna receivers.

Improved interference cancellation for turbo-equalization

Sub-optimal joint equalization and decoding is performed by iterated equalization and decoding. This processing is named turbo-equalization with reference to the turbo-decoding of serially concatenated codes. We propose to optimize the equalizer structure (an interference canceler) thanks to the training sequence available to estimate the channel impulse response. The interference canceler, optimized at each iteration, permits one to reduce the number of iterations needed to achieve a given performance. The gain is all the more important that the channel is hard to equalize.

MMSE cyclic equalization

In digital communications, conventional linear and decision feedback equalizers are optimal for stationary signals. However, modulated signals are polycyclostationary (PCS): their correlation functions are time-polyperiodic. Cyclic equalization structures optimized by the MMSE (minimum mean square error) criterion are proposed. The proposed structure is based on a multidimensional input. The latter is built from the received signal possibly shifted by its cyclic frequencies. We show by simulation that the proposed cyclic structures give better performance in terms of binary error rate than the conventional equalizers. The paper is organized as follows. First, we review some background topics on cyclostationarity and optimum filtering of PCS signals. Then we calculate the expressions of the forward and backward filters for the cyclic DFE and deduce the expression of the linear cyclic equalizer. Finally, simulation results of conventional and cyclic equalizers are given for BPSK and MSK signals jammed by other modulated signals. The new cyclic structures are particularly suitable for FDMA networks disturbed by adjacent channel interference (ACI). They can lead to a significant improvement in the number of communication channels within a given bandwidth. Besides, cyclic structures can also be used for transmission links disturbed by jammers that are modulated signals with the same or different baud rates.

New studies about a high data rate HF parallel modem

This paper presents a new 9600 bps high data rate HF modem. This modem uses a coherent parallel modulation thanks to the addition in the waveform of known pilot tones, the channel as well as the the noise on each tone can be estimated during demodulation. The coding scheme uses the turbo-code principle. This new coherent parallel modem offers an interesting and extremely competitive solution for a high data rate HF modem in a frequency selective fading channel.