Didier Pirez

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.

Practical implementation of a multichannel equalizer for a propagation with ISI and CCI-application to a GSM link

For radiocommunication channels with propagation multipaths, the presence of intersymbol interference (ISI) at reception must be mitigated by an equalizer. Furthermore, when the link is perturbed by cochannel interference (CCI), the use of an adaptive array at reception may be very useful. In such contexts, it has been shown that the maximum likelihood sequence estimation (MLSE) multichannel receiver is composed of a spatio-temporal matched filter (STMF) followed by a symbol rate sampler and the MLSE-based modified Viterbi algorithm for a mono channel receiver. However, this optimal receiver has not been implemented in a practical context. The purpose of this paper is to fill the gap previously mentioned by introducing a new multichannel receiver, which is an implementation of the optimal one in temporally white noise (TWN). The performance of this new receiver is then analyzed in the context of a GSM link with ISI and CCI.

Joint equalization and decoding: why choose the iterative solution?

This paper deals with turbo-equalization as a joint equalization and decoding algorithm. The performance analysis shows that there is a trigger point in this iterative process, followed by a breakdown effect. Actually after a given point (the trigger one), the BER decreases steeply as a function of the decoding step p. We compare the performance of the turbo-equalizer with that of the optimal joint receiver and show that it can match the bound over the optimal disjoint receiver.

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.

Multi-user detection applied to GSM communications

In this paper we propose to apply to the GSM system multi-user detection (MUD) techniques generally used in CDMA networks. The constraint of using the GSM wave form makes it impractical to use the optimum receiver due to its high complexity. Thus, in this paper, we derive the multi-user sub-optimum DFE-MMSE adapted to the GSM wave form. It relies on a linear approximation of the GSM modulation leading to a staggered modulation and thus requires a specific, (and multi-user in our case) equalizer to achieve good performances (Tu 1993). Not taking into account the fact that the modulation is staggered (as in Tripathi et al. 2000) leads to poor performances. The symbol detection is made possible by means of multiple antennas at the receiver. This technique is actually being tested on real data for underground transmissions in an IST funded project called ESCORT.

Low-cost Turbo receiver for coded asynchronous DS-CDMA in frequency selective channels

When channel coding is used on a CDMA transmission, the most appropriate approach for reception is joint Multiuser detection and decoding. This joint problem can be successfully processed by the Turbo-techniques. Therefore, we consider a low-complexity iterative receiver for the CDMA uplink transmission, called Turbo-CDMA, and we propose to further improve it. First, we derive a new DFE to be used at the first iteration. Then, we propose to fully use the decoded information in a new structure called Full Turbo-CDMA. Finally, we derive a new demodulation order that is well suited to the Turbo-CDMA receiver. This improved receiver, called Full Turbo-CDMA, aims to eliminate MUI and ISI almost completely in the presence of frequency selective channels. Improved performance is illustrated by simulations.

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.

Iterative multi-user algorithm for convolutionally coded asynchronous DS-CDMA systems: turbo-CDMA

A low-complexity and iterative receiver is proposed for the CDMA uplink, consisting of a low-complexity equalizer and single-user decoders. Iterations are used to feed the equalizer with more reliable data, which improves performance. This receiver called turbo-CDMA is aimed to eliminate MUI almost completely in the presence of multipath and to be robust to the near-far effect. Simulation results show that the performance approaches single-user performance at moderate and also low signal-to-noise ratios, even for a severe distortion channel.

Dynamic frequency allocation in ad hoc networks

This paper presents a new frequency allocation scheme for ad hoc networks. Unlike other proposals, the scheme allocates frequencies to groups of nodes, for their intra-group communication needs, in order to guarantee a high level of availability of the needed frequency resources. This scheme uses a distributed decision process with interference measurements and a simple collaboration protocol between groups. Performance figures concerning the optimality of the obtained solution and the convergence time are given. The paper also presents a work that has been done to simulate this allocation scheme in a realistic environment with a mobility scenario on a 3D terrain model. Finally some ideas for future work are proposed.