Charlotte Langlais

Linear precoding with low complexity MMSE turbo-equalization and application to the wireless LAN system

An improved turbo-equalization process is proposed for linear precoding with iterative decoding. Using recent results in turbo-equalization, we derive the equalization coefficients for an optimized preceding decoder thanks to the SINR criterion. The proposed method keeps a low complexity even with large size precoders and high order modulations. Theoretical and simulation results show that full diversity is achieved after three or four iterations over a flat Rayleigh fading channel. An application of this transmission scheme is presented in the context of the OFDM wireless LAN 802.11a. We observe a gain of several decibels compared to the conventional unprecoded transmission scheme.

Optimization of the equalization and the interleaving in turbo-equalization for a frequency-selective fading channel

An analysis of two crucial devices of turbo-equalization, the equalization part and the interleaving part, is developed for a frequency-selective fading channel, representative of an HF channel. First of all, we explain why the interference canceller of A. Gersho and T.L.Lim (see Bell System Technical Journal, vol.60, no.11, p.238-47, 1981) plays a key role in the convergence of turbo-equalization. Since this technique is an iterative process including the concatenation of an equalizer and a channel decoder, interleaving is also a very important task. Thanks to an analogy with coded modulation, we pinpoint the influence of the type of interleaving (bit or symbol) on the global performance.

Which interleaver for turbo-equalization system on frequency and time selective channels for high order modulations?

The development engineer has to face the question of where to place an interleaver in a digital communication system. In this paper we propose to solve this problem according to the channel transmission characteristics in the particular case of turbo-equalization system. Using a state of the art in coded modulations and some simulation results, we deduce the best interleaver, among the coordinate, bit and symbol interleavers, depending on the channel time selectivity. The coordinate interleaver is then proposed as a trade-off between the two latter types of interleaver.

Comparison between a hybrid digital and analog beamforming system and a fully digital Massive MIMO system with adaptive beamsteering receivers in millimeter-Wave transmissions.

Massive MIMO systems are known to be a very promising solution for future 5G systems. The attractiveness of using such a system for millimeter Wave (mmWave) transmissions comes partly from the reduced size of a many antenna base station. Furthermore, the beamforming gains that they allow is highly suited to combat the high path losses experienced at such wavelengths. Unfortunately the complexity to implement a mmWave Massive MIMO system comes with the difficulty to have a high number of stacked Radio Frequency (RF) chains. Hybrid Analog and Digital Beamforming systems have then emerged to retain a high number of antennas without as many RF chains in order to keep high beamforming gains. In this paper we first describe a User Equipment (UE) solution allowing the system to form a beam that adapts to its own movement so that it always focuses its energy toward the base station, using an on board analog array and an Inertial Measurement Unit. Then we compare the performance of the known Hybrid solution with a fully digital Massive MIMO system, having as many RF chains as the Hybrid system, but serving UEs with beamforming abilities. Mostly we emphasize how such a system allows for great flexibility and evolution, both traits being invaluable features in many future networks.