Carlo Luschi

Nonparametric trellis equalization in the presence of non-Gaussian interference

We consider the problem of trellis equalization of the intersymbol interference channel in the presence of thermal noise and cochannel interference (CCI). Conventional maximum-likelihood sequence estimation (MLSE) and maximum a posteriori probability (MAP) trellis equalizers treat the sum of noise and interference as additive white Gaussian noise, while CCI is generally a colored non-Gaussian process. We propose a novel nonparametric approach based on the estimation of the probability density function of the noise-plus-interference. Given the availability of a limited volume of data, the density is estimated by kernel-smoothing techniques. The use of a whitening filter in the presence of temporally colored disturbance is also addressed. Simulation results are provided for the global system for mobile communications (GSM), showing a significant performance improvement with respect to the equalizer based on the Gaussian assumption. Major advantages of the proposed strategy are its intrinsic robustness and general applicability to those cases where accurate modeling of the interference is difficult or a model is not available.

Advanced signal-processing algorithms for energy-efficient wireless communications

Substantial progress has been made in the receiver signal-processing algorithms for wireless communications to minimize the requirements on signal-to-noise (and/or interference) power ratio and computational complexities for the same quality of service. In cellular infrastructure systems, one of the key system design objectives in the base stations is to maximize the receiver sensitivity, so that the required signal level from the mobile stations can be minimized. The use of advance signal-processing algorithms, based on maximum a posteriori (MAP) estimation, iterative (turbo) channel estimation, equalization, and decoding, allows for a reduction of the required transmitter power by one-third to one-half. Lower computational complexities in the terminals, which implies a reduced power drain on the digital circuits, can be achieved by using techniques that adapt the state complexity of the receiver to the propagation channel. We give an in-depth review of these algorithms, and discuss their performance and implementation requirements.

Low complexity source controlled channel decoding in a GSM system

In this paper we investigate source controlled channel decoding with a hard output channel decoder. Various methods have been devised in the past for source controlled channel decoding, but most of them assume that a soft output channel decoder is used. Most receivers in mobile wireless communications have a standard Viterbi channel decoder which produces only hard outputs. It is shown that a simple sliding histogram is capable of improving the speech quality significantly. The ideas and methods in this paper are applied for the full-rate and enhanced full-rate speech codecs in the GSM system.

Performance of soft-output space-time equalization for EGPRS

The use of antenna arrays can help to combat cochannel interference (CCI) in wireless cellular systems. In interference-limited scenarios, spatio-temporal filtering techniques based on a deterministic least-square (LS) approach may be competitive with respect to trellis based algorithms, because of the difficulties of accurately modelling the space-time characteristics of CCI. We consider the impact on the receiver performance of the quality of the reliability information on the coded bits, computed at the output of the deterministic space-time equalizer. Simulation results are provided for an EGPRS receiver employing linear and decision-feedback training-based filtering techniques.

Capacity calculation of a packet switched voice cellular network

A semi-analytical method for the calculation of the teletraffic capacity of a packet switched voice cellular network is provided. The capacity is defined as the maximum traffic load that can be accommodated subject to certain service quality criteria and can be limited either by co-channel interference levels or by the number of available traffic channels. Focusing on GSM-type systems it is demonstrated that there is a potential for a significant capacity improvement when packet switching is used instead of circuit switching over the air interface, due to the inherent statistical multiplexing gain associated with packet switching.

Estimation of the output error statistics of space-time equalization in an antenna array EGPRS receiver with soft-decision decoding

The use of antenna arrays can help combat cochannel interference (CCI) in wireless cellular systems. In this paper, we consider an enhanced general packet radio service diversity receiver based on least squares spatio-temporal equalization and soft-decision decoding in the presence of decision feedback and/or asynchronous CCI. We compare known and novel estimators of the error mean and variance at the output of the deterministic space-time equalizer. The collected simulation data indicate that the estimation of the error mean and variance is critical to the performance of soft-in/hard-out Viterbi decoding in the presence of nonstationary input disturbance. Moreover, the use of short-term error statistics provides receiver performance gains of up to 15-20 dB in terms of signal-to-interference ratio, with respect to the use of burst statistics based on the training sequence midamble and tentative decisions on the payload symbols.