In this paper we review the most peculiar and interesting information-theoretic and communications features of fading channels. We first describe the statistical models of fading channels which are frequently used in the analysis and design of communication systems. Next, we focus on the information theory of fading channels, by emphasizing capacity as the most important performance measure. Both single-user and multiuser transmission are examined. Further, we describe how the structure of fading channels impacts code design, and finally overview equalization of fading multipath channels.

Spread spectrum (SS) communications offers a promising solution to an overcrowded frequency spectrum amid growing demand for mobile and personal communications services. The proposed overlay of spread spectrum signals on existing narrowband users implies strong interference for the SS system. This paper discusses how system performance can be improved by preprocessing to suppress narrowband interference. Linear prediction filters have been proposed since the 1980s for suppression of narrowband interference. In 1991 Vijayan and Poor proposed nonlinear methods of suppressing the narrowband signal with significant increase in the SNR improvement. We derive an enhancement to this nonlinear prediction and achieve further improvement by applying the technique to interpolating filter structures. Finally, we extend results to the case of multiple spread spectrum users and demonstrate how nonlinear filtering can dramatically outperform linear filtering. I. Introduction IEEE Transactions on Com...

Ultra-Wide band (UWB) radio systems have drawn a lot of attention during the last few years. These systems use very low transmission power, spreaded over a bandwidth of several gigahertz. The very low transmission power and the large bandwidth used, enable UWB radio systems to co-exist with other narrow band systems over the same frequency band without interfering the narrow band systems. Nevertheless, these narrow band systems may cause interference which may jam the UWB receiver completely. Since standard narrow band interference suppression techniques are not applicable, techniques for interference suppression have to be developed. This paper presents novel narrow band interference suppression algorithms for UWB radio systems. Theoretical analysis of these algorithms reveal that they can eliminate the narrow band interference almost completely. 1.

This research investigates robust demodulation of Gaussian Minimum Shift Keying (GMSK) signals, using demodulator diversity and real-time bit-error-rate (BER) estimation. GMSK is particularly important because of its use in prominent wireless standards around the world (GSM, DECT, CDPD, DCS1800, and PCS1900). The dissertation begins with a literature review of GMSK demodulation techniques (coherent and noncoherent) and includes an overview of singlechannel interference rejection techniques in digital wireless communications. Various forms of GMSK demodulation are simulated, including the limiter discriminator and di#erential demodulator (i.e., twenty-five variations in all). Ten represent new structures and variations. The demodulator performances are evaluated in realistic wireless environments, such as additive white Gaussian noise, co-channel interference, and multipath environments modeled by COST207 and SMRCIM. Certain

We consider the problem of estimating and suppressing many unknown independent and time-varying interferers in a spread-spectrum communication system. The interferers are assumed to be present in a wide frequency range. In order to detect, estimate and track the interference, we use a bank of hidden Markov model lters operating in the frequency domain. The hidden Markov model lters' outputs are then used to suppress the existing interference. The computational complexity of our scheme is only linear in the number of interferers. The simulation studies show that our proposed novel schemes adapt quickly in tracking the time-varying nature of the interference. 1 Introduction In this paper, we consider the problem of detecting, tracking and suppressing interference in a spread-spectrum communication system. The interference is assumed to consist of many unknown, independent and time-varying narrowband interferers. There are several advantages with spreading the spectrum of the signal th...

The spread spectrum modulation presents a more robust interference rejection capability than the non code-based modulations. However, the increasing of the communication services has augmented the power of the interference signals. In particular, the unlicensed bands such as the Industrial Scientific and Medical (ISM) can seriously saturate the interference rejection capability of the spread spectrum systems. To overcome this drawback it is possible to resort to an Interference Canceller (IC) previously to the detection. Then to design an IC the Conditional Mean (CM) and the Maximum-APosteriori criteria are the optimum methods to suppress analog and digital interferences respectively. However, in general both criteria present a high computational burden which reduces their real feasibility. In addition, the communication scenery can be corrupted by Gaussian or Non-Gaussian noise, which makes more complex the structure of the optimum detector. This paper proposes an interference canceller based on Fuzzy Logic and set theory that allows to suppress either analog or digital interferences and reduces the computational burden of the CM estimator and MAP detector.

Communication over a waveform channel corrupted by additive white Gaussian noise and by an unknown and arbitrary interference of bounded power is considered. For this channel, an upper bound is presented for the worst-case error probability of a communication system comprising a direct-sequence spread-spectrum modulator and a nonlinear correlation receiver. It is shown that this bound is exponentially tight as the number of chips used in the modulator becomes large. This bound is evaluated for several detector nonlinearities. Numerical examples and comparisons to the performance of a pure Gaussian noise channel are also given. Index Terms --- spread-spectrum, direct-sequence modulation, signal detection, robust detection, nonlinear detection I. Introduction The ability to suppress a wide variety of interfering signals is among the most important properties of spread-spectrum communication systems. In the particular case of directsequence (DS) systems, this ability has been extensively...

Ultra Wide-band (UWB) radio systems have drawn attention during the last few years. These systems use low transmission power, spread over a bandwidth of several gigahertz, which enable them to co-exist with other narrow band systems over the same frequency band without interfering them. Nevertheless, the narrow band systems can cause interference which may jam the UWB receiver completely. This paper presents novel narrow band interference suppression algorithms for UWB radio systems, where standard narrow band interference suppression techniques cannot be applied. The presented algorithms use a RAKE receiver with MMSE weights and correlators at delays that does not correspond to channel taps. Theoretical analysis of these algorithms reveal that they can eliminate the narrow band interference almost completely. Simulation results support the theoretical ones.

We consider blind demodulation of multiple digital signals in a cellular environment under the assumption that training data is unavailable, and that the receiver has no a priori knowledge of user signatures in space or time. Our approach to this problem is to exploit diversity in the time, frequency, and spatial domains through the application of a linear receiver with multiple antenna elements. To separate the users, a subspace decomposition is used to obtain an initial estimate of the users' signatures. Once estimates of the signatures are available, the anchored algorithm described in [1] can be used to obtain initial bit estimates. These bit estimates are then used as the input to a decision-directed least squares (LS) algorithm (either full-rank or reduced-rank), which provides refined bit estimates for the packet of interest. This process can be iterated until the bit estimates remain fixed. Simulation results indicate that for the Time-Division Multiple-Access (TDMA) model cons...

This paper presents an adaptive code-aided technique for the suppression of narrowband interference (NBI) in direct-sequence code-division multiple access (DS-CDMA) systems. This technique uses a multiuser detector based on the Interacting Multiple Model (IMM) algorithm. This detector is based on the concept that the effective model of the received signal at a time instance can be approximated by one of models in the IMM algorithm. Simulations are used to compare the performance of the proposed technique with that of the recursive least squares (RLS) version of the minimum mean-square error (MMSE) for multiuser detection [1].