this paper is to provide an overview of these techniques. An excellent review of those methods developed prior to 1988 can be found in a survey paper authored by Milstein [12]. Thus, although we will review all of the methods that have been proposed, we will treat these earlier methods only briefly, and we will focus instead on those that have been developed more recently. It should also be noted that much of the work in this area has been motivated by the application of spread spectrum as an anti-jamming signaling method for military use; however it is equally applicable to the problem of NBI in cellular CDMA. Current research makes a more conscious effort to incorporate methodologies that allow for performance improvements when considering the spread signal as a CDMA signal, as we will discuss later. We begin, in the following section, with a brief review of NBI mitigation techniques based on the linear signal processing regimes of adaptive linear transversal filtering, and Fourier-domain filtering. These methods represent the original approaches to this problem, and they are quite well developed. Most of this paper, on the other hand, will be concerned with recently developed model-based techniques that employ non-standard signal processing methods to enhance the interference rejection capabilities in CDMA systems beyond those of standard methods. In particular, in the second section we consider techniques based on nonlinear filtering [15] in which the spread spectrum CDMA signal (which is digital) is modeled as a non-Gaussian noise in the interference suppression process. Further, in the third section we consider the situation in which the NBI is also a digital communications signal. In this situation, multiuser detection techniques [16] can be used to give quite ...

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...

Due to the scarcity of frequency allocations for emerging personal communications services, it has been proposed that spread spectrum (SS) signals be used in such services so that they can be overlaid on existing frequency band occupants. Such SS signals would be constrained in power so as not to interfere with pre-existing, narrowband users. In view of this application, suppression of strong narrowband signals that interfere with commercial SS communications systems is being investigated with renewed interest. Previous research has modeled narrowband users as sinusoidal or autoregressive signals, and consequently has advocated use of predictive filtering to suppress such signals. In this paper it is shown that when the narrowband interferer is in fact a digital communications signal these methods are less effective than techniques derived from multiuser communications. In particular, by modeling a narrowband user as a digital signal, optimal and asymptotically (low background noise) o...

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...

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].