Blind Adaptive Signal Reception for MC-CDMA Systems ... - IEEE Xplore

Blind Adaptive Signal Reception for MC-CDMA Systems in Rayleigh Fading Channels under multiple narrow-band interference signals Mohamed Samir, Ehab M. Shaheen and Ahmed Abd El Tawab Department of Electronic Warfare Military Technical College Cairo, Egypt

Abstract—This paper investigates the impact of multiple narrow-band interference (NBI) signals or jamming signals on the signal reception of multi-carrier code-division multipleaccess (MC-CDMA) systems in single path Rayleigh fading channels. Different blind adaptive algorithms are proposed which determine a weight vector that optimally combines the desired signal contributions from different carriers while suppressing noise and interference. A performance comparison among these algorithms is provided via extensive simulations. No knowledge of the channel conditions (fading coefficients, signature sequences and timing of interferers, statistics of other noises, etc.) nor any training sequence is required. Performance results show that the proposed algorithms perform well and are robust against jamming or NBI.

I. I NTRODUCTION Code-division multiple-access (CDMA) based on spread spectrum techniques have been recognized as having a significant role in cellular and personal communications. In particular, the direct sequence (DS) CDMA scheme has many attractive features such as its potential capacity increase, anti-multipath, and anti-jamming capabilities [ref.]. It is well known that the multiple-access capability of DSCDMA systems is achieved by assigning a distinct signature waveform for each user from a set of waveforms with low cross-correlation [ref.]. When different CDMA user signals are simultaneously received on the same frequency, the transmitted information can be recovered by using a filter matched to each user signature sequence (conventional detector). MC-CDMA has been proposed as an efficient multicarrier transmission scheme for supporting multiple access communications which combines CDMA and orthogonal frequency division multiplexing (OFDM) techniques [ref.]. In these systems, DS spreading is employed together with multi-carrier modulation to derive the benefits of both techniques. MC-CDMA system adapting maximal ratio combiner has the ability to mitigate the effect of NBI and multi-path fading. However, the use of the MRC requires the estimation of each fading coefficient associated with each carrier. The complexity of this problem becomes bigger in the presence of interference, especially multiple access interference (MAI), and noise other than additive white Gaussian noise 978-1-4673-4810-2/12/$31.00 ©2012 IEEE

(AWGN). In such case, not only does it become more difficult to estimate the fading coefficients, but to achieve an optimal reception the statistics of the interference and the noise also have to be estimated. In [1] a blind adaptive receiver has been proposed for DS systems. This method can be viewed as a form of minimum mean-squared error (MMSE) multi-user detection in the frequency domain and proved its robustness against MAI and single NBI signal. Whereas in [2] . This paper proposes an enhancement for the blind adaptive algorithm suggested in [1] in Rayleigh flat fading channels under the impact of multiple NBI signals and the presence of MAI. Such proposed algorithm determines a weight vector which optimally combines the contributions from different carriers and maximizes the signal-to-noise and interference ratio (SNIR). The desired signal contributions are constructively combined, while noise and interference are suppressed. Neither a prior knowledge of the fading coefficients such as the signature sequences, the timing of the interferers and the statistics of other noise nor a training sequence is assumed or required. The paper is organized as follows. The system model is shown in section II. In section III an explanation to the basic blind adaptive technique algorithm suggested in [1] is presented. The stochastic gradient only algorithm is presented in section IV. Section V presents the proposed algorithm which is based on a combination between the basic algorithm suggested in [1] and the stochastic gradient algorithm in order to enhance the system’s capability against NBI. Section V. presents representative simulation results of these various proposed algorithms and finally, section VI draws the conclusions. II. S YSTEM M ODEL Assuming the presence of simultaneous K users, which use the same M carriers in the system. The k th user, for 1