Performance of adaptive linear interference suppression in the presence of dynamic fading

Abstract

Adaptive linear interference suppression for directsequence (DS) code-division multiple access (CDMA) is studied in the presence of time-and frequency-selective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decision-directed and blind adaptive algorithms, which do not require a training sequence, are considered. Modifications to least squares adaptive algorithms are presented which are compatible with differential coding and detection. For frequency-selective fading, adaptive algorithms are presented based upon different assumptions concerning knowledge of the desired user's channel. Specifically, the cases considered are as follows: 1) perfect knowledge of the desired channel; 2) knowledge of only the relative path delays; and 3) knowledge of only one delay corresponding to the strongest path. Computer simulation results are presented which compare the performance of these algorithms with the analogous RAKE receivers. These results show that for case 3), even slow fading can cause a significant degradation in performance. Effective use of channel parameters in the adaptive algorithm reduces the sensitivity to fade rate, although moderate to fast fading can significantly compromise the associated performance gain relative to the RAKE receiver.

Constrained MMSE receivers for CDMA systems in frequency-selective fading channels

“…When is fixed, the signal power is given by (16) and the interference and noise power can be written as (17) which results from the independence assumptions. The interference and noise power averaged over is given by (18) where is defined (7). The average signal-to-noise plus interference ratio (SINR), which is the ratio between and , is expressed as…”

Multiuser detection using the Taguchi method for DS-CDMA systems

“…Beside the detectors based on maximum likelihood estimation, there are many nonmaximum likelihood multiuser detectors as well. The minimum mean square error (MMSE) receiver [8], [9], [16], [17], [39] is a popular linear nonmaximum likelihood detector, which minimizes the mean square error between the filter output and the transmitted bit and also maximizes the output signal-to-interference ratio. A blind adaptive multiuser detector based on constant modulus algorithm is presented in [20], and a blind adaptive detector based on minimum output energy is proposed in [27] and [36].…”

Bidirectional algorithms for interference suppression in multiuser systems

“…Although a channel might be highly time variant, two adjacent fading coefficient will be similar and have a significant level of correlation as studied in [31][32][33]. These properties can then be exploited to obtain a sequence of faded symbols where the primary purpose of the filter is to suppress multiuser interference and track the ratio between successive fading coefficients; thus, not burdening it with estimation of the fading coefficients themselves.…”

“…The non-zero correlation between multiple time instants is exploited to improve the robustness, tracking, and convergence performance of existing MMSE schemes. Unlike existing adaptive solutions [8,[31][32][33], which do not fully exploit the fading correlation between multiple successive time instants, the proposed bidirectional approach exploits the correlation and adaptively weighs the output of the receive filter in order to optimize the estimation performance. Normalized least-mean square (NLMS) and conjugate gradient (CG)-type algorithms are presented that overcome a number of problems associated with applying the recursive least-squares (RLS) algorithm to bidirectional problems.…”

“…Approaches that avoid tracking and estimation of the fading coefficients were proposed in [31][32][33]. Although a channel might be highly time variant, two adjacent fading coefficient will be similar and have a significant level of correlation as studied in [31][32][33].…”