Linear Parallel Interference Cancellation Detector Research Articles

Competitive linear parallel interference cancellation detection based on monotone line‐search techniques

In this study, a competitive linear parallel interference cancellation (LPIC) detector that is memory efficient, enjoys fast convergence and low complexity and can be used to approximate the decorrelator/minimum-mean-square error detector in large-scale communication systems is proposed. Similar to the non-monotone gradient-based LPIC detectors developed recently by Bentrcia and Alshebeili, the proposed detector maintains its efficiency and does not break down quickly if matrix-vector products are not performed accurately. However, unlike the previous detectors, the proposed detector relies on a monotone line-search technique which renders it more attractive because early stopping methods such as the L-curve method can be used to stop the LPIC iterations prior to convergence in order to avoid the noise enhancement effect. Simulation results agree well with the authors theoretical findings.

Linear IC detectors for low to medium SNR ill-conditioned communication systems with unknown noise variance

In this paper, we introduce two new linear parallel interference cancellation (LPIC) detectors that are suitable for low to medium signal-to-noise ratio ill-conditioned communication systems and do not require knowledge of the noise variance but perform close to the linear minimum mean square error detector, which needs such information. Particularly, we focus in this work on fast linear parallel interference cancellation detectors that are asymptotically equivalent to the steepest descent and conjugate gradient algorithms, respectively, and show that they exhibit a spectral filtering property and semi-convergence behavior. Consequently, a deterministic stopping rule to stop the LPIC iterations that is independent of the noise level (known as the L-curve method) is investigated and tested. Simulation results are presented to support our theoretical findings.

Regularization property of linear interference cancellation detectors

In this article, we unveil a new property of linear interference cancellation detectors. Particularly, we focus in this study on the linear parallel interference cancellation (LPIC) detector and show that it exhibits a semi-convergence property. The roots of the semi-convergence behavior of the LPIC detector are clarified and the necessary conditions for its occurrence are determined. In addition, we show that the LPIC detector is in fact a regularization scheme and that the stage index and the weighting factor are the regularization parameters. Consequently, a stopping criterion based on the Morozov discrepancy rule is investigated and tested. Simulation results are presented to support our theoretical findings.

New linear PIC detectors based on non-monotone line-search techniques

In this communication, three novel linear parallel interference cancellation (LPIC) detectors that exhibit fast convergence, low complexity, low memory requirements and can be used to efficiently approximate the decorrelator/MMSE detector in large-scale wireless communication systems are proposed. Unlike conjugate gradient based LPIC detectors, the proposed LPIC detectors maintain their efficiency and do not break down quickly if the matrix–vector products are performed only approximately. These detectors employ non-monotone line search techniques and use a pre-conditioner that exploits the structure of the inter-carrier interference (ICI) matrix to speed up the convergence of the conventional LPIC detector. Moreover, the chaotic behavior observed in the convergence of the proposed detector in AWGN channel is greatly reduced, thanks to the time-varying nature of the fading channel. Simulation results support well our theoretical findings.

Soft Decision Parallel Interference Cancellationfor Scrambled/Unscrambled DS-CDMA System

A soft decision linear parallel interference cancellation for the direct sequence spread spectrum CDMA systems which can accommodate more users (K = N+M >N) than the processing gain of the system is analyzed in this paper. The proposed technique accommodates N users without any mutual interference and a number of additional users at the expense of a small signal-to-noise ratio. Two sets of orthogonal Gold codes are used, one for the first N users and the other for the additional users. Both the N users and additional users are overlaid by a set-specific pseudonoise scrambling sequence. The performance of scrambled and unscrambled system for matched filter and linear parallel interference cancellation is analyzed and the scrambled system is found better than unscrambled system. The allowable load of this scheme is investigated, so that the uncoded performance with linear parallel interference cancellation detector using soft (SDIC) decision functions remains close to the single user bound for N= 64 at 33% overloading.

SINR, power efficiency, and theoretical system capacity of parallel interference cancellation

This paper analytically derives exact expressions for the SINR of the two-stage linear parallel interference cancellation (LPIC) and two-stage hard-decision parallel interference cancellation (HPIC) multiuser detectors in a synchronous, nonorthogonal, binary, CDMA communication system with deterministic short spreading sequences. We consider approximations to the SINR expressions that are justified in typical operating scenarios to obtain a more intuitive understanding of the SINR performance of the HPIC detector. We consider the case where a specific SINR requirement is given for each user in the system and derive expressions for the set of transmit powers necessary to meet this requirement when two-stage LPIC or HPIC detection is used. We also derive expressions for a measure of the theoretical system capacity using LPIC and HPIC detection, defined as the maximum number of users possible in a system with finite available transmit power. Numerical results are presented that compare the HPIC and LPIC detectors to the hard-decision successive interference cancellation (SIC) detector and matched filter (MF) detector. Our results suggest that HPIC detection may offer the best SINR and power efficiency performance when the number of users in the system is low to moderate and that SIC detection may offer superior performance when the number of users in the system is large.

On the performance of linear parallel interference cancellation

This paper analyzes the performance of the linear parallel interference cancellation (LPIC) multiuser detector in a synchronous multiuser communication scenario with binary signaling, nonorthogonal multiple access interference, and an additive white Gaussian noise channel. The LPIC detector has been considered in the literature lately due to its low computational complexity, potential for good performance under certain operating conditions, and close connections to the decorrelating detector. In this paper, we compare the performance of the two-stage LPIC detector to the original multistage detector proposed by Varanasi and Aazhang (1990, 1991) for CDMA systems. The general M-stage LPIC detector is compared to the conventional matched filter detector to describe operating conditions where the matched filter detector outperforms the LPIC detector in terms of error probability at any stage M. Analytical results are presented that show that the LPIC detector may exhibit divergent error probability performance under certain operating conditions and may actually yield error probabilities greater than 0.5 in some cases. Asymptotic results are presented for the case where the number of LPIC stages goes to infinity. Implications of the prior results for code division multiple access (CDMA) systems with random binary spreading sequences are discussed in the large-system scenario. Our results are intended to analytically corroborate the simulation evidence of other authors and to provide cautionary guidelines concerning the application of LPIC detector to CDMA communication systems.