Linear Parallel Interference Cancellation 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.

A Projected Parallel Interference Cancellation for Asynchronous Upstream OCDMA-PON

OCDMA is a promising candidate for next generation passive optical networks (NG-PON). OCDMA-PON can potentially provide all customers with a Gb/s-class BW upstream with inherent flexibility. Unfortunately OCDMA suffers from multiple access interference (MAI) and various detection noises. To mitigate MAI, we propose a novel parallel interference cancellation technique for incoherent DS-OCDMA that better exploits the non-negativity of the solution. The proposed PIC scheme incorporates this additional information into its structure. The proposed PIC detector is shown to outperform the conventional correlator detector, the decorrelator detector, the linear parallel interference Cancellation (LPIC) detector and even the linear minimum mean square error (LMMSE) detector.

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.

A novel technique to increase the capacity of code division multiple access system using scrambled spreading sequences

A multistage Linear Parallel Interference Cancellation (LPIC) approach is presented to mitigate the effect of multiuser interference in uplink Direct Sequence Code Division Multiple Access (DS CDMA) system. The system is overloaded to accommodate more number of users than the processing gain of the system. The performance of this overloading scheme is evaluated by assigning one set of orthogonal Gold codes to the first N users and another set of orthogonal Gold codes to extra users. Both the sets are overlaid by a common pseudo noise scrambling sequence. This paper proposes a novel technique of using Weighted Linear Parallel Interference Cancellation (WLPIC) for overloaded CDMA system resulting in bandwidth saving, better performance and increase in the speed of interference cancellation. Also this technique considerably reduces the multipleaccess interference, since set-1 users suffer from interference of the set-2 users only, while the set-2 users suffer from interference of the set-1 users. This provides a way for accommodating additional users in the same available bandwidth. Considerable conservation of time is achieved by avoiding the usage of conventional successive Interference Cancellation (IC) for scrambled CDMA system. The empirical results show that this scheme provides 50% overloading with three-stage weighted linear parallel interference cancellation (WLPIC) technique for N = 64 at a Bit Error Rate (BER) of 10-3 and 75% overloading at a BER of 10-2. The three-stage WLPIC scheme clearly outperforms matched filter detector, Conventional LPIC and the twostage WLPIC on Additive White Gaussian Noise (AWGN) channel.

Asynchronous CDMA Systems With Random Spreading—Part II: Design Criteria

Totally asynchronous code-division multiple-access (CDMA) systems are addressed. In Part I, the fundamental limits of asynchronous CDMA systems are analyzed in terms of spectral efficiency and SINR at the output of the optimum linear detector. The focus of Part II is the design of low-complexity implementations of linear multiuser detectors in systems with many users that admit a multistage representation, e.g. reduced rank multistage Wiener filters, polynomial expansion detectors, weighted linear parallel interference cancellers. The effects of excess bandwidth, chip-pulse shaping, and time delay distribution on CDMA with suboptimum linear receiver structures are investigated. Recursive expressions for universal weight design are given. The performance in terms of SINR is derived in the large-system limit and the performance improvement over synchronous systems is quantified. The considerations distinguish between two ways of forming discrete-time statistics: chip-matched filtering and oversampling.

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.

Improved Linear Parallel Interference Cancellers

In this paper, taking the view that a linear parallel interference canceller (LPIC) can be seen as a linear matrix filter, we propose new linear matrix filters that can result in improved bit error performance compared to other LPICs in the literature. The motivation for the proposed filters arises from the possibility of avoiding the generation of certain interference and noise terms in a given stage that would have been present in a conventional LPIC (CLPIC). In the proposed filters, we achieve such avoidance of the generation of interference and noise terms in a given stage by simply making the diagonal elements of a certain matrix in that stage equal to zero. Hence, the proposed filters do not require additional complexity compared to the CLPIC, and they can allow achieving a certain error performance using fewer LPIC stages. We also extend the proposed matrix filter solutions to a multicarrier DS-CDMA system, where we consider two types of receivers. In one receiver (referred to as Type-I receiver), LPIC is performed on each subcarrier first, followed by multicarrier combining (MCC). In the other receiver (called Type-II receiver), MCC is performed first, followed by LPIC. We show that in both Type-I and Type-II receivers, the proposed matrix filters outperform other matrix filters. Also, Type-II receiver performs better than Type-I receiver because of enhanced accuracy of the interference estimates achieved due to frequency diversity offered by MCC.

A New Linear Group-Wise Parallel Interference Cancellation Detector

In this paper, a new linear group-wise parallel interference cancellation (LGPIC) detector is proposed. Four different group-detection schemes are derived, namely, the linear group matched filter PIC (LGMF-PIC) detector, the linear group decorrelator PIC (LGDEC-PIC) detector, the linear group minimum mean square error PIC (LGMMSE-PIC) detector and the linear group parallel interference cancellation weighted PIC (LGPIC-PIC) detector. The convergence behavior of the proposed detector is analyzed and conditions of convergence are derived. Finally, extensive simulations regarding the convergence behavior and the effect of the grouping on the convergence behavior of the proposed LGPIC detector are conducted.

Low-Complexity MIMO Multiuser Receiver: A Joint Antenna Detection Scheme for Time-Varying Channels

This paper deals with the uplink of a wireless multiple-input multiple-output (MIMO) communication system based on multicarrier (MC) code division multiple access (CDMA). We focus on time-varying channels for users moving at vehicular speeds. The optimal maximum a posteriori (MAP) receiver for such a system is prohibitively complex and can be approximated using iterative linear minimum mean-square error (LMMSE) multiuser detection and parallel interference cancellation (PIC). For time-varying channels, two LMMSE filters for channel estimation and multiuser detection need to be computed at every time instant, making implementation in a real-time system difficult. We develop a novel low-complexity receiver that exploits the multiple antenna structure of the system and performs joint iterative multiuser detection and channel estimation. Our receiver algorithms are based on the Krylov subspace method, which solves a linear system with low complexity, trading accuracy for efficiency. The computational complexity of the channel estimator can be reduced by one order of magnitude. For multiuser detection, a PIC scheme in the user space, i.e., after the matched filter, allows simultaneous detection of all users as well as drastic computational complexity reduction by more than one order of magnitude.

Cancellation of Multiuser Interference Due to Carrier Frequency Offsets in Uplink OFDMA

In uplink orthogonal frequency division multiple access (OFDMA) systems, multiuser interference (MUI) occurs due to different carrier frequency offsets (CFO) of different users at the receiver. In this paper, we present a multistage linear parallel interference cancellation (LPIC) approach to mitigate the effect of this MUI in uplink OFDMA. The proposed scheme first performs CFO compensation (in time-domain) followed by K DFT operations (where K is the number of users) and multistage LPIC on these DFT outputs. We scale the MUI estimates by weights before cancellation and optimize these weights by maximizing the average signal-to-interference ratio (SIR) at the output of the different stages of the LPIC. We derive closed-form expressions for these optimum weights. The proposed LPIC scheme is shown to effectively cancel the MUI caused by the other user CFOs in uplink OFDMA. While our proposed approach performs CFO compensation in time-domain, an alternate approach proposed recently by Huang and Letaief performs CFO compensation and interference cancellation in frequency-domain. We show that our approach performs better than the Huang & Letaief's approach when the magnitude of the CFO differences (between desired user CFO and other user CFOs) are small, whereas their approach performs better when the magnitude of the individual CFOs (of other users) are small. Since the CFO values can be arbitrary at the receiver, in order to make the receiver robust under various CFO conditions, we propose simple metrics based on CFO knowledge, which the receiver can compute and use to choose between the time-domain (ours) and the frequency-domain (Huang & Letaief's) cancellers so that better performance among the two approaches is achieved under various CFO conditions.

BER-Optimal Linear Parallel Interference Cancellation for Multicarrier DS-CDMA in Rayleigh Fading

In this paper, we consider the design and bit-error performance analysis of linear parallel interference cancellers (LPIC) for multicarrier (MC) direct-sequence code division multiple access (DS-CDMA) systems. We propose an LPIC scheme where we estimate and cancel the multiple access interference (MAI) based on the soft decision outputs on individual subcarriers, and the interference cancelled outputs on different subcarriers are combined to form the final decision statistic. We scale the MAI estimate on individual subcarriers by a weight before cancellation. In order to choose these weights optimally, we derive exact closed-form expressions for the bit-error rate (BER) at the output of different stages of the LPIC, which we minimize to obtain the optimum weights for the different stages. In addition, using an alternate approach involving the characteristic function of the decision variable, we derive BER expressions for the weighted LPIC scheme, matched filter (MF) detector, decorrelating detector, and minimum mean square error (MMSE) detector for the considered multicarrier DS-CDMA system. We show that the proposed BER-optimized weighted LPIC scheme performs better than the MF detector and the conventional LPIC scheme (where the weights are taken to be unity), and close to the decorrelating and MMSE detectors.

BER-Optimal Linear Parallel Interference Cancellation for Multicarrier DS-CDMA in Rayleigh Fading

BER-Optimal Linear Parallel Interference Cancellation for Multicarrier DS-CDMA in Rayleigh Fading

SIR-optimized weighted linear parallel interference canceller on fading channels

In this letter, we present a weighted linear parallel interference canceller (LPIC) where the multiple access interference (MAI) estimate in a stage is weighted by a factor before cancellation on Rayleigh fading and diversity channels. We obtain exact expressions for the average signal-to-interference ratio (SIR) at the output of the cancellation stages which we maximize to obtain the optimum weights for different stages. We also obtain closed-form expressions for the optimum weights for the different stages. We show that this SIR-optimized weighted LPIC scheme clearly outperforms both the matched filter (MF) detector as well as the conventional LPIC (where the weight is taken to be unity for all stages), in both near-far as well as non-near-far conditions on Rayleigh fading and diversity channels

Analytical Performance Analysis of the Linear Multistage Partial PIC Receiver for DS-CDMA Systems

In this letter, we derive a recursive expression for the decision statistic of a linear multistage partial parallel interference cancellation (PIC) receiver for direct-sequence code-division multiple-access systems that allow us to present two closed-form expressions (exact and approximate) for the bit-error probability of this receiver. The bit-error rate is computed in terms of the number of PIC stages, set of cancellation parameters, crosscorrelation between the users' codes, amplitude of the users' signals, and noise power.

Parallel Interference Cancellation for Uplink Multirate Overlay CDMA Channels

To provide new and/or higher rate wireless services with limited spectrum resources, frequency overlay has been naturally proposed to accommodate the new and legacy systems in a common band. We address the multiuser detection problem for overlaid code-division multiple-access (CDMA) scenarios. However, in contrast to the well-studied conventional single-rate CDMA, miscellaneous systems overlay almost always indicates the presence of multirate traffic that introduces an additional degree of freedom in receiver design-i.e., differences in the symbol rates. We concentrate on receiver design for multirate traffic, while assuming a lack of information exchange between the constituent (new and legacy) systems, as is commonplace in practice. We propose a receiver architecture based on linear parallel interference cancellation where the out-of-rate intersystem interference is estimated and subtracted by means of its characteristic subspace, thereby avoiding the need for the exact knowledge of signature waveforms of the interfering system. Simulation results validate our solution and show that the proposed receiver has better performance in various aspects than several other solutions for the same purpose.

A graphic approach to performance analysis of multistage linear interference canceller in long-code cdma systems

The signal-to-interference-plus-noise-ratio performance of the multistage linear parallel and successive interference cancellers (LPIC and LSIC) in a long-code code-division multiple-access system is analyzed using a graphical approach. The decision statistic is modeled as a Gaussian random variable, whose mean and variance can be expressed as functions of moments of R for the LPIC and L for the LSIC, respectively, where R is the correlation matrix of signature sequences and L is the strict lower triangular part of R. Since the complexity of calculating these moments increases rapidly with the growth of the stage index, a graphical representation of moments is developed to facilitate the computation. Propositions are presented to relate the moment calculation problem to several well-known problems in graph theory, i.e., the coloring, the graph decomposition, the biconnected component finding, and the Euler tour problems. It is shown that the derived analytic results match well with simulation results.

Ping-pong effects in linear parallel interference cancellation for CDMA

The convergence behavior of linear parallel interference cancellation is investigated. Especially the so-called ping-pong effect, where the bit-error rate performance is found to oscillate between two different convergence patterns is studied in detail. This effect is shown to be a direct consequence of the extreme eigenvalues of the correlation matrix, allowing for an analytical approach. Intervals for the dominating eigenvalues within which ping-pong effects can occur are specified and illustrated by examples. It is shown that the decision statistic for traditional parallel cancellation will always exhibit oscillating behavior with either short or long codes. Relaxation factors, leading to weighted cancellation, are shown to be effective for alleviating oscillations and ping-pong effects at the expense of convergence rate. Asymptotic analysis for large systems is applied to uncover the convergence behavior for long code systems.

Large system analysis of linear multistage parallel interference cancellation

In this paper, we derive an expression for the signal to interference-plus-noise ratio of a linear multistage parallel interference cancellation receiver. We focus on a linear multistage receiver which converges to the linear minimum mean-squared error receiver as the number of stages increases. The signal to interference-plus-noise ratio is given in terms of the system loading, the partial cancellation factor, the number of stages, and the signal-to. noise ratio. Our expression also allows a simple approximation for the bit error rate at each stage. Finally, we perform a numerical optimization to maximize the signal to interference-plus-noise ratio expression with respect to the partial cancellation factor of the resulting linear multistage receiver.