Performance of multi-rate DS-CDMA system with multi-stage partial parallel interference cancellation

In this paper, we introduce a simple, closed-form expression for finding the optimum partial cancellation factors (PCF) for the linear multistage partial parallel interference cancellation (PPIC) receiver. These factors are found in a direct- sequence code division multiple access (DS-CDMA) system over a frequency-flat fading channel for a large-system case. In this case, the number of active users and the processing gain tend to infinity while their ratio is finite. It will be shown that the expression for the PCF is a function of the moments of the eigenvalues of the correlation matrix, the number of interference cancellation (IC) stages, the system load, and the signal-to-noise ratio (SNR). Compared to recently-proposed methods, our method has the following advantages: a) It is less complex because the calculated PCF will be a direct function of the moments of the eigenvalues of the large correlation matrix; b) there is no need for ordering the PCF resulting in lower complexity; and c) it is not necessary to know the number of IC stages ap riori .

We derive an expression for the signal to interference-plus-noise ratio of a multistage parallel interference cancellation receiver. We focus on a linear multistage receiver employing partial parallel interference cancellation which converges to the linear minimum mean squared error receiver as the number of stages increase. 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 background noise variance. The expression allows a simple approximation for the bit error rate at each stage. Finally, we verify and apply our results to optimise the signal to interference-plus-noise ratio of the resulting linear multistage receiver.

The paper introduces an analytical tool to find the large-system performance of a multistage linear partial parallel interference cancellation (PPIC) receiver using the moments of the eigenvalues of the covariance matrix for a code division multiple access (CDMA) system over a frequency-flat fading channel. The figure of merit to evaluate the performance is the signal-to-interference-plus-noise ratio (SINR) that is calculated under a large-system condition. In this case, the number of active users and the processing gain tend towards infinity while their ratio is a fixed value. It is shown that the large-system performance is a function of the system load, the partial cancellation factor (PCF), the number of interference cancellation stages, the signal-to-noise ratio (SNR), and the received powers of the interfering users. Furthermore, for practical applications, the physical meaning of the large system is described by numerical simulations.

We present a comparison of four of the most discussed multiuser receiver structures: the decorrelator, the minimum mean square error (MMSE) receiver, the multistage parallel interference cancellation receiver, and the successive interference cancellation receiver. Comparisons are based on both theoretical analysis and simulation results. We examine the bit error rate (BER) performance in AWGN, Rayleigh fading, and near/far channels, as well as the computational complexity. Finally, we present results for non-coherent implementations of the aforementioned receivers.

We investigate the performance of adaptive interference cancellation receivers for the wideband code division multiple access (WCDMA) uplink physical data channel. For the WCDMA uplink waveform, we derive a blind adaptive receiver (BAR) based an the constant modulus algorithm (CMA), and an multistage adaptive parallel interference cancellation (APIC) receiver. In order to improve the performance of the APIC receiver, we propose a hybrid APIC (H-APIC) receiver structure which uses the CMA based BAR as the first stage of the APIC. We evaluate and compare the performance of the above receivers in a near-far scenario and show that the proposed H-APIC receiver performs better than the APIC receiver.

This paper proposes a new application of fuzzy inference mechanism to the interference cancellation in a direct-sequence code-division multiple-access (DS/CDMA) communication system over a non-perfect power control channel. Multistage parallel interference cancellation (PIC) is an effective method to cancel the multiple access interference (MAI) for equal-power channels; nevertheless, there is a significant degradation in performance for non-equal power ones. The effectiveness of interference cancellation is related to the reliability of the tentative decision involved in the interference estimates. We established a fuzzy logic system to adapt the weight of each interference cancellation path. According to the received signal's reliability, the weights of interfering users can be estimated by the fuzzy inference mechanism, so that the error propagation would be minimized. The simulation results show that the proposed adaptive fuzzy PIC scheme is robust to the interference and channel noise and is superior to the conventional PIC and constant weight PIC schemes under a severe near-far environment.

Multistage successive interference cancellation (SIC) and multistage parallel interference cancellation (PIC) are compared under two different realistic dual rate asynchronous DS-CDMA schemes. The effect of power control and imperfect channel side information are also considered. Multistage SIC outperforms single- or multi-stage PIC for any conditions considered.

Multi-User Shared Access is a non-orthogonal multiple access scheme of 5G,which has a high computational complexity and a large time delay due to the usage of successive interference cancellation detection algorithm. This paper proposes a multi-stage partial parallel interference cancellation detection algorithm, which does not require repeated ordering and repeated matrix inversion. In the first stage of detection, the bits of these users with good channel conditions will be outputted, and the influence of multiple access interference on users with bad channel conditions in the second stage will be decreased. Theoretical analysis and simulation results show that the symbol error rate of the proposed algorithm is slightly better than that of the two-stage MMSE-PIC, and the complexity is reduced. In the meanwhile, the computational complexity is significantly reduced without SER performance degradation when compared with MMSE-SIC algorithm.

Multiuser detection (MUD) techniques are mandatory in direct-sequence code division multiple access (DS-CDMA) systems. They allow to mitigate multiple access interference (MAI) and hence increase the capacity and allow high data rates. Multistage parallel interference cancellation (MPIC) detectors are considered serious candidates for practical implementation showing a good tradeoff between performance and complexity. In this paper, FPGA implementation of MPIC detectors with and without decision feedback (DF) is presented. Although the two techniques present the same arithmetic complexity, data dependency of the DF-MPIC causes a non negligible increase in FPGA resources.

In multistage parallel interference cancellation, the tentative decisions for different users at each stage are mutually correlated, and they are also correlated with Gaussian noise. By taking the correlations into consideration and employing Price's theorem, an analytical model is proposed to refine the variance evaluation of the final decision variable. It is shown that the refined variance is very close to that obtained by simulation and can improve the accuracy in bit error rate evaluation.

Identification (ID) systems utilizing visible light are an interesting topic, and have been attracting the growing attention of researchers recently. However, LED transmitters with a unique ID illuminate a limited coverage area, therefore, limiting the communication coverage. In order to extend the communication coverage, we proposed wireless mesh networks (WMNs) for relaying the information. Routing operations in WMNs might be challenging due to various factors. In this paper we analyze the link error probability and optical power loss. Based on our proposed network topology, we further found an effective scheme, multi-stage parallel interference cancellation (PIC), to alleviate the negative impacts due to interference which comes from multi-user data transmission.

We study the improvement technique for multistage parallel interference cancellation (PIC) DS-CDMA multiuser receivers, which cancel only a fraction of the estimated multiple access interference (MAI) at each receiver stage. This technique takes into account the fact that the MAI estimate, especially in the earlier stages, contains substantial error, and a relatively conservative strategy of using the estimate may lead to better system performance (i.e., lower average bit error rate). We start with simulating the BER performance as a function of the fraction of cancellation, which shows that the fraction can make a significant difference and a best fraction can be far less than one. Then the PIC with partial cancellation and the standard PIC are compared under various near-far effects, channel SNR, and number of users. The simulations indicate how the partial cancellation technique can provide significant improvement over the standard PIC.

Turbo-encoded multiple-input multiple-output (MIMO) radio links have been recently proposed for high-speed downlink packet access (HSDPA) in UMTS, where the reuse of spreading codes across the transmitter antennas results in high levels of interference. A state-of-the-art receiver chain for such a link incorporates space-time channel equalization, despreading, prewhitening, and detection. In this paper, the impact of modeling errors at the equalizer output on the receiver performance is investigated. Both a precise model and a more pragmatic approximate model are considered. Degradations resulting from imperfect knowledge of the channel state information at the receiver are also evaluated. The a posteriori probability (APP) detector and its max-log variant as well as the multistage partial parallel interference canceller (MS-PPIC) are examined as detection candidates in 1/3-rate and 1/2-rate coded 4/spl times/4 MIMO links. It is shown that, surprisingly, the MS-PPIC can provide superior performance compared to max-log-APP.

An effective design of multistage parallel interference cancellation (PIC) receiver using blind adaptive (BA) despreader and pre‐respreader interference estimator for uplink CDMA is proposed and analysed. A novel algorithm is designed, which exploits constant modulus (CM) property of the users' transmitted signals and inherent channel condition to perform adaptive despreading based on minimum error variance criteria. This is carried out by BA weighting of each chip signal for accurate tracking of the desired user's signal power and hence for more improved data detection at the output of each stage of PIC. Furthermore, the despreader weights are used within the adaptive pre‐respreader interference estimation and cancellation to obtain online scaling factors during every symbol period, without any knowledge of users' channels or the use of training sequences. It is found that this way of estimation is optimal in minimum mean squared error sense, and hence, significant reduction in interference and noise variance is observed in detection and estimation of the desired users' signals compared with conventional PIC. Bit error probability of the proposed PIC is obtained using Gaussian Approximation method. Extensive simulation results are shown, which demonstrate impressive performance advantage in fading environments, high system loading, and severe near far conditions. Copyright © 2008 John Wiley & Sons, Ltd.

A hybrid Interference cancellation (HIC) technique employing both Parallel and Successive Interference Cancellation (PIC/SIC) is proposed for Overloaded Direct Sequence Code Division Multiple Access (OCDMA) system under imperfect synchronization. Two types of receivers are proposed to efficiently mitigate the multiuser interference (MUI). In type I receiver, the MUI within group-1 users are first cancelled using Parallel Interference Cancellation (PIC). Then the multistage Successive Interference Cancellation (m-SIC) is performed on the resultant output of PIC to efficiently cancel the interference of group-2 users on group-1 users. The type II receiver is designed by performing PIC immediately followed by SIC and this combined process is repeated till a major chunk of multi user interference is cancelled. The BER performance of type II receiver is proved to be better than other cancellation techniques such as PIC, SIC and the proposed type I receiver.