Performance improvement in wireless system due to parallel interference cancellation (PIC) algorithm

In this paper, we propose a novel differentially coherent (DC) parallel interference cancellation (PIC) scheme for direct-sequence code-division multiple access (DS-CDMA) employing differential phase-shift keying (DPSK). The proposed scheme combines decision-feedback differential detection (DF-DD) and PIC. For optimization of the DF-DD and interference cancellation (IC) filters three different criteria are adopted. The first (DC-PIC I) and the second (DC-PIC II) criteria assume a constant channel and a channel with a small constant frequency offset, respectively, whereas the third criterion (DC-PIC III) optimizes the filters in the minimum mean-squared error (MMSE) sense taking into account the statistical properties of the underlying channel. Simulations show the high achievable performance and the robustness of the novel DC-PIC receiver. A comparison with linear DC receivers and a previously proposed DC-PIC scheme show the superiority of the proposed approach.

Performance analysis of an adaptive two-stage PIC CDMA receiver in AWGN channels

This paper introduces an adaptive ( /spl mu/ + /spl lambda/ ) evolutionary algorithm (EA) for multiuser detection (MUD) in direct sequence code division multiple access (DS-CDMA) systems. The EA based multiuser detector takes the output of parallel interference cancellation (PIC) detector as the initial population and adopts maximum likelihood decision rule to detect the user bit sequences. During an EA running, the offspring population size is adaptively adjusted according to the current success probability. The major advantage of the adaptive EA based multiuser detection scheme is that it obtains rather good bit error rate (BER) performance with lower computational complexity. Monte Carlo simulation results show the EA based multiuser detector can always converge to the optimal solution with a small number of generations. Numerical analysis shows its computational time is polynomial complexity and less than other heuristic search algorithms.

The multiuser detection is a signal processing tech- nique which enhances the performance and capacity in DS- CDMA technology. Since its complexity issue was too expensive to handle, there was focus on the suboptimal detectors. In this paper we propose a hybrid method called adaptive differential interference cancellation method (ADIC) which is a hybrid of the differential interference cancellation method and which is then combined in a adaptive way to determine a weight matrix which will map the outcome of the differential interference cancellation method to come out with a better performance. This paper also looks into the computational cost of different methods using the floating point operation (flops) which is a good estimation of the comparative computational cost if not the exact one. I. INTRODUCTION The conventional DS-CDMA match filter detector fails to combat these issues due three prominent drawbacks: unrealis- tic ucorrelated code, near-far problem (even with strict power control (1)), and tough multiple access interference (MAI). Many advanced signal processing approaches have been made to improve the performances of these kinds of detectors. In this paper we will study the different approaches that were made in this field and implement these methods to reach a conclusion about the detection methods. As the name of the topic suggests the focus of our work will be study and propose cost effective better performing multiuser detection method. We will explore Multiuser Detection Techniques in Synchronous CDMA environment. Multi-user detection deals with the development and ap- plication of joint demodulation and interference cancellation cancellation techniques for improved detection of a desired set of digital signals in the presence of multiple access interfer- ence, intersymbol interference and noise. Since veterbi forward dynamic algorithm (2) is used in case of Optimum multiuser detection (3) the complexity grows in with the increase in the number of users. Hence the complexity is immense which increases exponentially with the increase in the number of users. Looking ahead for lower complexity hence lesser cost of implementation, the paper proposes the use of sub-optimal linear detectors (4). The sub-optimal detector the paper next focuses on is Decorrelator type of Detector (5) which is optimal in least squares. This detector out performs the optimal detector in many respects. Then the Minimum Mean Square Error (MMSE) linear detector is proposed which tries to minimize the squared error from the required transformation. It is one of the most popular detectors in use due to its easiness to implement and its lower complexity. But then in the above detection methods the decisions made were based on certain linear transformation criterion and there are missing efforts to check the performance in between the detection process to improvise the transformation method better detection. So then the paper proposes Adaptive multiuser detection. Then the paper focuses on Interference cancellation methods which utilize the feedback to reduce Multiple Access Interference (MAI) in the detected bits. Under the Interference Cancellation umbrella the Successive Interference Cancellation (SIC) (6), (7) method is explored and simulated to observe the results. Then the next method which performs better than SIC and more commonly used in real life, called Parallel Interference Cancellation (PIC) (6), (8) is also implemented. Before going ahead to implement the final proposed detection method, this paper implements Differential Interference Cancellation (DIC) (9) method. And then this paper finally proposes an Inter- ference Cancellation method called the Adaptive Differential Interference Cancellation Multiuser Detection method which is cost effective in comparison to other Interference Cancellation methods as we will require lesser hardware (multipliers as in the SIC or Parallel Interference Cancellation (PIC)) for that matter. More ever since we are using adaptive method the desired BER can be further reduced. This proposed method will be discussed in detail in the following section.

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 the conventional zero-forcing ordered successive interference cancellation (ZF-OSIC) or minimum mean square error ordered successive interference cancellation (MMSE-OSIC) detection algorithm of vertical Bell-Labs layered space-time (V-BLAST) architecture, there exists an undesirable processing delay due to the calculation of the matrix pseudo-inverse and power reordering in detecting the symbol in each layer. The parallel interference cancellation (PIC) algorithm has a lower processing delay but poorer performance. Based on the features of SIC and PIC algorithms, an integrated PIC and OSIC detection (IPOD) algorithm is proposed which can lower the complexity as well as the processing delay of the whole algorithm with little performance loss. Moreover, there is an alterable parameter in IPOD algorithm which can be changed to obtain different trade off between complexity and performance. Thus, the proposed algorithm is more feasible for practical applications.

A CDMA scheme based on perfect Gaussian integer sequences

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.

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.

A simple direct sequence-code division multiple access receiver that combines adaptive beamforming with parallel interference cancellation in a multipath fading channel is proposed and analyzed. A fast adaptation, conjugate gradient algorithm is used to find the optimum beamformer weights. By beamforming, the desired user's signal is enhanced and the cochannel interference from other directions is reduced. For in-beam multiple access interference reduction, a parallel interference canceller is used in each RAKE finger. In the demodulation process, we propose a new demodulation method in which the incoming signal is correlated with the effecting spreading code rather than the physical spreading code called the effective matched filter. A new combining method called equivalent maximal ratio combining is also proposed and analyzed. The average uncoded bit error rate as a function of the average antenna signal-to-noise ratio and the number of receiving antennas is examined in a frequency selective Rayleigh fading channel for all proposed receiver structures. Both simulation and analysis show an increase in system capacity as a function of the number of antennas and the number of interferers canceled per finger.

The paper makes a brief introduction of Multiple Carrier-Code Division Multiple Access (MC-CDMA) scheme based on the principles of Orthogonal Frequency Division Multiplexing (OFDM) and CDMA, and a mathematical model of the uplink is established in MC-CDMA system.The simulation research on different multi-user detection methods in MC-CDMA system is implemented by using MATLAB software, and it also compares the impacts to system performance by several different multi-user detection methods, such as De-Correlation (DEC) multi-user detection, Minimum Mean Square Error (MMSE) multi-user detection and Parallel Interference Cancellation (PIC) multi-user detection, based on understanding and analyzing the experimental results.In addition, the paper also focuses on the research of multi level PIC multi-user detection performance in different primary detection methods and different series detections.

This paper investigates the effect of other-cell multiple-access interference (MAI) on some suboptimal multiuser detectors such as decorrelating multiuser detector, parallel interference canceller and adaptive minimum mean-squared error (MMSE) multiuser detector. Based on the Gaussian approximation for other-cell MAI, the bit-error rate (BER) of each detector is derived. Both analytical results and simulations indicate that the performance degradation due to other-cell MAI is significant. It is also observed that the other-cell MAI introduces more errors to multiuser detectors that require prior information on users' signature waveforms.

In this paper, we examine the performance of the iterative minimum mean-squared error (MMSE) adaptive receiver structure over asynchronous direct-sequence code-division multiple-access (DS-CDMA) channels. The receiver base-station uses estimates of the detected users' symbols to improve the convergence rate of the adaptive algorithm. The iterative algorithm used is based on the parallel interference cancellation (PIC) algorithm, and we examine the transient behaviour of both the least-mean-square (LMS) and the recursive-least squares (RLS) algorithms in asynchronous (i.e., reverse-link) DS-CDMA systems. Our results demonstrate a significant convergence improvement for the asynchronous case. It is shown that for the more severe asynchronous case, the convergence speed of the iterative algorithm is even better than the one achieved for the synchronous channel model. Also, for systems with no power control, the iterative algorithm is shown to be near-far resistance.

A low complexity Parallel Interference Cancellation (PIC) technique is proposed to suppress Multi-Access Interference (MAI) and minimize near-far effect for multi-user communication using Direct-Sequence Code Division Multiple Access (DS-CDMA). Interference signal is estimated using a relationship between cyclic correlation of the received signal and user's code of the interferer without requiring any knowledge of the channel condition. The user's codes for the multi-user DS-CDMA communication are constructed by using a set of m-sequences. The cyclic correlation is performed using Fast Walsh-Hadamard Transform (FWHT) which exhibits low computational complexity, i.e., it does not require any multiplication/division operations. In addition to low complexity, the proposed technique does not require complicated channel condition estimation and has no convergence issues. The uncoded Bit Error Rate (BER) performance of the proposed interference cancellation is calculated and compared with the conventional one over three different types of channels: AWGN, slow Rayleigh fading, and multipath channels.

A parallel interference cancellation (PIC) detection scheme is proposed to suppress the impact of imperfect synchronisation. By treating as interference the extra components in the received signal caused by timing misalignment, the PIC detector not only offers much improved performance but also retains a low structural and computational complexity.