Adaptive semi-blind linear parallel interference cancellation multiuser detection for DS-CDMA systems

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.

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.

In the multicarrier code division multiple access (MC-CDMA) uplink, the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper, we present a novel semi-blind adaptive MMSE multi-user detector based on subspace tracking for MC-CDMA systems in such scenarios. We develop an modified MMSE detector which can combat the performance deterioration induced by eigenvalues estimation errors. Moreover, the proposed method exploits the orthogonal projection approximation subspace tracking (OPAST) algorithm for adaptive subspace estimation. This proposed detector makes use of the spreading sequences of all known users to suppress the intracell interference, while suppresses the intercell interference using the modified MMSE detector. Simulation results demonstrate that the proposed semi-blind multi-user detector offers substantial performance gains over the blind multi-user detection and the traditional semi-blind method in a MC-CDMA uplink environment.

In this paper, the problem of multiuser detection for synchronous code division multiple access systems in both additive white Gaussian noise and multipath channels is addressed. A new multiuser detection receiver that uses an adaptive blind array along with an adaptive parallel interference canceler is proposed. The replacement of a conventional antenna array with a two-dimensional RAKE receiver is also considered for frequency selective Rayleigh fading channels. By using a constrained optimization criterion along with the gradient-projection algorithm, a blind algorithm for the adaptation of the array response vector is proposed. The new algorithm is superior to a few typical blind algorithms in the literature in terms of both performance and computational complexity. The proposed receiver has the ability to cancel very strong multiple access interference coming from the same direction as the desired signal. Simulation results are presented to show the excellent performance of the proposed combination scheme in comparison to that of using either a multiuser detection or adaptive antenna arrays in a severe near-far situation.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

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

In this paper, an adaptive parallel interference cancellation (PIC) receiver is introduced for asynchronous multicarrier code division multiple access based (MC-CDMA) communications systems. The multiple access interference (MAI) estimates are obtained using the bit estimates from the previous stage and the adaptive weights for the bit estimates. The adaptive weights are updated by minimizing the mean squared error between the received signal and its estimate based on the adaptation of the normalized least mean square (LMS) algorithm. The results from the simulations show that the performance of conventional PIC (CPIC) and adaptive PIC (APIC) can be different depending on the combining schemes used.

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.

In multiuser detection for a direct-sequence code division multiple access system, the genetic algorithm based multiuser detector (GA-MUD) is regarded as a feasible scheme that exhibits the near optimum performance yet requires less computational complexity than the optimum multiuser detector does. Through a new measurement called the computational efficiency, this paper indicates that the improper configuration of the fixed generation number makes most computations in the GA-MUD redundant. To overcome this problem, we propose a novel redundancy-saving genetic algorithm (RSGA) in this paper. The simulation result shows that the RSGA can effectively remove the unnecessary iterations with slight performance degradation and exploit the real potential of the GA-MUD. Hence, the computational complexity of the GA-MUD can be significantly reduced by eliminating the redundancy. Due to its simplicity, the proposed scheme can be applied to all varieties of GA-MUD and does not cause additional computation burden.

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 presents an adaptive multiuser receivers scheme for MIMO OFDM over Turbo-Equalization for Single-Carrier Transmission. It involves the joint adaptive minimum mean square error multiuser detection and decoding algorithm with prior information of the channel and interference cancelation in the spatial domain. A partially filtered gradient LMS (Adaptive) algorithm is also applied to improve the convergence speed and tracking ability of the adaptive detectors with slight increase in complexity. The proposed technique is analyzed in a slow and fast Rayleigh fading channels in MIMO OFDM systems. The adaptive multiuser detection for MIMO OFDM (AMUD MIMO OFDM) perform as well as the iterative equalization for single-carrier.

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.

This paper presents an iterative, yet effective minimum mean square error (MMSE) space-time multiuser detector (MUD) with the assistance of soft interference to enhance the performance in the multipath code division multiple access (CDMA) systems. The proposed space-time MUD is of rank one and comprises of a spatial filter and a temporal filter. To offset the performance loss due to such a constraint, the new MUD alternatively updates the spatial filter or the temporal filter with the soft interference cancellation in between so as to refine the received signal. Computer simulations show that the new MUD can drastically enhance the performance compared to previous works with modest computations, especially in highly loaded scenarios.

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

OCDMA is a promising candidate for Next Generation Passive Optical Networks (NG-PON). OCDMA-PON can potentially provide all customers with a Gb/s-cIass bandwidth upstream with inherent flexibility. Unfortunately OCDMA suffers from Multiple Access Interference (MAl) and various detection noises. To mitigate MAl we propose a novel parallel interference cancellation technique for incoherent DS-OCDMA that better exploits the positivity 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. As a matter of fact, our detector achieves more than 1 dB enhancement in SNR at 10.3 average BER for 32 active users compared to the LMMSE detector. This gain can be used to reduce the code length required to support a higher number of users.