Multiuser Access Interference Research Articles

Performance Analysis of Two Dimensional Spreading for OFCDM Femtocell Users Overlaid with OFDM Macrocell

Femtocells provide indoor coverage for voice and high speed data services and are self-deployed by end-users. Orthogonal frequency code division multiplexing (OFCDM) is a multi-carrier spread spectrum technique that utilizes two dimensional spreading in time and frequency domains simultaneously to improve frequency diversity and minimize multiuser access interference. In this paper, variable spreading factor (VSF)-OFCDM is employed in subcarrier allocation for femto users (FUEs) in a hybrid femto/macro network, and the effect of FUE deployment on macro users (MUEs) is studied. We evaluate the bit error rate (BER) performance of FUEs and MUEs through Monte Carlo simulation under various load conditions in noise and interference-limited scenarios. Relationship between the channel load and optimum spreading factor employed by FUEs for the given network configuration, is also analyzed. We provide various graphs showing the impact of FUE spreading factor on the MUE BER. We note that in interference-limited conditions, $$4\times 8$$ spreading $$(\text{ time}\times \text{ freq}.)$$ proves to be the best choice for macro users; and for the noise-limited case, when there are limited number of users and channel noise is dominant, $$2\times 16$$ spreading is the optimum choice for the considered hybrid network configuration. We also evaluate the effect of femto wall penetration loss on macro BER for various spreading factors. The favorable spreading factor for MUEs is tabulated for different loads and different number of subcarriers, which can be used as a deployment guide in this heterogeneous network.

Impact of GVD and SPM on the Performance of DS-OCDMA

The bit error rate (BER) performance of direct sequence optical code division multiple access with cascaded optical amplifiers is analytically investigated in presence of fiber group velocity dispersion (GVD) and self phase modulation (SPM). Super Gaussian-shaped optical orthogonal codes are employed as address sequence for intensity modulation direct detection technique. The signal to noise power ratio for the proposed system is calculated on account of receiver, optical amplifier, and multiuser access interference noises. The system performance is determined as a function of optical signal power, code length, code weight, number of simultaneous users, fiber length, and order of Super Gaussian. The power penalty suffered by the system is evaluated at BER of 10-9. The numerical results show that the BER performance of the proposed system is highly dependent on the input signal power, bit rate, fiber length, and shape of the chip. It is found that the performance of the proposed system can be improved significantly in presence of GVD and SPM using Gaussian shaped chip.

Spectrally Phase Coded Waveform Discrimination at 10 GHz for Narrow Band Optical CDMA within 100 GHz Spectral Window

We demonstrate binary spectral phase coded waveform discrimination at 10 GHz for narrow band optical code-division multiple-access (NB-OCDMA) via direct electrical detection without using any optical hard-limiter. Only 9 phase-locked, 10 GHz spaced, spectral lines within a 100 GHz spectral window are used for the phase coding. Considerably high contrast ratio of 5 between signal and multiuser access interference noise can be achieved for <TEX>$4{\times}10\;G\;pulse/sec$</TEX> timing coordinated OCDMA at a simple electrical receiver with 50 GHz bandwidth.

Adaptive Subcarrier Allocation in Synchronous Reverse Links of a Multicarrier CDMA System with Time and Frequency Spreading

Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement, ranging from 1.5 dB with 2times16 (time times frequency) and spreading to 7 dB with 16times2 (time times frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of Eb/No, a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the Eb/No threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB.

A Blind RAKE Receiver With Robust Multiuser Interference Cancelation for DS/CDMA Communications

In this paper, a blind RAKE receiver with robust multiuser access interference cancellation is presented for frequency-selective Rayleigh fading channels. In contrast to a conventional receiver, here, only knowledge of the spreading code and rough timing of the desired user is required. By investigating the code space of the multipath signals and the data vector space, a RAKE filtering vector is developed to extract the desired data from all the paths of the desired user. Our proposed technique not only exploits the characteristics of multipath propagation but also the characteristics of timing offsets that may occur in the receiver, to facilitate the application of a blind linear filter-optimization technique for robust interference suppression. Based on the RAKE filtering vector, interference rejection is implemented by using the auxiliary-vector (AV) technique. Our approach, however, effectively overcomes the sensitivity of the original AV method to multipath propagation and timing offsets. To mitigate the signal cancellation at relatively high signal-to-interference and noise ratios (SINR) resulting from the estimation errors of the RAKE filtering vector, robust strategies are introduced in addition to the linear filter optimization. Simulation results are presented to show the effectiveness of the proposed techniques.

Influence of Single-mode Fiber Dispersion and Pulse Linear Chirp on Direct-detection Optical CDMA Systems

In this paper, the influence of single-mode fiber (SMF) dispersion and pulse linear chirp on direct-detection optical code-division multiple-access (OCDMA) systems is analyzed. In the analysis, optical orthogonal codes (OOCs) are employed as address sequence codes. Avalanche photodiode (APD) is selected as the system receiver. The effect of APD shot noise, bulk dark current, surface leakage current, thermal noise current and multiuser access interference (MAI) is considered. The loss of system devices is omitted. The system performance is evaluated under the assumption of optical signal power, code length and code weight are constant. The numerical results show that, if the effect of SMF dispersion is considered, the system performance, i.e., bit error rate (BER) is degradation. With pulse width decreasing and transmission distance increasing, the system performance becomes aggravation. Also, the pulse linear chirp has an important effect on the system BER performance.

Meeting mobile's demands with multicarrier systems

With the increasing requirements for future wireless applications, OFDM, MC-CDMA, and MC-DS-CDMA have all been considered for 4G wireless systems. These systems have the ability to incorporate very large band widths without sacrificing equalization complexity. The long symbol duration is effective at mitigating ISI, and adaptive modulation or frequency diversity can be used to provide protection against destructive fades. The benefit of MC CDMA is that it experiences frequency diversity because each bit is transmitted over several independently faded subcarriers. If some subcarriers experience destructive fades, diversity combining can be used at the receiver to recover the data. This improves the BER performance over OFDM, and this improvement is more significant as the number of subcarriers is increased. The draw back of MC-CDMA is that it may experience high levels of multiuser access interference (MAI) when the channel is heavily loaded. This occurs because each chip of the PN sequence experiences independent fading, which tends to destroy the orthogonality between spreading sequences. This increases the MAI and degrades the BER performance. Although OFDM, MC-CDMA, and MC-DS-CDMA signals experience a high PAPR, synchronization issues, and ICI, the benefits greatly outweigh these disadvantages.

Frequency-domain interference cancellation and nonlinear equalization for CDMA systems

In wireless broadband communications using code division multiple access (CDMA), interference cancellation (IC) techniques have been proposed to reduce multiuser access interference (MAI); however, their performance is limited by complexity constraints that still favor the use of rake receivers. In this paper, we propose an IC architecture that makes use of a block decision feedback equalizer (DFE) to remove intersymbol interference (ISI) and whose overall complexity is much lower than equivalent (in terms of performance) existing structures. In fact, by making use of a particular block data transmission format, all filters are implemented in the frequency domain (FD), and in particular, the DFE is iteratively designed according to the reliability of the detected data in the previous iteration. Due to the strong interaction of the FD-IC and the block DFE, error propagation in the receiver is reduced by simply interleaving (INT) chips before transmission. Simulations performed for an uplink communication on a wireless multipath channel show that the combination of FD-IC, block DFE, and chip INT provides an efficient solution with good performance for CDMA systems in dispersive channels.

A RAKE Receiver Design for WCDMA FDD Uplink With an RLS-Based Adaptive Beamforming Scheme

In this paper, we present a RAKE receiver design with adaptive antenna arrays for the wide-band code-division multiple-access (WCDMA) frequency-division duplexing (FDD) uplink. The RLS-based adaptive beamforming scheme is proposed and can be built with the existing one-dimensional RAKE receiver. We adaptively calculate the beamforming weight vector for each multipath of the desired user, and use maximum ratio combining (MRC) to combine each multipath signal in the demodulation process. Two matched filters based on the spreading waveforms are designed in our scheme for WCDMA FDD uplink application. The proposed scheme has the ability of suppressing strong multiuser access interference and the other types of interferers through spatial nulling. The tracking capability of the proposed algorithm is demonstrated in the simulation results.

A hybrid interference cancellation scheme for blind multiuser detection in multipath fading channels

This paper introduces and analyzes a detection scheme for adaptive suppression of Multiuser Access Interference (MAI) and MultiPath Distortion (MPD) for mobile station of DS/CDMA system. The proposed detection scheme may amount to a RAKE receiver structure, wherein each branch is considered as a linear multiuser filter designed under a Linear Constrained Minimum Variance (LCMV) optimization strategy to suppress MAI, followed by a proper combining rule to suppress MPD. The adaptive blind multiuser detecting and optimum combining of the proposed receiver are realized, based on the Least-Mean-Square (LMS) algorithm and an adaptive vector tracking algorithm respectively. Finally, the feasibility of the above two algorithms is proved by the numerical results provided by computer simulation.

Quadratic programming neural network based integrated space-time interference suppression in CDMA systems

Quadratic programming models for integrated space-time interference suppression in CDMA systems are proposed in this paper. The models integrate the advantages of smart antenna and RAKE receiver, mitigate multiuser access interference (MAI) and interchip interference (ICI), and combine multipath components. The zero-forcing conditions are derived. Neural network implementation of the models is also studied.

Blind RLS based space-time adaptive 2-D RAKE receivers for DS-CDMA communication systems

The goal of a blind two-dimensional (2-D) RAKE receiver for DS-CDMA is to cancel strong multiuser access interference (MUAI) while optimally combining the signal of interest's (SOI) multipath. The weight vector yielding the optimum SINR for bit decisions may be computed as the largest generalized eigenvector of a matrix pencil composed of the SOI plus MUAI and MUAI alone space-time correlation matrices. An alternative blind 2-D RAKE receiver is presented based on RLS-type space-time adaptive filtering that offers competitive performance. The applicability of the scheme to the IS-95 uplink is addressed.

Reduced-dimension blind space-time 2-D RAKE receivers for DS-CDMA communication systems

Blind space-time RAKE receivers for classical DS-CDMA that cancel strong multiuser access interference while optimally combining the desired user's multipath are presented. The delay spread is assumed to be fraction of a symbol interval as in the IS-95 CDMA standard. The post-correlation symbol interval is segmented into that which encompasses the RAKE fingers and that away from the RAKE fingers. The signal-plus-interference and interference alone space-time correlation matrices are estimated during the former and latter, respectively. Reduced-dimension space-time RAKE receivers are investigated for alleviation of the computational burden of computing the largest generalized eigenvector of the resulting matrix pencil. Theoretical analysis and supporting simulations reveal that if the space and time compressing transformations are designed judiciously, reduced-dimension two-dimensional (2-D) RAKE receivers offer faster convergence at the expense of only a slight loss in asymptotic signal-to-interference-plus-noise ratio (SINR) relative to the full-dimension space-time RAKE receiver. Applications to the IS-95 uplink are also presented along with supporting simulations.

Low-complexity space-time processor for DS-CDMA communications

A novel wideband beamforming technique for cellular CDMA systems is presented in this paper. The proposed algorithm asymptotically provides the maximum SINR estimate of the signal with the desired code (SDC) by optimally combining desired signals from different paths and canceling strong multiuser access interference (MUAI). A two-dimensional (2-D) matched filter structure is used where not only different temporal samples of the matched filter output are processed but where those from matched filters connected to different antennas are processed as well. In contrast to previously proposed techniques, an exact code synchronization for the SDC is not required. The algorithm presented herein asymptotically provides the arrival time of the multipaths within a bit period and the optimum beamformers for extracting each of them. Space-time filters for combining the fingers across both space and time while canceling the MUAIs are constructed correspondingly. The instrumental property exploited by this technique is the fact that although the respective spectra of the SDC and MUAI components at the matched filter output are statistically identical, the respective spectra of their squared values differ. A simplified RAKE structure-based receiver is also proposed. The 2-D RAKE receiver considerably decreases the computations but requires a coarse SDC code synchronization. A technique to achieve coarse SDC code synchronization is also proposed.

Constrained optimisation based space-time-code blind receiver in CDMA systems

A space-time-code (STC) blind receiver is proposed. The receiver has the advantages of constrained minimum output energy (CMOE) blind multiuser detectors and space-time (ST) 2D-Rake receivers and overcomes their limitations. It can make full use of spatial, temporal and code signatures involved in the desired user&apos;s signal to suppress multiuser access interference and combine multipath components.

On hetero-associative neural networks and adaptive interference cancellation

We discuss two novel adaptive algorithms for generalized eigendecomposition that are derived from a two-layer linear feedforward hetero-associative neural network. In addition, we provide a rigorous convergence analysis of the adaptive algorithms by using stochastic approximation theory. Finally, we use these algorithms for on-line multiuser access interference cancellation in code-division-multiple-access-based cellular communications. Numerical simulations are reported to demonstrate their rapid convergence.

A low-complexity space-time RAKE receiver for DS-CDMA communications

An algorithm is presented for estimating the quantities needed by a space-time RAKE receiver for DS-CDMA to achieve maximum SINR for the desired user. The proposed algorithm, which is applicable in the case of either periodic or aperiodic spreading codes, asymptotically provides the exact time of arrival of each dominant multipath within a bit period, and the optimum beamformer for extracting each multipath. This is achieved by using cyclostationarity to exploit the difference between the respective spectra of the desired user and each multiuser access interferer at the output of the correlator based on the desired user's code. Estimates of the relative time delay and optimal beamformer for each RAKE finger are used by the space-time RAKE receiver to optimally combine the desired user's multipath while simultaneously canceling strong multiuser access interference. Simulations of a near-far scenario are presented demonstrating the efficacy of the proposed algorithm.