Presence Of Multiple Access Interference Research Articles

Noncoherent Multiuser Chirp Spread Spectrum: Performance With Doppler Shift and Asynchronism

In this paper, we investigate multi user chirp spread spectrum with noncoherent detection as a continuation of our work on coherent detection in [1]. We derive the analytical bit error ratio (BER) expression for binary chirp spread spectrum (BCSS) in the presence of multiple access interference (MAI) caused by correlation with other user signals because of either asynchronism or Doppler shifts, or both, and validate with simulations. To achieve this we analyze the signal cross correlations, and compare traditional linear chirps with our recently-proposed nonlinear chirps introduced in [1] and with other nonlinear chirps from the literature. In doing so we illustrate the superior performance of our new nonlinear chirp designs in these practical conditions, for the noncoherent counterpart of [1].

MAI statistics estimation and analysis in a DS-CDMA system

A primary limitation of Direct Sequence Code Division Multiple Access DS-CDMA link performance and system capacity is multiple access interference (MAI). To examine the performance of CDMA systems in the presence of MAI, i.e., in a multiuser environment, several works assumed that the interference can be approximated by a Gaussian random variable. In this paper, we first develop a new and simple approach to characterize the MAI in a multiuser system. In addition to statistically quantifying the MAI power, the paper also proposes a statistical model for both variance and mean of the MAI for synchronous and asynchronous CDMA transmission. We show that the MAI probability density function (PDF) is Gaussian for the equal-received-energy case and validate it by computer simulations.

Pre-equalization Schemes for MIMO CC-CDMA Systems Over Frequency-Selective Fading Channels

Complementary coded code-division multiple access (CC-CDMA) system offers many attractive features over traditional CDMA systems due to its potential to provide ideal correlation properties and interference resistance feature in multiuser communication. However, the presence of multiple access interference (MAI) is one of the main drawbacks in reducing system performance under frequency-selective fading channels. In this paper, time domain pre-equalization (TDPE) is considered for downlink CC-CDMA systems to resist MAI over frequency-selective fading channels with perfect knowledge of channel state information. Pre-equalization shifts the complicated signal processing to the base station reducing the receiver complexity and improving the battery lifetime at the mobile station. We perform a comparison study between multiple-input multiple-output CC-CDMA (MIMO CC-CDMA) with pre-equalization and traditional equalization schemes towards overcoming MAI. The comparison study revealed that TDPE at the transmitter enriches diversity gain and reduces MAI in MIMO CC-CDMA systems with less receiver complexity.

Mean Spectral Efficiency Evaluation of the Uplink of MC-CDMA Cellular Systems

The mean spectral efficiency of the uplink of MC-CDMA cellular systems is evaluated. The complete system employs the multicarrier-code division multiple access (MC-CDMA) technique, a linear antenna array at the base stations, frequency interleaving, maximal ratio combining, adaptive modulation, perfect power control and a cyclic prefix large enough to eliminate the intersymbol and intercarrier interferences effects. This scenario assumes the presence of multiple access interference, co-channel interference, additive white Gaussian noise, exponential path-loss and slow and frequency selective Rayleigh fading channel. In this context, a procedure to calculate the mean spectral efficiency of the cellular network is proposed, which depends of the mean bit error rate, the channel reuse factor, the increase of bandwidth due to the cyclic prefix, the system load, the spreading factor, the cell radius and the modulation schemes. Results show that an unitary channel reuse factor is the most efficient way to use the spectrum, but it is critical in relation to the transmission rates. Also, as orthogonal frequency division multiple access technique is a particular case of MC-CDMA, the spectral efficiency of both techniques is compared.

A Novel Technique to Detect Code for SAC-OCDMA System

Abstract The main task of optical code division multiple access (OCDMA) system is the detection of code used by a user in presence of multiple access interference (MAI). In this paper, new method of detection known as XOR subtraction detection for spectral amplitude coding OCDMA (SAC-OCDMA) based on double weight codes has been proposed and presented. As MAI is the main source of performance deterioration in OCDMA system, therefore, SAC technique is used in this paper to eliminate the effect of MAI up to a large extent. A comparative analysis is then made between the proposed scheme and other conventional detection schemes used like complimentary subtraction detection, AND subtraction detection and NAND subtraction detection. The system performance is characterized by Q-factor, BER and received optical power (ROP) with respect to input laser power and fiber length. The theoretical and simulation investigations reveal that the proposed detection technique provides better quality factor, security and received power in comparison to other conventional techniques. The wide opening of eye in case of proposed technique also proves its robustness.

Novel multiuser ultra‐wideband receiver based on adaptive modelling of generalised normal‐Laplace distribution

This study takes a new receiving approach to detect an ultra-wideband (UWB) signal in the presence of multiple access interference (MAI). A generalised normal-Laplace (GNL) distribution is exploited for designing a novel multiuser UWB receiver. To more accurately comply with real statistical behaviours of the MAI-plus-noise in time-hopping multiple access UWB systems, a modified parameter estimator, which is an adaptive method of moments estimation (aMME), is proposed. In the existing studies, the analysis about GNL model shows appropriate results for UWB simulations, while it requires heavy complexity to obtain the precise distribution because of no existence of a closed-form probability density function (PDF) expression. To practically evaluate the GNL PDF, the fast Fourier transform, which can significantly reduce the computational complexity is considered. The GNL using the aMME outperforms the conventional matched filter UWB receiver, the soft-limiting receiver, the Gaussian–Laplace mixture receiver, the p-order metric receiver and the previous GNL receivers, especially in high SNR ranges. Furthermore, the presented GNL receiving method is applied to each finger of the conventional Rake receiver for signal detection in IEEE UWB multipath channels. The proposed Rake receiver based on the GNL distribution performs better than the conventional Rake receiver in multipath fading channels.

PN CODE ACQUISITION USING SMART ANTENNA AND ADAPTIVE THRESHOLDING CFAR BASED ON ORDERED DATA VARIABILITY FOR CDMA COMMUNICATIONS

Recently, a novel approach for PN code acquisition of direct sequence code division multiple- access (DS-CDMA) systems in Rayleigh fading multipath channel was proposed in (1). The authors considered a combination of adaptive thresholding constant false alarm rate (CFAR) and smart antennas to increase the system capacity and consequently enhance the detection performance. This paper considers still the problem of PN code acquisition for DS-CDMA communication systems over Rayleigh fading channels under the presence of multipath and multiple-access interference (MAI) signals. We propose and analyze an adaptive array acquisition system, which integrates an adaptive thresholding technique based on ordered data variability (ODV) index constant false alarm rate (ODV-CFAR) and digital beamforming where a low complexity least mean square (LMS) algorithm is used to calculate the optimal weighting coe-cients. This approach is expected to mitigate interferences caused by the presence of multiple access interference and multipath. Unlike other approaches based on a flxed censoring point when the number of interferences is assumed known, ODV-CFAR processing does not require prior knowledge about the number of interferences. The simulation results show a robust performance of the proposed system in varying mobile communication channels.

Capacity Analysis of Asynchronous Optical CDMA Systems Utilizing Wrapped OPPM: Multiple Access Interference Limited Case

In this paper the capacity of asynchronous incoherent optical CDMA (OCDMA) systems employing wrapped overlapping pulse position modulation (WOPPM) for two types of receiver structures is analyzed. The analysis is performed in the presence of multiple access interference as the main limiting factor. Throughput analysis for uncoded systems utilizing WOPPM is conducted and a lower bound on the capacity for two types of correlator based receiver structures is derived. Furthermore, for a fixed code and a fixed maximum number of symbols, a numerical solution for maximizing the capacity is presented. Moreover, to minimize the probability of error when the throughput and the number of users are fixed, a numerical solution is introduced. Finally, for two common design problems, algorithms are proposed for achieving the system parameters well close to the optimal settings. The results of our different analyses for uncoded systems using WOPPM have been compared with unwrapped OPPM. Our results have also been compared with on-off keying (OOK) and pulse position modulation (PPM). The results of this work show that, for a fixed throughput, the performance of WOPPM is significantly superior to OOK and PPM.

Photon-counting chip-interleaved iterative PIC detector over atmospheric turbulence channels

A photon-counting-based iterative parallel interference cancellation (PIC) scheme for free-space opticalcommunications in the presence of multiple-access interference, shot noise, background radiation, and turbulence fading is designed. An efficient chip-level iterative equivalent noise estimation algorithm is also derived. Simulation results show that the proposed scheme can achieve a single-user performance,bound with the fast convergence property. More importantly, it can eliminate the bit-error rate floor of the conventional optical code-division multiple-access system with the aid of a relatively short spreading code length.

Blind adaptive receiver based on RLS-CMA for TH-UWB system

A novel receiver is proposed in this paper that uses the blind adaptive multi-user detection (MUD) technique to deal with the presence of multiple-access interference (MAI) and inter-symbol interference (ISI) in terms of high speed time-hopping ultra-wideband (TH-UWB) system. The receiver firstly adopts constrained recursive least squares constant modulus algorithm (RLS-CMA) to conduct adaptive decorrelation for eliminating MAI and ISI, and then uses projection approximation subspace tracking with deflation (PASTd) algorithm to launch channel estimation, do maximal ratio combining (MRC) and improve signal to interference plus noise ratio (SINR). The simulation results show that the proposed receiver can go to convergence rapidly under all four IEEE's UWB channel models, the bit error ratio (BER) performance is better than that of the traditional receiver and rake receiver.

Bit Error Probability of Space-Shift Keying MIMO Over Multiple-Access Independent Fading Channels

In this paper, we study the performance of space-shift keying (SSK) modulation for multiple-input–multiple-output (MIMO) wireless systems in the presence of multiple-access interference. More specifically, a synchronous multiuser scenario is considered. The main technical contributions of this paper are given as follows: Two receiver structures based on the maximum-likelihood (ML) criterion of optimality are developed and analytically studied, i.e., single- and multiuser detectors. Accurate frameworks for computing the average bit error probability (ABEP) over independent identically distributed (i.i.d.) Rayleigh fading channels are proposed. Furthermore, simple and easy-to-use lower and upper bounds for performance analysis and system design are introduced. The frameworks account for the near–far effect, which significantly affects the achievable performance in multiple-access environments. In addition, we extend the analysis to generalized SSK (GSSK) modulation, which foresees multiple active antennas at the transmitter. With respect to SSK modulation, GSSK modulation achieves higher data rates at the cost of increased complexity at the transmitter. The performance of SSK and GSSK modulations is compared with that of conventional phase-shift keying (PSK) and quadrature amplitude modulation (QAM) schemes, and it is shown that SSK and GSSK modulations can outperform conventional schemes for various system setups and channel conditions. In particular, the performance gain of SSK and GSSK modulations increases for increasing values of the target bit rate and of the number of antennas at the receiver. Finally, we put forth the concept of coordinated multipoint (or network MIMO) SSK (CoMP-SSK) modulation as a way of exploiting network cooperation and the spatial-constellation diagram to achieve high data rates. Analytical derivations and theoretical findings are substantiated through extensive Monte Carlo simulations for many setups.

An analysis of delay-locked loop tracking binary Markovian sequence in the presence of multiple access interference and channel noise

This paper presents an analysis of delay-locked loops in direct spreading code division multiple access systems with Markovian spreading sequences. The established analysis with piecewise constant approximation is similar to the mean field analysis which estimates macroscopic characteristics of a spatially-distributed probabilistic system from the local interaction between the elements of the system with low computational complexity. The low complexity makes it possible to analyze the behavior of not only second-order archetype delay-locked loops but also higher-order non-coherent delay-locked loops in the presence of both multiple access interference and channel noise. It is found from the presented analysis that replacing i.i.d. spreading sequences with Markovian spreading sequences with non-vanishing negative autocorrelation reduces the phase tracking error of a non-coherent 1Δ delay-locked loop. The time spent for estimating the error by the presented method is about 1/1000 compared with that by the path integral method.

Estimation and equalization of fiber-wireless uplink for multiuser CDMA 4G networks

Fiber-wireless (Fi-Wi) access fronts can support 100s of Mb/s envisioned by 4G networks. However, a major issue associated with Fi-Wi links is the nonlinear distortion of the radio-over-fiber (ROF) link coupled with the multipath dispersion of the wireless channel. Estimation and subsequent equalization of the concatenated fiber-wireless channel needs to be done, especially at high bit rates. The uplink is severely affected due to large fluctuations in the radio signal. This paper proposes an estimation and subsequent equalization algorithm for the Fi-Wi CDMA uplink. The estimation employs the properties of pseudo noise (PN) sequences and the equalization uses a novel Hammerstein type decision feedback equalizer (HDFE). The estimation and equalization are performed in the presence of multiple access interference (MAI) and wireless and optical channel noise. The cumulative effects of multiuser interference, multipath dispersion, nonlinear distortion, and noise are all considered in our analysis. Correlation properties of white-noise like PN sequences enable decoupling of the linear (wireless) and nonlinear (optical) channel portions. Furthermore, we propose a unique algorithm to mitigate MAI. Numerical evaluations show a good estimation and equalization of both the linear and nonlinear channels. Bit error rate (BER) simulations show that this algorithm leaves only small residual MAI.

Asymptotic Statistics of the Mutual Information for Spatially Correlated Rician Fading MIMO Channels With Interference

The statistics of the mutual information of a separately correlated Rician fading multiple-input multiple-output (MIMO) channel in the presence of multiple-access interference are addressed in this paper. The approach followed is asymptotic in the number of transmit, receive, and interfering antennas, which are all assumed to grow asymptotically large while approaching finite ratios. In this asymptotic regime: i) the mean and the variance of the mutual information are calculated; and ii) the mutual information distribution is shown to converge to the Gaussian distribution, specified completely by the mean and variance, under some mild technical conditions. The asymptotic method adopted relies on two powerful tools developed in the context of theoretical physics: the replica method and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">superanalysis</i> . The former has been already successfully applied in several research studies on MIMO systems. The application of these asymptotic results takes advantage of the fact that, in spite of being developed under the assumption of an asymptotically large number of antennas, they still represent a very accurate approximation even when the number of antennas is limited to a few units, as supported by the ample set of numerical results obtained by Monte Carlo simulations and reported in this paper.

Demonstration and Analysis of Asynchronous and Survivable Optical CDMA Ring Networks

We propose an incoherent optical code-division multiple access (CDMA) ring network configured such that each node in the network includes all-optical add/drop multiplexers to provide truly asynchronous operation and a self-healing protection monitoring circuit to ensure service availability during link failure. We also analyze the performance of such a network considering the presence of multiple-access interference and noise from incomplete dropping of the desired data and accidental dropping of the undesired data. Furthermore, we experimentally demonstrate the proposed design with a two-node system operating at 2.5 Gbits/s. Error-free transmission is obtained and a 5.3 dB power penalty at a bit error rate of 10−9 is measured for the worst connection configuration. The complete optical CDMA ring network is successfully demonstrated in truly asynchronous operation.

Code-multiplexed transmitted-reference UWB systems in a multi-user environment

Recent studies have investigated alternative methods to the realization of transmitted-reference ultra-wideband communication systems. In addition to the conventional scheme, in which data and reference are separated in time, two other methods have been proposed in which the separation is achieved through a frequency shift or via code multiplexing. In a single-user scenario the alternative methods have advantages in terms of error rate and receiver implementation. In this paper the three schemes are compared in a multi-user environment. It is shown that even in the presence of multiple access interference the unconventional solutions are preferable and, in particular, the code multiplexing method has the best performance and is the simplest to implement.

Evaluation of Multiusers’ Interference on Radiolocation in CDMA Cellular Networks

Radiolocation has been previously studied for CDMA networks, the effect of Multiple Access Interference has been ignored. In this paper we investigate the problem of Radiolocation in the presence of Multiple Access Interference. An extensive simulation technique was developed, which measures the error in location estimation for different network and user configurations. We include the effects of lognormal shadow and Rayleigh fading. Results that illustrate the effects of varying shadowing losses, number of base stations involved in position location, early-late discriminator offset and cell sizes in conjunction with the varying number of users per cell on the accuracy of radiolocation estimation was presented.

Performance improvement in detection and estimation of MC-CDMA systems over MIMO channels

Multiple-input Multiple-output (MIMO) multi-carrier code division multiple access (MC-CDMA) is a strong candidate for the downlink of future mobile communications to obtain high data rates. Nevertheless, during any transmission over fading channel, performance of MC-CDMA systems are highly degraded due to the presence of multiple access interference (MAI). Multi-user detection (MUD) and channel estimation play a major role in overcoming MAI and characterising the channel, respectively. In this paper, space time serial interference cancellation (STSIC) detection using random and Gold codes and turbo aided iterative channel estimation (ICE) techniques are extended for MC-CDMA system MIMO channels to overcome MAI. Simulation results show STSIC outperforms optimal MUD and linear MUD techniques in mitigating MAI and turbo aided ICE surpasses ICE in characterising the channel with reduced error rates.

Multiple-Access Performance Analysis of Combined Time-Hopping and Spread-Time CDMA System in the Presence of Narrowband Interference

We consider a combined time-hopping (TH) and spread-time (ST) multiple-access technique that uses an internal code. In this method, the duration of each bit is divided into <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ns</i> frames. The outputs of the encoder and a pseudorandom (PN) sequence specify the number of the frame in which the data bit is transmitted in ST code-division multiple-access (ST-CDMA) form using the second PN sequence. We consider the correlator receiver, followed by the channel decoder. We obtain the performance of the combined method in additive white Gaussian noise (AWGN) and fading channels in the presence of multiple-access interference (MAI) and narrowband interference (NBI). We also consider the conventional ST-CDMA system and compare its performance with our method. The results indicate that the proposed method has much better performance while it has the same bandwidth and spectral efficiency as the conventional ST-CDMA system. In the new method, like in the conventional ST-CDMA system, an external error-correcting code can be applied for further performance enhancement.

Multiple-Access Interference Plus Noise-Constrained Least Mean Square (MNCLMS) Algorithm for CDMA Systems

Since multiuser code-division multiple-access (CDMA) communications systems suffer significantly from multiple-access interference (MAI) and from classical white Gaussian noise, it is therefore necessary to consider their impact on the performance of these systems. It is well known that the learning speed of any adaptive filtering algorithm is increased by adding a constraint to it. In this paper, a constrained least-mean-square (LMS) algorithm, which incorporates the knowledge of the number of users, spreading sequence length, and additive noise variance, is developed subject to the new combined constraint comprising the MAI and noise variance for a synchronous downlink direct-sequence CDMA system. The novelty of this constraint resides in the fact that the MAI variance was never used as a constraint. In our approach, a Robbins-Monro algorithm is used to minimize the conventional mean-square-error criterion subject to the variance of the new constraint (MAI plus noise). This constrained optimization technique results in an (MAI plus noise)-constrained LMS (MNCLMS) algorithm. The MNCLMS algorithm is a type of variable step-size LMS algorithm where the step-size rule arises naturally from the constraints on MAI and noise variance. Convergence and tracking analysis of the proposed algorithm are carried out in the presence of MAI. Finally, a number of simulations are conducted to compare the performance of the MNCLMS algorithm with other adaptive algorithms.