Multi-access Interference Research Articles

Multiaccess Interference Suppression in Orthogonal Space–Time Block Coded MIMO Systems by Adaptive Projected Subgradient Method

This paper introduces adaptive filters that are effective to suppress multiple access interference (MAI) in orthogonal space-time block coded/ multiple-input multiple-output (OSTBC-MIMO) systems. We define an optimal linear filter that minimizes the mean-square error between the filter output and a scaled version of the desired output under a constraint defined by the available channel state information (CSI). The adaptive filters refine a given estimate of the optimal filter by suppressing a sequence of closed convex functions with the adaptive projected subgradient method (APSM) at each iteration. To provide robustness against imperfect CSI, the adaptive filters use not only the available CSI but also estimates of previously transmitted symbols, which usually belong to a small finite set in digital communication systems. The resulting algorithms employ computationally efficient projections onto hyperplanes or hyperslabs and do not require any matrix inversion. An efficient recursive scheme based on such an algorithm is also presented. Convergence analysis and simulation results show the excellent performance of the proposed schemes.

Precoded Multiuser OFDM Transceiver in Timing Asynchronous Environment

The performance of a multiuser orthogonal frequency-division multiplexing (OFDM) transceiver, called the precoded multiuser OFDM (PMU-OFDM), with time asynchronous access is investigated in this paper. It is shown that multiaccess interference (MAI) due to time asynchronism is asymptotically zero as the number of parallel input symbols <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> becomes large in the PMU-OFDM system. Then, we show that PMU-OFDM with even or odd Hadamard-Walsh codewords can significantly suppress the MAI effect due to time asynchronism. For the half-loaded case, no sophisticated signal processing technique is needed by PMU-OFDM for MAI suppression. For the fully loaded situation, PMU-OFDM demands less complexity than does conventional OFDMA for interference suppression since PMU-OFDM deals with only one half of interferers.

Optical plankton: an optical oceanic probing scheme

In this paper, we evaluate the possibility of a wireless sensor network concept operating in the oceanic environment. A population of underwater sensor nodes, termed plankton, form a distributed sensor network characterized by the deployment of many miniature and low-cost sensors capable of profiling the particulate composition in the immediate marine environs. The stimulation of backscatter signals from the surrounding medium is one principle of operation studied, but the potential for additional probing methods is discussed. The optically probed signal is communicated to a base station, where the large number of received signals is fused to obtain an accurate estimate of the nature of the local aqueous medium. The many similarities and differences between the atmospheric and oceanic sensing and wireless communication environments are discussed and the distinctive features of an oceanic probing system are underlined. Specific scientific applications are briefly reviewed. The challenges to be met are addressed in general and a focused analysis of the specific issue of multi-access interference (MAI), common to all optical wireless-based sensor networks, is presented. Novel analytical approaches have been employed to evaluate and quantify the MAI. It is the object of this paper to assess the feasibility of a novel miniaturized oceanic probing system and explore some of the challenges. It would appear that the proposed scheme could be the basis of an innovative oceanic probing method and fill a niche not as yet catered for with existing technologies.

Carrier Frequency Offset Compensation with Successive Cancellation in Uplink OFDMA Systems

Similar to OFDM systems, OFDMA systems also suffer from frequency mismatches between the receiver and the transmitter. However, the fact that each uplink user has a different frequency offset makes the compensation more challenging than that of OFDM systems. This letter proposes successive interference cancellation (SIC) for compensating the frequency offset in the uplink OFDMA systems. A decorrelator is used to remove the inter-carrier interference (ICI) within a user's signal and successive cancellation is applied to mitigate the multi access interference (MAI) arising due to the frequency difference among uplink users. The proposed algorithm is shown to eliminate the interference and has a manageable complexity.

Performance of Multiple Antenna DS-CDMA UWB System With Noisy Channel Estimates and Narrow-Band Interference

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this work, the performance of a DS-CDMA ultra-wideband (UWB) system with multiple antennas at the receiver, in the presence of multiple users and narrow-band interference (NBI), is evaluated. A spatio-temporal UWB receiver is proposed which first forms an estimate of the channel in the presence of multi-access interference and NBI, and then uses this estimate to optimally combine the multipath components such that the output signal-to-interference-plus-noise-ratio is maximized. The performance of this system, through an analytically-derived expression for the probability of error, is investigated. In particular, a closed-form expression for the probability of error is established when the temporal correlation between the NBI during the channel-estimation phase and the data-detection phase is negligible. The effects of both quality of channel estimates and antenna correlation on the system performance are analyzed. </para>

Multiaccess interference in a non-line-of-sight ultraviolet optical wireless sensor network

The concept of exploiting both the scattering properties and the absence of solar radiation in the "solar blind ultraviolet" spectral range for achieving a non-line-of-sight (NLOS) communication link for wireless sensor networks has been discussed in scientific literature. We address the issue of the multiaccess interference (MAI) that would be encountered in a simple and low-cost sensor network operating on the above NLOS principle, for different sensor node densities and traffic levels, and use a Poisson model for the sensor node distribution. A metric for evaluation and comparison of sensor node distribution scenarios is derived and used to discuss the performance limitations of NLOS wireless sensor networks operating in the solar blind ultraviolet spectrum. Guidelines for NLOS wireless sensor network design are outlined taking into consideration the cumulative effect of interference from distant sensor nodes, the expected number of hops, and the trade-off between node redundancy and node isolation. The significant contribution of network traffic control to system operability is demonstrated.

Fuzzy Integration Detector for Multi-Access UWB Impulse Radio

A fuzzy integration (FI) detector aiming to alleviate the adverse effect due to multi-access interference (MAI) is presented. The Fl detector suppresses MAI by clipping the magnitude of the possible corrupted signal, hence avoids the domination of any interfered oversampling pulses in the detection process. Simulation results illustrate that the proposed detector could improve the bit-error-rate (BER) performance of a multi-access ultrawide bandwidth (UWB) impulse radio.

Performance Analysis of Pre-Rake Diversity With Generalized Orthogonal Codes for DS-CDMA

The pre-rake technique is mainly adopted in the time-division duplexing (TDD) direct-sequence code-division multiple access (DS-CDMA) system to provide multipath diversity. In this paper, generalized orthogonal (GO) codes are applied to the TDD/DS-CDMA system employing the pre-rake technique to reduce the self and multiaccess interference and improve system capacity. Based on the theoretical analysis and simulation results, it is shown that the pre-rake diversity with GO codes outperforms the one with other spreading codes.

An Approximately MAI-Free Multiaccess OFDM System in Fast Time-Varying Channels

A new multiuser orthogonal frequency division multiplexing (OFDM) transceiver with preceding, called the precoded multiuser OFDM (PMU-OFDM) system, was recently introduced by Tsai et al. (2005). The PMU-OFDM system can reduce multiaccess interference (MAI) due to the carrier frequency offset (CFO) to a negligible amount by preceding the data of each user with a codeword selected from either even or odd Hadamard-Walsh codes. The performance of the PMU-OFDM system in a mobile environment, where the channel response varies within one OFDM symbol due to the Doppler effect, is evaluated in this paper. It is shown by analysis and simulation that the use of even or odd Hadamard-Walsh codewords can greatly reduce the MAI effect due to the Doppler effect as well. Generally, all Hadamard-Walsh codewords except for the all-one codeword result in considerable suppression of intercarrier interference (ICI) induced by the Doppler effect. Furthermore, we discuss codeword priority schemes to minimize ICI as much as possible. Finally, simulations results are given to demonstrate the advantage of PMU-OFDM over OFDMA with a codeword priority scheme in the Doppler environment. We show that the PMU-OFDM system with odd Hadamard-Walsh codewords outperforms a half-loaded OFDMA system in a mobile environment.

Information and multiaccess interference in a complexity-constrained vector channel

A noisy vector channel operating under a strict complexity constraint at the receiver is introduced. According to this constraint, detected bits, obtained by performing hard decisions directly on the channel's matched filter output, must be the same as the transmitted binary inputs. An asymptotic analysis is carried out using mathematical tools imported from the study of neural networks, and it is shown that, under a bounded noise assumption, such complexity-constrained channel exhibits a non-trivial Shannon-theoretic capacity. It is found that performance relies on rigorous interference-based multiuser cooperation at the transmitter and that this cooperation is best served when all transmitters use the same amplitude.

Frequency-Domain Adaptive Antenna Array for Multi-Code MC-CDMA

SUMMARY MC-CDMA is an attractive multi-access method for the next generation high-speed mobile communication systems. The uplink transmission performance is limited by the multi-access interference (MAI) from other users since all users share the same bandwidth. Adaptive antenna array can be used to suppress the MAI and to improve the uplink transmission performance. In this paper, we propose a frequencydomain adaptive antenna array for multi-code MC-CDMA. The proposed frequency-domain adaptive antenna array uses a simple normalized LMS (NLMS) algorithm. Although the NLMS algorithm is used, very fast weight convergence within one MC-CDMA symbol duration is achieved since the weight updating is possible as many times as the number of subcarriers within one MC-CDMA symbol duration.

A 320-Gb/s Capacity (32-User 10 Gb/s) SPECTS O-CDMA Network Testbed With Enhanced Spectral Efficiency Through Forward Error Correction

This paper demonstrates a high-performance optical-code-division-multiple-access (O-CDMA) network testbed using the spectral phase encoded time spreading (SPECTS) method. Through additional time and polarization multiplexing, a total of 32 10-Gb/s users are supported while sharing eight O-CDMA spreading codes. User detection is achieved with time gating and nonlinear thresholding to suppress the multiaccess interference of other users. Incorporation of forward error correction successfully reduces the performance loss imposed by coherent beat interference, resulting in error-free performance (BER<10-11), significant per-user power penalty reduction, and the elimination of a bit-error-rate noise floor. The testbed also applies bandwidth suppression within the encoders and decoders, yielding a 52% increase in spectral efficiency

Evaluation of beat noise in OCDMA system with non-Gaussian approximated method

With the development of phase encoding technologies, beat noise becomes one of the most predominant limitations of optical code division multiaccess (OCDMA) systems. In order to analyze beat noises' impact more accurately, a non-Gaussian analysis with consideration of a decision variable's moment generation function is first introduced in this paper to evaluate both the multiaccess interference and the beat noises simultaneously, where prime, secondary, and partial coherent beat noises are discussed in detail. In this paper, the saddle-point approximation (SPA) method is first utilized to investigate beat noise's impact on an OCDMA system. Comparison of probability density functions as well as bit error rates between the proposed method and the traditional Gaussian approximation method is given here. The resultant achievement shows the outstanding performance of SPA on this occasion, which proves the necessity of such a moment generation function-based non-Gaussian analysis

Chip-Level DS-CDMA Downlink Interference Suppression With Optimized Finger Placement

In direct-sequence code-division multiple-access (DS-CDMA) downlink, multiaccess interference arises due to the loss of orthogonality of the spreading codes for each user in frequency-selective channels. We analyze the receiver structure where a linear combiner with a RAKE structure is used. A generic model is developed to account for arbitrary finger placement, which allows many CDMA downlink linear receivers to be treated as special cases. For fixed finger placement, we derive the optimal weights for the receiver under both chip-level and symbol-level criteria. We show that the weights designed to minimize the chip mean-square error (MSE) also maximizes the output symbol signal-to-interference-noise ratio (SINR) and differs only by a scaling factor from the weights that minimizes the symbol MSE. The performance of the receiver depends heavily on the finger placement scheme. We propose two strategies for finger placement design. The first one is analytical and is based on the sampling design theory. It gives the optimal finger placement in an asymptotic sense as the number of fingers becomes large and sheds important light on how the performance of the system depends on the number of fingers and the smoothness of the underlying chip pulse waveform. The other design is numerical and is based on a greedy search algorithm. An efficient recursive algorithm is developed to reduce the computational complexity associated with the search. For Rayleigh-fading channels, numerical results show that both design schemes achieve significantly better performance than uniformly spaced finger placement with the same number of fingers, or achieve performance similar to that of uniformly spaced finger placement but with a much smaller number of fingers

A novel multi‐access scheme for UWB‐PPM communication systems

AbstractIn this paper, it is shown that some spikes exist in power spectral density (PSD) of ultra‐wideband‐time‐hopping‐pulse‐position‐modulation (UWB‐TH‐PPM) transmitted signal. Analytical and numerical simulation results show that these spikes severely degrade UWB‐TH‐PPM system bit‐error‐rate performance and must be properly removed. This paper proposes a novel multi‐access scheme that uses both direct‐sequence (DS) and time‐hopping (TH) multi‐access schemes to remove these harmful spikes. Analytical and numerical simulation results show that the proposed scheme removes the spikes from the PSD of the UWB‐TH‐PPM transmitted signal and yields better performance. In this paper, the performance of the proposed multi‐access scheme is examined with numerical simulations in the presence of additive white Gaussian noise (AWGN), multipath interference (MPI), narrow‐band interference (NBI), and multi‐access interference (MAI); and furthermore closed form representations for the probability of error of both proposed and conventional multi‐access UWB‐PPM schemes are given. Copyright © 2006 AEIT

Multi-access Interference Cancellation Receiver for Time-Hopping Ultra Wideband Communication

In this paper, a low complexity coherent joint maximum-likelihood detection receiver is developed for canceling multi-access interference in impulse radio (IR) based ultra wideband (UWB) wireless c...

MAI-Free MC-CDMA Systems Based on Hadamard–Walsh Codes

It is known that multicarrier code-division multiple-access (MC-CDMA) systems suffer from multiaccess interference (MAI) when the channel is frequency-selective fading. In this paper, we propose a Hadamard-Walsh code-based MC-CDMA system that achieves zero MAI over a frequency-selective fading channel. In particular, we will use appropriately chosen subsets of Hadamard-Walsh code as codewords. For a multipath channel of length L, we partition a Hadamard-Walsh code of size N into G subsets, where G is a power of two with GgesL. We will show that the N/G codewords in any of the G subsets yields an MAI-free system. That is, the number of MAI-free users for each codeword subset is N/G. Furthermore, the system has the additional advantage that it is robust to carrier frequency offset (CFO) in a multipath environment. It is also shown that the MAI-free property allows us to estimate the channel of each user separately and the system can perform channel estimation much more easily. Owing to the MAI-free property, every user can enjoy a channel diversity gain of order L to improve the bit error performance. Finally, we discuss a code priority scheme for a heavily loaded system. Simulation results are given to demonstrate the advantages of the proposed code and code priority schemes

Performance of multicarrier CDMA in frequency-selective fading via statistical physics

This correspondence extends previous work on direct-sequence code-division multiple access (DS-CDMA) to give a single-user characterization of multicarrier (MC) CDMA systems. Results on the error performance and information rate of a family of multiuser detectors for MC-CDMA are obtained using the replica method, which was originally developed in statistical physics. The central result is the "decoupling principle", namely, an MC-CDMA channel with frequency-selective fading followed by a generic multiuser detection front end can be decoupled into a bank of single-user fading channels in the large-system limit. Thus, conditioned on one's fading coefficients, each user essentially experiences an equivalent single-user Gaussian channel with a degradation in the signal-to-noise ratio (SNR) in lieu of multiaccess interference. A set of joint equations is identified, which determines the degradation for each user, known as the multiuser efficiency. The spectral efficiencies under both optimal joint decoding and separate single-user decoding following multiuser detection are obtained analytically. The result applies to arbitrary input distribution and SNRs, and to optimal multiuser detection as well as various suboptimal schemes.

PERFORMANCE ANALYSIS OF MULTICARRIER CDMA SYSTEM BASED ON COMPLEX WAVELET PACKET AND SPACE–TIME BLOCK CODES IN RAYLEIGH FADING CHANNEL

On the basis of analyzing the principle of multicarrier CDMA (MC-CDMA) technique and space–time coding technique, using the optimized complex wavelet packet as multicarrier modulation, a novel MC-CDMA system based on complex wavelet packet and space–time block codes is proposed. The bit error rate (BER) performance of the system is investigated in Rayleigh fading channel, and the corresponding BER expression is derived in detail. The system can overcome the decrease of spectrum efficiency of conventional MC-CDMA due to inserting cyclic prefix (CP). Moreover, the application of space–time coding technique strengthens the capability against fading channel significantly and perfects the system performance. Theoretical analysis shows that the proposed system with 2-transmitter antenna and 2-receiver antenna space–time block codes performs four times better than the single-antenna MC-CDMA system based on complex wavelet packet (CWP-MC-CDMA); the same conclusion is also reached by the simulation results in fading channel. As a result, the performance of the proposed CWP-MC-CDMA system outperforms a conventional MC-CDMA system based on DFT, it is close or superior to that of the conventional MC-CDMA with CP. Especially, the CWP-MC-CDMA system with space–time coding has superior ability to combat spatial fading and multi-access interference, and it slightly outperforms conventional MC-CDMA with CP and space–time coding.

Frequency-Domain Pre-Equalization for MC-CDMA/TDD Uplink and Its Bit Error Rate Analysis

SUMMARY In multi-carrier code division multiple access (MCCDMA) uplink (mobile-to-base station), since different users’ signals go through different frequency-selective fading channels, large multi-access interference (MAI) is produced. The use of frequency-domain equalization reception can only partially restore the orthogonality among different users’ signals, resulting in a severe degradation in the bit error rate (BER) performance. Hence, frequency-domain pre-equalization transmission, which equalizes the MC-CDMA signal before transmission, is recently attracting attention. In this paper, we present a generalized minimum mean square error (GMMSE) frequency-domain pre-equalization transmission suitable for MC-CDMA/TDD uplink. The pre-equalization weight is derived based on the method of Lagrange multipliers. The theoretical analysis of BER performance using the GMMSE frequency-domain pre-equalization transmission in a frequency-selective Rayleigh fading channel is presented and the result is confirmed by computer simulation.