Asynchronous Code-division Multiple-access Systems Research Articles

Method of MAI cancellation in asynchronous underwater acoustic CDMA system

In the underwater passive positioning systems (UPPS), it involves a key technology of multi-user communication that the beacons synchronously send positioning messages to the targets. Interferences from other users, referred to multi-access interferences (MAIs), can induce significant bit errors for the desired user. In this paper, we designed an underwater acoustic Code Division Multiple Access (CDMA) system using the coded cyclic shift key (CCSK) modulation, and proposed a hybrid MAI cancellation scheme based on cyclic shift inverse matrix transform (CSIMT) and recursive least square (RLS) in the CDMA system. The hybrid scheme can suppress MAI better than RLS or CSIMT, and meets the requirements of strong stability to support reliable transmission for UPPS. The CSIMT interference cancellation performs well under the far-near effects, which doesn’t depend on the accuracy of the estimation of transmitted data, and the strong MAI can be reduced by transformation of the zero-forcing correlation values using m sequence cyclic shift inverse matrix. The RLS interference cancellation is used to mitigate MAI form the detected users with comparable power, by using the estimated the channel impulse response between the receiver and the detected users, and the regenerated signal of detected users form decoder output, which allows of strong error-correcting codes, and it’s robust in an asynchronous environment. The theoretical analysis and experimental result show that the hybrid MAI cancellation scheme can effectively suppress MAI better. The shallow water sea trial was carried out near Dalian Tiger Beach Ocean Park in China, at a distance of about 5 km, the data rates of 100bps, where three users could communicate asynchronously with low failure ratio and can match the UPPS application.

Application of Adaptive Chaotic Simulated Annealing Network to Uplink Power Control in Asynchronous CDMA System

A kind of adaptive Chaotic Stimulated Annealing (CSA) network approach used in the uplink power control of asynchronous Code Division Multiple Access (CDMA) mobile communication system is proposed in this study. The particular influence of near-far effect can be overcome by utilizing the global optimal searching function of CSA method, and the Multiple Access Interference (MAI) in certain cell can be conquered by introducing adaptive multiuser interference cancellation network in corresponding close loop control strategy. Computer simulations give the comparison results between proposed approach and conventional method with heterogeneous Signal-to- Interference Ratio (SIR) threshold decision. It shows that this new approach can achieve higher channel capability and lower average outrage probability with some small increase of iteration steps, which can improve the performance of CDMA mobile communication system effectively.

Phase locked loop assisted phase tracking in asynchronous-code division multiple access systems over slow fading and time-varying channels using simplified expression for improved Gaussian approximation

In this study, a simplified expression for improved Gaussian approximation is considered to model multiple access interference in an A-code division multiple access system. Punctured convolutional codes, which are one of the subclasses of convolutional codes, are employed to provide error protection to the information transmitted over the wireless channel. A detailed theoretical analysis is carried out initially and then compared with the simulated results for the various code rates. The simulated performance of the codes is then compared for different power levels and different number of interferers of the desired user. No phase information is assumed to be available for the information signal of the desired user throughout this study.

Diversity incorporated energy adaptation for A-CDMA systems over slow fading channels

In this paper, a theoretical analysis of diversity incorporated Variable Energy Adaptation (VEA) in an Asynchronous Code Division Multiple Access (A-CDMA) system is discussed for Rayleigh, Rician and Nakagami slow fading channels. The adaptation is accomplished by providing the receiver with the capability of measuring the signal energy-to-noise ratio, and controlling the transmitted signal energy by means of a noise-free feedback loop. System parameters such as fading margin, maximum signal-to-noise ratio, and mean transmitter energy gain are derived and plotted for fading channels as a function of the probability of error specification and the probability of unsatisfactory operation. The mean and median probabilities of error are plotted as a function of energy-to-noise ratios for different fading channels. Error probability distribution and density functions are derived and plotted for various signal-fading distributions.

Distributed power control algorithms for asynchronous CDMA systems in frequency-selective fading channels

In Code Division Multiple Access (CDMA) radio environments, the maximum number of supportable users per cell is limited by multipath fading, shadowing, multiple access interference and near-far effects which cause fluctuations of the received power at the base station. In this context, power control and signal detection are essential to provide satisfactory Quality of Service (QoS) and to combat the near-far problem in CDMA systems. In this paper, we raised the uplink power control problem for a generalize asynchronous direct-sequence (DS) CDMA system that explicitly incorporate into the analysis: (1) the propagation delays in the network (generally neglected in the literature), (2) the adverse effect of multipath fading for wireless channels, and (3) the asynchronous transmissions in the uplink channels. This framework is used to propose a distributed power control strategy enhanced with linear multiuser receivers. It is shown that through a proper selection of an error function, the nonlinear coupling among active users is transformed into individual linear loops. A Linear-Quadratic-Gaussian (LQG) power control strategy is derived and compared with other approaches from the literature. Simulation results show that the uplink channel variations do not destroy the stability of these power control structures. However, delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. It is also shown that the use of multiuser detection at the base station can bring significant improvements to the performance of power control.

Performance of Convolutional Codes in A-CDMA Systems over Time-Varying Rician Fading Channels

In this letter, the analytical and simulated performance of punctured convolutional codes in an Asynchronous Code Division Multiple Access (A-CDMA) system under imperfect phase tracking conditions are discussed. A time-varying Rician fading channel is assumed. Analytical results show the variation of the Bit Error Rate (BER) with nominal loop Signal-to-Noise Ratio (SNR) and the ratio of the loop bandwidth to the local oscillator bandwidth, $${\dfrac{B_o}{B_{Lo}}}$$ , in a phase locked loop. Simulated results are found to agree well with the analytical results.

Asynchronous CDMA Systems With Random Spreading—Part II: Design Criteria

Totally asynchronous code-division multiple-access (CDMA) systems are addressed. In Part I, the fundamental limits of asynchronous CDMA systems are analyzed in terms of spectral efficiency and SINR at the output of the optimum linear detector. The focus of Part II is the design of low-complexity implementations of linear multiuser detectors in systems with many users that admit a multistage representation, e.g. reduced rank multistage Wiener filters, polynomial expansion detectors, weighted linear parallel interference cancellers. The effects of excess bandwidth, chip-pulse shaping, and time delay distribution on CDMA with suboptimum linear receiver structures are investigated. Recursive expressions for universal weight design are given. The performance in terms of SINR is derived in the large-system limit and the performance improvement over synchronous systems is quantified. The considerations distinguish between two ways of forming discrete-time statistics: chip-matched filtering and oversampling.

Mitigation techniques of channel collisions in the TTFR-based asynchronous spectral phase-encoded optical CDMA system

In this paper, we propose a chip-level detection and a spectral-slice scheme for the tunable-transmitter/fixed-receiver (TTFR)-based asynchronous spectral phase-encoded optical code-division multiple-access (CDMA) system combined with time-encoding. The chip-level detection can enhance the tolerance of multiple access interference (MAI) because the channel collision does not occur as long as there is at least one weighted position without MAI. Moreover, the spectral-slice scheme can reduce the interference probability because the MAI with the different frequency has no adverse effects on the channel collision rate. As a result, these techniques mitigate channel collisions. We analyze the channel collision rate theoretically, and show that the proposed system can achieve a lower channel collision rate in comparison to both conventional systems with and without the time-encoding method.

Employing LSF at Transmitter Eases MMSE Adaptation at Receiver in Asynchronous CDMA Systems

The Lebesgue spectrum filter (LSF), a finite impulse response (FIR) filter whose coefficients decay exponentially with a negative factor r := √3 - 2, is shown to be effective preprocessing for spreading code in asynchronous code-division multiple-access (CDMA) systems. The LSF has only been studied independently from the well-known minimum mean-square error (MMSE) filter, an optimal FIR filter in the mean-square error sense. In this paper, we propose an efficient structure, employing the LSF at the transmitter and the MMSE filter at the receiver, for asynchronous CDMA systems. We employ a spreading code preprocessed by the LSF (referred to as LSF-code), and the LSF-code supplies a best initial estimate (among the ones obtained without any a priori information) to an adaptive algorithm for the MMSE filter, leading to significant reduction of iterations in adaptation. This is verified by computer simulations. Also we investigate the link between the LSF and the MMSE filter by examining their autocorrelation properties.

Iterative Joint Channel Estimation and Multiuser Detection for DS-CDMA in Frequency-Selective Fading Channels

An iterative joint channel estimation, symbol detection, phase recovery and interference cancellation structure is proposed for asynchronous code-division multiple-access systems over frequency-selective fading channels. Based on the expectation maximization (EM) algorithm, a recursive channel estimator is developed for blind channel tracking, using a novel stochastic signal processing technique. To perform symbol detection given the phase ambiguities of the resultant EM channel estimates, a noncoherent scheme is developed to compute the a posteriori probabilities (APPs) of data symbols. Moreover, by incorporating the APPs into the proposed recursive channel estimator, phase ambiguity due to the EM channel estimation can be resolved, which enables soft multiple access interference cancellation for multiuser detection. Based on these new signal processing schemes, an iterative structure is proposed for joint channel estimation and multiuser detection over fast fading channels.

Performance of SCHT Sequences in Asynchronous CDMA System

In this letter, the performance of a new set of spreading sequences for a direct sequence (DS) code division multiple access (CDMA) system is examined. These new sequences are derived from sequency-ordered complex Hadamard transform (SCHT). Various correlation properties are presented and the performance of the new sequence is analyzed by comparing it with the other well-known existing spreading sequences, such as UCHT, WHT, Gold, small set of Kasami and m-sequences. According to the simulation results, it is shown that the SCHT sequences provide smaller mean square cross correlation values and offer better average bit error rate for an asynchronous CDMA system over the multipath fading channel.

Blind Schemes for Asynchronous CDMA Systems on Dispersive MIMO Channels

The problem of blind detection in a dispersive multiple-input multiple-output (MIMO) code division multiple access (CDMA) channel is considered in this paper. Unlike previous studies, each user is assigned one spreading code to be employed on all of the transmit antennas, which poses a problem of data re-association at the receiver-end in the absence of prior information as to the channel state. Focusing on the differential Alamouti scheme, we propose a two-stage receive structure. The first stage performs a linear interference-blocking transformation, which allows user separation and inter-symbol interference (ISI) suppression. The second stage is, instead, a novel differential space-time block (STB) decoder suitable for frequency-selective channels. Interestingly, the proposed detector allows decoupling of the decisions on the transmitted symbols, while its blind implementation only requires a cubic (in the processing gain) complexity. A thorough performance assessment is undertaken to investigate, on one hand, the capability of acquiring the missing information, such as the system and the encoder timings, on the other hand, the interplay between the diversity gain provided by the MIMO structure of the communication system, and the additional co-channel interference that multiple transmit antennas produce in multi-path, multiple-access channels.

Intersymbol decorrelating detector for asynchronous CDMA systems

Estimated channel information, especially multipath length and multipath channel of the desired user, is necessary for most previously reported linear blind multiuser detectors in code division multiple access (CDMA) systems. This paper presents a new blind intersymbol decorrelating detector in asynchronous CDMA systems, which uses the cross correlation matrix of the consecutive symbols. The proposed detector is attractive for its simplicity because no channel estimation is required except the synchronization of the desired user. Compared with other reported multiuser detectors, simulation results show that the proposed detector provides a good performance when the active users have significant intersymbol interference and the multipath length is short.

Optimal Sequences and Sum Capacity of Symbol Asynchronous CDMA Systems

The optimal signature sequences that maximize the sum capacity of a direct sequence code-division multiple-access (CDMA) system are characterized in the general case of symbol delay profile and user power constraints. It is shown that the optimal sum capacity of the symbol asynchronous system equals that of the symbol synchronous system with the same user power constraints. With the optimal signature sequence set, the maximum sum capacity is achieved with white Gaussian input signals. The existence of the optimal signature sequence set is proved by the proposal of an explicit construction method for arbitrary user delay profiles and power constraints.

Analysis of Asynchronous Long-Code Multicarrier CDMA Systems With Correlated Fading

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code-division multiple-access (CDMA) systems with correlated fading between sub-carriers are investigated in this research. To conduct theoretical analysis for the maximal ratio combining (MRC) receiver, random parameters including asynchronous delays, correlated Rayleigh fading, and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein (1996) can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences, where code sequences are assumed perfectly time-synchronized. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation. We conclude that, when properly choosing a sufficiently large number of sub-carriers, the benefit of mitigating multipath interference and exploiting potential hidden frequency diversity shall prevail the impairment brought by increased ICI.

Performance of punctured convolutional codes in asynchronous CDMA communications under perfect phase-tracking conditions

In this paper, the performance of punctured convolutional codes of short constraint lengths is discussed. The punctured codes are used to provide error protection to a particular user in an asynchronous code division multiple access (A-CDMA) system. Perfect channel estimation is assumed at the receiver. A slow fading Rician or Rayleigh channel is assumed. Maximum likelihood decoding through a Viterbi algorithm is used to decode the received symbols. Soft decision decoding for perfect phase tracking of the received signal is considered. Analytical bounds, which are useful in predicting the performance of the A-CDMA system are derived and plotted for the cases of infinite and finite channel memory. The upper bounds with Viterbi decoding are derived and plotted for the various punctured codes considered. The simulated results are found to agree very well with their upper bounds and predicted results.

User Capacity of Asynchronous CDMA Systems With Matched Filter Receivers and Optimum Signature Sequences

For a symbol-asynchronous (but chip-synchronous) single-cell code-division multiple-access (CDMA) system, we define a system-wide quantity called the total squared asynchronous correlation (TSAC) which, for arbitrary signature sets, depends on the users' delay profile. We develop a lower bound for TSAC that is independent of the users' delays. We show that if the signature set achieves this TSAC lower bound, then the user capacity of the asynchronous CDMA system using matched filters becomes the same as that of a single-cell synchronous CDMA system; in this case, there is no loss in user capacity due to asynchronism. We present iterative signature adaptation algorithms, which, when executed sequentially by the users, appear to converge to these optimum signature sequences; however, the existence, for all user delay profiles, of signature sequences achieving this lower bound remains a significant open problem.

Blind Multiuser Detection in Uplink CDMA With Multipath Fading: A Sequential EM Approach

We consider joint channel estimation and data detection in uplink asynchronous code-division multiple-access systems employing aperiodic (long) spreading sequences in the presence of unknown multipath fading. Since maximum-likelihood (ML) sequence estimation is too complex to perform, multiuser receivers are proposed based on the sequential expectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. Moreover, the snapshot estimates of each path are tracked by linear minimum mean-squared error filters. The user data are detected by a ML sequence detector, given the channel estimates. The proposed receivers that use the exact expressions have a computational complexity O(2/sup K/) per bit, where K is the number of users. Using the EM algorithm, we derive low-complexity approximations which have a computational complexity of O(K/sup 2/) per bit. Simulation results demonstrate that the proposed receivers offer substantial performance gains over conventional pilot-symbol-assisted techniques and achieve a performance close to the known channel bounds. Furthermore, the proposed receivers even outperform the single-user RAKE receiver with Nyquist pilot-insertion rate in a single-user environment.

Multirate asynchronous optical CDMA system with product code

The authors propose a new architecture for a multirate asynchronous optical CDMA (code division multiple access) system. An optical orthogonal code (OOC) is employed as the signature sequence. The serial multirate data is converted into parallel information bits and then transmitted simultaneously via spatial channels. Exploiting the essence of parallel transmission of spatial channels, the product code (i.e. rectangular code) through an extra protection channel performs error control, which can significantly improve the system performance. The authors' proposed scheme requires only one laser diode, sequence encoder and product-coded encoder/decoder. Moreover, the adverse impact of multiple access interference, APD (avalanche photodiode) noise and thermal noise are taken into consideration in system performance analysis.

MMSE detection in asynchronous CDMA systems: an equivalence result

The analysis of linear minimum mean-square error (MMSE) detection in a band-limited code-division multiple-access (CDMA) system that employs random spreading sequences is considered. The key features of the analysis are that the users are allowed to be completely asynchronous, and that the chip waveform is assumed to be the ideal Nyquist sinc function. It is shown that the asymptotic signal-to-interference ratio (SIR) at the detector output is the same as that in an equivalent chip-synchronous system. It is hence been established that synchronous analyses of linear MMSE detection can provide useful guidelines for the performance in asynchronous band-limited systems.