Direct-sequence Code-division Multiple-access Systems Research Articles

Multiuser detection for DS-CDMA systems in non-Gaussian channels

An adaptive multi-user detector was developed for direct-sequence code division multiple access (DS-CDMA) systems corrupted by non-Gaussian channel noise, which can be quite detrimental to the performance of the multi-user detectors based on classical white Gaussian assumption. This receiver simultaneously combats multipleaccess interference (MAI) and non-Gaussian impulsive noise. The channel parameters are estimated and transmitted signals are jointly detected by a simple recursive algorithm derived from the EM/SAGE algorithm. Analytical and simulation results show that the proposed technique is robust with wider applicability than conventional multi-user detectors in terms of near-far resistance and bit-error ratio (BER) when either MAI or non-Gaussian impulsive noise is dominant.

SMC-Based Blind Detection for DS-CDMA Systems Over Multipath Fading Channels

This letter derives a computationally efficient sequential Monte Carlo solution for blind detection of direct-sequence code-division multiple-access systems over multipath fading channels by decomposing the observed data into a number of signal components. Then the parameters of each component can be estimated by the sequential importance sampling and Kalman filtering. In comparison with other similar receivers, simulation results demonstrate that the proposed solution achieves the desirable performance with a significantly reduced computational complexity

Blind adaptive CDMA processing for smart antennas using the block shanno constant modulus algorithm

A space-time blind adaptive processing algorithm is proposed for the uplink base station receiver in a direct-sequence code-division multiple-access (DS-CDMA) cellular system. The proposed algorithm, called the space-time block Shanno constant modulus algorithm (STBSCMA), minimizes a constant modulus algorithm (CMA) objective function in a bilinear function structure combined with penalty terms for both spatial and temporal domains. The proposed algorithm rapidly converges to the stationary points at fewer iterations than block conjugate gradient CMA (CGCMA) and block gradient descent CMA (GDCMA). STBSCMA finds the critical points of the objective function. STBSCMA also recovers rapidly from an abrupt change of channel. The proposed algorithm structure requires relatively low computational complexity to improve the receiver performance for space-time processing in the DS-CDMA system.

Minimum probability of error-based adaptive multiuser decision feedback detection

For synchronous direct-sequence code-division multiple access (DS-CDMA) systems, a new criterion based on minimum probability of error (MPE) is proposed for designing decision feedback detectors (DFDs) instead of the conventional minimum mean-squared error criterion. The MPE criterion-based DFD (MPEDFD) tries to minimize the average/conditional probability of error adaptively. Study of bit-error rate and convergence performance shows that the MPEDFDs work pretty well after a short training sequence. Moreover, the modified MPEDFD is applicable for the asynchronous DS-CDMA systems with multipath.

Performance Evaluation of CDMA Reverse Links With Imperfect Beamforming in a Multicell Environment Using a SimplifiedBeamforming Model

Reverse link capacity of a direct-sequence codedivision multiple-access (DS-CDMA) system in a multicell environment has been studied recently, and significant capacity improvements due to the use of beamforming have been observed. However, system performance with beamforming will be affected by several impairments, such as direction of arrival estimation errors, array perturbations, mutual coupling, and signal spatial spreads. In this paper, reverse link performance of CDMA systems with beamforming under these impairments (imperfect beamforming) is investigated. A simplified beamforming model is developed to evaluate the system performance in terms of user capacity, bit-error rates (BER), and outage probabilities. Both signal-to-interference-ratio-based power control and strengthbased power control are considered in this paper. The capacity and BER degradations due to different impairments are shown, and outage probabilities under different power control schemes are examined.

The Use of Delay-Locked Loop Signals in DS/CDMA Receiver for Multiple-Access Interference Reduction

This paper presents a coherent demodulation scheme that reduces the multiple-access interference (MAI) considerably in a band-limited direct-sequence code-division multiple-access system. The despreading signal is obtained from a combination of the conventional despreading signal and its weighted early and late versions that are used in the delay-locked loop (DLL). It is shown that with appropriate choice of the weighting parameter of the early-late signals of the DLL, the signal-to-interference-plus-noise ratio of the decision variable is increased, which leads to improvement in the bit-error performance and hence the potential increase in the system capacity by about 25%. The analyses are performed in the frequency domain. The effect of imperfect synchronization on the system performance is also analyzed

Spectral efficiency of a single-cell multi-carrier DS-CDMA system in Rayleigh fading

The spectral efficiency of a multi-carrier direct-sequence code-division multiple-access (MC/DS-CDMA) system operating in a Rayleigh fading environment is investigated and evaluated in terms of the theoretically achievable channel capacity (in the Shannon sense) per user, estimated in an average sense. This short paper covers operation of the considered system over broadcast communication randomly time-varying channels as applicable to wireless radio networks and single-cell indoor mobile systems and leads to the derivation of a closed-form expression for the achieved spectral efficiency. Furthermore, the relation between the number of the employed sub-carriers and the achieved spectral efficiency is revealed.

A geometrical-based microcell mobile radio channel model

In this paper we present a geometric multipath propagation model for a microcell mobile environment. The proposed model provides the statistics for the direction-of-arrival (DOA) of multipath components. These statistics are required to test adaptive array algorithms for cellular applications. The proposed model assumes that (1) a line-of-sight (LOS) path exists between the transmitter and the receiver, (2) the scatterers lie within a circle of radius R around the mobile station, and (3) the base station lies within this circle. The distances between the scatterers and the mobile station are subject statistically to a hyperbolic distribution. The model also provides the multipath power delay profiles (PDP), which are used to evaluate the bit error rate (BER) and the signal to interference ratio (SIR) for the direct-sequence code division multiple access (DS-CDMA). We derive and simulate the joint probability density functions (pdfs) of the power--DOA and the power--Doppler shift. Further we determine expressions for the BER performance and for the SIR of a DS-CDMA system over the proposed channel model. A simplified expression based on the improved Gaussian approximation (SEIGA) is used to evaluate the BER and the SIR in a wideband multipath channel. Although the proposed model is applicable for downlink as well, in this paper we will analyze the uplink environment only.

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.

Performance analysis of multicarrier CDMA systems with frequency offsets and random spreading under optimum combining

The carrier-frequency offset effect on the performance of asynchronous multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems with aperiodic random spreading and correlated Rayleigh fading is studied in this paper. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise ratio (SINR) of the combined statistics and exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability formula, is derived by averaging several random parameters including asynchronous delays, correlated Rayleigh fading, and signature sequences. The analytic results obtained are applicable to MC-CDMA with appropriate modifications. Furthermore, we show that the MC-CDMA system with a common random signature sequence over all subcarriers for a given user outperforms that with distinct sequences over different subcarriers. Finally, the performance of MC-CDMA systems using the optimum combining technique is compared with that of different combining filters in the simulation.

Iterative vector channel estimation/MAP/PIC for CDMA systems in time-selective correlated multipath fading channels

Recently, Gao et al. proposed the expectation-maximization (EM) vector channel estimation for single-user direct-sequence code-division multiple-access (DS-CDMA) systems to estimate the correlated fading channel coefficients of the paths of a user. They also used one D-lag decision-feedback maximum a posteriori (MAP) detector to detect the information bits. A straightforward extension (noniterative) to multiuser synchronous CDMA cases, i.e., adding the parallel interference cancellation (PIC) multiuser detector into their scheme, results in poor performance. Therefore, we propose an iterative scheme which combines vector channel estimation, decision-feedback MAP detection, and PIC. The partial PIC is applied in the sequence hypothesis stage of MAP in each iteration before the final MAP decision. The proposed scheme has twice the decision delay of the previous scheme for single-user cases. The performance of the proposed system in the time-selective, correlated multipath fading channel environment is investigated. Simulation results show that the proposed scheme significantly outperforms the noniterative scheme. Simulation results also show that the proposed scheme performs better than its scalar channel-estimation version.

Performance analysis of space-time MMSE multiuser detection for coded DS-CDMA systems in multipath fading channels

This paper investigates the use of convolutional coding in space-time minimum mean-square-error (MMSE) multiuser-based receivers over asynchronous multipath Rayleigh fading channels. We focus on the performance gain attained through error control coding when used with binary-phase-shift-keyed modulation (BPSK) and multiuser access based on direct sequence-code-division multiple access (DS-CDMA). In our analysis, we derive an approximation for the uncoded probability of bit-error in multipath fading channels. This bit-error rate (BER) approximation is shown to be very accurate when compared to the exact performance. For a convolutionally coded system, we obtain a closed form expression for the bit-error rate upper bound. This error bound is noted to be tight as the number of quantization levels increased beyond eight. Using our theoretical results, we obtain an estimate for the achieved user-capacity that accrues due to error control coding. It is found that using convolutional coding with 3-bit soft-decision decoding, a user-capacity gain as much as 300% can easily be achieved when complete fading state information plus ideal channel interleaving are assumed.

Capacity improvement of multicarrier DS-CDMA system with antenna array

An antenna array based base station receiver for multicarrier Direct Sequence Code Division Multiple Access (DS-CDMA) system is proposed. The main advantage of the receiver is that the spatial diversity is achieved by combining signals of array elements. Based on the detailed analysis of multiuser interference and noise characteristics, the performance of the proposed receiver is analyzed. Theoretical analysis shows significant performance improvement in terms of system capacity due to the use of antenna arrays compared with the conventional single antenna multicarrier DS-CDMA approach. Simulation results confirm the theoretical analysis.

Performance of optimum wavelet waveform for DS-CDMA chip waveform over QS-AWGN channel: Research Articles

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence-code division multiple access (DS-CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi-synchronous additive white Gaussian noise channel for DS-CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS-CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half-sine and rectangular waveforms. Especially, the Pe of the optimum wavelet-based scheme becomes significantly better than those of the conventional chip waveforms-based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet-based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half-sine and rectangular-based systems, respectively, for a Pe value of 10-3. Copyright © 2005 John Wiley & Sons, Ltd.

An enhanced widely linear CDMA receiver with OQPSK modulation

This paper studies an enhanced widely linear (WL) receiver for direct-sequence code-division multiple-access (DS-CDMA) systems that employ aperiodic spreading sequences with offset quadrature phase-shift keying (OQPSK) modulation. The modulation scheme generates improper complex multiple-access interference (MAI) and necessitates the use of WL receivers. Focus is on bandlimited pulse shapes and the inherent cyclostationarity (CS) of the uncoded received signal. The enhanced WL receiver replaces the conventional chip-matched filter with new filters that exploit the CS of the received signal through frequency shifting. The proposed WL receiver is shown to outperform the classical strictly linear (SL) receiver when the interfering users are (quasi-)synchronous with respect to the user of interest. High-powered interfering users, that may exist to support high data rates, increase the performance improvement delivered by the WL receiver. Moreover, it is shown that MAI can become proper, either identically or asymptotically, when users are asynchronous and equally powered. This is despite the fact that individual interfering signals are improper. Numerical results demonstrate that the WL receiver can outperform the SL receiver by 1-3 dB under the examined scenarios with current CDMA standards settings. In asynchronous or quasi-synchronous transmission modes, performance gain of the WL receiver degrades unless the number of high-powered active users remains small. An example for implementation of the WL receiver is proposed and compared with that of the SL receiver when minimum-shift keying modulation, a special case of OQPSK, is used. The implementation is based on a fractionally spaced equalizer whose taps are updated by an adaptive algorithm. It is shown that the proposed structure is capable of delivering the maximum signal-to-noise ratio predicted by theory.

Recursive algorithms for multiuser detection over DS-CDMA channels

We focus on the synthesis and the analysis of recursive receivers for direct-sequence code-division multiple-access systems aimed at achieving satisfactory performance at the price of a moderate computational complexity. At the analysis stage, some interesting properties shared by the proposed procedures are proven. Finally, the performance assessment shows that the new schemes are superior to the linear detectors, and some of them achieve a bit-error rate close to that of the optimum receiver.

Performance of optimum wavelet waveform for DS-CDMA chip waveform over QS-AWGN channel

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence-code division multiple access (DS-CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi-synchronous additive white Gaussian noise channel for DS-CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS-CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half-sine and rectangular waveforms. Especially, the Pe of the optimum wavelet-based scheme becomes significantly better than those of the conventional chip waveforms-based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet-based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half-sine and rectangular-based systems, respectively, for a Pe value of 10−3. Copyright © 2005 John Wiley & Sons, Ltd.

Transmitter adaptation algorithm for multicellular synchronous DS-CDMA systems with multipath

A procedure for the design of complex sequence/signature sets for a multicell system under synchronous multipath transmission is presented. The optimization criterion is the total weighted square correlation (TWSC) that incorporates transmission power and channel impulse response values. By solving an inverse eigenvalue problem related to block structured matrices, the bandwidth of initial generalized Welch bound equality signals is preserved in the extended signal space associated to multiple cell system. Numerical results for overloaded uplink multicellular direct-sequence code-division multiple-access (DS-CDMA) systems are presented in order to support our analysis.

A robust channel estimator for DS-CDMA systems under multipath fading channels

This paper addresses the problem of channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. The multipath fading channels are modeled as autoregressive (AR) models. A method is first proposed to convert the time-varying regression model due to the time-varying nature of users' information symbols into a time-invariant one. Then, a polynomial approach is proposed to obtain the minimum mean square error (MMSE) estimator. The uncertainty of the channel model and decision errors of the DS-CDMA detector are taken into consideration in the design of the MMSE estimator. Compared with the Kalman estimator, the computational complexity of the proposed algorithm is much lower. The simulation results show that the proposed estimator provides a comparable estimation performance with the Kalman estimator and is robust for fast-fading channels.

Generalized Neural Method for Shaping the Exponential Chip Weighting Waveforms in Direct Sequence CDMA Systems

Generalized Neural Method for Shaping the Exponential Chip Weighting Waveforms in Direct Sequence CDMA Systems