Performance analysis of asynchronous direct sequence code division multiple access for general chip waveforms over multi-path Rayleigh fading channels

The evaluation of a hybrid acquisition performance has been considered for the pilot signal in the direct sequence code division multiple access (DS/CDMA) forward link. The hybrid acquisition is introduced by the combination of two schemes, parallel and serial acquisitions. The mean acquisition time of the proposed scheme is derived by considering both case 1 (the correct code-phase offsets are included in one subset) and case 2 (the correct code-phase offsets exist at the boundary of two subsets), which are caused by the distribution of correct code-phase offsets in the subset. Detection, false alarm, and miss probabilities are derived for the cases of multiple correct code-phase offsets and multipath Rayleigh fading channel. Results are provided for the acquisition performance with respect to design parameters such as postdetection integration length in search and verification modes, subset size, and number of I/Q noncoherent correlators. Also, comparison between the hybrid acquisition and the fully parallel acquisition is provided in terms of the minimum mean acquisition time.

The probability of packet success for asynchronous direct sequence code division multiple access (DS-CDMA) over the multipath fading channel is investigated under different fading conditions and for different cases. We studied the bit-to-bit dependence caused by the multipath fading and by the multiple access interference (MAI) originating from the chip and phase offsets between the desired and interfering signals. We investigated the effect of using the Gaussian versus the improved Gaussian process on the computation of probability of packet success as well as the effect of the bit-to-bit dependence within a packet. The improved Gaussian approximation provides more accurate values for the probability of bit error for any number of simultaneous users, while it also allows us to incorporate the effects of bit-to-bit dependence into the calculations of the probability of packet success.

The conventional minimum mean squared error (MMSE) receiver which actively suppresses multiuser interference (MUI), outperforms the RAKE receiver for direct sequence code division multiple access (DS-CDMA) in multipath slow fading channels. However the performance gap between the conventional MMSE receiver and the theoretical bound is still large. We therefore propose an extended MMSE receiver which spans three symbol intervals the desired symbol interval as well as the previous and the next symbol interval. The extended MMSE receiver outperforms the conventional MMSE receiver especially for large system load and large number of propagation paths in the channel. Its computational complexity is approximately three times the complexity of the conventional MMSE receiver but still independent of the number of active users and the number of paths in the channel. As a consequence the extended MMSE receiver allows a low-complexity low-power implementation in the mobile user-terminal for communication over multipath slow fading channels in the forward link.

In this paper, a multi-antenna based receiver structure for direct sequence code division multiple access (DS/CDMA) system is proposed. The proposed scheme exploits the excellent time resolution of a CDMA RAKE receiver and uses an antenna array beamforming structure to resolve multipath returns in both angular and time domains. A much higher diversity gain than that based only on the time domain diversity can be achieved. This work suggests a new space-time diversity paradigm, namely angular-time diversity, which differs from traditional Alamouti-type space-time coded schemes. The impairments caused by multipath and multiuser interference are analyzed. The performance of the proposed receiver in multipath fading channel is explicitly evaluated. An expression for uncoded system bit error probability is derived. Simulation results show the performance improvement in terms of BER due to the use of multi-antenna in the receiver, and the results illustrate that the multi-antenna based receiver works effectively in resolving multipaths in both angular and time domains to achieve performance improvement due to angular and time diversity gain provided by the multi-antenna system.

In an asynchronous DS-CDMA system using random signature sequences and correlation receivers, the signal-to-interference-plus-noise ratio (SINR) at the output of each correlation receiver depends on the shape of the chip waveform employed. The use of double chip waveforms instead of a single chip waveform is investigated, in order to increase the SINR performance. An analytical expression for the SINR when double chip waveforms are used is obtained. To evaluate the performance of the proposed technique, an extension to Holtzman's approximation of the error probability for the case of double chip waveforms and signalling over an additive white Gausian noise channel is derived. Numerous numerical examples are given to demonstrate the advantage of using double chip waveforms over the single chip waveform. Moreover, the advantage of using double chip waveforms for improving the SINR performance over a multipath Rayleigh fading channel is also demonstrated.

This paper investigates the effect of chip waveform shaping on multiple access interference (MAI) in band-limited direct sequence code-division multiple access (DS-CDAM) systems. In particular, it concentrates on the family of band-limited waveforms with zero interchip interference (ICI) and with arbitrary excess bandwidth /spl beta/ in the range 0 /spl les/ /spl beta/ /spl les/ 1. A criterion for the performance comparison of various band-limited chip waveforms based on the elementary density function is introduced. The comparison is made among different chip waveforms, including the optimal chip waveform, the square-root raised cosine proposed for 3G wireless systems and the previously proposed better than Nyquist pulse. Finally, the effect of varying the roll-off factor of a band-limited chip waveform on the MAI level is also investigated.

An approximate maximum likelihood (ML) method for joint estimation of time delays and directions-of-arrivals (DOAs) for the direct-sequence code-division multiple-access (DS-CDMA) systems over the multipath Rayleigh fading channels is developed. The proposed estimation scheme is developed by modeling the known training sequence of the desired user as the desired signal and the multiple access interference (MAI)-plus-AWGN as unknown colored Gaussian noise uncorrelated with the desired signal. An efficient iterative algorithm for estimating channel parameters (the time delays and DOAs of multipath signals) is proposed to reduce the computational complexity. The algorithm transforms the multi-dimensional maximization problem into two simple one-dimensional (1-D) maximization problems based on an iterative scheme. Analytical and simulation results are presented to illustrate the performance in comparison with the Cramer-Rao bounds (CRBs). It is shown that the proposed algorithm is near-far resistant for both time delay and DOA estimation and little performance loss is observed in comparison with the CRBs at reasonable SNRs.

The reverse link performance of direct sequence code division multiple access (DS-CDMA) system in wireless communication over a frequency selective multipath fading channel has been investigated in this paper. The channel has been modeled as Nakagami-m distributed frequency selective multipath fading channel. A conventional correlator type receiver has been considered for the analysis. Noise is taken as additive white Gaussian noise (AWGN) with two sided power spectral density N/sub 0//2 . To model multiple access interference (MAI), standard Gaussian approximation (SGA) has been considered in this paper. To mitigate the near-far problem, a power control scheme, but imperfect has been assumed in this paper. The bit error rate (BER) expression for the system has been derived and simulated by varying number of users, number of fading path, processing gain, fading parameter, SNR per bit E/sub b//N/sub 0/ and variation in the amplitude of the received signal. The analysis shows that as the number of simultaneous active user, the number of fading path and the variation in the power of the received signal increases, the BER of the system increases indicating the performance degradation. As the value of the processing gain, the ratio E/sub b//N/sub 0/ and the fading parameter increases, the BER performance of the system improves.

A DS/CDMA (direct-sequence code division multiple access) system using BPSK (binary phase shift keying) modulation is simulated to determine the bit error probabilities for various numbers of users with a processing gain close to that of a cellular network. The results are compared to results obtained using the Gaussian approximation equations. The results obtained are to the feasibility of a DS/CDMA cellular network. It is concluded that DS/CDMA cellular networks appear to offer higher subscriber capacities than existing analog or TDMA (time division multiple access) formats. In addition, DS/CDMA offers additional advantages such as inherent privacy through the use of spreading codes and the ability to combat problems such as multipath fading and narrowband noise. >

In a recent paper, closed-form formulas for the Cram??r-Rao bounds (CRBs) on the error variance of any unbiased estimator of the amplitudes, timing offsets, phase offsets, and directions-of-arrival (DoAs) of multiplexed asynchronous direct-sequence code division multiple access (DS/CDMA)signals adopting aperiodic spreading codes have been derived. These formulas depend on the active users' spreading codes and pilot information symbols. In this paper, we obtain formulas for the modified CRB under the assumption that the spreading codes can be modeled as random nuisance parameters. Moreover, we also consider the case that the timing offsets are modeled as random variates and provide formulas for the hybrid CRB, i.e., the CRB corresponding to the situation that some of the parameters to be estimated are random variates rather than unknown deterministic quantities. Numerical examples showing the impact of the training length, signal-to-noise ratio (SNR), and the number of active users on the CRB are also reported and commented upon.

This paper introduces a new multi-user detection method, successive interference cancellation based on the order of log-likelihood-ratio (LLR-SIC), for asynchronous code division multiple access (CDMA) systems. LLR-SIC operates on the fact that the most reliable user, i.e. the user with the largest log- likelihood-ratio (LLR) absolute value, should be first detected and cancelled from received signal. Simulation results show that LLR-SIC significantly outperforms partial parallel interference cancellation (P-PIC) over multi-path Rayleigh fading channels. And the results also show that LLR-SIC can even support overloaded systems in which the number of users is greater than the spreading factor.

In this paper, we introduce a novel fuzzy adaptive algorithm for blind detection of direct sequence code division multiple access (DS-CDMA) signals in a multiuser communication scenario. We make use of fuzzy control systems to wisely determine the appropriate step size of the adaptive algorithm based on the quality of the received signal. We have used the popular minimum output energy (MOE) blind detector proposed by Honig et al. as our blind adaptive detector whose step size is to be determined by the fuzzy control system. This blind detector requires no more knowledge than does the conventional single-user receiver, i.e. the desired user's signature waveform and its timing. We have proposed to use signal to interference ratio (SIR) and its variations as the key inputs to our fuzzy algorithm. By simulation, we show the competency of our algorithm in terms of the convergence characteristics of the adaptive algorithm. Likewise, by simulation results we show that these convergence improvements yield to a lower bit error rate (BER) for the demodulation process in the receiver.

In this paper we present two new, reduced complexity algorithms for multistage detection of direct sequence code division multiple access (DS-CDMA) signals in additive white Gaussian noise (AWGN) channels. The selective and successive selective multistage detection algorithms are designed to reduce the amount of computation performed by the conventional multistage detection algorithm while providing roughly the same bit error rate performance. Bit error rate performance of the new algorithms is determined via Monte Carlo simulation.

In this paper, we propose an adaptive multistage parallel interference cancellation (PIC) structure for multipath fading channels. Similarly to the previous partial interference cancellation (IC) approach suggested in Divsalar et al. (1998), only part of the multiaccess interference (MAI) estimate is canceled at each stage. However, the weights employed in this proposed scheme are derived from an LMS algorithm which tries to minimize the mean-square error between the actual signal received and its replica. The complexity of the proposed adaptive multistage PIC scheme is much lower than that of linear multiuser detectors. Performance of the proposed structure in single-path Rician and multipath Rayleigh fading channels is evaluated by simulations. Our results show that the proposed technique outperforms the existing interference cancellation methods in multipath fading channels.

In this paper, the probability of error (Pe) expression of asynchronous direct sequence‐code division multiple access (DS‐CDMA) signals using band‐limited chip waveforms is derived over multi‐path Rayleigh fading channels. In receiver, a matched filter‐based rake receiver in conjunction with maximum ratio combiner (MRC) is considered. Numerical values for the Pe are calculated for various chip waveforms including an optimum wavelet waveform. Analytical results are verified by conducting simulations. Results show that the optimum wavelet‐based scheme outperforms time‐limited raised cosine, half sine, rectangular and band‐limited square‐root raised cosine chip waveform‐based schemes in terms of the Pe and the capacity defined as the number of users per Hertz for a same Pe level. Copyright © 2006 John Wiley & Sons, Ltd.