Alexey Dudkov

Optimization of Signature Ensembles for SI DS CDMA using Random Search

Performance of direct sequence code division multiple access system (DS CDMA) in a multipath channel can be severely impaired due to high level of multiple access interference (MAI). Usage of signature interleaved (SI) DS CDMA mitigates this problem, reducing dependence of MAI to easily controllable correlations of non-shifted or just one-chip-shifted signatures independently of the maximal multipath delay. In the article selection criterion for signature ensembles for SI DS CDMA is considered. Using random search, correlation properties of the latter can be significantly improved, providing a visible gain in terms of bit error rate (BER).

Precoding Welch-bound signature sets

In this paper signal-to-interference ratio for precoded Welch-bound signature sets is derived for oversaturated synchronous CDMA system. For this, a novel definition for CDMA precoding is given and a general correlation result is presented. It is shown that the system with precoding outperforms the system without it, assuming that conventional receiver is used.

RAKE Reception for Signature-Interleaved DS CDMA in Rayleigh Multipath Channel

Performance of RAKE receiver in a multipath channel for direct sequence code division multiple access system (DS CDMA) can be severely impaired due to high level of multiple access interference (MAI), whose value is controlled by correlation functions of user signatures. Due to data modulation MAI becomes dependent on aperiodic user signature correlations of big shifts, which are hard to control and become even less so as multipath delay grows, leading to significant MAI increase. In this article we analyze performance of RAKE receiver for Signature-Interleaved (SI) DS CDMA in synchronous multipath Rayleigh channel in comparison with that of conventional DS CDMA. It is shown that SI DS CDMA provides better performance in terms of bit error rate (BER), especially when using specifically tailored user signatures.

Precoding for improved performance of Welch-bound signature sets

Welch-bound signature sets are optimal for oversaturated synchronous CDMA in terms of minimizing the multiple access interference. In this paper, precoding of the sum signal of Welch-bound signatures, in order to maximize the mean signal-to-interference ratio for conventional receivers, is analysed. The gain compared to normal sum signal is calculated, and also the effect on the energy of the sum signal is derived.

Oversaturated Group Orthogonal CDMA in Multipath Channels using Chip Interleaving with Zero Padding

Oversaturated group orthogonal CDMA (GO-CDMA) offers an elegant way of increasing capacity of synchronous DS-CDMA, which allows existence of number of users exceeding signal space dimension. The system provides excellent results in synchronous single-path channels; however, in multipath channels complicated equalizers of big length are required to secure good performance due to significant MAI. In this paper implementation of chip interleaving with zero padding in GO-CDMA system is considered to alleviate the effect of MAI. It is shown that this approach results in bit-synchronous multipath signals at the receiving end. Computer simulations demonstrate that in such a system the usage of the simplest zero-forcing equalizer of moderate length provides performance very close to that of GO-CDMA in an AWGN channel.

Single-User Receiver for Signature-Interleaved DS CDMA in Multipath Channel

Multipath propagation can significantly impair performance of DS CDMA due to presence of the odd correlations of user signatures caused by delayed replicas of the transmitted signal. Traditionally to neutralize this effect some form of equalizing is used, which, along with receiver complication, leads to SNR loss. In the paper signature-interleaved (SI) DS CDMA is proposed and analyzed in application to a multipath channel. It is shown that with signature interleaving the output of a single user correlator contains only correlations of either non-shifted at all or just one-chip-shifted signatures. Thereby a deteriorating impact of odd correlations on the system performance is drastically reduced. As a result, SI DS CDMA provides performance improvement versus conventional DS CDMA with no need in complex equalizing.

Asynchronous Oversaturated Group Orthogonal CDMA Using Chip Interleaving with Zero Padding

In this article we consider usage of chip interleaving with zero padding in oversaturated systems. It is shown that this approach makes user signals at the receiving end bit-synchronous independently of the user delay profile, making thus chip group orthogonal (GO-CDMA) applicable in asynchronous transmission conditions as well. Theoretical calculations and computer simulations demonstrate that at high signal to noise ratio (SNR) values the performance of the system is equivalent to the one operating in synchronous channels.

Eliminating odd correlations in asynchronous DS CDMA employing cyclic prefix

In an asynchronous DS CDMA system performance of a single-user receiver suffers from the presence of odd cross-correlations of user signatures due to information modulation and variable delays between user signals. To neutralize this effect, employing a cyclic prefix is recommended. Both analytical and simulation results demonstrate that scenarios exist where such a measure brings about a substantial gain in performance with no compromising of spectral efficiency

Asymptotic optimality of random PSK signature ensembles

Selection of a proper signature ensemble is the point of a special care in designing code division multiple access (DS CDMA) systems. When the receiver complexity beyond that of a single user algorithm is not affordable, Welch-bound ensembles minimizing intensity of the multiple access interference (MAI) are optimal. In this paper we show that for random signature sets the mean square deviation of MM power from the Welch bound drops in nearly inverse proportion with growth of K/N, where K is number of users and N is the spreading factor. Thereby, the fact of asymptotic (K/spl Gt/N) convergence in probability of random sets to the Welch-bound ones is demonstrated.