In non-cooperative scenarios, the estimation of direct sequence spread spectrum (DS-SS) signals has to be done in a blind manner. In this letter, we consider the spreading sequence estimation problem for DS-SS signals. First, the maximum likelihood estimate (MLE) of spreading sequence is derived, then a semidefinite relaxation (SDR) approach is proposed to cope with the exponential complexity of performing MLE. Simulation results demonstrate that the proposed approach provides significant performance improvements compared to existing methods, especially in the case of low numbers of data samples and low signal-to-noise ratio (SNR) situations.

In this letter, we give a generalized construction for sets of frequency-hopping sequences (FHSs) based on power-residue sequences. Our construction encompasses a known optimal construction and can generate new optimal sets of FHSs which simultaneously achieve the Peng-Fan bound and the Lempel-Greenberger bound.

In this paper, a joint blind synchronization and demodulation scheme is developed for ultra-wideband (UWB) impulse radio systems. Based on the prior knowledge of the direct-sequence (DS) spread codes, the proposed approach can achieve frame-level synchronization with the help of frame-rate samples. Taking advantage of the periodicity of the DS spread codes, the frame-level synchronization can be carried out even in one symbol interval. On the other hand, after timing acquisition, these frame-rate samples can be re-utilized also for demodulation. Thus the acquisition time and the implementation complexity are reduced considerably. The performance improvement can be justified by both theoretical analysis and simulation results, in terms of acquisition probability and bit error rate (BER).

This paper proposes a combination of novel Received Response (RR) sequence at the transmitter and Matched Filter-Equalizer-RAKE (MF-EQZ-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. When binary code sequence such as M sequence is used, there is a possibility of generating extra Inter-Symbol Interference (ISI) in the UWB system. Therefore, it is quite a challenging task to collect the energy efficiently although RAKE reception method is applied at the receiver. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by improving the RAKE reception performance using RR sequence and suppressing the ISI effect with the equalizer. Simulation results verify that significant improvement can be obtained by the proposed system especially in UWB multipath channel models such as channel CM4 that suffered severe ISI effect.

This paper proposes a combination of novel Received Response (RR) sequence at the transmitter and a Matched Filter-RAKE (MF-RAKE) combining scheme receiver system for the Direct Sequence-Code Division Multiple Access Ultra Wideband (DS-CDMA UWB) multipath channel model. This paper also demonstrates the effectiveness of the RR sequence in Multiple Access Interference (MAI) reduction for the DS-CDMA UWB system. It suggests that by using conventional binary code sequence such as the M sequence or the Gold sequence, there is a possibility of generating extra MAI in the UWB system. Therefore, it is quite difficult to collect the energy efficiently although the RAKE reception method is applied at the receiver. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of MAI during multiple accessing in the DS-CDMA UWB system. The proposed system improves the system performance by improving the RAKE reception performance using the RR sequence which can reduce the MAI effect significantly. Simulation results verify that significant improvement can be obtained by the proposed system in the UWB multipath channel models.

This paper proposes a combination of adaptive equalizer and Least Mean Square-RAKE (LMS-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by mitigating the multipath effect using LMS-RAKE receiver and suppressing the ISI effect with the adaptive equalizer. Simulation results verify that significant equalization gain can be obtained by the proposed system especially in UWB multipath channel models such as channel CM3 and channel CM4 that suffered severe ISI effect.

The computation involved in multiuser detection (MUD) for multicarrier CDMA (MC-CDMA) based on maximum likelihood (ML) principle grows exponentially with the number of users. Particle swarm optimization (PSO) with soft decisions has been proposed to mitigate this problem. The computational complexity of PSO, is comparable with genetic algorithm (GA), but is much less than the optimal ML detector and yet its performance is much better than GA.

This paper extends previous analytical approaches for the study of CDMA systems to the relevant case of multipath environments where users can operate at different bit rates. This scenario is of interest for the Wideband CDMA strategy employed in UMTS, and the model permits the performance comparison of classic and more innovative spreading signals. The method is based on the characteristic function approach, that allows to model accurately the various kinds of interferences. Some numerical examples are given with reference to the ITU-R M.1225 Recommendations, but the analysis could be extended to different channel descriptions.

We propose a novel asynchronous direct-sequence code-division multiple access (DS-CDMA) using feedback-controlled spreading sequences (FCSSs) (FCSS/DS-CDMA). At the receiver of FCSS/DS-CDMA, the code-orthogonalizing filter (COF) produces a spreading sequence, and the receiver returns the spreading sequence to the transmitter. Then the transmitter uses the spreading sequence as its updated version. The performance of FCSS/DS-CDMA is evaluated over time-dispersive channels. The results indicate that FCSS/DS-CDMA greatly suppresses both the intersymbol interference (ISI) and multiple access interference (MAI) over time-invariant channels. FCSS/DS-CDMA is applicable to the decentralized multiple access.

In this paper, we propose a new code set which has very low spectral peak to average ratio (SPAR) and good correlation properties for DS-UWB. The codes which have low SPAR are suitable for DS-UWB system which operates in UWB (3.110.4 GHz) because they can utilize more power than high SPAR codes can do. And, in order to reduce inter symbol interference (ISI) and inter piconet interferences, the codes which have good auto- and cross-correlation properties must be used. In this paper, we propose three items; (1) a new code generation method which can generate good SPAR and auto-correlation codes, (2) code selection criteria, and (3) a code set, which has been selected according to the proposed selection criteria. The proposed code set has SPAR reduced about 0.22 dB and GMF improved by 30% compared to the previous code set while it is maintaining almost same cross-correlation properties. Each code of the proposed code set, therefore, has gained 1.43 dB SIR on an average compared to that of the previous code set.

The computational complexity of the optimum maximum likelihood detector (OMLD) does not allow its utility for multi-user detection (MUD) in code division multiple access (CDMA) systems. As proposed in this letter, particle swarm optimization (PSO) with soft decision offers a much more efficient option with few parameters to be adjusted, flexibility to implement, that gives a much faster convergence compared to OMLD. It outperforms the conventional detector, the genetic algorithm approach and the standard suboptimal detectors considered in the literature.

In this paper, the performance of narrow band interference (NBI) rejection scheme for direct sequence spread spectrum (DS/SS) is analyzed. A 2-tapped complex FIR filter is used for filtering a chip code to suppress NBI. In this system, the spectrum of transmitted signal has a null at an arbitrary frequency. By choosing filter coefficients, the authors place this null at NBI center frequency to mitigate the effect of NBI. In this paper, an OFDM signal is considered as NBI. The performance of this scheme is theoretically analyzed by introducing Queueing model, and validated via simulation.

In this letter, the effects of transmit beamforming on downlink performance in DS-CDMA communication systems are examined. We present a simple-to-use expression for the conditional instantaneous SINR after Rake combining. Assuming BPSK modulation, the performance of average bit error rate is evaluated. We compare the average BER performance obtained by different beamforming methods under frequency selective multipath fading channels.

An enhanced Ultra Wideband (UWB) signaling scheme that employs PSWF (Prolate Spheroidal Wave Functions)-based orthogonal chip pulses and ternary complementary code sets is proposed for Direct-Sequence (DS) UWB systems. Every information bit of each user is modulated and transmitted over a set of parallel sequences of PSWF-based orthogonal chip pulses and are further assigned to a ternary complementary code set with additional zero padding if necessary. Moreover, the ternary complementary code sets are generated to be mutually orthogonal and assigned to any pair of multiple users. Hence, the mitigation of multipath interference as well as multiple user interference (MUI) can be expected. Furthermore, the ternary code length can be greatly shortened by taking advantage of pulse and code orthogonality. Thus, the proposed transmission scheme is especially suitable for high data rate DS-UWB systems that offer very high flexibility.

In this paper, the performance of narrow band interference (NBI) rejection scheme for direct sequence spread spectrum (DS/SS) is analyzed. A single-tapped complex FIR filter is used for filtering a chip code to suppress NBI. In this system, the spectrum of transmitted signal has a null at an arbitrary frequency. By choosing filter coefficients, we place this null at NBI center frequency to mitigate the effect of NBI. The performance of this scheme is theoretically analyzed and validated by simulation. We also compare the effectiveness against BPSK interference between the chip code filtering and received signal filtering. The results indicate the chip code filtering is effective against single-tone and BPSK interference, and gains better performance than the received signal filtering at certain level of SNR.

Coexisting with many concurrent narrowband services, the performance of UWB systems will be affected considerably by them. Specifically, IEEE 802.11a systems which operate around 5 GHz and overlap the band of UWB signals will interfere with UWB systems significantly. In this paper, a novel narrow-band interferences (NBI) suppression technique based on singular value decomposition (SVD) algorithm for a direct sequence ultra-wideband (DS-UWB) communication system is presented. SVD is used to approximate the interferences which then are subtracted from the received signals. The proposed technique is simple and robust. Simulation results show that the proposed new technique is very effective.

An unsupervised adaptive signal processing method of principal components analysis (PCA) neural networks (NN) based on signal eigen-analysis is proposed to permit the eigenstructure analysis of lower signal to noise ratios (SNR) direct sequence spread spectrum (DS) signals. The objective of eigenstructure analysis is to estimate the pseudo noise (PN) of DS signals blindly. The received signal is firstly sampled and divided into non-overlapping signal vectors according to a temporal window, which duration is two periods of PN sequence. Then an autocorrelation matrix is computed and accumulated by these signal vectors one by one. Lastly, the PN sequence can be estimated by the principal eigenvector of autocorrelation matrix. Since the duration of temporal window is two periods of PN sequence, the PN sequence can be reconstructed by the first principal eigenvector only. Additionally, the eigen-analysis method becomes inefficient when the estimated PN sequence is long. We can use an unsupervised adaptive method of PCA NN to realize the PN sequence estimation from lower SNR input DS-SS signals effectively.

In this letter, we propose a multi-user detection scheme based on a hidden training sequence for DS-UWB systems. The hidden training sequence, which uses a fraction of the informative sequence's transmitting power as training information, is utilized for the receiver filter adaptation and channel estimation. By using this, the proposed scheme offers increased bandwidth efficiency (no period dedicated for training) and also shows reasonably good performance and near-far resistance in single and multiple-access UWB indoor multipath channel environment.

Severe frequency-selective fading, encountered in a broadband wireless mobile communication, significantly degrades the bit error rate (BER) performance of direct sequence spread spectrum (DSSS) signal transmission with rake combining. In this paper, frequency-domain pre-equalization transmission, called pre-FDE transmission, is presented for orthogonal multicode DSSS signal transmission. It is confirmed by the computer simulation that pre-FDE transmission can achieve a BER performance almost identical to that attainable by FDE reception.

For suppressing inter symbol interference, the support vector machine mutliuser detector (SVM-MUD) was adopted as a nonlinear method in direct sequence code division multiple access (DS-CDMA) signals transmitted through multipath channels. To solve the problems of the complexity of SVM-MUD model and the number of support vectors, based on recursive least squares support vector machine (RLS-SVM) and Riemannian geometry, a new algorithm for nonlinear multiuser detector is proposed. The algorithm introduces the forgetting factor to get the support vectors at the first training samples, then, uses Riemannian geometry to train the support vectors again and gets less improved support vectors. Simulation results illustrated that the algorithm simplifies SVM-MUD model at the cost of only a little more bit error rate and decreases the computational complexity. At the same time, the algorithm has an excellent effect on suppressing multipath interference.