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Hua Guo ZHANG Qing MOU Hong Shu LIAO Ping WEI
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.
Pinhui KE Zhihua WANG Zheng YANG
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.
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.
Fumiyuki ADACHI Kazuaki TAKEDA Hiromichi TOMEBA
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.
Wei-Lung MAO Hen-Wai TSAO Fan-Ren CHANG
GPS receivers are susceptible to jamming by interference. This paper proposes a recurrent neural network (RNN) predictor for new application in GPS anti-jamming systems. Five types of narrowband jammers, i. e. AR process, continuous wave interference (CWI), multi-tone CWI, swept CWI, and pulsed CWI, are considered in order to emulate realistic conditions. As the observation noise of received signals is highly non-Gaussian, an RNN estimator with a nonlinear structure is employed to accurately predict the narrowband signals based on a real-time learning method. The node decoupled extended Kalman filter (NDEKF) algorithm is adopted to achieve better performance in terms of convergence rate and quality of solution while requiring less computation time and memory. We analyze the computational complexity and memory requirements of the NDEKF approach and compare them to the global extended Kalman filter (GEKF) training paradigm. Simulation results show that our proposed scheme achieves a superior performance to conventional linear/nonlinear predictors in terms of SNR improvement and mean squared prediction error (MSPE) while providing inherent protection against a broad class of interference environments.
This paper presents a novel transmission diversity scheme for code division multiple access system. Conventional diversity receivers in mobile stations require space and complicated circuits, however, the proposed diversity schemes present significant diversity effect without any diversity equipment at the mobile station. It is possible to use the transmitter diversity at the base station by using the feature of time division duplex (TDD) which has strongly correlated fading patterns in both forward and reverse link. Computer simulation is performed to evaluate the performance of the proposed systems for single user environment. The performance of the system 1, which select best situated antenna, is analyzed and the BER performance for multiple access is presented.
For direct sequence spread spectrum systems, the performance of PN sequence acquisition can be significantly affected if data modulation is present. However, the data modulation often exists during the reacquisition of a PCS radio channel. This study proposes and analyzes two shemes which are designed to improve acquisition process for PN sequence under data modulation. Both designs are based upon a PN acquisition receiver with parallel I-Q matched filters. The first scheme employs a serial search strategy with verification mode. The second scheme, which is still based upon the same parallel acquisition receiver, employs the parallel search strategy. We show that the second scheme is capable of providing faster acquisition under data modulation than the first serial search scheme using the same number of I-Q matched filter. We believe it should become a very good alternative for the acquisition of data modulated PN sequences in personal communications.
Riaz ESMAILZADEH Masao NAKAGAWA
A new method of multipath diversity combination is proposed for Direct Sequence Spread Spectrum (DS-SS) mobile communications. In this method, the transmitted signal from the base staion is the sum of a number of the same spread signal, each one delayed and scaled according to the delay and the strength of the multipaths of the transmission channel. As a result the received signal at the mobile unit will already be a Rake combination of the multipath signals. This new method is called Pre-Rake diversity combination because the Rake diversity combination process is performed before transmission By this method the size and complexity of the mobile unit can be minimized, and the unit is made as simple as a non-combining single path receiver. A theoretical examination of the Signal to Noise Ratio (SNR) and the Bit Error Rate (BER) results for the traditional Rake and the Pre-Rake combiners as well as computer simulations show that the performance of the Pre-Rake combiner is equivalent to that of the Rake combiner.
Tetsushi IKEGAMI Shinichi TAIRA Yoshiya ARAKAKI
The bit error performance of a Direct Sequence Spread Spectrum Communication system in actual land mobile satellite channel is evaluated with experiments. Field test results with the ETS-V satellite in urban and suburban environments at L-band frequency show that this land mobile satellite channel of 3MHz bandwidth can be seen as a non-frequency selective Rician fading channel as well as shadowing channel. The bit error performance can be estimated from signal power measurement as in the case of narrow band modulation signals.
Riaz ESMAILZADEH Masao NAKAGAWA
A time division duplex (TDD) direct sequence spread spectrum communication (DS-SS) system is proposed for operation in channels with Rayleigh fading characteristics. It is shown that using the TDD method is advantageous because the devices can be designed more simply, the method is more frequency efficient and as a result the systems will be less costly and less power consuming. It is also shown that an efficient power control method can be implemented for the TDD systems. In contrast to the traditional access techniques such as frequency division multiple access (FDMA) and time division multiple access (TDMA) that are mainly frequency limited, the code division multiple access (CDMA) method which uses the DS-SS technique is interference limited. This means that an efficient power control method can increase the capacity of the DS-SS communications system. Computer simulations are used to evaluate the performance of the TDD power control method. Performance improvement of order of 12 to 17dB at bit error rate (BER) of 10-3 can be obtained for different methods of power control. The advantages of the TDD technique for the future DS-SS systems operating in the Industrial, Scientific and Medical (ISM) band are explained in an appendix to this paper.