An adaptive direct-sequence spread-spectrum receiver for burst typeinterference

The hybrid system is an extension of the state-space system model which allows for random and abrupt changes in the model parameters. In this paper we use the hybrid system to model burst type interference in a direct-sequence spread-spectrum (DS/SS) communication system. The resulting optimum receiver is easily derived but too computationally complex to be practical. Two suboptimal but more practical receivers are presented. In an example, the hybrid system is used to model pulsed narrowband interference. The performance of each suboptimal receiver is evaluated using computer simulations     

Third-generation TDRSS-compatible direct-sequence spread-spectrumdigital receiver

A digital communication receiver, called a third-generation receiver, has been developed. This receiver takes samples of the direct-sequence spread signal at a nonzero intermediate frequency (IF) instead of the zero IF (baseband), and quantizes the samples by employing a 1-b analog-to-digital (A/D) converter at the receiver front end. These 1-b samples are digitally processed for pseudonoise (PN) code, carrier, bit synchronization, and bit decision with the use of an application-specific integrated circuit. The effects of the IF sampling and 1-b A/D conversion on PN code synchronization are analyzed for a PN spread-spectrum communication system with oversampling rate, e.g., 12 samples per chip. In addition, the bit-error rate (BER) degradation due to the 1-b A/D conversion is studied by assuming perfect PN code, carrier, and bit synchronization. It is observed that the BER degradation due to the 1-b A/D is significant, e.g., 2.4 dB, when decimation is made after IF sampling such that only one sample per chip is used for bit decision. These analyzed BER results agree well with the simulated results. However, if no decimation is made and oversampling is used for bit decision, BER degradation due to 2-b A/D conversion is insignificant, e.g., 0.6 dB     

An adaptive multiuser receiver for CDMA systems

A new real-time, digital adaptive multiuser receiver structure is proposed for the uplink in a mobile communications system employing code division multiple access (CDMA). The receiver efficiently implements the decorrelating detector of Lupas and Verdu (1989) and can be adapted to incorporate decision feedback to further improve the detector performance. While the basic receiver design is presented for synchronous CDMA over AWGN channels, experimental evaluation of the receiver for the asynchronous case verifies its robustness for cases when the relative user delays are small compared to the symbol duration as in microcellular scenarios. An efficient decorrelating RAKE combiner for frequency-selective multipath channels is also proposed and analyzed. Performance evaluation of the detector via computer simulation scenarios is conducted in support of analytical results to substantiate its potential for real-time operation     

Blind adaptive interference suppression for direct-sequence CDMA

Direct sequence (DS) code division multiple access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity of DS-CDMA systems is interference-limited and can therefore be increased by techniques that suppress interference. In this paper, we present developments in interference suppression using blind adaptive receivers that do not receive knowledge of the signal waveforms and propagation channels of the interference, and that require a minimal amount of information about the desired signal. The framework considered generalizes naturally to include additional capabilities such as receive antenna diversity. The most powerful application of the methods described here is for linearly modulated CDMA systems with short spreading waveforms (i.e., spreading waveforms with period equal to the symbol interval), for which they provide substantial performance gains over conventional reception. Implications for future system design due to the restriction of short spreading waveforms and directions for further investigation are discussed     

An adaptive non-AWGN SSMA receiver

This article deals with the issue of optimum detection of a known spread spectrum (SS) signal in nonwhite noise and/or narrowband interference. Prior to detection, the input signal is whitened according to the reciprocal of an interfering spectrum, I(z)=1/AG(z). Next, in force of the maximum likelihood (ML) principle, the replica of signal is carried through the same filter, I(z), to match it to a new signal shape. Results of simulation show a significant gain in comparison to conventional single-matched detection.     

基于码匹配滤波的数字化直扩接收机

推导出了一种基于码匹配滤波的数字直扩接收机模型，该方法通过在解扩之前对本地扩频码进行匹配滤波实现对变带宽扩频信号的最佳接收。由于不需直接对输入基带信号进行滤波，而对本地扩频码的滤波又可以通过查表的方式实现，因此与传统的扩频接收机相比，占用的硬件资源大大减少。文中还提出了一种易于实现的次最佳方案。仿真结果表明，次最佳接收机的误码率性能和最佳接收机的误码率性能基本一致。     

Microwave photonic direct-sequence transmitter and heterodyne correlation receiver

This paper proposes and demonstrates a new architecture for the transmission, heterodyne reception, and correlation of direct-sequence (DS) signals encoded onto an optical carrier. The approach is practical for pseudo-noise in-modulated laser radar and free-space optical-code-division multiple-access communications. Although the local oscillator is free running, we show that the received signal is free from laser phase noise. Furthermore, by applying DS coding to both the transmitted signal and the local oscillator, the delay required for correlation can be realized via a combination of electrical and optical means.     

An adaptive maximum likelihood receiver for correlatedRayleigh-fading channels

The paper develops a receiver structure for random Gaussian signals in additive noise based on the classic maximum likelihood (M-L), estimator-correlator derivation of Kailath [1960], and applies it to differential phase shift keying (DPSK) on the correlated Rayleigh-fading channel. It is shown to lower the error floor found in the performance of conventional DPSK receivers by orders of magnitude. In addition, the maximum-likelihood procedure is shown to make uncorrelated symbol decisions. The performance of both conventional and optimal receivers, which require knowledge of the channel statistics, is examined analytically. A recursive, channel-adaptive version of the optimal receiver, utilizing decision feedback to estimate the channel statistics, is developed. Its simulated performance shows no penalty compared to theoretical calculations which require explicit knowledge of the channel statistics     

MLSE receiver for direct-sequence spread-spectrum systems on a multipath fading channel

To accommodate high-speed data transmissions, it may be necessary to substantially reduce the processing gain of a direct-sequence spread-spectrum (DSSS) system. As a result, intersymbol interference effects may become more severe. In this paper, we present a new structure for maximum-likelihood sequence estimation equalization of DSSS signals on a multipath fading channel that performs the function of despreading and equalization simultaneously. Analytical upper bounds are derived for the bit-error probability when random spreading sequences are used, and comparisons to simulation results show that the bounds are quite accurate. The results also show that significant performance improvement over the conventional RAKE receiver is obtained.     

An adaptive receiver for the time- and frequency-selective fadingchannel

An adaptive receiver is presented in this paper for the reception of linearly modulated signals transmitted over a time- and frequency-selective fading channel. The channel is modeled as a truncated power series which represents the dispersive fading channel as a sum of three elementary flat-fading channels. The proposed receiver consists of a sequence estimator with a parallel channel estimator. The channel estimator recovers the instantaneous fading processes associated with each elementary channel and filters them to generate one-step predictions of each fading process. Some implementation difficulties and solutions are also discussed. Computer simulations using quadrature phase-shift keying (QPSK) and channels with moderate delay spreads and fade rates have been used to evaluate the performance of the receiver. The results show that our technique has potential in channels with delay spread of about 20%, signal-to-noise ratio (SNR) greater than 15 dB, and applications requiring bit-error rates (BER's) less than 10^-2     

An adaptive spatial diversity receiver for non-Gaussianinterference and noise

Standard linear diversity combining techniques are not effective in combating fading in the presence of non-Gaussian noise. An adaptive spatial diversity receiver is developed for wireless communication channels with slow, flat fading and additive non-Gaussian noise. The noise is modeled as a mixture of Gaussian distributions and the expectation-maximization (EM) algorithm is used to derive estimates for the model parameters. The transmitted signals are detected using a likelihood ratio test based on the parameter estimates. The new adaptive receiver converges rapidly, its bit error rate performance is very close to optimum when relatively short training sequences are used, and it appears to be relatively insensitive to mismatch between the noise model and the actual noise distribution. Simulation results are included that illustrate various aspects of the adaptive receiver performance     

An adaptive truncated MLSE receiver for Japanese personal digitalcellular

This paper describes a dual-mode Japanese personal digital cellular receiver that uses an adaptive truncated symbol-spaced maximum-likelihood sequence-estimation (MLSE) equalizer in one mode and a tangent type differential detector in the other. The receiver employs a channel estimation and symbol synchronization procedure that uses the known phase shifts between successive symbols in the synchronization word. Per-survivor processing is used to track the channel variations and carrier frequency offset. Simulation results are presented for multipath Rayleigh fading channels having various delay profiles. Comparisons between the regular symbol-spaced truncated MLSE equalizer and a fractionally spaced truncated MLSE equalizer are also furnished     

An adaptive MLSE receiver for TDMA digital mobile radio

The authors present a simulation study of an adaptive receiver, based on the concept of maximum-likelihood sequence estimation (MLSE), which compensates for the heavy selective distortions caused by multipath propagation. The receiver includes a matched filter and a modified Viterbi processor and is suitable for implementation in a digital form. It operates adaptively, in a training mode at the beginning of each burst, as well as in a tracking mode during message detection. This makes the receiver robust both to fast Doppler shifts and to a large frequency offset. Simulation results are presented which show the performance in different multipath environments, with echo delay in excess of 20 μs and vehicle speed up to 250 km/h     

Switched correlator receiver architecture for optical CDMA networkswith bipolar capacity

A new optically switched correlator receiver architecture, based on the principle of unipolar-bipolar correlation, is proposed for optical fibre CDMA networks. The receiver BER performance is assessed as a function of the incident optical power for noncoherent transmission and direct detection, with the number of simultaneous users as a parameter     

一种零中频直接序列扩频数字接收技术

直接序列扩频技术在现代无线通信系统中有着广泛的应用,该信号的数字接收是实现对其侦察截获的关键技术之一。本文介绍了直接序列扩频技术原理及其在cdma2000移动通信系统中的应用,提出一种针对cdma2000直接序列扩频信号的零中频数字接收算法,并详细给出了码捕获及跟踪算法、载波同步算法的数学模型推导。实验结果显示,该算法成功地实现了码同步及载波同步,有较好的解调效果。     

An improved blind adaptive MMSE receiver for fast fading DS-CDMAchannels

Blind adaptive minimum mean-squared errors (MMSE) receivers for multiuser direct-sequence code-division multiple access (DS-CDMA) systems that assume knowledge of the steering vector, i.e., the cross-correlation between the desired output and the input signal, are known for their robustness against channel fading as they do not attempt to explicitly track the channel of the user of interest. However, these receivers often have higher excess mean squared error and, hence, poorer performance than training-sequence based adaptive MMSE receivers. In this paper, an improved correlation matrix estimation scheme for blind adaptive MMSE receivers is provided. The new scheme takes advantage of the fact that the desired linear receiver can be expressed as a function of the interference correlation matrix only, rather than the total data correlation matrix. A theoretical analysis is performed for the flat fading case which predicts that the new estimation scheme will result in significant performance improvement. Blind adaptive MMSE receivers with the new estimation scheme appear to achieve performance comparable to the training-sequence based adaptive MMSE receivers. Detailed computer simulations for the fast multipath fading environment verify that the proposed scheme yields strong performance gains over previous methods     

Split-sum correlator simplifies range computations in GPS receiver

A novel correlator architecture for handling the satellite-to-receiver time skew in a Global Positioning System (GPS) receiver is presented. The correlator's signal integration and numerically-controlled oscillator updates are referenced to receiver time epochs rather than satellite epochs as in traditional architectures. To avoid straddling navigation data bits, the correlation is split into two parts. The technique was successfully implemented and integrated into a field programmable gate array-based multichannel software GPS receiver     

An optimal receiver using a time-dependent adaptive filter

The optimal time-dependent receiver (OTDR) is presented and its performance is compared to that of the matched filter receiver. The OTDR, a time-dependent adaptive filter, is shown through simulation to be superior to the matched filter for signals corrupted by cyclostationary interference because it exploits statistical periodicities of the interference     

A linear receiver for direct-sequence spread-spectrummultiple-access systems with antenna arrays and blind adaptation

A linear receiver for direct-sequence spread-spectrum multiple-access communication systems under the aperiodic random sequence model is considered. The receiver consists of the conventional matched filter followed by a tapped delay line with the provision of incorporating the use of antenna arrays. It has the ability of suppressing multiple-access interference (MAI) and narrowband interference in some weighted proportions, as well as combining multipath components without explicit estimation of any channel conditions. Under some specific simplified channel models, the receiver reduces to the minimum variance distortionless response beamformer, the RAKE receiver, a notch filter, or an MAI suppressor. The interference rejection capability is made possible through a suitable choice of weights in the tapped delay line. The optimal weights can be obtained by straightforward but computationally complex eigenanalysis. In order to reduce the computational complexity, a simple blind adaptive algorithm is also developed     

Performance of direct-sequence spread-spectrum receiver usingdecision feedback and transversal filters for combatting narrowbandinterference

A direct-sequence spread-spectrum (DSSS) receiver using both decision feedback (DFB) and two-sided transversal filters for combatting narrowband interference (NBI) is proposed. The receiver is made up of two branches. In the first branch, the conventional demodulator is followed by a DFB filter, while in the second, auxiliary branch, a demodulator with the carrier in quadrature is followed by a two-sided adaptive transversal (AT) filter. Performance of this receiver was analyzed on the basis of the calculated mean-square error and the probability of error at the output of the receiver. Special attention was paid to the effects caused by the propagation of errors in the DFB filter. The results obtained show that NBI rejection is fairly high and practically does not depend upon the difference of frequencies of the desired and interfering carriers or upon the interfering carrier level