Adaptive interference rejection in a digital direct sequence spread spectrum receiver

In an effort to transmit information over communication channels that contain both Gaussian and non-Gaussian noise components, military and commercial applications frequently rely upon the use of direct sequence spread spectrum (DSSS) techniques. Whilst DSSS is an interference tolerant modulation scheme, situations may be encountered when the processing gain offered by the pseudonoise (PN) spreading code is inadequate. Enhancements to the interference immunity of a DSSS system can be accomplished through the use of signal processing techniques that suppress unwanted interference prior to despreading. To date, the majority of interference suppression techniques developed for DSSS applications, have concentrated on improving detector performance exclusively, and perfect synchronisation between transmitter and receiver is assumed. However, the process of PN code acquisition and tracking within a DSSS receiver is arguably one of the most critical functions within the receiver structure since, if it fails to operate correctly, the wanted signal will not be successfully recovered. An evaluation of the benefits of any interference suppression technique within a DSSS receiver would not be complete without considering at least one level within the synchronisation hierarchy. We examine the improvements in receiver performance, when the process of PN code tracking and detection employs a novel wavelet packet denoising (WPD) procedure for a variety of non-Gaussian channel scenarios.

The Widrow IMS algorithm is considered for the implementation of an adaptive prewhitening filter in a direct sequence (DS) spread spectrum receiver. Using an approximation for the statistics of the output of the adaptive filter, expressions for the resulting bit-error-rate (BER) when the adaptive algorithm is used to suppress a fading tone jammer are developed, and numerical results obtained from these expressions are compared with simulation results for the DS receiver.

Code synchronization in direct-sequence spread-spectrum (DSSS) receivers can be divided into the acquisition and the tracking phases. Here we assume that the acquisition is performed with a periodic pseudo-random signal of period N. Acquisition of pseudo-random signals is similar to target detection with a pulse compression radar. The appropriate tool for the description of a pulse compression radars behaviour is the ambiguity function. The present paper describes the code acquisition in DSSS receivers in terms of cross ambiguity functions. Moreover the importance of the merit factor for incoherent acquisition is represented. The signal to interference ratio and an equation giving the optimum matched filter length as a function of the Doppler shift is derived.

This paper presents a comparison of direct sequence spread spectrum (DS-SS) and filter bank multi carrier spread spectrum (FB-MC-SS) systems when applied for communications over HF channels. Such systems by their spread spectrum nature are, clearly, broadband. On the other hand, noting that the presence of narrowband interferers are inevitable in HF channels, interference suppression performance has to be given a special consideration when such systems are designed. In this paper DSSS and FB-MC-SS are compared using measured HF interference data. FB-MC-SS is shown to have very good performance while using a low complexity and robust maximum ratio combining receiver. The partitioned structure of the FB-MC-SS waveform enables this implementation. Among the DS-SS receivers in the literature, we find that only the highly complex minimum mean square error (MMSE) receiver can provide a comparable performance to its FB-MC-SS counterpart. Considering the receiver complexity, it becomes obvious that FB-MC-SS is the right choice for further developments of HF spread spectrum modems in the future.

Performance of a direct sequence spread spectrum communication system operating in an impulsive noise and multipath channel is evaluated using computer simulation techniques. The results indicate that the direct sequence spread spectrum receiver tends to convert the block error pattern in to random error pattern and further application of convolutional coding with Viterbi decoding gives substantial improvement in performance. Balanced Gold codes are found to perform better than Gold codes and m-sequences.

The performances of a direct sequence spread spectrum receiver has been carried out by considering a differential coherent phase shift keying detection of the data. After the baseband down conversion of the received signal an hard limiter is included in both the in-phase and quadrature paths. So that we can built a fully digital receiver. The penalty we pay in the presence of noise jamming is of 5 dB with respect to analog receiver for a quadriphase transmission. Moreover, increasing the signal to jamming ratio a saturation of the error rate appear.

A self-referencing direct sequence spread spectrum receiver is presented which is based on an appropriate single lag of the auto-triple-correlation. The receiver is self-acquiring, in the sense that no local reference code is necessary. The received signal is auto-triple-correlated at a particular lag such that the transmitted code acts as its own reference. The proposed receiver is characterized by a bound on its probability of bit error in additive white Gaussian noise, and compared with a conventional coherent BPSK receiver. >

In a digital matched filter (DMF) the degradation of signal quality depends on the pre-sampling filter bandwidth and on the number of analogue to digital) converter (ADC) bits. Digital matched filters are used in the acquisition of Global navigation satellite systems (GNSS) signals. Automatic gain control (AGC) in the GNSS receivers is used to determine the maximum level of quantization by the ADC. In (Chang, 1982), the quantization degradation for different pre sampling bandwidths and adc bits is analysed in the presence of Additive Gaussian white noise (AGWN). Bandwidths of more than once (or twice) the data bit rate contribute only about 0.4 dB (or (0.2 dB) to the overall degradation of the DMF. Also 2 bit uniform step quantization with optimum ADC threshold setting is shown to recover most of the digital implementation degradation. In this paper by focusing on pre-sampling filter bandwidth equal to the data bit rate and 2 bit ADC, the effect of quantization on the received GNSS signal quality in the presence of continuous wave (CW) radio frequency interference (RFI) is analysed. It is shown that a one bit ADC can have much higher degradation than the 3.5 dB in the AGWN case. It is also shown that the effect of AGC on the quantization degradation has to do with both power and frequency of CW RFI. In the presence of CW RFI, the optimal value for ratio of maximum threshold of ADC to the effective noise RMS is shown to be about 1.35 which is higher than the case when RFI does not exist (0.8).

An experimental design approach is used to determine which factors (design parameters) of spectrally modulated, spectrally encoded (SMSE) waveforms have the greatest impact on coexistence with other communication waveforms. The SMSE framework supports cognition-based, software defined radio (SDR) applications and is well-suited for coexistence analysis. For initial proof-of-concept, a two factor (parameter), three-level (value) experimental design technique is applied to a coexistent scenario to characterize SMSE waveform impact on direct sequence spread spectrum (DSSS) receiver performance. The experimental design methodology reliably captures factor-level sensitivities and identifies those factors having greatest impact on system coexistence behavior (bit error variation). Given these initial results and its effectiveness in other engineering fields, it is believed that experimental design may pave the way for developing more rigorous waveform design methods and allow more robust coexistence analysis of conventional, DSSS and SMSE waveforms.

In traditional feedback structure of direct sequence spread spectrum (DS-SS) receiver, delay lock loop is a prevailing approach to estimate the timing error. However, in burst communication, it is hard for DLL to be in transient state or lock the signal with high accuracy. In this paper, an efficient feedforward PN code timing recovery scheme is proposed for DS-SS systems. The received signal after the chip-matched filter is two times over-sampled. The characteristics of DS-SS signal are explored to make the synchronization scheme efficient and simple in such a low sampling rate. Three sets of correlation values with different timing offsets are stored to estimate the code timing and carrier phase error. With very low computational complexity, the proposed scheme can make the performance approaches to the DS- SS systems with ideal synchronization in an AWGN channel.

Ergodic chaotic parameter modulation (ECPM) has recently been proposed as a viable modulation technique for power-line communications (PLC) due to its robustness to multipath conditions and low complexity in receiver design. While performance of ECPM has been found to be satisfactory in limited noise conditions, performance issues for other hostile challenges in the power-line environment, such as narrowband and impulsive noise, have not been extensively studied for this technique. Numerous studies have been performed on direct-sequence spread-spectrum (DSSS) modulation used in a wireless channel. The technique has been found to be well suited for a mobile environment due to its resistance to multipath fading and inherent narrowband interference suppression capability. For this study, PLC performance is investigated for both the DSSS and ECPM receiver for PLC. A signal-to-noise ratio (SNR) analysis is presented which investigates the noise-limiting properties of the power-line channel. This work provides a detailed insight to communication system performance in a power-line environment that includes the presence of significant narrowband and impulsive noise. It is concluded that while DSSS significantly outperforms ECPM in terms of bit-error rate for a Gaussian white noise-limited channel, the performance of the two techniques is similar for low SNR communication dominated by narrowband or impulsive noise. While the implementation of DSSS is more complex, ECPM is found to be less robust to low-frequency interference

This paper presents the results of an investigation concerning the performance degradation of a phase-locked loop (PLL) due to continuous wave (CW) interference and additive white Gaussian noise. The performance measures evaluated include the probability density function (pdf) of phase error and the cycleslipping rate. The measures are characterized in terms of the ratio of interference to the desired signal power and the loop signal-to-noise ratio. Since the interference term in the loop phase error signal is periodic, the conditional pdf of the phase error process is cyclostationary. A Fourier series expansion is applied as an approach to obtain a solution to the cyclostationary process. Linear performance results are derived and compared to the analytic results.

It has been shown previously that the performance of a direct-sequence spread-spectrum (DS/SS) receiver which incorporates transform-domain excision of narrowband interference is improved by the use of time-weighting when the interference-to-signal power ratio (ISR) is relatively large, but is degraded by its use otherwise. The demodulator employed in this receiver is a filter which is matched to the chip sequence. While the matched filter gives minimum probability of error for additive white Gaussian noise (AWGN) in the absence of time-weighting and excision processing, the matched filter is suboptimal in their presence. Alternative demodulation rules which are adapted to time-weighting and excision processing are developed in the present paper. It is shown that, for the weighting functions considered, the matched filter requires as much as 3 dB more signal power than an adapted demodulator, to obtain a given probability of error. Furthermore, the performance with an adapted demodulator is generally superior to that for a receiver which does not use time-weighting when the ISR is at least moderately large, and is comparable otherwise. The potential benefit that may be derived from the use of an adapted demodulator in a CDMA network overlay is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Code tracking performance analysis of GNSS signal in the presence of CW interference

This paper investigates several maximum selection and serial search based code acquisition techniques for direct sequence spread spectrum receivers, which utilize either a single or multiple antenna elements. The techniques are studied with different combinations of threshold comparison and verification. Paper includes large number of analytical results calculated in static, fast and slow fading channels to obtain an insight of the performances of these methods. Numerical results will give an overview of the methods relative performance in different channels.