An Experimental Design Approach for Optimizing SMSE Waveforms to Minimize Coexistent Interference

A genetic algorithm (GA) is used to design Spectrally Modulated, Spectrally Encoded (SMSE) waveforms while characterizing the impact of parametric variation on coexistence. As recently proposed, the SMSE framework supports cognition-based, software defined radio (SDR) applications and is well-suited for coexistence analysis. For initial proof-of-concept, two SMSE waveform parameters (number of carriers and carrier bandwidth) are optimized in a coexistent scenario to characterize SMSE impact on Direct Sequence Spread Spectrum (DSSS) bit error performance. Given optimization via GA techniques have been successfully applied in many engineering fields, as well as operations research, they are viable candidates for robust SMSE waveform design. As demonstrated, the analytic SMSE framework is well-suited for parametric optimization via GA techniques.

A spectrally modulated spectrally encoded (SMSE) based overlay cognitive radio has been implemented and demonstrated in [1] via GNU software define radio (SDR). However, like most of the current cognitive radio implementations and demonstrations, this work does not consider the mobility between cognitive radio nodes. In a high mobility environment, the frequency offset introduced by Doppler shift leads to loss of the orthogonality among subcarriers. As a direct result, severe inter-carrier interference (ICI) and performance degradation is observed. In our previous work, we have proposed a new ICI cancellation method (namely Total ICI Cancellation) for OFDM [2] and MC-CDMA [3] mobile communication systems, which eliminates the ICI without lowering the transmission rate nor reducing the bandwidth efficiency. In this paper, we apply the total ICI cancellation algorithm onto the SMSE base overlay cognitive radio to demonstrate a high performance cognitive radio in high mobility environment. Specifically, we demonstrate an SMSE based overlay cognitive radio that is capable of detecting primary users in real time and adaptively adjusting its transmission parameters to avoid interference to (and from) primary users. When the primary user transmission changes, the cognitive radio dynamically adjusts its transmission accordingly. Additionally, this cognitive radio maintains seamless real time video transmission between the cognitive radio pair even when large frequency offset is introduced by mobility between CR transmitter and receiver.

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.

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.

To improve system performance in the presence of continuous-wave (CW) interference, a 2-b analog-to-digital converter (ADC) with adaptive thresholds has been implemented in a DSSS (direct-sequence spread-spectrum) receiver for code-shift-keyed signals. The theoretical and measured system performance of a hard limiter and the adaptive 2-bit ADC to accomplish analog-to-digital (A/D) conversion in the DSSS receiver are compared for AWGN (additive white Gaussian noise), CW, and combined AWGN-CW interference. The system's dynamic response to changes in signal or interference conditions is investigated by means of Monte-Carlo simulations. Both steady-state and dynamic behavior of the 2-b ADC are satisfying in the presence of strong CW interference. However, if the AWGN contribution is dominant, there is no significant difference in the performance of the hard limiter and the 2-b ADC.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

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.

Based on spread spectrum technique, two wireless video transmission schemes are designed and implemented in this paper, namely direct sequence spread spectrum (DSSS) scheme and multi-code spread spectrum (MCSS) scheme. We use software defined radio (SDR) method for hardware implementation of two schemes, where a XILINX Virtex-4 field programmable gate array (FPGA) for the transmitter and another Virtex-5 FPGA for the receiver. In order to evaluate their performance, we compared the two designed schemes with our previous work on orthogonal frequency division multiplexing (OFDM) scheme. The experimental results show that two schemes of DSSS and MCSS have better anti-jamming performance than OFDM scheme at the cost of spectrum efficiency. Also, DSSS scheme possesses higher power efficiency than MCSS scheme and OFDM scheme due to its lower peak-to-average power ratio.

: This technical report summarizes the results of the research we have been engaged in regarding the use of surface acoustic wave devices in direct sequence spread spectrum receivers. The heart of this research has been the use of the device as a real-time Fourier transformer. A system of this type is sometimes referred to as a compressive receiver, and our use of the system has been primarily as a means to implement a narrowband interference rejection filter. In addition, we have studied many other topics such as rapid acquisition, Hilbert transform generation, etc. and these topics are all overviewed in this report. (Author)

Six configurations of direct sequence spread spectrum receiver using surface acoustic wave convolvers is presented and analyzed. These configurations are compared on the basis of the average bit error probability and complexity of realization.

A new algorithm for narrowband interference suppression in direct-sequence spread-spectrum (DS-SS) communications is presented. The algorithm combines the undecimated wavelet packet transform (UWPT) with frequency shifts to confine the interference energy in a subband that is subsequently eliminated. Computer simulation shows a robust performance that appears to be independent of the interference frequency

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>

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