A knowledge-based approach to interference rejection for direct-sequence spread spectrum (DSSS) systems

This paper presents performance evaluation and comparison analysis of Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) systems. The evaluation and analysis are done based on the systems performance against wideband interferences. The interferences are signals with similar spectrum characteristic to the transmitted signals of DSSS and FHSS systems. Bit Error Ratio (BER) is used as evaluation parameter to assess the performance of both systems. Simulation and mathematical analysis are performed to test and verify the performance of both systems. Mathematical analysis also verifies that increasing Spreading Frequency on certain conditions will reduce the BER. This research also points out that FHSS system has a better performance compared to DSSS system indicated by smaller BER.

The performance of anti narrowband noise of direct sequence spread spectrum (DSSS) communication system is investigated in this paper. The new result given in the paper shows that performance of anti noise in DSSS system is different from classic result. The attenuation of narrowband noise in DSSS system is the function of the relative distance between the narrowband noise and carrier frequency of DSSS system, and it is discontinuous. The attenuation of broadband noise is little, and the process gain is about 0.46dB. The results can be utilized in design and application of spread spectrum system.

In this paper, a fast convergent nonlinear adaptive algorithm for narrow-band interference rejection in direct sequence spread spectrum (DS-SS) systems is proposed. The new algorithm can significantly increase the convergence rate of the nonlinear adaptive filter by computer simulations.

The performance of the data-aided nonlinear filter (DNF), for combating narrowband interference (NBI) in direct sequence spread spectrum (DS-SS) systems, is analyzed. Closed-form analytical expressions for improvement in signal to interference-plus-noise ratio (SINR) are derived under the condition of single tone interference. Results show that the DNF structure outperforms conventional nonlinear prediction filter.

This paper proposes an algorithm that improves the mean code acquisition time using an energy estimation mode (EEM) and dwell time values from direct-sequence spread spectrum (DS-SS) System. Until recently, DS-SS System has been used in many areas from Wireless LAN, cellular phone, and GPS to telecommunication. Meanwhile, the development of reducing of pseudonoise (PN) code acquisition time, the core technique of DS-SS system, is continuously explored. The algorithm in this paper added the EEM before a search mode (SM) from the conventional PN code acquisition system, which is composed of a SM and a verification mode (VM). In the EEM, dwell time and threshold values are determined for use in the SM and the VM depending on the correlation energy. This paper applies the proposed technique under Rayleigh-fading channel in performing a simulation of PN code acquisition time by a mathematical modeling to evaluate the proposed algorithm. Numerical results show that the proposed method improves the detection performance and the rejection performance of false code phases not only in the VM but also in the SM. Furthermore, the proposed algorithm reduces the mean code acquisition time by maximum 29% than the conventional algorithm.

Tone interference suppression in DS-SS systems with modified DFT

The study on the Doppler frequency search strategy in direct sequence spread spectrum (DSSS) systems has been addressed in this article. To reduce the estimation error and alleviate the possible false detection, a novel Doppler search algorithm that combines the serial-based search and the fuzzy-logic (FL)-based search is presented. At first, the serial-based search is used until the decision variable exceeds a preset threshold; the FL-based search is then performed for a second search and the final decision is made. In the second search stage, Doppler search step size is adaptively adjusted by an FL controller. To design a robust FL controller, the characteristics of the spectrum distribution and several possible abnormal conditions are analyzed, input and output fuzzy membership functions are derived, and a set of fuzzy rules is formulated. Finally, the effectiveness of the proposed approach is evaluated through Monte Carlo simulations. The obtained results show that a smaller estimation error is achieved when the proposed algorithm is used; further, reasonable mean acquisition times are attained compared with other existing search techniques. Furthermore, the proposed algorithm can alleviate possible false detection in the abnormal conditions.

Closed-form analytical expressions for narrow-band interference rejection of nonlinear prediction filters in direct-sequence spread-spectrum (DS-SS) systems are established. It is shown that nonlinear prediction filters outperform linear ones.

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.

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.

Spread spectrum communication has good performances such as anti-noise, anti-interference and fast transmission rate, so it has been widely used in military communication and civil communication fields and others. The M-ary spread spectrum system has its unique advantages and strong practical application background with respect to direct sequence spread spectrum, and effectively solves the problem of limited bandwidth of direct spread spectrum. This paper firstly expounds the principle of traditional direct sequence spread spectrum system and then focuses on the research of the principle and system model of M-ary spread spectrum system. The error performance of M-ary spread spectrum system is also simulated and analyzed. The simulation results show that M-ary spread spectrum has a greater advantage in the bandwidth-constrained field, where high-speed data transmission is required and certain anti-noise capability is required.

Time diversity is achieved in direct sequence spread spectrum by receiving different faded delayed copies of the transmitted symbols from different uncorrelated channel paths when the transmission signal bandwidth is greater than the coherence bandwidth of the channel. In this paper, a new time diversity scheme is proposed for spread spectrum systems. It is called code-time diversity. In this new scheme, N spreading codes are used to transmit one data symbol over N successive symbols interval. The diversity order in the proposed scheme equals to the number of the used spreading codes N multiplied by the number of the uncorrelated paths of the channel L. The paper represents the transmitted signal model. Two demodulators structures will be proposed based on the received signal models from Rayleigh flat and frequency selective fading channels. Probability of error in the proposed diversity scheme is also calculated for the same two fading channels. Finally, simulation results are represented and compared with that of maximal ration combiner (MRC) and multiple-input and multiple-output (MIMO) systems.

Deployment of wireless sensor networks (WSNs) with a large number of nodes have reinstated interest in spread spectrum (SS) technologies as they allow to establish multiple access for nodes with simple hardware and limited power budget. Chaotic spreading is one of the still insufficiently explored areas which can lead to higher security and lower multiple access interference (MAI). This research is intended to bring new information about the performance of binary direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) communication systems which employ chaos-based spreading codes and compare them against classical systems employing m-sequences. Unlike existing research, this investigation is based on the simulation of passband communication systems with up/down-conversion and respective filtering. Performance comparison in terms of bit error ratio (BER) versus signal-to-noise ratio (SNR) at the different number of users and MAI dependence on carrier frequency offset among users in case of additive white Gaussian noise (AWGN) channel are presented. Obtained results highlight several scenarios where chaotic-sequences-based binary DSSS and FHSS systems have advantages over classical solutions.

Under the software environment of MATLAB/Simulink, the communication system of direct sequence spread spectrum (DSSS) is dynamically modeled and simulated over a new practical Rayleigh channel, and the relationship curves of the bit error rate (BER) with the signal-to-noise ratio (SNR) of DSSS communication systems are presented. It is analyzed that DSSS system can effectively overcome multipath fading in multipath Rayleigh Fading Channel (RFC). In addition, the anti-multipath fading performance improved by the way of increasing pseudo noise (PN) code length is better than that by the way of increasing the spread spectrum gain in the condition of strong multipath fading.

Direct sequence spread spectrum (DSSS) system needs some anti-jam algorithms for communication when strong interference exists. Both Normalized least mean square (NLMS) algorithm in time domain and anti-jam algorithm based on FFT in frequency domain are widely used for the excellent performance and small hardware costs. This paper implemented the two algorithms on FPGA and researched their system performances and hardware costs, finally some useful guides for later system design were given.