Sequence Spread Spectrum Receiver Research Articles

Low-Density Parity-Check Coded Direct Sequence Spread Spectrum Receiver Based on Analog Probabilistic Processing

Forward error correction (FEC) coding is an indispensable technique in the direct sequence spread spectrum (DS-SS) systems for satellite communication applications. Both the FEC and DS-SS can be regarded as specific cases of probabilistic computing based on analog circuits, which is expected to be a promising solution for power-limited scenarios. The combination of FEC and DS-SS techniques can provide sufficient link margin and robustness for communication systems. In this paper, a probabilistic receiver chain for the Low-Density Parity-Check (LDPC) coded DS-SS system is proposed. Generically, an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> -sequence can be regarded as a codeword of cyclic linear codes. Similar to the decoding procedure of LDPC codes, the joint detection and decoding process of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> -sequences can be performed by factor graph-based iterative message-passing algorithms (iMPAs). In terms of the iterative signal processing, we first present an improved approach of iterative stopping criterion which can reduce the average number of iteration by 90% for the LDPC decoding approach. Furthermore, a joint detection and decoding method is developed to provide quick synchronization of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> -sequence. Meanwhile, stopping criterion-based iMPAs are especially suitable for analog implementation with low complexity. Finally, cascading to the analog LDPC decoder, the implementation of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> -sequence detector is designed. The prototyping chip is fully integrated into a 0.35- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu\text{m}$</tex-math></inline-formula> CMOS technology, which can achieve higher throughput than 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\textbf {Gcps}}$</tex-math></inline-formula> with a core chip area of 2.79 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\textbf {mm}}^2$</tex-math></inline-formula> and power consumption of 6.99 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\textbf {mW}}$</tex-math></inline-formula> for its core circuit. Experimental results demonstrate the effectiveness of our proposed receiver mechanism.

Square Wave Decomposition for Fast Correlation in DSSS Receivers

Despreading is the most computationally demanding task of a direct sequence spread spectrum (DSSS) receiver. This operation can be efficiently performed by exploiting bit-wise parallel algorithms using quantized local carriers. The correlation loss due to the use of a quantized local carrier is determined, and a new class of bit-wise parallel correlation algorithms is obtained by decomposing the local carrier into a linear combination of bipolar square waves minimizing the correlation loss.

Research on Interpolation Algorithm for DSSS Signal Receiver

For Direct Sequence Spread Spectrum (DSSS) receiver, a parallel acquisition algorithm based on FFT-IFFT is proposed to improve speed of the DSSS signal acquisition. In order to ensure the FFT data points is equal to a power of 2(2N), Interpolation Algorithm is always used. In this paper, three interpolation algorithms are introduced in detail, and they are analyzed theoretically and compared by quantitative simulation, the resources occupied and test performance verification. We draw the conclusion that the linear interpolation is the optimal algorithm which offers the effective technique for the implementation of DSSS signal receiver.

Adaptive median threshold algorithm used in FDIS of DSSS receivers

For direct sequence spread spectrum (DSSS) receivers, the capability of rejecting narrow-band interference can be significantly improved by a process of frequency-domain interference suppression (FDIS). The key issue of this process is how to determine a threshold to eliminate interference in the frequency domain, which has been extensively studied. However, these previous methods are tedious or very complex. A simple and efficient algorithm based on medians is proposed. The elimination threshold is only related to the median by a scale factor, which can be obtained by the numerical analysis. Simulation results show that the algorithm provides excellent narrow-band interference suppression while only slightly degrading the signal-to-noise ratio (SNR). A one-pass algorithm using logarithmic segmentation is further derived to estimate medians with low computational complexity. Finally, the FDIS is implemented in a field programmable gate array (FPGA) of Xilinx. Experiments are carried out by connecting the FDIS FPGA to a DSSS receiver, and the results show that the receiver has an effective countermeasure for a 60 dB interference-to-signal ratio (ISR).

APPARATUS AND METHOD FOR OPERATING A HEARING AID

A programmable hearing aid including means for receiving and transmitting data wirelessly from and to a portable module being in proximity to said hearing aid. Said portable module has means for transmitting audio signals, fitting data or special instructions to the hearing aid processor and including means for receiving data transmitted from said hearing aid, including data representing a monitoring of real-time signal processing parameters in the hearing aid. A preferred embodiment of the hearing aid/portable module combination utilizes Miller-coded direct sequence spread-spectrum radio signal transmitters and receivers for transmitting and receiving data between the heading aid and the portable module. This enables remote controlling or monitoring of, transmitting audio to, or programming of a hearing aid without the need for external connectors.

Serial search and maximum selection based code acquisition techniques for single and multi antenna systems

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.

Narrowband Interference Suppression Using Undecimated Wavelet Packets in Direct- Sequence Spread-Spectrum Receivers

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

Fast-matched filters in time domain for global positioning system receivers

A fast-matched filter algorithm in time domain is presented to correlate signals for optimal detection of signals in noise, to extract ranging measurements, for synchronisation, and so on. The matched filters are discussed in the context of global positioning system receivers, but the algorithms presented here can be directly used for other direct sequence spread spectrum receivers as no constraining assumptions are made in the derivations. The method proposed here outperforms the conventional time-domain method several times in arithmetic complexity. It is also competitive with transform-domain techniques based on fast Fourier transform (FFT). Unlike FFT-based methods, however, this approach is accurate and does not use rounding or scaling operations.

Subband transforms for adaptive, RLS direct sequence spread spectrum receivers

Adaptive Direct Sequence Spread Spectrum (DSSS) receivers have advantages over their fixed matched filter counterparts including interference cancellation capabilities and simplification of PN code acquisition. However, convergence using the LMS algorithm will be very slow in situations with relatively high SNR and/or a large number of users. The use of the RLS algorithm will improve convergence speed but at significantly increased computational cost, especially for long PN codes. Unfortunately, computationally efficient, fast RLS algorithms cannot be used because the filter is updated at the symbol rate rather than at every sample. In this paper, we propose a subband version of the RLS-based receiver that utilizes multiple, shorter length adaptive filters. This approach significantly reduces computation and introduces architectural parallelism into the system implementation. We design an optimal subband transform and provide simulation results demonstrating the improved convergence properties as compared with the fullband system.

DS-CDMA Receiver Based on a Five-Port Technology

High data rates, low-power consumption, and low complexity will be the most important parameters in the design of the next-generation mobile terminals. In this paper we are introducing a new paradigm in the design of direct sequence spread spectrum receiver by combining analog and digital signal processing. The main difference with respect to the conventional all-digital receiver design approach is that the proposed mixed analog/digital processing results in a symbol rate sampling rather than the high-rate subchip sampling. Analog signal despreading is the key part of the proposed receiver solution, which is based on a five-port device, a passive RF square-law-type device. It is used to perform two important tasks at the same time, namely, the direct conversion and analog despreading. To achieve lower complexity, the proposed receiver uses rectangular instead of pulse-matched despreading at the cost of only a small performance degradation. Also, we propose a new noncoherent pseudonoise (PN) code tracking scheme based on error signal generated through the L1 norm. This results in comparable or even better PN code tracking performance than L2 norm circuitry, using less complex hardware. Further, we explore how this technology can be applied in the design of DS-CDMA RAKE receiver for mobile terminals. Depending on how the pilot signal is multiplexed, we propose two types of RAKE receivers. It is shown that under Rayleigh fading channel such receiver structures offer robustness and high performance, while maintaining the low complexity achievable through the five-port device.

A programmable pipelined digital differential matched filter for DSSS receiver

A programmable pipelined digital differential matched filter (PDMF) implemented for a direct sequence spread spectrum receiver is proposed in this paper. To reduce the power consumption, the PDMF architecture is based on the synchronization combined PN code phase acquisition algorithm. Compared with the conventional PN code phase acquisition algorithm, the theoretical analysis result indicates that the PDMF acquires both power efficient and preferable detection. Depending on different applications, programmability allows the PDMF to implement 3-tap, 5-tap, or 11-tap Barker codes with the same hardware but different precisions for each tap coefficient. For short tap Baker codes, the architecture could adopt more precision on each tap coefficient to resist the channel noise. Simulation results also show that there are fewer errors of high sample precision with the same tape.

On the threshold setting principles in code acquisition of DS-SS signals

This paper compares the meaning of different threshold setting principles in the code acquisition process of a direct sequence (DS) spread spectrum (SS) receiver. The consideration is made mainly in one-path additive white Gaussian noise (AWGN) channel. A consideration to a fixed multipath channel is given to see its effect on the results. Also, a consideration for a certain type of fading is given in a case where the signal power is assumed to be considerably lower, i.e., faded, part of the time. For the possible performance measures of code acquisition, the main interest is in the mean acquisition time T/sub MA/. The probability of the acquisition in a given observation interval P/sub acq/ is also considered to see if different measures have different demands. A matched filter (MF) acquisition is used with and without a verification mode using an active integration. In the comparisons, fixed thresholds, thresholds based on constant false alarm rate (CFAR) criteria, and optimal thresholds in the sense to give either the minimum T/sub MA/ or the maximum P/sub acq/ are used. The results, which are obtained by using a method of selecting the maximum value at the output of the MF, are compared to the threshold cases. The results can be summarized as follows: when the performance measure is T/sub MA/, the best results are obtained by using CFAR-based threshold comparison. By a proper selection of the probability of a false alarm, the same performance is obtained as by using the optimal thresholds. When the performance measure is P/sub acq/, the maximum-selection method is the best choice.

An FPGA receiver for CPSK spread spectrum signaling

In this paper, we present the design, implementation and testing of an M-ary direct sequence spread spectrum receiver suitable for wireless home networking applications. The receiver employs a novel code-phase-shift keying (CPSK) signaling scheme, in which each of the M signaling waveforms is derived from a different phase shift of a single pseudonoise code sequence. The receiver consists of an IF demodulator and a CPSK baseband decoder, implemented using discrete components and an FPGA (field programmable gate array) chip, respectively. A modified double-dwell serial search scheme is used for code acquisition and tracking, and the carrier-phase synchronization is solved by a Costas loop in the IF demodulator and a double threshold detection scheme in the CPSK decoder. Measurements of receiver performance are presented and compared with theoretical calculations.

The performance of a direct-sequence spread-spectrum acquisition system using a linear prediction filter for narrowband interference suppression

The coarse acquisition performance of a direct sequence spread-spectrum receiver is analyzed when a linear prediction filter is used for narrowband interference suppression. We show that once an appropriate matching strategy is identified, the linear prediction filter can provide favorable performance when narrowband interference is present over a considerable range of both interference power and bandwidth. In addition, the presence of the filter dramatically improves the performance over the case where there is no filter, except when the interference bandwidth and the power are both small (i.e., when the processing gain provides sufficient interference immunity without the filter). If long spreading sequences are used with moderately sized observation windows, the acquisition performance can be severely degraded when a parallel acquisition scheme is used due to the linear predication filter. We show, however, that a slower serial receiver will provide reliable performance.

A unified time-domain analysis of serial search with application to spread spectrum receivers

The paper presents a time-domain approach to the analysis of the code phase acquisition process in direct sequence spread spectrum receivers. This approach is suitable for a variety of search and verification/lock strategies. The time-domain approach is more direct than the unified z transform-domain approach and permits computation of the acquisition time distribution. The paper also provides equations for calculating the mean acquisition time under various search and verification/lock strategies. The usefulness of the approach is demonstrated by application to a specific nontrivial case of practical significance.

Single and dual multi-dwell synchronization for a direct-sequence spread spectrum receiver

Investigates factors affecting the coarse synchronization performance of a multi-dwell circuit for direct sequence spread spectrum receivers. The influence of the search/verification/lock (SVL) strategy, detector threshold and dwell time on the speed of acquisition is analyzed. For a selected multi-dwell SVL strategy, the mean, variance and distribution of the acquisition time are presented for several values of the detector threshold. A system design example is presented which demonstrates the high sensitivity to detector threshold. Two dual multi-dwell circuits that reduce the acquisition time are presented. The first approach searches two mutually exclusive areas, while the other searches the full region in opposite directions. It is shown that there is little difference in performance between these dual multi-dwell circuits.

Simulation of a Coded Spread Spectrum System for the Impulsive and Multipath Channel

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.

Suppression of Narrow-Band Interference in a PN Spread-Spectrum Receiver Using a CTD-Based Adaptive Filter

Analytical results have shown that adaptive filtering can be a powerful tool for the rejection of narrow-band interference in a spreadspectrum receiver. However, the complexity of adaptive filtering hardware has hindered the experimental verification of these results. This paper describes a new adaptive filter architecture for implementing the Widrow-Hoff LMS algorithm while using only two multipliers regardless of filter order. This hardware simplification is achieved through the use of a burst processing technique. A 16-tap version of this adaptive filter constructed using charge-transfer devices (CTD's) is used to suppress a single tone jammer in a direct sequence spread-spectrum receiver. Probability of error measurements demonstrating the effectiveness of the adaptive filter for suppressing the single tone jammer along with simulation results for the optimal Weiner-Hopf filter are presented and discussed.

Spread-spectrum synchronizer using a SAW convolver and recirculation loop

Segments of a long biphase coded waveform were correlated in a degenerate acoustic convolver, and the resulting correlation peaks were summed in a SAW recirculation loop. Using a 9 Mchip/s wave-form, 730 µs of waveform was correlated, giving an experimental SNR increase of 40 dB. This system can be used to reduce substantially the acquisition time for synchronization of a direct sequence spread-spectrum communication receiver.