A low energy architecture for fast PN acquisition

Compared with the SAW convolver and the digital correlator, the SAW matched filter (tapped delay line) has many advantages in commercial wireless LAN applications, because of its high efficiency and simple configuration. A new type of SAW device for the spread spectrum (SS) transceiver and a low-loss SAW matched filter for the wireless LAN system is described. Two matched filters which correspond to bi-phase data modulation and bandpass filters for the intermediate frequency (IF) section are combined into one device. This SAW matched filter fabricated on ST cut quartz has been practically used in commercial wireless LAN systems and has good temperature stability. To reduce the conversion loss further we have developed an extremely low loss SAW matched filter. The output correlation peak of the matched filter is 3 dB larger than the input level at a 50 /spl Omega/ input and output impedance. This low loss SAW matched filter is fabricated on 128/spl deg/ Y-X LiNbO/sub 3/ substrate and consists of unidirectional transducers. The phase error caused by the temperature coefficient of delay is compensated by using a temperature dependent change of local frequency in the spread spectrum receiver. Under the critical conditions of a 2.4 GHz wireless LAN (data rate is 1.4 Mbps, C/N>2 dB, that is E/sub b//N/sub 0/>13.6 dB, and operation temperature is from -20/spl deg/C to +60/spl deg/C), good performance is achieved with less than a 10/sup -5/ bit error rate.

We propose a closed-loop noncoherent Pseudo-Noise (PN) acquisition scheme for Direct-Sequence Spread-Spectrum (DS/SS) systems using an auxiliary sequence, previously used for coherent acquisition. The proposed scheme consists of an alignment detection and a VCC loop for updating the local PN signal and the direction of update. The performance of the proposed scheme is obtained by simulation and compared with the noncoherent fixed-dwell serial scheme. Results show that the proposed scheme acquires approximately three to four times faster than the conventional noncoherent serial scheme.

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.

Preface. 1 Spread spectrum signals and systems. 1.1 Basic definition. 1.2 Historical sketch. 2 Classical reception problems and signal design. 2.1 Gaussian channel, general reception problem and optimal decision rules. 2.2 Binary data transmission (deterministic signals). 2.3 M-ary data transmission: deterministic signals. 2.4 Complex envelope of a bandpass signal. 2.5 M-ary data transmission: noncoherent signals. 2.6 Trade-off between orthogonal-coding gain and bandwidth. 2.7 Examples of orthogonal signal sets. 2.8 Signal parameter estimation. 2.9 Amplitude estimation. 2.10 Phase estimation. 2.11 Autocorrelation function and matched filter response. 2.12 Estimation of the bandpass signal time delay. 2.13 Estimation of carrier frequency. 2.14 Simultaneous estimation of time delay and frequency. 2.15 Signal resolution. 2.16 Summary. Problems. 3 Merits of spread spectrum. 3.1 Jamming immunity. 3.2 Low probability of detection. 3.3 Signal structure secrecy. 3.4 Electromagnetic compatibility. 3.5 Propagation effects in wireless systems. 3.6 Diversity. 3.7 Multipath diversity and RAKE receiver. Problems. 4 Multiuser environment: code division multiple access. 4.1 Multiuser systems and the multiple access problem. 4.2 Frequency division multiple access. 4.3 Time division multiple access. 4.4 Synchronous code division multiple access. 4.5 Asynchronous CDMA. 4.6 Asynchronous CDMA in the cellular networks. Problems. 5 Discrete spread spectrum signals. 5.1 Spread spectrum modulation. 5.2 General model and categorization of discrete signals. 5.3 Correlation functions of APSK signals. 5.4 Calculating correlation functions of code sequences. 5.5 Correlation functions of FSK signals. 5.6 Processing gain of discrete signals. Problems. 6 Spread spectrum signals for time measurement, synchronization and time-resolution. 6.1 Demands on ACF: revisited. 6.2 Signals with continuous frequency modulation. 6.3 Criterion of good aperiodic ACF of APSK signals. 6.4 Optimization of aperiodic PSK signals. 6.5 Perfect periodic ACF: minimax binary sequences. 6.6 Initial knowledge on finite fields and linear sequences. 6.7 Periodic ACF of m-sequences. 6.8 More about finite fields. 6.9 Legendre sequences. 6.10 Binary codes with good aperiodic ACF: revisited. 6.11 Sequences with perfect periodic ACF. 6.12 Suppression of sidelobes along the delay axis. 6.13 FSK signals with optimal aperiodic ACF. Problems. 7 Spread spectrum signature ensembles for CDMA applications. 7.1 Data transmission via spread spectrum. 7.2 Designing signature ensembles for synchronous DS CDMA. 7.3 Approaches to designing signature ensembles for asynchronous DS CDMA. 7.4 Time-offset signatures for asynchronous CDMA. 7.5 Examples of minimax signature ensembles. Problems. 8 DS spread spectrum signal acquisition and tracking. 8.1 Acquisition and tracking procedures. 8.2 Serial search. 8.3 Acquisition acceleration techniques. 8.4 Code tracking. Problems. 9 Channel coding in spread spectrum systems. 9.1 Preliminary notes and terminology. 9.2 Error-detecting block codes. 9.3 Convolutional codes. 9.4 Turbo codes. 9.5 Channel interleaving. Problems. 10 Some advancements in spread spectrum systems development. 10.1 Multiuser reception and suppressing MAI. 10.2 Multicarrier modulation and OFDM. 10.3 Transmit diversity and space-time coding in CDMA systems. Problems. 11 Examples of operational wireless spread spectrum systems. 11.1 Preliminary remarks. 11.2 Global positioning system. 11.3 Air interfaces cdmaOne (IS-95) and cdma2000. 11.4 Air interface UMTS. References. Index.

A wireless underwater acoustic communication system based on the combination of chirp modulation and direct-sequence spread-spectrum signaling is presented. The use of chirp signals takes advantage of the low Doppler sensitivity in the matched filter operation whereas the choice of pseudo noise (PN) sequences allows one to reduce narrowband interference arising from other users and self-interference due to multipath propagation. The system here described is made of a transmitter encoding the bits with linear chirps multiplied by PN sequences, and a rake receiver that allows one to positively exploit the energy present in the most significant propagation paths. Moreover a tracking procedure that allows an adaptation to the instantaneous Doppler shift has been devised and tested. Results, obtained on real data over a long-range shallow-water channel with a moving transmitting platform, are presented and discussed.

A novel spread spectrum (SS) system based on the unconventional self encoding principle has been proposed for highly secured data transmissions. This system inherits the antijamming (AJ) and low probability of interception (LPI) characteristics like any other SS system with the aid of truly random spreading codes, which are generated from the source stream itself. Therefore, the proposed system is unique in that the traditional transmit and receive PN code generators are not needed. In fact, the enhanced transmission security arises not only due to the SS nature of the signal, but also from the true randomness of the unique spreading processes. This paper describes the proposed system and presents the simulation results of the system performance under various kinds of jamming conditions, which is not been so far dealt. The obtained performance curves are compared with existing SS systems like direct sequence and frequency hopped SS.

In order to more effectively monitor and interfere with enemy signals, it is particularly important to accurately and efficiently identify the intercepted signals and estimate their parameters in the increasingly complex electromagnetic environment. Therefore, in non-cooperative situations, it is of great practical significance to study how to accurately detect direct sequence spread spectrum (DSSS) signals in real time and estimate their parameters. The traditional time-delay correlation algorithm encounters the challenges such as peak energy leakage and false peak interference. As an alternative, this paper introduces a Pseudo-Noise (PN) code period estimation method utilizing a one-dimensional (1D) convolutional neural network based on the residual network (CNN-ResNet). This method transforms the problem of spread spectrum code period estimation into a multi-classification problem of spread spectrum code length estimation. Firstly, the In-phase/Quadrature(I/Q) two-way of the received DSSS signals is directly input into the CNN-ResNet model, which will automatically learn the characteristics of the DSSS signal with different PN code lengths and then estimate the PN code length. Simulation experiments are conducted using a data set with DSSS signals ranging from -20 to 10 dB in terms of signal-to-noise ratios (SNRs). Upon training and verifying the model using BPSK modulation, it is then put to the test with QPSK-modulated signals, and the estimation performance was analyzed through metrics such as loss function, accuracy rate, recall rate, and confusion matrix. The results demonstrate that the 1D CNN-ResNet proposed in this paper is capable of effectively estimating the PN code period of the non-cooperative DSSS signal, exhibiting robust generalization abilities.

According to the demand of spread spectrum signal processing in lower SNR conditions, a novel blind estimation algorithm of the pseudo-noise (PN) sequence for the direct sequence spread spectrum (DSSS) signal is proposed based on the similarities among the DSSS signals and the eigenanalysis technique. At first, through segmentation processing and average cross-correlation calculation, the starting symbol of PN sequence is determined. Next, by utilizing the eigenanalysis technique PN sequence can be estimated accurately without any information of the DSSS signal. Simulation results show that compared with the existing algorithms under the same condition, the proposed algorithm not only overcomes the partial-encode problem, but also improves estimation performance by 2dB. Therefore, the proposed algorithm achieves an accurate estimation of PN sequence in lower SNR conditions.

This paper considers sequence acquisition in direct-
\nsequence spread-spectrum (DS/SS) systems that use Gold
\nsequences as the multiple access pseudonoise (PN) sequence.
\nWhen long PN sequences are used, the design of an acquisition
\nscheme requires the characteristics of partial correlation of PN
\nsequences. For ease of analytic design, the partial correlation
\nof Gold sequences is approximated using a piecewise linearized
\nupper bound model. Since the acquisition of PN sequence in direct-
\nsequence code-division multiple access (DS-CDMA) systems
\ncan be formulated as testing two simple hypotheses, we apply fixed
\nsample size (FSS) and sequential tests to detection of phase alignment.
\nWhen a sliding correlator-type structure is employed for
\nthe acquisition scheme, the acquisition performance of Gold-sequence-
\nbased DS-CDMA systems is analyzed and compared to
\nthat of -sequence-based systems. Analytical results show that
\nthe use of Gold sequences requires a mean acquisition time quite
\ncomparable to the use of -sequences. Finally, analytical results
\nare verified by computer simulation.

For direct sequence (DS) spread spectrum communication systems with differential phase-shift keying (DPSK) modulation, we have calculated low autocorrelation sidelobe pseudo noise (PN) sequences. We constructed a matched filter comprising localized layered structure on a quartz crystal to obtain low matching loss. As a result of the sequences and the construction, 14 dB (6 dB improvement on that of the conventional sequences) peak vs sidelobe of correlated output signal and 9 dB lower loss than that of a conventional filter was achieved. We have applied these matched filters to SS rf-modem units for weak-power digital communications. In these modem units high data rate (1 Mbps), long communication distance (about 20 m) and low current (210 mA when received) performances are obtained.

This paper introduces analysis of synchronization probabilities for direct sequence (DS) spread spectrum (SS) system when the receiver is equipped with a smart antenna system and the matched filter (MF). The analyzed method first estimates direction-of-arrival (DOA) of the DS-signal, then performs beamforming, and finally makes synchronization decision based on the maximum value of the output of the MF. The analysis takes into account the fact that the DOA estimates are not perfect when finite number of samples is available. Therefore, beamforming is not necessarily perfect, which results power losses. The obtained results are compared to simulated and semianalytical ones. The sensitivity for DOA estimation error increases when the number of antennas in the array increases. It is also shown that at low signal-to-noise ratios the simulated accuracy of the DOA estimator is far away from the theoretical results. This behavior reflects also on the synchronization probabilities.

The authors have analyzed and compared performances of PN (pseudo noise) codes to find optimal PN codes which can be used in a DS/SS (direct sequence/spread spectrum) wireless LAN (local area network) system over a multipath fading channel. A wireless LAN system is susceptible to multipath fading that gives rise to intersymbol interference. The authors have analyzed and compared performances of Barker sequences with those of Gold sequences. As results, Barker sequences are better than Gold sequences for DS/SS wireless LAN systems, from the SNR and BER points of view.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small.

In this paper, we theoretically analyse the influence by intersymbol interference (ISI) on the bit error rate (BER) performance of the spread spectrum (SS) system using a real-valued Huffman sequence whose aperiodic correlation function has zero sidelobes except at the left and right shift-ends, under the quasi-static Rayleigh fading environment. The system can give the unified communication and ranging system because the output of a matched filter (MF) is the ideal impulse by overlap of 1 chip. As a result, the BER performance of the system under the quasi-static Rayleigh fading environment is improved with decrease in the absolute value of the shift-end value, is not influenced by ISI if the shift-end value is almost zero-value.

This paper investigates the application of spread spectrum code-division multiple access (SS-CDMA) techniques to subband image transmission over a bandlimited radio channel. For the transmission of images over SS-CDMA AWGN channels, a subband coding scheme that divides the image information into a number of independent data streams using an analysis filter bank, each of which is multiplied by its unique signature PN code, enables the transmission of these data streams via multiple parallel virtual channels created by their corresponding PN codes. With a sufficiently large number of streams, the total signal is able to fit within the narrow radio channel bandwidth even though the total bandwidth of all the signals may exceed the channel bandwidth. At the receiver, each received signal is separately recovered at the decoder by multiplying its PN code and integrating over the code length in order to obtain the desired subband. All the recovered subbands are then reassembled by a synthesis filter bank into a close reproduction to the original image. Additionally, for color subband image transmission, color images are first transferred to luminance (Y) and two chrominance components (I, Q). Each component is then decomposed independently into several subbands for SS-CDMA transmission. Therefore, a number of additional PN codes are required to support the transmission of the chrominance signals over the CDMA channels whereas the luminance signal was treated in the same manner as monochrome pictures. Moreover, SS-CDMA allows more than one image to be transmitted and be accessed simultaneously at the same limited channel bandwidth. If all the product signals for different images are transmitted over the SS-CDMA AWGN channel with perfect power control, the performance is the same as if the data were transmitted in a single stream. However, the quality of the farther received images would be degraded when the near-far problem occurs. The image quality of both the monochrome and color subband image transmission via SS-CDMA AWGN channels is evaluated for both perfect and imperfect power control.