Code Acquisition Technique Research Articles

Ultra-Fast L2-CL Code Acquisition for a Dual Band GPS Receiver

GPS L2C signal is a recently added civil signal to L2 frequency and is constructed by time division multiplexing of civil moderate (L2-CM) and civil long (L2-CL) code signals. While the L2-CM code is 20 ms-periodic and modulates satellite navigation message, the L2-CL code is 1.5s-periodic with 767,250 chips long code sequence and carries no data. Therefore, the L2-CL code signal allows receivers to perform a very long coherent integration. However, due to the length of the L2-CL code, the acquisition of the L2-CL code signal may take too long or require too much hardware resources. In this paper, we propose a three-step ultra-fast L2-CL code acquisition (TSCLA) technique for dual band GPS receivers. In the proposed TSCLA technique, a dual band GPS receiver sequentially acquires the coarse/acquisition (C/A) code signal at L1 frequency, the L2CM code signal, and the L2-CL code signal to minimize mean acquisition time (MAT). The theoretical performance analysis and numerous Monte Carlo simulations show the significant advantage of the proposed TSCLA technique over conventional techniques introduced in the literature.

PN Code Acquisition Using Belief Propagation with Adaptive Parity Check Matrix

Pseudonoise (PN) code acquisition technique based on iterative message passing algorithm (iMPA) has been proposed due to its short acquisition time and low complexity. However, the cyclic and regular nature of constructed tanner graph makes it difficult to achieve promising performance. To address this problem, this correspondence proposes a new message passing algorithm based on adaptive parity check matrix. We find multiple sets of linear sparse constraints for PN sequence by squaring the generator polynomial. The topology of the graphic models as well as the parity check matrix is adapted every a few iterations to avoid local optima. The performance of proposed algorithm is evaluated in terms of detection probability. Simulation results show that this method provides more than 3 dB gains over iMPA with fixed parity check matrix.

Code acquisition of random optical codes in optical code-division multiple-access

In this study, the authors present a technique for code acquisition in optical code-division multiple-access systems using the recently introduced random optical codes. In these systems, the communication is divided in two parts: a synchronisation process and the data transmission. The main target of the study is to describe and analyse the new algorithm of synchronisation using Markov processes. Once described the code acquisition technique and established an estimation for the probability of false alarm (FA) of the proposed scheme, two different proposals for the optimisation of the code parameters are described. They are based on two different estimators for the probability of FA. The first one considers the expected value for the probability of FA. The second one is very much restrictive. It guarantees that the probability of FA is in a confidence interval. It takes the upper bound of the confidence interval as estimator for the probability of FA. Although the second one presents worse acquisition time, the authors consider that the gain provided by the first method is not sufficient enough to assume its loss in the reliability. Thus, the authors propose to guarantee the probability of FA in a confidence interval. A compromise between reliability and acquisition time is obtained with this second method.

Performance Analysis and Comparison of Full Chip and Half Chip Rate DC and NC Code Acquisition in MIMO DSCDMA over Uncorrelated Rayleigh Wireless Channel

This paper presents the performance analysis and comparison of full chip and half chip rate of noncoherent (NC) and differentially coherent (DC) code acquisition scheme in (multiple input-multiple output) MIMO assisted by direct sequence spread spectrum (DS-CDMA) wireless system when communicated over uncorrelated Rayleigh channel. Four schemes are investigated, namely, SISO with full chip rate, SISO with Half chip rate, MIMO with full chip rate, and MIMO with half chip rate by varying the code acquisition technique. The simulation is done in RF signal processing Lab using matlab tool box, and the performance metrics are considered, namely, Bit Error Rate (BER) and mean acquisition time (MAT). The simulation results indicate that DC performance is superior than NC in both full chip and half chip rate and also shown that half chip performance is better than full chip in both DC and NC code acquisition methods.

Direct P-Code Acquisition Technique Based on the Data Resource Pool

In GPS system, the P-code can offer higher accuracy and better anti-jamming property than C/A code. However, the direct P-code acquisition is difficult as its long period and high chip rate. To solve the problem, the idea of expanding the code sequence to a three-dimensional space is put forward. The concept of data resource pool is defined, and the fast direct P code acquisition technique based on the data resource pool is proposed to improve the acquisition efficiency. By effect compression, the acquisition technique can enlarge the uncertainty search coverage and accelerate the search process. Furthermore, the technique based on the data resource pool can get different sub-algorithms according to the different compression method. The simulation results show that the proposed technique can achieve fast direct P code acquisition by the reasonable choice of compression parameters and the compression mode of code sequence.

A self-synchronizing method for asynchronous code acquisition in band-limited spread spectrum communications

This paper addresses the problem of initial synchronization of pseudo-noise code. A new code acquisition technique for spread-spectrum communication systems using band-limited chip waveforms is presented. Unlike conventional power detector based on testing the estimated maximum of the ambiguity function, the devised detector exploits a fast parabolic interpolation, running on three estimated ambiguity samples in the neighborhood of the coarse estimate. Performance analysis is carried out in comparison with conventional detector. Mathematical expressions for the probability of false alarm and detection are derived. They are numerically evaluated, under operating settings, by a reduced Tayloriquests expansion up to the second order. The theoretical results, substantiated by computer simulations, have evidenced that the devised method is well suited for asynchronous spread-spectrum communications. In particular, the acquisition performance depends on the actual offset between the received and the reference code waveforms, which are randomly distributed (in chip-asynchronous systems) within one sampling period. In fact, the parabolic interpolation technique outperforms the conventional detector for a wide range of code offsets because it is able to self-synchronize the testing variable around the true offset.

Code Detection and Acquisition Techniques in the DS/CDMA Communications

In the direct sequence- code division multiple access (DS/CDMA) communications, one of the primary functions of the receiver is to despread the received PN code. This is accomplished by generating a local replica of the PN code in the receiver and then synchronizing it to the one superimposed on the received waveform. In general, this synchronization process is accomplished in two steps are; code acquisition, which is a coarse alignment process bringing the two PN sequences within one chip interval, and code tracking, which is a fine tuning and synchronization-maintaining process. However, in many situations, such as communications intelligence, the PN code utilized to spread the received signal is unknown to the intercepting receiver. In this paper, we focus on the fundamental concepts of both code estimation and code acquisition techniques. An algorithm based on the higher order statistics (HOS) for code estimation is provided. This HOS algorithm is complex and requires high signal to noise ratio, SNR for operating correctly. For that reason a much simpler algorithm based on the structure of the PN code itself and short observation of the received signal is proposed. The proposed algorithm, based on short observation, performs faster and better than the HOS algorithm, especially in dense environments. Finally we compare the serial search, the parallel search and the Z- search methods, in terms of the average delay time to acquire code acquisition for different spreading code lengths.

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.

On the Performance of Code Acquisition in MIMO CDMA Systems

This letter investigates the effects of using multiple transmit antennas on code acquisition for preamble search in the CDMA uplink when MIMO is used for signal transmission and reception. The performance of a ML code acquisition technique in the presence of MIMO channel is analyzed by considering the detection and miss probabilities. The acquisition performance is numerically evaluated on a frequency selective fading channel. It is found that the performance of code acquisition scheme for a SIMO system is better than that for the case of MIMO on the low thresholds in terms of detection performance and MAT.

Return-Link Code Acquisition for 1-D and 2-D With DS-CDMA for High-Capacity Multiuser Systems

Acquisition of the code timing in a direct-sequence code-division multiple-access (DS-CDMA) system at the base station must take place before signal detection and decoding is possible. Code acquisition under severe multiple-access-interference (MAI) conditions with time-varying codes makes the task even more difficult. Inefficient designs lead to a large number of false alarms and/or missed detections. This requirement is needed for conventional single-antenna (1-D) designs, as well as for multielement antenna (2-D) designs. This paper details a powerful code-acquisition technique for the uplink of DS-CDMA systems under high-loaded situations for both 1-D and 2-D schemes, where the number of users is greater than the processing gain. Under this high-MAI condition, the DS-CDMA acquisition problem becomes very difficult, and conventional search methods simply fail. The method discussed utilizes soft data from a multiuser detector to reduce the interference received by the acquisition unit. Analytical performance is compared to simulation results in terms of the number of users, processing gain, interferer-signal power, cancellation factor, antenna configuration, and noise variance. Numerical results validate performance under realistic conditions with amplitude, phase, and frequency impairments.

Effect of spatial fading correlation on CDMA code-acquisition performance

The effect of correlations among the fading signals at the multiple antennas on the code-acquisition performance for preamble search in a code-division multiple-access (CDMA) uplink is investigated. Previous results for frequency-selective fading assumed independent fading at each antenna. A general case of spatial fading correlation as a function of antenna spacing and azimuth spread is presented for the performance analysis of code-acquisition systems. The correlated antennas are exploited to derive the probability density function of a decision variable. The performance of a maximum-likelihood (ML) code-acquisition technique based on spatially correlated antennas is analysed by considering the detection, miss, and false-alarm probabilities. The result of performance analysis of a code-acquisition scheme utilising uncorrelated diversity antennas is also described. The detection performance and mean acquisition time given by partial correlation in spatial fading are presented and compared with those offered by antenna decorrelation via numerical evaluation. It is shown that without exploiting time diversity, the code-acquisition approach with multiple antennas provides much better detection performance than the corresponding scheme with a single antenna in a frequency-selective fading channel and that the performance enhancement depends on the degree of spatial fading correlation.

Acquisition Performance of CDMA Systems With Multiple Antennas

This paper discusses the code-acquisition problem for preamble search in the code-division multiple-access (CDMA) uplink when multiple antennas are employed for signal reception. The multiple antennas are used either in the form of antenna array for beamforming or in the form of antenna diversity. The performance of a maximum-likelihood (ML) code-acquisition technique utilizing beamforming is analyzed by considering the detection, miss, and false-alarm probabilities. This analysis is based on the statistics of the CDMA noncoherent demodulator and beamformer output. In the case of a code-acquisition scheme utilizing antenna diversity, the probability density function of a decision variable is analytically derived for multiple antennas. The expressions for the probabilities of detection, miss, and false alarm are also presented. The acquisition performance and mean acquisition time for both code-acquisition techniques are numerically evaluated and compared. The code-acquisition approaches with multiple antennas are shown to provide much better performance than the corresponding scheme with a single antenna in a frequency-selective fading channel.

Differentially coherent PN code acquisition based on a matched filter for chip-asynchronous DS/SS communications

A pseudonoise code acquisition technique based on a matched filter with differentially coherent detection is proposed for direct-sequence spread-spectrum communications in chip-asynchronous environments. Such a technique can be confidently exploited as an alternative to noncoherent detectors, which have been extensively investigated, because it can provide very attractive improvements over its noncoherent counterpart as well as overcome the problems caused by chip asynchronization without increasing hardware complexity. The results of statistical analyses show that the proposed technique certainly can achieve much lower error probabilities.

Code acquisition in coherent optical pulse CDMA systems utilizing coherent correlation demodulation

A code acquisition technique based on active optical correlation detection is introduced for coherent optical pulse code-division multiple-access (CDMA) systems implemented through coherent correlation demodulation. Passive matched filter acquisition, commonplace in most radio CDMA systems, does not translate directly into the optical domain since synchronization between the incoming optical signal and a local optical code of the intended user at the receiver has to take place in several picosecond duration. Parallel and serial acquisition schemes under the same search algorithm are described and the effect of optical device noise, partial autocorrelation, and multiuser interference on acquisition performance is theoretically analyzed. Numerical results are given for Gold codes. In general, the degree of parallelism inherent in code acquisition techniques for radio systems allows a tradeoff between acquisition speed and hardware cost; however, in optical acquisition schemes, increasing the degree of parallelism with the concomitant hardware cost penalty does not necessarily reduce the time to acquisition. This is due to optical power budget limitations brought about by the need to split the optical signal, although optical amplifier technology can improve the performance of these parallel search schemes. The expressions for the partial correlation and multiuser interference derived in the paper can also be utilized for the analysis of other radio and optical acquisition schemes.

A parallel code acquisition technique for M-ary orthogonal signals in a cellular CDMA system

Addressed in this paper is a parallel code acquisition technique for M-ary orthogonal signals in a cellular code division multiple access (CDMA) system where pilot signal or preamble is not available for code acquisition. The proposed code acquisition system consists of parallel M-ary orthogonal demodulators each having M correlators. The system also employs two operating modes, i.e., search and verification modes with the decision variables formulated with successive maximums of M correlator outputs. The mean acquisition time is first derived for a multiple H/sub 1/ cell case and then the probabilities of detection, miss, and false alarm are obtained for frequency-selective Rayleigh fading environments. Also provided is a computationally efficient solution for detection performance analysis by using quantized decision variables. The performance of the proposed code acquisition technique is numerically evaluated when it is applied to the initial acquisition of the mobile station requesting hand over.

Acquisition performance of an adaptive receiver for DS-CDMA

This paper presents a single-user code timing estimation algorithm for direct-sequence code-division multiple access that is based on processing the weight vector of an adaptive filter. The filter weight vector can be shown to adapt in the mean to a scaled time-shifted version of the spreading code of the desired user. Therefore, our algorithm requires very little side information in order to form its estimate. The acquisition performance of the algorithm is investigated when the filter is adapted using the least mean square (LMS) or the recursive least square (RLS) algorithm. The proposed algorithm is shown through experimental results to be resistant to the near-far problem when the RLS adaptation algorithm is used, but not when the LMS algorithm is used. However, the performance of this code-acquisition technique is still substantially better than the traditional correlator-based approach, even when the computationally simple LMS algorithm is used. As an extension to the basic timing estimator algorithm, we consider the effect of frequency synchronization error on the performance of the timing estimate. As expected, frequency-offset error degrades the performance of the timing estimate. However, a modified version of the adaptive filter is presented to combat this effect.

Code acquisition of DS/SS signals in fading channels using an LMS adaptive filter

The mean acquisition time performance for a direct-sequence spread-spectrum (DS/SS) code acquisition technique utilizing a finite impulse response (FIR) least-mean-square (LMS)-adaptive filter is investigated in fading conditions. The optimum tap-weight vector for the adaptive filter is derived and computer simulation results are presented to assess the acquisition performance for the system. It is shown that fading causes a slight degradation to the mean acquisition time (less than 3-dB signal-to-noise ratio (SNR) degradation) which is far less than that incurred by other systems utilizing matched filters (about 10 dB). In frequency selective fading channels, the adaptive nature of the proposed scheme is exploited to acquire the strongest received path and as the paths fade the new strongest available path is acquired.

On the performance of a maximum-likelihood code-acquisition technique for preamble search in a CDMA reverse link

The performance of a maximum likelihood (ML) code-acquisition technique is analyzed for slotted-mode preamble search in the code-division multiple access (CDMA) reverse link. The probabilities of detection, miss, and false alarm are derived analytically for multiple H/sub 1/ cell cases, based on the statistics of the CDMA noncoherent demodulator output in a frequency-selective Rayleigh fading channel. The probability density function (PDF) of a decision variable is also derived by considering the fading characteristics of the received signal for both single- and dual-antenna cases. The performance of the code-acquisition technique is evaluated numerically with emphasis on the effects of postdetection integration, fading rate, antenna diversity, and so on.

Parallel code acquisition technique for preamblesearch in the CDMA reverse link

A parallel PN code acquisition technique for slotted-mode preamble search in a CDMA reverse link is presented. For received PN codes with different signal-to-noise ratios (SNRs), this scheme provides a sub-optimal threshold value such that the mean acquisition time is minimised.

New fast GPS code-acquisition technique using FFT

A new spread-spectrum code-acquisition technique for the navigation systems Navstar/GPS and Glonass is introduced. This technique uses the FFT to compute the correlation function, thereby eliminating the time-consuming code phase shift process. Comparisons with existing systems show a theoretical reduction in acquisition time of about 2000 times.