Code Acquisition Time Research Articles

An Adaptive Threshold Technique for Fast PN Code Acquisition in DS-SS Systems

This paper presents an efficient technique to considerably reduce pseudonoise (PN) code acquisition time by adjusting threshold values, depending on the pre-estimated correlation energy in direct-sequence spread spectrum (DS-SS) systems. The proposed technique utilizing a threshold generation mode (TM) prior to a search mode is employed to a conventional double-dwell serial-search algorithm, and the TM determines the threshold values of both search and verification modes, depending on its correlation energy. We present a mathematical model of the proposed scheme to analyze it with respect to the mean code acquisition time in synchronous code-division multiple-access (CDMA) systems such as Third-Generation Partnership Project 2 (3GPP2) under a Rayleigh-fading channel. Numerical results show that the proposed method can reduce the mean code acquisition time by more than 60% and compensate for the performance degradation in high signal-to-noise ratios (SNRs) with no increase in hardware costs, compared with the conventional algorithm that adopts a soft-decision technique.

Design, demonstration and analysis of a modified wavelength-correlating receiver for Incoherent OCDMA system

A novel wavelength-correlating receiver for incoherent Optical Code Division Multiple Access (OCDMA) system is proposed and demonstrated in this paper. Enabled by the wavelength conversion based scheme, the proposed receiver can support various code types including one-dimensional optical codes and time-spreading/wavelength-hopping two dimensional codes. Also, a synchronous detection scheme with time-to- wavelength based code acquisition is proposed, by which code acquisition time can be substantially reduced. Moreover, a novel data-validation methodology based on all-optical pulse-width monitoring is introduced for the wavelength-correlating receiver. Experimental demonstration of the new proposed receiver is presented and low bit error rate data-receiving is achieved without optical hard limiting and electronic power thresholding. For the first time, a detailed theoretical performance analysis specialized for the wavelength-correlating receiver is presented. Numerical results show that the overall performance of the proposed receiver prevails over conventional OCDMA receivers.

Modified pseudo-noise code regeneration method

A modified pseudo-noise (PN) code regeneration method is proposed to improve the clock tracking accuracy without impairing the code acquisition time performance. Thus, the method can meet the requirement of high accuracy ranging measurements in short time periods demanded by radio-science missions. The tracking error variance is derived by linear analysis. For some existing PN codes, which can be acquired rapidly, the tracking error variance performance of the proposed method is about 2.6 dB better than that of the JPL scheme (originally proposed by Jet Propulsion Laboratory), and about 1.5 dB better than that of the traditional double loop scheme.

Low-Complexity Code Acquisition Method in DS/CDMA Communication Systems: Application of the Maximum Likelihood Method to Propagation Delay Estimation

Code acquisition performance in the Direct-Sequence Code-Division Multiple-Access (DS/CDMA) communication system is strongly related to the quality of the communication systems. The performance is assessed by (i) code acquisition time; (ii) precision; and (iii) complexity for implementation. This paper applies the method of maximum likelihood (ML) to estimation of propagation delay in DS/CDMA communications, and proposes a low-complexity method for code acquisition. First, a DS/CDMA system model and properties of outputs with a passive matched-filter receiver are reviewed, and a statistical problem in code acquisition is mentioned. Second, an error-controllable code acquisition method based on the maximum likelihood is discussed. Third, a low-complexity ML code acquisition method is proposed. It is shown that the code acquisition time with the low-complexity method is about 1.5 times longer than that with the original ML method, e. g. 13 data periods under 4.96dB.

PN code acquisition using smart antenna for spread-spectrum wireless communications-Part I

The smart antenna (a blind adaptive antenna array) has brought much attention to its ability to improve the future code-division multiple-access (CDMA) wireless communications systems. However, Adachi et al. (1998), employed only one out of M elements that are in a smart antenna array during the pseudonoise (PN) code acquisition process due to the difficulty in obtaining the weight coefficients of the smart antenna and PN code acquisition jointly and adaptively. Yet, as the PN code is acquired, all M elements are used by Adachi et al. to improve the signal-to-noise ratio (SNR) at a reverse link. We propose an efficient PN code acquisition scheme where all elements are employed during PN code acquisition. Also, we show that by using all elements, the PN code acquisition time can be shortened by at least 40% for a given signal-to-noise ratio and M=5, compared to the single element case. Furthermore, the power of a random access user signal in the reverse link can be lowered at least 3 dB. The additional computational load and complexity to use the proposed PN code acquisition are insignificant.

PN code acquisition for DS/CDMA systems employing smart antennas. II

For pt. I see IEEE Trans. Veh. Technol. A smart antenna, i.e., a blind adaptive antenna array, has attracted much attention to improve the capacity of a future code-division multiple-access wireless communications system. It has been demonstrated that there is significant improvement in data demodulation through lab simulations and field experiments by employing a smart antenna of multiple elements. However, only one element is used for the pseudonoise (PN) code acquisition process, which is a coarse PN code synchronization prior to data demodulation. This paper proposes a simple and practical PN code acquisition scheme, which employs all elements in the smart antenna. Also, this paper uses an adaptive threshold for the PN code acquisition. Simulation results demonstrate that the proposed scheme can significantly improve the PN code acquisition performance, e.g., the PN code acquisition time will be half as long by employing five elements rather than the single element at a given bit-energy-to-interference power spectral density ratio.

Performance analysis of DS-SS PN code acquisition systems using soft-decision techniques in a rayleigh-fading channel

A soft-decision method employing multiple threshold values for PN code acquisition in direct sequence spread spectrum (DS-SS) systems is described. We apply this technique to conventional search algorithms, double dwell serial search and hybrid search, and analyze them in terms of mean code acquisition time. For the analysis, we present mathematical models of proposed algorithms under a Rayleigh-fading channel. Numerical results show that our proposed schemes outperform conventional ones because multiple threshold values mitigate the possible decline in search performance caused by the use of a single threshold in conventional algorithms.

DS-CDMA용 개선된 PN 코드 포착 시스템의 ASIC 설계

The existing method in PN code acquisition process have a problem in PN code acquisition time because PN code searching is accomplished in one epoch. In this paper, we propose algorithm that can reduce PN code acquisition time because PN code searching is accomplished in each other two epoches. The designed ASIC chip using proposed algorithm confirmed that the area (the number of gates) increase more than existing method in PN code acquisition, but the performance of PN code acquisition is better than existing method.

Effects of multiple threshold values in double dwell DS-SS code acquisition systems

We describe a decision method using multiple threshold values for direct-sequence spread-spectrum (DS-SS) code acquisition systems. We apply this technique to a conventional double dwell serial search algorithm and analyze it in terms of mean code acquisition time. Numerical results show that our proposed scheme outperforms the conventional one by 0.2-0.5 sec with respect to the mean code acquisition time because multiple threshold values mitigate the possible decline in search performance caused by the use of a single threshold.

Cell search in W-CDMA

In a CDMA cellular system, the process of the mobile station searching for a cell and achieving code and time synchronization to its downlink scrambling code is referred to as cell search. Cell search is performed in three scenarios: initial cell search when a mobile station is switched on, idle mode search when inactive, and active mode search during a call. The latter two are also called target cell search. This paper presents algorithms and results for both initial and target cell search scenarios for the wideband CDMA (W-CDMA) standard. In W-CDMA, the cell search itself is divided into five acquisition stages: slot synchronization, frame synchronization and scrambling code group identification, scrambling code identification, frequency acquisition, and cell identification. Initial cell search needs all five stages, while target cell search in general does not need the last two stages. A pipelined process of the first three stages that minimizes the average code and time acquisition time, while keeping the complexity at a reasonable level, is considered. The frequency error in initial cell search, which may be as large as 20 kHz, is taken care of by partial symbol despreading and noncoherent combining. Optimization of key system parameters such as the loading factors for primary synchronization channel, synchronization channel, and common pilot channel for achieving the smallest average code and time acquisition time is studied. After code and time synchronization (the first three stages), a maximum likelihood (ML)-based frequency acquisition method is used to bring down the frequency error to about 200 Hz. The gain of this method is more than 10 dB compared to an alternative scheme that obtains a frequency error estimate using differential detection.

Modeling of code acquisition process in CDMA networks-asynchronous systems

In this paper, we discuss modeling of the serial code acquisition process in a code division multiple access (CDMA) network. Due to multiple access interference (MAI), the process is characterized by a different probability of signal detection P/sub D/ and probability of a false alarm P/sub FA/ in each cell of the code delay uncertainty region. We derive exact expressions for average code acquisition time and its variance. In addition to this, we also present several useful approximations, which enable easy engineering use of these results for practical applications in future Universal Mobile Telecommunications System (UMTS) solutions. Numerical results based on this analysis are used for decision threshold optimization in code acquisition process for asynchronous CDMA networks.

Spread Spectrum PN Code Acquisition Using Hybrid Correlator Architectures

A hybrid correlator architecture is described which combines the serial structure of an active correlator with the parallel structure of a matched filter correlator. The mean PN code acquisition time performance of this hybrid serial-parallel correlator structure is analysed. Results are shown which compare the acquisition performance of the serial, parallel, and serial-parallel structures. The results are for a PN code length of 64 code chips and assumes a Gaussian channel with the receiver detection threshold set to obtain a constant false alarm rate. An enhancement to the serial-parallel acquisition algorithm is also described which can increase the acquisition time performance by about 15% for typical operating conditions. Overall the results demonstrate that the hybrid correlator can provide rapid code acquisition with a limited receiver complexity.

Analysis of Direct P(Y)-Code Acquisition

Global Positioning System (GPS) signal acquisition consists of a two-dimensional search in a time/frequency space to synchronize a receiver-generated reference signal with the received GPS signal in code phase and fre- quency. User uncertainties in code phase and frequency offsets determine the size of the region to be searched, and hence, the code acquisition time. Encrypted P(Y) code, transmitted on L 1 and L2 fiequen- ties, provides more precise (16 m - spherical error proba- ble (SEP)) user positioning inherently more jamming immunity than does C/A code. The purpose of this study is to explore and use existing signal detection concepts to quantify the time to achieve direct acquisition of the P(Y) code in a jamming environ- ment. Of particular interest is the mitigating effect paral- lel processing has in shrinking acquisition time in the presence of initial (receiver) position, time, velocity, and frequency errors and/or uncertainties.

On the distribution function of the spread-spectrum code acquisition time

By using the direct approach to the analysis of the serial-search spread-spectrum code acquisition, the author presents the approximate analysis of the cumulative probability density function (CDF) of the acquisition time. This approach is based on the binomial approximation to the distribution of the partial acquisition times, instead of the Gaussian one used previously. The binomial approximation gives very accurate results but, more importantly, it shows that the analysis of the arbitrary detection/verification scheme can be reduced to the much simpler analysis of the equivalent single-dwell system, whose parameters depend only on the mean test times and not on their distribution. Based on the exact results, simple closed-form approximations are derived for the CDF in the case of the straight-line serial search with uniform a priori distribution, which might be useful in practice.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of strategies for serial-search spread-spectrum code acquisition-direct approach

A 'direct' approach to obtain statistics of the code acquisition time for serial-search spread-spectrum receivers is presented, which combines the idea of algebraic characterization of the search with transform domain methods. This is a very general unified technique, yielding many other known results as special cases. One of the major advantages of the direct approach is that the effect of the search strategy on moments of the acquisition times can be isolated from the effect of the detection/verification logic. This fact will permit one both to analyze all known search strategies-confirming several conclusions regarding their comparative performance-and to propose two novel strategies (alternate search) which outperform the conventional ones when the code rewinding time is small in comparison to the dwell times. Optimum search strategy based on the maximum a posteriori criterion (MAP) is analyzed, thus ensuring an absolute and uniform basis of comparison. The two new strategies perform well by this measure. >