Code Acquisition Research Articles

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part II: Application to Cellular Networks

This paper applies the theoretical framework for the evaluation of code acquisition in the presence of fading developed in the companion Part I paper. Code synchronization for code-division multiple-access (CDMA) cellular networks, such as IS-95 and cdma2000, is considered. A multidwell testing procedure is adopted. Notably, a procedure for the optimization of the multidwell parameters (number of dwells, dwell lengths, thresholds) to achieve minimum acquisition time is introduced and enforced. The effects of fading, interchip interference (ICI), frequency offset, multiple-access interference, and noise are taken into account in the determination of false alarm and detection probabilities. In particular, anomalous ICI despreading and internal interference-cancellation effects are described. Numerical and simulation results, in terms of time for correct acquisition, confirm the accuracy of the proposed approach and show how it can be effectively used in the design and evaluation of a code-acquisition subsystem.

A Deterministic Multiuser Code-Timing Estimator for Long-Code Band-Limited CDMA Systems

In this letter, we present a deterministic multiuser code-timing estimator for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems with aperiodic long spreading codes and band-limited chip waveforms. A key feature of the proposed estimator is that it captures and capitalizes a deterministic structure of the overall interference, namely multi-access interference (MAI) and intersymbol interference (ISI), in the frequency domain. This allows complete interference elimination in a deterministic manner, which is in general more effective and data-efficient than stochastic approaches. Numerical results show that the proposed estimator can achieve fast acquisition; it is also near-far resistant, providing accurate code acquisition for even overloaded systems (i.e., systems with more users than the processing gain) in multipath fading environments.

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part I: Theoretical Aspects

This paper deals with the analysis of the code-acquisition process performed by a mobile terminal in a code-division multiple-access cellular system. The fundamental contribution is the setting of a theoretical framework for code-acquisition analysis, accounting for the presence of fading and shadowing with specified coherence time. A multidwell architecture is considered, accounting for different verification strategies. The MAX/TC detection criterion is used, and the acquisition performance is analyzed, following both the direct and flow-graph approaches. Expressions for the acquisition time probability density function under different assumptions are given, as well as closed-form expressions for the mean and variance of the acquisition time. Practical applications and numerical results are reported in the Part II companion paper.

A Simple Expression for the Optimization of Spread-Spectrum Code Acquisition Detectors Operating in the Presence of Carrier-Frequency Offset

In this letter, we present a simple expression for the optimization of the threshold detection performance for direct-sequence spread-spectrum code acquisition in the presence of carrier-frequency offset. The proposed scheme divides the total integration time into subintervals, and the results of the coherent integrations performed over these subintervals are noncoherently combined prior to detection. The proposed expression allows obtaining the optimum number of coherent-integration subintervals for a given total integration time.

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part II: Application to Cellular Networks

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part II: Application to Cellular Networks

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part I: Theoretical Aspects

DS-CDMA Code Acquisition in the Presence of Correlated Fading—Part I: Theoretical Aspects

Code Acquisition for DS/SS Communications in Non-Gaussian Impulsive Channels

In this letter, we propose a new detector for code acquisition systems in non-Gaussian noise channels. Modeling the acquisition problem as a hypothesis testing problem, a detector is derived for non-Gaussian, symmetric /spl alpha/-stable noise, based on the locally optimum detection technique. Numerical results show that the proposed detector can offer robustness and substantial performance improvement over the conventional schemes in non-Gaussian channels.

Two-Dimensional Code Acquisition in Environments With a Spatially Nonuniform Distribution of Interference: Algorithms and Performance

In this paper, we investigate code acquisition in the delay and angular domain in environments where interference and noise are nonuniformly distributed in the spatial (angular) domain. It is seen that performance is clearly degraded by the presence of nonuniform spatial distributions of interference. The degree of impairment is related to the actual shape of the distribution and becomes more significant when large amounts of interfering power are concentrated in one or more angular directions. As compared to a reference case with a uniform distribution of equivalent interfering power, the mean acquisition time could increase by factors of up to four. Some effective approaches to compensate for the loss of performance due to the uneven nature of interference are also studied. First, the effect of search strategies in the delay and angular domains on acquisition performance is assessed. A searching algorithm up-ranking angular cells according to the amount of associated interference is proposed as one solution to reduce the performance gap. It is shown that applying this search strategy can considerably reduce the mean acquisition time. Finally, two adaptive schemes based on setting integration times or threshold values according to the level of interference prevailing in each angular cell, are discussed. The former approach returns performance figures similar to those obtained with the uniform equivalent interference distribution while the latter provide moderate performance gains.

Adaptive acquisition of PN code using excision CFAR detector in multipath fading mobile channels

An adaptive acquisition of PN sequences is presented for DS/SS communications. The E-CFAR detector is introduced to excise strong signals before the cell-averaging operation. Performance of the proposed detector suffers almost no degradation under the benign environment, and the degradation is quite small under multipath fading, in comparison with a conventional cell-averaging method.

Joint acquisition of PN code and adaptive beamforming weight in spread spectrum systems

A novel approach called spatial-temporal correlator (STC) is proposed for joint acquisition of pseudonoise (PN) code and adaptive beamforming weight in direct-sequence spread-spectrum systems with antenna array. Detection performance of the proposed STC is theoretically analysed and simulated under Gaussian jamming. These results show that the proposed scheme has improved acquisition performance and constant false alarm rate (CFAR) is achieved.

A Generalized Signal Flow Graph Approach for Hybrid Acquisition of Ultra-Wideband Signals

This paper establishes a general framework for the study of various search procedures in which a group of observers is searching for a group of items in some arbitrary manner. A generalized signal flow graph approach yields a complete statistical description of the search time as well as the probability of correctly terminating the search. The motivation for this study comes from the acquisition of an ultra-wideband (UWB) signal in a dense multipath channel. In this particular problem many resolvable paths tend to cluster at a receiver containing multiple correlators allowing for a hybrid serial/parallel search. This hybrid search, as well as a bound on a sorted hybrid search, is analyzed using the generalized signal flow graph approach. Rapid code and frame acquisition at the receiver is desired and it is shown that a specific nonconsecutive search, the bit reversal search, yields an optimum hybrid search procedure under certain conditions. The acquisition of UWB signals also leads to the concept of a self-similar signal flow graph, in which a new search is initiated upon the completion of the previous one.

A "rayleigh-ness" test for DS/SS code acquisition

Testing for statistical distribution has received increasing attention in signal processing for communications. Noncoherent receivers, based on correlation magnitude, are employed when the actual carrier phase of the received samples is unknown. This paper presents a new test, based on high-order statistics, to decide whether a real positive white series is a realization of a Rayleigh-distributed random process. Such a "Rayleigh-ness" test is based on a testing variable that measures the "Ricianity" of the series under investigation. That is, it estimates the possible presence of the mean of the complex Gaussian model generating both Rayleigh and Rice distributions. The asymptotic testing statistics have been derived as explicit functions of the higher order moments of the noise-plus-interference distribution. The performance of the Rayleigh-ness test has been analyzed in comparison with a conventional power detector. The devised test has application to noncoherent initial synchronization of the chip offset (code acquisition) in a symbol-length spreading sequence of direct-sequence code-division multiple-access systems. In such a case, the noise-plus-interference variance under the out-of-sync condition is much larger than the effective variance in the in-sync case. The obtained results evidence the robustness of the Rayleighness test, avoiding the severe performance degradation in the presence of large Gaussian and non-Gaussian noise, as well as multiuser interference in a downlink scenario.

Constrained optimization of noncoherent serial acquisition of spread-spectrum code by exploiting the generalized Q-functions

Initial code acquisition of direct-sequence spread-spectrum signals is typically based on serial multidwell hypothesis tests to limit the costs of mobile terminals. The procedure that searches for the correct code and its actual time offset usually adopts a user-oriented quality criterion based on constant error rates. The false-alarm and miss detection probabilities can be theoretically evaluated by means of the class of generalized Q (GQ) functions. We show that the GQ functions constitute a useful tool not only for analytic performance analysis, but also for the optimized design of initial code synchronization systems. Some examples of application of two-dwell (search/verification) procedures are provided. The mathematical problem consists of the minimization of an objective function (i.e., the mean acquisition time) depending on four parameters (two testing durations and two thresholds) with two constraints (the probabilities of miss detection and false alarm). In particular, we have implemented and analyzed the convergence of the steepest descent and the Newton-Raphson numerical algorithms. The computational cost of the method and the effect of multipath Rayleigh channels are also discussed. The optimized acquisition procedure has evidenced a significant reduction of the mean duration of serial tests in comparison with (suboptimum) previous attempts.

A combined code acquisition and symbol timing recovery method for tds-ofdm

A novel timing recovery method which combines code acquisition and symbol timing recovery for TDS-OFDM (time domain synchronous OFDM) is developed. The method is based on the searching and tracking of the correlation peaks of the PN sequences embedded in the signals. Simulations show that this algorithm has very good performance for both AWGN and multipath channels, even when a small frequency offset exists in the sampling clock of the receiver.

Code acquisition in transmit diversity ds-cdma systems

In this paper, two code acquisition schemes are studied for use in conjunction with transmit diversity direct-sequence code-division multiple access (DS-CDMA). One is a training-based single-user maximum-likelihood (SUML) estimator, which can achieve code acquisition very fast at low computational complexity. The other is a blind estimator based on the multiple signal classification (MUSIC) algorithm. Two recently proposed transmit diversity schemes known as orthogonal transmit diversity simulcast (OTD-S) and space-time selective spreading transmit diversity (STSTD) are considered. While the advantages of transmit diversity from the detection standpoint are well known, less is known about how code acquisition performance is affected by the use of transmit diversity. Through the analysis in this paper, it is proven that the SUMI. estimator should give the same performance in both the OTD-S and STSTD schemes in a single-user environment. In a multiple-user environment, simulation results show that the STSTD system offers slightly better code acquisition performance. It is also seen that the SUML estimators provide significantly better code acquisition performance than the MUSIC estimators in either transmit diversity system. From the standpoint of robustness to carrier frequency offset, it Is found that the training-based SUML estimator is very sensitive to frequency offset, while the MUSIC estimator is quite robust. A simple frequency offset estimator to be used in conjunction with the SUML estimator is also proposed and is shown to make the timing estimator quite tolerant of substantial frequency offsets.

Code acquisition of ZCZ‐CDMA systems based on complete complementary codes

AbstractIn recent years, a new quasi‐synchronous CDMA (QS‐CDMA) system based on complete complementary codes with zero correlation zones (ZCZ), named ZCZ‐CDMA system, has been proposed. In a ZCZ‐CDMA system, data transmission efficiency can be greatly improved by accurately detecting the multi‐path properties of the communication channel utilizing the unique correlation features of the ZCZ sequence. As for the despreading operation and the code acquisition, the ZCZ‐CDMA system however is more complicated and difficult than the conventional DS‐CDMA systems. In this paper, we discuss the issues of the code acquisition in ZCZ‐CDMA systems, and propose an efficient code acquisition scheme, called two‐stage search (TSS) scheme. In the first stage of the TSS code acquisition scheme, a predictive code phase is coarsely searched, and then a fine search utilizing the unique features of the ZCZ‐sequences is carried out in the second stage. Simulation results show that the proposed scheme achieves better performance in terms of search time and hardware complexity than conventional code acquisition schemes. Copyright © 2003 John Wiley & Sons, Ltd.

Differentially coherent combining for double-dwell code acquisition in ds-cdma systems

The use of differentially coherent combining is proposed to improve the performance of a double-dwell acquisition system by increasing the reliability of a decision in the verification stage. The detection and mean acquisition time performance of the acquisition scheme with the proposed combining scheme is analyzed in frequency-selective Rayleigh fading channels, and compared with that of two previously published double-dwell acquisition schemes based on long correlation intervals and noncoherent combining. It is shown that the proposed acquisition scheme outperforms the previous ones, and that the performance improvement increases as the frequency offset increases.

Use of multiple antennas for DS/CDMA code acquisition

A generalized acquisition scheme is proposed for direct sequence code-division multiple-access systems with multiple antennas. The proposed scheme employs grouping of multiple antennas as a means of a tradeoff between two important factors determining the mean acquisition time, combining gain and search time. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh-fading channels with consideration of spatial correlations. Numerical results show that the use of the largest number of antenna groups is preferable to reducing the mean acquisition time at low signal-to-interference ratio (SIR) values. At high SIR values, on the contrary, the mean acquisition time is found to increase in proportion to the number of antenna groups. In a typical environment, the presence of spatial correlation is shown to increase or decrease the mean acquisition time within 50% compared with the uncorrelated fading case.

Acquisition for DS/CDMA systems with multiple antennas in frequency-selective fading channels

A generalized code acquisition scheme for direct-sequence code-division multiple-access systems with multiple antennas is proposed over frequency-selective fading channels. The proposed scheme is developed on the framework of a generalized configuration of multiple antennas and correlators. The nonconsecutive search method is generalized and extended to multiple antenna systems to exploit multipath signals in improving acquisition performance over frequency-selective fading channels. The proposed scheme also adopts a hybrid decision strategy to make effective decisions on acquisition. The mean acquisition time performance of the proposed acquisition scheme is analyzed and evaluated in frequency-selective Rayleigh-fading channels with general multipath delay profiles and spatial-fading correlations. The effects of nonconsecutive search on mean acquisition time are investigated for various channel environments, and the optimal choice of decision strategy is discussed. Furthermore, effects of various configurations of multiple antennas and correlators, decision thresholds, and correlation interval on the performance are also investigated.

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.