Code Acquisition Research Articles

Design, implementation and verification through a real‐time test‐bed of a multi‐rate CDMA adaptive interference mitigation receiver for satellite communication

AbstractThis paper presents the design, the implementation, and the main performance results of a multi‐rate code division multiple access (CDMA) interference mitigation receiver for satellite communication. Such activity was performed within a research project supported by the European Space Agency (ESA), whose aim was to demonstrate the suitability of the linear adaptive interference mitigation detector (IMD) named extended complex‐valued blind anchored interference‐mitigating detector (EC‐BAID) for single‐user detection of a CDMA signal in third‐generation (3G) satellite networks. Such a detector, which exhibits a remarkable robustness to multiple access interference, operates in a blind mode, i.e. it only requires knowledge of the timing of the wanted user's signature code, and is therefore very well suited for integration into handheld user terminals.Experimental results in terms of bit error rate with respect to the theoretical behaviour were derived through a specifically developed test bed. Signal plus multiple access interference generation is performed via a computer‐controlled arbitrary waveform generator, followed by frequency up‐conversion to the standard intermediate frequency of 70 MHz. Additive white Gaussian noise is then injected with the aid of a precision noise generator. The core of the test bed is a flexible digital receiver prototype featuring the EC‐BAID detector plus all functions ancillary to IMD (multi‐rate front‐end, automatic gain control, code acquisition and tracking, carrier synchronization, etc.). Those functions were implemented through careful mixing of different technologies: field programmable gate arrays (FPGAs) for computing‐intensive signal processing functions, digital signal processor (DSP) for housekeeping and monitoring, and application specific integrated circuit (ASIC) for adaptive IMD. The adopted design flow also allows an easy re‐use of the prototype architecture to come to an overall integration of the receiver into a single ASIC with modest complexity and power consumption increase with respect to a conventional detector. Copyright © 2003 John Wiley & Sons, Ltd.

Performance of low-complexity code acquisition for direct-sequence spread spectrum systems

The authors consider a low-complexity technique for direct-sequence spread spectrum code acquisition. They present performance analysis in Rayleigh fading channels, and with carrier frequency offset in the receiver. They derive analytical expressions for acquisition probabilities, and use them to determine the mean and variance of the acquisition time. It is shown that acceptable performance can be achieved with significant reduction in hardware requirements, compared to conventional detectors. Simulations confirm the analytical results. Performance comparisons are presented under a range of scenarios including significant frequency offsets and fading channels.

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.

A Performance Comparison of Code Acquisition Techniques in DS-CDMA System

This paper compares parallel acquisition with hybrid acquisition in terms of mean acquisition time in code acquisition system of forward link and reverse link of direct sequence code division multiple access (DS-CDMA) system. The mean acquisition time of the hybrid and parallel acquisitions is derived from the distribution of the multiple correct code-phase offsets. Especially, the mean acquisition time of the hybrid acquisition scheme is derived by considering both case 1 (the correct code-phase offsets are included in one subset) and case 2 (the correct code-phase offsets exist at the boundary of two subsets), which are caused by the distribution of the correct code-phase offsets in the subset. Results are provided for the performance comparison between hybrid acquisition and parallel acquisition under the same hardware complexity in the forward and reverse links. In our performance evaluation of code acquisition in DS-CDMA system, the hybrid acquisition shows a better performance than the parallel acquisition in terms of mean acquisition time of the both links in the frequency-selective Rayleigh fading channel.

A New Phase Acquisition Method of Address Code Utilizing the Combination of Logistic Map and BP Network

A novel phase acquisition method of code division multiple access (CDMA) address code is proposed in this paper. Based on the combination of chaotic nonlinear maps and backpropagation neural networks, this new approach can be applied to realize the phase acquisition of a CDMA address code, which plays an important role in wireless mobile communications. Both the theoretical analysis and computer simulation results show that it can increase the detection probability of the received signals compared with the conventional series acquisition method, which can improve the performance of the CDMA receiving system significantly.

Acquisition performance improvement by barker sequence repetition in a preamble for ds-cdma systems with symbol-length spreading codes

We investigate the effect on code acquisition performance of using Barker sequence repetition in a preamble for direct-sequence code-division multiple-access systems with symbol-length spreading codes. To do so, we first introduce the properties of repeated Barker sequences; then, using those properties, we derive the false alarm and detection probabilities for the acquisition system with Barker sequence repetition. Some numerical examples show that the acquisition performance of a system using Barker sequence repetition is better than that of a system not using Barker sequence repetition. Also, the performance improvement by use of Barker sequence repetition is shown to be greater with a longer period of the Barker sequence.

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.

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.

Serial acquisition performance of single-carrier and multicarrier DS-CDMA over Nakagami-m fading channels

We investigate the issue of pseudo noise (PN) code acquisition in single-carrier and multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems, when the channel is modeled by frequency-selective Nakagami-m (1960) fading. The PN code acquisition performance of single-carrier and MC DS-CDMA systems is analyzed and compared when communicating over Nakagami-m fading channels under the hypothesis of multiple synchronous states (H/sub 1/ cells) in the uncertainty region of the PN code. In the context of MC DS-CDMA, the code acquisition performance is evaluated, when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined by using equal gain combining (EGC) or selection combining (SC) schemes. The performance comparison of the above mentioned schemes shows that the code acquisition performance of the MC DS-CDMA scheme, especially when using the EGC scheme, is more robust, than that of single-carrier DS-CDMA schemes communicating over the multipath Nakagami-m fading channels encountered. However, our code acquisition performance comparison also shows that if the detection threshold was set inappropriately, the performance might be degraded, even if the channel fading becomes less severe.

Low-power adaptive pseudo noise code acquisition for spread-spectrum systems

A novel pseudo noise code acquisition combined with the newly proposed adaptive sampling rate and threshold control (ASTC) algorithm is derived for low-power spread-spectrum systems with complementary metal-oxide-semiconductor implementations. Low-power performance can be achieved by reducing the sampling rate of the proposed system while maintaining the system performance. The sampling rate is dynamically updated due to the change of the channel noise level. Under the assumptions that the channel is additive white Gaussian noise slow-fading channel, up to 74.3% reduction in power consumption compared to the conventional fixed-sampling rate and fixed-threshold architecture is demonstrated with insignificant increase of system complexity. The proposed architecture can be applied to the design of low-power and controllable-performance spread-spectrum communication systems.

Nonparametric PN code acquisition using the signed-rank statistic for DS/CDMA systems in frequency-selective Rician fading channels

In this paper, signed-rank based nonparametric detectors are used for pseudonoise (PN) code acquisition in frequency-selective Rician fading channels. We first derive the locally optimum rank detector and then propose the locally suboptimum rank (LSR) and kth order modified signed-rank (MSR/sub k/) detectors using approximate score functions. We compare the hybrid parallel double-dwell scheme using the LSR and MSR/sub k/ detectors with that using the conventional squared-sum (SS) and modified sign (MS) detectors. From the simulation results, it is shown that the LSR and MSR/sub k/ detectors perform better than the SS detector using the cell averaging constant false-alarm rate processor and the MS detector by about 2-3 dB.

Adaptive PN code acquisition using instantaneous power-scaled detection threshold under Rayleigh fading and pulsed Gaussian noise jamming

An adaptive serial search pseudonoise (PN) code acquisition scheme is proposed, in which the detection threshold is scaled by the instantaneous received power measured prior to PN code correlation. We observe that the proposed scheme achieves significantly improved mean acquisition times compared to the conventional nonadaptive schemes under Rayleigh fading and pulsed Gaussian noise jamming. Furthermore, the proposed scheme is shown to be optimum under pulsed Gaussian noise jamming in the sense that it forces the worst case jamming fraction to unity.

Code acquisition schemes using antenna arrays for DS-SS systems and their performance in spatially correlated fading channels

We propose parallel and serial code-acquisition schemes using antenna arrays for initial acquisition of direct-sequence spread-spectrum signals, which can lower substantially the range of detectable signal-to-noise ratio (SNR) as compared to the corresponding conventional schemes with a single antenna. The proposed schemes use the sum of all the single-antenna decision samples from antenna arrays, corresponding to an identical subsequence of a given pseudonoise code as a decision variable in order to enhance SNR of the resulting signal. We analyze the mean acquisition time performance under an additive white Gaussian noise channel and spatially correlated time-varying flat Rayleigh fading channels by deriving the expressions for the probabilities of detection, missing, and false alarm. We consider two extreme cases of spatial fading correlation among antenna arrays, the case of uncorrelated diversity antennas and the case of perfectly correlated antennas. From numerical results, we see that the acquisition performance of the proposed schemes using antenna arrays becomes improved continually as the number of antennas increases and the performance improvement depends on the degree of spatial fading correlation.

Optimal and suboptimal decision rules for parallel code acquisition in chip-asynchronous DS/SS systems

We investigate optimal and suboptimal decision rules for parallel code acquisition in chip-asynchronous direct-sequence spread-spectrum (DS/SS) systems. The conventional decision rule for parallel acquisition is to choose the largest correlator output of a receiver. However, such a scheme is optimum only for chip-synchronous models. In this letter, an optimal decision rule is derived based on the maximum-likelihood criterion for chip-asynchronous models. A simpler suboptimal decision rule is also discussed. The performance of the optimum and suboptimum decision rules is compared to that of the conventional decision rule. Numerical results show, that for chip-asynchronous models, both the optimal and suboptimal decision rules outperform the conventional decision rule.

Differentially coherent PN code acquisition with full-period correlation in chip-synchronous DS/SS receivers

Differentially coherent detection techniques for PN code acquisition in direct-sequence spread-spectrum (DS/SS) receivers are designed and analyzed in this letter. The full-period cross correlation (FPC) can be modeled as deterministic for certain types of PN sequences whose autocorrelation functions have ideal shapes the same as those of maximum length shift registration (MLSR) sequences. This model is exploited here to simplify the design and performance evaluation of the proposed detection techniques. Such techniques can be confidently employed as alternatives to noncoherent detectors because of their attractive improvements in the signal-to-noise ratio (SNR) and, thus, are very suitable for commercial code-division multiple-access (CDMA) systems, which often operate in severely noisy environments. The results of statistical analyses show that the proposed techniques can provide rapid acquisition and improved performance in SNR.

Joint detection and channel estimation algorithms for QS-CDMA signals over time-varying channels

We consider a quasi-synchronous code-division multiple access (QS-CDMA) cellular system, where the code delay uncertainty at the base station is limited to a small number of chips. For such QS-CDMA systems, the need for code acquisition is eliminated, however, the residual code tracking and channel estimation problems still have to be solved. An extended Kalman filter (EKF) is employed to track the user delays and channel coefficients. By separating data detection, based on the QR decomposition combined with the M-algorithm (QRD-M) from the delay/channel estimation process, the computational complexity can be significantly reduced as the number of users increases. Simulations show that the EKF channel estimator performance is improved when the QRD-M algorithm is used instead of the MMSE detector or decorrelator for data decisions.

Noncoherent sequential PN code acquisition using sliding correlation for chip-asynchronous direct-sequence spread-spectrum communications

Noncoherent sequential pseudonoise (PN) code acquisition using sliding correlation is proposed in this paper. Noncoherent detection and chip asynchronization should be taken care of to handle a severely noisy environment, while frequency offset and data modulation effects can be simultaneously dealt with in the proposed technique. To realize sequential detection, the cross-correlation sequences at the output end of the integrate/dump (LID) filter under out-of-lock conditions have to be modeled as either a Gaussian random sequence for chip-asynchronous applications or as their upper bound for chip-synchronous applications in order to avoid significantly high probabilities of false alarm caused by the conventional zero-sequence model. Meanwhile, the in-lock sequence also has to be modified by taking the frequency offset and chip-asynchronization effects into account in order to avoid the occurrence of high probabilities of miss. Extensive computer simulation results indicate that the proposed technique can achieve low probabilities of false alarm and miss and can outperform its fixed-sample-size (FSS) counterparts by roughly 2/spl sim/4 dB. This superiority, furthermore, increases with decreasing SNRs and/or decreasing desired error probabilities.

Wide-band CDMA for the UMTS/IMT-2000 satellite component

This paper describes the main aspects relevant to the development of a third-generation radio transmission technology (RTT) concept identified as satellite wide-band CDMA (SW-CDMA), which has been accepted by the International Telecommunications Union (ITU) as one of the possible RTTs for the satellite component of International Mobile Telecommunications-2000 (IMT-2000). The main outcomes of the extensive system engineering effort that has led to the above ITU RTT are described. In particular, we address propagation channel characteristics, satellite diversity, power control, pilot channel, code acquisition, digital modulation and spreading format, interference mitigation, and resource allocation. Due to its similarity with respect to the terrestrial W-CDMA proposal from which it is derived, the SW-CDMA open air interface solution is described briefly, with emphasis only on the major adaptation required to best cope with the satellite environment. Quantitative results concerning the physical-layer performance over realistic channel conditions, for both forward and reverse link, are reported. A system capacity study case for a low-Earth-orbit constellation is also provided.

Optical orthogonal code acquisition in fiber-optic CDMA systems via the simple serial-search method

We analyze, for the first time, the performance of a serial-search algorithm for optical code-division multiple access (CDMA) systems using optical orthogonal codes (OOC). For our analysis, we use advance photon-counting methods and we consider the effect of shot noise and dark current and multi-access interference (MAI). We introduce a new method for modeling the effect of MAI, then we use two approaches to consider this effect on the performance of the synchronization system. For the first approach, we consider the average effect of interference in our analysis, while for the second approach two specific forms of interference are taken into account where one form represents a heavily localized interference pattern and the other shows a uniformly distributed one. The effects of different parameters of the system including system threshold, number of active users in the network, and mean number of photons in each optical pulse on the performance of the synchronization system are evaluated and discussed.

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.