Variable dwell-time code acquisition for direct-sequencespread-spectrum systems on time-variant Rayleigh fading channels

Variable dwell-time code acquisition based on multiple-dwell or sequential linear tests is investigated for direct-sequence spread-spectrum systems on time-variant Rayleigh fading channels. Unlike in the conventional additive white Gaussian noise channels, the channel memory incurred by fading renders the exact analysis of the acquisition systems extremely difficult, if not impossible. In this paper, a novel method is developed to evaluate the mean acquisition time of the acquisition systems very accurately. The effects of Rayleigh fading are evaluated, and comparisons are made between double-dwell and sequential linear tests. Numerical results show that Rayleigh fading may result in 1-4-dB loss in performance, and the sequential linear test can outperform double-dwell test by a margin of 1-2 dB. The analytical results are verified by computer simulations     

采用多相关峰值的PN码频域匹配滤波捕获实现方法

现有的伪噪声(PN)码捕获算法大多是基于相关峰值进行门限判决,而捕获性能依赖于门限实时估计的正确性。针对这种情况,该文提出了一种利用多次PN码相关峰值位置间的相互关系进行判决的算法,避免了对信道噪声的实时估计,并给出了基于频域匹配滤波的实现方法和算法复杂度分析,推导了所提算法的检测概率和虚警概率,并且使用计算机仿真验证了推导结果的正确性;结果表明,所提算法避免了噪声估计的开销,增强了PN码捕获的鲁棒性,提高了捕获性能,适合工程应用。      

Effects of multiple threshold values for PN code acquisition inDS-CDMA systems

A decision method using multiple threshold value for PN code acquisition in DS-CDMA systems is describe. This technique is applied to a conventional hybrid search algorithm and analysis in terms of the probability of detection and false alarm. Simulation results show that the proposed scheme outperforms the conventional scheme in terms of detection probability because multiple threshold values mitigate any possible decline in performance caused by the used of a single threshold     

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 modified PN code tracking loop for direct-sequencespread-spectrum communication over arbitrarily correlated multipathfading channels

A modified fully digital pseudonoise code tracking loop is proposed for direct-sequence spread-spectrum communication. By taking advantage of the inherent diversity, a modified code tracking loop is embedded into a RAKE receiver in order to avoid problems caused by unstable locked points of error signals. Such unsteadiness of locked points often occurs with a conventional code tracking loop because the error signals may be randomly biased by multipath fading. Thus, a robust pull-in capability can be provided over a time-variant fading channel where multiple propagation paths are arbitrarily correlated. Furthermore, an effective multipath interference regeneration and cancellation technique is also proposed to improve the error characteristics of the proposed technique. Analytical expressions of the error characteristics and error signals are derived and then confirmed by means of extensive computer simulation results. In addition, several simulation results for the timing jitter and the mean time to lose lock are also presented. The very attractive behavior obtained using the proposed technique is verified     

Automatic decision threshold level control in direct-sequencespread-spectrum systems

An algorithm for automatic decision threshold-level control in direct-sequence spread-spectrum systems is presented and analyzed. Using this algorithm, instantaneous setting of the threshold is possible. An additional loop called a threshold loop is used to improve the system performance. This loop is based on a constant false alarm rate (CFAR) criterion. The purpose of the algorithm is to set a decision threshold in the system which will provide a small probability of false alarm (P₃→0) and a large probability of signal detection (P_d→1) at the same time. The analysis has shown that the threshold probability distribution function (PDF) is narrower (equaling better performance) in the case of the CFAR algorithm, but this comes at the cost of a considerably larger integration time for the system (the system time response is slower). As a compromise, the instantaneous algorithm is modified so that k successive samples of the signal are used to determine the value of the threshold. This permits better control of the shape and position of the threshold PDF with respect to the P₃ and P _d curves. At the same time, the time response of the system is good and can be easily controlled     

PN code acquisition using signed-rank-based nonparametric detectorsin DS/SS systems

In this paper, signed-rank based nonparametric detectors are used for pseudonoise (PN) code acquisition in direct-sequence spread-spectrum (DS/SS) systems. We first derive the locally optimum rank (LOR) detector and then propose the locally suboptimum rank (LSR) and modified signed-rank (MSR) detectors using approximate score functions. We compare the single-dwell scheme without the verification mode using the proposed LSR and MSR detectors with that using the conventional squared-sum (SS) and modified sign (MS) detectors. From the simulation results, it is shown that the proposed LSR and MSR detectors perform better than the MS detector by about 2-3 dB and are nearly optimum     

一种快速的PN码同步捕获方法

针对直扩系统中长PN码同步捕获速度慢的问题,利用推导的串接短PN码与长PN码具有相似相关性的结论,对直扩系统中PN码同步捕获的并行匹配滤波捕获方法进行改进,得到了简单可行的串并结合捕获方法。然后,结合一个周期内PN码相关值,采用迭代的方式,选取简单、实用的自适应门限,实现了突发通信中PN码的快速同步捕获。采用该自适应门限的捕获方法无需估计噪声,计算量小。仿真实验表明所提方法捕获概率高,抗噪性能强。     

Performance of digital code tracking loops for direct-sequencespread-spectrum signals in mobile radio channels

The performance of first- and second-order non-coherent digital delay lock loops (DDLL) for direct-sequence spread-spectrum (DS-SS) signals is investigated in the mobile radio environment. The mobile radio channel is first characterized by Rayleigh fading and Doppler shift. A closed-form expression for the timing error transition probability density function of the Chapman-Kolmogorov (C-K) equation is proposed. The probability density function of the steady-state timing error for the first- and second-order DDLL is obtained by solving the C-K equation numerically, and the results are confirmed by computer simulations. Furthermore, the mean time to lose lock (MTLL) of the first-order loop is evaluated, and some numerical results and simulation results are reported. Finally, the steady-state timing error and MTLL of the first-order loop for DS-SS signals in the log-normal fading environment are also presented, and the results are compared with those of Rayleigh and AWGN channels     

Distributed code assignments for CDMA packet radio networks

Code-division multi-access (CDMA) techniques allow many users to transmit simultaneously in the same band without substantial interference by using approximately orthogonal (low cross-correlation) spread-spectrum waveforms. Two-phase algorithms have been devised to assign and reassign spread-spectrum codes to transmitters, to receivers and to pairs of stations in a large dynamic packet radio network in polynomial times. The purpose of the code assignments is to spatially reuse spreading codes to reduce the possibility of packet collisions and to react dynamically to topological changes. These two-phase algorithms minimize the time complexity in the first phase and minimize the number of control packets needed to be exchanged in the second phase. Therefore, they can start the network operation in a short time, then switch to the second phase with the goal of adapting to topological changes. A pairwise code-assignment scheme is proposed to assign codes to edges. Simulations based on well-controlled topologies (sparse topologies) show that the scheme requires much fewer codes than transmitter-based code assignment, while maintaining similar throughput performance     

DSSS系统PN码捕获的相关峰优化及自适应门限检测

针对直接扩频系统的PN码捕获问题提出了一种基于恒虚警概率自适应门限算法的改进方案。方案通过将相邻两个符号长度内接收信号的相关峰值及平均能量对应相乘构造出新的统计量,并用随噪声能量及信噪比变化的自适应门限代替原有的固定门限判决统计量,以达到提高系统检测性能的目的。理论分析和仿真结果表明,改进方案较原方案的PN码捕获性能有很大的提高。     

Noncoherent hybrid parallel PN code acquisition for CDMA mobilecommunications

This paper proposes a noncoherent hybrid parallel pseudonoise (PN) code acquisition scheme for code-division multiple access (CDMA) mobile communication systems and analyzes the effect of multiple access interference (MAI) on the code acquisition performance for Rayleigh and Rician fading channels. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. It has a much simpler acquisition hardware structure than the total parallel acquisition and can achieve the mean acquisition time slightly inferior to that of the total parallel acquisition in the case of severe MAI; on the other hand, it provides the flexibility in the tradeoff between the mean acquisition time and system complexity if no MAI is considered. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. Numerical analysis quantifies the severe performance degradation of code acquisition due to both MAI and channel fading, and demonstrates the dependence of the increase of mean acquisition time (due to MAI) on the number of users in the CDMA system, system design parameters, and channel fading statistics     

A differentially coherent PN code acquisition receiver for CDMAsystems

New differentially coherent detectors for acquisition of direct sequence spread-spectrum signals are introduced. These detectors are alternatives to the noncoherent detectors that have been considered almost exclusively in the past. The proposed detectors are suitable for commercial code-division multiple-access (CDMA) systems which operate with a relatively large noise floor and provide a surprisingly large signal-to-noise ratio (SNR) improvement over the noncoherent detectors of approximately 5 dB. Under the random code sequence assumption, an exact analysis of the differentially coherent detection performance for both full period correlation (FPC) and partial period correlation (PPC) is carried out. The detector performance in terms of detection and false alarm probabilities for both partial and full period correlations is investigated, and the results are compared with those of classical noncoherent detection. The mean acquisition time for both single-dwell and multiple-dwell acquisition schemes are compared with their noncoherent counterparts     

PN码滑动相关捕获方法的改进

在扩频通信中,PN码的同步一直是研究的重点和难点.同步包括两部分:捕获和跟踪.捕获是实现同步的第一步,为了适应不同的环境,人们研究出了许多捕获的方法,其中,滑动相关捕获法是一种最简单、最实用的方法.针对该方法捕获时间较长的不足,提出了增加一个相位搜索控制器的方法来改进PN码捕获电路.在完全不影响其优点的情况下,缩短了捕获时间,使其捕获时间变为了原来的3/4,在工程中有重要的应用价值.     

Rapid code acquisition algorithm employing PN matched filters

The performance of four algorithms using pseudonoise matched filters (PNMFs), for direct-sequence spread-spectrum systems, is analyzed. They are: parallel search with fix dwell detector (PL-FDD), parallel search with sequential detector (PL-SD), parallel-serial search with fix dwell detector (PS-FDD), and parallel-serial search with sequential detector (PS-SD). The operation characteristic for each detector and the mean acquisition time for each algorithm are derived. All the algorithms are studied in conjunction with the noncoherent integration technique, which enables the system to operate in the presence of data modulation. Several previous proposal using PNMF are seen as special cases of the present algorithms     

Noncoherent sequential PN code acquisition using slidingcorrelation for chip-asynchronous direct-sequence spread-spectrumcommunications

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 2~4 dB. This superiority, furthermore, increases with decreasing SNRs and/or decreasing desired error probabilities     

基于滑动相关检测法的伪码同步研究

文章介绍了滑动相关检测法的基本原理,对用时钟脉冲加减法来调整本地伪码相位的过程进行了说明。采用systemview软件构建了滑动相关的伪码同步捕获仿真模型,通过仿真波形可知,滑动相关检测法可以很好地完成伪码的同步捕获。     

Adaptive acquisition of PN code in multipath fading mobile channels

The adaptive acquisition processor (AAP), which controls false alarms and reduces miss detection of multipath signals by estimating the threshold level after censoring some highest samples in the sliding window, was introduced by the present author. In addition, an AAP was analysed in a single path situation. Here, the detection performance of an AAP in multipath situations, which may exist in the sliding window, is investigated. Results show that the AAP has good performance if the number of excision samples, Ne, is greater than or equal to the number of multipath signals in the window     

低轨卫星扩频系统中的PN码混合并行捕获技术研究

以低轨卫星信道多普勒特性为背景,给出一种适用于大频偏条件下突发通信系统直接扩频序列混合并行捕获结构,并以门限信噪比下平均捕获时间最小为设计目标,对检测积分时间和判决门限设置进行讨论,给出最优配置方案。