Correlation-aided distributed sample acquisition scheme for fast and robust code synchronization in cellular DS-CDMA systems

We introduce a correlation-aided distributed sample acquisition (CDSA)-based intercell synchronous direct-sequence code-division multiple-access (DS/CDMA) system for enhancing the robustness of the previously proposed distributed sample acquisition (DSA)-based system. The CDSA system incorporates the state symbol correlation process in the existing DSA process to maintain the fast acquisition performance even under very poor channel environment. For its realization, each CDSA-based mobile station stores in its memory a period of the possible state symbol sequences, which are determined by the long-period scrambling PN sequences used in the system and the sampling time parameters for the state samples. In the first stage of the synchronization procedure, the mobile station (MS) attempts to acquire the psuedo noise (PN) sequence by taking the comparison-correction based synchronization approach of the original DSA scheme. However, if it cannot acquire the synchronization until it collects a predetermined number of state symbols (e.g., a period of the state symbol sequence), then the MS determines the PN sequence timing by correlating the received symbol sequence with each shift of the prestored state symbol sequences. Performance analysis and simulation results reveal strong robustness of the CDSA-based system in low-signal-to-noise ratio, fading, and large frequency-offset channels.     

Distributed sample acquisition-based fast cell search in inter-cellasynchronous DS/CDMA systems

We introduce a fast fell-search scheme based on the distributed sample acquisition (DSA) technique for inter-cell asynchronous DS/CDMA systems. The proposed DSA scheme enables the long-code acquisition extremely fast even without employing any passive matched filter which used to dominate synchronization circuit complexity in asynchronous DS/CDMA systems. In the DSA-based asynchronous DS/CDMA system, each base station spreads its DQPSK-modulated long-code generator information with an assigned short-period igniter sequence, and broadcasts it as a common pilot signal. A mobile station first identifies and acquires the igniter sequence of the cell group currently located, then detects the conveyed long-code generator information by despreading the acquired igniter sequence, thereby identifying and synchronizing the long-code of the current cell. The mean acquisition time of this proposed DSA-based inter-cell asynchronous DS/CDMA system, evaluated under the assumption that each of 512 cell-specific long-codes is a 10-ms segment (38400-chips for the chip rates of 3.84 Mchips/s) of complex Gold codes of period 2¹⁸-1 and the igniter sequence (or, group code) set is composed of 7 complex orthogonal Gold codes of period 256, turned out even shorter than that of the inter-cell synchronous DS/CDMA systems employing the conventional serial search method. Furthermore, simulation results showed that the DSA scheme employing a passive matched filter is much superior in terms of acquisition time and robustness, to the 3GPP W-CDMA synchronization scheme having comparable complexity     

DSA techniques for fast and robust acquisition of DS/CDMA scrambling codes

This article introduces the key concepts, organization, and operations of distributed sample-based acquisition (DSA) systems, which have previously been introduced for fast and robust synchronization of the long-period scrambling codes in DS/CDMA environments. In DSA systems, the transmitter samples and sends the state of its main sequence generator, or main shift register generator (SRG), in a distributed manner over the short-period igniter sequence, and the receiver detects and applies the state samples to correct the state of its main SRG, thereby acquiring SRG synchronization after a round of state reception. Acquisition performance of DSA techniques is extremely fast and robust compared to typical correlation-based acquisition techniques of comparable complexity. This article discusses the operation and performance of DSA techniques in the DS/CDMA communication environment as well as their applications to intercell synchronous and asynchronous cellular systems.     

低信噪比下超长PN码的快速捕获技术

直扩系统中,为了硬件实现上的简便,PN码的捕获一般都采用串行搜索的方法。但当PN码很长时,由于要搜索的相位点很多,串行搜索法将使捕获时间过长。并且当信噪比较低时,会有较高的虚警概率。这些都会使系统的捕获性能下降。文献[6]针对长码提出了分布采样的快速捕获方案(DSA)。本文就此提出了改进,利用积累使之能够适应低达-50dB的低信噪比环境。仿真结果表明,改进后的算法改善了原算法在极低信噪比下的性能,降低了平均捕获时间。     

一种基于二分试探法的相干扩频快捕系统

利用辅助序列与接收PN码的线性相关性,在搜索方向判决准确的前提下,折半的查找范围对应折半的误差范围,达到应用二分法大步进缩小查找范围,从而有效的加快捕获速度,与常规步进滑动相关法相比,该捕获系统能够有效改善长PN码的捕获性能。     

A Generalized Analysis for a Dual Threshold Sequential Detection PN Acquisition Receiver

Spread spectrum techniques have become very useful in modern communications. A common spread spectrum technique is to directly modulate a pseudorandom noise (PN) code onto the carrier. Systems employing this technique are called direct sequence (DS) systems. A primary concern with direct sequence systems involves PN code synchronization (acquisition) of the incoming and local signals to within at least one PN symbol followed by a fine alignment (tracking) which must be maintained throughout transmission. This paper is concerned with the PN acquisition aspect of code synchronization. Specifically, a dual threshold sequential detection receiver is analyzed and a general expression is derived which addresses the probability that the sequential detection procedure ceases after an arbitrary number of samples. This expression is needed in order to support a characteristic function approach in deriving acquisition Probability as a function of time which will be the topic of a subsequent paper.     

PN acquisition for DS/SS using a preloop parallel binary search phase estimator and a closed-loop selective search subsystem

An acquisition method of long pseudo-noise (PN) sequences in direct-sequence spread spectrum (DS/SS) systems is presented. The concurrent cross correlation of locally generated auxiliary signals with the incoming PN sequence produces binary search estimates leading to an initial approximation of the input signal's PN offset. The PN receiver then uses a closed-loop subsystem to determine the true offset of the incoming PN signal. New phase estimates are generated from successive corrections of likely errors in the binary search. The acquisition scheme's average acquisition time and its variance are analytically determined. Simulation results show that the proposed acquisition scheme is faster than the conventional serial receiver. A hybrid serial/parallel PN acquisition receiver is used for comparing the acquisition performance given similar hardware complexity. The hybrid receiver used the same number of parallel correlators as used in the proposed receiver's input phase estimator. Simulation results show that the proposed system provides an improved performance under normal operating conditions.     

Blind adaptive multiuser detection for DS/SSMA communications withgeneralized random spreading

A new spreading scheme and an accompanying blind adaptive receiver structure are proposed for direct-sequence spread-spectrum multiple-access communications in a slowly-varying, frequency-selective fading channel. Each user's spreading sequence is given by the Kronecker product of a long-period pseudonoise (PN) sequence, which is accurately modeled by a random sequence, and a short-length deterministic signature code. This spreading scheme bridges the gap between pure PN spreading and pure short-code spreading schemes. It is shown that with this spreading scheme, the channel response to the desired signal component is easily estimated without relying on the spectral decomposition of the signal correlation matrix. With the estimate of the channel response, the receiver suppresses interference based on the maximum signal-to-interference ratio criterion. The blind adaptive receiver requires only coarse timing information and a priori knowledge of the desired user's PN sequence for adaptation. Numerical results show that the adaptive receiver significantly suppresses interference by successfully estimating the channel response and the interference statistics with a low computational complexity. An extension to spatio-temporal processing using an array antenna is also discussed     

Performance analysis of DSA-based DS/CDMA acquisition

We analyze the acquisition time performance of the distributed sample acquisition (DSA) scheme proposed for a fast DS/CDMA signal acquisition. To analyze the composite acquisition mechanism, we first define several acquisition-related probability parameters which can describe all the essential events in the DSA synchronization, along with the basic states that constitute the eventual DSA state transition diagram. Then, we construct the state transition diagram employing these basic states and the moment generating functions which are entirely based on the above defined probability parameters. Applying this state transition diagram and determining the acquisition-related parameters, we finally analyze the mean acquisition time performance of the DSA     

组合导航接收机的码捕获技术

针对组合导航环境下的PN码捕获特点,采用分段并行捕获技术＋多次最大值判决策略,解决了高动态、大多普勒频移下PN码快速捕获的工程实现问题。从理论分析和仿真结果看是切实可行的。     

高动态短时突发扩频信号的快速捕获

针对低信噪比和大多普勒频移环境下,短帧突发直接序列扩频通信系统要求在短时间内以很低虚警和漏警概率捕获信号这一难题,设计了仅使用独特码的短时突发扩频信号帧结构,在接收端将扩频码与独特码作为一个复合扩频码看待,提出了基于FFT的高动态突发信号捕获方法,可同时快速完成突发信号检测、扩频码捕获、频偏估计、帧同步检测和位定时捕获等任务。仿真结果表明:该算法能有效消除多普勒频移对短时突发扩频信号捕获性能的影响。     

基于DS扩频信号的自适应门限双积分滑动相关捕获算法

提出了一种基于直接序列扩频信号的双积分滑动相关捕获算法,同时使用两组相关器联合进行码相位的捕获,捕获的门限采用CAFR(常虚警率)自适应控制方式。文中给出了这个捕获方案的数学模型,并进行了具体分析。实验表明,利用FPGA实现这种捕获算法,不仅具有较高的性价比,而且衡量PN码捕获的最关键指标——平均捕获时间也大大少于常规捕获方法。     

A coherent acquisition method for a PN sequence using binary searchand an auxiliary sequence

Direct sequence spread spectrum systems require a local replica of the pseudonoise (PN) sequence to be synchronized with the received signal's spreading code. A method is proposed to improve the PN sequence acquisition time and decrease the acquisition time variability. The proposed method is based on a binary search algorithm. At each stage of the algorithm, an auxiliary signal is used to formulate a new search region within the PN sequence, and at each stage the search region is reduced by half. The region containing the incoming PN phase is therefore obtained within O(log₂ (N+1)) steps. A closed loop coherent PN acquisition algorithm presented by Salih and Tantaratana (1996) motivated the use of the auxiliary signal, and the algorithm of Salih et al. is also used for comparison. Results indicate the algorithm presented in this paper can achieve a factor of two to three times improvement in the mean acquisition time with a significant decrease in the acquisition time variability     

扩频通信系统中一种改进的快速编码捕捉方法

针对扩频通信系统中传统的滑动相关捕捉法对于较长的刚码序列编码捕捉时间较长的问题，提出了一种改进的编码捕捉方法，该方法采用三条相同支路同时搜索的方式来实现PN码的快速捕捉，不仅不受PN码长度限制，而且电路实现简单，可以有效的降低虚警概率。理论分析和仿真实验均表明：提出的改进方法大大缩短了系统的捕捉时间。     

An improved sequential estimation scheme for PN acquisition

An improved sequential estimation (ISE) pseudonoise (PN) acquisition scheme based on an extended characteristic polynomial is proposed in which the PN despreader can work or both the m sequence and the inverted m sequence. The scheme can be easily implemented by a front-end bit detector followed by full digital circuitry. The mean acquisition time of ISE is derived     

A programmable pipelined digital differential matched filter forDSSS receiver

A programmable pipelined digital differential matched filter (PDMF) implemented for a direct sequence spread spectrum receiver is proposed in this paper. To reduce the power consumption, the PDMF architecture is based on the synchronization combined PN code phase acquisition algorithm. Compared with the conventional PN code phase acquisition algorithm, the theoretical analysis result indicates that the PDMF acquires both power efficient and preferable detection. Depending on different applications, programmability allows the PDMF to implement 3-tap, 5-tap, or 11-tap Barker codes with the same hardware but different precisions for each tap coefficient. For short tap Baker codes, the architecture could adopt more precision on each tap coefficient to resist the channel noise. Simulation results also show that there are fewer errors of high sample precision with the same tape     

Sequential acquisition of PN sequences for DS/SS communications:design and performance

The performance of sequential acquisition of m sequences, based on the sequential probability ratio test (SPRT), is studied. The authors consider a sliding correlator-type structure for the acquisition scheme, which the mean acquisition time is proportional to the average number of samples used for a synchronization test. Sequential acquisition requires the knowledge of the partial correlation of the pseudonoise (PN) sequence, however, as the partial correlation of an m sequence is difficult to model the authors propose an approximate upper bound. This is then piecewise linearized and used for designing the SPRT which is then compared to the fixed-dwell time technique, a scheme using the fixed sample size test. Numerical results show that the saving of acquisition time by the SPRT increases as the desired probabilities of errors and/or the input SNR decrease. Analytic results are verified by computer simulation     

A Robust and Adaptive Carrier Recovery Method for Chinese DTTB Receiver

This paper presents a robust and adaptive carrier recovery method for Chinese digital terrestrial television broadcasting (DTTB) system in which pilot signal and pseudonoise (PN) sequence are adopted to help carrier recovery. The conventional methods utilize pilot or PN sequence respectively. In this paper, we try to combine the advantage of each method together and propose a well designed state machine to control system state automatically. Moreover, as for using PN sequence, a fine PN tracking state is introduced to ensure the robustness of the proposed method. Software simulations show that the proposed method can provide large acquisition range, short acquisition time and small tracking jitter in severely distorted static and dynamic channels. Lab tests and field trials also prove its good performance in real propagation environments.     

高动态突发扩频通信中伪码和多普勒频移的快速捕获

在高动态突发扩频通信系统中,要求接收机在很短时间内实现伪码和多普勒频移的快速捕获。目标的高速运动使接收机基带信号中存在较大的多普勒频移。针对伪随机序列相关峰的多普勒频移敏感特性,分析了多普勒频移对伪码捕获和数据解调的影响,提出了采用单通道匹配滤波进行伪码捕获,采用三通道并行相关器搜索多普勒频移的接收机捕获方案。该方案可有效减少硬件资源消耗,同时缩短捕获时间。     

A CDMA scheme based on perfect Gaussian integer sequences

Both m-sequences and Gold sequences have been applied as the pseudonoise (PN) code in the code division multiple access (CDMA) scheme to enable distinct users to simultaneously share the available capacity of a common channel, which is called direct sequence CDMA (DS-CDMA). However, this scheme does not provide the required ideal periodic cross-correlation function property among various members of sequences within the same set. This paper proposes a new perfect Gaussian integer sequence (PGIS)-based CDMA scheme called PGIS-CDMA; in this scheme, the PN code of DS-CDMA is replaced by a set of PGISs. The intrinsic orthogonal property of PGIS can be applied to a CDMA system to achieve perfect cochannel interference separability under the ideal synchronism; it can also be adapted at the receiver to obtain diversity gain and improve the carry-to-interference ratio. These two concepts are further developed to implement two PGIS-CDMA configurations, and they can outperform DS-CDMA on the basis of Gold sequences that possess a favorable cross-correlation property for the CDMA scheme.