A closed-loop coherent acquisition scheme for PN sequences using anauxiliary sequence

We propose a closed-loop system for the acquisition of the pseudo-noise (PN) signal in direct-sequence spread-spectrum (DS/SS) systems. We introduce a novel idea of using an auxiliary signal, as opposed to the PN signal itself, for correlation with the incoming signal. The cross-correlation function of the auxiliary signal and the PN signal has a triangle shape that covers essentially the entire period of the PN signal. Consequently, their correlation provides the direction for the phase update of the local signal generator in the acquisition scheme. With coherent demodulation, the mean and variance of the acquisition time are derived under additive white Gaussian noise (AWGN). They are compared to those of the conventional serial-search acquisition system. Results suggest that the proposed system acquires the PN phase at least twice faster, with significantly smaller acquisition time variance, than the conventional system     

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.     

基于相位估计器的直接序列快捕方法

该文提出一种基于直接序列扩频系统(DS/SS)的码相位估计器。它能根据输入信号与一个特殊序列不同相位下的互相关值,相当准确、有效地估计出本地伪随机序列的改进方向和大小,成倍地缩短了捕获的时间。理论分析和计算机模拟表明,对于周期511以上的伪随机码而言,该算法在正常工作条件下平均捕获时间仅为常规捕获方法的一半。     

A closed-loop coherent PN acquisition system with a pre-loopestimator

We propose a closed-loop system with a pre-loop code phase estimator (PLE) for acquisition of the pseudonoise (PN) signal in direct-sequence spread-spectrum systems. It consists of two major parts: a PLE and a closed-loop acquisition subsystem. The system uses an auxiliary signal, as opposed to the PN signal itself, to correlate with the incoming signal. The PLE uses the correlation result to obtain an estimate of the incoming code phase. Then, starting with this initial estimate, the closed-loop subsystem will acquire the code phase of the incoming PN signal. With coherent carrier demodulation, mean and variance of the acquisition time of the proposed scheme are obtained and compared to those of the conventional serial-search acquisition receiver and the closed-loop system without the pre-loop estimator. Results show that, with proper choices of parameters, the proposed system acquires the PN phase significantly faster than the other two systems     

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 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     

Direct sequence spread spectrum Walsh-QPSK modulation

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel     

一种新的直扩超宽带系统快速同步方案

本文提出了一种新的基于能量窗的直扩超宽带系统码片同步方法，通过设计具有重复特性的码来提高能量窗的信噪比，从向可以较为准确地获得码片同步，并将窗口中的信息作为接收端本地信号与接收信号进行匹配接收，这样可以存未知接收信号相位的情况下有效地接收多径能量，提高接收信噪比。在完成码片同步之后再进一步进行PN码的相位同步，这里以线性搜索为例分析得到超宽带信号总的平均同步时间，并与现有的超宽带同步方案进行了比较，新方案将信号捕获分成两部分完成，大大降低了超宽带信号捕获时间。     

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。     

数字差动匹配滤波器技术在PN码捕获系统中的应用

重点介绍了数字差动匹配滤波器(DDMF)的结构和原理，提出了一种采用DDMF进行PN捕获的方案。研究结果表明：此方案能实现快速捕获，与传统的数字匹配滤波器(CDMF)相比能节约硬件资源。     

DSA: a distributed sample-based fast DS/CDMA acquisition technique

We propose a new acquisition scheme called distributed sample acquisition (DSA) which is applicable to the direct-sequence code-division multiple-access (DS/CDMA) system with a long-period pseudonoise (PN) sequence. The proposed scheme lays its foundations on an auxiliary PN sequence called the igniter sequence, which has relatively short period, and the distributed sample scrambling (DSS) technique previously developed for the asynchronous transfer mode cell transmission. In the DSA scheme, acquisition is done in three steps. First, the igniter sequence is synchronized, which normally takes a very short time due to its short period. Second, the state samples of the long-period PN sequence generator in the transmitter, which are conveyed to the receiver over the igniter sequence stream, are determined out of the synchronized igniter sequence. Third, the long-period PN sequence is synchronized by applying the state samples to the DSS receiver. Since the operation of each step takes a very short time, the overall acquisition is done very quickly. We analyze the acquisition time performance of the proposed scheme by taking the transform domain approach, confirming that the resulting mean acquisition time is dramatically reduced. If compared with the existing serial-search scheme, the DSA scheme can perform acquisition about 100 times faster when the period of the PN sequence is 2¹⁵-1. None the less, the additional circuit complexity for its implementation is very small     

Median-prefiltering-based robust acquisition of direct-sequencespread-spectrum signals in wide-band pulse jamming

We propose robust acquisition schemes for chip time-synchronous direct-sequence spread-spectrum (DS/SS) signals in wide-band pulse jamming. To mitigate the performance degradation due to the impulse-like wide-band jamming signal, the received signal is preprocessed by employing simple order statistic filters. We consider the use of two types of median filtering schemes: a running median filter operating at a chip rate and a block median filter processing the received signal sampled at an oversampling rate. For analytic design, the partial correlation of median filtered pseudonoise (PN) signals is approximated by a piecewise linear model using an upper bound. The analytic design is verified by computer simulation. Finally, the acquisition performance of the proposed schemes is compared to that of conventional schemes     

Acquisition of m-sequences using recursive soft sequential estimation

A novel sequential estimation method is proposed for the initial synchronization of pseudonoise (PN) signals derived from m-sequences. This sequential estimation method is designed based on the principle of recursive soft-in/soft-out (SISO) decoding, and we refer to it as the recursive soft sequential estimation (RSSE) acquisition scheme. The RSSE acquisition scheme exhibits a complexity similar to that of a conventional m-sequence generator, which increases only linearly with the number of stages in the m-sequence generator. Our simulation results also show that the acquisition time of the proposed RSSE acquisition scheme is also linearly dependent on the number of stages in the m-sequence generator. Owing to the above properties, the employment of the proposed RSSE acquisition scheme is beneficial for the acquisition of long m-sequences.     

Noncoherent SPRT-based acquisition scheme for DSSS

A noncoherent sequential PN code acquisition scheme is proposed. The out-of-phase and on-phase sequences are properly modelled to avoid significantly high error probabilities occurring with the conventional SPRT-based acquisition. In addition, data modulation and frequency offset can be effectively overcome using this technique     

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

Rapid acquisition of PN sequences with a new decision logic

In this paper, a fast acquisition scheme of pseudonoise (PN) sequences for direct-sequence spread-spectrum systems is proposed. The scheme exploits a new decision logic not only to estimate chip values, but also to check the reliability of the chip estimates. As a result, the internal state of a PN sequence generator is estimated more accurately. We derive the probability of finding a correct state estimate and show that the proposed scheme can reduce the average number of chips for acquisition when compared with the conventional scheme. It is also shown that the performance improvement is more noticeable in the moderate signal-to-noise-ratio range.     

Phase Noise Pre-correction Scheme for SC-FDMA Signals in LTE-Uplink

This paper proposes a novel carrier Phase noise (PN) pre-correction scheme with an adaptive PN prediction algorithm for Single-carrier Frequency-division multiple-access (SC-FDMA) systems to alleviate degrada-tion due to the PN. Our proposed PN prediction algorithm is a modified polynomial fitting algorithm which is based on receding horizon principle. The parameters of the pre-diction model are optimized by using the algorithm on PN samples of the local oscillator signal in a training window. By using the optimized prediction model parameters and the latest PN samples, we can predict future PN samples. Then these predicted PN samples are put into our pro-posed PN pre-correction scheme and the SC-FDMA sym-bols at the transmitter are pre-compensated. Due to the absence of the radio frequency delay device, the proposed scheme has a low hardware complexity. Simulation results show that our proposed scheme can greatly reduce the ef-fect of the PN on the transmitted SC-FDMA signal.     

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.     

Acquisition of Pseudonoise Signals by Recursion-Aided Sequential Estimation

An improved method of acquisition of pseudonoise signals (RARASE) is described and analyzed. This method is an outgrowth of the RASE method, rapid acquisition by sequential estimation. In the new method, the known recursion relation of the PN signal is used to determine if a short estimate of the state of the received PN signal is probably correct and if, consequently, an attempt should be made to track with that estimate. A high proportion of the incorrect initial state estimates can be discarded with relatively simple logic. Analyses are provided for a range of implementations from the simplest, a single 3-input modulo-2 gate, to multiple logic gates and for both a simple in-lock detector and a sequential detector. Significant improvement in acquisition time is shown for long PN signals and for the signal-to-noise range of -13 dB to +3 dB. Acquisition time reduction by a factor of 7.5 is calculated for a PN signal of length2^{15}-1bits compared to the sequential estimation (RASE) method using a simple in-lock detector. Reduction by factors of 1.5 to 3.5 is calculated using the sequential detector.     

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.