摘要
在超宽带无线通信系统中,接收机需要清楚地知道接收信号的时延才能够实现正确的解扩和解调。典型的同步算法分为两个阶段,第一个阶段在一定的时间精度内实现粗同步,称为捕获过程。第二个阶段被称为跟踪过程,作用是实现更精确的定时同步以及通过时钟调整来维持发射机和接收机之间的精同步。
本文就基于估计理论的同步算法进行了深入研究。分析了两种基于极大似然原理的超宽带帧级同步算法:基于极大似然准则(ML)和基于广义似然比检验(GLRT)的同步算法。通过结合以上两种算法,对无跳时情况下的GLRT算法中本地模板信号进行改进;其次对有跳时码情况下的有噪模板进行平均处理。仿真结果表明,改进后的GLRT算法具有高捕获精度和低复杂度,是一种有效的帧级同步算法。
在帧级精度同步捕获的基础之上,进行脉冲级精度的细同步。首先分析了一种传统的基于信号循环平稳特性的盲同步算法,接下来提出一种基于有噪模板的脉冲级同步捕获算法,此算法利用接收信号作为相关模板,这样的相关模板包含了全部的多径分量和脉冲畸变信息,因此能够获得较好的同步性能。分析和仿真结果表明,该算法的精度相比于帧级同步算法有了较大幅度的提高,从而进一步完善了利用导频的超宽带同步方案。
A class of spread spectrum techniques known as ultra-wideband (UWB) communication has recently received a significant amount of attention from academic researchers as well as from the industry. UWB signaling is being considered for high data rate wireless multimedia applications for the home entertainment and personal computer industry, as well as for low data rate sensor networks involving low power devices. It is also considered a potential candidate for alternate physical layer protocols for the high-rate IEEE 802.15.3 and the low rate IEEE 802.15.4 wireless personal area network (WPAN) standards. UWB technology becomes more and more attractive because of its advantages, such as high transmission rate, low power consumption, high security, robustness to multi-path and low cost. UWB wireless communication technology uses the principle of overlapping to share the spectrum occupied, it can coexist with the existing wireless communication systems at the same frequency band, in which data are transferred using non-sinusoidal pulses with very short duration.
In any communication systems, the receiver needs to know the timing information of the received signal to accomplish demodulation. The subsystem of the receiver which performs the task of estimation of this timing information is known as the synchronization stage. Synchronization is especially a difficult task in spread spectrum systems which employ spreading codes to distribute transmitted signal energy over a wide bandwidth. The receiver needs to be precisely synchronized to the spreading code to be able to de-spread the received signal and proceed with demodulation. In spread spectrum systems, synchronization is typically performed in two stages. The first stage achieves coarse synchronization with a reasonable amount of accuracy in a short time, and is known as the acquisition stage. The second stage is known as the tracking stage and is responsible for achieving fine synchronization and maintaining synchronization through clock drifts occurring in the transmitter and the receiver.
Generally speaking, timing acquisition in UWB system can be broadly classified into detection-based approaches and estimation-based strategies. In this paper, estimation-based schemes are presented. And the estimation-based schemes can also be broadly classified into data aided estimation and non-data aided estimation. Non-data aided timing estimation exploits the signal structure inherent in UWB to estimate timing information of the received signal, which overcomes disadvantages of resources waste. While data aided schemes uses pilot symbols. It needs to design appropriate pilot symbols in order to improve accuracy and reduce complexity. These estimation-based schemes are provided with characteristic of quick acquisition speed and high accuracy, but waste pilot width and power resource, thus decreases efficiency of the system.
1. Frame-level acquisition schemes in UWB communications
In dense multi-path environments, timing acquisition for UWB communications faces major challenges due to the stringent requirements to resolve and capture ultra-short transmitted pulses. This paper develops efficient maximum-likelihood principle acquisition methods that offer explicit design options to trade off acquisition accuracy and complexity.
(1) ML-based schemes for timing acquisition in UWB system The proposed ML approach performs computationally affordable channel estimation based on a tapped delay line model, where the tap spacing is set at a low frame-level rate. Both the data-aided and non-data-aided scheme are derived in this paper. Data-aided ML timing acquisition method assuming the transmitted symbol is known for us, and it estimates the frame-level acquisition via maximizing the log-likelihood function. In Non-data-aided ML timing acquisition the likelihood expression entails unknown information-bearing symbols, which can be averaged out over the probability density of symbols to produce marginal likelihood function, and acquire the simplest form via the low-SNR approximation. The procedure for this approach involves a frame level-point grid search course to complete the acquisition.
(2) GLRT-based schemes for timing acquisition in UWB system Generalize likelihood ratio test (GLRT) is also a method based on the maximum likelihood. But the unknown parameters are replaced with maximum likelihood estimation. This scheme is data-aided mode to estimate timing offset. The proposed algorithms only employ digital samples collected at a low symbol rate. Starting with the no-TH case, we derive a generalized likelihood ratio test for joint detection and frame-level timing acquisition, where channel-dependent unknowns are regarded as nuisance parameters. Frame-level timing offset acquisition amounts to an amplitude estimation problem accepting a closed-form solution. Based on symbol-rate samples, the GLRT yields channel-dependent amplitude estimates, the timing acquisition solution, the symbol detection rule, as well as the associated estimation performance bounds. Then, extend the results to the TH case by properly selecting a noisy template to generate correlator output samples. Such a template inherently accounts for the unknown TH pattern under mistiming, thus being able to retain the desired symbol-rate sample structure that links timing offset parameters with amplitude scaling factors. Use of a noisy template also enjoys effective multi-path energy capture and asymptotically maximizes the received signal-to-noise ratio (SNR), which in turn improves timing accuracy.
(3) Summary
Simulations of the two algorithms based on ML and GLRT principle are made and improve the template used in GLRT scheme. The template which used in ML method can be used in no-TH GLRT algorithm, and average the noisy template used in TH GLRT scheme in order to decrease the noisy nuisance. The simulation results show that the algorithm that link two algorithms above is more practical and effective.
2. Pulse-level acquisition schemes in UWB communications
(1) Cyclostationarity (CS) based algorithm for pulse-level tracking CS is naturally present in UWB signals due to the inherent pulse repetition across multiple frames comprising each symbol. A cyclostationary approach is used to recover timing of UWB transmissions over rich multi-path environment. It based on the received signal’s second-order cyclic statistics, and non-data-aided time offset estimation algorithms are developed.
(2) Timing with dirty templates in pulse-level tracking
Most existing synchronizers are based on the unique maximum of the received pulse’s autocorrelation function, which requires a“clean template”of the received pulse to be available. Evidently, the later is not feasible when the multi-path channel is unknown. This novel criterion relies on the fact that the cross correlation of“dirty templates”extracted from the received waveform exhibits a unique maximum at the correct timing. TDT based scheme not only achieves timing, but also improves acquisition performance over competing alternatives by collecting the rich multi-path energy provided in UWB channels.
(3) Summary
Simulations of the two algorithms and the normalized MSE and BER of the two pulse-level algorithms are made. The simulation results show that the capability of TDT-based scheme is better than traditional CS-based scheme. It also shows that the timing offset estimation after tracking is better than that the method with only a coarse acquisition, and it can achieve high pulse level synchronization accuracy.
引文
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