【作者】 王甲池；

【导师】 胡修林；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2006， 博士

【摘要】 近年来,由于多天线系统在滤除空域干扰、提高频谱效率等方面的优异性能,多天线阵应用得到了广泛的重视,在直接序列扩频系统中的应用也越来越多。但是作为直扩系统瓶颈之一的捕获问题在多天线系统中的研究进展缓慢,大多数的算法没有充分开发多天线阵的潜在优势。本文详尽研究了基于天线阵的直接序列扩频系统的多维捕获问题,特别注意利用多天线增强捕获判决的可靠性,取得了一些有益的结果。论文首先提出了一种基于多天线阵的高动态DS/SS三维捕获算法,捕获最大允许的多普勒频移达到40kHz,这是常规捕获算法根本无法达到的。为了加快频域的搜索和精确估计多普勒频移,算法使用了FFT变换,值得注意的是,在低信噪比环境中这种变换是无法精确估计单天线系统多普勒频移的,这是因为大多普勒频偏的情况下捕获预积分的累积时间必须要短,也就无法获得足够的扩频增益来改善估计的信噪比,而低信噪比下的FFT变换是不准确的。值得庆幸的是,多天线阵的应用改善了系统工作的信噪比,保证了多普勒频移估计的准确性。理论分析和计算机仿真表明,由于充分利用了多天线阵的潜在优势,提出捕获算法与同等复杂程度的常规匹配滤波算法相比性能有了很大改善,特别是在低信噪比条件下,改善更明显。针对移动快衰落信道的特点,本文给出了一种新的自适应门限设置方法。鉴于多径分量一般会成簇出现的事实,算法在估计信噪比时不是简单地剔除数据样本中的几个最高值,而是根据一定的规则只剔除那些含有信号分量的样本,仍能保留那些数值较大但仅含有噪声分量的样本,与已有的自适应门限算法相比,对多径衰落信道下背景噪声功率估计更准确,门限设置更合理。针对DS/SS长周期伪随机序列,提出了一种码相位估计器辅助加速算法。基本原理是利用短时内信道的平稳和时不变特性,将两个相对移相1/4伪码周期的特殊辅助序列分别与接收信号互相关,给出一个本地伪码产生器的相对码相位改进值和改进方向。这样,搜索的起始位置就以较大概率集中在同步相位附近,大大节省了捕获时间。另外,为了进一步增加估计的准确性,算法对多个估计按改进方向分别进行累加,并用大数判决的方法决定最终的结果。理论分析和计算机仿真证实,码相位估计器可以几倍地提高捕获速度,且伪码周期越长效果越明显。文章最后给出了一种基于多天线的DS-UWB捕获算法。算法利用UWB信号的循环平稳特点,将接收信号与它自身延迟一个信息位产生的模板相关,应用最大似然方法就可得到一个粗略的同步时间估计。在充分利用线性天线阵空间分集增益的情况下,这种估计相当准确,误差一般不超过一个信息位时间长度的20%,特别在高信噪比条件下,估计误差能小于一帧。更多还原

【Abstract】 Recently, the incorporation of antenna arrays to direct-sequence spread-spectrum (DS/SS) systems has been widely considered and becomes more and more popular in modern communication systems, due to the significant performance improvements in spatial filtering and bandwidth efficiency. But the researching advance on acquisition, a bottleneck for DS/SS systems, is not satisfied and most schemes do not exploit the potential of antenna arrays well. In this dissertation, we study multi-dimensional acquisition of DS/SS systems based on antenna arrays and achieve some satisfied results through exploiting the potential of antenna arrays to increase the detection probability of code acquisition process.First, a three-dimensional acquisition scheme of DS/SS systems based on antenna arrays is proposed in this dissertation, which can work well even if the Doppler offset is up to 40kHz. And those conventional schemes can not work in this larger Doppler offset. For accelerating the searching process in frequency domain and estimating the Doppler offset accurately, FFT transform is used in this scheme. It is worth noticed that FFT can not work well in those systems with single antenna because the pre-intregation period must be shortly due to existing larger Doppler offset so that the appropriate signal-noise ratio(SNR) can not be obtained after despreading to guarantee the correctness of the Doppler estimation. But it is different for the systems with multi-antenna arrays because the gain obtained from antenna arrays can improve the SNR. Analysis in theory and numerical results show that the proposed scheme outperforms the conventional match-filtering scheme with the same complexity of hardware, especially in the lower SNRs.Second, a novel scheme of adaptive threshold setting is proposed for the rapidly-varying and fading channel. Based on the fact of the multipath component arriving in cluster, the scheme can eliminate the samples containing multipath signal components according to some rules and does not discard those larger samples simply. In this way, some larger samples merely containing noise component is reserved so that the estimation of noise power is more precisely and the threshold setting is more reasonably compared with the conventional schemes.Third, a code phase estimator is presented to improve the acquisition performance for DS/SS systems with larger Pseudo-Noise(PN) periods. In this scheme, the improving value and direction of the local PN generator is given based on the cross correlation between the incoming PN signal and the special auxiliary signal assuming that the channel is stationary and time-invariant in short time. In this way, the search can start from those code phase positions nearby sync cell with larger probability and reduce the mean acquisition time. For increasing the precision of phase estimator, some repetitious estimating outputs are also summed and averaged according to their sign and adopted the strategy of majority to decide which result is chosen at last. Analysis in theory and numerical results show that the proposed scheme can speedup the acquisition process several times compared with the conventional methods and the performance improves as the PN period increases.At last, a rapid timing acquisition scheme with multiple antennas is proposed for direct-spread ultra wideband(DS-UWB) systems, in which coarse estimation of initial frame is obtained in this dissertation based on the Cyclostationarity(CS) property inherent to UWB signals. We develop a data-aided approach to correlate the received waveform with the template taken from the received signals a symbol earlier to obtain the estimation based on the maximum likelihood(ML) criterion. Through utilizing spatial diversity from multiple antennas, the results of coarse estimation are very precise and the estimating errors are 20% at most, even less than one frame at higher SNRs.更多还原