【作者】 谭晓衡；

【导师】 杨士中；

【作者基本信息】 重庆大学 ， 电路与系统， 2003， 博士

【摘要】 对于特种小卫星扩频通信系统，衰落及干扰状况是系统设计时必须考虑的重要方面。特别是卫星在低仰角进行通信时，多径衰落尤其严重，并且由于卫星与地面接收点（地面站或移动用户）的相对运动较快，高多普勒频移可能使信号经历快衰落。信号在传播过程中还将遭受人为和自然噪声的干扰，通常表现为窄带干扰、阻塞干扰、单频连续波干扰、脉冲干扰及多址干扰的形式，它们可以等效为同道干扰（CCI），对星上及地面信号的接收造成严重影响。由于这些干扰和多径衰落的存在，导致了接收信号强度变化剧烈，波动范围较大，把已经开发成功的特种小卫星通信系统在实际的信道环境中应用时，系统出现了同步慢和可靠性低等问题。分析发现，造成这一问题的原因除了接收信号波动范围较大外，还与系统采用固定的判决门限有关。结合这一情况，为了进一步确保特种小卫星通信系统在干扰和衰落环境下可靠地运行，在已有的数字匹配滤波器（DMF）研究基础上，本论文提出了采用自适应门限技术。由于自适应门限技术在信噪比非常低的环境中性能都不佳，本文还提出了运用时域1D Rake接收和空时2D Rake接收技术先改善接收信号质量，再结合自适应门限技术，以期能为改善特种小卫星扩频通信系统的性能提供有价值的参考。下面是本文在“特种小卫星扩频系统”课题中所完成的主要研究工作： （1）详细分析了固定门限时，基于DMF的接收系统在非衰落信道和衰落信道（非频率选择性和频率选择性衰落信道）中混有各种噪声和干扰时的符号差错性能及捕获性能；利用状态转移图获得了基于DMF的捕获系统的平均捕获时间与判决门限的关系式；通过数值分析，揭示了系统采用固定的判决门限不能适应信号衰落及干扰等信道环境的动态变化，反映出的问题是虚警率高或漏检率大。（2）提出了两种自适应门限算法。针对固定的判决门限设置不当会增大系统的虚警概率，采用了恒虚警门限设置法；恒虚警门限设置法需要进行功率估计，本文在介绍了几种经典功率估计方法后，重点讨论了基于有序统计（OS）和剔除平均（TM）的最大选择恒虚警（OSTMGO-CFAR）检测器，该检测器在均匀背景及多目标和杂波边缘引起的非均匀背景中均具有很好的检测性能。由于恒虚警门限设置法没有充分利用信号能量导致信号的检测概率低，为了使接收机能够有效接收，当输入信号幅度的动态范围很大时，提出了运用自动电平控制电路（ADTLC）来设置门限；进一步分析了ADTLC电路在各种噪声和干扰环境下都能够同时提供虚警概率和检测概率的判决条件。如果影响系统性能的主要因素是单频<WP=6>连续波干扰或脉冲干扰，提出了利用中值滤波器辅助数字匹配滤波器的捕获方法（MF+MeF法），MF+MeF法能够彻底消除单频连续波干扰或脉冲干扰对系统性能的影响。不过在信噪比非常低的环境中，自适应门限技术的性能都不佳。（3）由于卫星信道为频率选择性衰落信道，接收信号中存在多径衰落。如果采用常规的接收机，没有充分利用多径分量，故接收信号的信噪比很低。本文在1D Rake接收机的理论基础上，给出了一种具有工程实现价值的基于DMF的准最佳接收方案。数值分析说明，当接收的多径分量路数超过4时，该方案能够较好抵抗多径衰落。（4）特种通信系统中，同信道干扰（CCI）和多径衰落通常一起存在，而时域Rake接收机对CCI无能为力。为了同时抑制CCI和多径衰落，本文提出在特种通信系统的接收装置中运用空时2D Rake接收技术。该技术采用天线阵列在空域收集期望用户所有信息，经过空域处理，形成期望用户的多径信号分量，同时消除CCI；期望用户的多径信号分量在时域进行Rake合并。2D Rake接收机结合了空域和时域的优点，是抑制CCI和多径衰落的有效方法，能明显提高接收信号的信噪比。本文介绍了利用CDMA信号的恒模特性以及用户已知的扩频码来确定2D Rake接收机的加权系数，使用这样一种综合技术构成的算法称为最小二乘解扩重扩恒模算法（空时LS－DRCMA）。空时LS－DRCMA的优点是不需要训练序列和避免了权矢量的正交化，降低了运算量，且性能好于除了空时最小均方误差算法（ST-MMSE）外的其它算法。（5）为了分析空时2D Rake接收机的性能，采用了等效空域滤波器。利用空域滤波器把干扰等效处理以及阵列增益获得了空时2D Rake接收机误码性能的数学模型；还利用空域滤波器和状态转移图，对空时2D Rake接收机的捕获性能进行了深入分析。 （6）给出了硬件实验系统。用FPGA实现了自适应门限算法和时域Rake接收。给出了大量的测试结果。更多还原

【Abstract】 When designing a special type small satellite (STSS) communication system with spread-spectrum technology, the fading and the interference must be considered carefully. The satellite system is doomed to suffer from the multipath fading in the presence of little elevation. Because the speed of the satellite comparing to it of the ground station is fast, the high Doppler shifts may lead to fast fading of the signal. There are many interference sources in the transmission channels of the low-orbit satellite communications, such as the narrow band interference, the barrage interference, the continuous carrier interference, the pulse interference and the multi-access interference. Because the existing of these interferences and fading, the received signal will be characterized by very low signal to noise (SNR) and large amplitude dynamics of the input signal. When it was used in the actual channel, the STSS communication system can’t operate properly with slow synchronization. Through analyzing these problems, the author found the reason included the system using the fixed threshold except large amplitude dynamics of the input signal.For efficient operation of the STSS communication system under the environments full of fading and interferences, the adaptive threshold must be adopted. The adaptive decision thresholds are offered based on the studies of the digital matched-filter (DMF) in this paper. Aims at the bad effect of the adaptive decision thresholds under the very low SNR, the author give a new means which combines the adaptive decision threshold with Rake receiver or 2D-Rake receiver. By this way, we can obtain the optimum performance of the STSS communication system. The study of the STSS communication system is as follows:(I) The author fully analyzed the bit error ratio (BER) performance and acquisition performance over non-fading channel and fading channel (including non-frequency selective and frequency selective fading channel) with noise and interference when the system adopts fixed threshold. Using the state transition diagram, the analytic expression of the mean acquisition time of the PN code acquisition by one dwell decision approach is derived. Numerical analysis is also given. It is shown that the system can’t operate properly at fixed threshold.(II) Two adaptive algorithms about the decision threshold setting are presented in this <WP=8>paper. In constant false alarm (CFAR), the case of the probability of a false alarm is fixed independently of the SNR. In order to keep constant false alarm, the system needs to estimate the noise power to set an adaptive threshold. After introducing some classic approaches for noise power estimation, a new CFAR detector (OSTMGO) based on ordered statistics (OS) and trimmed mean (TM) is proposed in this paper. The detection performance of OSTMGO is good both in homogeneous background and in non-homogeneous environment caused by strong interfering targets and clutter edges. For efficient operation of the receiver the threshold must be automatically controlled to provide the decision conditions where the sync false alarm probability and sync correct detection probability even in the case of large amplitude dynamics of the input signal. If they are main ingredients to deteriorate the system performance, the influence of the continuous carrier interference and the pulse interference can be eliminated almost totally, by using a median filter as an assistant of the DMF. (III) Because the satellite communication channel is modeled as frequency selective fading, the communication is doomed to suffer from the multipath fading. The performance of the conventional receiver is bad over the frequency selective fading channel because it utilizes only one of multipath components. Therefore, a creative sub-optimum diversity receiver based on the theory of Rake receiver is proposed. The results of emulation indicate that the Rake receiver can largely eliminate the deleterious effects of the multi-path transmission when the received multi-path signal number exceeds更多还原