摘要
强调以开放性的最简硬件为通用平台，尽可能地用可升级、可重配置(reconfigurable)的应用软件(发送接收算法)来实现各种无线电功能的软件无线电(Software Radio或Soft-Defined Radio)，自1992年Jeo Mitola提出以来，在近几年取得了引人注目的进展，引起了包括军事通信、个人移动通信、微电子以及计算机等电子领域的巨大关注和广泛兴趣，被认为是未来通信技术的一个重要发展方向。
本文在介绍软件无线电的相关技术前提下，提出了一种适合线性软件无线电的GMSK接收机结构。GMSK调制方式因其良好的频谱效率和恒定包络特性，被广泛应用到无线通信系统中。但是GMSK是连续相位(CPM)调制，其调制相位是非线性的；现在较通用的软件无线电接收机是针对线性调制方式设计。本文依据GMSK的线性近似，提出了适合线性软件无线电的GMSK非相干接收机整体结构，以及匹配滤波器设计方法。
同步是数字接收机中最重要的组成部分之一，本文利用最大似然准则，提出了一种不需要前导字的前馈GMSK载波相位和时钟误差联合估计算法。这个算法基于GMSK的线性近似，可以用在通用的软件无线电线性结构中。在算法推导后，本文给出了这种算法的性能仿真。从仿真结果可以看出，这个算法的误差方差接近于参数估计的理论下界——修正的Cramer-Rao界，在高信噪比情况下性能比目前的其他算法更优，而且可以工作在更低采样率下，算法复杂度低，可以节省硬件资源，增加软件无线电的信号处理带宽。
数字通信的发展越来越多的依靠现代信号处理理论。周期平稳的性质被证明可以用来作平坦衰落信道下，信号的载波频偏和时钟恢复。本文在介绍衰落信道的基础上，通过进一步的推导，证明了满足一定条件下，周期平稳的频偏估计方法同样适合于频率选择性衰落信道。仿真的结果证实了这个推导的可行性。在频率选择性衰落信道下的频偏估计的性能，只比在平坦衰落信道和高斯白噪声信道下的性能略差些。
最后，本文介绍了航空数据链中的软件无线电的硬件平台的搭建。
With the very rapid advancement of wireless communications, a new challenge to this industry is the integration of multiple systems and applications on a single device. Software Radio, firstly introduced by Jeo Mitola in 1992, addresses the goal of developing transceivers that will operate with several standards and in several frequency bands on a common hardware platform. Such a platform would allow flexible and programmable transceiver operations. This type of software radio is expected to be a key technology in several application scenarios of wireless communications, including cellular telephony, military communications, aeronautic data link and etc.
The practical structure of the hardware platform of software radio is designed to cope with the linear modulation schemes, such as MPSK, OQPSK, QAM and MSK. In this thesis, we introduce a linear software radio architecture that can be extended to handle GMSK, which is a nonllnear modulation scheme. The synchronization algorithm and the design of the matched filter of this architecture suitable for GMSK signals are also studied.
Synchronization is very important in digital receivers. A new non data-aided (NDA) and all-digital algorithm is proposed for join estimation of timing and carrier phase in GMSK modulation. It is based on maximum-likelihood arguments and has a feed forward structure. The proposed algorithm is also suitable for linear software radio. Performance in AWGN channel is assessed by computer simulation. and is compared with the modified Cramer-Rao bound and other existing algorithms. Simulation results have shown that the performance of the proposed algorithm is close to MCRB and other algorithms. Moreover, the proposed algorithm can work at an over sampling rate as low as 2, implying wider bandwidth and higher transmission rate, or lower hardware resource and power consumption.
Cyclostationarity (CS) method is very useful in signal processing for communications. Frequency offset and symbol timing recovery in flat-fading channels based on cyclostationarity was introduced by Gini and Giannakis, and this method has many advantages. In this thesis, we will show that under some additional assumptions, the CS method can be extended to frequency-selective fading channels. Simulations have proved the feasibility of this extension, with only a small degraded performance.
Finally, the implementation of the hardware platform of software radio for aeronautic data link is introduced.
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