| The modulation recognition technology of communication signals is widely used in both civilian and military fields, with the primary mission of analyzing, judging out and classifying the modulation method of the intercepted communication signals. For many years, many scholars have done a lot of researches on the aspect of communication signal recognition, and proposed a lot of new recognition methods. Based on these achievements, this paper carried out a deep analysis and research about the present commonly used methods for modulation recognition in the AWGN ideal channel and fading channel respectively. Finally, the paper studied a FPGA based realization scheme for modulation recognition systems. The primary coverage is as follows:1. Introduced the research background and present situation of the modulation recognition technology.2. Described the basic principles and characteristics of the commonly used analog modulation schemes (AM, DSB, SSB, FM) and digital modulation schemes (MASK, MFSK, MPSK), and then simulated each of these modulation schemes using the MATLAB tools.3. Researched on the preprocessing part of modulation recognition, which mainly included the carrier frequency estimation and symbol rate estimation of digital modulation signals. Then several commonly used preprocessing methods are presented.4. Studied the Nandi-Azzouz modulation recognition algorithm which is widely used at present. The complete modulation recognition process from the selective extraction of recognition characteristic parameters to classification judging of modulation schemes was investigated through a MATLAB simulation under the circumstances of AWGN ideal channel. Then we did deep analysis for several key questions such as the extraction of characteristic parameters and the choice of decision threshold for modulation signals.5. We stressed the research about the influence on modulation recognition imposed by fading circumstance in wireless channel. The performance of Nandi-Azzouz modulation recognition algorithm under fading circumstance is studied with a new method. First, the Clarke model of wireless channel was simulated with MATLAB, and the transmission rules and characteristics of mobile wireless channel was analyzed; Then, by changing the parameters of channel model, we simulated three special situations in which the channel respectively has only one of the three effects (magnitude effect, Doppler shift effect or inter-symbol interference effect) on signals; Finally, the influence on the modulation recognition process was investigated by the above three effects respectively . Through the analytical study, some conclusion was obtained, which is meaningful for the study of modulation recognition technology under fading circumstances.6. Investigated a FPGA based realization scheme for modulation recognition system and designed the FPGA implementation. The simulation results under ModelSim figured out its feasibility. |
