Research And FPGA Implementation Of Modulation And Digital Down Conversion In Cognitive Frequency Hopping System

With the continuous development of social economy and technology, higher requirements are put forward to the effectiveness and reliability of communication whether in civilian or military fields. And whether it can realize high efficiency, high reliability and high security communication has become an important indicator to measure the performance of a communication system. Owing to its excellent anti-jamming, anti-interception capabilities and easy networking, frequency hopping(FH) communication has been widely used in the field of military communication and civil communication. However, with the increasing demand for the communication and channel electromagnetic environment increasingly complex and changeful, and interference technologies constantly growing, the traditional frequency hopping communication has been unable to meet the requirements of modern communication, especially the military communications. Cognitive frequency hopping communication technology, introduced the cognitive radio technology into the frequency hopping communication, senses channel electromagnetic environment through the spectrum sensing technology real-time, dynamically allocates the spectrum, timely changes the system parameters,so as to achieve efficient, reliable and security information transmission. And it is considered to be one of the effective ways to solve the traditional frequency hopping dilemma?In this paper, the cognitive frequency hopping communication system is studied, and A practicable design scheme of the cognitive frequency hopping communication system is introduced. Then, we focus on the researching of the modulation and digital down conversion technologies which are applied in the cognitive frequency hopping system, and then implement the two parts in FPGA.Contraposing the special requirements for bit error performance and spectrum efficiency in this system, TCM modulation and CPM modulation technologies are researched emphatically. TCM modulation make the error correction coding and modulation as a whole to consider for the best design, and can effectively improve the system ber performance. CPM modulation is a kind of continuous phase modulations. Characteristics of continuous phase, constant envelope and memory between symbols, make it have high spectrum efficiency, not nonlinear effect, as well as good performance of error performance, and very suitable for occasions with high transmission efficiency and high error performance requirements. Then, the TCM8 PSK, 8CPFSK and GMSK modulations applied in this system are studied particularly and realized in FPGA.Digital down converter is an important part of digital receiver. In this paper, the digital down conversion technologies are studied. Considering the system is a broadband and high sampling speed receiver, this paper designs a multi-level filters architecture based on polyphase structure. Finally, the digital down converter is implemented in FPGA, and meets the system requirements.