The Design And Implementation Of Spread Spectrum Gmsk Modem

The spread spectrum technology which is result of the military communication has incomparable advantages on confidentiality, anti-interference, low intercept and high precision measurement. Similarly, as a way of continuous phase modulation with constant envelope, GMSK(gaussian minimum frequency shift keying) has good spectrum characteristics and ber performance. The DS-GMSK is combined with the advantages of GMSK and direct spread spectrum, it has been widely used in military command communication, so the research on spread spectrum GMSK modulator and demodulator design and implementation has important significance.The paper firstly introduces the basic principle of spread spectrum communication, at the same time studies the algorithms of the carrier synchronization, symbol synchronization, demodulation and detection in GMSK system. On this basis, the DS-GMSK modulation demodulation processing system is designed. The modulator includes framing, direct spread spectrum, gaussian filtering and phase forming modules; The demodulator contains digital down-conversion, carrier synchronization, bit synchronization, noncoherent demodulation and despread spectrum modules. And then the Matlab simulation system of the DS-GMSK modem has been builded, through the simulation each module’s processing algorithms get the feasibility and performance. Algorithm simulation mainly includes the carrier synchronization algorithm based on FFT frequency measurement, symbol synchronization algorithmbased on Gardner detection, and the noncoherent demodulation algorithm with different detection. At last the paper studies the FPGA implementation of DS-GMSK modulation and demodulation system, and realizes the logical function of each module using Verilog language.The correctness of the design and FPGA implementation of DS-GMSK modulation and demodulation system is verified by the timing simulation and the test results of logic analyzer.The paper also designs the hardware of the DS-GMSK digital modulator and demodulator. The hardware adopts the software radio architecture,it includes high speed ADC,DAC,and FPGA. At first the device is be selected by the hardware requirements of system, then use Cadence tools to do circuit diagram and PCB design. Finally complete the general platform design, commissioning and performance test. This paper also introduces the related content of high-speed PCB design based on Cadence.