Research And Implementation Of Modulation And Demodulation Algorithm For Long Code Spread Spectrum Signal

Spread spectrum communication systems are provided with superior advantages in stronger narrow band interference suppression ability, multiple access communication achievability, security and confidentiality, digitization and analogue compatibility and so on compared with traditional ones. This paper is researched on modulation and demodulation algorithms of direct sequence spread spectrum signals using long PN code sequence and FPGA realization based on Quartus Ⅱ designing platform of Altera.Firstly, algorithms of PN code acquisition, tracking and carrier synchronization are researched by means of simulation and analysis. The matched filter algorithm is used to accomplish code acquisition, the delay locking loop algorithm is utilized to maintain code tracking, and the Costas loop with hard decision is applied for carrier synchronization, which is able to decrease hardware complexity.Then, the whole realization sheme of modulation and demodulation alogrithms based on FPGA is designed, realization measures and details of each function block are discussed explicitly as well. Corresponding simulation waveforms using Modelsim and real-time waveforms obtained by Signaltap during board-level debugging are given out as well. Besides, the bit error (BER) curve is drawn out with data acquired by system test. Results indicate the great performance of algorithms used in this paper.Finally, a fast acquisition method of long PN code sequence combining FFT and partial matched filtering is elaborated, whereas long spread spectrum sequence is Doppler sensitive. Considering narrow band interference of spread spectrum communication, a transform domain suppression algorithm based on FFT is utilized and simulated. These two algorithms are proved of great performance. Meanwhile, the FPGA realization of these two algorithms are also accomplished.