Study On M-ary Orthogonal Spread Spectrum System Based On Software-defined Radio Technology

Under circumstances like fire fighting headpiece equiped uncooled focus plane array(UFPA),building security monitoring and forest fire monitoring,electric equipments monitoring and other abominable environments, there are urgent demand for real-time wireless transmittal of infrared images. And mostly the system has a rigorous limitation of the SNR and security of image information. So an ideal solution is to transmit the infrared image using spread-spectrum schemes. Direct sequence spread spectrum system has its special merits, but under actual applications the system band is often limited, so we can't achieve too high a data rate by it. To solve the contrary of spread spectrum gain and high data rate, a new M-ary orthogonal spread spectrum system is proposed in this paper.At first, the uncooled focus plane array technology and image compressing technology are introduced, and then the principle of M-ary orthogonal spread spectrum system is discussed.Then the design and implementation of the M-ary orthogonal spread spectrum system are discussed in detail. The whole design contains two branches, one branch is for synchronization of Pseudo Noise code, and the other is for data transmittal, which is based on 16-ary orthogonal spread spectrum technology and could offer a higher data rate. The author illuminates the following issues emphatically:· The method of choosing codes for M-ary orthogonal spread spectrum system· The modulation method of variable data rate in M-ary orthogonal spread spectrum system · The acquisition and tracking method for M-ary orthogonal spread spectrum system· The automatic carrier frequency capturation and tracking circuits· The method of soft-judging correlation outputsAt last, the author makes an introduction of the hardware circuits of this system. The IF sampling circuit and FPGA's configuration scheme are demonstrated briefly. Then the author analyzes the security performance of FPGA and proposes a scheme of encrypting FPGA devices.All the research work is verified upon our experiment board, and the results shows that the whole system works well.With a deeper research into system performance under multi-user and complex channel environment and the miniaturization and modulation of the hardware system, there will be a bright future for our system in circumstances that has a high demand on data rate and spread spectrum gain at the same time.