Study On The Spread Spectrum Soft Receiver

In this thesis, a kind of software method is researched to realize the functions of fast acquisition , tracking and demodulation of PCM-CDMA-BPSK intermediate frequency signals.Conventional research on spread spectrum signals is in the case of ideal conditions, such as low code rate , narrow dynamic range , narrow frequency-changing range and so on. The specialities and difficulties of spread spectrum signals in this thesis are high dynamic. high code rate. low signal-to-nosie radio. wide Doppler range. varied code length, varied code rate, which have more difficulties.Little acquisition time and high acquisition probability are required during the capturing process of spread spectrum signals. In this thesis, a kind of 2-dimention signal acquisition method is presented which combines parallel PN code phase and carrier frequency acquisition, as a result the acquisition time is shortened greatly. Besides, according to the fact that the detection of spread spectrum signals is always the key point in the receiving system. A new method is presented in this thesis by which accumulates the energy to improve the signal-to-nosie ratio of input-signals, consequently improve the detection performance.It is necessary that the receiver tracks the acquired signals steadily and credibly to ensure the successful demodulation process. In this thesis, an improved tracking loop design method is presented which adopts self-adaptation parameters and varied bandwidth tracking technique to adapt to the input signals, and improves the acquisition and tracking performance.The performance targets of acquisition and tracking: the code rate of pseudorandom signals is 1M-5MHz, the code length is 255. 511, the receiving sensitivity is -118dBm,the dynamic range is 70dB. the Doppler of carrier is 75kHz,the time that 10 targets are captured within 0.2s.