Key Technology For All-digital Spread Spectrum Receiver

The synchronization is the key technology which affect the performance of the receiver heavily in the DS-SS (Direct-sequence Spread-spectrum) system. After achieving the synchronization of the PN code and carrier, despreading and demodulating the received DSSS signal can be executed with the local synchronized code and carrier. So, lots of energy and fund has been expended on it. There are two uncertain factor about it: the phase of the PN code and the Doppler-shift. After capturing the received signal successfully, the traditional DS receiver always uses a delay locked loop (DLL) to synchronize the PN code and then uses a Costas loop to realize the carrier synchronization. This complex closed-loop structure not only take long time to realize the synchronization, but also has the defect of"hang up". In recent years, with the development of the DS-SS and the theory of all digital receiver, the open-loop structure based forward estimation attracts more attentions.Based searching the code phase parallel and carrier frequency serially at the same time, this dissertation uses differentially coherent combining to combine the coherent result of multiple correlators to improve the SNR of the received signal, so that it can acquire the weak GPS signals, i.e. signals down to–147dBm received power. By complementing the proposed scheme with methods to estimate the fractional code delay, the acquisition unit an provide high quality delay estimates such that it can instead of the delay locked loop in the traditional DS receiver.After dispreading successfully, this dissertation introduces a method to estimate the Doppler-shift directly from some samples based Maximum Likelihood Estimation, and then revise it forwardly. The result of simulation proves that it can replace the Costas loop in the traditional DS receiver to realize the carrier synchronization.