(A) study on digital synchronization loop for tracking pseudo noise sequences

Abstract

Spread spectrum communication has many advantages such as inherent message privacy and security, interference rejection (anti-jamming), and strong noise immunity. Also, the spread spectrum system is used in high resolution ranging, selective calling, position location, and multiple access system. In this thesis, a digital synchronization loop for tracking pseudo noise sequences is studied. The proposed digital tracking loop is an early/late time gated tracking loop which controls the reference PN code clock toward the direction to increase the correlation value between the locally generated PN code and the received signal. For the performance analysis of the tracking system we have used a new method based on a finite absorbing Markov chain. We appended discrete dummy states in the Markov chains transition matrix to remedy the error effect due to frequency difference of the source clock frequencies between the receiver and the transmitter. As a result we could obtain exact analysis results. In our system the mean hold-in tracking time can be maintained over 10 hours at the SNR of as low as-15 dB. This result was verified by computer simulation. The direct sequence spread spectrum communication system whose processing gain is 22.2 dB has been implemented employing the proposed digital tracking loop. This system has been implemented using digital devices, and thus its circuits are simple and flexible.