SPREAD SPECTRUM CODE SYNCHRONIZATION USING SIGNAL TRANSITIONS
Lau, Floyd Larb Fore
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Direct Sequence (DS) Spread Spectrum (SS) techniques are finding more frequent use in today's commercial communication systems. Serial search is a widely used scheme for DS SS code phase acquisition. The problem of serial search is that it requires long acquisition time which results in a long set-up time before the communication link is useable. In this thesis, a new acquisition scheme called Rapid Acquisition by Accumulation of Transition Sequence (RAATS) is introduced. The RAATS system detects and accumulates the transitions of the transmitted pseudonoise (PN) sequence. A code phase estimate is made by matching a periodic pattern in the accumulated transition sequence. The RAATS method is nearly insensitive to polarity reversals of the data and allows fast acquisition in presence of high noise. Prototypes of both the serial search and the RAATS acquisition scheme were successfully implemented and measured for their acquisition time performance. Source data at 2400 bits/s was spread by a 1023 chips/cycle maximal length PN sequence. The chip rate was set at 32 times the data rate. Test results indicate that the RAATS system has a negative exponential acquisition time probability density function. Significant improvement in acquisition time is achieved by the RAATS system when compared with a serial search acquisition system.