Multiple Access -6.Synchronization Capture and Synchronization Tracking
In order to correctly restore the received signals, it is necessary to accurately synchronize the received frequency and clock generation timing to those of the transmission side. The synchronization technology used consists of synchronization capture and synchronization tracking, and processing is conducted by their respective independent circuits.
Synchronization capture assumes that the received signals exhibit PN series generation timing. The receiver establishes PN series generation timing with a precision of better than one chip (at the spread code transmission speed), and uses the timing to control the reception-side PN series generator.
For DS signals, there are two typical synchronization capture methods: one is called serial search in which the PN series on the receiver side is initiated at any timing and gradually shifted little by little during reception; the other is called matched filter in which synchronization capture is instantaneously established by detecting the correlation value of signals via a correlator that incorporates CCD and SAW elements.
Synchronization tracking establishes control over the tracking process so that the spread series on the reception side does not lead to any time lags with the timing information obtained from synchronization capture.
For synchronization capture it is necessary to maintain high precision for long periods so that the spread series on the reception side does not cause any modulation- or noise-induced time lags. Accordingly, unlike synchronization tracking, a loop circuit is used. Delay lock loops and tau dither loops are often used for DS circuits.