Synchronization
Application:
Whenever a symbol stream is combined with a same or higher rate Walsh-chip word or PN-chip sequence at the transmitter for scrambling, spreading, or addressing; the combining process must eventually be reversed or undone at the receiver to recover the symbol stream. In each case, the signal s(t) is in effect multiplied by a wideband signal PN(t) and the transmitted signal is of the form PN(t)s(t). After propagation delay, the received signal is of the form PN(t-t)s(t-t). At the receiver, a local replica of the PN or Walsh generator produces a signal PN(t-x), where x is an adjustable timing offset. The receiver forms the product PN(t-x)PN(t-t)s(t-t). The Synchronization circuit adjusts x so that it is close enough to t that PN(t-x)PN(t-t) is approximately PN(t-t)2, which is always one. This recovers s(t-t), the desired signal. This process applies to PN spreading and despreading, Privacy Scrambling and descrambling, and encoding and decoding of Walsh words on the forward link in IS-95.
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© 1999 DR. ERNEST SIMO - SPACE2000
Synchronization
Example:
Several schemes have been developed to synchronize two digital streams. In one method used in Direct Sequence (DS) spread spectrum systems, initially the PN generator at the receiver, while the same generator as used at the transmitter, is clocked at a slightly different rate causing the chip rate of the receiver sequence to be slightly different that the chip rate of the signal received. Hence, the PN signals PN(t-x) and PN(t-t) tend to slide by one another as time passes because there rate differ. A circuit continually monitors the product PN(t-x)PN(t-t)s(t-t). The power in this signal will be spread over a wide bandwidth whenever x and -t differ by more than about half a chip time. When x and -t differ by less that half a chip, the power in PN(t-x)PN(t-t)s(t-t) lies mostly within the bandwidth of s(t). The Synchronization circuit detects this condition and sets the chipping rate at the receiver equal to the chipping rate at the transmitter. Other tracking circuitry brings the two signals into closer alignment and keeps them aligned. When the received signal is very weak, the product PN(t-x)PN(t-t)s(t-t) may be processed an extended period of time. In such cases it is best if s(t-t) does not change over the processing interval. This is done in the IS-95-pilot case.
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© 1999 DR. ERNEST SIMO - SPACE2000
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