Symbol Repetition
Definition: The process of repeating the symbols of an input data stream to produce a new output symbol stream whose symbol rate is an integer times greater than the rate of the input stream.
Symbol Repetition
Application: All the channels in IS-95 use symbol repetition except the pilot channel.
In the case of the Synchronization channel, the primary purpose in repeating the symbols to produce a 4.8 ksym/sec stream from a 2.4 ksym/sec stream appears to be to improve signal reliability by transmitting the symbols of the message twice with interleaving. Increasing the symbol rate on the Access Channels from 14.4 ksym/sec to 28.8 ksym/sec improves reliability and also produces a 28.8 ksym/sec stream, which is common to all reverse-link channels. In all other cases, the primary motivation appears to be to produce a convenient common symbol rate. The forward link, for example, repeats each symbol of the half-rate paging channel to increase the symbol rate from 9.6 ksym/sec to 19.2 ksym/sec, which is the same rate as in the full-rate paging channels. Forming the common 19.2 ksym/sec rate allows the use of one interleaver and one decimator to scramble the paging messages. In the case of half-rate paging, repeating the paging symbols also improves the reliability of the channel, which has no power control. The forward and reverse traffic channels are both greatly simplified using repetition. The four possible rates produced by the variable rate Vocoder are transformed to a common rate. This allows common circuitry following the repetition function. In the case of the traffic channels, after repetition is used to form a convenient common symbol rate, some other function is used to achieve the capacity increase associated with the voice activity factor. On the forward traffic links, the repeated symbols are transmitted but at a reduced power level which keeps the energy per bit fixed. On the reverse traffic links, the average power is reduced by eliminating in the data burst randomizer the redundant bits produced by repetition. The resulting transmitted signal has a duty cycle equal to 1, 1/2, 1/4, and 1/8 when the Vocoder bit frame rate is full-rate, 1/2, 1/4, or 1/8 respectively.
See Data Burst Randomizer.
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