Puncturing
Definition: The name of the procedure used in IS-95 to combine the power-control bits and the binary voice symbols on the forward-link traffic channels.
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© 1999 DR. ERNEST SIMO - SPACE2000
Puncturing
Application: The forward link traffic channels generate a stream of binary voice symbols at the rate of 19.2 ksym/sec. However, up to 800 times a second, a power control bit must be transmitted on the forward traffic channels to control the power transmitted by the mobiles on the reverse traffic channels. Together the binary voice symbols and the power-control bits constitute a rate of 20 ksym/sec. However, the forward link can only accommodate a rate of 19.2 ksym/sec so a procedure must be established to combine the voice and the power control messages. The combining procedure used in IS-95 calls for the loss or discard of two voice symbols in each 1.25 msec power control group in which a power control bit is transmitted. These two voice symbols are essentially cut or punctured out of the voice stream leaving a time slot for one power control bit whose duration is twice that of a voice symbol. That is, the time slot opened-up by the removal of two voice symbols in a 19.2 ksym/sec stream equals 2/(19200) or 104.166 µsec, which corresponds to a bit rate of 9600 bits/sec. The power level used to transmit the power control bit is at least as high as the level used to transmit the full-rate digital-voice bits, which are also sent at a 9600 bit/sec rate.
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Puncturing
Example: At the symbol rate of 19.2 sym/sec, a 1.25 msec power control group contains 24 symbols. Two of these 24 symbols will be punctured to make room for the power control bit. The 24 symbols correspond to 12 bits using the traffic-channel rate 1/2 code. Only 22 of these 24 symbols are actually transmitted. This increases the code rate from 12 bits/24 symbols (1/2) to 12 bits/22 symbols (.545). In his CDMA text, Viterbi states that the slight increase in code rate results in slightly degraded performance.
In IS-95, the two punctured symbols always occur in the first 17 symbols of the power control group. The first 17 voice symbols are numbered zero through 16. Exactly which two of the first 17 symbols will be punctured is randomized using four chips from the 19.2 kchip/sec output of the decimated long code. The four chips are used to calculate a random number from zero to 15. This number identifies the first symbol to be punctured. The second symbol to be punctured is this number plus one. For example, suppose the four chips are used to derive the binary number 1011, which is equivalent to decimal 11. Then the 11th and 12th voice symbols are punctured in that power control group to make room for the power control bit.
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© 1999 DR. ERNEST SIMO - SPACE2000
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