Multiplexing and Demultiplexing
Application:
IS-95 uses Time Multiplexing (TM) on both the forward and reverse links. That is, each individual signal to be transmitted is given a time segment or slot which does not overlap with the slot used by any other signal going to the same receiver. On the forward link on traffic channels TM is used to send digital voice traffic, power control bits, and signaling traffic. The reverse link traffic channels use TM primary digital voice traffic and signaling traffic. As an option, IS-95 may send secondary digital traffic TM with the primary digital voice traffic.
On the forward link, the power control bits replace two coded symbols of the primary digital voice stream. This replacing of voice symbols causing these voice symbols not to be transmitted is called puncturing.
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© 1999 DR. ERNEST SIMO - SPACE2000
Multiplexing and Demultiplexing
Example:
The IS-95 links when operating at the 9600 bit/sec rate transmit 172 bits per frame at a frame rate of 50 frames/second. The rate of 172(50) or 8600 bits/sec is increased to 9600 bits/sec by adding 12 frame quality bits and 8 convolutional encoding tail bits per frame. When the Vocoder identifies a half-rate frame, the rate may drop to 4800 bits/sec. At the 4800 bit/sec rate the Vocoder provides 80 bits per frame yielding 4000 bits/sec with the increase to 4800 bits/sec provided by 8 frame quality bits and 8 encoding tail bits per frame. However, if a signaling message must be sent to a particular user when a half-rate frame to that user occurs, the transmitter may TM the primary and traffic messages together and send a frame having a rate of 9600 bits/sec instead of 4800 bits/sec. In this case, all 80 Vocoder bits and 88 signaling bits are sent for a total of 168 bits or a rate of 168(50) or 8400 bits/sec. The usual 20 frame quality and tail bits and four other over-head bits and are then added to bring the rate up to 9600 bits/sec.
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© 1999 DR. ERNEST SIMO - SPACE2000
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