Frame Quality Indication or Cyclic Redundancy Coding
Application: The forward and reverse traffic channels in IS-95 employ CRC to allow quick assessment of frame quality at the receiver. Measurements of frame quality are based on satisfying certain CRC relationships. A frame error is declared when the bits in a frame at the receiver fail the CRC relation test. A frame error rate of 1% or less is generally required for reliable voice recovery. The Frame Quality Indication (FQI) is also used to select frames to be passed to the Vocoder on reverse links in soft handoff. Full-rate frames from the variable 8600 bit/sec Vocoder have 12 CRC check bits added and half-rate frames have 8 CRC check bits added to the digital-voice bits in the frame. Frames with less than half rate have no CRC check bits added. At a frame rate of 50 per second, the 12 CRC bits produce a rate increase of 600 bits/sec. The 8 CRC bits per frame increase the bit rate by 400 bits/sec.
Frame Quality Indication or Cyclic Redundancy Coding
Example: The CRC used in IS-95 adds either 12 or 8 check bits. Using Viterbi's result noted above, the probability of detecting one or more error in a frame equals about .99976 using 12 check bits and about .9961 using 8 check bits. Hence, with probability near one, FQI checks allow IS-95 receivers to avoid sending frames containing bit errors to their Vocoders for voice recovery. The procedures for adding the CRC check bits are specified in detail in the IS-95 documentation. The CRC relation test which must be satisfied at the receiver follows directly from the CRC polynomial used as the basis for adding the CRC check bits at the transmitter.
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