At the UMTS physical layer, convolutional codes are used to perform channel encoding for AMR voice services and power control is used to ensure AMR voice quality.

As shown in above figure, the UE uses convolutional codes to encode AMR voice data and sends the encoded data to the NodeB. The NodeB decodes the received data and performs a cyclic redundancy check (CRC) on the decoded data. The NodeB then sends the decoded data and a CRC indicator (CRCI) to the RNC. The RNC calculates the block error rate (BLER) for outerloop power control based on all the CRCIs received. The RNC also sends the decoded data and the CRCI to the Core Network (CN) for AMR voice data decoding (source decoding).

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