How Forward Error Protection in CDMA
- Uses Half-Rate Convolutional Encoder
- Outputs Two Bits of Encoded Data for Every Input Bit
Unlike many digital cellular systems, CDMA provides powerful error correction to all voice data bits. This is desirable in CDMA since the idea is to increase the occupied bandwidth (spread the data).
The forward link uses a half-rate convolutional encoder to provide error correction capabilities. This type of encoder accepts incoming serial data and outputs encoded data derived from a series of delay taps and summing nodes.
A half-rate encoder produces two output symbols for every symbol input. For the CDMA forward link, the half-rate encoder produces two 9,600 bps serial data streams when driven by a single 9,600 bps data stream.
These two 9,600 serial data streams are combined at a higher rate to produce a single 19,200 bps data stream. The resulting redundancy in the digital data after convolutional encoding imparts powerful error correction capability to the TIA CDMA cellular system.