Trellis decoder for decoding data stream including symbols coded with multiple convolutional codes

A trellis decoder for decoding symbols within a stream of symbols, said stream comprising symbols of a first type and a second type, said symbols of said first type encoded using a first convolutional code, said symbols of said second type encoded using a second convolutional code, said trellis decoder comprising:

• a path metric calculator for calculating a path error metric for a path through each state of the trellis decoder along a first trellis including a previous path error metric for that path and a minimum incremental error metric for at least two arriving paths for said each state;
• path metric registers for storing path metrics for each state along said first trellis;
• path memory operable to store a path indicator of each leg of each path associated with the minimum incremental error for that state along said first trellis, for arriving symbols of said second type, said path memory being further operable to store states of said trellis decoder along each of said paths along said first trellis for arriving symbols of said second type, arriving immediately prior to symbols of said first type along each of said paths;
• a path trace-back calculator in communication with said path memory for using said stored path indicators and said stored states to trace back along a path to associated with a minimum path error metric to decode a symbol in said stream.

In a trellis decoder, a method of trellis decoding symbols within a stream of symbols, said stream comprising symbols of a first type and a second type, said symbols of said first type encoded using a first convolutional code, said symbols of said second type encoded using a second convolutional code, said method comprising:

• a. calculating an incremental error metric for each leg of at least two arriving paths for each state of said trellis decoder;
• b. calculating a path error metric for a path through each state of said trellis decoder including a previous path error metric for that path and the minimum incremental error metric for said at least two arriving paths for said each state;
• c. for arriving symbols of said first type, storing states of said trellis decoder along each of said paths as those states existed immediately prior to symbols of said second type along each of said paths;
• d. for arriving symbols of said second type, storing in memory a path indicator of each leg of each path associated with the minimum incremental error for that state;
• e. using said stored path indicators and said stored states to determine a maximum likelihood path along a trellis defined by said trellis decoder to trace back along one of said candidate paths to decode a symbol of said second type in said stream.