Method and apparauts for symbol decoding

A communications system, comprising:

• a transmitter having an encoder for trellis encoding a sequence of symbols and transmitting the trellis encoded symbols over a communications channel;
• a receiver for receiving the trellis encoded symbols transmitted over the communications channel and having a trellis decoder including electronic circuitry for performing the steps of:
• (a) developing a trellis structure of nodes and branches for decoding a sequence of symbols received on the communications channel, each set of nodes in the trellis corresponding to a fixed number of states representing possible values of a received symbol at one time instant, each branch defining a specific transition between nodes at different time instants and having an associated branch weight, wherein a linkage of branches between nodes in the trellis defines a path through the trellis that represents a possible symbol sequence;
• (b) determining a current condition of the communications system;
• (c) determining a variable number of survivor paths through the trellis, wherein the value of the variable number of survivor paths depends on the condition of the communications system determined in step (b) and may differ from the fixed number of states in the trellis and wherein the value of the variable number of survivor paths is less than the fixed number of states in the trellis when the condition of the communication system satisfies a predetermined condition;
• and
• (d) decoding the sequence of received symbols using the the variable number of survivor paths survivor paths.

In a communications system, a method for decoding encoded symbols transmitted over a communications channel, comprising the steps of:

• (a) developing a trellis structure of nodes and branches for decoding a sequence of symbols received on the communications channel, each set of nodes in the trellis corresponding to a fixed number of states representing possible values of a received symbol at one time instant, each branch defining a specific transition between nodes of nodes sets at different time instants and having an associated branch weight, wherein a linkage of branches between nodes in the trellis defines a path through the trellis that represents a possible symbol sequence;
• (b) determining a quality of the communications channel;
• (c) comparing the channel quality to a threshold;
• (d) determining a variable number of survivor paths through the trellis,
• (e) if the compared channel quality equals or exceeds the threshold, setting the value of the variable number of survivor paths to a small value less than the number of fixed states in the trellis, and
• (f) performing a trellis decoding stage of the sequence of received symbols using only the smaller value of survivor paths, and returning to step (b)
• if the compared channel quality is less than the threshold, setting the value of M to a larger value and performing one or more stages of trellis decoding before returning to step (c).

In a communications system, a method comprising:

• encoding a sequence of symbols;
• separating portions of the sequence of encoded symbols in time or frequency;
• transmitting the separated portions;
• receiving the separated portions and combining the separated portions into the sequence of symbols;
• and
• trellis decoding the received signal using a variable number of trellis survivor paths where the variable number of survivor paths used in the trellis decoding is reduced as a result of the separating step.