Estimation of symbols of transmitted signals using hypotheses

1. A method of estimating a symbol that has been transmitted into a channel connecting a modulator for producing modulation symbols and an equaliser, wherein a channel estimate describes said channel and the method comprises:

a. calculating a metric for each of a number of hypotheses of a sequence commencing with the symbol under estimation followed by a number of subsequently transmitted symbols, each symbol in the sequence occupying a respective node of the sequence; and

b. deeming the symbol at the beginning of the hypothesis that has the best metric to be the symbol under estimation;

wherein calculating a metric for a given hypothesis comprises:

c. calculating a discrepancy metric at each node of the given hypothesis; and

d. combining the discrepancy metrics of nodes in the given hypothesis to produce the metric;

and wherein a discrepancy metric for a given node of a given hypothesis is an assessment of the difference between:

e. a sample received from said channel at a time point corresponding to said given node;

f. and a sample that would be received at that time point if said channel estimate were acting on a model signal comprising the symbols of the given hypothesis up to the given node preceded by any estimates of symbols transmitted into the channel immediately prior to said time point as may be required to fill the estimated channel.

2. A method according to claim 1 , wherein the channel comprehends a physical channel between a transmitter containing the modulator and a receiver containing the equaliser and a filtering process for conditioning multipath delays embodied by the physical channel.

3. A method according to claim 2 , wherein the filtering process is a feed forward filtering process acting on samples received from the physical channel.

4. A method according to claim 1 , wherein the number of hypotheses is determined by a pruning process.

5. A method according to claim 4 , wherein the pruning process comprises eliminating those possible hypotheses whose discrepancy metrics at a particular node are the worst.

6. A data carrier containing a program which when run on a data processor will cause the data processor to perform the method of claim 1 .

7. Apparatus for estimating a symbol that has been transmitted into a channel connecting a modulator for producing modulation symbols and an equaliser, wherein a channel estimate describes said channel and the apparatus comprises:

a) a calculator arranged to calculate a metric for each of a number of hypotheses of a sequence commencing with the symbol under estimation followed by a number of subsequently transmitted symbols, each symbol in the sequence occupying a respective node of the sequence; and

b) a selector arranged to deem the symbol at the beginning of the hypothesis that has the best metric to be the symbol under estimation;

wherein said calculator is arranged to:

c) calculate a discrepancy metric at each node of the given hypothesis; and

d) combine the discrepancy metrics of nodes in the given hypothesis to produce the metric;

and wherein said calculator is arranged to produce a discrepancy metric for a given node of a given hypothesis in the form of an assessment of the difference between:

e) a sample received from said channel at a time point corresponding to said given node; and

f) a sample that would be received at that time point if said channel estimate were acting on a model signal comprising the symbols of the given hypothesis up to the given node preceded by any estimates of symbols transmitted into the channel immediately prior to said time point as may be required to fill the estimated channel.

8. Apparatus according to claim 7 , wherein the channel comprehends a physical channel between a transmitter containing the modulator and a receiver containing the equaliser and a filtering process for conditioning multipath delays embodied by the physical channel.

9. Apparatus according to claim 7 , wherein the filtering process is a feed forward filtering process acting on samples received from the physical channel.

10. Apparatus according to claim 7 , wherein said calculator is arranged to determine said number of hypotheses through pruning a set of possibilities for the hypotheses.

11. Apparatus according to claim 10 , wherein said calculator is arranged to prune said set through eliminating those possible hypotheses whose discrepancy metrics at a particular node are the worst.