Receiver using pseudo partial response maximum likelihood sequence detection

1. A receiver comprising:

a decision element that derives initial symbol decisions from samples of an equalized receive signal;

a difference element that calculates a full-response error between each sample of the equalized receive signal and a level of a corresponding initial symbol decision;

a filter that converts the full-response errors into partial response errors; and

a reduced-complexity maximum likelihood sequence detector (rMLSD) that derives final symbol decisions by evaluating state metrics only for each initial symbol decision and its competing symbol decision, the rMLSD comprising:

calculation circuitry that combines the partial response errors with initial symbol decisions and with associated competing symbol decisions to determine corresponding branch metrics, and that sums each branch metric with an associated state metric to obtain path metrics; and

comparators that determine a minimum path metric for each initial symbol decision and each competing symbol decision.

2. The receiver of claim 1 , wherein the equalized receive signal is produced by a feedforward equalizer that diminishes a receive signal's intersymbol interference.

3. The receiver of claim 1 , further comprising: a circuit that uses the full-response errors to determine a competing symbol decision for each initial symbol decision.

4. The receiver of claim 1 , wherein the decision element determines a competing symbol decision for each initial symbol decision.

5. The receiver of claim 1 , wherein the rMLSD further comprises:

a pair of copy-shift registers that respond to outputs of the comparators to assemble a most likely symbol decision sequence ending in that symbol decision and that competing symbol decision.

6. A receiver comprising:

a feedforward equalizer that diminishes a receive signal's intersymbol interference to produce an equalized receive signal;

a decision element that derives initial symbol decisions from samples of the equalized receive signal;

a filter that converts the equalized receive signal into a partial response signal; and

a reduced-complexity maximum likelihood sequence detector (rMLSD) that derives final symbol decisions by evaluating state metrics only for each initial symbol decision and its competing symbol decision, the rMLSD including:

calculation circuitry that combines the partial response signal with initial symbol decisions and with associated competing symbol decisions to determine corresponding branch metrics, and that sums each branch metric with an associated state metric to obtain path metrics; and

comparators that determine a minimum path metric for each initial symbol decision and each competing symbol decision.

7. The receiver of claim 6 , further comprising:

an element that determines a sign of an equalization error between each sample of the equalized receive signal and a level of a corresponding initial symbol decision; and

a circuit that uses the sign to determine a competing symbol decision for each initial symbol decision.

8. The receiver of claim 6 , wherein the decision element determines a competing symbol decision for each initial symbol decision.

9. The receiver of claim 6 , wherein the rMLSD further includes:

a pair of copy-shift registers that respond to outputs of the comparators to assemble a most likely symbol decision sequence ending in that symbol decision and that competing symbol decision.

10. A receiving method comprising:

deriving initial symbol decisions from samples of an equalized receive signal;

calculating a full-response error between each sample of the equalized receive signal and a level of a corresponding initial symbol decision;

filtering the full-response errors to obtain partial response errors; and

using a reduced-complexity maximum likelihood sequence detector (rMLSD) to determine final symbol decisions by evaluating state metrics only for each initial symbol decision and its competing symbol decision, wherein the rMLSD performs said determining by:

combining the partial response errors with initial symbol decisions and with associated competing symbol decisions to determine corresponding branch metrics;

summing each branch metric with an associated state metric to obtain path metrics; and

determining a minimum path metric for each initial symbol decision and each competing symbol decision.

11. The receiving method of claim 10 , further comprising producing the equalized receive signal by diminishing intersymbol interference in a receive signal.

12. The receiving method of claim 10 , further comprising: using the full-response errors to determine a competing symbol decision for each initial symbol decision.

13. The receiving method of claim 10 , wherein said deriving includes determining a competing symbol decision for each initial symbol decision.

14. The receiving method of claim 10 , wherein the rMLSD further performs said determining by:

assembling a most likely sequence of symbol decisions that ends with that initial symbol decision; and

assembling another most likely sequence of symbol decisions that ends with that competing symbol decision.

15. A receiving method comprising:

diminishing a receive signal's intersymbol interference to produce an equalized receive signal;

deriving initial symbol decisions from samples of the equalized receive signal;

filtering the equalized receive signal to obtain a partial response signal; and

using a reduced-complexity maximum likelihood sequence detector (rMLSD) to determine final symbol decisions by evaluating state metrics only for each initial symbol decision and its competing symbol decision, wherein the rMLSD performs said determining by:

combining the partial response signal with initial symbol decisions and with associated competing symbol decisions to determine corresponding branch metrics;

summing each branch metric with an associated state metric to obtain path metrics; and

determining a minimum path metric for each initial symbol decision and each competing symbol decision.

16. The receiving method of claim 15 , further comprising:

determining a sign of an equalization error between each sample of the equalized receive signal and a level of a corresponding initial symbol decision; and

combining each sign with the corresponding initial symbol decision to determine a competing symbol decision.

17. The receiving method of claim 15 , wherein said deriving includes determining a competing symbol decision for each initial symbol decision.

18. The receiving method of claim 15 , wherein the rMLSD further performs said determining by:

assembling a most likely sequence of symbol decisions that ends with that initial symbol decision; and

assembling another most likely sequence of symbol decisions that ends with that competing symbol decision.

19. A semiconductor intellectual property core that generates circuitry comprising:

a feedforward equalizer that diminishes a receive signal's intersymbol interference to produce an equalized receive signal;

a decision element that derives initial symbol decisions from samples of the equalized receive signal, each initial symbol decision having a competing symbol decision;

a filter that applies a partial response to the equalized receive signal or to an equalization error signal to produce a filter output; and

a reduced-complexity maximum likelihood sequence detector (rMLSD) that derives final symbol decisions by evaluating state metrics only for each initial symbol decision and its competing symbol decision, the rMLSD including:

calculation circuitry that combines the filter output with initial symbol decisions and with associated competing symbol decisions to determine corresponding branch metrics, and that sums each branch metric with an associated state metric to obtain path metrics;

comparators that determine a minimum path metric for each initial symbol decision and each competing symbol decision; and

a pair of copy-shift registers that respond to outputs of the comparators to assemble a pair of most likely symbol decision sequences ending in each symbol decision and its competing symbol decision.