Methods and circuits for decision-feedback equalization with early high-order-symbol detection
A PAM-4 DFE receives an input signal distorted by inter-symbol interference (ISI) and expressing a series of symbols each representing one of four pulse amplitudes to convey two binary bits of data per symbol. High-order circuitry resolves the most-significant bit (MSB) of each two-bit symbol, whereas low-order circuitry 115 resolves the immediate least-significant bit (LSB). An immediate value of the MSB is used to select a set of ISI offsets used to resolve the LSB. Resolved values of the prior values of the MSB and LSB are then used to select the ISI offset for the immediate symbol.
1. A decision-feedback equalizer (DFE) comprising:
an input node configured to receive an input signal expressing a series of symbols, each of the symbols representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value;
high-order circuitry coupled to the input node to receive the series of symbols, the high-order circuitry configured to resolve the high-order bit of the immediate symbol; and
low-order circuitry coupled to the input node to receive the series of symbols, the low-order circuitry configured to resolve the low-order bit of the immediate-symbol value based on the high-order bit of the immediate-symbol value and the prior-symbol value.
2. The DFE of claim 1 , the low-order circuitry configured to resolve the low-order bit of the immediate-symbol value further based on the high-order bit of the prior-symbol value.
3. The DFE of claim 2 , the low-order circuitry configured to resolve the low-order bit of the immediate-symbol value further based on the low-order bit of the prior-symbol value.
4. The DFE of claim 1 , the high-order circuitry configured to resolve the high-order bit of the immediate-symbol value based on the prior-symbol value.
5. The DFE of claim 4 , the high-order circuitry configured to resolve the high-order bit of the immediate-symbol based on the high-order bit of the prior-symbol value.
6. The DFE of claim 1 , the low-order circuitry comprising a first low-order multiplexer configured to select between alternatively offset immediate-symbol levels responsive to the high-order bit of the immediate-symbol value.
7. The DFE of claim 6 , the low-order circuitry further comprising low-order samplers configured to sample the selected alternatively offset immediate-symbol level with low-order offsets to acquire speculative low-order samples of the immediate symbol.
8. The DFE of claim 7 , the low-order circuit further comprising a second low-order multiplexer configured to select between the speculative low-order samples responsive to the prior-symbol value.
9. The DFE of claim 8 , the second low-order multiplexer configured to select between the speculative low-order samples further responsive to the low-order bit and the high-order bit of the prior-symbol value.
10. The DFE of claim 1 , wherein the high-order bit is a most-significant bit and the low-order bit is a least-significant bit.
11. A method for resolving an input signal expressing a series of symbols, each of the symbols expressed over a symbol time and representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value, the method comprising:
sampling the immediate symbol to produce first and second tentative high-order samples;
sampling the immediate symbol to produce first and second tentative low-order samples;
selecting one of the first and second tentative high-order samples as the high-order bit of the immediate-symbol value based on the prior-symbol value; and
selecting one of the first and second tentative low-order samples based on the high-order bit of the immediate-symbol value.
12. The method of claim 11 , wherein the selected one of the first and second tentative low-order samples is an analog sample.
13. The method of claim 12 , further comprising applying offsets to the analog sample to provide tentative analog samples.
14. The method of claim 13 , further comprising sampling the tentative analog samples to obtain tentative low-order bits of the immediate symbol.
15. The method of claim 14 , further comprising selecting between the tentative low-order bits of the immediate symbol responsive to the prior-symbol value.
16. The method of claim 11 , wherein the input signal expresses the series of symbols at a symbol rate, the method further comprising sampling the symbols at a slice rate less than the symbol rate.
17. A receiver comprising:
an input node configured to receive an input signal expressing a series of symbols, each of the symbols representing a symbol value having a high-order bit and a low-order bit, the series of symbols including an immediate symbol representing an immediate-symbol value and a prior symbol representing a prior-symbol value;
high-order circuitry configured to select the high-order bit of the immediate-symbol value; and
means for selecting the low-order bit of the immediate-symbol value based on the high-order bit of the immediate-symbol value.
18. The receiver of claim 17 , the means for selecting further selecting the low-order bit of the immediate symbol value based on the low-order bit of the prior-symbol value.
19. The receiver of claim 18 , the means for selecting further selecting the low-order bit of the immediate symbol value based on the high-order bit of the prior-symbol value.