Equalizer for heavily clipped or compressed communications signals
View Patent ↗Apparatus and methods mitigate a problem of equalizing communications signals that have been distorted by severe non-linearities such as clipping or harsh compression. For example, severe non-linearity occurs when signal compression or signal clipping occurs at rates above 20% of the data transmission interval. Severe non-linearities may significantly reduce system performance. Disclosed techniques selectively apply DSP equalization based on the detection of non-linearity for a present sample or one or more samples prior to the present sample. These techniques can be implemented in relatively low-cost high-speed SerDes designs to improve eye openings and reduce sensitivity to InterSymbol Interference (ISI) and to improve bit error rate (BER).
1. A method of equalization in a receiver, the method comprising:
receiving a communications signal having symbols embedded therein;
converting, by a detection circuit, the received communications signal from analog form to digital form to generate a digital received signal comprising soft symbols, wherein the digital received signal is quantized to at least 3 bits; and
selectively equalizing a current soft symbol sample based at least partly on a determination of whether or not the current soft symbol sample, a first prior soft symbol sample, or a second prior soft symbol sample have been determined by the detection circuit to have been non-linearly distorted beyond a threshold, wherein the first prior soft symbol sample is an immediately prior soft symbol sample to the current soft symbol sample, wherein the second prior soft symbol sample is an immediately prior soft symbol sample to the first prior soft symbol sample, wherein distortion beyond the threshold indicates clipping, harsh compression, or both clipping and harsh compression, wherein selectively equalizing further comprises:
using a normal coefficient c 1 for equalization when none of the current soft symbol sample, the first prior soft symbol sample, and the second prior soft symbol sample have been non-linearly distorted beyond the threshold;
using a first weighted coefficient w 1 c 1 when the first prior soft symbol sample has been non-linearly distorted beyond the threshold; and
using a second weighted coefficient w 2 c 1 when the second prior soft symbol sample has been non-linearly distorted beyond the threshold.
2. The method of claim 1 , wherein selectively equalizing further comprises:
selecting a tap value from a plurality of tap values of the coefficients, wherein the selected tap value varies depending upon whether or not non-linear distortion is indicated;
multiplying the selected tap value and at least one of the first and second prior soft symbol samples to generate a weighted prior soft symbol sample; and
summing the current soft symbol sample and the weighted prior soft symbol sample to generate an equalized current soft symbol sample.
3. The method of claim 2 , further comprising slicing the equalized current soft symbol sample to generate a hard symbol sample.
4. The method of claim 3 , wherein at least a portion of values for the plurality of tap values is determined based on eye diagnostics.
5. The method of claim 2 , wherein the current soft symbol sample is equalized using a plurality of prior soft symbol samples.
6. The method of claim 5 , wherein equalizer coefficients are selected based on a non-linear distortion status of the current soft symbol sample and the plurality of prior soft symbol samples.
7. A non-transitory computer-readable recording medium comprises computer-executable instructions stored thereon, which when executed by a processor configured to execute the computer-executable instructions, are adapted to perform the method of claim 1 .
8. An apparatus for equalization in a receiver, the apparatus comprising:
means for receiving a communications signal having symbols embedded therein;
a detection circuit for converting the received communications signal from analog form to digital form to generate a digital received signal comprising soft symbols, wherein the digital received signal is quantized to at least 3 bits; and
means for selectively equalizing a current soft symbol sample based at least partly on a determination of whether or not the current soft symbol sample, a first prior soft symbol sample, or a second prior soft symbol sample have been determined by the detection circuit to have been non-linearly distorted beyond a threshold, wherein the first prior soft symbol sample is an immediately prior soft symbol sample to the current soft symbol sample, wherein the second prior soft symbol sample is an immediately prior soft symbol sample to the first prior soft symbol sample, wherein the distortion beyond the threshold indicates clipping, harsh compression, or both clipping and harsh compression, wherein the means for selectively equalizing is configured to:
use a normal coefficient c 1 for equalization when none of the current soft symbol sample, the first prior soft symbol sample, and the second prior soft symbol sample have been non-linearly distorted beyond the threshold;
use a first weighted coefficient w 1 c 1 when the first prior soft symbol has been non-linearly distorted beyond the threshold; and
use a second weighted coefficient w 2 c 1 when the second prior soft symbol has been non-linearly distorted beyond the threshold.
9. An apparatus for equalization in a receiver, the apparatus comprising:
a detection circuit configured to receive a digital received signal comprising soft symbols, wherein the digital received signal is quantized to at least 3 bits, wherein the detection circuit is configured to determine whether a sample has been non-linearly distorted beyond a threshold, wherein distortion beyond the threshold indicates clipping, harsh compression, or both clipping and harsh compression; and
an equalizer configured to generate an equalized current soft symbol sample and to adjust equalization based at least partly on whether or not a current soft symbol sample, a first prior soft symbol sample, or a second prior soft symbol sample have been determined by the detection circuit to have been non-linearly distorted beyond the threshold, wherein the first prior soft symbol sample is an immediately prior soft symbol sample to the current soft symbol sample, wherein the second prior soft symbol sample is an immediately prior soft symbol sample to the first prior soft symbol sample, wherein the equalizer is further configured to use a normal coefficient c 1 for equalization when none of the current soft symbol sample, the first prior soft symbol sample, and the second prior soft symbol sample have been non-linearly distorted beyond the threshold, wherein the equalizer is further configured to use a first weighted coefficient w 1 c 1 when the first prior soft symbol has been non-linearly distorted beyond the threshold, and wherein the equalizer is further configured to use a second weighted coefficient w 2 c 1 when the second prior soft symbol has been non-linearly distorted beyond the threshold.
10. The apparatus of claim 9 , wherein the equalizer comprises:
a multiplexer circuit configured to receive a plurality of tap values from the coefficients, wherein the plurality of tap values vary in value;
an equalizer control block configured to control the multiplexer circuit to select a tap value based on a pattern of non-linear distortion observed in the current soft symbol sample and at least one of the first and second prior soft symbol samples;
a multiplier circuit configured to multiply the selected tap value from the multiplexer circuit and the at least one of the first and second prior soft symbol sample to generate a weighted prior soft symbol sample; and
a summing circuit configured to sum the current soft symbol sample and at least the weighted prior soft symbol sample to generate the equalized current soft symbol sample.
11. The apparatus of claim 10 , further comprising a slicer configured to slice the equalized current soft symbol sample to generate a hard symbol sample.
12. The apparatus of claim 10 , wherein at least a portion of values for the plurality of tap values is determined based on eye diagnostics.
13. The apparatus of claim 10 , wherein the equalizer is further configured to equalize the current soft symbol sample with a plurality of prior soft symbol samples.
14. The apparatus of claim 13 , wherein the equalizer is further configured to apply equalizer coefficients based on a non-linear distortion status of the current soft symbol sample and the plurality of prior soft symbol samples.