Constellation detection in a multi-mode QAM communications system
Systems and methods for determining an unknown QPSK or QAM constellation from a set of possible received constellations are described. One method utilizes a histogram of the power of the signal after inter-symbol-interference has been minimized with a modified constant modulus algorithm equalizer. The constellation may be determined before carrier frequency and phase has been fully recovered. An unknown QPSK or QAM constellation may be identified before or after equalization using disclosed methods for analyzing an output power histogram.
1. A method for identifying a constellation of symbols in a multi-mode quadrature amplitude modulated communications system symbols, the method being performed by a modem, the method comprising: characterizing power distribution in a signal, wherein the power distribution statistically tracks occurrences of power levels at a plurality of frequencies detected in the signal; determining one or more peak occurrences of power levels within the power distribution; and identifying the constellation of symbols in a multi-mode quadrature amplitude modulated communications system symbols based on distribution of the peak occurrences.
2. The method of claim 1 , wherein the constellation is determined based on a variance of the one or more peak occurrences relative to a predefined constellation power.
3. The method of claim 1 , wherein the constellation is determined by examining a plurality of sections in a histogram of the power distribution, wherein each of the sections corresponds to a range of power levels associated with only one of a plurality of constellation candidates.
4. The method of claim 3 , wherein the plurality of constellation candidates includes a quadrature phase shift key constellation and a quadrature amplitude modulation (QAM) constellation.
5. The method of claim 4 , wherein the plurality of constellation candidates includes 16-QAM and 64-QAM constellations.
6. The method of claim 4 , wherein the plurality of constellation candidates includes a 256-QAM constellation.
7. The method of claim 1 , further comprising establishing reliability of an identified constellation by performing steps for each of a succession of constellation determinations by:
incrementing a counter when a succeeding determination confirms the identity of the constellation;
decrementing the counter when a succeeding determination identifies a different constellation; and
providing a measure of reliability based on the value of the counter.
8. The method of claim 7 , wherein the constellation is reliably identified when the counter exceeds a threshold value.
9. The method of claim 7 , wherein a counter is provided for each of a plurality of constellation candidates and wherein the constellation is reliably identified when its corresponding counter exceeds a threshold value.
10. The method of claim 1 , wherein the peak occurrences of power levels correspond to corner symbols of the constellation.
11. The method of claim 1 , wherein the constellation is identified before the signal is equalized.
12. The method of claim 1 , further comprising equalizing the signal, wherein the constellation is identified after the signal is equalized.