Constellation detection in a multi-mode qam communications system

Abstract

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.

What is claimed is: 
     
         1 . An apparatus comprising a modem configured to:
 detect a plurality of synchronization signals; and   responsive to detecting one of the plurality of synchronization signals:
 extract mode information from a frame of data corresponding to the one synchronization signal; 
 determine a current constellation by identifying one of a plurality of valid constellation code words that most closely matches a received code word corresponding to the mode information; and 
 update a confidence metric associated with a previously identified constellation by increasing the confidence metric if the current constellation matches the previously identified constellation, and decrease the confidence metric if the current constellation is different from the previously identified constellation, 
   wherein the modem is further configured to decode the frame of data using the previously identified constellation if the confidence metric exceeds a predetermined threshold.   
     
     
         2 . The apparatus of  claim 1 , wherein the modem is configured to determine a current constellation using cross-correlations performed for each of the plurality of valid constellation code words with the received code word. 
     
     
         3 . The apparatus of  claim 1 , wherein the frame of data is one of a plurality of frames received in an unequalized signal received by the modem. 
     
     
         4 . The apparatus of  claim 3 , wherein the modem is configured to determine the current constellation while recovering a carrier from the unequalized signal. 
     
     
         5 . The apparatus of  claim 3 , wherein the modem is configured to:
 calculate an error signal using a constant modulus algorithm (CMA) to converge equalizer filter taps; and   use the converged equalizer filter taps to obtain an equalized signal from the unequalized signal.   
     
     
         6 . The apparatus of  claim 5 , wherein the error signal is calculated using a scaled CMA parameter to improve equalization performance. 
     
     
         7 . The apparatus of  claim 5 , wherein the modem is configured to determine the current constellation by:
 calculating power associated with a plurality of symbols in the equalized signal; and   identifying corner symbols of the current constellation by identifying power levels corresponding to the corner symbols in the equalized signal.   
     
     
         8 . The apparatus of  claim 5 , wherein the modem is configured to determine the current constellation based on an analysis of histograms of power in the equalized signal using a probability mass function. 
     
     
         9 . The apparatus of  claim 5 , wherein the modem is configured to determine the current constellation by examining a power distribution of the equalized signal, wherein each of a plurality of sections of the power distribution corresponds to a range of power levels, each power level being associated with only one of a plurality of candidate constellations. 
     
     
         10 . The apparatus of  claim 9 , wherein the plurality of candidate constellations comprise quadrature amplitude modulation (QAM) constellations. 
     
     
         11 . A method comprising:
 detecting a plurality of synchronization signals in a signal received by a modem; and   responsive to detecting each synchronization signal:
 extracting mode information from a frame of data corresponding to the each synchronization signal, 
 determining a current constellation by identifying one of a plurality of valid constellation code words that most closely matches a received code word corresponding to the mode information, 
 increasing a confidence metric associated with a previously identified constellation if the current constellation matches the previously identified constellation, 
 decreasing the confidence metric if the current constellation is different from the previously identified constellation, and 
 decoding the frame of data using the previously identified constellation if the confidence metric exceeds a predetermined threshold. 
   
     
     
         12 . The method of  claim 11 , wherein determining the current constellation includes:
 performing cross-correlations for each of the plurality of valid constellation code words to obtain a plurality of correlation values; and   determining the current constellation based on magnitudes of the plurality of correlation values.   
     
     
         13 . The method of  claim 11 , wherein the frame of data is one of a plurality of frames received in an unequalized signal received by the modem. 
     
     
         14 . The method of  claim 13 , wherein the determining the current constellation is performed while recovering a carrier from the unequalized signal. 
     
     
         15 . The method of  claim 13 , further comprising:
 converging taps of a filter of an equalizer filter with an error signal calculated using a constant modulus algorithm (CMA); and   obtaining an equalized signal from the unequalized signal using the equalizer filter.   
     
     
         16 . The method of  claim 15 , wherein determining the current constellation includes:
 calculating power associated with a plurality of symbols in the equalized signal; and   identifying corner symbols of the current constellation by identifying power levels in the equalized signal corresponding to the corner symbols.   
     
     
         17 . The method of  claim 15 , wherein determining the current constellation includes determining a power distribution in the equalized signal, wherein each of a plurality of sections of the power distribution corresponds to a range of power levels, each power level being associated with only one of a plurality of candidate constellations. 
     
     
         18 . The method of  claim 15 , further comprising:
 determining at least one peak within a power distribution of the equalized signal; and   identifying a constellation of symbols based on a frequency at which the at least one peak occurs.   
     
     
         19 . A computer program product, comprising a computer-readable medium comprising code for:
 detecting a synchronization signal in a signal received by a modem;   extracting mode information from a frame of data corresponding to the synchronization signal;   determining a current constellation by identifying one of a plurality of valid constellation code words that most closely matches a received code word corresponding to the mode information;   increasing a confidence metric associated with a previously identified constellation if the current constellation matches the previously identified constellation;   decreasing the confidence metric if the current constellation is different from the previously identified constellation; and   decoding the frame of data using the previously identified constellation if the confidence metric exceeds a predetermined threshold.