Method and apparatus to track gain variation in orthogonal frequency division multiplexing (OFDM) systems

Abstract

Gain variations during a packet can lead to significant performance degradation in communications systems that use high order quadrature amplitude modulation (QAM). A method and the associated apparatus track such variations in an OFDM system and completely eliminate any performance degradation. Gain estimation and compensation is employed with the use of pilot subcarriers in the payload of an OFDM data packet. Estimated pilot magnitude ratios are averaged, throughout the processing life of a packet, to yield accurate gain estimations. A gain compensation factor is used to adjust data carriers. An exclusion method is also employed to eliminate pilot carriers which contribute to noise.

1 . A method for gain compensation of a packet in an orthogonal frequency division multiplexing (OFDM) system comprising: 
 establishing an initial magnitude estimate for a packet;    determining a variation in magnitude of the packet over time; and    applying to the packet a gain that varies over time, and is based on the initial magnitude estimate and the variation in magnitude.    
     
     
         2 . The method of  claim 1 , wherein the initial magnitude estimate and the variation in magnitude are with respect to pilot signals.  
     
     
         3 . The method of  claim 1 , wherein the magnitudinal variation is a pilot magnitude averaged over successive OFDM payload symbols.  
     
     
         4 . The method of  claim 1 , wherein the initial magnitude estimation is normalized by a lead pilot.  
     
     
         5 . The method of  claim 1  further comprising: 
 computing a gain compensation factor by obtaining a ratio of the initial magnitude estimate and the variation in magnitude.    
     
     
         6 . The method of  claim 1  further comprising: 
 computing a gain compensation factor by obtaining a ratio of the initial magnitude estimate and the variation in magnitude, the gain compensation factor further comprising an operator that returns an index of a pilot associated with the data subcarrier.    
     
     
         7 . The method of  claim 1 , wherein determining the variation in magnitude further comprising: 
 excluding a pilot whose magnitude is low in accordance with an exclusion algorithm.    
     
     
         8 . The method of  claim 1 , wherein the variation in magnitude is updated for every OFDM symbol.  
     
     
         9 . An apparatus for gain compensation gain compensation of a packet in an orthogonal frequency division multiplexing (OFDM) system comprising: 
 an initial gain estimator, the initial gain estimator establishing the gain applied to a packet at a time zero; and    a gain compensator, the gain compensator compensating the gain applied to the packet over a time of the packet.    
     
     
         10 . The apparatus of  claim 9 , wherein further comprising: 
 a variation in magnitude calculator, the variation in magnitude calculator determining a variation in magnitude of the packet over time.    
     
     
         11 . The apparatus of  claim 10 , wherein the variation in magnitude calculator further comprises: 
 a pilot magnitude ratio estimator, the pilot magnitude ratio estimator determining a pilot magnitude ratio estimation over time; and    a pilot magnitude estimator, the pilot magnitude estimator determining a pilot magnitude estimation based on the pilot magnitude ratio estimation.    
     
     
         12 . The apparatus of  claim 11  wherein the pilot magnitude ratio estimation is normalized by a lead pilot.  
     
     
         13 . The apparatus of  claim 9  wherein the gain compensation factor comprises an operator that returns an index of a pilot associated with a data subcarrier.  
     
     
         14 . The apparatus of  claim 10 , wherein the variation in magnitude calculator further comprises: 
 an excluder, the excluder excluding a pilot whose absolute whose magnitude is low in accordance with an exclusion algorithm.    
     
     
         15 . The apparatus of  claim 10 , wherein the variation in magnitude is updated for every OFDM symbol.  
     
     
         16 . A method for gain compensation of a packet in an orthogonal frequency division multiplexing (OFDM) system comprising: 
 an initial magnitude estimation means for establishing an initial magnitude estimate for a packet;    a variation in magnitude means for establishing a a variation in magnitude of the packet over time;    an adjusting means for adjusting a gain of the packet by applying a gain compensation factor to a data subcarrier.    
     
     
         17 . The method of  claim 16 , wherein the a variation in magnitude means further comprises: 
 a pilot magnitude ratio estimation means for estimating a pilot magnitude ratio over time; and    a pilot magnitude estimation means for estimating a pilot magnitude based on the pilot magnitude ratio.    
     
     
         18 . The method of  claim 17 , wherein the pilot magnitude estimation means further comprising: 
 an exclusion means for excluding a pilot whose magnitude is low in accordance with an exclusion algorithm.