Carrier frequency acquisition method and apparatus having improved reliability for detecting carrier acquisition or loss thereof

Abstract

A carrier signal acquisition technique is disclosed that improves the reliability of the generated carrier frequency offset. An improved course carrier frequency offset algorithm is employed in conjunction with a conventional fine carrier frequency offset algorithm. The course carrier frequency offset algorithm associated with the present invention estimates large offsets that are multiples of the carrier spacing that may occur at system startup. A spectral null is placed in the center of the transmit spectrum and is thereafter located in a received signal. The position of the spectral null provides an estimate of the local oscillator carrier offset. A frequency finite state machine (FSM) is disclosed that processes a number of metrics to ensure the reliability of the course carrier frequency offset and of transitions between acquisition and tracking modes. The frequency FSM will utilize the frequency offset (modin) generated by a MODSC algorithm provided one or more predefined thresholds are satisfied. A first metric value, c2adj, is the normalized ratio of average power (excluding null carrier power) of carriers to the null carrier power. A second metric, p2av, is the normalized ratio of average power (excluding minimum carrier power) of carriers to the minimum carrier power. If the average difference of the ratios of the powers of the carriers (p2av and c2adj) is less than a predefined threshold, then frequency acquisition is completed. Similarly, while in tracking mode, if the differences are consistently greater than the threshold, then loss of track is achieved and the frequency FSM process will switch operation to the acquisition mode.

I claim:  
     
         1 . A method for determining a local oscillator carrier offset frequency in a communication system, said method comprising the steps of: 
 receiving a signal having at least one spectral null inserted therein;    locating a position of said spectral null in said signal; and    using said position of said spectral null to estimate said local oscillator carrier offset frequency if one or more predefined conditions are satisfied.    
     
     
         2 . The method according to  claim 1 , wherein said signal is an OFDM signal.  
     
     
         3 . The method according to  claim 1 , wherein said signal is a DMT signal.  
     
     
         4 . The method according to  claim 1 , wherein said predefined conditions ensure that a signal-to-noise ratio satisfies predefined conditions.  
     
     
         5 . The method according to  claim 1 , wherein said predefined conditions evaluate a ratio of the powers of the carriers to determine if their averaged difference is less than some predefined threshold.  
     
     
         6 . The method according to  claim 5 , wherein an acquisition mode is complete if said averaged difference is less than said predefined threshold.  
     
     
         7 . The method according to  claim 1 , wherein said predefined conditions evaluate a ratio of the powers of the carriers during a tracking mode to determine if their averaged difference are consistently greater than a threshold indicating a that a loss of tracking has occurred.  
     
     
         8 . The method according to  claim 1 , wherein said local oscillator carrier offset frequency is determined by said position of said spectral null in a tracking mode and is zero in an acquisition mode.  
     
     
         9 . The method according to  claim 1 , further comprising the steps of: 
 extracting a plurality of sub-carriers of said signal after a fast fourier transform (FFT) operation;    determining a minimum energy sub-carrier of said plurality of sub-carriers;    calculating at least one average energy of said plurality of sub-carriers, excluding an energy of said minimum energy sub-carrier;    comparing a ratio of said average energy and said energy of said minimum energy sub-carrier to a predefined threshold; and    multiplying an index of said minimum energy sub-carrier by an inter-carrier spacing to generate said local oscillator carrier offset frequency.    
     
     
         10 . A system for determining a local oscillator carrier offset frequency in a communication system, comprising: 
 a memory that stores computer-readable code and information relating to local oscillator carrier offset calculations; and    a processor operatively coupled to said memory, said processor configured to implement said computer-readable code, said computer-readable code configured to:    receive a signal having at least one spectral null inserted therein;    locate a position of said spectral null in said signal; and    use said position of said spectral null to estimate said local oscillator carrier offset frequency if one or more predefined conditions are satisfied.    
     
     
         11 . The system according to  claim 10 , wherein said signal is an OFDM signal.  
     
     
         12 . The system according to  claim 10 , wherein said signal is a DMT signal.  
     
     
         13 . The system according to  claim 10 , wherein said predefined conditions ensure that a signal-to-noise ratio satisfies predefined conditions.  
     
     
         14 . The system according to  claim 10 , wherein said predefined conditions evaluate a ratio of the powers of the carriers to determine if their averaged difference is less than some predefined threshold.  
     
     
         15 . The system according to  claim 14 , wherein an acquisition mode is complete if said averaged difference is less than said predefined threshold.  
     
     
         16 . The system according to  claim 10 , wherein said predefined conditions evaluate a ratio of the powers of the carriers during a tracking mode to determine if their averaged difference are consistently greater than a threshold indicating a that a loss of tracking has occurred.  
     
     
         17 . The system according to  claim 10 , wherein said local oscillator carrier offset frequency is determined by said position of said spectral null in a tracking mode and is zero in an acquisition mode.  
     
     
         18 . The system according to  claim 10 , wherein said processor is further configured to: 
 extract a plurality of sub-carriers of said signal after a fast fourier transform (FFT) operation;    determine a minimum energy sub-carrier of said plurality of sub-carriers;    calculate at least one average energy of said plurality of sub-carriers, excluding an energy of said minimum energy sub-carrier;    compare a ratio of said average energy and said energy of said minimum energy sub-carrier to a predefined threshold; and    multiply an index of said minimum energy sub-carrier by an inter-carrier spacing to generate said local oscillator carrier offset frequency.    
     
     
         19 . A communication system, comprising: 
 a receiver for receiving a signal having at least one spectral null inserted therein;    means for locating a position of said spectral null in said signal; and    means for using said position of said spectral null to estimate said local oscillator carrier offset frequency if one or more predefined conditions are satisfied.    
     
     
         20 . The communication system according to  claim 19 , wherein said signal is an OFDM signal.  
     
     
         21 . The communication system according to  claim 19 , wherein said signal is a DMT signal.  
     
     
         22 . The communication system according to  claim 19 , wherein said predefined conditions ensure that a signal-to-noise ratio satisfies predefined conditions.  
     
     
         23 . The communication system according to  claim 19 , wherein said predefined conditions evaluate a ratio of the powers of the carriers to determine if their averaged difference is less than some predefined threshold.  
     
     
         24 . The communication system according to  claim 23 , wherein an acquisition mode is complete if said averaged difference is less than said predefined threshold.  
     
     
         25 . The communication system according to  claim 19 , wherein said predefined conditions evaluate a ratio of the powers of the carriers during a tracking mode to determine if their averaged difference are consistently greater than a threshold indicating a that a loss of tracking has occurred.