Carrier tracking circuit and method including dual numerically controlled oscillators and feedforward phase correction coefficient

A carrier tracking circuit, comprising:

• a first phase adjustment circuit adapted to receive input samples and operable to adjust a phase value of each input sample responsive to a first phase adjustment signal to thereby generate a corresponding phase-adjusted input sample, with groups of the input samples corresponding to sequentially received symbols;
• a delay element coupled to the first phase adjustment circuit and operable to generate a group of output components responsive to a group of the phase-adjusted input samples corresponding to a particular symbol;
• a second phase adjustment circuit coupled to the delay component to receive the output components, the second phase adjustment circuit operable to adjust a phase value of each output component responsive to a second phase adjustment signal to thereby generate phase-adjusted output components corresponding to a particular symbol;
• and
• a phase correction circuit coupled to the first and second phase adjustment circuits,
• the phase correction circuit operable to generate the first phase adjustment signal having a value that is a function of the values of the phase-adjusted output components for a particular symbol, and operable to delay application of the phase adjustment signal to the first phase adjustment circuit for approximately a first delay time at which a first input sample of a subsequent symbol is applied to the first phase adjustment circuit, and
• the phase correction circuit further operable to generate the second phase adjustment signal having a value that is equal to the value the first phase adjustment signal for a second delay time and that is thereafter equal to a new value of the first phase adjustment signal associated with a subsequent symbol minus an initial value of the first phase adjustment signal associated with the prior symbol, the second delay time being equal to approximately the time between when the initial value of the first phase adjustment signal is generated and the time when the output components for a subsequent symbol that have been phase adjusted using that initial value are output from the delay element.

An OFDM receiver, comprising:

• a radio frequency receiver circuit;
• an analog-to-digital converter coupled to the radio frequency receiver circuit;
• a serial-to-parallel converter coupled to the analog-to-digital converter;
• an FFT circuit coupled to the serial-to-parallel converter;
• a parallel-to-serial converter coupled to the output of the FFT circuit;
• a symbol demodulation circuit;
• and
• a carrier tracking circuit coupled to the analog-to-digital converter and to the parallel-to-serial converter, the carrier tracking circuit including,
• a first phase adjustment circuit coupled to receive input samples from the analog-to-digital converter and operable to adjust a phase value of each input sample responsive to a first phase adjustment signal to thereby generate a corresponding phase-adjusted input sample, with groups of the input samples corresponding to sequentially received OFDM symbols;
• a second phase adjustment circuit coupled to the FFT circuit to receive output components from the FFT, the second phase adjustment circuit operable to adjust a phase value of each output component responsive to a second phase adjustment signal to thereby generate phase-adjusted output components corresponding to a particular OFDM symbol;
• and
• a phase correction circuit coupled to the first and second phase adjustment circuits,
• the phase correction circuit operable to generate the first phase adjustment signal having a value that is a function of the values of the phase-adjusted output components for a particular OFDM symbol, and operable to delay application of the phase adjustment signal to the first phase adjustment circuit for approximately a first delay time at which a first input sample of a subsequent OFDM symbol is applied to the first phase adjustment circuit, and
• the phase correction circuit further operable to generate the second phase adjustment signal having a value that is equal to the value the first phase adjustment signal for a second delay time and that is thereafter equal to a new value of the first phase adjustment signal associated with a subsequent OFDM symbol minus an initial value of the first phase adjustment signal associated with the prior OFDM symbol, the second delay time being equal to approximately the time between when the initial value of the first phase adjustment signal is generated and the time when the output components for a subsequent OFDM symbol that have been phase adjusted using that initial value are output from the FFT.

An OFDM communications system, comprising:

• an OFDM transmitter operable to communicate a modulated OFDM signal over a wireless communications channel;
• and
• an OFDM receiver including,
• a radio frequency receiver circuit adapted to receive the modulated OFDM signal;
• an analog-to-digital converter coupled to the radio frequency receiver circuit;
• a serial-to-parallel converter coupled to the analog-to-digital converter;
• an FFT circuit coupled to the serial-to-parallel converter;
• a parallel-to-serial converter coupled to the output of the FFT circuit;
• a symbol demodulation circuit;
• and
• a carrier tracking circuit coupled to the analog-to-digital converter and to the parallel-to-serial converter, the carrier tracking circuit including,
• a first phase adjustment circuit coupled to receive input samples from the analog-to-digital converter and operable to adjust a phase value of each input sample responsive to a first phase adjustment signal to thereby generate a corresponding phase-adjusted input sample, with groups of the input samples corresponding to sequentially received OFDM symbols;
• a second phase adjustment circuit coupled to the FFT circuit to receive output components from the FFT, the second phase adjustment circuit operable to adjust a phase value of each output component responsive to a second phase adjustment signal to thereby generate phase-adjusted output components corresponding to a particular OFDM symbol;
• and
• a phase correction circuit coupled to the first and second phase adjustment circuits,
• the phase correction circuit operable to generate the first phase adjustment signal having a value that is a function of the values of the phase-adjusted output components for a particular OFDM symbol, and operable to delay application of the phase adjustment signal to the first phase adjustment circuit for approximately a first delay time at which a first input sample of a subsequent OFDM symbol is applied to the first phase adjustment circuit, and
• the phase correction circuit further operable to generate the second phase adjustment signal having a value that is equal to the value the first phase adjustment signal for a second delay time and that is thereafter equal to a new value of the first phase adjustment signal associated with a subsequent OFDM symbol minus an initial value of the first phase adjustment signal associated with the prior OFDM symbol, the second delay time being equal to approximately the time between when the initial value of the first phase adjustment signal is generated and the time when the output components for a subsequent OFDM symbol that have been phase adjusted using that initial value are output from the FFT.

A method of correcting for frequency offset in a communications system that communicates data symbols, the method comprising:

• generating a first group of frequency components that correspond to a data symbol by applying an FFT algorithm to a group of time domain input samples that correspond to the symbol;
• calculating a phase adjustment value from the group of frequency components;
• adjusting the phase values of subsequent groups of time domain input samples corresponding to subsequent symbols in using the calculated phase adjustment value for a prior symbol, the phase values of the subsequent time domain input samples being adjusted prior to applying the FFT algorithm to these input samples;
• adjusting the phase values of the groups of frequency components generated by the FFT algorithm for a given symbol using the phase adjustment value calculated from the frequency components of a prior symbol;
• and
• when a given symbol has the phase values of the corresponding time domain input samples adjusted using the phase adjustment value calculated from a particular prior symbol, compensating for this adjustment to the time domain input samples when adjusting the phase values of the group of frequency components corresponding to this given symbol.

A method of correcting for frequency offset in an OFDM communications system that communicates OFDM symbols, each OFDM symbol including a plurality of corresponding time domain input samples and a plurality of corresponding frequency components generated by applying a frequency analysis algorithm to the time domain input samples, each time domain input sample and each frequency component having an associated phase and magnitude, the method comprising:

• calculating a phase adjustment value from the frequency components of each OFDM symbol;
• adjusting the phases of the time domain input samples of at least one prior OFDM symbol using the calculated phase adjustment value for a prior OFDM symbol prior to applying the input samples to the frequency analysis algorithm;
• adjusting the phases of the frequency components of at least one prior OFDM symbol using the calculate phase adjustment value for a prior OFDM symbol;
• and
• adjusting the phases of the frequency components of a given OFDM symbol using the calculated phase adjustment value for that OFDM symbol.