Frequency domain equalizer for wireless commuications system
View Patent ↗The present invention relates to wireless communications and is particularly applicable to devices and modules for correcting errors introduced to a wireless signal after its transmission. An equalizer is provided which compensates for undesirable effects on received radio signals introduced by either signal processing or by the transmission medium. In operation, the equalizer multiples the complex received signal with a complex corrective signal that compensates for these effects. A tap corrective signal corrects for time-varying channel effects (i.e. channel distortions), a timing tracking signal corrects for carrier frequency offset errors, and a phase tracking signal corrects for sampling frequency offset errors.
1. A frequency domain equalization system to compensate for wireless communication time-constant and time-varying channel effects, residual carrier frequency offset, and sampling frequency offset arising in a received complex signal comprising:
an equalizer tap calculation circuit cooperating with an equalizer tap tracking circuit to correct the time-constant and time-varying channel effects;
a phase tracking circuit to correct the residual carrier frequency offset; and
a timing tracking circuit to correct the sampling frequency offset;
where the received complex signal is inputted from a Fast Fourier Transform (FFT) circuit and a corrected complex signal is outputted to a soft decision demapper;
where the equalizer tap calculation circuit is configured to generate a corrective tap signal to use in correcting the complex signal;
where pilot signals are extracted for use in the phase and timing tracking circuits;
where the complex signal comprises 52 complex subcarriers; and
wherein the corrective tap signal is generated by applying a sign least mean squares algorithm to the 52 complex subcarriers.
2. The frequency domain equalization system of claim 1 where the equalizer tap tracking circuit further includes a slicer and where the equalizer tap tracking circuit updates the corrective tap signal with a running time average of a slicer error.
3. A frequency domain equalization system to compensate for wireless communication time-constant and time-varying channel effects, residual carrier frequency offset, and sampling frequency offset arising in a received complex signal comprising:
an equalizer tap calculation circuit cooperating with an equalizer tap tracking circuit to correct the time-constant and time-varying channel effects;
a phase tracking circuit to correct the residual carrier frequency offset; and
a timing tracking circuit to correct the sampling frequency offset;
where the received complex signal is inputted from a Fast Fourier Transform (FFT) circuit and a corrected complex signal is outputted to a soft decision demapper;
where the equalizer tap calculation circuit is configured to generate a corrective tap signal to use in correcting the complex signal;
where pilot signals are extracted for use in the phase and timing tracking circuits; and
where respective corrective tap signals are calculated for each subcarrier associated with the received complex signal.
4. The frequency domain equalization system of claim 3 where the complex signal comprises 52 complex subcarriers.
5. The frequency domain equalization system of claim 3 wherein said timing tracking circuit tracks the phases of said pilot signals using time averaging.
6. A frequency domain equalization system to compensate for wireless communication time-constant and time-varying channel effects, residual carrier frequency offset, and sampling frequency offset arising in a received complex signal comprising:
an equalizer tap calculation circuit cooperating with an equalizer tap tracking circuit to correct the time-constant and time-varying channel effects;
a phase tracking circuit to correct the residual carrier frequency offset; and
a timing tracking circuit to correct the sampling frequency offset;
where the received complex signal is inputted from a Fast Fourier Transform (FFT) circuit and a corrected complex signal is outputted to a soft decision demapper;
where the equalizer tap calculation circuit is configured to generate a corrective tap signal to use in correcting the complex signal;
where pilot signals are extracted for use in the phase and timing tracking circuits; and
where the equalizer tap calculation circuit is configured to perform spectral smoothing.
7. A frequency domain equalization system to compensate for wireless communication time-constant and time-varying channel effects, residual carrier frequency offset, and sampling frequency offset arising in a received complex signal comprising:
an equalizer tap calculation circuit cooperating with an equalizer tap tracking circuit to correct the time-constant and time-varying channel effects;
a phase tracking circuit to correct the residual carrier frequency offset; and
a timing tracking circuit to correct the sampling frequency offset;
where the received complex signal is inputted from a Fast Fourier Transform (FFT) circuit and a corrected complex signal is outputted to a soft decision demapper;
where the equalizer tap calculation circuit is configured to generate a corrective tap signal to use in correcting the complex signal;
where pilot signals are extracted for use in the phase and timing tracking circuits;
where the complex signal comprises 52 complex subcarriers; and
where a respective phase and timing rotor is applied to said pilots and the 52 complex subcarriers to correct the received complex signal.
8. A frequency domain equalization system to compensate for wireless communication time-constant and time-varying channel effects, residual carrier frequency offset, and sampling frequency offset arising in a received complex signal comprising:
an equalizer tap calculation circuit cooperating with an equalizer tap tracking circuit to correct the time-constant and time-varying channel effects;
a phase tracking circuit to correct the residual carrier frequency offset; and
a timing tracking circuit to correct the sampling frequency offset;
where the received complex signal is inputted from a Fast Fourier Transform (FFT) circuit and a corrected complex signal is outputted to a soft decision demapper;
where the equalizer tap calculation circuit is configured to generate a corrective tap signal to use in correcting the complex signal;
where pilot signals are extracted for use in the phase and timing tracking circuits; and
where four pilot signals are calculated.
9. A method for correcting a received complex signal, comprising:
estimating a channel response from long sequence training symbols FFT 1 and FFT 2 contained in a received data packet preamble;
processing pilot tones in each of the FFT 1 and FFT 2 long sequence training symbols to evaluate a carrier frequency offset and a sampling frequency offset; compensating for the carrier frequency offset or the sampling frequency offset in a subsequently received data packet;
tracking channel distortion during subsequent reception of data packets; and
modifying the channel response to compensate for any detected distortion;
where estimating the channel response includes comparing the received amplitude and phase of the long sequence training symbols FFT 1 and FFT 2 with a reference.
10. The method of claim 9 where the comparing the received amplitude includes averaging over the long sequence training symbols FFT 1 and FFT 2 and demodulating each subcarrier associated with the received complex signal.
11. A method for correcting a received complex signal, comprising:
estimating a channel response from long sequence training symbols FFT 1 and FFT 2 contained in a received data packet preamble;
processing pilot tones in each of the FFT 1 and FFT 2 long sequence training symbols to evaluate a carrier frequency offset and a sampling frequency offset;
compensating for the carrier frequency offset or the sampling frequency offset in a subsequently received data packet;
tracking channel distortion during subsequent reception of data packets; and
modifying the channel response to compensate for any detected distortion;
where the estimating the channel response includes producing a corrective tap signal and applying the corrective tap signal to the received long sequence training symbols FFT 1 and FFT 2 .
12. The method of claim 11 where the tracking includes refining the corrective tap signal by tracking residual phase and timing error, and channel variations.
13. The method of claim 11 where the estimating includes spectral smoothing.
14. A method for correcting a received complex signal, comprising:
estimating a channel response from long sequence training symbols FFT 1 and FFT 2 contained in a received data packet preamble;
processing pilot tones in each of the FFT 1 and FFT 2 long sequence training symbols to evaluate a carrier frequency offset and a sampling frequency offset;
compensating for the carrier frequency offset or the sampling frequency offset in a subsequently received data packet;
tracking channel distortion during subsequent reception of data packets; and
modifying the channel response to compensate for any detected distortion;
where the compensating includes applying respective phase and timing rotors to the pilots and subcarriers associated with the received complex signal to correct the respective carrier frequency and sampling frequency offsets.
15. The method of claim 14 where processing the pilot tones includes tracking the phase across the pilot tones using time averaging.