Mitigation of DC distortion in OFDM receivers
View Patent ↗A baseband receiver and corresponding methods are arranged and configured to mitigate effects of direct current (DC) distortion and process an Orthogonal Frequency Division Multiplexing (OFDM) signal as provided from a direct conversion radio or receiver. The baseband receiver includes an OFDM demodulator configured to demodulate the OFDM signal, a post processor coupled to the OFDM demodulator and configured to provide symbols corresponding to the OFDM signal, and a compensator coupled to at least one of the OFDM demodulator and the post processor and configured to reduce error rates out of the baseband receiver that result from DC distortion in the direct conversion radio.
1. A baseband receiver configured to mitigate effects of direct current (DC) distortion and process an Orthogonal Frequency Division Multiplexing (OFDM) signal as provided from a direct conversion radio, the baseband receiver comprising:
an OFDM demodulator configured to demodulate the OFDM signal;
a post processor coupled to the OFDM demodulator and configured to provide symbols corresponding to the OFDM signal; and
a compensator coupled to at least one of the OFDM demodulator and the post processor and configured to reduce error rates out of the baseband receiver that result from DC distortion in subcarrier amplitude and phase in the direct conversion radio,
wherein the compensator further comprises a selector responsive to an estimate of frequency error and configured to select channel estimation filters where one or more particular filters are selected in accordance with a subcarrier spacing relative to the DC distortion and
wherein the compensator is configured to erase pilot symbols that have been impacted by the DC distortion before using the channel estimation filters to determine characteristics of a channel for the OFDM signal.
2. The baseband receiver of claim 1 wherein the compensator further comprises a compensation filter coupled to the OFDM signal and operable to reverse one or more DC distortion components with an output of the compensation filter coupled to an input of the OFDM demodulator.
3. The baseband receiver of claim 2 wherein the compensation filter further comprises a DC compensation filter configured to substantially remove attenuation due to a high pass response of the direct conversion radio and a DC offset corrector where the DC offset corrector estimates a DC offset prior to and then applies the estimate while an OFDM packet is received.
4. The baseband receiver of claim 1 wherein the compensator further comprises an automatic frequency controller (AFC) and an offset generator inter coupled and collectively configured to steer a local oscillator frequency of the direct conversion radio so as to change the location in frequency of the DC distortion.
5. The baseband receiver of claim 4 wherein the offset generator is responsive to an estimate of frequency error from the AFC and further configured to steer the local oscillator frequency such that adjacent pilot subcarriers in the OFDM signal are spared away from zero (0) hertz.
6. The baseband receiver of claim 1 wherein the compensator further comprises a symbol de-mapper that is configured to set soft symbols to an erasure when the soft symbols are from subcarriers that have been subjected to the DC distortion.
7. The baseband receiver of claim 6 wherein the symbol de-mapper is coupled to a frequency error estimate and the frequency error estimate is used to determine what subcarriers have been subjected to the DC distortion.
8. The baseband receiver of claim 1 wherein the one or more particular filters are selected to have a filter length that is shorter when a subcarrier is closer to the DC distortion than when the subcarrier is further from the DC distortion.
9. A method of reducing effects resulting from direct current (DC) distortion of amplitude and phase in a signal executed by a baseband receiver, the signal provided from a direct conversion radio, the method comprising:
adaptively modifying the signal to provide a compensated signal;
demodulating the compensated signal to provide a demodulated signal; and
post processing the demodulated signal to provide data,
where the adaptively modifying the signal reduces an error rate in the data due to the DC distortion of amplitude and phase in the signal as caused by the direct conversion radio,
wherein the adaptively modifying the signal further comprises compensating the signal using a compensation filter, selected in accordance with a subcarrier spacing relative to the DC distortion, to reduce the DC distortion, and substantially removing attenuation due to a high pass response of the direct conversion radio and estimating a DC offset prior to and then applying the estimate while a packet is received.
10. The method of claim 9 wherein the adaptively modifying the signal further comprises steering a local oscillator frequency of the direct conversion radio to change the location in frequency of the DC distortion.
11. The method of claim 9 wherein the adaptively modifying the signal further comprises steering, responsive to an estimate of frequency error, a local oscillator frequency of the direct conversion radio such that adjacent pilot subcarriers in an orthogonal frequency division multiplexing (OFDM) signal are substantially equally distant from zero (0) hertz.
12. The method of claim 9 wherein the post processing the demodulated signal further comprising adaptively modifying the demodulated signal to further reduce the error rate in the data.
13. A method of reducing effects resulting from direct current (DC) distortion of subcarrier amplitude and phase in an Orthogonal Frequency Division Multiplexing (OFDM) signal executed by a baseband receiver, the OFDM signal provided from a direct conversion radio, the method comprising:
demodulating the OFDM signal to provide a demodulated OFDM signal;
adaptively modifying the demodulated OFDM signal to compensate for the DC distortion of subcarrier amplitude and phase in the demodulated OFDM signal; and
post processing the demodulated OFDM signal, as compensated, to provide data
wherein the adaptively modifying the demodulated OFDM signal further comprises selecting channel estimation filters
where one or more particular filters are selected in accordance with a subcarrier spacing relative to the DC distortion, and
wherein the adaptively modifying the demodulated OFDM signal further comprises estimating a frequency error for the ODFM signal and
wherein the selecting channel estimation filters further comprises selecting, based on the frequency error, channel estimation filters that have a filter length that is shorter when a subcarrier is closer to the DC distortion than when the subcarrier is further from the DC distortion.
14. The method of claim 13 wherein the adaptively modifying the demodulated OFDM signal further comprises setting symbols in the demodulated ODFM signal to an erasure when the soft symbols are from subcarriers that have been subjected to the DC distortion.
15. The method of claim 14 wherein the adaptively modifying the demodulated OFDM signal further comprises determining, based on the frequency error, what subcarriers have been subjected to the DC distortion.
16. The method of claim 13 wherein the adaptively modifying the demodulated OFDM signal further comprises erasing, based on the frequency error, pilot symbols that have been impacted by the DC distortion before using the channel estimation filters to determine characteristics of a channel for the OFDM signal.