Method for DC offset removal in OFDM systems
View Patent ↗Systems and methods for removing a DC offset from an orthogonal frequency division multiplexed (OFDM) signal transmitted over a plurality of subcarrier frequencies. The system includes a receiver. The system further includes a high pass DC component filter configured to reduce a DC component of the orthogonal frequency division multiplexed signal, the high pass DC component filter shaping noise in the orthogonal frequency division multiplexed signal which results in a non-uniform power spectral density of the noise across the plurality of subcarrier frequencies. The system further includes a noise whitener configured to compensate for the noise shaping by the high pass DC component filter by normalizing the non-uniform power spectral density of the noise across the plurality of subcarrier frequencies.
1. A system comprising:
a receiver configured to receive an orthogonal frequency division multiplexed (OFDM) signal that has been transmitted over a plurality of subcarrier frequencies; and
a high pass filter configured to filter the OFDM signal to reduce a DC component of the OFDM signal,
wherein the high pass filter includes a plurality of subfilters, wherein a sum of filter coefficients of the plurality of subfilters is equal to zero, and wherein a filter length of the plurality of subfilters does not increase an effective channel delay beyond an OFDM cyclic prefix interval.
2. The system of claim 1 , further comprising:
a serial-to-parallel converter configured to convert the filtered OFDM signal into multiple parallel data signals, one data signal for each subcarrier frequency; and
a noise whitener configured to normalize power spectral density of noise across the plurality of subcarrier frequencies by multiplying each data signal by a whitening factor that is specific to the respective subcarrier frequency.
3. The system of claim 2 , wherein the whitening factor for each subcarrier frequency is a function of noise power at the respective subcarrier frequency.
4. The system of claim 1 , further comprising a decoder that is configured to determine a likelihood ratio for each bit on each subcarrier frequency.
5. The system of claim 1 , wherein the high pass filter is bypassable.
6. The system of claim 5 , wherein the system is configured to bypass the high pass filter when a bypass criteria is present from a group of bypass criteria consisting of: a DC magnitude in the received OFDM signal being below a first threshold value, a carrier frequency offset being above a second threshold, and a carrier frequency offset being below a third threshold.
7. A system comprising:
a receiver configured to receive an orthogonal frequency division multiplexed (OFDM) signal that has been transmitted over a plurality of subcarrier frequencies;
a high pass filter configured to filter the OFDM signal to reduce a DC component of the OFDM signal;
a serial-to-parallel converter configured to convert the filtered OFDM signal into multiple parallel data signals, one data signal for each subcarrier frequency; and
a noise whitener configured to normalize power spectral density of noise across the plurality of subcarrier frequencies by multiplying each of the data signals by a whitening factor that is specific to the respective subcarrier frequency, wherein the whitening factor for each subcarrier frequency is (i) a function of noise power at the respective subcarrier frequency and (ii) inversely proportional to the square root of noise power at the respective subcarrier frequency.
8. The system of claim 7 , wherein the high pass filter includes a plurality of subfilters, wherein a sum of filter coefficients of the plurality of subfilters is equal to zero.
9. The system of claim 7 , wherein the high pass filter includes a plurality of subfilters, wherein a filter length of the plurality of subfilters does not increase an effective channel delay beyond an OFDM cyclic prefix interval.
10. The system of claim 7 , wherein the high pass filter and the noise whitener are bypassable.
11. The system of claim 10 , wherein the system is configured to bypass the high pass filter and the noise whitener when a magnitude of a DC component of the OFDM signal exceeds a threshold.
12. The system of claim 10 , wherein the system is configured to bypass the high pass filter and the noise whitener when a carrier frequency offset is above a threshold.
13. The system of claim 7 , further comprising a controller configured to control the noise whitener and to select which of the data signals are to be multiplied by a whitening factor, to enable only a subset of the data signals to be multiplied by a whitening factor.
14. The system of claim 7 , further comprising:
a Fourier transform module configured to perform a Fourier transform on each data signal before the respective data signal is multiplied by the respective whitening factor.
15. A method comprising:
receiving an orthogonal frequency division multiplexed (OFDM) signal that has been transmitted over a plurality of subcarrier frequencies;
high-pass filtering the OFDM signal;
converting the high-pass filtered OFDM signal into multiple parallel data signals, one data signal for each subcarrier frequency; and
normalizing power spectral density of noise across the plurality of subcarrier frequencies by multiplying each data signal by a whitening factor that is specific to the respective subcarrier frequency, wherein the whitening factor for each subcarrier frequency is (i) a function of noise power at the respective subcarrier frequency (ii) inversely proportional to the square root of noise power at the respective subcarrier frequency.
16. The method of claim 15 , wherein, in the high-pass filtering, a sum of filter coefficients of a plurality of subfilters is equal to zero, and wherein an effective channel delay caused by the high-pass filtering is not increased beyond an OFDM cyclic prefix interval.