Notch filtering for OFDM system with null postfix
Transmission systems and methods for reducing interference by wideband communication systems with narrowband communications systems are disclosed. In some embodiments an ultrawideband system employing orthogonal frequency division multiplexing (OFDM) with a null post-fix nulls symbols for sub-carriers at potentially interfering frequencies prior to transformation to the time domain and filters a time domain representation using a notch filter to further reduce spectral components at the potentially interfering frequencies. In further embodiments pre-emphasis is applied to a frequency domain representation of symbols to reduce ripples introduced by the notch filter.
1. A transmitter for a frequency division multiplexing system, comprising:
a tone nuller block to null symbols or portions of symbols representative of potentially interfering subcarriers of a frequency division multiplexed symbol;
an inverse fast Fourier transform (IFFT) block to transform information indicative of the frequency division multiplexed symbol with nulled subcarriers to a time domain representation; and
a notch filter to filter at least portions of the time domain representation about the interfering subcarriers.
2. The transmitter of claim 1 wherein the notch filter includes a FIR filter.
3. The transmitter of claim 1 wherein the notch filter comprises a low pass FIR filter and frequency shifters.
4. The transmitter of claim 1 further comprising a pre-emphasis block configured to provide an emphasis to the frequency division multiplexed symbol with the nulled subcarriers, and the information indicative of the frequency division multiplexed symbol is the emphasized frequency division multiplexed symbol with the nulled subcarriers.
5. The transmitter of claim 4 wherein the pre-emphasis block is configured to provide the emphasis by effectively multiplying the frequency division multiplexed symbol with the nulled subcarriers by an inverse of a frequency domain impulse response of the notch filter.
6. The transmitter of claim 5 wherein the notch filter includes a delay line with thirty one taps.
7. The transmitter of claim 5 wherein the notch filter includes a tapped delay line with taps multiplied by coefficients in accordance with a truncated sinc impulse.
8. A method of processing symbols in a frequency division multiplexing system in order to reduce interference with other communication systems, comprising:
nulling information for subcarriers, in a frequency domain representation, intended for transmission at frequencies indicated as being used by the other communication systems, the information for the subcarriers being part of a frequency division multiplexing symbol;
transforming a representationof the frequency division multiplexing symbol with nulled subcarriers to a time domain representation;
adding null symbols to the time domain representation;
filtering the time domain representation with the added null symbols using a frequency selective filter so as to reduce signal components of the subcarriers intended for transmission at the frequencies indicated as being used by the other communication systems; and
transmitting the processed symbols to reduce the interference with the other communication systems.
9. The method of claim 8 further comprising reducing ripples in a spectrum of the transmitted processed symbols, the ripples resulting from the filtering.
10. The method of claim 9 wherein reducing the ripples is performed by modifying at least portions of the frequency division multiplexing symbol in the frequency domain representation.
11. The method of claim 10 wherein modifying at least portions of the frequency division multiplexing symbol in the frequency domain representation comprises effectively multiplying the frequency division multiplexing symbol with the nulled subcarriers by an inverse of a frequency domain impulse response of the frequency selective filter.
12. The method of claim 11 wherein the frequency selective filter comprises a low pass FIR filter and frequency shifting.
13. A method of reducing interference by an ultrawideband orthogonal frequency division multiplexing (OFDM) communication transmitter with narrowband communication systems, comprising:
receiving an indication of frequencies used by the narrowband communication systems;
nulling, in a frequency domain representation, information of an OFDM symbol otherwise expected to be transmitted at the frequencies used by the narrowband communication systems; and
filtering, in a time domain representation, information of the QFDM symbol otherwise expected to be transmitted at the frequencies used by the narrowband communication systems, thereby
reducing the interference caused by the ultrawideband OFDM communication system transmitter.
14. The method of claim 13 wherein the filtering is performed by a filter, and further comprising effectively multiplying, in the frequency domain representation, information of the OFDM symbol by an inverse of a frequency domain impulse response of the filter, thereby reducing frequency spectrum ripples caused by the filtering.
15. The method of claim 13 wherein the filtering comprises frequency shifting the OFDM symbol by a first amount, low pass filtering the frequency shifted OFDM symbol, and frequency shifting the OFDM symbol in the direction reverse of the first amount.