Same-channel interference detection and early receiver powerdown for OFDM signal processor
A power saving receiver has a controller which is operative to remove power from the receiver when a threshold is exceeded during reception of a packet. The threshold level is formed by comparison of any of: signal energy of unoccupied subcarriers less the signal energy in occupied subcarriers; signal energy in a first range of occupied subcarriers compared to signal energy in a different range of occupied subcarriers; error vector magnitude from a first set of subcarriers to a second set of subcarriers in a different spectral region of the channel; cyclic prefix cross-correlation, or common phase error increase.
1. A power saving receiver for detection of interference in a communication channel having a plurality of subcarriers, the power saving receiver comprising:
a controller operative to remove power from a receiver;
the receiver operative to measure a first signal strength of a first plurality of subcarriers over a first frequency range of the communication channel;
the receiver also operative to measure a second signal strength of a second plurality of subcarriers over a second frequency range of the communication channel which is non-overlapping with the first frequency range;
the receiver comparing the first signal strength of the first plurality of subcarriers to the second signal strength of the second plurality of subcarriers;
the controller removing power from the receiver if the first signal strength of the first plurality of subcarriers is different than the second signal strength of the second plurality of subcarriers by an amount greater than a threshold, the threshold being greater for a Binary Phase Shift Keying (BPSK) modulation than for a Quadrature Amplitude Modulation (QAM);
where the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is measured during a packet short preamble or a packet long preamble; and
the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is an error vector magnitude.
2. The power saving receiver of claim 1 where the first plurality of subcarriers or the second plurality of subcarriers is generated by a fast Fourier transform or an inverse fast Fourier transform.
3. The power saving receiver of claim 1 where the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is also measured during a data segment of a packet.
4. The power saving receiver of claim 1 where the first signal strength of the first plurality of subcarriers is a maximum signal strength subcarrier.
5. The power saving receiver of claim 1 where the first plurality of subcarriers or the second plurality of subcarriers is modulated by a modulation type which is at least one of: Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-QAM (Quadrature Amplitude Modulation), or 64-QAM.
6. The power saving receiver of claim 1 where the threshold is a fixed value for any modulation type.
7. The power saving receiver of claim 1 where the threshold is a value which depends on a modulation type.
8. A method for power savings in a receiver operative in a communication channel having a plurality of subcarriers and subject to interferers, the method comprising:
measuring a first signal strength of a first plurality of subcarriers over a first frequency range of the plurality of subcarriers;
measuring a second signal strength of a second plurality of subcarriers over a different frequency range of the second plurality of subcarriers which is exclusive of any subcarriers in the first plurality of subcarriers;
disabling power to the receiver if a difference in the first signal strength from the first plurality of subcarriers to the second signal strength from the second plurality of subcarriers is greater than a threshold, the threshold being greater for a Binary Phase Shift Keying (BPSK) than for Quadrature Amplitude Modulation (QAM) 16-QAM or 64-QAM modulation;
where the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is determined during a preamble interval or during a data interval of a packet using occupied subcarriers;
the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is determined using an error vector magnitude for one or more associated subcarriers.
9. The method of claim 8 where the first signal strength of the first plurality of subcarriers or the second signal strength of the second plurality of subcarriers is determined using one or more maximum signal strength subcarriers associated with the first plurality of subcarriers or the second plurality of subcarriers.
10. A method for power savings in a receiver operative in an OFDM channel having a plurality of subcarriers and subject to interferers, the receiver having an input for a baseband signal applied to a fast Fourier transform (FFT), the FFT providing a plurality of outputs, each output corresponding to a respective subcarrier of the plurality of subcarriers, the method comprising:
identifying an FFT output corresponding to a pilot subcarrier;
computing a common phase error for each FFT output corresponding to a pilot subcarrier by cross correlating a previous symbol pilot subcarrier with a current symbol pilot subcarrier to generate a current symbol correlation result;
disabling power to the receiver if a reduction by a factor of 2 or 4 in the current symbol correlation result compared to previous correlation result occurs.
11. A method for power savings in a receiver operative in a channel having a plurality of subcarriers and subject to interferers, the receiver having an input for a baseband signal applied to a fast Fourier transform (FFT), the FFT providing a plurality of outputs, the receiver powering down for an interval of time upon detection of any of:
a reduction of a cross correlation result for an OFDM symbol cyclic prefix cross correlated with a previous OFDM symbol cyclic prefix by ½ or ¼ compared to a previous cross correlation result;
an increase in common phase error (CPE) as measured by pilot subcarrier phase error from a previous symbol to a current symbol or subsequent symbol, the CPE measurement performed on least one pilot subcarrier, where the increase in the CPE measurement for a current symbol is more than 2× or 4× of the CPE measurement for a previous symbol.