Coherent track for FM IBOC receiver using a switch diversity antenna system
View Patent ↗A method is provided for coherently tracking a radio signal including a plurality of digitally modulated reference subcarriers. The method comprises the steps of receiving symbols transmitted on the reference subcarriers, combining the reference subcarrier symbols with a known reference sequence conjugate to produce a plurality of samples, median filtering the samples to produce filtered samples, and smoothing the samples for each of the reference subcarriers over the plurality of reference subcarriers to produce a coherent reference signal estimate for each of the subcarriers. A receiver for coherently tracking a radio signal including at least one digitally modulated reference carrier is also provided.
1. A method for coherently tracking a radio signal including a plurality of digitally modulated reference subcarriers, the method comprising the steps of:
using a plurality of antenna elements to receive symbols transmitted on the reference subcarriers; and
using processing circuitry to coherently track the received symbols by combining the reference subcarrier symbols with a known reference sequence conjugate to produce a plurality of samples,
median filtering the samples to produce filtered samples, and to maintain a step transient in the received signal, and
smoothing the samples for each of the reference subcarriers over the plurality of reference subcarriers to produce a coherent reference signal estimate for each of the subcarriers.
2. The method of claim 1 ,
wherein the step transient is caused by switching among the antenna elements.
3. The method of claim 1 , wherein the processing circuitry further performs the steps of:
using the plurality of samples and the coherent reference signal estimate for each of the subcarriers to produce a noise variance signal;
filtering the noise variance signal to produce a filtered noise variance signal;
selecting between the noise variance signal and the filtered noise variance signal to determine a selected noise variance signal; and
smoothing the selected noise variance signal for each of the reference subcarriers over the plurality of reference subcarriers to produce a signal noise variance estimate for each of the subcarriers.
4. The method of claim 3 , wherein the step of using the plurality of samples and the coherent reference signal estimate for each of the subcarriers to produce a noise variance signal comprises the steps of:
subtracting the coherent reference signal estimate from the plurality of samples to produce a noise sample; and
squaring the magnitude of the noise sample to produce the noise variance signal.
5. The method of claim 3 , wherein the step of filtering the noise variance signal to produce a filtered noise variance signal comprises the step of:
sequentially passing the noise variance signal through a median filter and an additional filter to smooth the noise variance signal over frequency.
6. The method of claim 5 , wherein the additional filter comprises:
an infinite impulse response filter or a finite impulse response filter.
7. The method of claim 3 , wherein the processing circuitry further performs the step of:
scaling the noise variance signal prior to the step of selecting between the noise variance signal and the filtered noise variance signal to determine a selected noise variance signal.
8. A receiver for coherently tracking a radio signal including at least one digitally modulated reference carrier, the receiver comprising:
an input for receiving the radio signal; and
a processor for coherently tracking a radio signal including a plurality of digitally modulated reference subcarriers, by receiving symbols transmitted on the reference subcarriers, combining the reference subcarrier symbols with a known reference sequence conjugate to produce a plurality of samples, median filtering the samples to produce filtered samples and to maintain a step transient in the received signal, and smoothing the samples for each of the reference subcarriers over the plurality of reference subcarriers to produce a coherent reference signal estimate for each of the subcarriers.
9. The receiver of claim 8 , further comprising:
a plurality of antenna elements for receiving the radio signal, wherein an impulsive noise transient is caused by switching among the antenna elements.
10. The receiver of claim 8 , wherein the processor further uses the plurality of samples and the coherent reference signal estimate for each of the subcarriers to produce a noise variance signal; filters the noise variance signal to produce a filtered noise variance signal; selects between the noise variance signal and the filtered noise variance signal to determine a selected noise variance signal; and smoothes the selected noise variance signal for each of the reference subcarriers over the plurality of reference subcarriers to produce a signal noise variance estimate for each of the subcarriers.
11. The receiver of claim 10 , wherein the processor further subtracts the coherent reference signal estimate from the plurality of samples to produce a noise sample; and squares the magnitude of the noise sample to produce the noise variance signal.
12. The receiver of claim 10 , wherein the processor sequentially passes the noise variance signal through a median filter and an additional filter.
13. The receiver of claim 10 , wherein the additional filter comprises:
an infinite impulse response filter or a finite impulse response filter.
14. The receiver of claim 10 , wherein the processor scales the noise variance signal prior to selecting between the noise variance signal and the filtered noise variance signal to determine a selected noise variance signal.
15. The method of claim 1 , further comprising the step of:
removing data from the reference subcarriers prior to the combining step.