Peak-to-average power ratio reduction for hybrid FM HD radio transmission

1. A method comprising:

(a) modulating a set of subcarriers with a set of data to produce a modulated OFDM symbol vector;

(b) modulating a FM carrier signal with the set of data to produce a modulated FM symbol;

(c) combining the modulated OFDM symbol and the modulated FM symbol to produce a modulated hybrid symbol;

(d) limiting the magnitude of the modulated hybrid symbol to produce a first limited modulated hybrid symbol;

(e) demodulating the first limited modulated hybrid symbol to recover distorted QPSK constellations;

(f) constraining the distorted QPSK constellations recovered from the first limited modulated hybrid symbol to values greater than or equal to a minimum threshold value to produce constrained QPSK constellations in a hybrid symbol vector;

(g) demodulating the modulated FM symbol to produce an FM symbol vector;

(h) subtracting the FM symbol vector from the hybrid symbol vector to produce a modified OFDM symbol vector;

(i) applying a mask to inactive subcarriers in the plurality of subcarriers of the OFDM symbol vector;

(j) modulating an OFDM symbol vector with OFDM modulation to produce a modified modulated OFDM symbol;

(k) combining the modified modulated OFDM symbol and the modulated FM symbol to produce a modified modulated hybrid symbol; and

(l) outputting the modified modulated hybrid symbol.

2. The method of claim 1 , further comprising:

scaling the modulated FM symbol prior to step (c).

3. The method of claim 1 , further comprising:

removing equalization compensation prior to step (f); and

restoring equalization compensation prior to step (j).

4. The method of claim 1 , wherein steps (d) through (k) are repeated prior to step (l).

5. The method of claim 1 , wherein the step of constraining the distorted QPSK constellations comprises:

assigning the minimum threshold value to bits of the distorted QPSK constellations having values less than the minimum threshold value.

6. The method of claim 1 , further comprising:

measuring a modulation error rate;

filtering the modulation error rate; and

adjusting the minimum threshold value in response to the filtered modulation error rate.

7. The method of claim 1 , wherein step (d) applies a negative clip level to the modulated hybrid symbol.

8. The method of claim 7 , wherein the negative clip level is −3 dB.

9. The method of claim 1 , wherein the steps (a) through (l) are performed for multiple sets of the subcarriers in upper and lower sidebands.

10. The method of claim 9 , wherein subcarriers in one of the upper or lower sidebands is set to a magnitude of 1 and the subcarriers in the other sideband are scaled relative to 1.

11. A transmitter for reducing peak-to-average power ratio in an OFDM signal, the transmitter comprising:

a modulator for modulating a set of subcarriers with a set of data symbol vectors including an OFDM symbol vector and an FM symbol vector to produce a modulated OFDM symbol and a modulated FM symbol;

processing circuitry for combining the modulated OFDM symbol and the modulated FM symbol to produce a modulated hybrid symbol, limiting the magnitude of the modulated hybrid symbol to produce a first limited modulated hybrid symbol, demodulating the first limited modulated hybrid symbol to recover distorted QPSK constellations, constraining the distorted QPSK constellations recovered from the first limited modulated hybrid symbol to values greater than or equal to a minimum threshold value to produce constrained QPSK constellations in a hybrid symbol vector, demodulating the FM hybrid symbol to produce an FM symbol vector, subtracting the FM symbol vector from the hybrid symbol vector to produce an OFDM symbol vector, applying a mask to inactive subcarriers in the OFDM symbol vector, modulating the OFDM symbol vector to produce a modulated OFDM symbol, combining the modified modulated OFDM symbol vector with the modulated FM symbol to produce a modified modulated hybrid symbol, and outputting the modified modulated hybrid symbol; and

an amplifier for amplifying the modified modulated hybrid symbol.

12. The transmitter of claim 11 , wherein the circuitry scales the modulated FM symbol prior to combining the modulated OFDM symbol and the modulated FM symbol to produce a modulated hybrid symbol.

13. The transmitter of claim 11 , wherein the circuitry removes equalization compensation prior to constraining the distorted QPSK constellations; and

restoring equalization compensation prior to modulating the OFDM symbol vector.

14. The transmitter of claim 11 , wherein the circuitry assigns a threshold value to bits of the distorted QPSK constellations having values less than the threshold value.

15. The transmitter of claim 11 , wherein the circuitry measures a modulation error rate, filters the modulation error rate, and adjusts the minimum threshold value in response to the filtered modulation error rate.

16. The transmitter of claim 11 , wherein the processing circuitry applies a negative clip level to the modulated hybrid symbol.

17. The transmitter of claim 16 , wherein the negative clip level is −3 dB.

18. The transmitter of claim 11 , wherein the processing circuitry performs a peak-to-average power ratio reduction process on multiple sets of the subcarriers in upper and lower sidebands.

19. The transmitter of claim 18 , wherein subcarriers in one of the upper or lower sidebands is set to a magnitude of 1 and the subcarriers in the other sideband are scaled relative to 1.