Peak-to-average power ratio reduction for QAM modulation with HD radio signals
View Patent ↗A peak-to-average power reduction method includes: (a) quadrature amplitude modulating a signal including a set of subcarriers with a set of data to produce an OFDM modulated symbol vector; (b) limiting the magnitude of the modulated symbol vector to produce a first limited OFDM modulated symbol; (c) demodulating the first limited OFDM modulated symbol to recover distorted QAM constellations; (d) constraining points in the distorted QAM constellations recovered from the first limited OFDM modulated symbol to produce constrained QAM constellations; (e) modulating an OFDM symbol vector with the constrained QAM constellations to produce a modified modulated OFDM symbol; and (f) output the modified modulated OFDM symbol. A transmitter that implements the method is also provided.
1. A method comprising:
(a) using a modulator to quadrature amplitude modulate a signal including a set of subcarriers with a set of data to produce an Orthogonal Frequency Division Multiplexing (OFDM) modulated symbol vector; and
using processing circuitry to (b) limit the magnitude of the modulated symbol vector to produce a first limited OFDM modulated symbol; (c) demodulate the first limited OFDM modulated symbol to recover distorted Quadrature Amplitude Modulation (QAM) constellations; (d) constrain points in the distorted QAM constellations recovered from the first limited OFDM modulated symbol to produce constrained QAM constellations wherein: points at corners of the distorted QAM constellations are constrained both in-phase and in-quadrature to values greater than or equal to a first minimum threshold; points along edges, said points along edges not including any points at corners of the distorted QAM constellations are constrained to either in-phase values greater than or equal to a second minimum threshold value and in quadrature to a first predetermine range of values on either side of the undistorted constellation point, or in-quadrature to values greater than or equal to a third minimum threshold value and in phase to a second predetermine range of values on either side of the undistorted constellation point; and internal points of the distorted QAM constellations are constrained to both in-phase or in-quadrature values less than or equal to a third predetermined range of values on either side of the undistorted constellation point; (e) modulate an OFDM symbol vector with the constrained QAM constellations to produce a modified modulated OFDM symbol; and (f) output the modified modulated OFDM symbol.
2. The method of claim 1 , wherein steps (b) through (e) are repeated prior to step (f).
3. The method of claim 1 , wherein first, second and third predetermined ranges have equal values.
4. The method of claim 1 , wherein first, second and third minimum thresholds have equal absolute values.
5. The method of claim 1 , wherein signal includes a main AM carrier and BPSK modulated subcarriers, and the method further comprises:
restoring training symbols and the BPSK subcarriers to original undistorted values; and
removing imaginary component distortion from the main AM carrier after peak-to-average power reduction.
6. The method of claim 5 , further comprising:
reducing a time domain peak of the AM main carrier by adjusting the AM main carrier amplitude, wherein symbol pulse-shaping of the AM main carrier keeps the amplitude adjustments orthogonal to other subcarriers; and a constant average main carrier amplitude is maintained.
7. The method of claim 1 , further comprising:
applying a mask to inactive subcarriers in the plurality of subcarriers of the OFDM symbol vector prior to step (e).
8. The method of claim 7 , wherein enhancement sidebands in an AM core-only signal are replaced with intermodulation distortion.
9. The method of claim 8 , wherein the mask in the enhancement sidebands is adjustable to balance peak-to-average power ratio reduction performance versus interference.
10. The method of claim 1 , wherein signal includes an FM-modulated analog component and BPSK modulated reference subcarriers, and the method further comprises:
restoring the BPSK subcarriers to original undistorted values.
11. A transmitter for reducing peak-to-average power ratio in an Orthogonal Frequency Division Multiplexing (OFDM) signal, the transmitter comprising:
a modulator configured to quadrature amplitude modulate a signal including a set of subcarriers with a set of data to produce an OFDM modulated symbol vector;
processing circuitry configured to (a) limit the magnitude of the modulated symbol vector to produce a first limited OFDM modulated symbol; (b) demodulate the first limited OFDM modulated symbol to recover distorted Quadrature Amplitude Modulation (QAM) constellations; (c) constrain points in the distorted QAM constellations recovered from the first limited OFDM modulated symbol to produce constrained QAM constellations wherein: points at corners of the distorted QAM constellations are constrained both in-phase and in-quadrature to values greater than or equal to a first minimum threshold; points along edges, said points along edges not including any points at corners of the distorted QAM constellations are constrained to either in-phase values greater than or equal to a second minimum threshold value and in quadrature to a first predetermine range of values on either side of the undistorted constellation point, or in-quadrature to values greater than or equal to a third minimum threshold value and in phase to a second predetermine range of values on either side of the undistorted constellation point; and internal points of the distorted QAM constellations are constrained to both in-phase or in-quadrature values less than or equal to a third predetermined range of values on either side of the undistorted constellation point; (d) modulate an OFDM symbol vector with the constrained QAM constellations to produce a modified modulated OFDM symbol; and (e) output the modified modulated symbol; and
an amplifier for amplifying the modified modulated OFDM symbol.
12. The transmitter of claim 11 , wherein the processing circuitry is configured to repeat functions (a) through (d) before performing function (e).
13. The transmitter of claim 11 , wherein first, second and third predetermined ranges have equal values.
14. The transmitter of claim 11 , wherein first, second and third minimum thresholds have equal absolute values.
15. The transmitter of claim 11 , wherein signal includes a main AM carrier and BPSK modulated subcarriers, and the processing circuitry is configured to restore training symbols and the BPSK subcarriers to original undistorted values and remove imaginary component distortion from the main AM carrier after peak-to-average power reduction.
16. The transmitter of claim 15 , wherein the processing circuitry is configured to reduce a time domain peak of the AM main carrier by adjusting the AM main carrier amplitude, wherein symbol pulse-shaping of the AM main carrier keeps the amplitude adjustments orthogonal to other subcarriers, and a constant average main carrier amplitude is maintained.
17. The transmitter of claim 11 , wherein the processing circuitry is configured to apply a mask to inactive subcarriers in the plurality of subcarriers of the OFDM symbol vector prior to modulating the OFDM symbol vector with the constrained QAM constellations to produce the modified modulated OFDM symbol.
18. The transmitter of claim 17 , wherein enhancement sidebands in an AM core-only signal are replaced with intermodulation distortion.
19. The transmitter of claim 18 , wherein the mask in the enhancement sidebands is adjustable to balance peak-to-average power ratio reduction performance versus interference.
20. The transmitter of claim 11 , wherein signal includes an FM-modulated analog component and BPSK modulated reference subcarriers, and the processing.