Carrier frequency and phase recovery in quadrature encoded E-band communications
View Patent ↗A mechanism for retrieval of carrier frequency and carrier phase in a received modulated carrier waveform. Retrieval of carrier frequency and carrier phase can be implemented in an analog electrical circuit, using a field programmable gate array (FPGA), or in computer code. Independent of the implementation, the mechanism performs frequency and primary phase recovery by forcing transforms of a pilot tone in the upper and lower sidebands to the same frequency using a feedback loop. The difference-in-magnitudes of the channelized pilot are used by a phase lock loop to perform secondary phase recovery in a manner that also resolves phase sign ambiguity. Benefits of this mechanism include improved phase lock loop tracking performance and a reduction of noise in the data demodulated from the received carrier waveform.
1. A system for performing carrier recovery in a radio-frequency communications receiver, the system comprising:
an input subsystem configured to receive a quadrature-encoded signal comprising a carrier waveform having a carrier frequency and a pilot tone having a pilot frequency;
a frequency recovery subsystem configured to:
process the quadrature-encoded signal to produce an upper sideband signal and a lower sideband signal;
identify, in the upper sideband signal, an upper sideband transform of the pilot tone;
identify, in the lower sideband signal, a lower sideband transform of the pilot tone;
determine a frequency difference between the upper sideband transform and the lower sideband transform; and
adjust a frequency of a local oscillator of the receiver based on the frequency difference.
2. The system of claim 1 , wherein the frequency difference between the upper sideband transform and the lower sideband transform is twice a difference between the frequency of the local oscillator before adjustment and the carrier frequency.
3. The system of claim 1 , wherein the input subsystem is further configured to mix the quadrature encoded signal with a reference signal generated by a local oscillator to produce an I channel and a Q channel, the I channel and the Q channel separated by a non-zero number of degrees relative to each other, and the frequency recovery subsystem is further configured to:
produce the upper sideband signal by combining a leading portion of the I channel with a lagging portion of the Q channel; and
produce the lower sideband signal by combining a lagging portion of the I channel with a leading portion of the Q channel.
4. The system of claim 3 , wherein the frequency recovery subsystem is further configured to:
phase shift a portion of the I channel forward by a number of degrees to generate the leading portion of the I channel; and
phase shift a portion of the Q channel backward by a corresponding number of degrees to generate the lagging portion of the Q channel,
wherein the lagging portion of the Q-channel lags the leading portion of the I-channel by approximately ninety degrees.
5. The system of claim 3 , wherein the frequency recovery subsystem is further configured to:
phase shift a portion of the I channel backward by a number of degrees to generate the lagging portion of the I channel; and
phase shift a portion of the Q channel forward by a corresponding number of degrees to generate the leading portion of the Q channel,
wherein the leading portion of the Q-channel leads the lagging portion of the I-channel by approximately ninety degrees.
6. The system of claim 1 , wherein the carrier frequency is in a range of 71 GHz to 76 GHz, 81 GHz to 86 GHz, and 92 GHz to 95 GHz, and the pilot frequency is in a range from 500 kHz to a data clock frequency.
7. The system of claim 1 , wherein the input subsystem is further configured to mix the quadrature encoded signal with a reference signal generated by the local oscillator to produce an I channel and a Q channel, the I channel and the Q channel separated by a non-zero number of degrees relative to each other, the system further comprising a phase recovery subsystem configured to:
determine a difference-in-magnitudes between the I channel and the Q channel;
determine a phase error comprising a magnitude and a direction based on the difference-in-magnitudes; and
adjust a phase of the local oscillator based on the magnitude and the direction of the phase error.
8. The system of claim 7 , wherein the phase recovery subsystem determines the difference-in-magnitudes by:
determining a difference between an amplitude of the I channel squared and an amplitude of the Q channel squared.
9. The system of claim 7 , wherein the difference-in-magnitudes is determined by a difference circuit.
10. A non-transitory computer-readable storage medium storing computer executable code for performing carrier recovery in a radio-frequency communications system, the computer executable code comprising instructions for:
receiving a quadrature-encoded signal comprising a carrier waveform having a carrier frequency and a pilot tone having a pilot frequency;
processing the quadrature-encoded signal to produce an upper sideband signal and a lower sideband signal;
identifying, in the upper sideband signal, an upper sideband transform of the pilot tone;
identifying, in the lower sideband signal, a lower sideband transform of the pilot tone;
determining a frequency difference between the upper sideband transform and the lower sideband transform; and
adjusting a frequency of a local oscillator of the receiver based on the frequency difference.
11. The computer-readable medium of claim 10 , wherein the instructions for processing the quadrature-encoded signal comprise instructions for:
mixing the quadrature encoded signal with a reference signal generated by the local oscillator to produce an I channel and a Q channel, the I channel and the Q channel separated by a non-zero number of degrees relative to each other;
producing the upper sideband signal by combining a leading portion of the I channel with a lagging portion of the Q channel; and
producing the lower sideband signal by combining a lagging portion of the I channel with a leading portion of the Q channel.
12. The computer-readable medium of claim 11 , wherein the instructions for producing the upper sideband signal comprise instructions for:
phase shifting a portion of the I channel forward by a number of degrees to generate the leading portion of the I channel; and
phase shifting a portion of the Q channel backward by a corresponding number of degrees to generate the lagging portion of the Q channel,
wherein the lagging portion of the Q-channel lags the leading portion of the I-channel by approximately ninety degrees.
13. The computer-readable medium of claim 11 , wherein the instructions for producing the lower sideband signal comprise instructions for:
phase shifting a portion of the I channel backward by a number of degrees to generate the lagging portion of the I channel; and
phase shifting a portion of the Q channel forward by a corresponding number of degrees to generate the leading portion of the Q channel,
wherein the leading portion of the Q-channel leads the lagging portion of the I-channel by approximately ninety degrees.
14. The computer-readable medium of claim 10 , wherein the carrier frequency is in a range of 71 GHz to 76 GHz, 81 GHz to 86 GHz, and 92 GHz to 95 GHz, and the pilot frequency is in a range from 500 kHz to a data clock frequency.
15. The computer-readable medium of claim 10 , wherein the computer executable code further comprises instructions for:
mixing the quadrature encoded signal with a reference signal generated by the local oscillator to produce an I channel and a Q channel, the I channel and the Q channel separated by a non-zero number of degrees relative to each other;
determining a difference-in-magnitudes between the I channel and the Q channel
determining a phase error comprising a magnitude and a direction based on the difference-in-magnitudes; and
adjusting a phase of the local oscillator based on the magnitude and the direction of the phase error.
16. The computer-readable medium of claim 10 , wherein determining the difference-in-magnitudes comprises:
determining a difference between an amplitude of the I channel squared and an amplitude of the Q channel squared.