US-12627552-B2 - Loopback signal, receiver and transmitter for synchronization

Abstract

A loopback receiver to synchronize timing and frequency with a loopback signal relayed to the loopback receiver, the loopback receiver including: an Rx signal representing the loopback signal received at the loopback receiver; and a common Fast Fourier Transform (FFT) to estimate, during an acquisition mode and a tracking mode, an estimated timing offset and an estimated frequency offset of the Rx signal compared to the loopback signal, wherein the loopback signal includes a burst including a GOLD Pseudo Noise (PN) sequence having a good circular correlation and the GOLD PN sequence has cross-correlations within a set. Some embodiments may eliminate a bias of the estimated timing offset and the estimated frequency offset with double linearization.

Inventors

• James Jehong JONG
• Tahereh FAZEL

Assignees

• HUGHES NETWORK SYSTEMS, LLC

Dates

Publication Date

20260512

Application Date

20230315

Claims (20)

1. 1 . A loopback receiver to synchronize timing and frequency with a loopback signal relayed to the loopback receiver, the loopback receiver comprising: an Rx signal representing the loopback signal received at the loopback receiver; and a common Fast Fourier Transform (FFT) to estimate, during an acquisition mode and a tracking mode, an estimated timing offset and an estimated frequency offset of the Rx signal compared to the loopback signal, wherein the loopback signal comprises a burst comprising a GOLD Pseudo Noise (PN) sequence having a good circular correlation and the GOLD PN sequence has cross-correlations within a set.
2. 2 . The loopback receiver of claim 1 , further comprising a square time estimator, in the tracking mode of the Rx signal, to calculate the estimated timing offset.
3. 3 . The loopback receiver of claim 1 , further comprising a generalized complex interpolator, in the acquisition mode and the tracking mode of the Rx signal, to calculate the estimated timing offset and the estimated frequency offset.
4. 4 . The loopback receiver of claim 3 , further comprising a filter to linearize, in the acquisition mode of the Rx signal, a bias of the estimated timing offset and the estimated frequency offset.
5. 5 . The loopback receiver of claim 3 , further comprising a recursive filter tuned by a configurable forgetting factor (Y) to follow frequency variations of the Rx signal, wherein the configurable forgetting factor is based on one more of a target SNR, a timing drift rate and a frequency drift rate.
6. 6 . The loopback receiver of claim 3 , further comprising a first order filter having a forgetting factor (γ) of 0.1 to follow frequency and timing variations of the Rx signal.
7. 7 . The loopback receiver of claim 3 , further comprising a double linearizer, in the tracking mode, to correct a bias of the estimated timing offset and the estimated frequency offset.
8. 8 . The loopback receiver of claim 1 , wherein the common FFT transitions from the acquisition mode to the tracking mode after N trans acquisitions of the Rx signal.
9. 9 . The loopback receiver of claim 1 , wherein a Signal to Noise Ratio (SNR) of the Rx signal is less than 5 dB.
10. 10 . The loopback receiver of claim 1 , wherein a Signal to Noise Ratio (SNR) of the Rx signal is less than 0 dB.
11. 11 . The loopback receiver of claim 1 , wherein the FFT provides an outage flag metric to indicate that the loopback receiver has detected an outage of the Rx signal and is operating in an outage state.
12. 12 . The loopback receiver of claim 11 , wherein, the common FFT in the tracking mode, saves a filter state when not in outage and sets the filter state to the saved filter state when exiting from the outage state.
13. 13 . A method for synchronizing timing and frequency with a loopback signal relayed to a loopback receiver, the method comprising: receiving, at the loopback receiver, an Rx signal representing the loopback signal; and estimating with a common Fast Fourier Transform (FFT), during an acquisition mode and a tracking mode, an estimated timing offset and an estimated frequency offset of the Rx signal compared to the loopback signal, wherein the loopback signal comprises a burst comprising a GOLD Pseudo Noise (PN) sequence having a good circular correlation and the GOLD PN sequence has cross-correlations within a set.
14. 14 . The method of claim 13 , further comprising generalized complex interpolating, in the acquisition mode and the tracking mode, the estimated timing offset and the estimated frequency offset of the Rx signal.
15. 15 . The method of claim 14 , further comprising linearizing, in the acquisition mode, a bias of the estimated timing offset and the estimated frequency offset of the Rx signal.
16. 16 . The method of claim 14 , further comprising using a recursive filter tuned by a configurable forgetting factor (γ) to follow frequency variations of the Rx signal, wherein the configurable forgetting factor is based on one more of a target SNR, a timing drift rate and a frequency drift rate.
17. 17 . A receiver to synchronize timing and frequency with a signal relayed to the receiver, the receiver comprising: an Rx signal representing the signal received at the receiver; a common Fast Fourier Transform (FFT) to estimate, during an acquisition mode and a tracking mode, an estimated timing offset and an estimated frequency offset of the Rx signal compared to the signal; a generalized complex interpolator, in the acquisition mode and the tracking mode of the Rx signal, to calculate the estimated timing offset and the estimated frequency offset; and a double linearizer, in the tracking mode, to correct a bias of the estimated timing offset and the estimated frequency offset.
18. 18 . The receiver of claim 17 , further comprising a square time estimator, in the tracking mode of the Rx signal; and setting a linearized timing offset equal to the estimated timing offset.
19. 19 . The receiver of claim 17 , wherein the double linearizer comprises a look up table and a linearized frequency offset is based on using a multiplier BF determined as a value associated with the estimated timing offset in the look up table.
20. 20 . The receiver of claim 17 , further comprising a filter to linearize, in the acquisition mode of the Rx signal, a bias of the estimated timing offset and the estimated frequency offset.

Description

REFERENCE The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application Ser. No. 63/370,079 filed Aug. 1, 2022, which is incorporated herein by reference in its entirety. FIELD A loopback signal, a loopback receiver and a loopback transmitter use a signal including a GOLD Pseudo Noise (PN) sequence having a circular correlation, for example, a good circular correlation. The loopback receiver uses a common Fast Fourier Transform (FFT) module for acquisition and tracking of a loopback signal. The receiver provides finer estimation using a generalized complex interpolator and operates at a very low Signal-to-Noise Ratio (SNR), for example, a negative SNR. The receiver also handles an outage. The receiver provides reliable frequency and timing offsets due to signal propagation in the presence of impairments. The loopback signal may be relayed by a satellite. BACKGROUND A satellite provides a loopback path for ground equipment to be synchronized with respect to the satellite, Accurate measurement of the delay and Doppler between the ground equipment and satellite is used for efficient signaling and reception. A channel between the satellite and ground equipment can be impaired or attenuated by an atmospheric loss in various frequency bands. This problem is more acute at higher frequency bands such as the Ka-band. The channel between the satellite and ground equipment can be impaired or attenuated by phase noise. Severe attenuation of the channel can lead into an outage. A loopback transmitter and receiver to provide reliable frequency and timing in the presence of these impairments is needed. SUMMARY This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The present teachings disclose a loopback signal waveform definition, and an efficient loopback receiver using a common a Fast Fourier Transform (FFT) algorithm for acquisition and tracking. The teachings permit finer estimation using a generalized complex interpolator. In some embodiments, double linearization may be used to handle timing and frequency estimation bias. The receiver of the present teachings operates at a very low Signal to Noise Ratio (SNR), for example, a negative SNR. In some embodiments, the receiver handles outage of the loopback signal. In some aspects, the techniques described herein relate to a loopback receiver to synchronize timing and frequency with a loopback signal relayed to the loopback receiver, the loopback receiver including: an Rx signal representing the loopback signal received at the loopback receiver; and a common Fast Fourier Transform (FFT) to estimate, during an acquisition mode and a tracking mode, an estimated timing offset and an estimated frequency offset of the Rx signal compared to the loopback signal, wherein the loopback signal includes a burst including a GOLD Pseudo Noise (PN) sequence having a good circular correlation and the GOLD PN sequence has cross-correlations within a set. In some aspects, the techniques described herein relate to a loopback receiver, further including a square time estimator, in the tracking mode of the Rx signal, to calculate the estimated timing offset. In some aspects, the techniques described herein relate to a loopback receiver, further including a generalized complex interpolator, in the acquisition mode and the tracking mode of the Rx signal, to calculate the estimated timing offset and the estimated frequency offset. In some aspects, the techniques described herein relate to a loopback receiver, further including a filter to linearize, in the acquisition mode of the Rx signal, a bias of the estimated timing offset and the estimated frequency offset. In some aspects, the techniques described herein relate to a loopback receiver, further including a recursive filter tuned by a configurable forgetting factor (γ) to follow frequency variations of the Rx signal, wherein the configurable forgetting factor is based on one more of a target SNR, a timing drift rate and a frequency drift rate. In some aspects, the techniques described herein relate to a loopback receiver, further including a first order filter having a forgetting factor (γ) of 0.1 to follow frequency and timing variations of the Rx signal. In some aspects, the techniques described herein relate to a loopback receiver, further including a double linearizer, in the tracking mode, to correct a bias of the estimated timing offset and the estimated frequency offset. In some aspects, the techniques described herein relate to a loopback receiver, wherein the common FFT transitions from the acquisition mode to the tracking mode after Ntrans acquisitions of the Rx signal. In some aspects, the techniques described herein rela