Orthogonal signal demodulation method capable of reducing bit error rate and having Doppler frequency shift correction function

Abstract

The invention provides an orthogonal signal demodulation method with a Doppler frequency shift correction function for reducing the bit error rate, which is characterized in that when a satellite moves to a terminal for transmitting a data packet, the carrier frequency observed by a transmitter is increased, and the carrier frequency is reduced; the observed carrier frequency is reduced along with the distance between the satellite and the terminal, the change of the observed frequency is caused by Doppler frequency shift and is equal to v * f / c, v is the relative speed from the satellite to the terminal, f is the carrier frequency, c is the light velocity, and the Doppler frequency shift of the satellite is displayed; the position of the satellite changes between-600 kilometers and 600 kilometers relative to the position of the terminal in the advancing direction, for 2.4 GHz carrier frequency, when the height of the satellite is 600 kilometers and the speed of the satellite is 7.6 kilometers per second, Doppler frequency shift ranges from-35 kilometers to 35 kilometers, and the problem of frequency synchronization caused by Doppler frequency shift in satellite communication is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of a correction method for Doppler frequency shift of a low-orbit satellite Internet of Things communication system, and in particular to an orthogonal signal demodulation method with a Doppler frequency shift correction function for reducing a bit error rate. Background Art

[0002] In satellite communications, the relative motion between the satellite and the terminal causes Doppler shift, which causes the received signal frequency to change. For orthogonal modulation schemes such as M-ARY FSK, the change in Doppler shift may cause frequency synchronization failure, thus affecting the accuracy of data demodulation. Summary of the invention

[0003] The invention provides an orthogonal signal demodulation method with a Doppler frequency shift correction function for reducing bit error rate, aiming at the frequency synchronization problem caused by Doppler frequency shift in satellite communication.

[0004] The technical solution of the present invention is as follows: When the satellite moves toward the terminal transmitting the data packet, the carrier frequency observed by the transmitter will increase. The observed carrier frequency will decrease as the satellite moves away from the terminal. This change in observed frequency is due to the Doppler shift, which is equal to v*f / c, where v is the relative speed from the satellite to the terminal, f is the carrier frequency, and c is the speed of light. Figure 1 The Doppler shift for a satellite is shown whose position varies from -600 km to 600 km relative to the terminal position along the direction of travel. For a 2.4 GHz carrier frequency, the Doppler shift varies from -35 kHz to 35 kHz when the satellite is at an altitude of 600 km and a velocity of 7.6 km / s. This would not be a problem if the shift was constant, as the receiver would be able to lock onto the carrier frequency when searching for the preamble sequence. Variations in the Doppler shift over multiple data symbols can cause the receiver to lose frequency synchronization and bit errors. In the example of M-ARY FSK, we can clearly see that, assuming each symbol is spaced 125 Hz apart, small variations in the Doppler shift around 125 Hz can cause bits to be decoded incorrectly.

[0005] When the carrier frequency is 2.4 GHz and the typical pre-conference duration is 50 mS, the Doppler frequency changes as follows: Figure 2 As shown in the figure, the worst case frequency variation is about 30 Hz during the entire preamble sequence. Even with this variation, the preamble detection circuit is still able to lock onto the preamble sequence and only suffers from performance degradation (about 0.5 dB reduction in preamble detection sensitivity for an 8mS long symbol).

[0006] The Doppler correction process of the orthogonal or non-orthogonal signal demodulator is applicable to all orthogonal and non-orthogonal modulated signals, including but not limited to M-ARY FSK, chirp spread spectrum, direct sequence spread spectrum, BPSK and QPSK signals. Multiple signal demodulators 62 process input signals from all beams and each spoke. The preamble detector 63 operates along each beam and searches for the preamble symbol sequence. When the preamble detection occurs on both the row beam and the column beam, the position of the preamble transmission is estimated 64. An example of spoke 1 beam m and spoke 2 beam n detecting the preamble simultaneously is shown in the figure. Using the preamble position estimate, the Doppler shift change 65 of the remaining packet data symbols is estimated by looking up the deterministic Doppler frequency change over time curve 66 of the estimated terminal position. Unlike the lookup table, the change in Doppler frequency over time can be calculated analytically as a function of the terminal position relative to the satellite position and the known satellite velocity. Each data symbol received from the correct spatial position after the preamble sequence is then frequency adjusted 67 based on an estimated Doppler correction factor calculated from the Doppler frequency versus time curve. The frequency adjusted symbols are then sent to a packet demodulator 68 which converts the data symbols into bits and optionally applies bit deinterleaving and error correction to the bits.

[0007] The working principle and beneficial effects of the present invention are:

[0008] The purpose of the present invention is to provide an effective Doppler correction method to ensure that the received signal can still be correctly demodulated during the satellite movement process, thereby improving the reliability of the communication system.

[0009] The beneficial effect of the present invention is that it solves the frequency synchronization problem caused by Doppler frequency shift in satellite communication and improves the performance and reliability of the system. By real-time monitoring and correction of Doppler frequency shift, it ensures that the signal received during satellite movement can be accurately demodulated, reduces the bit error rate, and improves the communication quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0011] Figure 1 The Doppler frequency shift of the wireless carrier of the receiving terminal data packet observed by the low earth orbit satellite

[0012] Figure 2 A graph showing the change in carrier Doppler frequency shift of the receiving terminal data packet observed by the low earth orbit satellite over a duration of 50 ms ago;

[0013] Figure 3 Block diagram of the M-ARY quadrature signal demodulator with Doppler frequency correction. DETAILED DESCRIPTION

[0014] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0015] like Figure 1-2 As shown, this embodiment provides an orthogonal signal demodulation method with Doppler frequency shift correction function to reduce the bit error rate. When the satellite moves toward the terminal transmitting the data packet, the carrier frequency observed by the transmitter will increase. The observed carrier frequency will decrease as the satellite moves away from the terminal. This change in observed frequency is due to the Doppler frequency shift, which is equal to v*f / c, where v is the relative speed from the satellite to the terminal, f is the carrier frequency, and c is the speed of light. Figure 1 The Doppler shift for a satellite is shown whose position varies from -600 km to 600 km relative to the terminal position along the direction of travel. For a 2.4 GHz carrier frequency, the Doppler shift varies from -35 kHz to 35 kHz when the satellite is at an altitude of 600 km and a velocity of 7.6 km / s. This would not be a problem if the shift was constant, as the receiver would be able to lock onto the carrier frequency when searching for the preamble sequence. Variations in the Doppler shift over multiple data symbols can cause the receiver to lose frequency synchronization and bit errors. In the example of M-ARY FSK, we can clearly see that, assuming each symbol is spaced 125 Hz apart, small variations in the Doppler shift around 125 Hz can cause bits to be decoded incorrectly.

[0016] When the carrier frequency is 2.4 GHz and the typical pre-conference duration is 50 mS, the Doppler frequency changes as follows: Figure 2 As shown in the figure, the worst case frequency variation is about 30 Hz during the entire preamble sequence. Even with this variation, the preamble detection circuit is still able to lock onto the preamble sequence and only suffers from performance degradation (about 0.5 dB reduction in preamble detection sensitivity for an 8mS long symbol).

[0017] The Doppler correction process of the orthogonal or non-orthogonal signal demodulator is applicable to all orthogonal and non-orthogonal modulated signals, including but not limited to M-ARY FSK, chirp spread spectrum, direct sequence spread spectrum, BPSK and QPSK signals. Multiple signal demodulators 62 process input signals from all beams and each spoke. The preamble detector 63 operates along each beam and searches for the preamble symbol sequence. When the preamble detection occurs on both the row beam and the column beam, the position of the preamble transmission is estimated 64. An example of spoke 1 beam m and spoke 2 beam n detecting the preamble simultaneously is shown in the figure. Using the preamble position estimate, the Doppler shift change 65 of the remaining packet data symbols is estimated by looking up the deterministic Doppler frequency change over time curve 66 of the estimated terminal position. Unlike the lookup table, the change in Doppler frequency over time can be calculated analytically as a function of the terminal position relative to the satellite position and the known satellite velocity. Each data symbol received from the correct spatial position after the preamble sequence is then frequency adjusted 67 based on an estimated Doppler correction factor calculated from the Doppler frequency versus time curve. The frequency adjusted symbols are then sent to a packet demodulator 68 which converts the data symbols into bits and optionally applies bit deinterleaving and error correction to the bits.

[0018] One embodiment includes the coordinated use of a preamble detector, a position estimation module, a Doppler frequency change calculation module, and a frequency adjuster. The preamble detector searches for the preamble symbol sequence, the position estimation module determines the position of the preamble transmission, and the Doppler frequency change calculation module parses and calculates the Doppler frequency change curve over time. Finally, the frequency adjuster adjusts the frequency of the received data symbols according to the calculated Doppler correction factor to ensure accurate frequency synchronization.

[0019] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for orthogonal signal demodulation with Doppler frequency shift correction function for reducing bit error rate, characterized in that: As the satellite moves toward the terminal transmitting the packet, the carrier frequency observed by the transmitter will increase. The observed carrier frequency will decrease as the satellite moves away from the terminal. This change in observed frequency is due to the Doppler shift, which is equal to v*f / c, where v is the relative velocity of the satellite to the terminal, f is the carrier frequency, and c is the speed of light. The Doppler shift of a satellite is shown for a satellite whose position varies from -600 km to 600 km relative to the terminal position along the direction of travel. For a 2.4 GHz carrier frequency, the Doppler shift varies from -35 kHz to 35 kHz when the satellite is at an altitude of 600 km and a velocity of 7.6 km / s. If the frequency shift was constant, this would not be a problem because the receiver would be able to lock onto the carrier frequency when searching for the preamble sequence. Changes in the Doppler shift over multiple data symbols can cause the receiver to lose frequency synchronization and suffer bit errors. In the example of M-ARY FSK, we can clearly see that, assuming each symbol is spaced 125 Hz apart, then small changes in the Doppler shift around 125 Hz can cause bits to be decoded incorrectly.

2. The orthogonal signal demodulation method with Doppler frequency shift correction function for reducing bit error rate according to claim 1, characterized in that: At a carrier frequency of 2.4 GHz, with a typical preamble duration of 50 mS, the Doppler frequency variation is, in the worst case, about 30 Hz over the entire preamble sequence. Even with this variation, the preamble detection circuit is still able to lock onto the preamble sequence and only suffers from a degradation in performance (about 0.5 dB reduction in preamble detection sensitivity for an 8 mS long symbol).

3. The orthogonal signal demodulation method with Doppler frequency shift correction function for reducing bit error rate according to claim 1, characterized in that: The Doppler correction process of the orthogonal or non-orthogonal signal demodulator is applicable to all orthogonal and non-orthogonal modulated signals, including but not limited to M-ARY FSK, chirp spread spectrum, direct sequence spread spectrum, BPSK and QPSK signals. Multiple signal demodulators 62 process input signals from all beams and each spoke. The preamble detector 63 operates along each beam and searches for the preamble symbol sequence. When the preamble detection occurs on both the row beam and the column beam, the position of the preamble transmission is estimated as 64. The figure shows an example of spoke 1 beam m and spoke 2 beam n detecting the preamble at the same time. Using the preamble position estimation, the terminal position is estimated by finding the deterministic Doppler frequency variation curve 66 over time. The Doppler shift of the remaining packet data symbols changes 65. Unlike the lookup table, the change of Doppler frequency over time can be calculated analytically as a function of the terminal position relative to the satellite position and the known satellite velocity. Then, based on the estimated Doppler correction factor calculated from the Doppler frequency change over time curve, each data symbol received from the correct spatial position after the preamble sequence is frequency adjusted 67, and the frequency adjusted symbols are then sent to the packet demodulator 68, which converts the data symbols into bits and optionally applies bit deinterleaving and error correction to the bits.

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