CN110545247B - Low earth orbit satellite multi-carrier communication system downlink carrier frequency offset estimation and compensation method - Google Patents
Low earth orbit satellite multi-carrier communication system downlink carrier frequency offset estimation and compensation method Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
Abstract
The invention discloses a method for estimating and compensating downlink carrier frequency offset of a low-orbit satellite multi-carrier communication system, belonging to the field of satellite communication. Estimating downlink common Doppler carrier frequency offset by a gateway station based on ephemeris, and pre-compensating a downlink transmitting signal by using the downlink common Doppler carrier frequency offset estimation value; (2) the user terminal estimates the downlink residual carrier frequency offset by using the downlink synchronous signal or the reference signal, and compensates the downlink receiving signal by using the downlink residual carrier frequency offset estimation value. By using the method of the invention, the system can estimate and compensate the carrier frequency offset of the downlink with lower realization complexity under the condition that the user terminal has no ephemeris and only the gateway station has the ephemeris, thereby meeting the demodulation requirement of multi-carrier communication under the large Doppler carrier frequency offset of the low-orbit satellite.
Description
Technical Field
The invention belongs to the field of wireless communication, and particularly relates to a method for estimating and compensating downlink carrier frequency offset in low-earth orbit satellite multi-carrier communication.
Technical Field
With the development of communication demand, in order to meet the demand for high-speed data transmission in remote areas without base station coverage, it is an important issue to implement communication coverage for remote areas by low-earth orbit satellites in future mobile communication. A typical low earth orbit satellite communications system is shown in fig. 1, the system components comprising: (1) gateway station, (2) satellite, (3) user terminal. The satellite (2) is only responsible for transparent forwarding, namely, only performs frequency shift forwarding processing on the received signal, and does not perform modulation and demodulation on the signal. The link between the gateway station and the satellite is a (4) feeder link. The link between the user terminal and the satellite is (5) a user link. Wherein the user terminal is located within a user beam of the satellite. Here, a transmission link (gateway station- > satellite- > user terminal) transmitted by the gateway station, retransmitted via the satellite, and received by the user terminal is referred to as a downlink, and a transmission link (user terminal- > satellite- > gateway station) transmitted by the user terminal, retransmitted via the satellite, and received by the gateway station is referred to as an uplink.
The communication technology using the low-orbit satellite has the characteristics of low time delay and high signal-to-noise ratio, and can well meet the high-speed communication requirement of remote areas by combining multi-carrier technologies such as OFDM, DFT-S-OFDM, MC-DS-CDMA and the like. However, for low-orbit satellites, because the low-orbit satellites move at a high speed, the doppler effect is significant in a high frequency band, and serious carrier frequency offset in the system is caused.
Carrier frequency offset refers to the carrier frequency offset between the transmitter and the receiver. When the multi-carrier communication system demodulates, in order to correctly demodulate each path of data, it is necessary to ensure that very good orthogonality among the sub-carriers is maintained. When a large carrier frequency offset exists between a transmitting end and a receiving end, the sub-carrier frequency offset can be caused, the orthogonality among the sub-carriers of the receiving end is damaged, serious sub-carrier interference is caused, and the demodulation performance is rapidly deteriorated. For a low-orbit satellite with the flight altitude of 1200km, when the low-orbit satellite works in a Ka frequency band, the typical value of Doppler carrier frequency offset is larger than 600kHz and is far larger than the subcarrier spacing of a common multicarrier system, and under the condition, the system cannot demodulate correctly. Therefore, in order to ensure the effective work of the system, the carrier frequency offset is reduced by adopting a frequency offset estimation and compensation technology, which is very necessary for a low-orbit satellite multi-carrier communication system.
The carrier frequency offset of the low earth orbit satellite communication system is composed of two parts, namely Doppler frequency offset caused by relative motion between a transmitter and a receiver and frequency offset caused by frequency difference between crystal oscillators of the transmitter and the receiver. The frequency offset caused by the crystal oscillator is related to the manufacturing accuracy of the crystal oscillator, is inherent frequency offset in the system, and for a satellite communication system, the frequency offset is generally much smaller than carrier frequency offset caused by the Doppler effect.
In a conventional multi-carrier system, a user terminal generally performs downlink frequency offset estimation based on a received signal, and converts an uplink frequency offset value based on a downlink frequency offset estimation value to correct frequency offsets in uplink and downlink signals, respectively. However, in a low-earth orbit satellite communication system, under a condition of large doppler, the downlink carrier frequency offset estimation based on the received signal will greatly increase the complexity of terminal implementation. Meanwhile, because a satellite participates in forwarding, a transmission link of the low earth orbit satellite communication system consists of a feed link and a user link, and the feed link and the user link use different carrier frequencies. The downlink frequency offset estimation method based on the received signal cannot separate frequency offsets introduced by a feeder link and a user link, and if the downlink frequency offset estimation value is directly used as a frequency offset compensation value when a terminal transmits a signal, an additional compensation error is introduced into an uplink.
One solution is to compensate for the large doppler frequency offset caused by satellite movement by means of satellite ephemeris, and the gateway station and the user terminal calculate the doppler frequency offset of the feeder link and the user link based on the ephemeris and their own positions, respectively. However, it is not easy for the user terminal to acquire real-time ephemeris, and this solution still has problems when the user terminal cannot acquire ephemeris or has no real-time ephemeris.
Therefore, in order to solve the problems of high implementation complexity, introduction of additional uplink compensation errors, dependence of the user terminal on ephemeris and the like in the downlink carrier frequency offset estimation and compensation of the low-orbit satellite multi-carrier communication system, a novel downlink carrier frequency offset estimation and compensation method which is adaptive to the low-orbit satellite multi-carrier communication scene and in which the user terminal does not depend on ephemeris needs to be provided.
Disclosure of Invention
The invention provides a method for estimating and compensating downlink carrier frequency offset of a low-orbit satellite multi-carrier communication system, which aims to solve the problems of high implementation complexity, introduction of additional uplink compensation errors, dependence of a user terminal on ephemeris and the like of the downlink carrier frequency offset estimation and compensation of the low-orbit satellite multi-carrier communication system.
Firstly, estimating downlink common Doppler carrier frequency offset by adopting a gateway station based on ephemeris, and pre-compensating a downlink transmitting signal by using a downlink common Doppler carrier frequency offset estimation value; and then, the user terminal estimates the downlink residual carrier frequency offset by using the downlink synchronous signal or the reference signal and compensates the downlink receiving signal by using the downlink residual carrier frequency offset estimation value.
Therefore, the technical scheme of the invention is a method for estimating and compensating the frequency offset of the downlink carrier wave of a low-orbit satellite multi-carrier communication system, which comprises the following steps:
step 1: the downlink common Doppler carrier frequency offset consists of two parts, namely downlink feeder link Doppler carrier frequency offset and downlink user link common Doppler carrier frequency offset; the gateway station calculates the Doppler carrier frequency offset of the downlink feeder link according to the ephemeris information, the position of the gateway station and the carrier frequency of the downlink feeder link; selecting a beam coverage center position as a cell reference point, wherein the position is uniquely determined by a satellite position and a satellite beam direction; when the user terminal is positioned at a cell reference point, the Doppler carrier frequency offset introduced by the downlink user link is defined as the common Doppler carrier frequency offset of the downlink user link; the gateway station calculates the common Doppler carrier frequency offset of the downlink user link according to the ephemeris information, the cell reference point position and the carrier frequency of the downlink user link; the gateway station uses the frequency offset estimation value of the downlink common Doppler carrier to pre-compensate the downlink transmitting signal;
step 2: the downlink residual carrier frequency offset consists of Doppler carrier frequency offset between a user terminal and a cell reference point, crystal oscillator frequency offset and the pre-compensation error in the step 1; the receiving end carries out joint processing on the received downlink synchronous signal or reference signal and a locally generated synchronous signal or reference signal, and estimates the downlink residual carrier frequency offset by utilizing the correlation characteristic of the signal; and the user terminal compensates the downlink residual frequency offset when receiving the signal according to the estimated value of the downlink residual carrier frequency offset.
Further, in the step 1, the frequency offset f of the doppler carrier wave of the downlink feeder linkfeeder，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the feeder link,
is the direction of the satellite's connection to the gateway station,
is the velocity vector of the satellite, c is the speed of light;
common Doppler carrier frequency offset f of downlink user linkuser，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the user link,
is the direction of the connection of the satellite to the user terminal,
is the velocity vector of the satellite, c is the speed of light;
the frequency offset estimation value of the downlink common Doppler carrier wave is as follows:
fcommon，downlink＝ffeeder，downlink+fuser，downlink。
further, the specific method of step 2 is as follows:
step 2.1: let z + ∈ represent relative frequency offset, defined as the ratio between frequency offset and subcarrier spacing Δ f, z is the integer part of frequency offset, called integer frequency offset, and epsilon is the fractional part of frequency offset, called fractional frequency offset; when there is a relative frequency offset of z + ∈, the synchronization signal or reference signal sequence x (n) at the transmitting end is expressed as received at the receiving end
Wherein, N represents the number of sampling points, N is the serial number of sampling, and w (N) represents noise;
step 2.2: the user terminal locally generates a synchronous signal sequence copy X (k) on a frequency domain, then transforms the synchronous signal sequence copy X (k) to a time domain, records the synchronous signal sequence copy X (k) as x (n), performs cross-correlation operation with r (n), and calculates to obtain the residual frequency offset with the relative decimal multiple of the size as follows:
in the formula (DEG)*Representing taking the conjugate of the signal;
step 2.3: size the received signal to
And performing fractional frequency offset compensation, performing FFT (fast Fourier transform) on the received signal to transform the received signal to a frequency domain to perform integral frequency offset estimation, and recording the frequency domain received signal on the kth subcarrier as R (k). When the synchronization signal adopts an m sequence, the integer frequency offset performs sliding correlation operation by using the property of the m sequence to obtain the integer frequency offset as follows:
step 2.4: compensating for integer frequency offsets of signals
The invention provides a method for estimating and compensating carrier frequency offset of a downlink of a low-orbit satellite multi-carrier communication system, which has the following beneficial effects compared with the prior art:
in the invention, after the gateway station compensates the frequency offset of the downlink common Doppler carrier based on ephemeris, the user terminal only needs to compensate the frequency offset of the downlink residual carrier, thereby reducing the frequency offset estimation range of the downlink user terminal and further reducing the implementation complexity of a frequency offset estimation module of the user terminal; after the gateway station compensates the frequency offset of the downlink common Doppler carrier based on ephemeris, the received signal of the user terminal does not contain the Doppler frequency offset introduced by the feeder link any more, so that the separation of the Doppler frequency offset of the feeder link and the user link is realized, and the converted uplink frequency offset estimation value is more accurate; the method is suitable for the scene that the user terminal can not rely on ephemeris to calculate the Doppler carrier frequency offset of the relative satellite.
Drawings
FIG. 1 is a schematic diagram of a low-earth orbit satellite multi-carrier communication system
FIG. 2 is a schematic diagram of the frequency offset compensation process of the downlink carrier of the multi-carrier communication system of the low earth orbit satellite according to the present invention
Fig. 3 is a schematic diagram illustrating the calculation principle of doppler carrier frequency offset of the feeder link according to the present invention.
Detailed Description
The invention is further illustrated with reference to the figures and examples.
The invention provides a method for estimating and compensating downlink carrier frequency offset of a low earth orbit satellite multi-carrier communication system. Fig. 1 shows a low-earth orbit satellite multicarrier communication system to which the present invention is applied, and the system comprises: (1) gateway station, (2) satellite, (3) user terminal. The satellite (2) is only responsible for transparent forwarding, namely, only performs frequency shift forwarding processing on the received signal, and does not perform modulation and demodulation on the signal. The link between the gateway station and the satellite is a (4) feeder link. The link between the user terminal and the satellite is (5) a user link. Wherein the user terminal is located within a user beam of the satellite. Here, a transmission link (gateway station- > satellite- > user terminal) transmitted by the gateway station, retransmitted via the satellite, and received by the user terminal is referred to as a downlink, and a transmission link (user terminal- > satellite- > gateway station) transmitted by the user terminal, retransmitted via the satellite, and received by the gateway station is referred to as an uplink.
Referring to fig. 2, a specific example of the method for estimating and compensating the frequency offset of the downlink carrier is as follows:
step 1, the gateway station estimates the downlink common Doppler carrier frequency offset based on ephemeris and pre-compensates the downlink transmitting signal by using the downlink common Doppler carrier frequency offset estimation value. Specifically, the gateway station calculates the downlink doppler carrier frequency offset according to the ephemeris information, the position of the gateway station, and the downlink carrier frequency. Referring to fig. 3, the downlink feeder link doppler carrier frequency offset ffeeder，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the feeder link,
is the direction of the satellite's connection to the gateway station,
is the velocity vector of the satellite, c is the speed of light;
selecting a beam coverage center position as a cell reference point, wherein the position is uniquely determined by a satellite position and a satellite beam direction; when the user terminal is positioned at a cell reference point, the Doppler carrier frequency offset introduced by the downlink user link is defined as the common Doppler carrier frequency offset of the downlink user link; the gateway station calculates the common Doppler carrier frequency offset of the downlink user link according to the ephemeris information, the cell reference point position and the carrier frequency of the downlink user link; referring to fig. 3, the downlink user link common doppler carrier frequency offset fuser，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the user link,
is the direction of the connection of the satellite to the user terminal,
is the velocity vector of the satellite, c is the speed of light;
calculating the frequency offset estimation value of the downlink common Doppler carrier wave as follows:
fcommon，downlink＝ffeeder，downlink+fuser，downlinkand the gateway station pre-compensates the downlink transmitting signal by using the downlink common Doppler carrier frequency offset estimation value.
And 2, estimating the downlink residual carrier frequency offset by the user terminal by using the downlink synchronous signal or the reference signal, and compensating the downlink receiving signal by using the downlink residual carrier frequency offset estimation value. Specifically, the signal received by the user terminal after pre-compensation still has residual carrier frequency offset. The residual carrier frequency offset comprises Doppler carrier frequency deviation between the user terminal and a cell reference point, crystal oscillator frequency offset and step 1 precompensation error. And the user terminal carries out residual carrier frequency offset estimation by utilizing the downlink synchronous signal. Let z + ∈ denote relative frequency offset, defined as the ratio between frequency offset and subcarrier spacing Δ f, where z is the integer part of frequency offset, called frequency offset, and epsilon is the fractional part of frequency offset, called fractional frequency offset. When there is a relative frequency offset of z + ∈, the synchronization signal or reference signal sequence x (n) at the transmitting end is expressed as received at the receiving end
Where N represents the number of sampling points, N is the number of samples, and w (N) represents noise. The user terminal locally generates a synchronization signal sequence copy X (k) on a frequency domain, then transforms the synchronization signal sequence copy X (k) to a time domain, records the synchronization signal sequence copy X (n), and performs cross-correlation operation with r (n). Calculating to obtain the residual frequency offset with the relative decimal multiple as:
in the formula (DEG)*Meaning taking the conjugate of the signal. The receiving end carries out the size of the received signal to
And performing FFT on the received signal to transform the received signal to a frequency domain for further integral frequency offset estimation.
Note that the frequency domain received signal on the k-th subcarrier is r (k). When the synchronization signal adopts an m sequence, the integer frequency offset performs sliding correlation operation by using the property of the m sequence to obtain the integer frequency offset as follows:
the receiving end estimates the result according to the integer carrier frequency deviation
Compensating for integer frequency offset of the received signal.
The following specific examples of the present invention are given in conjunction with the flow chart:
the flight height of the satellite is 1175km, the flight speed of the satellite is 7.2km/s, the carrier frequency of a downlink user link is 20GHz, and the carrier frequency of a downlink feeder link is 40 GHz; the elevation angle of the gateway station to the satellite is 5 degrees, the width of a satellite beam is 3 degrees, the maximum elevation angle of a cell reference point and the satellite is 30 degrees, and a user terminal is positioned at the edge of the cell. The actual Doppler carrier frequency offset of the downlink feeder link is 1040kHz, the Doppler carrier frequency offset of the public part of the downlink user link is 350kHz, the Doppler carrier frequency offset between the user terminal and the cell reference point is-10 kHz, and the total Doppler carrier frequency offset of the downlink is 1380 kHz.
According to the step 1, the frequency offset of the downlink common Doppler carrier is calculated based on ephemeris, and the typical calculation error is 1 kHz.
According to the step 2, the user terminal estimates the downlink residual carrier frequency offset by using the downlink synchronous signal or the reference signal. The downlink residual carrier frequency offset to be estimated is 11kHz, which is the doppler carrier frequency offset (-10kHz) between the user terminal and the cell reference point, plus typical crystal frequency offset (20kHz) + step 1 pre-compensation error (1 kHz).
For the conventional downlink carrier frequency offset estimation and compensation method without pre-compensation, the carrier frequency offset value to be estimated when the user terminal receives the signal is 1400kHz, which is the total downlink doppler carrier frequency offset (1380kHz) + typical crystal oscillator frequency offset (20kHz), which is much larger than the carrier frequency offset to be estimated in the step 2. Therefore, the method effectively reduces the carrier frequency offset range to be estimated when the user terminal receives the carrier frequency offset, reduces the estimation difficulty of the carrier frequency offset, and reduces the implementation complexity of the frequency offset estimation module of the user terminal.
The present invention has been described in detail with reference to the embodiments thereof, which are set forth herein in the detailed description, but are intended to be exemplary and explanatory only and are not restrictive of the invention, as claimed. Meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.
Claims (3)
1. A method for estimating and compensating downlink carrier frequency offset of a low-earth orbit satellite multi-carrier communication system comprises the following steps:
step 1: the downlink common Doppler carrier frequency offset consists of two parts, namely downlink feeder link Doppler carrier frequency offset and downlink user link common Doppler carrier frequency offset; the gateway station calculates the Doppler carrier frequency offset of the downlink feeder link according to the ephemeris information, the position of the gateway station and the carrier frequency of the downlink feeder link; selecting a beam coverage center position as a cell reference point, wherein the position is uniquely determined by a satellite position and a satellite beam direction; when the user terminal is positioned at a cell reference point, the Doppler carrier frequency offset introduced by the downlink user link is defined as the common Doppler carrier frequency offset of the downlink user link; the gateway station calculates the common Doppler carrier frequency offset of the downlink user link according to the ephemeris information, the cell reference point position and the carrier frequency of the downlink user link; the gateway station uses the frequency offset estimation value of the downlink common Doppler carrier to pre-compensate the downlink transmitting signal;
step 2: the downlink residual carrier frequency offset consists of Doppler carrier frequency offset between a user terminal and a cell reference point, crystal oscillator frequency offset and the pre-compensation error in the step 1; the receiving end carries out joint processing on the received downlink synchronous signal or reference signal and a locally generated synchronous signal or reference signal, and estimates the downlink residual carrier frequency offset by utilizing the correlation characteristic of the signal; and the user terminal compensates the downlink residual frequency offset when receiving the signal according to the estimated value of the downlink residual carrier frequency offset.
2. The method of claim 1, wherein the step 1 comprises a step of estimating and compensating a downlink carrier frequency offset f of the multi-carrier communication system of the low earth orbit satellite, wherein the step of estimating and compensating the downlink carrier frequency offset f comprises a step of estimating a downlink carrier frequency offset f of the multi-carrier communication system of the low earth orbit satellite according to the received downlink carrier frequency offset ffeeder，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the feeder link,
is the direction of the satellite's connection to the gateway station,
is the velocity vector of the satellite, c is the speed of light;
common Doppler carrier frequency offset f of downlink user linkuser，downlinkIs calculated as:
wherein the content of the first and second substances,
for the downlink carrier frequency of the user link,
is the direction of the connection of the satellite to the user terminal,
is the velocity vector of the satellite, c is the speed of light;
the frequency offset estimation value of the downlink common Doppler carrier wave is as follows:
fcommon，downlink＝ffeeder，downlink+fuser，downlink。
3. the method of claim 1, wherein the step 2 comprises the following steps:
step 2.1: let z + ∈ represent relative frequency offset, defined as the ratio between frequency offset and subcarrier spacing Δ f, z is the integer part of frequency offset, called integer frequency offset, and epsilon is the fractional part of frequency offset, called fractional frequency offset; when there is a relative frequency offset of z + ∈, the synchronization signal or reference signal sequence x (n) at the transmitting end is expressed as received at the receiving end
Wherein, N represents the number of sampling points, N is the serial number of sampling, and w (N) represents noise;
step 2.2: the user terminal locally generates a synchronous signal sequence copy X (k) on a frequency domain, then transforms the synchronous signal sequence copy X (k) to a time domain, records the synchronous signal sequence copy X (k) as x (n), performs cross-correlation operation with r (n), and calculates to obtain the residual frequency offset with the relative decimal multiple of the size as follows:
in the formula (DEG)*Representing taking the conjugate of the signal;
step 2.3: size the received signal to
The fractional frequency offset compensation, then the received signal is FFT transformed to the frequency domain to carry out integer frequency offset estimation, the frequency domain received signal on the kth subcarrier is recorded as R (k), when the synchronous signal adopts an m sequence, the integer frequency offset is subjected to sliding correlation operation by utilizing the property of the m sequence, and the obtained integer frequency offset is as follows:
step 2.4: compensating for integer frequency offsets of signals
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