RU2787776C1 - Method for quasi-synchronous multiple access with code separation based on clock frequency synchronization conditions - Google Patents

Abstract

FIELD: communication technology.

SUBSTANCE: invention relates to communication technology and can be used in satellite communication systems. Based on the clock frequency synchronization conditions with an open radio channel, many transmitting stations share energy resources to transmit signals via the CDMA transmission mode with an extended spectrum; with the help of an external guiding signal, clock frequency synchronization is provided for CDMA signals transmitted by a plurality of terminal stations in an open radio channel and the difference between delays in signal propagation from a plurality of terminal stations to a relay node or receiving station is eliminated, so that a plurality of CDMA signals received by a relay node or receiving station are synchronized in clock frequency and phase-matched, providing quasi-synchronous reception of multiple CDMA signals.

EFFECT: ensuring the operation of CDMA in quasi-synchronous conditions in channels with a relay node and without a relay node.

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Description

Cross-reference to related applications

This application is based on Chinese Patent Application No. CN 2019106968235 filed on July 30, 2019, the entire content of which is incorporated herein by reference and claims priority.

FIELD OF TECHNOLOGY

The present invention relates to the field of wireless communication technology, in particular, to a method for quasi-synchronous code division multiple access based on clock synchronization by providing a pilot signal.

BACKGROUND OF THE INVENTION

In the CDMA (Code Division Multiple Access) access mode, different spread spectrum address codes are allocated to different users to share satellite resources. CDMA users occupy the same frequency and transmit at the same time. Users differ in the orthogonality of the codes. Compared with FDMA (Frequency Division Multiple Access), CDMA technology is relatively complex and is mainly used in a star network.

CDMA technology has been recognized as the second generation (IS-95) and third generation (CDMA2000) communication standards and has become one of the main communication systems. At the same time, CDMA technology is also widely used in military communication systems due to its anti-jamming, strong privacy protection and low transmission power. Compared to FDMA and TDMA, CDMA has the advantage of flexible bandwidth. A communication system using FDMA and TDMA (Time Division Multiple Access) may not meet the communication needs of other users if the number of users is greater than the number of frequency bands and time slots. Therefore, TDMA and FDMA have a hard bandwidth limitation. CDMA technology can use multi-user discovery to improve user throughput.

The CDMA multiplexing technology is widely used in satellite communications with the previous element. First, the transmission power of CDMA is lower than that of TDMA and FDMA, so that it provides strong privacy protection. Second, as mentioned above, CDMA has the advantage of flexible bandwidth compared to FDMA and TDMA, and can meet the traffic demand of more users through efficient multi-user discovery technology. Third, CDMA can counteract narrowband interference. Finally, CDMA can reduce the backhaul EIRP and prevent interference with neighboring satellites, in particular, meeting the communication requirements of a satellite communication system for "mobile communication". Therefore, whether it is a wireless communication system or a satellite communication system, CDMA holds the promise of a wide range of applications.

In DS-CDMA (Direct Sequence Code Division Multiple Access), each user's symbols are modulated by a group of orthogonal waveforms. Each user is assigned a waveform that is orthogonal to the waveform of the other users. In FIG. 1 shows the DS-CDMA system model. In general, DS-CDMA is a wideband spread spectrum signal obtained by multiplying a narrowband information containing signal with a high speed address code signal. The receiving end must use the same address code signal synchronized with the transmitting end to control the carrier phase of the input frequency converter to perform compression. DS-CDMA has the advantages of narrowband interference prevention, multipath fading prevention, and strong privacy protection. Other advantages of DS-CDMA are as follows: multiple users can share frequency resources without complicated frequency allocation and management; it has the property of "flexible bandwidth", that is, due to the increase in the number of users within a certain limit, only the signal-to-noise ratio will decrease without interrupting the connection, which means that DS-CDMA does not have an absolute bandwidth limit.

However, due to the strict synchronization of conventional CDMA, it is difficult to apply it directly to geosynchronous satellite communications.

The cycle of operation of a geosynchronous satellite is equal to the period of rotation of the planet Earth. If the eccentricity and inclination of the orbit are zero, this means that the position of the trajectory of the sub-satellite point remains unchanged. From any point on Earth, the satellite is stationary. The orbit is called geostationary orbit. In a geosynchronous circular orbit with non-zero inclination, its trajectory of the sub-satellite point has the shape of "8" and the highest north-south latitude over which the satellite flies is equal to the inclination of its orbit. If the operation of the satellite is affected by disturbances, the trajectory of the sub-satellite point will have the shape of a curved "8". As a result, the ground station must monitor the satellite's elevation in real time and deal with the problem of inaccurate synchronization.

SHORT DESCRIPTION

To overcome the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a method for quasi-synchronous code division multiple access based on clock synchronization by providing a pilot signal. It can provide CDMA operation in quasi-synchronous conditions, so that it is suitable for open radio links with relay nodes, such as satellite communication systems, and open radio links without relay nodes.

The technical scheme of the present invention is as follows.

This quasi-synchronous code division multiple access method based on clock synchronization by providing a pilot signal includes:

application of the spread spectrum CDMA transmission mode for sharing energy resources for transmitting signals to several transmitting stations over open radio channels;

use an external pilot to provide clock synchronization for CDMA signals transmitted by multiple terminal stations over open radio channels, eliminate propagation delay differences from multiple terminal stations to a relay node or receiver station, and provide clock synchronization and phase matching for multi-channel signals CDMA received at a relay node or a receiving station in order to realize quasi-synchronous reception of multi-channel CDMA signals.

The present invention adopts a spread spectrum CDMA transmission mode to share power resources for transmitting signals to multiple transmitting stations over open radio channels; apply an external pilot to provide clock synchronization for CDMA signals transmitted by multiple terminal stations over open radio channels, eliminate propagation delay differences from multiple terminal stations to a relay node or receiver station, and provide clock synchronization and phase matching for multi-channel signals CDMA received at a relay node or a receiving station in order to realize quasi-synchronous reception of multi-channel CDMA signals. Therefore, it can provide CDMA operation in quasi-synchronous conditions, so that it is suitable for open radio links with relay nodes, such as satellite communication systems, and open radio links without relay nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1 is a block diagram illustrating a DS-CDMA system model.

In FIG. 2 is a block diagram illustrating the trajectory of a sub-satellite point of a non-geosynchronous satellite.

In FIG. 3 is a block diagram of a pilot request method.

In FIG. 4 is a block diagram illustrating a pilot frame structure according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

This quasi-synchronous code division multiple access method based on clock synchronization by providing a pilot signal includes:

application of the spread spectrum CDMA transmission mode for sharing energy resources for transmitting signals to several transmitting stations over open radio channels;

use an external pilot to provide clock synchronization for CDMA signals transmitted by multiple terminal stations over open radio channels, eliminate propagation delay differences from multiple terminal stations to a relay node or receiver station, and provide clock synchronization and phase matching for multi-channel signals CDMA received at a relay node or a receiving station to realize quasi-synchronous reception of multi-channel CDMA signals, that is, quasi-synchronous code division multiple access.

The present invention adopts a spread spectrum CDMA transmission mode to share power resources for transmitting signals to multiple transmitting stations over open radio channels; apply an external pilot to provide clock synchronization for CDMA signals transmitted by multiple terminal stations over open radio channels, eliminate propagation delay differences from multiple terminal stations to a relay node or receiver station, and provide clock synchronization and phase matching for multi-channel signals CDMA received at a relay node or a receiving station in order to realize quasi-synchronous reception of multi-channel CDMA signals. Therefore, it can provide CDMA operation in quasi-synchronous conditions, so that it is suitable for geosynchronous satellite communications.

Preferably, the external pilot signal contains a stable and continuous stream of clock synchronization information and a continuous stream of reference clock information. Reception of an external pilot signal ensures that several terminal stations perform quasi-clock synchronization in the system based on an external clock generator and set a time reference.

In addition, the relay node independently transmits and independently receives a trial frame to each terminal station if the relay node is present in open radio channels and the position or trajectory of the relay node is known; measuring the absolute distance between the terminal station and the relay node; performing phase compensation for the difference in distances of several terminal stations transmitting CDMA signals, relative to the relay node for the implementation of the transmitted multi-channel CDMA signals phase matching when reaching the relay node; perform clock and phase matching based on the external pilot signal by combining with quasi-clocking based on the external pilot signal to provide quasi-synchronous combining of multi-channel CDMA signals at the relay node and perform quasi-synchronous reception at the receiving terminal station.

Alternatively, a test frame is transmitted to the receiving terminal station in an appropriate manner from several transmitting terminal stations based on the implementation of quasi-clocking by the external pilot signal, the response to the test provided by the receiving terminal station is received, and the distance between each transmitting terminal station and the receiving terminal station is measured, if there is no relay node in open radio channels, and several terminal stations transmit CDMA signals to the same terminal station; performing phase compensation for a distance difference of the plurality of terminal stations transmitting CDMA signals with respect to respective receiving terminal stations so that the transmitted multi-channel CDMA signals are phase-matched upon reaching the receiving terminal stations; and performing clock and phase matching based on the external pilot by combining with quasi-clocking based on the external pilot to achieve quasi-synchronous combining of multi-channel CDMA signals at receiving terminal stations.

Preferably, clock synchronization is maintained by continuous or intermittent reception of the external pilot signal after performing quasi-clock synchronization based on the external pilot signal; periodically transmitting a pilot frame to the relay node or the receiving terminal station from the transmitting terminal station due to a change in the distance from the transmitting terminal station to the relay node or the receiving terminal station accumulated over time; re-measure the distance and re-set the phase to achieve phase matching. The implementation of Quasi-Synchronous CDMA depends on the accuracy of the phase matching and clock timing, which correspond to the change in relative position between the transmitting terminal station and the relay node or receiving terminal station during a ranging cycle.

In addition, in order to achieve quasi-synchronous reception in CDMA, the phase matching accuracy is less than 1/4 of the spread spectrum chip width.

In addition, the Global Positioning System (GPS) and BeiDou Satellite Navigation System are external pilot sources used to support quasi-synchronous CDMA reception. The GPS/BeiDou system can provide accurate clock synchronization for terminal stations and provide geographic location information for each terminal station. If there is no relay node in the channel, the distances between the transmitting terminal stations and the receiving terminal stations of multiple CDMA signals are determined by direct measurement, and the distance differences are compensated at the transmitting ends to achieve quasi-synchronous reception of the multiple CDMA signals by the receiving terminal station. If there is a relay node in the channel, after knowing the location or trajectory information of the relay node, measure the distance between the transmitting terminal station and the relay node, compensate for the difference in distances between the transmitting end of the multi-channel CDMA and the relay node, perform quasi-synchronous combining of multi-channel CDMA signals at the relay node. node and further perform quasi-synchronous reception of multi-channel CDMA signals at the receiving end. The phase matching effect of the CDMA signal depends on the clock frequency accuracy and positioning accuracy of the GPS/BeiDou signal.

In addition, as shown in FIG. 3, the reference clock is obtained by means of a pilot signal; while the pilot signal:

selects a terminal station as a reference in the open radio channels and transmits a pilot signal; occupies all or some frequency bands of open radio channels for pilot signal transmission; while all or some frequency bands of open radio channels occupied by pilot signals are pilot signal bands, and the ratio of pilot signal power to the total power of pilot signal frequency bands is 0.1‰~5%; spreads with the spread spectrum code and is superimposed on the communication signal in the pilot bands in a signal mode with a low power spectrum, similar to the noise in the transmission of the pilot signal; the effect on the received signal-to-noise ratio for the signal of the receiving end is less than the effect on the local thermal noise of the receiving end;

wherein the pilot signal provides a pilot frequency, a carrier waveform, and clock synchronization, standard clock information, and pointing information for building a network and interconnecting multiple types of terminal stations over channels.

In addition, as shown in FIG. 4, the pilot signal contains a frame header, a frame number, and a data array with pointing information;

wherein the frame header contains a sync header and a pilot frequency; wherein the synchronizing header is configured to time the pilot signal frame and recover the carrier waveform, and the pilot frequency is configured to eliminate the frequency offset of the pilot signal frame;

the frame number is configured to identify the cyclic sequence of the pilot frame;

the index information data array is configured to contain index information for indicating the channel, network status, and control information;

while demodulating and receiving a pilot signal, extracting the carrier and information about the clock frequency in each terminal station; the reference clock is obtained by the duration of the fixed signal frame and the frame identification number in the signal frame; identifying a reference station transmitting pilot information according to the pilot information in the pilot signal, and receiving other indication information.

Preferably, the timing is in the range of 1/2~1/32 timeslot. In this case, the effect of 1/32 time interval is the most optimal, which can ensure the implementation of CDMA in a state close to synchronous.

Specifically, for a signal transmitted by a transmitting station, the length of each frame is fixed, the bit rate of each frame is fixed, so the length of time represented by each frame is also fixed. Therefore, the receiving station can determine the time interval by the interval between frames of the signal transmitted by the transmitting terminal station. The interval between signal frames is taken as the basic element of the clock frequency, and time calibration is performed by the time difference between frames with different frame numbers. If the count of the time interval between the leading and trailing frame is not equal to the duration of the signal frames, the clock frequency of the receiving station must be adjusted.

The ground receiving station continuously receives the pilot signal and can recognize each frame in the received pilot signal in accordance with the frame structure. As for the pilot signal, the length of each frame is fixed, the bit rate of each frame is fixed, thus the time represented by each frame is also fixed. Therefore, the ground receiving station may determine the time interval by the interval between pilot frames. The interval between signal frames (ie, signal frame duration) is taken as the fundamental element of the clock frequency, and time calibration is performed by the time difference between frames with different frame numbers. If the time interval between the leading and trailing frames is not equal to the duration of the pilot frames, the ground receiving station clock must be adjusted.

In addition, the signal frame duration is in the range of 50ms~250ms. When the signal frame duration is an integer multiple of 50 ms, such as 50 ms, 100 ms, 150 ms, 200 ms, and 250 ms, it is optimal as the main clock and calibration element.

When the transmitting end of the pilot is a satellite,

a pilot signal is superimposed on each carrier wave of the satellite data signal and broadcast by the satellite; communicating with the spread spectrum for the pilot signal through the spread code, and the power is 1‰~1% of the power of the transceiver of the communication satellite;

monitoring the satellite pilot signal and performing clock synchronization between the user receiving station and the satellite clock by receiving the pilot signal at the user receiving station in the network.

When the transmitting end of the pilot signal is a control center station among ground stations,

a pilot signal is superimposed on each carrier wave of the ground station data signal as a transmitting end signal;

demodulating and receiving the pilot signal to the ground station as a receiving end signal, extracting information about the carrier wave, and identifying the ground station as the transmitting end of the signal;

communicating with the spread spectrum for the pilot signal through the spread code, and the power is 0.1‰~5% of the power of the transceiver of the communication satellite.

In particular, the pilot signal contains: a frame header, a frame number, and a data array with pointing information. In FIG. 4 is a block diagram illustrating a pilot frame structure according to an embodiment of the present invention.

In addition, the frame header is configured to calculate from the time frame of the pilot signal and restore the carrier wave, the length of which is in the range of 16~128 bits. The pilot part is the all-0 part, and its length is more than 60 bits. It is configured to eliminate pilot frame frequency offset. The frame number field is longer than 8 bits and is configured to measure the absolute distance of the ground station. The data array contains the network management information and is configured to manage the satellite network information and store it at other stations in the network. The length is in the range of 256~1008 bits.

In addition, the OVSF code (Orthogonal Variable Spreading Factor. The OVSF code is mainly configured for orthogonal spreading. The length of the OVSF code used depends on the overhead channel rate) is configured to distinguish between a channel and a spreading rate. The reason the OVSF code is used is because it has orthogonality and variable length.

In addition, the spread spectrum sequence uses the OVSF code. This provides support for variable speeds.

In addition, the pilot signal is circularly modulated by binary phase shift keying and continuously transmitted; each user station is equipped with a pilot receiver. The pilot receiver despreads the pilot signal and then directly receives it to obtain a pilot signal frame. An information bit is determined by a frame header, and a pilot signal is continuously received to perform quasi clock synchronization. Because the frame length and the Geometric Data Processing (GDP) time are fixed, quasi clock synchronization can be realized by continuously receiving the GDP.

In this case, for a large signal (a signal whose power is more than half the power of the entire transceiver is considered a large signal), reception is carried out by reordering the TDMA frame header, the large signal is eliminated to obtain a mixed signal of small signals (a signal whose power is less than 10% of the power of the entire transceiver, considered a small signal) and then the OVSF orthogonal code is configured to compress the mixed signal of small signals for reception.

Meanwhile, for an average signal (a signal whose power is 10% ~ 50% of the power of the entire repeater is considered an average signal), due to the orthogonality of the OVSF code, the corresponding spread code can be directly configured to perform compression reception to obtain the target signal.

Preferably, the signal frame duration is in the range of 50ms~250ms.

When the signal frame duration is an integer multiple of 50 ms, such as 50 ms, 100 ms, 150 ms, 200 ms, and 250 ms, it is optimal as the main clock and calibration element.

Preferably, the change in absolute distance between the satellite and the ground station is derived from the phase difference between the transmitting end and the receiving end.

In the process in which the control center station continuously transmits the pilot signal, the stationary satellite continues to move around the Earth, describing the trajectory in the form of "8" relative to the sub-satellite point. Therefore, the distance between the satellite and the control center station changes continuously, resulting in a continuous change in the pilot propagation delay. However, the movement of the satellite shows a certain pattern with the change from far to near and then from near to far. Therefore, the change in propagation delay also shows a certain pattern.

For the control center station, by self-reception of the pilot signal, the change in the propagation delay of the link transmitted to the satellite corresponds to the change in the propagation delay of the link received from the signal of the satellite. Therefore, the phase delay between the receiving end and the transmitting end of the pilot signal is equivalent to twice the change in propagation delay from satellite to ground. Therefore, when the spread spectrum communication receiver detects that the maximum correlation peak is moving in a certain direction, a phase delay can be obtained while tracking the maximum correlation peak. Half the phase delay is the propagation delay. The change in absolute distance between the satellite and the reference station can be obtained by multiplying the propagation delay by the speed of light.

Preferably, the determination of the satellite position is carried out by changing the approximate absolute distance between the earth stations of the satellite communication system to the exact absolute distance between the earth stations of the satellite communication system.

Through the pilot signal, the absolute time of a control center station transmitting a frame with a specific frame number and receiving a frame with a specific frame number can be used to provide a difference to obtain an absolute propagation delay between earth stations of a satellite communication system, which can be multiplied by the speed of light to obtain a value absolute satellite-to-ground distance. However, since the satellite communication propagation delay is approximately 280 ms, the absolute distance sampling in this time period is smaller and it is not possible to make a very accurate determination of the satellite position.

However, the absolute distance can be supplemented by adding a change in the absolute distance measured while moving the maximum correlation peak of the spread spectrum communication receiver, since the number of samples in this time period is very large. Between the two absolute distances, the absolute distance in the intermediate time period can be predicted by the absolute distance change value to realize highly accurate satellite position determination.

Preferably, the pilot signal is circularly modulated by binary phase shift keying and continuously transmitted. Thus, all ground stations in the network can receive the pilot signal.

The above content is only the preferred embodiments of the present invention and does not limit the present invention in any way. Any improvements, additions and alternative changes made to the above embodiments in accordance with the technical essence of the present invention are within the claimed scope of the present invention.

Claims (30)

1. A method for quasi-synchronous code division multiple access based on clock synchronization by providing a pilot signal, comprising: application of the spread spectrum CDMA transmission mode for sharing energy resources for transmitting signals to several transmitting stations over open radio channels; using an external pilot to provide clock synchronization for CDMA signals transmitted by multiple terminal stations over open radio channels; elimination of delay differences in signal propagation from several terminal stations to a relay node or receiving station; and providing clock synchronization and phase matching of multi-channel CDMA signals received at the relay node or receiving station to realize quasi-synchronous reception of multi-channel CDMA signals, wherein the external pilot signal comprises a stable and continuous stream of clock synchronization information and a continuous stream of reference clock information; at the same time, an external pilot signal is received to ensure that the multiple terminal stations implement quasi-clock synchronization based on the external clock generator and set the time reference in the system. 2. The method according to claim 1, characterized in that the relay node independently transmits and independently receives a trial frame to each terminal station if there is a relay node in open radio channels and the position or trajectory of the relay node is known; measuring the absolute distance between the terminal station and the relay node; performing phase compensation for the difference in distances of several terminal stations transmitting CDMA signals, relative to the relay node for the implementation of the transmitted multi-channel CDMA signals phase matching when reaching the relay node; and performing clock and phase matching based on the external pilot signal by combining with quasi-clocking based on the external pilot signal to provide quasi-synchronous combining of multi-channel CDMA signals at the relay node and perform quasi-synchronous reception at the receiving terminal station. 3. The method according to claim. 1, characterized in that the test frame is transmitted to the receiving terminal station in an appropriate manner from several transmitting terminal stations based on the implementation of quasi-clock synchronization by an external pilot signal; receiving a response to the test provided by the receiving terminal station; measuring the distance between each transmitting terminal station and the receiving terminal station if there is no relay node in open radio channels and several terminal stations transmit CDMA signals to the same terminal station; performing phase compensation for a distance difference of the plurality of terminal stations transmitting CDMA signals with respect to respective receiving terminal stations so that the transmitted multi-channel CDMA signals are phase-matched upon reaching the receiving terminal stations; and performing clock and phase matching based on the external pilot by combining with quasi-clocking based on the external pilot to provide quasi-synchronous combining of multi-channel CDMA signals at receiving terminal stations. 4. The method according to any one of paragraphs. 1 to 3, characterized in that clock synchronization is maintained by continuously or intermittently receiving an external pilot signal after performing quasi-clock synchronization based on the external pilot signal; periodically transmitting a pilot frame to the relay node or the receiving terminal station from the transmitting terminal station due to a change in the distance from the transmitting terminal station to the relay node or the receiving terminal station accumulated over time; and re-measure the distance and re-set the phase to achieve phase matching. 5. The method according to claim 4, characterized in that the phase matching accuracy is less than 1/4 of the width of the spread spectrum chip to ensure the efficiency of quasi-synchronous reception in CDMA. 6. The method according to claim 5, characterized in that the global positioning system and satellite navigation system "BeiDou" are external pilot signal sources used to support quasi-synchronous CDMA reception; while the GPS / BeiDou system provides accurate clock synchronization for terminal stations and provides information about the geographical location of each terminal station; determine by direct measurement of the distance between the transmitting terminal stations and the receiving terminal stations of several CDMA signals, if there is no relay node in the channel, and compensate for distance differences at the transmitting ends to achieve quasi-synchronous reception of several CDMA signals by the receiving terminal station; and measuring the distance between the transmitting terminal station and the relay node after knowing the location or trajectory information of the relay node, if the relay node is present in the channel, and compensating for the distance difference between the transmitting end of the multi-channel CDMA and the relay node to realize quasi-synchronous combining of the multi-channel CDMA signals on relay node and further perform quasi-synchronous reception of multi-channel CDMA signals at the receiving end. 7. The method according to p. 5, characterized in that the reference clock frequency is obtained by means of a pilot signal; for pilot signal: a terminal station is selected as a reference in open radio channels and a pilot signal is transmitted; all or some frequency bands of open radio channels are occupied for pilot signal transmission; all or some frequency bands of open radio channels occupied by pilot signals are pilot signal bands, and the ratio of pilot signal power to the total power of pilot signal frequency bands is 0.1‰~5%; spread by the spread spectrum code and superimposed on the communication signal in the pilot bands in a signal mode with a low spectrum power similar to the noise in the transmission of the pilot signal; the effect on the received signal-to-noise ratio for the signal of the receiving end is less than the effect on the local thermal noise of the receiving end; wherein the pilot signal provides a pilot frequency, carrier waveform and clock synchronization, standard clock information, and pointing information for building a network and interconnecting multiple types of terminal stations over channels. 8. The method according to p. 7, characterized in that the pilot signal contains a frame header, frame number and data array with pointing information; wherein the frame header contains a sync header and a pilot frequency; wherein the synchronizing header is configured to time the pilot signal frame and recover the carrier waveform, and the pilot frequency is configured to eliminate the frequency offset of the pilot signal frame; the frame number is configured to identify the cyclic sequence of the pilot frame; the index information data array is configured to contain index information for indicating the channel, network status, and control information; while demodulating and receiving a pilot signal, extracting the carrier and information about the clock frequency in each terminal station; the reference clock is obtained by the duration of the fixed signal frame and the frame identification number in the signal frame; identifying a reference station transmitting pilot information according to the pilot information in the pilot signal, and receiving other indication information.

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