WO2021018123A1 - 一种基于时钟导引同步条件下的准同步码分多址方法 - Google Patents

一种基于时钟导引同步条件下的准同步码分多址方法 Download PDF

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Publication number
WO2021018123A1
WO2021018123A1 PCT/CN2020/105101 CN2020105101W WO2021018123A1 WO 2021018123 A1 WO2021018123 A1 WO 2021018123A1 CN 2020105101 W CN2020105101 W CN 2020105101W WO 2021018123 A1 WO2021018123 A1 WO 2021018123A1
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clock
pilot signal
synchronization
relay node
quasi
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PCT/CN2020/105101
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English (en)
French (fr)
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金野
吕国成
刘爱民
董明科
王宝吉
何涛
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北京大学
北京鑫星卫通科技有限公司
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Priority to US17/630,931 priority Critical patent/US20220278744A1/en
Publication of WO2021018123A1 publication Critical patent/WO2021018123A1/zh

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/1851—Systems using a satellite or space-based relay
    • H04B7/18519—Operations control, administration or maintenance
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
    • H04B7/0678—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission using different spreading codes between antennas
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W56/00—Synchronisation arrangements
    • H04W56/001—Synchronization between nodes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/04—Wireless resource allocation
    • H04W72/044—Wireless resource allocation based on the type of the allocated resource
    • H04W72/0446—Resources in time domain, e.g. slots or frames

Definitions

  • the present invention relates to the technical field of wireless communication, in particular to a plesiochronous code division multiple access method based on clock-guided synchronization conditions.
  • CDMA Code Division Multiple Access, code division multiple access
  • CDMA users occupy the same frequency and send at the same time, and the orthogonality of codes is adopted between users To distinguish users, compared with FDMA (Frequency Division Multiple Access, Frequency Division Multiple Access), CDMA technology is relatively complex and is mostly used in star networking.
  • FDMA Frequency Division Multiple Access, Frequency Division Multiple Access
  • CDMA technology has been adopted by the second-generation (IS-95) and third-generation (CDMA2000) communication standards and has become one of the mainstream communication systems.
  • CDMA technology is also widely used in military communications due to its anti-interference, good confidentiality, and low transmission signal power.
  • CDMA technology has the advantage of soft channel capacity.
  • FDMA and TDMA Time Division Multiple Access
  • TDMA and FDMA technologies have hard channel capacity limit.
  • the CDMA technology can use multi-user detection to increase user capacity.
  • CDMA multiple access multiplexing technology has been widely adopted.
  • the transmission power of CDMA technology is lower than that of TDMA and FDMA, so it has good confidentiality
  • CDMA has the advantage of soft channel capacity compared with FDMA and TDMA, through effective multi-user Detection technology can meet the communication needs of more users
  • CDMA technology can fight against narrowband interference
  • CDMA technology can reduce the EIRP value sent back to avoid interference to neighboring stars, especially in line with "on the move" satellite communication
  • CDMA technology can reduce the EIRP value sent back to avoid interference to neighboring stars, especially in line with "on the move" satellite communication
  • the communication requirements of the system Therefore, whether it is a wireless communication system or a satellite communication system, CDMA technology has a broad application prospect.
  • DS-CDMA Direct Sequence-Code Division Multiple Access, Direct Sequence Code Division Multiple Access
  • the symbols of each user are modulated by a set of orthogonal waveforms.
  • Each user is assigned a waveform that is orthogonal to the other users.
  • Figure 1 shows the system model of DS-CDMA technology.
  • DS-CDMA is a wideband spread spectrum signal obtained by multiplying a narrowband signal carrying information with a high-speed address code signal.
  • the receiving end needs to use the same address code signal synchronized with the sending end to control the carrier frequency phase of the input inverter to achieve despreading.
  • DS-CDMA system has the advantages of anti-narrowband interference, anti-multipath fading and good confidentiality.
  • DS-CDMA has no absolute capacity limit.
  • the period of a geosynchronous orbit satellite is equal to the orbit of the earth's rotation period. If the eccentricity and inclination of this orbit are zero, that is, the sub-satellite track position of the satellite always remains unchanged, and the satellite is stationary from any point on the earth. This orbit is called a stationary orbit.
  • its sub-satellite point trajectory is a figure of eight, and the highest latitude of the north-south latitude the satellite flies over is equal to its orbital inclination. If the satellite operation is affected by perturbation, the trajectory of the ground sub-satellite point will show a distorted figure of eight. This causes the ground station to track the elevation angle of the satellite in real time, and overcome the inaccuracy of synchronization caused by this.
  • the technical problem to be solved by the present invention is to provide a plesiochronous code division multiple access method based on clock-guided synchronization conditions, which can enable CDMA to operate under quasi-synchronous conditions, thus being applicable
  • a relay node such as in a satellite communication system
  • the technical scheme of the present invention is: this kind of plesiochronous code division multiple access method based on clock-guided synchronization conditions,
  • multiple transmitting stations use CDMA spread spectrum transmission to share power resources to transmit signals;
  • multiple sending stations use CDMA spread spectrum transmission mode to share power resources to send signals; use external pilot signals to provide clock synchronization for the CDMA signals sent by multiple terminal stations in the open wireless channel, and eliminate
  • the path delay difference between multiple terminal stations and the relay node or the receiving station makes the clock synchronization and phase alignment of the multiple CDMA signals received at the relay node or the receiving station realize the quasi-synchronous reception of the multiple CDMA signals.
  • Make CDMA operate under quasi-synchronous conditions, so that it is suitable for open wireless channels with relay nodes, such as satellite communication systems, and also for open wireless channels without relay nodes.
  • Figure 1 shows the system model of DS-CDMA technology.
  • Figure 2 shows a schematic diagram of the trajectory of the sub-satellite point of a non-stationary geosynchronous orbit satellite.
  • Fig. 3 shows a flow chart of obtaining a pilot signal.
  • Fig. 4 shows a schematic diagram of a pilot frame structure according to a preferred embodiment of the present invention.
  • multiple transmitting stations use CDMA spread spectrum transmission to share power resources to transmit signals;
  • multiple sending stations use CDMA spread spectrum transmission mode to share power resources to send signals; use external pilot signals to provide clock synchronization for the CDMA signals sent by multiple terminal stations in the open wireless channel, and eliminate
  • the path delay difference between multiple terminal stations and the relay node or the receiving station makes the clock synchronization and phase alignment of the multiple CDMA signals received at the relay node or the receiving station realize the quasi-synchronous reception of the multiple CDMA signals.
  • Make CDMA operate under quasi-synchronous conditions, which is suitable for geosynchronous orbit satellite communications.
  • the external pilot signal includes stable and continuous clock synchronization information and continuous reference timing information, and the external pilot signal is received to ensure that multiple terminal stations in the system realize clock quasi-synchronization based on the external clock and establish time Benchmark.
  • each terminal station realizes the self-transmission and self-reception of the detection frame through the relay node, and determines whether the terminal station and the middle Following the absolute distance of the node, multiple terminal stations sending CDMA signals make phase compensation for their respective distance differences relative to the relay node, so that the sent multiple CDMA signals achieve phase alignment when they reach the relay node.
  • the clock of the pilot signal is quasi-synchronized, realizing the clock and phase alignment based on the external pilot signal, ensuring the quasi-synchronous combination of multiple CDMA signals at the relay node and then achieving quasi-synchronous reception at the receiving interrupt station.
  • the multiple sending terminal stations each send to the receiving terminal station.
  • Phase compensation enables the transmitted multiple CDMA signals to achieve phase alignment when they reach the receiving terminal station, combined with the clock quasi-synchronization based on the external pilot signal, to achieve clock synchronization and phase alignment based on the external pilot signal to ensure multiple CDMA signals Quasi-synchronous reception at the receiving terminal station.
  • the clock synchronization is maintained by continuously or intermittently receiving the external pilot signal, and the distance change from the sending terminal station to the relay node or the receiving terminal station accumulates over time, and the sending terminal station sends The relay node or the receiving terminal station periodically sends detection frames, re-measures the distance, resets the phase, and realizes phase alignment.
  • the performance of quasi-synchronous CDMA is determined by the accuracy of phase alignment and clock synchronization, which corresponds to the change in the relative position between the transmitting terminal station and the relay node or the receiving terminal station within the ranging period.
  • the phase alignment accuracy is less than 1/4 of the spread spectrum chip width.
  • GPS and Beidou systems are external guidance signal sources used to support CDMA quasi-synchronous reception.
  • GPS/Beidou systems provide terminal stations with accurate clock synchronization and can provide geographic location information of each terminal station. If the channel is not If there is a relay node, the distance between multiple CDMA signal sending terminal stations and receiving terminal stations is directly measured and the distance difference is compensated at the sending end to realize the quasi-synchronous reception of multiple CDMA signals by the receiving terminal station; if If there is a relay node in the channel, after knowing the location or trajectory information of the relay node, the distance between the sending terminal station and the relay node is measured, and the distance difference between the relay node and the sending end of the multi-channel CDMA is determined. Compensation realizes the quasi-synchronous combination of multiple CDMA signals at the relay node, and further realizes the quasi-synchronous reception of multiple CDMA signals at the receiving end. The effect of CDMA signal phase alignment depends on the clock stability and positioning accuracy of GPS/Beidou signals.
  • the reference clock is obtained through a pilot signal;
  • the pilot signal is:
  • the pilot signal occupies all or part of the frequency band of the open wireless channel for transmission.
  • the pilot signal occupies all or part of the frequency band in the open wireless channel.
  • the pilot frequency band, the pilot signal power accounts for 0.1 ⁇ to 5% of the total power of the pilot frequency band.
  • the pilot signal is sent, it is spread by spreading code and superimposed on the pilot signal in a noise-like low power spectrum signal mode.
  • the influence on the signal-to-noise ratio of the receiving end signal is lower than the influence of the background thermal noise of the receiving end;
  • the pilot signal provides guidance, carrier and clock synchronization, standard timing and indication information for multiple types of terminal stations in the channel to build networks and communicate with each other.
  • the pilot signal includes: a frame header, a frame serial number, and an indication information data body.
  • the frame header includes a synchronization header and a pilot.
  • the synchronization header is used for pilot signal frame timing and carrier recovery.
  • the pilot field is used to eliminate the frequency offset of the pilot signal frame;
  • the frame sequence number field is an identification of the pilot signal frame cycle sequence;
  • the indication information data body field carries indication information, which is used to indicate the channel, network status and management information; each terminal station demodulates and receives the pilot signal, extracts the carrier and clock information, and fixes the signal frame period and the signal frame
  • the frame sequence number in the identification frame is used to obtain the clock reference, and the reference station sending the guidance information is identified according to the indication information in the pilot signal and the reception of other indication information is completed.
  • the synchronization range is 1/2 to 1/32 time slot.
  • the 1/32 time slot has the best effect, which can make CDMA operate in a synchronized state.
  • the length of each frame is fixed, and the information rate of each frame is fixed. Therefore, the time length represented by each frame is also fixed. Therefore, the receiving station sends signals through the sending station.
  • the time interval is determined by the interval between signal frames, and the interval between signal frames is used as the basic unit of the clock; the time calibration is completed by the time difference between frames with different sequence numbers. If the time interval count of the two frames before and after is not equal to the signal frame period, the receiving station The clock is adjusted.
  • the ground receiving station continuously receives the pilot signal, and can distinguish each frame in the received pilot signal according to the frame structure.
  • the pilot signal the length of each frame is fixed, and the information rate of each frame is fixed.
  • the time represented by the frame is also fixed, so the ground receiving station can determine the time interval through the interval between the pilot signal frames, and the interval between the signal frames (that is, the signal frame period) as the basic unit of the clock, through different serial numbers The time difference between frames is used to complete the time calibration. If the time interval count of the two frames before and after is not equal to the pilot signal frame period, the clock of the ground receiving station needs to be adjusted.
  • the period of the signal frame ranges from 50ms to 250ms.
  • the signal frame period is an integral multiple of 50ms, 100ms, 150ms, 200ms, 250ms, the basic unit of the clock and the calibration effect are best.
  • the pilot signal sending end is a satellite
  • the pilot signal is:
  • the pilot signal is used for spread-spectrum communication by spreading code, and the power is 1 ⁇ -1% of the power of the communication satellite transponder;
  • the user receiving station monitors the pilot signal of the satellite, and completes the clock synchronization between the user receiving station and the satellite clock by receiving the pilot signal.
  • the pilot signal is:
  • the pilot signal performs spread-spectrum communication through a spread-spectrum code, and the power is 0.1 ⁇ -5% of the power of the communication satellite transponder.
  • the pilot signal includes: a header (Header), a pilot (Pilot), a frame serial number (ID), and a data body (Data Frame).
  • Fig. 4 shows a schematic diagram of a pilot frame structure according to a preferred embodiment of the present invention.
  • the frame header is used for pilot signal frame timing and carrier recovery, and the length is 16-128bit;
  • the pilot field is all 0 fields, and the length is more than 60bit, used to eliminate the frequency deviation of the pilot signal frame;
  • frame sequence number field The length is more than 8bit, which is used to measure the absolute distance of the earth station;
  • the data body field carries network management information, which is used to manage and maintain satellite network information with other stations in the network, and the length is 256 ⁇ 1008bit.
  • OVSF codes Orthogonal Variable Spreading Factor, Orthogonal Variable Spreading Factor
  • OVSF codes are mainly used for orthogonal spreading.
  • the rate of the traffic channel is different, and the OVSF code used is different.
  • the length is also different).
  • the reason for using the OVSF code is that the OVSF code has orthogonality and the length is variable.
  • the spreading sequence uses OVSF code. This brings support for variable rates.
  • the pilot signal adopts binary phase shift keying BPSK mode to cyclically modulate, and keep sending continuously; each user station is equipped with a pilot signal receiver, which decodes the pilot signal Expand, and then directly receive hard to obtain the signal frame of the pilot signal, determine the information bit through the frame header, and continuously receive the pilot signal to complete the clock quasi-synchronization. Since the GDP frame length and time are fixed, the clock quasi-synchronization can be completed by continuously receiving GDP.
  • the frame header of the TDMA frame is used to rearrange to complete the reception, and then the large signals are eliminated to obtain small signals (which can be pushed The signal that is less than 10% of the power of the entire transponder is attributed to the small signal) mixed signal, and then the orthogonal OVSF codes are used to despread the small signal mixed signal to complete the reception.
  • the corresponding spreading code can be directly used to complete the despreading reception. To get the target signal.
  • the period of the signal frame ranges from 50 ms to 250 ms.
  • the signal frame period is an integral multiple of 50ms, 100ms, 150ms, 200ms, 250ms, the basic unit of the clock and the calibration effect are best.
  • the change in the absolute distance between the satellite and the earth station is obtained through the phase difference between the sending end and the receiving end.
  • the satellite In the process of the management center station continuously sending the guidance signal, the satellite will continuously move around the earth and move with the "8" trajectory relative to the sub-satellite point. Therefore, the distance between the satellite and the management center station is constantly changing.
  • the path delay of the pilot signal will continue to change, but the movement of the satellite presents a certain regularity, changing from far to near and then from near to far, so the path delay changes also show a certain regularity.
  • the path delay change of the link sent to the satellite is consistent with the path delay change of the link path received from the satellite signal, so at the pilot signal receiving end and the pilot signal sending end
  • the phase delay between the two is equivalent to the double satellite-to-ground path delay variation. Therefore, when the receiver of spread spectrum communication detects that the maximum correlation peak moves in a certain direction, it can track the maximum correlation peak while obtaining Phase delay, half of the phase delay is the path delay.
  • the path delay is multiplied by the speed of light to get the absolute distance change between the satellite and the earth station.
  • the satellite ranging is realized by changing the absolute distance between coarse-grained satellite ground stations and the absolute distance between fine-grained satellite ground stations.
  • the absolute time of sending a specific frame sequence number frame by the management center station and the absolute time of receiving a specific frame sequence number frame can be used to make a difference to obtain the absolute path delay between satellite ground stations, which is multiplied by the speed of light
  • the value of the absolute distance between the satellite and the ground can be obtained, but because the path delay of satellite communication is about 280ms, the absolute distance is sampled less in a period of time, and very accurate satellite ranging cannot be completed.
  • the pilot signal is cyclically modulated in a binary phase shift keying BPSK mode, and is continuously transmitted. This enables all earth stations in the network to receive the pilot signal.

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Abstract

公开一种基于时钟导引同步条件下的准同步码分多址方法，其能够使CDMA在准同步的条件下运行，适用于存在中继节点的开放无线信道，如卫星通信系统中，也适用于不存在中继节点的开放无线信道。这种基于时钟导引同步条件下的准同步码分多址方法，在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收。

Description

一种基于时钟导引同步条件下的准同步码分多址方法 技术领域
本发明涉及无线通信的技术领域，尤其涉及一种基于时钟导引同步条件下的准同步码分多址方法。
背景技术
CDMA(Code Division Multiple Access，码分多址)接入方式对不同用户分配不同的扩频地址码实现卫星资源共享，CDMA用户之间占用相同的频率同时发送，用户之间通过码子的正交性来区分用户，相比于FDMA(Frequency Division Multiple Access，频分多址)，CDMA技术相对复杂，多用于星状组网中。
CDMA技术已被第二代(IS-95)和第三代(CDMA2000)通信标准所采用，成为主流的通信体制之一。同时，CDMA技术因其抗干扰，保密性好，发送信号功率低等特点，也广泛地应用在军事通信中。CDMA技术相比于FDMA和TDMA技术，具有软信道容量的优势。采用FDMA和TDMA(Time Division Multiple Access，时分多址)的通信系统，当用户数多于频带数和时隙数时，系统就无法满足其他用户的通信需要，因此TDMA和FDMA技术具有硬信道容量极限。而CDMA技术可以利用多用户检测，提高用户容量。
在卫星通信返向链路中，CDMA多址复用技术已被广泛采用。首先，CDMA技术的发送功率相比于TDMA和FDMA都更低，因此具有良好的保密性；其次，如前所述，CDMA相比于FDMA和TDMA具有软信道容量的优势，通过有效的多用户检测技术，可以满足更多用户的通信需求；再次，CDMA技术可以对抗窄带干扰；最后，CDMA技术可以降低返向发送的EIRP值，避免对邻星的干扰，特别符合“动中通”卫星通信系统的通信要求。因此，无论是无线通信系统或者卫星通信系统，CDMA技术都具有广泛的应用前 景。
在DS-CDMA(Direct Sequence—Code Division Multiple Access，直接序列码分多址)中，各用户的符号被一组正交波形调制。每个用户被分配给一个与其他用户都正交的波形。图1为DS-CDMA技术的系统模型。从原理上来说，DS－CDMA是通过将携带信息的窄带信号与高速地址码信号相乘而获得的宽带扩频信号。接收端需要用与发端同步的相同地址码信号去控制输入变频器的载频相位即可实现解扩。DS－CDMA系统具有抗窄带干扰、抗多径衰落和保密性好的优点。DS－CDMA其余优点：许多用户可以共享频率资源，无须复杂的频率分配和管理；具有"软容量"特性，即在一定限度内的用户数增加，只会使得信噪比下降，而不会终止通信，也就是说DS－CDMA没有绝对的容量限制。
但是，传统的CDMA由于需要严格同步，直接应用于地球同步轨道卫星通信中存在着很大的困难。
地球同步轨道卫星运行的周期等于地球自转周期的轨道。如果这种轨道的偏心率和倾角都为零，即该卫星的星下点轨迹位置始终保持不变，从地球上任意一点来看卫星都是静止的，这种轨道称之为静止轨道。对于倾角不为零的地球同步圆轨道，其星下点轨迹是一个“8字”形，卫星飞越的南北纬的最高维度等于其轨道倾角。如果卫星运行受到摄动影响，地面星下点轨迹将呈现扭曲的“8字”形。这就造成地面站需要实时追踪卫星的仰角，并且克服由此带来的同步不精确问题。
发明内容
为克服现有技术的缺陷，本发明要解决的技术问题是提供了一种基于时钟导引同步条件下的准同步码分多址方法，其能够使CDMA在准同步的条件下运行，从而适用于存在中继节点的开放无线信道，如卫星通信系统中，也适用于不存在中继节点的开放无线信道。
本发明的技术方案是：这种基于时钟导引同步条件下的准同步码分多址方法，
在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；
使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收。
本发明在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收，因此能够使CDMA在准同步的条件下运行，从而适用于存在中继节点的开放无线信道，如卫星通信系统中，也适用于不存在中继节点的开放无线信道。
附图说明
图1示出了DS-CDMA技术的系统模型。
图2示出了非静止地球同步轨道卫星星下点轨迹的示意图。
图3示出了获取导引信号的流程图。
图4示出了根据本发明的一个优选实施例的导频帧结构的示意图。
具体实施方式
这种基于时钟导引同步条件下的准同步码分多址方法，
在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；
使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时 钟同步、相位对齐，实现多路CDMA信号的准同步接收，即，QS-CDMA(Quasi-Synchronized Code Division Multiple Access)。
本发明在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收，因此能够使CDMA在准同步的条件下运行，从而适用于地球同步轨道卫星通信中。
优选地，外部导引信号包含稳定连续的时钟同步信息、持续的基准定时信息，接收所述外部导引信号来保证系统内的多个终端站以外部时钟为基准实现时钟准同步，并建立时间基准。
更进一步地，如果开放无线信道中存在中继节点，且在中继节点位置或运动轨迹已知的条件下，各个终端站通过中继节点实现探测帧的自发自收，测定该终端站与中继节点的绝对距离，发送CDMA信号的多个终端站对各自相对于中继节点的距离差做出相位补偿，使得所发送的多路CDMA信号在达到中继节点时实现相位对齐，结合基于外部导引信号的时钟准同步，实现基于外部导引信号的时钟和相位对齐，保证多路CDMA信号在中继节点的准同步合路并进而在接收中断站实现准同步接收。
或者，如果开放无线信道中无中继节点，多个终端站向同一个终端站发送CDMA信号时，在基于外部导引信号实现时钟准同步的基础上，多个发送终端站各自向接收终端站发送测试帧，并接收终端站回应的测试应答，并进一步测定每个发送终端站和接收终端站之间的距离；发送CDMA信号的多个终端站对各自相对于接收终端站的距离差做出相位补偿，使得所发送的多路CDMA信号在达到接收终端站时实现相位对齐，结合基于外部导引信号的时钟准同步，实现基于外部导引信号的时钟同步和相位对齐，保证多路CDMA信号在接收终端站的准同步接收。
优选地，基于外部导引信号实现时钟准同步后，通过连续或间歇接收外部导引信号保持时钟同步，而发送终端站到中继节点或接收终端站的距 离变化随时间累积，发送终端站向中继节点或接收终端站周期性地发送探测帧，重新测定距离、重置相位，实现相位对齐。准同步CDMA的性能决定于相位对齐和时钟同步的精度，该精度对应于测距周期时间内发送终端站和中继节点或接收终端站之间相对位置的变化。
更进一步地，为保证CDMA中准同步接收的性能，所述相位对齐精度小于扩频码片宽度的1/4。
更进一步地，GPS和北斗系统是用来支持CDMA准同步接收的外部导引信号源，GPS/北斗系统为终端站提供精准的时钟同步并能够提供各终端站的地理位置信息，若信道中不存在中继节点，则直接测算出多个CDMA信号发送终端站和接收终端站之间的距离并在发送端对距离差做出补偿，实现接收终端站对多路CDMA信号的准同步接收；若信道中存在中继节点，则获知中继节点的位置或轨迹信息后，测定发送终端站和中继节点之间的距离，在多路CDMA的发送端对与中继节点间的距离差做出补偿，实现中继节点处多路CDMA信号的准同步合路，并进一步实现在接收端的多路CDMA信号的准同步接收。CDMA信号相位对齐的效果取决于GPS/北斗信号的时钟稳准度和定位精度。
更进一步地，如图3所示，所述基准时钟通过导引信号获取；该导引信号为：
在开放无线信道中选定一个终端站作为基准，发送一路导引信号，导引信号占用开放无线信道的全部频带或部分频带发送，导引信号在开放无线信道中占用的全部频带或部分频带为导引频带，导引信号功率占导引频带全部功率的比例范围为0.1‰-5％，导引信号发送时通过扩频码进行扩频，以类噪声的低功率谱信号模式叠加在导引频带内的通信信号上，对接收端信号接收信噪比的影响低于接收端本底热噪声的影响；
导引信号为信道中多类型的终端站构建网络和相互通信提供导引、载波与时钟同步、标准定时与指示信息。
更进一步地，如图4所示，所述导引信号包括：帧头、帧序号和指示信息数据体。帧头包括同步头、导频，同步头用于导引信号帧定时及载波恢复，导频字段用于消除导引信号帧的频偏；帧序号字段是对导引信号帧循环顺序的标识；指示信息数据体字段承载指示信息，用于指示信道、网络状态和管理信息；各终端站对该导引信号进行解调接收，提取出载波、时钟信息，通过固定信号帧的周期和在信号帧中的标识帧序号，获得时钟基准，根据导引信号中的指示信息对发送导引信息的基准站进行鉴别并完成其他指示信息的接收。
优选地，同步范围在1/2～1/32时隙。其中1/32时隙效果最佳，可以使CDMA接近在同步状态下运行。
具体地，发送站发送的信号，每一帧的长度固定，每一帧的信息速率固定，因此每一帧所代表的时间长度也固定，因此所述接收站通过发送站发送信号的信号帧之间的间隔来确定时间间隔，将信号帧之间的间隔作为时钟的基本单位；通过不同序号帧之间的时间差来完成时间标定，如果前后两帧时间间隔计数不等于信号帧周期，则接收站时钟进行调整。
地面接收站连续接收导引信号，并且根据帧结构可以对接收导引信号中的每一帧进行区分，作为导引信号，每一帧的长度固定，每一帧的信息速率固定，因此每一帧所代表的时间也固定，因此地面接收站可以通过导引信号帧之间的间隔来确定时间间隔，将信号帧之间的间隔(即，信号帧周期)作为时钟的基本单位，通过不同序号帧之间的时间差来完成时间标定，如果前后两帧时间间隔计数不等于导引信号帧周期，则地面接收站时钟需要进行调整。
更进一步地，信号帧的周期范围在50ms～250ms。当信号帧周期为50ms、100ms、150ms、200ms、250ms这些50ms的整数倍时，作为时钟的基本单位和标定的效果最好。
导引信号发送端是卫星时，导引信号为：
在卫星的每一个数据信号载波上叠加一个导引信号并通过卫星广播；所述导引信号通过扩频码进行扩频通信，且功率为通信卫星转发器功率的1‰-1％；
在网络中，用户接收站监听卫星的导引信号，通过接收导引信号，完成用户接收站和卫星时钟的时钟同步。
信号发送端是地球站中的管理中心站时，导引信号为：
在作为信号发送端的地球站的每一个数据信号载波上，叠加一个导引信号；
在作为信号接收端的地球站，对所述导引信号进行解调接收，提取出载波的信息，从而对作为信号发送端的地球站进行识别；
所述导引信号通过扩频码进行扩频通信，且功率为通信卫星转发器功率的0.1‰-5％。
具体地，所述导引信号包括：帧头(Header)、导频(Pilot)、帧序号(ID)和数据体(Data Frame)。图4示出了根据本发明的一个优选实施例的导频帧结构的示意图。
更进一步地，帧头用于导引信号帧定时及载波恢复，长度在16～128bit；导频字段为全0字段，长度在60bit以上，用于消除导引信号帧的频偏；帧序号字段长度在8bit以上，用于对地球站绝对距离进行测量；数据体字段承载网络管理信息，用于与网内其他站管理和维护卫星网络信息，长度在256～1008bit。
更进一步地，在区分信道和扩展速率时，使用OVSF码(正交可变扩频因子，Orthogonal Variable Spreading Factor。OVSF码主要用于正交扩频。业务信道的速率不同，使用的OVSF码的长度也不同)。使用OVSF码的原因是，OVSF码有正交性，长度又可变。
更进一步地，扩频序列采用OVSF码。由此带来支持可变速率。
更进一步地，所述导引信号采用二进制相移键控BPSK方式循环调制，连续不断地保持发送；为每一个用户站配备一个导引信号接收机，导引信号接收机对导引信号进行解扩，然后直接硬接收，得到导引信号的信号帧， 通过帧头来判断信息位，连续接收导引信号来完成时钟准同步。由于GDP帧长度和时间固定，因此连续接收GDP即可完成时钟准同步。
其中，对于大信号(可推满整个转发器功率的一半以上的信号归结为大信号)，通过TDMA帧的帧头来重新排列，从而完成接收，然后将大信号消去，得到小信号(可推满整个转发器功率的10％以下的信号归结为小信号)混合信号，再利用相互正交的OVSF码，来对小信号混合信号进行解扩，从而完成接收。
其中，对于中信号(可推满整个转发器功率的10％～50％的信号归结为中信号)，由于OVSF码的正交性，因此可以直接利用相应的扩频码来完成解扩接收，从而得到目标信号。
优选地，信号帧的周期范围在50ms～250ms。
当信号帧周期为50ms、100ms、150ms、200ms、250ms这些50ms的整数倍时，作为时钟的基本单位和标定的效果最好。
优选地，通过发送端和接收端的相位差，得到卫星与地球站之间绝对距离的变化。
在管理中心站连续发送导引信号的过程中，卫星会围绕着地球不断移动，并且相对于星下点呈现“8”字轨迹移动，因此卫星与管理中心站之间的距离不断发生变化，因此导引信号的路径时延会不断变化，但卫星的移动呈现一定的规律性，都是由远到近再由近到远地进行变化，因此路径时延的变化也呈现一定的规律性。
对于管理中心站，通过自身接收导引信号，发送到卫星的链路路径时延变化和接收到来自卫星信号的链路路径时延变化一致，因此在导引信号接收端和导引信号发送端之间的相位时延等效于双倍的星地路径时延变化，因此当扩频通信的接收机，探测到最大相关峰向某个方向移动，即可在跟踪最大相关峰的同时，得到相位时延，相位时延的一半即为路径时延，路径时延乘以光速即可得到卫星与地球站之间的绝对距离变化。
优选地，通过粗颗粒度的卫星地面站之间绝对距离和细颗粒度的卫星地面站之间绝对距离变化，实现卫星测距。
通过导引信号，可以利用管理中心站发送特定帧序号帧的绝对时间和接收到特定帧序号帧的绝对时间，进行做差即可得到卫星地面站之间的绝对路径时延，与光速相乘即可得到星地绝对距离的值，但因为卫星通信的路径时延在280ms左右，所以绝对距离在一段时间内采样较少，无法完成非常精确的卫星测距。
但通过加入绝对距离的变化，这个值是随着扩频接收机最大相关峰的移动来测量的，在一段时间内，采样数量极大，因此可以通过绝对距离的变化来对绝对距离进行补全，在两个绝对距离之间，通过绝对距离的变化值来预测中间时刻点的绝对距离，从而完成高精度的卫星测距。
优选地，所述导引信号采用二进制相移键控BPSK方式循环调制，连续不断地保持发送。这样能够使网络内所有地球站都可以接收到导引信号。
以上所述，仅是本发明的较佳实施例，并非对本发明作任何形式上的限制，凡是依据本发明的技术实质对以上实施例所作的任何简单修改、等同变化与修饰，均仍属本发明技术方案的保护范围。

Claims (10)

  1. 一种基于时钟导引同步条件下的准同步码分多址方法，其特征在于：
    在开放无线信道下，多个发送站使用CDMA扩频传输方式共享功率资源发送信号；
    使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步，并消除多个终端站到中继节点或接收站的路径时延差异，使得在中继节点或接收站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收。
  2. 根据权利要求1所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：外部导引信号包含稳定连续的时钟同步信息、持续的基准定时信息，接收所述外部导引信号来保证系统内的多个终端站以外部时钟为基准实现时钟准同步，并建立时间基准。
  3. 根据权利要求2所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：如果开放无线信道中存在中继节点，且在中继节点位置或运动轨迹已知的条件下，各个终端站通过中继节点实现探测帧的自发自收，测定各终端站与中继节点的绝对距离，发送CDMA信号的多个终端站对各自相对于中继节点的距离差做出相位补偿，使得所发送的多路CDMA信号在达到中继节点时实现相位对齐，结合基于外部导引信号的时钟准同步，实现基于外部 导引信号的时钟和相位对齐，保证多路CDMA信号在中继节点的准同步合路并进而在接收终端站实现准同步接收。
  4. 根据权利要求2所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：如果开放无线信道中无中继节点，多个终端站向同一个终端站发送CDMA信号时，在基于外部导引信号实现时钟准同步的基础上，多个发送终端站各自向接收终端站发送测试帧，接收终端站回应的测试应答，并进一步测定每个发送终端站和接收终端站之间的距离；发送CDMA信号的多个终端站对各自相对于接收终端站的距离差做出相位补偿，使得所发送的多路CDMA信号在达到接收终端站时实现相位对齐，再基于外部导引信号的时钟准同步，实现基于外部导引信号的时钟同步和相位对齐，保证多路CDMA信号在接收终端站的准同步接收。
  5. 根据权利要求1-4所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：基于外部导引信号实现时钟准同步后，通过连续或间歇接收外部导引信号保持时钟同步，而发送终端站到中继节点或接收终端站的距离变化随时间累积，发送终端站向中继节点或接收终端站周期性地发送探测帧，重新测定距离、重置相位，实现相位对齐。
  6. 根据权利要求5所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：为保证CDMA中准同步接收的性能，所述相位对齐精度小于扩频码片宽度的1/4。
  7. 根据权利要求6所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：GPS和北斗系统是用来支持CDMA准同步接 收的外部导引信号源，GPS/北斗系统为终端站提供精准的时钟同步并能够提供各终端站的地理位置信息，若信道中不存在中继节点，则直接测算出多个CDMA信号发送终端站和接收终端站之间的距离并在发送端对距离差做出补偿，实现接收终端站对多路CDMA信号的准同步接收；若信道中存在中继节点，则获知中继节点的位置或轨迹信息后，测定发送终端站和中继节点之间的距离，在多路CDMA的发送端对与中继节点间的距离差做出补偿，实现中继节点处多路CDMA信号的准同步合路，并进一步实现在接收端的多路CDMA信号的准同步接收。
  8. 根据权利要求6所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：所述基准时钟通过导引信号获取；该导引信号为：
    在开放无线信道中选定一个终端站作为基准，发送一路导引信号，导引信号占用开放无线信道的全部频带或部分频带发送，导引信号在开放无线信道中占用的全部频带或部分频带为导引频带，导引信号功率占导引频带全部功率的比例范围为0.1‰-5％，导引信号发送时通过扩频码进行扩频，以类噪声的低功率谱信号模式叠加在导引频带内的通信信号上，对接收端信号接收信噪比的影响低于接收端本底热噪声的影响；
    导引信号为信道中多类型的终端站构建网络和相互通信提供导引、载波与时钟同步、标准定时与指示信息。
  9. 根据权利要求8所述的基于时钟导引同步条件下的准同步码分多址方法，其特征在于：所述导引信号包括：帧头、帧序号和指示 信息数据体；帧头包括同步头、导频，同步头用于导引信号帧定时及载波恢复，导频字段用于消除导引信号帧的频偏；帧序号字段是对导引信号帧循环顺序的标识；指示信息数据体字段承载指示信息，用于指示信道、网络状态和管理信息；各终端站对该导引信号进行解调接收，提取出载波、时钟信息，通过固定信号帧的周期和在信号帧中的标识帧序号，获得时钟基准，根据导引信号中的指示信息对发送导引信息的基准站进行鉴别并完成其他指示信息的接收。
  10. 一种基于时钟导引同步条件下的准同步码分多址方法，其特征在于：
    在开放无线信道下，多个用于发送的终端站使用CDMA扩频传输方式共享功率资源发送信号；
    使用外部导引信号，为开放无线信道中多个终端站所发送的CDMA信号提供时钟同步；测定各终端站与中继节点或用于接收的终端站的绝对距离，各终端站对各自相对于中继节点或用于接收的终端站的距离差做出相位补偿，使得所发送的多路CDMA信号在达到中继节点或用于接收的终端站时实现相位对齐，结合基于外部导引信号的时钟同步，实现基于外部导引信号的时钟和相位对齐；并消除多个终端站到中继节点或用于接收的终端站的路径时延差异，使得在中继节点或用于接收的终端站所接收的多路CDMA信号时钟同步、相位对齐，实现多路CDMA信号的准同步接收； 外部导引信号包含稳定连续的时钟同步信息、持续的基准定时信息，接收所述外部导引信号来保证系统内的多个终端站以外部时钟为时钟基准实现时钟准同步，并建立时间基准。
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