CN113099527B - 5G wireless edge absolute time synchronization method based on timing message exchange - Google Patents

5G wireless edge absolute time synchronization method based on timing message exchange Download PDF

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CN113099527B
CN113099527B CN202110445930.8A CN202110445930A CN113099527B CN 113099527 B CN113099527 B CN 113099527B CN 202110445930 A CN202110445930 A CN 202110445930A CN 113099527 B CN113099527 B CN 113099527B
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time
base station
clock
msg2
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CN113099527A (en
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刘伟
柴子超
雷菁
朱锦锟
赵塑盾
李茂�
范瑞杰
王源鑫
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National University of Defense Technology
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    • 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
    • H04W56/00—Synchronisation arrangements
    • H04W56/0035—Synchronisation arrangements detecting errors in frequency or phase
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W56/00—Synchronisation arrangements
    • H04W56/004—Synchronisation arrangements compensating for timing error of reception due to propagation delay
    • H04W56/0045—Synchronisation arrangements compensating for timing error of reception due to propagation delay compensating for timing error by altering transmission time
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W74/00—Wireless channel access
    • H04W74/002—Transmission of channel access control information
    • H04W74/004—Transmission of channel access control information in the uplink, i.e. towards network
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W74/00—Wireless channel access
    • H04W74/002—Transmission of channel access control information
    • H04W74/006—Transmission of channel access control information in the downlink, i.e. towards the terminal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W74/00—Wireless channel access
    • H04W74/08—Non-scheduled access, e.g. ALOHA
    • H04W74/0833—Random access procedures, e.g. with 4-step access

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Abstract

The invention discloses a 5G wireless edge absolute time synchronization method based on timing message exchange, and aims to provide an absolute time synchronization method of 5G wireless edge equipment, which ensures synchronous cooperation among various industrial equipment such as sensors, actuators and the like. The technical scheme is that a base station is used as a main clock source, equipment in a cell is used as a slave clock, and timing message exchange is completed by relying on a random access process of a 5G network; selecting a group of timestamps with the minimum interrupt delay in order to reduce the synchronization error caused by the interrupt delay; calculating propagation delay by using the time advance, and calculating the relative frequency offset and phase offset of the local clock of the terminal according to the relation between the timestamps; and compensating a local clock of the terminal by adopting the FPGA or the voltage-controlled oscillator to complete absolute time synchronization of the wireless terminal. The invention comprehensively considers the problems of frequency deviation and phase deviation of the clock, utilizes the 5G existing mechanism to exchange time information, designs the corresponding timing message frame structure, and has high synchronization precision and low communication overhead and calculation complexity.

Description

5G wireless edge absolute time synchronization method based on timing message exchange
Technical Field
The invention belongs to the field of wireless communication, and particularly relates to a 5G wireless edge absolute time synchronization method based on timing message exchange. The method is suitable for the industrial application fields of audio and video production/sharing, motion control systems, unmanned aerial vehicle cluster formation, intelligent factories and the like which need synchronous cooperation of different types of equipment.
Background
With the advent of the world of everything interconnection, the industrial internet of things (IIoT) has pushed numerous industrial applications to digitization and intelligence, with the vision of quickly and reliably connecting human and industrial devices, enabling synchronized collaboration of different types of devices. For this reason, the industrial internet of things needs to transit from best-effort communication to deterministic communication, and all devices in the network are required to share a uniform time reference, that is, absolute time synchronization is realized.
The simplest and most efficient time synchronization method is to utilize GNSS for time service, and the precision of the method can reach 2 ns. However, the satellite signal is weak, the time service function is easily affected by factors such as weather and shelters, and the receiver needs to observe the satellite signal at a fixed position for a long time to realize high-precision time service. In order to ensure high-precision time synchronization in various environments and scenes, as a supplement to satellite time service, a network-based time synchronization technology has been proposed, and the main idea is to exchange time information in a communication network between terminals, and estimate and compensate phase offset and frequency offset between clocks.
Compared with an ideal wired channel, the uncertainty of a wireless channel, multipath effect, doppler effect and the like can greatly increase the difficulty of time synchronization, so that the time synchronization technology in a wired network and a wireless network has great difference. In wired networks, the time synchronization technology mainly includes Network Time Protocol (NTP), precision time protocol (IEEE 1588v2) and AS6802 protocol. The NTP is a time synchronization technology for packet switching networks, and uses time standard time (UTC) as reference time, and uses User Datagram Protocol (UDP) to exchange timing messages, so that millisecond-level synchronization accuracy can be achieved, but the NTP marks a timestamp on an application layer, and is easily affected by link delay and jitter; the IEEE 1588v2 protocol is based on Ethernet, performs synchronous message interaction on the original network and compensates network delay, can realize sub-microsecond synchronization precision, and the timestamp is marked between a physical layer and an MAC layer, so that the influence of link jitter can be alleviated to a certain extent, but special hardware support is required. NTP and IEEE 1588v2 are centralized time synchronization protocols with a master clock source, while AS6802 is a fault-tolerant distributed time synchronization protocol, which can remarkably enhance the time certainty of Ethernet data transmission.
For wireless networks, researchers have proposed three synchronization mechanisms, one-way message interaction, two-way message exchange, and recipient-recipient synchronization. The one-way message interaction mechanism means that a reference node broadcasts timing information to surrounding interception nodes, the interception nodes record message arrival time (ToA) and estimate clock parameters of the reference node, but clock phase deviation and message delay cannot be distinguished, and synchronization precision is insufficient. The bidirectional message exchange mechanism means that the node carries out clock frequency offset and phase offset estimation by exchanging time information with the reference node, but the required synchronization overhead is large because the random message delay is unknown. Receiver-receiver synchronization means that two nodes receiving timing messages from a reference node perform message exchange to achieve mutual synchronization, and although the mechanism can complete the elimination of the influence of a sender on synchronization, the computation complexity is high and the synchronization overhead is large.
As an effective industrial wireless communication solution, the 5G network also needs to integrate a time synchronization function to ensure synchronous cooperation among various industrial devices such as sensors and actuators. At the 5G radio edge, there is tight PHY synchronization between the terminal (UE) and the base station (gNB) to achieve stable communication, the downlink completes timeslot synchronization through the cell search process, and the uplink defines the Timing Advance (TA) to ensure orthogonality. However, to meet the requirement of IIoT, 5G needs to design a novel wireless access scheme to meet the delay and reliability requirements and achieve absolute time synchronization. For 5G networks, two absolute time synchronization methods have been proposed, the first is prediction time synchronization based on SIB9, but SIB9 has only 10ms resolution and insufficient timing accuracy; the second method is a one-way exchange synchronization method based on reference signals with time stamps, and although the influence of scheduling delay and retransmission on synchronization performance can be avoided by using a bottom-layer reference signal, clock frequency offset is not considered, a synchronization algorithm needs to be frequently executed, and communication resources are occupied.
In summary, the conventional time synchronization method at least has the following technical problems:
1. due to the limitation of the wired network, the time synchronization technology based on the wired network is difficult to meet the requirements of emerging industrial application on network flexibility and expansibility.
2. The existing time synchronization technology based on the wireless network is easily affected by uncertainty of a wireless channel, timing precision is insufficient, most algorithms are high in complexity, and required communication overhead is large.
In fact, the 5G network has a separate terminal access design and signaling, and the communication overhead and the computational complexity can be significantly reduced by making full use of the existing 5G mechanism to exchange time information, which is an effective means for realizing high-precision absolute time synchronization.
Disclosure of Invention
The invention aims to solve the technical problem of providing an absolute time synchronization method of 5G wireless edge equipment, and overcomes the defects of high communication overhead, high calculation complexity and insufficient precision of the existing time synchronization method. The method integrates time information exchange into a random access process, designs a timing message frame structure, can jointly estimate and compensate the clock frequency offset and the phase offset of the 5G terminal, has high synchronization precision and small required communication overhead, and ensures that the terminal equipment is synchronized when being accessed to the network.
The invention completes the timing message exchange by using the existing terminal access mechanism and signaling of 5G, thereby realizing the absolute time synchronization with high precision and low complexity at the wireless edge of 5G.
The technical scheme is as follows: assuming that all base stations are synchronized with the core network through the bearer network, all network access terminals need to achieve time synchronization with the cell base station according to the scene requirements. Therefore, a base station is used as a main clock source, equipment in a cell is used as a slave clock, a local clock model and a relative clock model are established firstly, timing message exchange is completed by relying on a 5G random access process, then a terminal selects an optimal timestamp, the relative frequency offset and the phase offset of the local clock are calculated, and finally the terminal compensates the local clock according to an estimated value to complete absolute time synchronization of the wireless terminal.
The specific technical scheme of the invention comprises the following steps:
the first step is as follows: assuming that the base station and the N terminals are both provided with local hardware clocks, the local clock model is
Ci(t)＝αit+βi, i＝0、1、2……N，
Where t represents world coordination time, αiRepresenting the clock frequency offset, beta, of the base station and the terminal in relation to the world coordinationiTo representThe base station and the terminal are offset relative to the clock of the world coordination, when i is 0, C0(t) represents a local clock of the base station; since the true frequency offset and phase offset of the local clock cannot be calculated, according to C0(t) and Ci(t) obtaining the relation
Ci *(t)＝αi0Ci(t)+βi0, i＝1、2……N，
Wherein alpha isi0＝α0/αi，βi0＝β0-αi0βiRespectively, the clock frequency offset and the phase offset of the wireless terminal relative to the base station; the invention aims to calculate the relative clock frequency offset and phase offset (alpha)i0,βi0) Compensating a local clock of the terminal to synchronize the local clock with the base station;
the second step is that: after the terminal obtains the cell identification and completes downlink synchronization, a proper physical random access channel is selected to send a random access preamble Msg1 to the base station; when the terminal detects the reference point in time of Msg1, i.e. the end of each preamble, the radio chip triggers an interrupt and records the local timestamp
Figure BDA0003036907670000031
Wherein M is the repetition number of the leader sequence, and M is the time stamp group number;
the third step: the base station receives a random access preamble Msg 1; when the base station detects the reference point in time of Msg1, the radio chip triggers an interrupt and records a local timestamp
Figure BDA0003036907670000032
Then the base station calculates the TA;
the fourth step: the base station sends a random access response Msg2 to the terminal; adding M frame start delimiters SFD after the frame header of the Msg2, taking the end of each SFD as a time reference point, triggering an interrupt by a radio chip and recording a local time stamp when the base station detects the time reference point of the Msg2
Figure BDA0003036907670000033
Simultaneous grouping of locally stored timestamps
Figure BDA0003036907670000034
Embedded in Msg 2;
the fifth step: the terminal receives Msg2 during the detection window; when the terminal detects the reference point in time of Msg2, the radio chip triggers an interrupt and records a local timestamp
Figure BDA0003036907670000035
Simultaneous demodulation of the TA values and time stamp sets contained in Msg2
Figure BDA0003036907670000036
Finally, the terminal obtains the timestamp group
Figure BDA0003036907670000037
If the terminal does not receive the Msg2 in the detection window, the random access process fails, and the terminal discards the stored timestamp group and records the timestamp group again in the next random access process;
and a sixth step: selecting an optimal timestamp by the terminal: because the terminal has limited processing capacity and uncertain interruption delay exists, in order to reduce errors caused by the interruption delay, one group with the minimum interruption delay is selected from M groups of timestamps, and the group is obtained by using an equation (1):
Figure BDA0003036907670000038
Figure BDA0003036907670000039
Figure BDA0003036907670000041
wherein the content of the first and second substances,
Figure BDA0003036907670000042
msg1 sending time recorded by the terminal, Msg1 receiving time recorded by the base station, Msg2 sending recorded by the base stationTime and Msg2 reception time of the terminal record,
Figure BDA0003036907670000043
a timestamp group number indicating a minimum interrupt latency;
the seventh step: calculating the relative frequency offset and the phase offset of a local clock of the terminal: terminal according to TA value and interruption delay minimum group time stamp
Figure BDA0003036907670000044
Obtain equations (2), (3)
Figure BDA0003036907670000045
Figure BDA0003036907670000046
Wherein, Ci、αi、βiI-1, 2,3 … are the local clock of the terminal and the clock frequency offset and phase offset of the terminal with respect to the world coordination, respectively, C0、α0、β0Respectively, the local clock of the base station and the clock frequency offset and phase offset, t, of the base station in relation to the world coordinationdThe message delay calculated for the UE according to TA is specified by the formula (4)
td＝16*κ*Tc*TA (4)
Where κ is a constant, TA is the timing advance, is an integer between 0 and 1228,
Figure BDA0003036907670000047
basic time unit of NR, f is the subcarrier spacing, NfIs the IFFT block size;
simultaneous equations (2) and (3) are obtained
Figure BDA0003036907670000048
Wherein the content of the first and second substances,
Figure BDA0003036907670000049
set of timestamps, α, for which interrupt latency is minimali0、βi0Respectively, the clock frequency offset and phase offset, t, of the wireless terminal relative to the base stationdA delay for a timing message;
solving equation to obtain relative clock frequency deviation and estimated value of phase deviation
Figure BDA00030369076700000410
Wherein the content of the first and second substances,
Figure BDA00030369076700000411
set of timestamps, t, for which interrupt latency is minimaldFor timing message delays, alphai0、βi0The finally obtained clock frequency offset and phase offset of the terminal relative to the base station;
eighth step: and compensating the local clock of the terminal according to the estimated value in the seventh step, wherein the compensation mode can be realized by adopting an FPGA or a voltage-controlled oscillator.
Further, a corresponding timing message frame structure is designed in the whole synchronization process, the end of each Preamble is taken as a time reference point in the Msg1, and M frame start delimiters SFD and C are added between the frame head of the Msg2 and a reserved bit R0(t2)、C0(t3) Characters and ends with each SFD as a reference point in time.
Further, the constant k is 64, the subcarrier spacing f is 480kHz, and the IFFT block size N f4096, or f 240kHz, Nf＝4096、f＝120kHz,Nf＝2048、f＝15kHz,Nf＝2048。
Further, the preamble repetition number M may be 1,2,4,6, 12.
The invention can achieve the following technical effects:
1. the invention introduces a logic clock model, realizes the combined estimation of the frequency offset and the phase offset of the local clock of the 5G wireless terminal by estimating the message delay by the time advance, and has high synchronization precision and low calculation complexity.
2. The invention integrates timing message exchange into the existing 5G random access process, designs the corresponding timing message frame structure, and has extremely low required synchronization overhead.
3. The invention puts the synchronization process in the initial access of the terminal, and ensures that the terminal is synchronized when accessing the network.
Drawings
FIG. 1 is a general flow diagram of the present invention;
fig. 2 is a schematic diagram of a 5G random access process (the left diagram shows a contention-based random access scheme, and the right diagram shows a non-contention-based random access scheme);
FIG. 3 is a timing message exchange process of the present invention;
FIG. 4 is a frame structure of timing message Msg 1;
FIG. 5 is a frame structure of timing message Msg 2;
FIG. 6 is a graph comparing the relative clock frequency offset estimation performance of the proposed method with a hybrid time synchronization scheme, TAP scheme;
FIG. 7 is a graph comparing the relative clock phase offset estimation performance of the proposed method with a hybrid time synchronization scheme, TAP scheme;
fig. 8 is a graph of absolute timing error of the proposed method versus the hybrid time synchronization scheme, the TAP scheme, when the terminal is one hundred meters from the base station;
fig. 9 is a graph of synchronization performance of the proposed method affected by an interruption versus preamble repetition number.
Detailed Description
For the purpose of promoting an understanding and enabling those of ordinary skill in the art to practice the present invention, reference will now be made in detail to the present embodiments of the invention as illustrated in the accompanying drawings.
FIG. 1 is a general flow diagram of the present invention. The present embodiment describes the specific procedure in detail by taking the contention-based random access scheme (fig. 2, left) as an example when the number of preamble repetitions M is 12.
First, consider the terminal UE and base station gNB local clock C shown in fig. 3i(t),C0(t), wherein gNB, UE's ownThe earth clock frequency offset and the phase offset are uniformly distributed in the range of alpha epsilon (0.99,1.01) and beta epsilon (-10,10) ms respectively.
Second, the UE sends a random access preamble Msg1 to the gNB: msg1 frame Structure As shown in FIG. 4, when the UE detects the end of each Preamble in Msg1, the radio chip triggers an interrupt and records the local timestamp
Figure BDA0003036907670000061
Third, the gNB receives the random access preamble Msg 1: when the gNB detects the end of each Preamble in Msg1, the radio chip triggers an interrupt and records a local timestamp
Figure BDA0003036907670000067
Then gNB calculates TA;
fourthly, the gNB sends a random access response Msg2 to the UE: msg2 frame Structure As shown in FIG. 5, start of frame delimiter SFD and local timestamp of second step recording
Figure BDA0003036907670000062
Adding the mark to the frame header of the original Msg2, and taking the tail of each SFD as a time reference point; when the gNB detects the end of each SFD in Msg2, the radio chip triggers an interrupt and records a local timestamp
Figure BDA0003036907670000063
At the same time stamping
Figure BDA0003036907670000064
Is embedded in C0(t2) Between the character and the reserved bit R;
fifth, the UE receives the Msg2 during the detection window, and when the UE detects the end of each SFD in Msg2, the radio chip triggers an interrupt and records the local timestamp
Figure BDA0003036907670000065
Simultaneous demodulation of the TA values and time stamp sets contained in Msg2
Figure BDA0003036907670000066
Sixthly, selecting a group of timestamps with the minimum UE interruption delay according to the formula (1);
seventhly, calculating the message delay t according to the formula (4)dAnd then, calculating the clock frequency offset and the phase offset of the UE relative to the gNB according to the formula (6). FIG. 3 illustrates a set of timestamps Ci(t1)、C0(t2)、C0(t3)、Ci(t4) The specific relationship of (c) };
and eighthly, compensating the local clock of the terminal according to the estimated value in the sixth step, wherein the compensation mode can be realized by adopting an FPGA or a voltage-controlled oscillator.
The method is compared with a hybrid synchronization method based on statistical signal processing and an air interface timing method TAP based on physical layer signals in a simulation mode, the synchronization performance is expressed by mean square error MSE, and the effectiveness of the method is verified.
As can be seen from fig. 6, the relative frequency offset estimation MSE of the proposed method is the lowest. The TAP scheme does not consider clock frequency deviation at all, and the MSE of the TAP scheme is the highest; the hybrid time synchronization scheme does not study the generation and transmission processes of the timing message, but estimates the fixed time delay and the time delay by transceiving the timing message for a plurality of times, and the MSE of the hybrid time synchronization scheme is about 40dB higher than that of the proposed method. In addition, since the dispersion of TA is related to the subcarrier spacing and IFFT block size, four different subcarrier spacing and IFFT block combinations commonly used in 5G systems are compared in the figure. It can be seen that the relative frequency offset estimate MSE of the proposed method gradually increases as the distance increases, but decreases at the boundary of TA. The reason is that the timing signal delay based on TA estimation is more accurate at TA boundaries, the larger the subcarrier spacing and IFFT blocks, the smaller the TA dispersion, and the estimation performance does not fluctuate much even if the timing signal delay occurs in the middle of two adjacent TA values.
It can be seen from fig. 7 that the relative phase offset estimation performance of the TAP scheme is the best, followed by the proposed method and the hybrid time synchronization scheme. For analysis reasons, the TAP method only estimates the relative phase offset of the local clock at the current moment, so that the TAP method is not affected by the relative phase offset and has the lowest MSE. Although the MSE of the proposed method is higher than the TAP method, the advantage of this method is the joint estimation of the local clock relative frequency offset and the initial phase offset. The hybrid time synchronization scheme is mainly affected by the random delay of the timing message, with an MSE about 40dB higher than the proposed method. Likewise, the larger the subcarrier spacing and IFFT blocks, the smaller the TA dispersion, and the better the estimation performance of the proposed method.
As shown in fig. 8, the zero time indicates that the local clock completes the parameter estimation and compensation. It can be found that the method and the mixed time synchronization scheme take clock relative frequency offset into consideration, the timing error of the method and the mixed time synchronization scheme is basically kept unchanged, the synchronization precision of the method can reach microsecond level, and the mixed time synchronization scheme only reaches millisecond level. The TAP scheme, while having a minimal timing error at time zero, has a gradual increase in timing error over time. At about 0.002 seconds, its timing error exceeds the proposed method; at 0.328 seconds, its timing error exceeds the hybrid time synchronization scheme.
As shown in fig. 9, the preamble length is set to 66.7ms, the gNB has no interruption delay, and the interruption delay e of the UE is typically uniformly distributed in the range of (0,5) μ s, but sometimes as high as 30 μ s. It can be seen that as the preamble repetition number increases, the relative frequency offset and the MSE of the phase offset estimate decrease. When the preamble repetition number is 12, the MSE is about 1dB higher than that of the case of only one preamble. In addition, it can be seen that the performance of repeating the preamble 2 times is worse than that of one time preamble, which indicates that repeating the preamble 2 times cannot completely avoid the impact of the interrupt delay on the synchronization performance.
The invention provides an absolute time synchronization method based on timing message exchange aiming at a 5G wireless edge, which completes the timing message exchange by relying on the random access process of a 5G network, designs a corresponding timing message frame structure, estimates the propagation delay according to the Time Advance (TA), can jointly estimate and compensate the phase offset and the frequency offset of a local clock, ensures the absolute time synchronization of all network access terminals, and has the advantages of high synchronization precision, small communication overhead and low calculation complexity. The invention can provide an absolute time synchronization function for a 5G network, and can be applied to the emerging industrial fields of audio/video production/sharing, motion control systems, unmanned aerial vehicle cluster formation, intelligent factories and the like which need synchronous cooperation of different types of equipment.

Claims (7)

1. A5G wireless edge absolute time synchronization method based on timing message exchange is characterized by comprising the following steps:
the first step is as follows: the base station and the N terminals are all provided with local hardware clocks, and the local clock model is
Ci(t)＝αit+βi,i＝0、1、2……N，
Wherein t represents world coordination time Ci(t)、αi、βiI-1, 2,3 … are the local clock of the terminal and the clock frequency offset and phase offset of the terminal with respect to the world coordination, respectively, C0(t)、α0、β0Respectively a local clock of the base station and a clock frequency offset and a phase offset of the base station relative to the world coordination; since the true frequency offset and phase offset of the local clock cannot be calculated, according to C0(t) and Ci(t) obtaining the relation
Ci *(t)＝αi0Ci(t)+βi0,i＝1、2……N，
Wherein alpha isi0＝α0/αi，βi0＝β0-αi0βiRespectively, the clock frequency offset and the phase offset of the wireless terminal relative to the base station;
the second step is that: after acquiring a cell identifier and finishing downlink synchronization, a terminal UE selects a proper physical random access channel to send a random access preamble Msg1 to a base station gNB; when the UE detects the reference point in time of Msg1, the radio chip triggers an interrupt and records the local timestamp
Figure FDA0003433996650000011
Wherein M is the repetition number of the leader sequence, and M is the time stamp group number;
the third step: the gNB receives a random access preamble Msg 1; when the gNB detects the reference point in time of Msg1, the radio chip triggers an interrupt and records a local timestamp
Figure FDA0003433996650000012
Then gNB calculates TA;
the fourth step: the gNB sends a random access response Msg2 to the UE; when the gNB detects the reference point in time of Msg2, the radio chip triggers an interrupt and records a local timestamp
Figure FDA0003433996650000013
Simultaneous grouping of locally stored timestamps
Figure FDA0003433996650000014
Embedded in Msg 2;
the fifth step: the UE receives Msg2 during the detection window; when the UE detects the reference point in time of Msg2, the radio chip triggers an interrupt and records the local timestamp
Figure FDA0003433996650000015
Simultaneous demodulation of the TA values and time stamp sets contained in Msg2
Figure FDA0003433996650000016
Eventually the UE obtains a set of timestamps
Figure FDA0003433996650000017
If the UE does not receive the Msg2 in the detection window period, the random access process fails, and the UE discards the stored timestamp group and records the timestamp group again in the next random access process;
and a sixth step: the terminal selects an optimal timestamp; because the terminal has limited processing capacity and uncertain interruption delay exists, in order to reduce errors caused by the interruption delay, one group with the minimum interruption delay is selected from M groups of timestamps, and the group is obtained by using an equation (1):
Figure FDA0003433996650000018
wherein the content of the first and second substances,
Figure FDA0003433996650000019
msg1 sending time recorded by the terminal, Msg1 receiving time recorded by the base station, Msg2 sending time recorded by the base station and Msg2 receiving time recorded by the terminal,
Figure FDA0003433996650000021
a timestamp group number indicating a minimum interrupt latency;
the seventh step: calculating the relative frequency offset and the phase offset of a local clock of the terminal: terminal according to TA value and interruption delay minimum group time stamp
Figure FDA0003433996650000022
Obtain equations (2), (3)
Figure FDA0003433996650000023
Figure FDA0003433996650000024
Wherein, tdThe message delay calculated for the UE according to TA is specified by the formula (4)
td＝16*κ*Tc*TA (4)
Where κ is a constant, TA is the timing advance, is an integer between 0 and 1228,
Figure FDA0003433996650000025
basic time unit of NR, f is the subcarrier spacing, NfIs the IFFT block size;
simultaneous equations (2) and (3) are obtained
Figure FDA0003433996650000026
Wherein，
Figure FDA0003433996650000027
Set of timestamps, α, for which interrupt latency is minimali0、βi0Respectively, the clock frequency offset and phase offset, t, of the wireless terminal relative to the base stationdA delay for a timing message;
solving equation to obtain relative clock frequency deviation and estimated value of phase deviation
Figure FDA0003433996650000028
Eighth step: and compensating the local clock of the terminal according to the estimated value in the sixth step, wherein the compensation mode is realized by adopting an FPGA.
2. The method of claim 1, wherein the timing message exchange is performed by means of a random access procedure of a 5G network.
3. The method of claim 1, wherein the corresponding frame structure of the timing message is designed according to the whole synchronization process, and M start of frame delimiters SFD and C are added between the header of Msg2 and the reserved bit R with the end of each Preamble as a time reference point in Msg10(t2)、C0(t3) Characters and ends with each SFD as a reference point in time.
4. The method for 5G wireless edge absolute time synchronization based on timing message exchange as claimed in claim 1, wherein the clock frequency offset and phase offset of the base station and the terminal are uniformly distributed within α e (0.99,1.01) and β e (-10,10) ms respectively.
5. The method of claim 1, wherein the constant k is 64 and the subcarrier spacing f is 64480kHz, IFFT Block size Nf4096, or f 240kHz, Nf＝4096、f＝120kHz,Nf＝2048、f＝15kHz,Nf＝2048。
6. The method of claim 1, wherein the preamble repetition number M is 1,2,4,6, 12.
7. The method of claim 1, wherein the compensating means is implemented by a voltage controlled oscillator.
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