CN111182594B - Low-orbit satellite constellation system cell switching method and device based on ephemeris information - Google Patents
Low-orbit satellite constellation system cell switching method and device based on ephemeris information Download PDFInfo
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Abstract
The invention discloses a cell switching method and device of a low-orbit satellite constellation system based on ephemeris information. The mobile terminal calculates the visible time of each wave beam in the neighbor cell list, screens out the wave beams with the visible time smaller than the wave beam visible time threshold, selects the wave beam with the maximum switching weight value in the screened neighbor cell list as a target wave beam, and finally sends a request for switching to the target wave beam to a source wave beam. The method has the characteristics of improving the beam cell switching reliability, reducing the ping-pong switching probability, reducing the number of system control signaling and improving the network efficiency.
Description
Technical Field
The invention discloses a cell switching method and device of a low-orbit satellite constellation system based on ephemeris information, and belongs to the field of mobility management in a mobile communication system.
Background
The low-earth-orbit satellite communication system has the characteristics of low time delay, high bandwidth utilization rate, low power consumption of the mobile terminal and the like, and is one of the development directions of future mobile communication technologies. Meanwhile, because the LEO system satellite orbit is low, the satellite moves at a high speed relative to the earth, so that communication connection is frequently switched between satellites or among beams, the overhead of system control signaling is greatly increased, and the call drop rate and the call termination probability are obviously improved. Therefore, mobility management technology is a focus and focus of research on low earth orbit satellite communication systems.
Currently, the research on the handover technology of the low earth orbit satellite communication system mainly includes the research on the network layer handover and the research on the link layer handover. The research of network layer handover mainly uses Mobile IP (MIP) technology to manage the Mobile terminal IP, and the research of link layer handover mainly discusses the influence of different handover criteria on the Mobile terminal drop call rate, call blocking rate, network utilization rate and other indexes. Link layer handovers may be classified as inter-satellite handovers and inter-beam handovers according to the handover beam and source beam satellite affiliation. Inter-satellite handover refers to a handover of a mobile terminal from a satellite coverage area to the coverage area of another satellite, and inter-beam handover refers to a handover of a mobile terminal between different beams of the same satellite. For low earth orbit satellite communication systems, the frequency of inter-beam handovers requires that the design of handover guidelines be heavily considered to avoid the occurrence of "ping-pong handovers".
Currently, common criteria for link layer handoff in low earth orbit satellite communication systems include the strongest signal criterion, the longest visible time criterion, the maximum elevation angle criterion, and the minimum load criterion. The study of these criteria is usually based on ideal cellular beams, taking into account less of the requirements of the actual system and the feasibility of the implementation. Therefore, the invention provides a cell switching method and a device of a low-orbit satellite constellation system based on ephemeris information.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the invention provides a switching method and a device thereof based on ephemeris track prediction aiming at the switching criterion of a low-orbit satellite communication system, which are used for predicting received broadcast ephemeris information, calculating a neighbor cell list at the switching triggering moment by combining the geographical position information of a mobile terminal, and screening an optimal access beam from the neighbor cell list according to the visible time of the beam and the received signal strength, thereby reducing the switching frequency of the mobile terminal, reducing the occurrence probability of ping-pong switching and improving the overall network utilization rate of the system.
The technical solution of the invention is as follows: the technical scheme adopted by the invention is as follows: a cell switching method of a low-orbit satellite constellation system based on ephemeris information comprises the following steps:
(1) the mobile terminal receives the signal of the current service cell at the current time t, and judges whether the received signal intensity of the current service cell is smaller than the signal intensity measurement threshold value P of any adjacent service cellthresh1If yes, continuing to execute the step (2); otherwise, the current serving cell does not need to be switched;
(2) the mobile terminal receives broadcast ephemeris information at the moment t, determines the position of the satellite-borne point at the moment t according to the position of the service satellite at the moment t in the broadcast ephemeris information at the moment t, and calculates the distance d from the mobile terminal to the satellite-borne point of the service satellite according to the position of the satellite-borne point at the moment tTH(t)；
(3) The mobile terminal is used for receiving the service satellite down-satellite distance d according to the mobile terminalTH(t) calculating the subsatellite half field angle of the serving satellite
And a movement direction declination angle mu (t) according to the half-aperture angle of the satellite under the satellite serving
And a motion direction deviation angle mu (t) for calculating the ideal received signal power sideal(t); judgment sideal(t) whether the communication time is less than or equal to the set ideal communication threshold value, if so, continuing to execute the step (4), otherwise, constantlyAdding t to a value obtained by increasing the interval delta t to t, replacing t in the step (2) and returning to the step (2);
(4) the mobile terminal calculates critical power of all beams in a coverage satellite set of the mobile terminal according to the satellite position at the moment t, and the critical power is compared with the ideal received signal power s in the step (3)ideal(t) comparing, if the comparison result is larger than or equal to the threshold power, putting the wave beam corresponding to the threshold power into a neighbor list of the mobile terminal, otherwise, discarding the wave beam corresponding to the threshold power;
(5) the mobile terminal simultaneously measures the signal intensity of the current service cell and the signal intensity of the wave beam in the neighbor cell list, and judges whether the received signal intensity of the current service cell is smaller than the set switching trigger threshold value Pthresh2If the current cell number is less than the preset value, continuing to execute the step (6), otherwise, the current serving cell does not need to be switched;
(6) calculating the visible time of the wave beams of the mobile terminal corresponding to the wave beams in the neighbor cell list one by one;
(7) mobile terminal screens out less-than-set wave beam visible time threshold t in neighbor listthreshIf the filtered neighbor list is empty, the set visible time threshold t of the wave beam is setthreshThe size of the neighbor list is reduced by half, and the original neighbor list is screened again until the neighbor list is not empty;
(8) and calculating the beam switching weight values of all the beams in the filtered neighbor cell list, selecting the beam with the maximum beam switching weight value in the neighbor cell list as a target beam, and switching the current beam to the target beam.
Preferably, in step (1), the received signal strength P of the serving cellmeasThe formula for calculation of (t) is:
where N is the length of the sampling window, Δ t is the interval of measurement, αnThe weight values of the received power at different sampling moments are shown, and P (t-n delta t) represents the actual received power at the t-n delta t moment;
preferably, the position of the point under the star(s)j,sw) The calculation formula of (2) is as follows:
wherein (x)r,yr,zr) The coordinate of the low earth orbit satellite in the geocentric inertial coordinate system, when GAST represents the Greenwich mean star at the current moment, the GAST is equal to the GMST of the Greenwich mean star in value, and the calculation formula of the GMST is as follows:
wherein, the parameter C4、C5The values of (1) are respectively the number of seconds contained in one solar day and the conversion coefficient of the solar day and the sidereal day, and the calculation formula of the parameter UT is as follows:
UT mod (JD +0.5,1) [ equation 4]
The JD represents the julian day corresponding to the current day, and the calculation formula is as follows:
wherein the parameter Y, M, D represents the year, month and date corresponding to the current day,
GMST0representing Greenwich mean time of 0 o' clock in world time, the calculation formula is:
GMST0＝C0+C1T+C2T2-C3T3[ equation 6)]
Wherein, the parameter C0、C1、C2And C3Have values of 24110.54841, 8640184.812866, 0.093104 and 6.2 x 10, respectively-6，T＝(JD-2451545.0-UT)/36525.0。
Preferably, the calculation formula of the terminal-substellar point distance is as follows:
wherein (u)j,uw) Indicating the current location of the mobile terminal,(s)j,sw) Representing the current position of the sub-satellite point, RERepresenting the radius of the earth.
The calculation formula of the half aperture angle under the satellite is as follows:
wherein d isTH(t) represents the terminal-to-subsatellite point distance at time t, RERepresenting the earth radius and h the orbit height.
The calculation formula of the deflection angle of the motion direction is as follows:
wherein R isERepresenting the radius of the earth, parameter dTQ(t) the calculation formula is:
parameter dHQ(t) the calculation formula is:
preferably, the calculation formula of the ideal received signal power is:
sideal(t)＝10logδ2-Lr(t) [ equation 12]
Wherein δ represents a signal field value at time t, and the calculation formula is as follows:
δ ═ H (u, v) [ equation 13]
Wherein,
and δ H (u, v) represents a UV coordinate system coordinate-field value mapping relationship of the on-satellite antenna corresponding to the mobile terminal. L isr(t) is the relative path loss of the signal, and the calculation formula is as follows:
wherein R isERepresenting the radius of the earth, h representing the orbit height,
representing the inferior star half opening angle at time t.
The conditions under which the beam is visible are:
sideal(t)>sthresh[ equation 15)]
Wherein s isthreshThe calculation formula represents the visible critical power value of the beam and is as follows:
sthresh＝sguarantee+soff[ equation 16]
Wherein s isguaranteeThe minimum relative received power for the mobile terminal communication is guaranteed and is determined by the link budget. soffThe value of the intensity drop of the mobile terminal due to signal fading is represented by the following formula:
wherein, Pideal(thandover) Representing the moment of handover trigger thandoverIdeal received power of Pmeas(thandover) Representing the actual measured signal strength value at the moment of handover trigger.
Preferably, the beam visibility time calculation formula is:
wherein, thandoverFor the handover trigger time, t is the time at which the beam visibility condition is first not satisfied, and Δ t is the calculation interval.
Preferably, the screening criteria of the best access cell beam [ i ] [ j ] are as follows:
beam[i][j]＝argmax{W[i][j]}
s.t.tvisble[i][j]≥tthresh[ equation 19]
Wherein, W [ i ] [ j ] represents the beam switching weight of the i number satellite j number beam in the neighbor list, and the calculation formula is as follows:
wherein, Pmeas(thandover)[i][j]And the measured signal strength represents the switching triggering moment of the j wave beam of the i satellite in the adjacent area list, M is a constant for judging whether the wave beam and the source wave beam are under the same satellite, the value under the same satellite is 1, and M is 0 under the condition that the visible time is the same if certain wave beam power values of the j satellite and the i satellite exist under different satellites, so that the probability of switching between the satellites is reduced, and the wave beam switching under the same satellite is ensured to be preferentially selected when the maximum value is calculated. t is tvisble[i][j]Representing the visible time of the mobile terminal to the j wave beam of the i-number satellite in the adjacent area list, T representing the maximum value of the visible time of the wave beam in all the adjacent area wave beams, P representing the maximum value of the intensity of the received signal at the switching triggering moment in all the adjacent area wave beams, and TthreshRepresenting the beam visibility time threshold.
As shown in fig. 2, preferably, the device for switching a cell of a low-earth satellite constellation system based on ephemeris information according to the present invention includes:
the signal measurement module is used for measuring the signal intensity of the service cell on a service channel and the signal intensity of a pilot signal of an adjacent cell;
the ephemeris information receiving module is used for receiving ephemeris information broadcasted by the satellite-borne base station in a downlink manner;
the mobile terminal positioning module is used for acquiring real-time position information of the mobile terminal;
the ephemeris prediction and satellite node calculation module is used for predicting ephemeris information at the current moment according to the received latest ephemeris information and calculating the satellite node position of the low-orbit constellation;
the signal field value storage module is used for pre-storing the coordinates and the signal field value pairs under the UV coordinate system of each beam internal branch point according to the antenna design condition;
the neighbor cell list calculation module is used for calculating and storing a neighbor cell list of the mobile terminal at the switching triggering moment according to the satellite point information, the mobile terminal position information and the signal field value;
the beam visible time calculation module is used for switching the trigger time and calculating the beam visible time of all beam cells in the neighbor cell list according to the satellite point information, the mobile terminal position information and the signal field value;
and the screening and judging module screens out the best access beam cell for the beam cell in the neighbor cell list according to the visible time of the beam and the intensity of the received signal.
Compared with the prior art, the invention has the beneficial effects that:
(1) the invention provides a method and a device for switching cells of a low-orbit satellite constellation system based on ephemeris information. This handover criterion reduces the frequency of mobile terminal handovers starting from the requirements of practical low earth orbit satellite systems.
(2) The method is simultaneously suitable for inter-satellite switching and intra-satellite switching, and has strong operability;
(3) the invention judges from two dimensions of power and time at the same time, the optimized switching target is real, and the reliability is high
(4) The invention effectively reduces the control signaling overhead brought by switching between the satellites.
Drawings
FIG. 1 is a process flow diagram of the present process;
FIG. 2 is a schematic diagram of a possible apparatus provided by the present method;
fig. 3 is a flow chart of possible intra-satellite beam switching signaling transmission provided by the present method;
fig. 4 is a flow chart of a possible inter-satellite beam switching signaling transmission provided by the present method;
fig. 5 is an application scenario diagram of the method.
Detailed Description
The present invention is further described with reference to the accompanying drawings and specific examples, which are intended to be illustrative only and not to be limiting of the scope of the invention, and various equivalent modifications of the invention will occur to those skilled in the art upon reading the present invention and fall within the scope of the invention defined by the appended claims.
The invention discloses a cell switching method and device of a low-orbit satellite constellation system based on ephemeris information. The mobile terminal judges the switching triggering time according to the signal intensity of the received service satellite, then predicts the position of the satellite points under the low orbit satellite according to the received satellite ephemeris parameters, calculates the covering satellite set of the switching triggering time according to the predicted position of the satellite points under the low orbit satellite, and compares the mobile terminal with the wave beam characteristic point set of all wave beams of each satellite in the covering satellite set to obtain a neighbor cell list. The mobile terminal calculates the visible time of each wave beam in the neighbor cell list, screens out the wave beams with the visible time smaller than the wave beam visible time threshold, selects the wave beam with the maximum switching weight value in the screened neighbor cell list as a target wave beam, and finally sends a request for switching to the target wave beam to a source wave beam. The method has the characteristics of improving the beam cell switching reliability, reducing the ping-pong switching probability, reducing the number of system control signaling and improving the network efficiency.
As shown in fig. 1, a method for switching a cell of a low-earth-orbit satellite constellation system based on ephemeris information according to the present invention includes the following steps:
(1) the mobile terminal receives the signal of the current service cell at the current time t, and judges whether the received signal intensity of the current service cell is smaller than the signal intensity measurement threshold value P of any adjacent service cellthresh1If yes, continuing to execute the step (2); otherwise, the current serving cell does not need to be switched;
(2) the mobile terminal receives broadcast ephemeris information at the moment t, determines the position of the satellite-borne point at the moment t according to the position of the service satellite at the moment t in the broadcast ephemeris information at the moment t, and calculates the distance d from the mobile terminal to the satellite-borne point of the service satellite according to the position of the satellite-borne point at the moment tTH(t)；
(3) The mobile terminal is used for receiving the service satellite down-satellite distance d according to the mobile terminalTH(t) calculating the subsatellite half field angle of the serving satellite
And a movement direction declination angle mu (t) according to the half-aperture angle of the satellite under the satellite serving
And a motion direction deviation angle mu (t) for calculating the ideal received signal power sideal(t); judgment sideal(t) whether the value is less than or equal to the set ideal communication threshold value, if so, continuing to execute the step (4), otherwise, adding a value after the interval delta t is increased at the moment t to the t, replacing the t in the step (2) and returning to the step (2);
(4) the mobile terminal calculates critical power of all beams in a coverage satellite set of the mobile terminal according to the satellite position at the moment t, and the critical power is compared with the ideal received signal power s in the step (3)ideal(t) comparing, if the comparison result is larger than or equal to the threshold power, putting the wave beam corresponding to the threshold power into a neighbor list of the mobile terminal, otherwise, discarding the wave beam corresponding to the threshold power;
(5) the mobile terminal simultaneously measures the signal intensity of the current service cell and the signal intensity of the wave beam in the neighbor cell list, and judges whether the received signal intensity of the current service cell is smaller than the set switching trigger threshold value Pthresh2If the current service is smaller than the preset service, continuing to execute the step (6), otherwise, keeping the current service smallThe zones do not need to be switched;
(6) calculating the visible time of the wave beams of the mobile terminal corresponding to the wave beams in the neighbor cell list one by one;
(7) mobile terminal screens out less-than-set wave beam visible time threshold t in neighbor listthreshIf the filtered neighbor list is empty, the set visible time threshold t of the wave beam is setthreshThe size of the neighbor list is reduced by half, and the original neighbor list is screened again until the neighbor list is not empty;
(8) and calculating the beam switching weight values of all the beams in the filtered neighbor cell list, selecting the beam with the maximum beam switching weight value in the neighbor cell list as a target beam, and switching the current beam to the target beam.
Mobile terminal, in particular satellite terminal
Neighbor measurement threshold Pthresh1The method includes that the mobile terminal starts signal intensity measured by a neighboring cell, and the neighboring cell is determined according to system requirements;
broadcast ephemeris information, including: the longitude and latitude position of the service satellite at the current time t also comprises information of the satellite such as time, running speed, running azimuth, satellite number, beam number and the like.
The set ideal communication threshold value is the minimum relative receiving power value for guaranteeing the communication of the mobile terminal and is determined according to the link budget.
The coverage satellite set refers to a set of satellites that the mobile terminal can receive signals at time t.
Set handover trigger threshold Pthresh2The power determination value when switching is required is determined by the transmission power (antenna design value) of the current access beam, the transmission loss of the distance power of the satellite point (path power loss of the position of the satellite point) and the minimum receiving power (mobile terminal design value) of the mobile terminal.
The mobile terminal beam visible time corresponding to each beam is the time when the mobile terminal can measure the signal corresponding to each beam.
The set beam visible time threshold tthreshThe size of the neighbor list is reduced by half, and the original neighbor list is screened out again until the size of the neighbor list is reduced by halfThe list of the adjacent cells is not empty; when the list screened out under the wave beam visible time threshold is empty, the value of the time is reduced to half, and the value is set as the wave beam visible time threshold again
The beam switching weight value is a value for judging the switching target beam, and the value is the maximum value of the sum of constants of the visible time of each beam covering the mobile terminal, the measurement power of each beam covering the mobile terminal at the switching moment and whether each beam covering the mobile terminal and the current access beam are in the same star.
And the current beam is switched to the target beam, so that cell switching is realized.
The critical power of all beams in the set of coverage satellites of the mobile terminal refers to the received power of the mobile terminal, and the value is determined according to the transmitting power of the satellite antenna.
Fig. 5 is a schematic view of an application scenario of the switching method based on ephemeris trajectory prediction in the low earth orbit satellite communication system according to the preferred embodiment of the present invention. The low-orbit constellation adopted in the embodiment totally comprises 6 orbits, the longitude difference of the orbit ascending point is fixed, 10 satellites are uniformly distributed in each orbit, the specific parameters of the system are shown in the following table 1, and the specific values and the constant values are shown in the following table 2, wherein the specific values of the thresholds can be properly adjusted according to the simulation result and the system requirement, the processes of inter-satellite switching and intra-satellite beam switching are illustrated in fig. 3 and 4, the switching method disclosed by the invention is suitable for two situations, and the satellite ephemeris information contains the information whether the current beam and the measurement beam are in the same satellite.
TABLE 1 System parameter Table
TABLE 2 threshold and constant tables
The invention discloses a further preferable scheme of a low-orbit satellite constellation system cell switching method based on ephemeris information, which comprises the following steps: :
the first step is as follows: and the mobile terminal receives the signal of the current service cell and judges whether the signal intensity of the service cell is smaller than the neighbor cell measurement threshold value.
In this step, the mobile terminal receives the serving cell signal on the traffic channel, and continuously receives N subframes, preferably according to the following formula:
obtaining the received signal strength value P of the current time tmeas(t), where P (t-n Δ t) is the power of the subframe received at time t-n Δ t, αnIs the power weight. After the next time slot arrives, the power of the sub-frame received by the first N-1 time slots and the power of the sub-frame received by the current time slot are weighted and averaged to obtain the received signal strength P at the momentmeas(t + Δ t), and so on. When:
Pmeas(t)≤Pthresh1
meanwhile, the mobile terminal starts the neighbor measurement, as shown in fig. 3 and 4. Wherein, Pthresh1The threshold value is the neighbor cell measurement threshold value, is the critical serving cell signal strength for the mobile terminal to start neighbor cell measurement, and is determined according to the system requirements.
The second step is that: and the mobile terminal calculates the distance between the terminal and the sub-satellite point. Firstly, receiving broadcast ephemeris information on a control channel, and obtaining the coordinate (x) of the service satellite at the current moment under the geocentric inertial coordinate system through orbit extrapolationr,yr,zr). Preferably according to the following formula:
UT mod (JD +0.5,1) [ equation 4]
Calculating to obtain the coordinate(s) of the satellite point of the service satellitej,sw). Wherein, C0、C1、C2、C3And C4Respectively being a parameter, GMST0Representing Greenwich mean time constant at world time 0, GMST representing Greenwich mean time constant at the current time, numerically equal to Greenwich mean time GAST, JD representing the Confucian day for that day, Y, M, D representing the year, month and date for that day.
Preferably according to the following formula:
calculating the terminal-substellar point distance d at the time tTH(t) wherein(s)j,sw) Is the position of the point under the satellite (u) calculated from the ephemerisj,uw) Is the current position that the mobile terminal obtained from the internal positioning module.
The third step: the mobile terminal calculates the ideal relative received signal power s to the serving cellidealAnd (t) predicting the switching trigger time so as to calculate the neighbor list.
Preferably according to the following formula:
computing mobile terminalHalf field angle of end-to-service satellite
Wherein d isTH(t) represents the terminal-to-subsatellite point distance at time t, RERepresenting the earth radius and h the orbit height.
Preferably according to the following formula:
and calculating the movement direction deflection angle mu (t) of the mobile terminal to the service satellite.
Preferably according to the following formula:
sideal(t)＝10logδ2-Lr(t) [ equation 12]
δ ═ H (u, v) [ equation 13]
Calculating the ideal relative received signal power s of the mobile terminal to the service cell at the time tideal(t) of (d). Where δ represents a signal field value at time t, and δ — H (u, v) represents an on-satellite day corresponding to the mobile terminalAnd the UV coordinate system coordinate-field value mapping relation of the lines is related to the design condition of the antenna. L isr(t) represents the relative path loss of the signal, RERepresenting the radius of the earth, h representing the orbit height,
represents the half field angle of the mobile terminal to the service satellite, and the mu (t) represents the movement direction deflection angle of the mobile terminal to the service satellite.
Preferably when:
sideal(t)≤sguarantee
then, predicting the time t as a switching trigger time; otherwise, let t be t + Δ t and jump to the second step. Wherein s isguaranteeThe minimum relative received power of the mobile terminal communication is guaranteed and is determined according to the link budget.
The fourth step: and the mobile terminal calculates the neighbor list. Firstly, calculating the half field angle of the mobile terminal to other satellites in the constellation at the time t, preferably according to the following formula:
a decision is made that satellite number i enters the set of overlay satellites, where,
is the ideal critical satellite lower half opening angle of the satellite No. i,
representing the inferior star half field angle of the mobile terminal to the i-number satellite, the preferred calculation formula is as follows:
wherein d isTH(t)[i]Represents the distance between the mobile terminal and the satellite-satellite point of the number i, RERepresents the earth radius, h represents the orbital height, (u)j,uw) Representing the position of the mobile terminal,(s)j[i],sw[i]) Representing the position of the satellite under the satellite No. i.
Still more preferably according to the following formula:
calculating the movement direction deflection angle mu (t) [ i ] of the mobile terminal to the No. i satellite]。dTH(t)[i]Representing the distance between the mobile terminal and the satellite-satellite point I, and the parameter dTQ(t)[i]The preferred calculation formula of (c):
parameter of
Parameter dHQ(t)[i]The preferred calculation formula of (c):
the mobile terminal is to
μ(t)[i]And comparing the critical power of all wave beams in the No. i satellite, and calculating to obtain a neighbor list of the mobile terminal. The critical power of the beam is determined by the antenna design.
The fifth step: and simultaneously measuring the signal intensity of the beam cell in the service cell and the neighbor cell list. If the intensity of the signal received by the serving cell is less than Pthresh2Then the handover trigger is decided. Pthresh2Is a handover trigger threshold, the preferred calculation formula:
Pthresh2＝10logPbeam-Ploss+sguarantee
wherein, PlossRepresents the satellite-borne point path loss of the service satellite, lambda represents the operating wavelength, h represents the orbital altitude, PbeamRepresenting the transmission power of a single beam, sguaranteeRepresenting the minimum relative received power at which the mobile terminal is guaranteed to communicate.
And a sixth step: the mobile terminal calculates the visible time of the pre-screened wave beams one by one through a snapshot method, preferably according to the following formula:
where Δ t is the interval of snapshots, r represents the number of snapshots, and preferably satisfies:
sideal(t+rΔt)≤sthresh[i][j]
sideal(t + r Δ t) represents the ideal relative received signal power of the mobile terminal to the j beam of satellite number i at time t + r Δ t, sthresh[i][j]The optimal calculation formula for the visible critical power value of the beam of satellite number j represents the number i:
sthresh[i][j]＝sguarantee+soff[i][j][ equation 16]
Wherein s isguaranteeIs to ensure the minimum relative received power, s, of the mobile terminal communicationoff[i][j]Representing the value of the intensity drop, P, of the mobile terminal due to signal fadingideal(t)[i][j]Representing the ideal receiving power, P, of the mobile terminal to the j wave beam of the i satellite at the switching triggering time tmeas(t)[i][j]Representing the actual measured signal strength value of the handoff trigger time t for satellite number i beam number j.
The seventh step: screening the beam cells in the neighbor cell list according to the beam visible time is preferably according to the following formula:
wherein, W [ i ] [ j ] represents the beam switching weight of the i number satellite j number beam in the neighbor list, and the preferred calculation formula is as follows:
wherein, Pmeas(thandover)[i][j]And the measured signal strength represents the switching triggering moment of the j wave beam of the i satellite in the adjacent area list, M is a constant for judging whether the wave beam and the source wave beam are under the same satellite, the value under the same satellite is 1, and M is 0 under the condition that the visible time is the same if certain wave beam power values of the j satellite and the i satellite exist under different satellites, so that the probability of switching between the satellites is reduced, and the wave beam switching under the same satellite is ensured to be preferentially selected when the maximum value is calculated. t is tvisble[i][j]Representing the visible time of the mobile terminal to the j wave beam of the i-number satellite in the adjacent area list, T representing the maximum value of the visible time of the wave beam in all the adjacent area wave beams, P representing the maximum value of the intensity of the received signal at the switching triggering moment in all the adjacent area wave beams, and TthreshRepresenting the beam visibility time threshold.
If the adjacent area list after screening is empty, t is addedthreshAnd (4) dividing the size by half, and re-screening the original neighbor list. Until the filtered neighbor list is not empty.
And selecting the wave beam cell with the maximum received signal intensity in the screened neighbor cell list as a new service cell and accessing the new service cell.
The invention provides a method and a device for switching cells of a low-orbit satellite constellation system based on ephemeris information. This handover criterion reduces the frequency of mobile terminal handovers starting from the requirements of practical low earth orbit satellite systems.
Claims (4)
1. A cell switching method of a low-orbit satellite constellation system based on ephemeris information is characterized by comprising the following steps:
(1) the mobile terminal receives the signal of the current service cell at the current time t, and judges whether the received signal intensity of the current service cell is smaller than the signal intensity measurement threshold value P of any adjacent service cellthresh1If yes, continuing to execute the step (2); otherwise, the current serving cell does not need to be switched;
(2) the mobile terminal receives broadcast ephemeris information at the moment t, determines the position of the satellite-borne point at the moment t according to the position of the service satellite at the moment t in the broadcast ephemeris information at the moment t, and calculates the distance d from the mobile terminal to the satellite-borne point of the service satellite according to the position of the satellite-borne point at the moment tTH(t)；
The calculation formula of the terminal-substellar point distance is as follows:
wherein (u)j,uw) Indicating the current location of the mobile terminal,(s)j,sw) Representing the current position of the sub-satellite point, RERepresents the radius of the earth;
the calculation formula of the half aperture angle under the satellite is as follows:
wherein d isTH(t) represents the terminal-to-subsatellite point distance at time t, RERepresents the earth radius, h represents the orbit height;
the calculation formula of the deflection angle of the motion direction is as follows:
wherein R isERepresenting the radius of the earth, parameter dTQ(t) is calculated by the formula
Parameter dHQ(t) the calculation formula is:
(3) the mobile terminal is used for receiving the service satellite down-satellite distance d according to the mobile terminalTH(t) calculating the subsatellite half field angle of the serving satellite
And a movement direction declination angle mu (t) according to the half-aperture angle of the satellite under the satellite serving
And a motion direction deviation angle mu (t) for calculating the ideal received signal power sideal(t); judgment sideal(t) whether the value is less than or equal to the set ideal communication threshold value, if so, continuing to execute the step (4), otherwise, adding a value after the interval delta t is increased at the moment t to the t, replacing the t in the step (2) and returning to the step (2);
the ideal received signal power is calculated as:
sideal(t)＝10logδ2-Lr(t)
wherein δ represents a signal field value at time t, and the calculation formula is as follows:
δ＝H(u,v)
wherein,
h (u, v) represents a UV coordinate system coordinate-field value mapping relationship of the on-satellite antenna corresponding to the mobile terminal; l isr(t) is the relative path loss of the signal, and the calculation formula is as follows:
wherein R isERepresenting the radius of the earth, h representing the orbit height,
represents the inferior star half opening angle at the moment t;
(4) the mobile terminal calculates critical power of all beams in a coverage satellite set of the mobile terminal according to the satellite position at the moment t, and the critical power is compared with the ideal received signal power s in the step (3)ideal(t) comparing, if the comparison result is larger than or equal to the threshold power, putting the wave beam corresponding to the threshold power into a neighbor list of the mobile terminal, otherwise, discarding the wave beam corresponding to the threshold power;
(5) the mobile terminal simultaneously measures the signal intensity of the current service cell and the signal intensity of the wave beam in the neighbor cell list, and judges whether the received signal intensity of the current service cell is smaller than the set switching trigger threshold value Pthresh2If the current cell number is less than the preset value, continuing to execute the step (6), otherwise, the current serving cell does not need to be switched;
(6) calculating the visible time of the wave beams of the mobile terminal corresponding to the wave beams in the neighbor cell list one by one;
(7) mobile terminal screens out less-than-set wave beam visible time threshold t in neighbor listthreshIf the filtered neighbor list is empty, the set visible time threshold t of the wave beam is setthreshThe size of the neighbor list is reduced by half, and the original neighbor list is screened again until the neighbor list is not empty;
(8) and calculating the beam switching weight values of all the beams in the filtered neighbor cell list, selecting the beam with the maximum beam switching weight value in the neighbor cell list as a target beam, and switching the current beam to the target beam.
2. The method of claim 1, wherein the serving cell has a received signal strength PmeasThe formula for calculation of (t) is:
where N is the length of the sampling window, Δ t is the interval of measurement, αnIs the weight of the received power at different sampling moments, and P (t-n Δ t) represents the actual received power at t-n Δ t.
3. The method of claim 1, wherein the position(s) of the sub-satellite point is a position(s) of the cell in the low earth orbit satellite constellation system based on ephemeris informationj,sw) The calculation formula of (2) is as follows:
wherein (x)r,yr,zr) The coordinate of the low earth orbit satellite in the geocentric inertial coordinate system, when GAST represents the Greenwich mean fixed star at the current moment, the GAST is approximately equal to GMST of Greenwich mean fixed star in the numerical value, and the calculation formula is as follows:
wherein, the parameter C4、C5The values of (1) are respectively the number of seconds contained in one solar day and the conversion coefficient of the solar day and the sidereal day, and the calculation formula of the parameter UT is as follows:
UT＝mod(JD+0.5,1)
the JD represents the julian day corresponding to the current day, and the calculation formula is as follows:
wherein the parameter Y, M, D represents the year, month and date corresponding to the current day,
GMST0representing Greenwich mean time of 0 o' clock in world time, the calculation formula is:
GMST0＝C0+C1T+C2T2-C3T3
wherein, C0、C1、C2And C3All are parameters, T ═ T (JD-2451545.0-UT)/36525.0.
4. A low earth orbit satellite constellation system cell switching apparatus based on ephemeris information, comprising:
the signal measurement module is used for measuring the signal intensity of the service cell on a service channel and the signal intensity of a pilot signal of an adjacent cell;
the ephemeris information receiving module is used for receiving ephemeris information broadcasted by the satellite-borne base station in a downlink manner;
the mobile terminal positioning module is used for acquiring real-time position information of the mobile terminal;
the ephemeris prediction and satellite node calculation module is used for predicting ephemeris information at the current moment according to the received latest ephemeris information and calculating the satellite node position of the low-orbit constellation;
the ephemeris prediction and satellite point calculation module firstly receives the broadcast ephemeris information on a control channel, and obtains the coordinates (x) of the service satellite at the current moment under the geocentric inertial coordinate system through orbit extrapolationr,yr,zr) According to the following formula:
UT＝mod(JD+0.5,1)
GMST0＝C0+C1T+C2T2-C3T3
T＝(JD-2451545.0-UT)/36525.0
calculating to obtain the coordinate(s) of the satellite point of the service satellitej,sw) (ii) a Wherein, C0、C1、C2、C3And C4Respectively being a parameter, GMST0Representing Greenwich mean time constant star time of world time 0, GMST representing Greenwich mean time constant star time of current time, being numerically equal to Greenwich mean time GAST, JD representing julian day corresponding to that day, Y, M, D representing year, month and date corresponding to that day;
according to the following formula:
calculating the terminal-substellar point distance d at the time tTH(t) wherein(s)j,sw) Is the position of the point under the satellite (u) calculated from the ephemerisj,uw) The current position of the mobile terminal is obtained from the internal positioning module;
the signal field value storage module is used for pre-storing the coordinates and the signal field value pairs under the UV coordinate system of each beam internal branch point according to the antenna design condition;
the neighbor cell list calculation module is used for calculating and storing a neighbor cell list of the mobile terminal at the switching triggering moment according to the satellite point information, the mobile terminal position information and the signal field value;
the neighbor list calculation module calculates the half satellite field angle of the mobile terminal to other satellites in the constellation at the time t according to the following formula:
a decision is made that satellite number i enters the set of overlay satellites, where,
is the ideal critical satellite lower half opening angle of the satellite No. i,
representing the half field angle of the mobile terminal to the satellite No. i,
wherein d isTH(t)[i]Represents the distance between the mobile terminal and the satellite-satellite point of the number i, RERepresents the earth radius, h represents the orbital height, (u)j,uw) Representing the position of the mobile terminal,(s)j[i],sw[i]) Representing the position of the satellite lower star point of the No. i satellite;
according to the following formula:
calculating the movement direction deflection angle mu (t) [ i ] of the mobile terminal to the No. i satellite]，dTH(t)[i]Representing the distance between the mobile terminal and the satellite-satellite point I, and the parameter dTQ(t)[i]The calculation formula of (2):
parameter dHQ(t)[i]The calculation formula of (2):
the mobile terminal is to
μ(t)[i]Comparing the critical power of all wave beams in the No. i satellite, and calculating to obtain a neighbor list of the mobile terminal;
the beam visible time calculation module is used for switching the trigger time and calculating the beam visible time of all beam cells in the neighbor cell list according to the satellite point information, the mobile terminal position information and the signal field value;
the beam visible time calculation module screens the beam cells in the neighbor cell list according to the beam visible time, preferably according to the following formula:
beam[i][j]＝argmax{W[i][j]}
s.t.tvisble[i][j]≥tthresh
wherein, W [ i ] [ j ] represents the beam switching weight of the i number satellite j number beam in the neighbor list, and the calculation formula is as follows:
wherein, Pmeas(thandove)r[i][j]The method comprises the steps of representing the strength of a measurement signal at the switching trigger time of a number i satellite and a number j satellite in a neighbor list, wherein M is a constant for judging whether a beam and a source beam are under the same satellite, the value under the same satellite is 1, if certain beam power values of the number j satellite and the number i satellite exist under different satellites, M is 0 under the condition that the visible time is the same, the probability of switching between the satellites is reduced, and when the maximum value is calculated, the beam under the same satellite is guaranteed to be preferentially selectedSwitching; t is tvisble[i][j]Representing the visible time of the mobile terminal to the j wave beam of the i-number satellite in the adjacent area list, T representing the maximum value of the visible time of the wave beam in all the adjacent area wave beams, P representing the maximum value of the intensity of the received signal at the switching triggering moment in all the adjacent area wave beams, and TthreshRepresents a beam visibility time threshold;
the screening and judging module screens out the best access beam cell for the beam cell in the neighbor cell list according to the visible time of the beam and the intensity of the received signal;
and selecting the wave beam cell with the maximum received signal intensity in the screened neighbor cell list as a new service cell and accessing the new service cell.
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