CN114465647A - Method and system for carrying out high-speed rail communication by dedicated coverage wide beam transmission of elliptical cell - Google Patents

Method and system for carrying out high-speed rail communication by dedicated coverage wide beam transmission of elliptical cell Download PDF

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CN114465647A
CN114465647A CN202210029567.6A CN202210029567A CN114465647A CN 114465647 A CN114465647 A CN 114465647A CN 202210029567 A CN202210029567 A CN 202210029567A CN 114465647 A CN114465647 A CN 114465647A
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cell
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eccentricity
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CN114465647B (en
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刘细妹
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Changshu Institute of Technology
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    • 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
    • H04B7/0615—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 of weighted versions of same signal
    • H04B7/0617—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 of weighted versions of same signal for beam forming
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
    • H04W16/24—Cell structures
    • H04W16/28—Cell structures using beam steering
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W36/00—Hand-off or reselection arrangements
    • H04W36/08—Reselecting an access point

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Abstract

The invention discloses a method and a system for high-speed rail communication by special coverage wide beam transmission of an elliptical cell, which comprises the following steps: establishing an elliptical cell, wherein the cell boundary passes through the two farthest ends of the orbit; establishing a wide beam optimization problem based on the elliptical cell, meeting the condition of eccentricity e, and maximizing the signal-to-noise ratio (SNR) of the edge of the elliptical cell to ensure that the SNR of the edge of the cell is equal; initializing e, converting the optimization problem into a relaxed semi-definite programming SDR problem, and iteratively updating e to obtain an optimal solution X★(ii) a By X★Obtaining a polynomial root with the beam vector as a coefficient, solving the polynomial coefficient according to the polynomial root to obtain the beam vector and X★Compared with no beam gain loss; the wave beam is calculated off line and stored, when the train passes through the cell, the base station does not need to switch the wave beam, and only one wave beam is needed to cover the whole cell. The invention establishes the elliptical cell, designs the wave beam, adjusts the eccentricity to obtain the optimal elliptical cell, and obtains the wide wave beam, thereby effectively improving the coverage rate of high-speed rail communication in a large range.

Description

Method and system for carrying out high-speed rail communication by dedicated coverage wide beam transmission of elliptical cell
Technical Field
The invention relates to the technical field of high-speed rail wireless communication, in particular to a method and a system for high-speed rail communication through coverage wide beam transmission special for an elliptical cell.
Background
The high-speed rail becomes an important green vehicle by the characteristics of rapidness, comfort, safety, reliability, high carrying capacity, low energy consumption and the like, and can be rapidly developed all over the world. According to UIC data reports, as of 2020, there have been over 5.2 kilometers of high speed railways around the world, and it is expected that the high speed rail history will exceed 8 kilometers by the time 2030 to 2035. The problem that follows is how to deploy a high-speed railway communication system, cover as long as possible high-speed railway sections with as few base stations and link budgets as possible, and provide ubiquitous network services for passengers anytime and anywhere.
In consideration of the long and narrow cell characteristics of a high-speed railway, the communication coverage efficiency of the traditional cellular network under the special scene of high-speed railway compromise is low, so that a special communication coverage network needs to be designed. Dedicated network coverage studies on high-speed rail communications have received extensive attention. Some existing coverage networks dedicated to high-speed rail cells are mainly implemented through hardware infrastructure, for example, data transmission is provided for high-speed rail communication through optical fiber wireless communication, a logical cell is formed by BBUs and RRUs, frequent cell switching is avoided through cell merging, and the cell coverage is improved. In addition, studies have shown that linear cell coverage can save link budget and avoid excessive coverage of the base station compared to circular cell coverage. However, the linear coverage provided by the large-scale narrow beam not only requires frequent handover in a high-speed mobile communication system, but also the communication performance is severely attenuated by the influence of doppler shift.
Disclosure of Invention
In order to solve the above problems, the present invention provides a method and a system for high-speed rail communication by elliptical cell dedicated coverage and wide beam transmission, which can effectively provide dedicated coverage for narrow strip-shaped high-speed rail cells and improve the coverage of the system.
The invention provides a method for carrying out high-speed rail communication by covering wide beam transmission specially for an elliptical cell, which comprises the following specific steps:
step 1, establishing a narrow strip-shaped elliptical cell, wherein the boundary of the cell passes through the farthest ends of an orbit, namely, the eccentricity is adjusted to enable the elliptical cell to be close to a linear cell;
step 2, establishing a wide beam optimization problem based on the elliptical cell, meeting the constraint condition of elliptical eccentricity e, and maximizing SNR (signal to noise ratio) of the edge of the elliptical cell to ensure that SNR of the edge of the elliptical cell are equal;
step 3, initializing an eccentricity e, converting the optimization problem into a relaxation semi-positive definite programming (SDR) problem, and iteratively updating the eccentricity e to obtain an optimal solution X ≧ C;
step 4, a polynomial root with elements in the beam vector w as coefficients is obtained through X ^ and the polynomial coefficient is solved according to the polynomial root, so that the obtained beam vector w has no beam gain loss compared with the optimal solution X ^ and the optimal solution X ^ is obtained;
and 5, calculating and storing the wave beams off line, wherein when the train passes through the cell, the base station does not need to switch the wave beams and only needs one wave beam to cover the whole cell.
Preferably, the step 1 specifically comprises:
step 1.1 establishing narrow strip-shaped elliptical cell
Assuming that a base station is projected to the track and positioned in the middle of the track, taking the base station as the center of an elliptical cell, establishing ellipses passing through the farthest ends of the cell track according to the length L of the high-speed rail covered by the base station and the given elliptical eccentricity e, and respectively obtaining a major semi-axis a and a minor semi-axis b of each ellipse as follows:
Figure BDA0003465739660000021
Figure BDA0003465739660000022
wherein d iss(0) Representing the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the initial 0 moment; obtaining an isoline of a receiving signal-to-noise ratio (SNR) of the receiving signal-to-noise ratio (SNR) in a plane containing all direct-view propagation paths as an elliptic curve through beam forming, and adjusting the eccentricity e to enable an elliptic cell to be close to the shape of a narrow strip-shaped linear track, namely improving the SNR of the farthest end of the track;
step 1.2 obtaining the direct-view propagation distance
According to the vertical height d of the transmitting end and the receiving endbsAnd dtrAnd obtaining the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the time t
Figure BDA0003465739660000023
Wherein d isminThe vertical distance from the base station to the track is represented, and theta represents the transmitting angle of a signal transmitted by the base station at the time t;
step 1.3 determination of elliptical cell boundaries
At time t, based on the train position information, the channel from the transmitting end of the base station to the receiving end of the train is represented as
Figure BDA0003465739660000024
At this time, the transmission angle is theta, and the channel from the base station transmitting end to the oval cell boundary under the same transmission angle is represented as
Figure BDA0003465739660000025
The direct-view propagation distance that a signal experiences to reach an elliptical cell boundary is denoted de(t) wherein the channels
Figure BDA0003465739660000026
Is a through channel
Figure BDA0003465739660000027
The direct-view path is obtained by extension, namely a point on an elliptic curve where a direct-view path extension line from a base station to a train receiving end passes is a virtual receiving point of a cell boundary; in addition, because the base station end is provided with M transmitting antennas, and the receiving end is a single antenna, then
Figure BDA0003465739660000031
And
Figure BDA0003465739660000032
column vectors of M elements each; thus, after the ellipse eccentricity e, the ellipse major semi-axis a and the ellipse minor semi-axis b are all determined, d is determined accordinglye(t) and channel
Figure BDA0003465739660000033
Preferably, the step 2 specifically comprises:
under the condition of satisfying the ellipse eccentricity e, the SNR of the ellipse boundary is maximized to improve the receiving SNR of the train at the farthest position, so as to improve the coverage rate, and if the receiving SNR of the ellipse boundary is gamma, the wide beam optimization problem based on the ellipse cell is written as
Figure BDA0003465739660000034
Figure BDA0003465739660000035
wHw＝1
0≤e<1
Where the upper right H represents the conjugate transpose of the matrix or vector, due to random variables in the channel
Figure BDA0003465739660000036
Is random in nature and is not only easy to be recognized,
Figure BDA0003465739660000037
to represent
Figure BDA0003465739660000038
W and Γ are unknown optimization variables, w is a beam vector and is a column vector consisting of M elements;
beamforming makes the received SNR at the oval cell boundary equal to Γ, and adopts a logarithmic path loss model to represent SNR, then the received SNR of the train on a linear track varies with time, denoted γ (t), and Γ and γ (t) have the following relationship
Figure BDA0003465739660000039
Where ε represents the path loss exponent, a known parameter; given eccentricity, i.e. de(t)-εGiven, gamma is maximized by obtaining the optimal beam w, thereby increasing gamma (t), and further increasing the coverage rate; and the closest d is obtained by adjusting the eccentricity esD of (t)eAnd (t), obtaining the elliptical cell closest to the straight line cell, wherein the obtained beam is optimal overall.
Preferably, the step 3 specifically comprises:
according to the expression
Figure BDA00034657396600000310
Figure BDA00034657396600000311
Representation matrix
Figure BDA00034657396600000312
Let X be wwHIf X is a semi-positive definite matrix, and rank (X) of matrix X is 1, removing the rank constraint condition; satisfies the constraint condition of eccentricity 0-e<1, giving initial eccentricity e ═ e0The optimization problem becomes a relaxed semi-definite programming (SDR) problem,
Figure BDA0003465739660000041
Figure BDA0003465739660000042
tr(X)＝1,
optimal solution X of SDR problem solved by CVX tool box in Matlab★And the upper right corner ″) represents the optimal solution; iteratively increasing the magnitude of the eccentricity e
Figure BDA0003465739660000043
Repeatedly solving the SDR problem by adopting new eccentricity until the SDR problem is not solved, and enabling the maximum eccentricity with the solution of the SDR problem to be the optimal eccentricity, wherein the gamma is also the maximum at the moment, and the corresponding X is★Is also the optimal solution of the original problem.
Preferably, the step 4 specifically comprises:
for the optimal solution X derived from the SDR problem★Decomposing the eigenvalues, if only one eigenvalue exists, taking the only eigenvector as the optimal beam vector w★(ii) a Such as X★For high ranks, the matrix X is extracted★Of diagonal elements, i.e.
Figure BDA0003465739660000044
Wherein
Figure BDA0003465739660000045
Denotes a Toeplitz matrix of M × M, - (M-1) ≦ τ ≦ M-1, i.e., a Toeplitz matrix having 1 element only on the τ -th minor diagonal, the remaining elements being 0, the minor diagonal referring to a diagonal parallel to the major diagonal, τ<0 denotes the lower diagonal, τ>0 represents the upper diagonal, e.g.
Figure BDA0003465739660000046
And is
Figure BDA0003465739660000047
It means that only the elements on the main diagonal are 1 and the remaining elements are 0;
when in use
Figure BDA0003465739660000048
Form the following polynomial relationship
f(x)＝(w0+w1x+…+wM-1xM-1)
×(w0 *+w1 *x-1+…+wM-1 *x-(M-1))
＝Σ(-(M-1))x-(M-1)+Σ(-(M-2))x-(M-2)+…
+Σ(0)+Σ(1)x+…+Σ(M-1)xM-1
Wherein w★＝[w0,w1,…,wM-1]TThe upper right "+" represents the conjugate of the complex number, and the upper right "T" represents the transpose of the vector or matrix; let f (x) be 0 for a total of 2(M-1) roots, and M-1 of these roots be x1,x2,…,xM-1Then another M-1 roots are
Figure BDA0003465739660000049
Thus will be 2 (M)-1) the roots are divided into two groups of roots which are conjugate and reciprocal to each other, for a total of M-1 pairs; two sets of roots are selected, one from each pair of roots, constituting the root of the following polynomial:
Figure BDA0003465739660000051
wherein z ism＝xmOr
Figure BDA0003465739660000052
Solving the coefficients of the above polynomial to obtain the optimal beam w★＝[w0,w1,…,wM-1]T。
Preferably, the step 5 specifically comprises:
according to the predictability of the train position information, the base station end calculates and stores the wave beam in an off-line mode, when the train enters the cell, the base station end adopts the wide wave beam of the elliptical cell to transmit data according to the real-time positioning information, only one wave beam is needed when the train passes through the cell, and frequent wave beam switching and frequent channel quality index CQI feedback can be avoided.
A high-speed rail communication scenario-oriented wide beam transmission system based on elliptical cell-specific coverage, comprising a memory in which a computer program is stored and a processor implementing the method steps as described when executing the computer program.
(1) The invention discloses a wide beam transmission method based on the special coverage of an elliptical cell, and when a train passes through the cell, a base station does not need to switch beams and only needs one beam to cover the whole cell.
(2) According to the invention, the beam design is carried out by establishing the elliptical cell, the optimal elliptical cell is obtained by adjusting the eccentricity, and the obtained wide beam can effectively improve the high-speed rail communication coverage rate in a large range.
Drawings
FIG. 1 is a flow chart of the present invention;
fig. 2 is an elliptical cell coverage model applied to a high-speed rail communication system according to the present invention;
FIG. 3 is a graph of the expected receive power for different locations per unit transmit power for a design using the present invention
Figure BDA0003465739660000053
Figure BDA0003465739660000054
FIG. 4 is a diagram illustrating the design received power expectation under a high-speed rail communication system applying the present invention
Figure BDA0003465739660000055
Figure BDA0003465739660000056
A performance schematic;
fig. 5 is a schematic diagram of coverage performance over different track lengths in a high-speed rail communication system applying the present invention.
Detailed Description
The technical solutions in the examples of the present invention are clearly and completely described below with reference to the drawings in the examples of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without inventive step, are within the scope of the present invention.
The present invention will be described in further detail with reference to the accompanying drawings.
Example 1
The method for performing high-speed rail communication by using the dedicated coverage wide beam transmission of the elliptical cell, which is provided by the invention, with reference to fig. 1 to 2, comprises the following steps:
s101: and (3) establishing an elliptical cell, wherein the boundary of the cell passes through the two farthest ends of the orbit, namely a special covering method for a high-speed railway cell, and the elliptical cell is close to the narrow strip-shaped cell by adjusting the eccentricity.
The method comprises the following steps:
assuming that a base station projects to the middle position of the track, the base station is used as the center of an elliptical cell, and the ellipse passing through the farthest ends of the cell track is established according to the length L of the high-speed rail track covered by the base station and the given elliptical eccentricity e, and the major semi-axis a and the minor semi-axis b of the ellipse can be respectively obtained as follows:
Figure BDA0003465739660000061
Figure BDA0003465739660000062
wherein d iss(0) And the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the initial moment is shown. By means of beam forming, the isoline of received signal-to-noise ratios (SNR) of the straight-looking propagation paths in a plane is obtained to be an elliptic curve, the eccentricity e is adjusted, the elliptic cell is close to the narrow strip-shaped cell, and the SNR of the farthest end of the track is improved. According to the vertical height d of the transmitting end and the receiving endbsAnd dtrAnd obtaining the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the time t
Figure BDA0003465739660000063
Wherein d isminRepresents the vertical distance from the base station to the track, and θ represents the transmission angle of the base station transmission signal at time t.
Determining the boundary of the elliptical cell, and at the moment t, based on the train position information, the channel from the transmitting end of the base station to the receiving end of the train can be expressed as
Figure BDA0003465739660000064
At this time, the transmission angle is theta, and the channel from the base station transmitting end to the oval cell boundary under the same transmission angle can be expressed as
Figure BDA0003465739660000065
The direct-view propagation distance that a signal experiences to reach an elliptical cell boundary is denoted de(t) the two location information based channels are distinguished by a channel
Figure BDA0003465739660000066
Is a through channel
Figure BDA0003465739660000067
The direct-view path is obtained by extension, which means that a point on an elliptic curve where a direct-view path extension line from a base station to a train receiving end passes is a virtual receiving point. In addition, considering that the base station end is provided with M transmitting antennas, and the receiving end is a single antenna, then
Figure BDA0003465739660000068
And
Figure BDA0003465739660000069
is a column vector of M elements. Thus, d is determined after the elliptical eccentricity e, the elliptical semi-major axis a and the elliptical semi-minor axis b are all determinede(t) and channel
Figure BDA00034657396600000610
S102: and establishing a wide beam optimization problem based on the elliptical cell, meeting the constraint condition of elliptical eccentricity e, and maximizing the SNR of the edge of the elliptical cell to ensure that the SNR of the edge of the elliptical cell is equal.
The method comprises the following steps:
under the condition of satisfying the ellipse eccentricity e, the SNR of the ellipse boundary is maximized to improve the receiving SNR of the train at the farthest position, so as to improve the coverage rate, and if the SNR of the ellipse boundary is gamma, the wide beam optimization problem based on the ellipse cell can be written as
Figure BDA0003465739660000071
Figure BDA0003465739660000072
wHw＝1
0≤e<1
Wherein the right partThe upper H represents the conjugate transpose of the matrix or vector, resulting in a random variation in the channel
Figure BDA0003465739660000073
Is random in nature and is not only easy to be recognized,
Figure BDA0003465739660000074
to represent
Figure BDA0003465739660000075
Is an unknown optimization variable, and w is a beam vector, which is a column vector consisting of M elements.
Beamforming makes the received SNR at the oval cell boundary equal to Γ, and logarithmic path loss model is used to represent SNR, then the received SNR of the train on a straight track varies with time, which can be represented as γ (t), and Γ and γ (t) have the following relationship
Figure BDA0003465739660000076
Where epsilon represents the path loss exponent, a known parameter. Given eccentricity, i.e. de(t)-εGiven, by obtaining the optimal beam w, Γ is maximized, which in turn enhances γ (t), which in turn enhances coverage. And the closest d is obtained by adjusting the eccentricity esD of (t)eAnd (t), obtaining the elliptical cell closest to the straight line cell, wherein the obtained beam is optimal overall.
S103: initializing eccentricity e, converting the optimization problem into a relaxed semi-definite programming (SDR) problem, iteratively updating the eccentricity e, and obtaining an optimal solution X★。
The specific process is as follows:
according to the expression
Figure BDA0003465739660000077
Figure BDA0003465739660000078
Representation matrix
Figure BDA0003465739660000079
Let X be wwHThen X is a semi-positive definite matrix and the rank (X) of matrix X is 1, the rank constraint is removed. Satisfies the constraint condition of eccentricity 0-e<1, giving initial eccentricity e ═ e0The optimization problem becomes a relaxed semi-definite programming (SDR) problem,
Figure BDA0003465739660000081
Figure BDA0003465739660000082
tr(X)＝1,
optimal solution X of SDR problem solved by CVX tool box in Matlab★And the upper right corner ″) represents the optimal solution. Iteratively increasing the magnitude of the eccentricity e
Figure BDA0003465739660000083
Repeatedly solving the SDR problem by adopting new eccentricity until the SDR problem is not solved, and enabling the maximum eccentricity with the solution of the SDR problem to be the optimal eccentricity, wherein the gamma is also the maximum at the moment, and the corresponding X is★Is also the optimal solution of the original problem.
S104: by X★Obtaining a polynomial root taking elements in the beam vector w as coefficients, solving the polynomial coefficient according to the polynomial root to obtain the beam vector w and an optimal solution X★Compared to no beam gain loss.
The specific process of the step is as follows:
for the optimal solution X derived from the SDR problem★Decomposing the eigenvalues, if only one eigenvalue exists, taking the only eigenvector as the optimal beam vector w★. But often X★Is of high rank, when the matrix X is extracted★Of diagonal elements, i.e.
Figure BDA0003465739660000084
Wherein
Figure BDA0003465739660000085
Denotes an M x M Toeplitz matrix, - (M-1) ≦ τ ≦ M-1, i.e., a Toeplitz matrix having only the element 1 on the τ -th minor diagonal, where the minor diagonal refers to a diagonal parallel to the major diagonal, τ is 0<0 denotes the lower diagonal, τ>0 represents the upper diagonal, e.g.
Figure BDA0003465739660000086
And is
Figure BDA0003465739660000087
Indicating that only the elements on the main diagonal are 1 and the remaining elements are 0.
When in use
Figure BDA0003465739660000088
Can form the following polynomial relation
f(x)＝(w0+w1x+…+wM-1xM-1)
×(w0 *+w1 *x-1+…+wM-1 *x-(M-1))
＝Σ(-(M-1))x-(M-1)+Σ(-(M-2))x-(M-2)+…
+Σ(0)+Σ(1)x+…+Σ(M-1)xM-1
Wherein w★＝[w0,w1,…,wM-1]TThe upper right-hand "+" denotes the conjugate of the complex number and the upper right-hand "T" denotes the transpose of the vector or matrix. Let f (x) be 0 for a total of 2(M-1) roots, and M-1 of these roots be x1,x2,…,xM-1Then another M-1 roots are
Figure BDA0003465739660000091
Thus, the 2(M-1) roots are divided into two groups of roots which are conjugate and reciprocal to each otherThere are M-1 pairs of such roots. Two sets of roots are selected, one from each pair of roots, constituting the root of a polynomial,
Figure BDA0003465739660000092
wherein z ism＝xmOr
Figure BDA0003465739660000093
The coefficients of the above polynomial are solved, i.e. the optimal beam w★＝[w0,w1,…,wM-1]T。
S105: and when the train passes through the cell, the base station does not need to switch the beam and only needs one beam to cover the whole cell.
The method comprises the following steps:
according to the predictability of the train position information, the base station end calculates and stores the wave beam in an off-line mode, when the train enters the cell, the base station end adopts the wide wave beam of the elliptical cell to transmit data according to the real-time positioning information, and before the train leaves the cell, the wave beam does not need to be switched and the Channel Quality Index (CQI) does not need to be fed back.
Referring to fig. 3 to 5, the simulation of the present invention will be described.
The invention is provided aiming at the special coverage problem of the linear high-speed rail communication cell in a large range, because the invention uses the channel based on the position information when designing the wave beam, but the signal experiences the instantaneous channel in the practical application, the instantaneous channel is adopted for simulation in the simulation of the coverage rate performance so as to be more accordant with the practical environment. Simulation conditions are as follows: the number M of base station transmitting antennas is 64, and the carrier frequency fc2.35GHz, high-speed railway speed 350km/h, path loss exponent epsilon 3.03, and vertical height d of base station antennabs25m, vertical height d of train receiving endtr＝5m。
FIG. 3 shows the result at dminWhen the eccentricity e is 0.95 and 70m, the wide beam shaping based on the elliptical district is carried out on the unit hairExpectation of designed received power at different positions on plane under transmitted power
Figure BDA0003465739660000094
The plane here refers to the plane containing all direct-view paths of the base station to the track. Assuming that the base station is located at the (0,0) point on the xy plane, as can be seen from the contour line of the g (t) three-dimensional curve on the xy plane, the contour line is an elliptic curve with the same eccentricity, which indicates that the coverage of the beamforming satisfies the elliptic cell shape, i.e. the curve of the expected value of the equal received power is an elliptic curve.
Next, simulation results compare the method with the method for designing beam forming directly on the track, and the beam forming is directly designed on the track, that is, the elliptical cells are not considered, and the receiving SNR of the train on the track is directly considered during beam design, which is called as linear coverage. FIG. 4 shows the expected designed received power of a train under different beamforming schemes
Figure BDA0003465739660000101
And (3) along with the change curve of the train position, wherein when the covering position is 0m, the position of the track closest to the base station is represented, the covering position is negative, the track on the left side of the base station is represented, and the covering position is positive, the track on the right side of the base station is represented. The expected designed received power g (t) of the linear coverage beamforming scheme at different coverage track lengths L is shown in fig. 4(a), and as shown in the figure, although the equality constraint is relaxed to be the inequality constraint, the optimization problem of the objective function to maximize the expected designed received power still results in the equality of all the expected received powers in the coverage range. Since the minimum g (t) of the covered edge is equal to the maximum value, the minimum g (t) on the rail line is maximized. Furthermore, linear coverage is limited by the length of the railway coverage when the minimum distance dminAt 70m, the maximum coverage length L does not exceed 600m, and above 600m, the beam optimization problem of linear coverage has no feasible solution, and thus is not suitable for large-range coverage.
The expected design received power g (t) of the elliptical cell-based wide beamforming scheme at different eccentricities e is shown in fig. 4(b), and it can be seen that g (t) follows the transmissionThe propagation distance increases and decreases gradually, and g (t) of the elliptical cell is in most cases larger than the circular coverage (e ═ 0), for example when the coverage track length is larger than 200 m. It was also found that g (t) at the farthest position increases with increasing elliptical eccentricity. When the eccentricity e is not very large, e.g. the minimum distance dmin70m, e is not more than 0.95, and the beam pattern is the same regardless of the total length L of the railway that the design covers. When the eccentricity is large, such as 0.98, the designed length L of the high-speed rail is limited, and the beam curves designed for different track lengths (L ═ 0.8km, 1km) do not overlap, that is, in the beam forming optimization problem, different beam patterns are caused by different railway distances L. Because when the eccentricity approaches 1, the path losses in different directions on the elliptic curve vary greatly, which results in different solutions when the coverage is expanded, i.e. when e is smaller, the path losses in different directions are relatively close, i.e. the expansion of the coverage has less influence on the optimization result.
FIG. 5 shows a cover
Figure BDA0003465739660000102
Graph with cover length L, assuming a break threshold for SNR of Γh3dB, transmission power Pt38 dBm. As can be seen, the maximum communication track length increases with increasing elliptical eccentricity. At 100% coverage, the 0.98 elliptical coverage rail length is over 1200m maximum, while the maximum coverage length for both circular and linear coverage is less than 600 m.
In summary, the method for high-speed rail communication by covering wide beam transmission for an elliptical cell provided by the present invention includes establishing an elliptical cell, wherein the boundary of the cell passes through the two ends of the farthest track, and adjusting the eccentricity to make the elliptical cell approach to the narrow strip-shaped cell; establishing a wide beam optimization problem based on the elliptical cell, meeting elliptical eccentricity constraint conditions, and maximizing the SNR of the edges of the elliptical cell to ensure that the SNR of the edges of the cell is equal; initializing e, converting the optimization problem into a relaxed semi-definite programming (SDR) problem, iteratively updating e to obtain an optimal solution X★(ii) a By X★Obtaining a polynomial root having the beam vector as a coefficient based onSolving polynomial coefficient by polynomial root to obtain beam vector and X★Compared with no beam gain loss; and when the train passes through the cell, the base station does not need to switch the beam and only needs one beam to cover the whole cell. According to the invention, the beam design is carried out by establishing the elliptical cell, the optimal elliptical cell is obtained by adjusting the eccentricity, and the obtained wide beam can effectively improve the high-speed rail communication coverage rate in a large range.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (7)

1. A method for carrying out high-speed rail communication by elliptical cell dedicated coverage wide beam transmission is characterized by comprising the following specific steps:
step 1, establishing a narrow strip-shaped elliptical cell, wherein the boundary of the cell passes through the farthest ends of an orbit, namely, the eccentricity is adjusted to enable the elliptical cell to be close to a linear cell;
step 2, establishing a wide beam optimization problem based on the elliptical cell, meeting the constraint condition of elliptical eccentricity e, and maximizing SNR (signal to noise ratio) of the edge of the elliptical cell to ensure that SNR of the edge of the elliptical cell are equal;
step 3, initializing eccentricity e, converting the optimization problem into a relaxed semi-definite programming SDR problem, and iteratively updating the eccentricity e to obtain an optimal solution X★；
Step 4 by X★Obtaining a polynomial root taking elements in the beam vector w as coefficients, solving the polynomial coefficient according to the polynomial root to obtain the beam vector w and an optimal solution X★Compared with no beam gain loss;
and 5, calculating and storing the wave beams off line, wherein when the train passes through the cell, the base station does not need to switch the wave beams and only needs one wave beam to cover the whole cell.
2. The method for communicating in a high-speed rail by transmitting the elliptical cell-specific coverage wide beam according to claim 1, wherein the step 1 specifically comprises:
step 1.1 establishing narrow strip-shaped elliptical cell
Assuming that a base station is projected to the track and positioned in the middle of the track, taking the base station as the center of an elliptical cell, establishing ellipses passing through the farthest ends of the cell track according to the length L of the high-speed rail covered by the base station and the given elliptical eccentricity e, and respectively obtaining a major semi-axis a and a minor semi-axis b of each ellipse as follows:
Figure FDA0003465739650000011
Figure FDA0003465739650000012
wherein, ds(0) Representing the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the initial 0 moment; obtaining an isoline of a receiving signal-to-noise ratio (SNR) of the receiving signal-to-noise ratio (SNR) in a plane containing all direct-view propagation paths as an elliptic curve through beam forming, and adjusting the eccentricity e to enable an elliptic cell to be close to the shape of a narrow strip-shaped linear track, namely improving the SNR of the farthest end of the track;
step 1.2 obtaining the direct-view propagation distance
According to the vertical height d of the transmitting end and the receiving endbsAnd dtrAnd obtaining the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the time t
Figure FDA0003465739650000013
Wherein d isminThe vertical distance from the base station to the track is represented, and theta represents the transmitting angle of a signal transmitted by the base station at the time t;
step 1.3 determination of elliptical cell boundaries
At time t, based on the train position information, the channel from the transmitting end of the base station to the receiving end of the train is represented as
Figure FDA0003465739650000021
At this time, the transmission angle is theta, and the channel from the base station transmitting end to the oval cell boundary under the same transmission angle is represented as
Figure FDA0003465739650000022
The direct-view propagation distance that a signal experiences to reach an elliptical cell boundary is denoted de(t) wherein the channels
Figure FDA0003465739650000023
Is a through channel
Figure FDA0003465739650000024
The direct-view path is obtained by extension, namely a point on an elliptic curve where a direct-view path extension line from a base station to a train receiving end passes is a virtual receiving point of a cell boundary; in addition, because the base station end is provided with M transmitting antennas, and the receiving end is a single antenna, then
Figure FDA0003465739650000025
And
Figure FDA0003465739650000026
column vectors of M elements each; thus, after the ellipse eccentricity e, the ellipse major semi-axis a and the ellipse minor semi-axis b are all determined, d is determined accordinglye(t) and channel
Figure FDA0003465739650000027
3. The method for communicating in a high-speed rail by transmitting the elliptical cell-specific coverage wide beam according to claim 2, wherein the step 2 is specifically:
under the condition of satisfying the ellipse eccentricity e, the SNR of the ellipse boundary is maximized to improve the receiving SNR of the train at the farthest position, so as to improve the coverage rate, and if the receiving SNR of the ellipse boundary is Γ, the wide beam optimization problem based on the ellipse cell is written as:
Figure FDA0003465739650000028
Figure FDA0003465739650000029
wHw＝1
0≤e<1
where the upper right H represents the conjugate transpose of the matrix or vector, due to random variables in the channel
Figure FDA00034657396500000210
Is random in nature and is not only easy to be recognized,
Figure FDA00034657396500000211
to represent
Figure FDA00034657396500000212
W and Γ are unknown optimization variables, w is a beam vector and is a column vector consisting of M elements;
beamforming makes the received SNR at the oval cell boundary equal to Γ, and adopts a logarithmic path loss model to represent SNR, then the received SNR of the train on a linear track varies with time, denoted γ (t), and Γ and γ (t) have the following relationship
Figure FDA00034657396500000213
Where ε represents the path loss exponent, a known parameter; given eccentricity, i.e. de(t)-εGiven, gamma is maximized by obtaining the optimal beam w, thereby increasing gamma (t), and further increasing the coverage rate; and the closest d is obtained by adjusting the eccentricity esD of (t)e(t) obtaining the closestAnd in the elliptical cell of the linear cell, the obtained wave beam is overall optimal.
4. The method for communicating in a high-speed rail by transmitting the elliptical cell-specific coverage wide beam according to claim 3, wherein the step 3 is specifically as follows:
according to the expression
Figure FDA0003465739650000031
Figure FDA0003465739650000032
Representation matrix
Figure FDA0003465739650000033
Let X be wwHIf X is a semi-positive definite matrix, and rank (X) of matrix X is 1, removing the rank constraint condition; satisfies the constraint condition of eccentricity 0-e<1, giving initial eccentricity e ═ e0The optimization problem becomes a relaxed semi-definite programming SDR problem,
Figure FDA0003465739650000034
Figure FDA0003465739650000035
tr(X)＝1,
optimal solution X of SDR problem solved by CVX tool box in Matlab★And the upper right corner ″) represents the optimal solution; iteratively increasing the magnitude of the eccentricity e
Figure FDA0003465739650000036
Repeatedly solving the SDR problem by adopting new eccentricity until the SDR problem is not solved, and enabling the maximum eccentricity with the solution of the SDR problem to be the optimal eccentricity, wherein the gamma is also the maximum at the moment, and the corresponding X is★Is also the optimal solution of the original problem.
5. The method for communicating in a high-speed rail by transmitting the elliptical cell-specific coverage wide beam according to claim 1, wherein the step 4 specifically comprises:
for the optimal solution X derived from the SDR problem★Decomposing the eigenvalues, if only one eigenvalue exists, taking the only eigenvector as the optimal beam vector w★. (ii) a Such as X★Is of high rank, the matrix X is extracted★Of diagonal elements, i.e.
Figure FDA0003465739650000037
Wherein
Figure FDA0003465739650000038
Denotes a Toeplitz matrix of M × M, - (M-1) ≦ τ ≦ M-1, i.e., a Toeplitz matrix having 1 element only on the τ -th minor diagonal, the remaining elements being 0, the minor diagonal referring to a diagonal parallel to the major diagonal, τ<0 denotes the lower diagonal, τ>0 represents the upper diagonal, e.g.
Figure FDA0003465739650000039
And is
Figure FDA00034657396500000310
It means that only the elements on the main diagonal are 1 and the remaining elements are 0;
when in use
Figure FDA00034657396500000311
Form the following polynomial relationship
f(x)＝(w0+w1x+…+wM-1xM-1)×(w0 *+w1 *x-1+…+wM-1 *x-(M-1))
＝∑(-(M-1))x-(M-1)+∑(-(M-2))x-(M-2)+…+∑(0)+∑(1)x+…+∑(M-1)xM-1
Wherein w★＝[w0，w1，…，wM-1]TThe upper right "+" represents the conjugate of the complex number, and the upper right "T" represents the transpose of the vector or matrix; let f (x) be 0 for a total of 2(M-1) roots, and M-1 of these roots be x1，x2，…，xM-1Then another M-1 roots are
Figure FDA0003465739650000041
Thus, 2(M-1) roots are divided into two groups of roots which are conjugated and reciprocal to each other, and have M-1 pairs; two sets of roots are selected, one from each pair of roots, constituting the root of the following polynomial:
Figure FDA0003465739650000042
wherein z ism＝xmOr
Figure FDA0003465739650000043
Solving the coefficients of the above polynomial to obtain the optimal beam w★＝[w0，w1，…，wM-1]T。
6. The method for communicating in a high-speed rail by transmitting the elliptical cell-specific coverage wide beam according to claim 1, wherein the step 5 specifically comprises:
and when the train enters the cell, the base station end adopts the wide beam of the elliptical cell to transmit data according to the real-time positioning information.
7. A system for carrying out high-speed rail communication by elliptical cell dedicated coverage wide beam transmission comprises a memory and a processor, wherein the memory stores a computer program and is characterized in that; the processor, when executing the computer program, realizes the method steps of any of claims 1-6.
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