CN114465647B - Method and system for high-speed railway communication by special coverage wide-beam transmission of elliptical cell - Google Patents
Method and system for high-speed railway communication by special coverage wide-beam transmission of elliptical cell Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04B7/00—Radio transmission systems, i.e. using radiation field
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- 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
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Abstract
The invention discloses a method and a system for high-speed railway communication by special coverage wide wave beam transmission of an elliptical cell, comprising the following steps: an elliptical cell is established, and the boundary of the cell passes through the two farthest ends of the track; establishing a wide beam optimization problem based on an elliptical cell, meeting the condition of eccentricity e, maximizing the signal-to-noise ratio (SNR) of the edge of the elliptical cell, and enabling the SNR of the edge of the cell to be 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 ★; obtaining a polynomial root taking a beam vector as a coefficient through X ★, solving the polynomial coefficient according to the polynomial root, wherein the obtained beam vector has no beam gain loss compared with X ★; the beams are calculated and stored offline, and when a train passes through the cell, the base station does not need to switch the beams, and only one beam is needed to cover the whole cell. According to the invention, an elliptical cell is established, the wave beam design is carried out, the eccentricity is adjusted to obtain an optimal elliptical cell, and the coverage rate of the wide wave beam in high-speed railway communication in a large range can be effectively improved.
Description
Technical Field
The invention relates to the technical field of high-speed railway wireless communication, in particular to a method and a system for high-speed railway communication by special coverage wide-beam transmission of an elliptical cell.
Background
The high-speed rail is an important green transportation means with the characteristics of rapidness, comfort, safety, reliability, high loading capacity, low energy consumption and the like, and can be rapidly developed worldwide. According to the data report of UIC, by 2020, there are already high-speed railways of over 5.2 kilometers around the world, and it is expected that the high-speed rail history will be over 8 kilometers between 2030 and 2035. A problem that follows is how to deploy a high-speed railway communication system to cover as long a high-speed railway section as possible with as few base stations and link budgets as possible, providing passengers with ubiquitous network services at any time and any place.
Considering the long and narrow cell characteristics of a high-speed railway, the communication coverage efficiency of the traditional cellular network in a special scene of compromising the high-speed railway is low, so that a special communication coverage network needs to be designed. Dedicated network coverage research on high-speed rail communications has received extensive attention. The existing special coverage network for the high-speed rail cells is mainly realized through hardware infrastructure, for example, data transmission is provided for the high-speed rail communication through optical fiber wireless communication, a logic cell is formed by BBU and RRUs, frequent cell switching is avoided through cell combination, 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 the high-speed mobile communication system, but also communication performance is severely attenuated by the doppler shift.
Disclosure of Invention
In order to solve the problems, the invention provides a method and a system for performing high-speed railway communication by special coverage wide beam transmission of an elliptical cell, which can effectively provide special coverage of a narrow strip-shaped high-speed railway cell and improve the coverage rate of the system.
The invention provides a method for performing high-speed railway communication by special coverage wide beam transmission of an elliptical cell, which comprises the following specific steps:
Step 1, establishing a narrow strip-shaped elliptic cell, wherein the boundary of the cell passes through the farthest two ends of the track, namely, the eccentricity is adjusted, so that the elliptic cell is close to a linear cell;
step2, establishing a wide beam optimization problem based on the elliptical cell, meeting constraint conditions of the elliptical eccentricity e, maximizing the edge SNR of the elliptical cell, and enabling the edge SNR of the elliptical cell to be 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, obtaining polynomial roots taking elements in the beam vector w as coefficients through X ∈, solving the polynomial coefficients according to the polynomial roots, wherein the obtained beam vector w has no beam gain loss compared with an optimal solution X ∈;
And 5, calculating and storing the wave beams offline, wherein when a train passes through the cell, the base station does not need to switch the wave beams, and only one wave beam is needed to cover the whole cell.
Preferably, the step 1 specifically includes:
Step 1.1 establishing a narrow strip-shaped elliptical cell
Assuming that the base station is projected to the middle position of the track and is positioned on the middle position of the track, taking the base station as the center of an elliptical cell, establishing ellipses passing through the farthest two ends of the track of the cell according to the length L of the high-speed rail covered by the base station and a given ellipse eccentricity e, and respectively obtaining a major half axis a and a minor half axis b of the ellipses as follows:
D s (0) represents 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; the method comprises the steps of obtaining a contour line of a received signal-to-noise ratio (SNR) of an elliptical curve in a plane containing all direct-view propagation paths through beam forming, and adjusting the magnitude of an eccentricity e to enable an elliptical cell to be close to a narrow linear track shape, namely improving the SNR of the farthest end of the track;
Step 1.2 obtaining the direct-view propagation distance
According to the vertical heights d bs and d tr of the transmitting end and the receiving end, obtaining the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the moment t as followsWherein d min represents the vertical distance from the base station to the track, and θ represents the emission angle of the signal transmitted by the base station at the time t;
step 1.3 determination of oval cell boundary
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 expressed asAt this time, the transmission angle is θ, and under the same transmission angle, the channel from the base station transmitting end to the oval cell boundary is expressed as/>The direct-view propagation distance that a signal experiences to reach an elliptical cell boundary is denoted d e (t), where channel/>Is a direct-view path through the channelThe direct-view path extension of the base station to the train receiving end is obtained by extending the direct-view path, namely, a certain point on the elliptic curve is a virtual receiving point of the cell boundary; in addition, since the base station is equipped with M transmitting antennas and the receiving end is a single antenna, thenAnd/>Column vectors of M elements; thus, after the ellipse eccentricity e, ellipse major half-axis a and ellipse minor half-axis b are all determined, d e (t) and channel/>, are determined
Preferably, the step2 specifically includes:
Under the condition of meeting the ellipse eccentricity e, the received SNR of the train at the farthest position is improved by maximizing the SNR of the ellipse boundary, so that the coverage rate is improved, and the wide beam optimization problem based on the ellipse cell is written on the assumption that the received SNR of the ellipse boundary is Γ
wHw＝1
0≤e<1
Wherein H in the upper right hand corner represents the conjugate transpose of the matrix or vector, resulting from the random variation in the channelIs random,/>Representation/>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 of the boundary of an elliptic cell equal to Γ, and adopts a logarithmic path loss model to represent SNR, so that the received SNR of a train on a linear track changes with time and is represented as gamma (t), and Γ and gamma (t) have the following relation
Where ε represents the path loss index, a known parameter; given the eccentricity, i.e. d e(t)-ε is given, by obtaining the optimal beam w, maximizing Γ, and thus increase y (t), and thus coverage; by adjusting the magnitude of the eccentricity e, d e (t) closest to d s (t) is obtained, i.e. an elliptical cell closest to the straight cell is obtained, and the resulting beam is overall optimal.
Preferably, the step 3 specifically includes:
according to the expression Representation matrix/>Let x=ww H, then X is a semi-positive definite matrix, and rank (X) =1 for matrix X, remove rank constraint; satisfying the eccentricity constraint 0.ltoreq.e1, giving an initial eccentricity e=e 0, the optimization problem becomes a relaxed semi-definite programming (SDR) problem,
tr(X)＝1,
The SDR problem is solved by a CVX tool box in Matlab to obtain an optimal solution X ★, wherein the upper right corner is the optimal solution; iteratively increasing the magnitude of the eccentricity eAnd repeatedly solving the SDR problem by adopting a new eccentricity until the SDR problem has no solution, so that the maximum eccentricity of the solution of the SDR problem is the optimal eccentricity, the gamma is also the maximum at the moment, and the corresponding X ★ is also the optimal solution of the original problem.
Preferably, the step4 specifically includes:
Performing eigenvalue decomposition on an optimal solution X ★ obtained by SDR problem, and taking a unique eigenvector as an optimal beam vector w ★ if only one eigenvalue exists; if X ★ is of high rank, then the sum of the diagonal elements of matrix X ★ is extracted, i.e Wherein/>Representing an M x M toeplitz matrix, - (M-1) τ.ltoreq.M-1, i.e., a toeplitz matrix having 1 element and 0 element on only the τ -th minor diagonal, the minor diagonal being a diagonal parallel to the major diagonal, τ <0 representing the lower diagonal and τ >0 representing the upper diagonal, e.g.
And is also provided withRepresenting that only the element on the main diagonal is 1 and the rest elements are 0;
When (when) 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]T, upper right "×" represents the conjugate of complex numbers, upper right "T" represents the transpose of the vector or matrix; let f (x) =0, 2 (M-1) roots in total, and wherein M-1 roots are x 1,x2,…,xM-1, then the other M-1 roots areThus, the 2 (M-1) roots are divided into two groups of roots which are conjugate and reciprocal, and M-1 pairs are shared; two sets of roots are selected, one from each pair of roots, constituting the roots of the following polynomials:
Wherein z m＝xm is or And solving coefficients of the polynomial to obtain the optimal beam w ★＝[w0,w1,…,wM-1]T.
Preferably, the step 5 specifically includes:
According to the predictability of the train position information, the base station side calculates and stores the wave beams offline, when the train enters the cell, the base station side adopts the wide wave beam of the elliptical cell to carry out data transmission according to the real-time positioning information, and the train only needs one wave beam during passing through the cell, so that frequent wave beam switching and frequent channel quality index CQI feedback can be avoided.
A wide beam transmission system based on elliptical cell dedicated coverage for a high-speed railway communication scenario, comprising a memory and a processor, the memory storing a computer program, the processor implementing the method steps as described when executing the computer program.
(1) According to the wide beam transmission method based on the special coverage of the elliptical cell, when a train passes through the cell, a base station does not need to switch beams, and only one beam is needed to cover the whole cell.
(2) According to the invention, the elliptical cell is established for beam design, and the optimal elliptical cell is obtained by adjusting the eccentricity, so that the obtained wide beam can effectively improve the communication coverage rate of high-speed rail 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 railway communication system provided by the invention;
FIG. 3 is a graph of the expected received power for different locations at a unit transmit power using the present invention
FIG. 4 is a diagram of the design of received power expectations for use in a high-speed rail communication system using the present invention A performance schematic;
fig. 5 is a schematic diagram of coverage performance over different track lengths for a high-speed rail communication system using the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made more apparent and fully by reference to the accompanying drawings, in which embodiments of the invention are shown, and in which it is evident that the embodiments shown are only some, but not all embodiments of the invention. All other embodiments, which can be made by a person skilled in the art without any inventive effort, are intended to be within the scope of the present invention.
Examples of the present invention will be described in further detail below with reference to the accompanying drawings.
Example 1
The method for performing high-speed railway communication by using coverage wide beam transmission special for elliptical cells provided by the invention, referring to fig. 1 to 2, comprises the following steps:
S101: an elliptical cell is established, and the boundary of the cell passes through the farthest two ends of the track, namely the special coverage method of the high-speed rail cell, and the elliptical cell is close to the narrow strip-shaped cell by adjusting the eccentricity.
The method specifically comprises the following steps:
Assuming that the base station is projected to the middle position of the track and is positioned at the middle position of the track, taking the base station as the center of an elliptical cell, establishing ellipses passing through the farthest two ends of the track of the cell according to the length L of the high-speed rail covered by the base station and a given ellipse eccentricity e, and respectively obtaining a major half axis a and a minor half axis b of the ellipses as follows:
Wherein d s (0) represents the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the initial moment. The contour line of the received signal-to-noise ratio (SNR) of the obtained direct-view propagation paths is an elliptic curve in a plane containing all the direct-view propagation paths through beam forming, and the magnitude of the eccentricity e is adjusted to enable the elliptic cell to be close to a narrow strip cell, namely the SNR of the farthest end of the track is improved. According to the vertical heights d bs and d tr of the transmitting end and the receiving end, obtaining the direct-view propagation distance from the transmitting end of the base station to the receiving end of the train at the moment t as follows Where d min represents the vertical distance from the base station to the track, θ represents the emission angle of the base station transmit signal at time t.
Determining oval cell boundary, at time t, based on train position information, the channel from the transmitting end of the base station to the receiving end of the train can be expressed asAt this time, the transmission angle is θ, and under the same transmission angle, the channel from the transmitting end of the base station to the boundary of the elliptical cell can be expressed as/>The direct-view propagation distance that a signal experiences to reach an oval cell boundary is denoted d e (t), and the two channels based on location information differ in channel/>Is through channel/>The direct-view path of the train is obtained by extending, which means that a certain point on the elliptic curve of the direct-view path extension line from the base station to the train receiving end is a virtual receiving point. In addition, consider that the base station end is provided with M transmitting antennas, and the receiving end is a single antenna, then/>And/>Are all column vectors of M elements. Thus, after the ellipse eccentricity e, ellipse major half-axis a and ellipse minor half-axis b are all determined, i.e., d e (t) and channel/>, are determined
S102: and establishing a wide beam optimization problem based on the elliptical cell, meeting the constraint condition of the elliptical eccentricity e, and maximizing the SNR of the edge of the elliptical cell to make the SNR of the edge of the elliptical cell equal.
The method specifically comprises the following steps:
Under the condition of meeting the ellipse eccentricity e, the received SNR of the train at the farthest position is improved by maximizing the SNR of the ellipse boundary, so that the coverage rate is improved, and the wide beam optimization problem based on the ellipse cell can be written as follows assuming that the received SNR of the ellipse boundary is Γ
wHw＝1
0≤e<1
Wherein H in the upper right hand corner represents the conjugate transpose of the matrix or vector, resulting from the random variation in the channelIs random,/>Representation/>W and Γ are unknown optimization variables, w is the beam vector, and is the column vector consisting of M elements.
Beamforming makes the received SNR of the boundary of the elliptic cell equal to Γ, and adopts a logarithmic path loss model to represent SNR, so that the received SNR of the train on the linear track changes with time and can be expressed as gamma (t), and Γ and gamma (t) have the following relationship
Where epsilon represents the path loss index and is a known parameter. Given an eccentricity, i.e. d e(t)-ε is given, by obtaining an optimal beam w, Γ is maximized, and hence y (t), and consequently coverage is improved. By adjusting the magnitude of the eccentricity e, d e (t) closest to d s (t) is obtained, i.e. an elliptical cell closest to the straight cell is obtained, and the resulting beam is overall optimal.
S103: initializing an 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 ★.
The specific process is as follows:
according to the expression Representation matrix/>Let x=ww H, then X is a semi-positive definite matrix, and rank (X) =1 for matrix X, the rank constraint is removed. Satisfying the eccentricity constraint 0.ltoreq.e1, giving an initial eccentricity e=e 0, the optimization problem becomes a relaxed semi-definite programming (SDR) problem,
tr(X)＝1,
The SDR problem is solved by the CVX toolbox in Matlab to obtain an optimal solution X ★, wherein the upper right corner is the optimal solution. Iteratively increasing the magnitude of the eccentricity eAnd repeatedly solving the SDR problem by adopting a new eccentricity until the SDR problem has no solution, so that the maximum eccentricity of the solution of the SDR problem is the optimal eccentricity, the gamma is also the maximum at the moment, and the corresponding X ★ is also the optimal solution of the original problem.
S104: polynomial roots taking elements in the beam vector w as coefficients are obtained through X ★, polynomial coefficients are solved according to the polynomial roots, and the obtained beam vector w has no beam gain loss compared with an optimal solution X ★.
The specific process of the steps is as follows:
and (3) carrying out eigenvalue decomposition on the optimal solution X ★ obtained by the SDR problem, and taking a unique eigenvector as an optimal beam vector w ★ if only one eigenvalue exists. But often X ★ is of high rank, when the sum of the diagonal elements of matrix X ★ is extracted, i.e Wherein/>Representing an M x M toeplitz matrix, - (M-1) τ.ltoreq.M-1, i.e., a toeplitz matrix having 1 element and 0 element only on the τ -th minor diagonal, where the minor diagonal refers to a diagonal parallel to the major diagonal, τ <0 represents the lower diagonal, τ >0 represents the upper diagonal, e.g.
And is also provided withIndicating that only the main diagonal is at 1 and the remaining elements are at 0.
When (when)Can be constructed as a polynomial relationship as follows
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
Where w ★＝[w0,w1,…,wM-1]T, upper right corner "×" represents the conjugate of complex numbers and upper right corner "T" represents the transpose of the vector or matrix. Let f (x) =0, 2 (M-1) roots in total, and wherein M-1 roots are x 1,x2,…,xM-1, then the other M-1 roots areThus, the pairs of 2 (M-1) roots are divided into two groups of roots which are conjugate and reciprocal to each other, and M-1 pairs of such roots are shared. Two sets of roots are selected, one from each pair of roots is selected, the roots of the following polynomials are formed,
Wherein z m＝xm is orAnd solving coefficients of the polynomial to obtain the optimal beam w ★＝[w0,w1,…,wM-1]T.
S105: the beams are calculated and stored offline, and when a train passes through the cell, the base station does not need to switch the beams, and only one beam is needed to cover the whole cell.
The method specifically comprises the following steps:
According to the predictability of the train position information, the base station side calculates and stores the wave beams offline, when the train enters the cell, the base station side adopts the wide wave beam of the elliptical cell to carry out data transmission according to the real-time positioning information, and before the train leaves the cell, the wave beams do not need to be switched, and the Channel Quality Index (CQI) does not need to be fed back.
The simulation of the present invention will be described with reference to fig. 3 to 5.
The invention is proposed for the special coverage problem of the linear high-speed railway communication cell in a large range, and because the invention uses the channel based on the position information in the wave beam design, but the signal in the practical application is the instantaneous channel, the instantaneous channel is adopted for simulation in the simulation of coverage rate performance so as to be more in line with the practical environment. Simulation conditions: the number of transmitting antennas at the base station end is M=64, the carrier frequency f c =2.35 GHz, the speed of the high-speed railway is 350km/h, the path loss index epsilon=3.03, the vertical height d bs =25M of the base station antenna and the vertical height d tr =5M of the train receiving end.
Fig. 3 shows the expected design received power for different locations on the plane per unit transmit power based on the wide beamforming of elliptical cells at d min = 70m and eccentricity e = 0.95The plane here refers to a plane containing all direct-view paths from the base station to the track. Assuming that the base station is located at a (0, 0) point on the xy plane, it can be seen from the contour line of the g (t) three-dimensional curve on the xy plane that the contour line is an elliptic curve with the same eccentricity, and the curve illustrating that the coverage of the beamforming meets the shape of the elliptic cell, that is, the curve of the desired value of the equal received power is an elliptic curve.
The simulation results compare the invention with the direct on-track design of beam forming, i.e. without considering elliptical cells, which directly considers the received SNR of the train on-track, herein referred to as linear coverage. Fig. 4 shows the expected design received power of a train under different beamforming schemesAnd (3) a change curve along with the train position, wherein when the coverage position is 0m, the position of the track closest to the base station is represented, the coverage position is negative, the left track of the base station is represented, and the coverage position is positive, the right track of the base station is represented. The desired design received power g (t) for a linear coverage beamforming scheme at different coverage track lengths L is given in fig. 4 (a), as shown, although the equality constraint is relaxed to an inequality constraint, the problem of optimizing the objective function to maximize the desired design received power still results in all desired received powers being equal within the coverage. Since the minimum g (t) on the rail line reaches a maximum when the minimum g (t) of the covered edge is equal to the maximum. Furthermore, linear coverage is limited by railway coverage length, and when the minimum distance d min = 70m, the maximum coverage length L does not exceed 600m, whereas when it exceeds 600m, the beam optimization problem of linear coverage is not feasible and thus not applicable to wide coverage.
The desired design received power g (t) for a wide beamforming scheme based on elliptical cells at different eccentricities e is given in fig. 4 (b), from which it can be seen that g (t) gradually decreases with increasing propagation distance, g (t) of elliptical cells being in most cases larger than circular coverage (e=0), for example when the coverage track length is larger than 200m. It was also found that g (t) at the farthest distance increases with increasing elliptical eccentricity. When the eccentricity e is not large, such as the minimum distance d min =70m, e is not more than 0.95, the beam pattern is the same regardless of the total length L of the railway the design covers. When the eccentricity is large, such as 0.98, the design of the high-speed rail length L is limited, and the beam curves of different rail lengths (l=0.8 km,1 km) are designed not to overlap, i.e. in the beam forming optimization problem, different railway distances L lead to different beam patterns. Because there is a large variation in path loss in different directions on the elliptic curve as the eccentricity approaches 1, there is a different solution when expanding the coverage, i.e. when e is smaller, the path loss in each direction is relatively close, i.e. expanding the coverage has less impact on the optimization result.
FIG. 5 shows an overlayThe graph over the coverage length L assumes an interruption threshold of Γ h =3 dB for SNR and a transmit power P t =38 dBm. As can be seen from the figure, the maximum communication track length increases with an increase in elliptical eccentricity. At 100% coverage, the maximum coverage length of the 0.98 oval coverage is over 1200m, while the maximum coverage length of both the circular coverage and the linear coverage is less than 600m.
In summary, in the method for performing high-speed railway communication by covering wide beam transmission special for an elliptical cell, firstly, an elliptical cell is established, and the boundary of the cell passes through the farthest two ends of a track, so that the elliptical cell is close to a narrow strip cell by adjusting the eccentricity; establishing a wide beam optimization problem based on an elliptical cell, meeting an elliptical eccentricity constraint condition, maximizing the edge SNR of the elliptical cell, and enabling the edge SNR of the cell to be 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 ★; obtaining a polynomial root taking a beam vector as a coefficient through X ★, solving the polynomial coefficient according to the polynomial root, wherein the obtained beam vector has no beam gain loss compared with X ★; the beams are calculated and stored offline, and when a train passes through the cell, the base station does not need to switch the beams, and only one beam is needed to cover the whole cell. According to the invention, the elliptical cell is established for beam design, and the optimal elliptical cell is obtained by adjusting the eccentricity, so that the obtained wide beam can effectively improve the communication coverage rate of high-speed rail in a large range.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the scope of the present invention should be included in the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.
Claims (5)
1. The method for performing high-speed railway communication by using the special coverage wide beam transmission of the elliptical cell is characterized by comprising the following specific steps of:
Step 1, establishing a narrow strip-shaped elliptic cell, wherein the boundary of the cell passes through the farthest two ends of the track, namely, the eccentricity is adjusted, so that the elliptic cell is close to a linear cell; the method comprises the following steps:
Step 1.1 establishing a narrow strip-shaped elliptical cell
Assuming that the base station is projected to the middle position of the track and is positioned on the middle position of the track, taking the base station as the center of an elliptical cell, establishing ellipses passing through the farthest two ends of the track of the cell according to the length L of the high-speed rail covered by the base station and a given ellipse eccentricity e, and respectively obtaining a major half axis a and a minor half axis b of the ellipses as follows:
，
Wherein, The direct-view propagation distance from the base station transmitting end to the train receiving end at the initial 0 moment is represented; the method comprises the steps of obtaining a contour line of a received signal-to-noise ratio (SNR) of an elliptical curve in a plane containing all direct-view propagation paths through beam forming, and adjusting the magnitude of an eccentricity e to enable an elliptical cell to be close to a narrow linear track shape, 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 of the transmitting end and the receiving endAnd/>Obtaining that the direct-view propagation distance from the base station transmitting end to the train receiving end at the moment t is/>Wherein/>The vertical distance from the base station to the track is represented, and theta represents the emission angle of the signal transmitted by the base station at the moment t;
step 1.3 determination of oval cell boundary
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 expressed asAt this time, if the transmission angle is θ, under the same transmission angle, the channel from the transmitting end of the base station to the boundary of the elliptical cell is expressed as/>The direct-view propagation distance that a signal experiences to reach an elliptical cell boundary is denoted/>Wherein channel/>Is a direct-view path through the channelThe direct-view path extension of the base station to the train receiving end is obtained by extending the direct-view path, namely, a certain point on the elliptic curve is a virtual receiving point of the cell boundary; in addition, since the base station is equipped with M transmitting antennas and the receiving end is a single antenna, thenAnd/>Column vectors of M elements; thus, after the ellipse eccentricity e, ellipse major half-axis a and ellipse minor half-axis b are all determined, thereby determining/>And channel/>；
Step 2, establishing a wide beam optimization problem based on the elliptical cell, meeting constraint conditions of the elliptical eccentricity e, maximizing the edge SNR of the elliptical cell, and enabling the edge SNR of the elliptical cell to be equal; the method comprises the following steps:
Under the condition of meeting the ellipse eccentricity e, the received SNR of the train at the farthest position is improved by maximizing the SNR of the ellipse boundary, so that the coverage rate is improved, and the received SNR of the ellipse boundary is assumed to be Then the elliptical cell based wide beam optimization problem is written as:
，
wherein H in the upper right hand corner represents the conjugate transpose of the matrix or vector, resulting from the random variation in the channel Is random,/>Representation/>W and/>For unknown optimization variables, w is a beam vector, which is a column vector composed of M elements; beamforming allows the received SNR at the oval cell boundary to be equal to/>Using a logarithmic path loss model to represent SNR, the received SNR of a train on a straight line track varies over time, expressed as/>And (2) andAnd/>The following relationship is provided:
，
Wherein the method comprises the steps of Representing the path loss index, a known parameter; given eccentricity, i.e./>Given, maximize/>, by obtaining an optimal beam wThereby improving/>Then the coverage rate is improved; and by adjusting the magnitude of the eccentricity e, the nearest/>, is obtained/>I.e. an elliptical cell closest to the linear cell is obtained, the beam obtained at this time is overall optimal;
Step3, 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 ；
Step 4 byObtaining a polynomial root taking elements in a beam vector w as coefficients, solving the polynomial coefficients according to the polynomial root, and obtaining the beam vector w and an optimal solution/>Compared with no beam gain loss;
and 5, calculating and storing the wave beams offline, wherein when a train passes through the cell, the base station does not need to switch the wave beams, and only one wave beam is needed to cover the whole cell.
2. The method for performing high-speed railway communication by using the coverage wide beam transmission special for the elliptical cell according to claim 1, wherein the step 3 is specifically: according to the expression，Representation matrix/>Trace of/>Then X is a semi-positive definite matrix and the rank of matrix X/>Removing rank constraint conditions; satisfy eccentricity constraint/>Given initial eccentricity/>The optimization problem becomes a relaxed semi-definite programming SDR problem,/>Optimal solution/>, of SDR problem through CVX toolbox in MatlabUpper right corner "/>"Means the optimal solution; iteratively increasing the magnitude of the eccentricity eRepeatedly solving the SDR problem by adopting the 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 eccentricityIs also maximum, corresponding/>Is also the optimal solution of the original problem.
3. The method for performing high-speed railway communication by using the coverage wide beam transmission special for the elliptical cell according to claim 1, wherein the step 4 is specifically:
for optimal solutions to SDR problems Performing eigenvalue decomposition, and if there is only one eigenvalue, taking the unique eigenvector as the optimal beam vector/>; Such as/>Is of high rank, then extract matrix/>Sum of diagonal elements, i.eWherein/>Representation/>Toeplitz matrix,/>I.e. only at the/>The elements on the secondary diagonal are 1 and the rest are 0 of the toeplitz matrix, the secondary diagonal refers to the diagonal parallel to the main diagonal, and the element on the secondary diagonal is/(is)Representing the lower diagonal,/>Representing upper diagonal lines, such as: /(I)，
And is also provided withRepresenting that only the element on the main diagonal is 1 and the rest elements are 0;
When (when) The following polynomial relationship is constructed:
，
Wherein the method comprises the steps of Upper right corner "/>"Represents the conjugate of complex numbers, the upper right hand corner" T "represents the transpose of the vector or matrix; let/>Common/>Root, and/>, thereinRoot is/>Then additionally/>Root is/>; Thus will/>The roots are divided into two groups of roots which are conjugate and reciprocal, and share/>Pairing; two sets of roots are selected, one from each pair of roots, constituting the roots of the following polynomials:，
Wherein the method comprises the steps of Solving the coefficients of the polynomial to obtain the optimal beam/>。
4. The method for performing high-speed railway communication by using the coverage wide beam transmission special for the elliptical cell according to claim 1, wherein the step 5 is specifically:
and according to the predictability of the train position information, the base station calculates and stores the wave beams offline, and when the train enters the cell, the base station adopts the wide wave beams of the elliptical cell to carry out data transmission according to the real-time positioning information.
5. A high-speed rail communication system for elliptical cell dedicated coverage wide beam transmission, comprising a memory and a processor, the memory storing a computer program, characterized in that; the processor, when executing the computer program, implements the method steps of any of claims 1-4.
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| WO2019174745A1 (en) * | 2018-03-16 | 2019-09-19 | Telefonaktiebolaget Lm Ericsson (Publ) | Determining beam settings for beam management |
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