KR101505118B1 - Apparatus and method for user selection in a multiuser MIMO system - Google Patents

Abstract

The present invention relates to a user selection apparatus and method in a multi-user MIMO system.

The apparatus and method for selecting a user in a multiuser MIMO system according to the present invention are characterized by receiving channel information from a plurality of user terminals and using the received channel information based on a bit error rate (BER) or a fast BER And selects a user terminal for MIMO.

Description

[0001] Apparatus and method for user selection in a multi-user MIMO system [

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a user selecting apparatus and method in a Multiuser Multiple Input Multiple Output (MIMO) system, and more particularly to a user selecting apparatus and method in a multiuser MIMO system supporting OFDMA (Orthogonal Frequency Division Multiple Access) And an error rate).

Recently, a MIMO (Multiple Input Multiple Output) system in which a plurality of antennas are used by a transmitter and a receiver has been studied in order to increase the transmission capacity of wireless communication.

The MIMO system can be divided into an open loop MIMO and a closed loop MIMO. The open loop MIMO is a method in which a transmitting terminal communicates without knowing a channel condition. In a closed loop MIMO, And refers to the channel information fed back from the base station. In the case of open-loop MIMO, complex coding techniques applied simultaneously in the time domain and the spatial domain must be used in order to reach the theoretical transmission capacity, which is disadvantageous in that the detection complexity increases greatly as the number of antennas increases. Accordingly, closed loop MIMO approaching the theoretical capacity of the open loop scheme is preferred by applying an independent modulation and coding scheme to each transmit antenna according to the channel state in an actual system.

Meanwhile, in the case of a multi-user MIMO system, selection of user terminals to which MIMO is applied is an important factor of system performance. In the prior art, a method of finding a set of user terminals maximizing the channel capacity based on the channel capacity has been used. In this case, the most accurate method is a method of measuring the sum of channel capacities considering a set of all possible user terminals. This is because the calculation complexity becomes large if the number of all user terminals is large, .

Therefore, many user selection techniques with low complexity have been developed. These methods are mainly based on the greedy method. First, a user terminal having the largest channel capacity is selected, and among the remaining unselected user terminals, And selects the selected user terminal and the user terminal that exhibits the best performance.

However, the user selection technique based on the channel capacity mainly used in the prior art is calculated on the assumption of an ideal environment, and does not necessarily match the actual system. In other words, since the channel capacity, which is a criterion of user selection in the prior art, means a data transmission amount that can be obtained in an ideal environment, a large channel capacity in an actual system did not necessarily exhibit a good performance. Therefore, a user selection technique with better performance that can be applied to an actual system is required.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an apparatus and method for selecting a user using a BER (Bit Error Rate) in a multiuser MIMO system.

It is another object of the present invention to provide an apparatus and method for selecting a user using Fast BER (Fast BER) in a multiuser MIMO system.

It is yet another object of the present invention to provide an apparatus and method for efficiently allocating power based on BER or fast BER in a multiuser MIMO system.

To this end, in a multi-user MIMO system according to an aspect of the present invention, a user selection apparatus includes: a channel information reception unit for receiving channel information from a plurality of user terminals; And a user selector for selecting a MIMO user terminal based on a bit error rate (BER) using the received channel information.

In addition, a method for selecting a user in a multi-user MIMO system according to an aspect of the present invention includes: a) receiving channel information from a plurality of user terminals; b) obtaining a precoding matrix for each user terminal using the received channel information; c) calculating a minimum of a singular value of the obtained precoding matrix for each user terminal; And d) selecting a user terminal having the largest minimum value among the minimum values calculated for each user terminal.

Also, a method for selecting a user in a multi-user MIMO system according to an aspect of the present invention includes: a) receiving channel information from a plurality of user terminals; b) obtaining a channel matrix set of already selected user terminals and preliminarily selected user terminals using the received channel information; c) calculating a minimum of a singular value of the obtained channel matrix set; And d) selecting a preliminarily selected user terminal corresponding to the channel matrix set having the largest minimum value among the calculated minimum values.

According to the present invention, system performance can be improved by using a channel capacity by selecting a user based on BER or fast BER in a multi-user MIMO system.

And, according to the present invention, it is possible to improve system performance by allocating power based on BER or fast BER in a multiuser MIMO system.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and preferred embodiments. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention unnecessarily obscure.

First, FIG. 1 illustrates a model of a multi-user MIMO system to which the present invention may be applied. For reference, in this system model, a channel is a downlink channel, which can be applied to both Time Division Duplex (TDD) and Frequency Division Duplex (FDD).

Referring to FIG. 1, in this system model, N _i (N _i is an integer equal to or larger than 2; i = 1, 2, 3,... , K _t ) antennas, and the base station selects K (K is an integer equal to or greater than 1) user terminals among the total K _t (K _t is an integer of 2 or more) user terminals Communication.

The relationship between the transmission and reception signals in the system model can be expressed by Equation (1).

[Equation 1]

는 i번째 사용자 단말로의 송신 신호,

는 i번째 사용자 단말의 수신 신호,

는 i번째 사용자 단말에 대한 프리코딩 행렬,

는 i번째 사용자 단말에 대한 채널 행렬,

는 i번째 사용자 단말에 할 당된 전력,

는 i번째 사용자 단말에 대한 노이즈(noise)이다. 그리고,

와

는 각각 독립적이고 동일한 분포(i.i.d.; independently and identically distributed)를 가지며, 평균이 0이고 표준편차가 1인 정규분포 N(0,1)을 따른다고 가정한다.Here, i is a user terminal index,

Is the transmission signal to the i < th > user terminal,

Is the received signal of the ith user terminal,

Is a precoding matrix for the ith user terminal,

Is the channel matrix for the ith user terminal,

Is the power allocated to the i < th > user terminal,

Is the noise for the ith user terminal. And,

Wow

Are assumed to follow the normal distribution N (0, 1), which is independent and identically distributed (iid), with an average of 0 and a standard deviation of 1.

In this case, assuming that the transmitting end knows the complete channel, Equation (2) holds.

&Quot; (2) "

FIG. 2 is a configuration diagram of a user selecting apparatus according to an embodiment of the present invention.

2, a user selection apparatus according to the present invention includes a channel information receiving unit 210, a user selecting unit 220, a precoding matrix generating unit 230, a modulation level assigning unit 240, a power allocating unit 250 A data transfer unit 260, and the like.

The channel information receiving unit 210 receives channel information fed back from a plurality of (K _t ) user terminals.

Then, the user selection unit 220 selects the user terminal by the user selection technique using the channel information fed back from each user terminal.

In the present invention, two algorithms are proposed as a user selection technique based on a BER (Bit Error Rate). FIG. 4 shows a first algorithm, which is referred to as a 'BER-based multiuser MIMO User Selection Algorithm'. FIG. 5 shows a second algorithm, hereinafter referred to as' Fast BER- BER based User Selection Algorithm. &Quot; 3, the user selecting unit 220 selects a user terminal by a BER-based user selecting unit 222 for selecting a user terminal by a BER-based user selecting method and a fast BER-based user selecting method And a fast BER-based user selection unit 224 for performing fast BER-based user selection.

First, a BER-based user selection scheme will be described with reference to FIG.

은 n번째 사용자 단말의 채널 행렬을 나타내고,

는 채널 행렬

의 특이값(singular value) 중 최소값이며, p는 T에 속하는 사용자 단말들에 해당하는

중에서 최대값을 나타내는 변수이다.

는 심볼 할당 함수(symbol allocation function)이며, 여기서 R은 총 전송 비트(total transmit bits) 수이고, i는 사용자 단말 인덱스이다. 그리고,

는 등최소거리함수(equal minimum distance function)이다.Step 1 is an initialization step. Where T denotes a set of selectable user terminals and S denotes a set of selected user terminals. And,

Denotes the channel matrix of the n-th user terminal,

The channel matrix

, P is the minimum of the singular values of T,

Which is the maximum value among the values.

Is a symbol allocation function, where R is the total number of transmitted bits and i is the user terminal index. And,

Is an equal minimum distance function.

For reference, in the initialization step T includes all user terminals, that is, T = {1,2, ..., K _t }, S is empty because there are no terminals selected yet. Then, d ₁ and G ₁ are set.

Step 2 (Step 2) is a step of selecting a user terminal.

을 구하여 행렬

=

를 구한 후, 다시

의 특이값(singular value) 중 최소값을 구하고, T에 속하는 사용자 단말들 중

이 가장 큰 사용자 단말을 선택한다. 그러면, 선택된 사용자 단말은 T에서 삭제되고, S에 속하게 된다.Since d ₁ and G ₁ are set in the initialization step, a loop is continued until K user terminals are selected starting from i = 2. In this case, a criterion for selecting a user terminal is a precoding matrix for each user terminal

Then,

=

After that,

Of the user terminals belonging to T, a minimum value among the singular values of T

And selects the largest user terminal. Then, the selected user terminal is deleted from T and belongs to S.

When the K user terminals are selected by repeating this process, the user selection process is completed. In step 3 (Sept 3), the K user terminals finally selected are left.

Hereinafter, a fast BER-based user selection scheme will be described with reference to FIG.

Step 1 is an initialization step. Where T denotes a set of selectable user terminals and S denotes a set of selected user terminals. And A is a channel matrix set of the temporarily selected user terminal.

For reference, in the initialization phase, T includes all user terminals, ie, T = {1,2, ..., K _t }, S and A are vacant since there are no terminals selected yet.

Step 2 (Step 2) is a step of selecting a user terminal.

은 n번째 사용자 단말의 채널 행렬을 나타내고,

는 임시 선택된 사용자 단말의 채널 행렬 집합

의 특이값(singular value) 중 최소값이며, p는 T에 속하는 사용자 단말들에 해당하는

중에서 최대값을 나타내는 변수이다.

는 심볼 할당 함수(symbol allocation function)이며, 여기서 R은 총 전송 비트의 수이고, i는 사용자 단말 인덱스이다. 그리고,

는 등최소거리함수(equal minimum distance function)이다.here,

Denotes the channel matrix of the n-th user terminal,

A channel matrix set of a temporarily selected user terminal

, P is the minimum of the singular values of T,

Which is the maximum value among the values.

Is a symbol allocation function, where R is the total number of transmitted bits and i is the user terminal index. And,

Is an equal minimum distance function.

(

는 물론 행렬임)의 특이값(singular value) 중 최소값

을 구하고, T에 속하는 사용자 단말들 중에서(즉, 예비적으로 선택된 단말들 중에서)

이 가장 큰 사용자 단말을 선택한다. 그러면, 선택된 사용자 단말은 T에서 삭제되고, S에 속하게 된다.Unlike the first algorithm, the second algorithm starts from i = 1 and continues to loop until K user terminals are selected. In this case, a criterion for selecting a user terminal is a channel matrix set of a temporarily selected user terminal (i.e., a user terminal already selected and preliminarily selected)

(

Of the singular value of the matrix

Among the user terminals belonging to T (that is, among the terminals selected preliminarily)

And selects the largest user terminal. Then, the selected user terminal is deleted from T and belongs to S.

When the K user terminals are selected by repeating this process, the user selection process is completed. In step 3 (Sept 3), the K user terminals finally selected are left.

For reference, fast BER-based user selection scheme, which is the second algorithm, can reduce the amount of computation significantly because it does not require a precoding matrix, unlike the first algorithm (BER-based user selection scheme) (See Figs. 8 to 10).

(K×M 행렬)는 하기 수학식 3과 같이 나타낼 수 있다. 참고로, 하기 수학식 3에서

(i=1,2,...,K)는 i번째 사용자 단말의 채널 정보(1×M 행렬)이다.On the other hand, channel information (K information) for the K user terminals selected by the user selection unit 220

(K x M matrix) can be expressed by the following equation (3). For reference, in the following equation (3)

(i = 1, 2, ..., K) is the channel information (1 x M matrix) of the ith user terminal.

&Quot; (3) "

The precoding matrix generator 230 generates a precoding matrix for the user terminals selected by the user selector 220. According to a preferred embodiment of the present invention, the precoding matrix generator 230 includes a first precoding matrix generator 232 and a second precoding matrix generator 234 as shown in FIG.

(

행렬)를 생성한다. 여기서, 제 1 프리코딩 행렬

는 블록 대각화(BD; Block Diagonalization) 프리코딩 기법을 이용하여 생성할 수 있으며,

으로 이루어져 있으며, 이때

는 i번째 사용자 단말의 제1 프리코딩 행렬(

행렬,

)이다. 따라서 제1 프리코딩 행렬

는 수학식 4를 만족한다.The first precoding matrix generator 232 includes a first precoding matrix for eliminating inter-user interference

(

Matrix). Here, the first precoding matrix

May be generated using a block diagonalization (BD) precoding technique,

In this case,

Lt; RTI ID = 0.0 > i < / RTI > of the ith user terminal

procession,

)to be. Thus, the first precoding matrix

Satisfies the expression (4).

&Quot; (4) "

(n_i×M_i 행렬)를 생성한다. 여기서, 제2 프리코딩 행렬

는

를 특이값 분해하여 얻을 수 있으며, 이를 상술하면 다음과 같다.The second precoding matrix generator 234 generates a second precoding matrix for separating each user terminal into eigenmodes

(n _i x M _i matrix). Here, the second precoding matrix

The

Can be obtained by singular value decomposition, which will be described in detail as follows.

를 특이값 분해한다.First, as shown in the following equation (5)

.

&Quot; (5) "

는

의 좌측 유니터리 행렬이며,

는

의 대각선 행렬이며,

는

의 우측 유니터리 행렬이다. 그리고,

는

의 왼쪽의 첫번째 부터 N_i번째까지 열이며이며

는

의 나머지 열에 해당하는 행렬이다.here,

The

Lt; RTI ID = 0.0 >

The

Lt; / RTI >

The

Lt; / RTI > And,

The

Is the column from the first left to the N _i th column

The

≪ / RTI >

Then, the final precoding matrix is expressed by Equation (6).

&Quot; (6) "

는 사용자간의 갑섭을 제거시키는 행렬로서 전체 시스템을 K개의 독립적인 다중 안테나 시스템으로 분해시키고, 제2 프리코딩 행렬

는 K개의 독립적인 다중 안테나 시스템을 고유모드(eigenmode)로, 즉 MIMO를 SISO(Single Input Single Output)로 분해시킨다.Thus, the first precoding matrix

Is a matrix for eliminating the inter-user interference, decomposes the entire system into K independent multi-antenna systems, and a second precoding matrix

Separates K independent multi-antenna systems into eigenmodes, i.e., MIMO into SISO (Single Input Single Output).

For reference, the received signal of each selected user terminal can be expressed as Equation (7).

&Quot; (7) "

는 S_i의 (k,k)성분이고,

는 i번째 사용자 단말의 k번째 SISO 채널의 수신 신호이다.here,

(K, k) component of S _i ,

Is the received signal of the kth SISO channel of the ith user terminal.

For the power P, the following equation (8) is satisfied.

&Quot; (8) "

Referring again to FIG. 2, the modulation level assigning unit 240 assigns a modulation level to the selected user terminal, and the power allocating unit 250 assigns power to the selected user terminal. This will be described in detail below.

는 선택된 사용자 단말의 집합 S에서 다음과 같은 방식으로 전력(power)을 적절히 할당해 주고, 등최소거리레벨(equal minimum distance level) a 값을 리턴(return)한다. Referring to Figures 4 and 5, an equal minimum distance function < RTI ID = 0.0 >

Appropriately allocates power in a set S of selected user terminals in the following manner and returns a value of equal minimum distance level a.

&Quot; (9) "

Where D _i is the minimum distance of the modulation level for the i th user terminal and has the values shown in Table 1 below.

[Table 1]

Then, power is allocated as shown in Equation (10).

&Quot; (10) "

Here, it is assumed that each user terminal having N antennas receives N streams, and that the antennas of the user terminals receive signals of the same modulation level.

Then, power is allocated as shown in Equation (11).

&Quot; (11) "

Finally, the power allocated to the k-th channel of the user terminal i is expressed by Equation (12).

&Quot; (12) "

는 하기 표 2와 같은 값을 가진다.For example, when N = 2, a symbol allocation function

Are as shown in Table 2 below.

[Table 2]

는 4가 되며, 따라서 4비트씩 변조하는 변조 레벨인 16QAM(표 1 참조)을 하나의 선택된 사용자 단말에 대하여 할당한다. 그리고, Rate(BPCU)가 8이고 i=3인 경우

는 (2,1,1)이 되는데, 이는 3개의 선택된 사용자 단말에 대하여 하나의 단말에는 QPSK(2비트씩 변조)를 할당하고 나머지 2개의 단말에는 BPSK(1비트씩 변조)를 할당한다 As described in detail with reference to Table 2, when Rate or Bit Per Channel Use (BPCU) is 8 and i = 1 (one selected user terminal)

Is allocated to one selected user terminal. Therefore, 16QAM (see Table 1), which is a modulation level for modulating 4 bits at a time, is allocated to one selected user terminal. If Rate (BPCU) is 8 and i = 3

(2, 1, 1), which allocates QPSK (modulation by 2 bits) to one terminal and allocates BPSK (modulation by 1 bit) to the remaining two terminals for the three selected user terminals

을 구하면 각 사용자 단말에 할당할 전력이 결정된다. And in this way,

The power to be allocated to each user terminal is determined.

Finally, the data transmission unit 260 transmits final signals (data) on which data to be transmitted to each user terminal is subjected to precoding, modulation, power allocation, and the like.

Hereinafter, a user selection method according to the present invention will be described with reference to FIG. For reference, the specific process or operation principle of the user selection method according to the present invention can be referenced to the description of the user selection device, so that redundant description will be omitted and briefly explained.

가 된다.In step S710, the base station (transmitting side) receives a respective feedback channel information from a plurality (K _t) of the user terminal (receiving end). For reference, the channel information fed back from the user terminal is channel vector

.

In step S720, the base station selects the user terminal by the user selection technique using the channel information fed back from each user terminal. In this case, the user selection scheme can use the BER-based user selection scheme or the fast BER-based user selection scheme.

및 각 사용자 단말을 고유모드(eigenmode)로 분리하기 위한 제2 프리코딩 행렬

를 생성한다. 그리고, 상기 제1 프리코딩 행렬과 제2 프리코딩 행렬을 곱하여 최종 프리코딩 행렬

을 생성한다. In step S730, the base station generates a precoding matrix for the selected user terminal. In this case, the precoding matrix includes a first precoding matrix for eliminating inter-user interference

And a second precoding matrix for separating each user terminal into eigenmodes

. Then, the first precoding matrix and the second precoding matrix are multiplied to generate a final precoding matrix

.

등에 의해 구할 수 있다(표 1 및 표 2 참조).In step S740, the base station assigns a modulation level for the selected user terminal. In this case, the modulation level to be allocated to each user terminal is determined by the number of selected user terminals, Rate (BPCU), symbol allocation function

(See Table 1 and Table 2).

, 등최소거리함수

, 등최소거리레벨 a 등에 의해 구할 수 있다(수학식 9 내지 12 참조).In step S750, the base station allocates power for the selected user terminal. In this case, the power to be allocated to the user terminal is determined by the symbol allocation function

, Etc. Minimum distance function

, Minimum distance level a, etc. (see Equations 9 to 12).

Finally, in step S760, the base station transmits the generated signal (data) to each user terminal

8 to 10 are graphs comparing BERs of various user selection schemes according to changes in the number of antennas.

More specifically, FIG. 8 shows SNRs for BER-based, fast BER-based, channel capacity-based, TDMA (i.e., single user MIMO) based user selection schemes when M = 4, N = 2, Kt = FIG. 9 is a graph showing SNR versus BER for BER-based, fast BER-based, channel capacity-based, and TDMA-based user selection schemes when M = 8, N = 2, Kt = 30 and BPCU = 10 is a graph showing SNR versus BER for BER-based, fast BER-based, channel capacity-based, and TDMA-based user selection schemes when M = 4, N = 1, Kt = 30 and BPCU = , M is the number of transmit antennas, N is the number of receive antennas, and K _t is the number of all user terminals in the cell).

As shown in the graph, the BER-based user selection scheme and the fast BER-based user selection scheme according to the present invention have much better BERs than the existing channel capacity based user selection scheme or TDMA based user selection scheme. It can be seen that both the BER-based user selection scheme and the BER-based user selection scheme have similar performance.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It is to be understood that the embodiments are to be considered in all respects only as illustrative and not restrictive.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. .

1 is a diagram illustrating a multi-user MIMO system model to which the present invention may be applied.

2 is a configuration diagram of a user selection device according to the present invention.

3 is a detailed configuration diagram of the user selection unit of FIG.

Figure 4 shows a BER-based algorithm.

Figure 5 shows a fast BER-based algorithm.

FIG. 6 is a detailed configuration diagram of the precoding matrix generator of FIG. 2. FIG.

7 is a flowchart of a method for selecting a user according to the present invention.

8 to 10 are graphs comparing BERs of various user selection schemes according to changes in the number of antennas.

Claims (18)

A user selection device in a multi-user MIMO system, A channel information receiver for receiving channel information from a plurality of user terminals; And And a user selector for selecting a MIMO user terminal based on a bit error rate (BER) using the received channel information, Wherein the user- A fast BER-based user selection for selecting a user terminal based on a minimum of a singular value of the channel matrix set after obtaining a channel matrix set of a temporarily selected user terminal using a channel matrix of each user terminal, Wherein the user selection unit comprises: 2. The apparatus according to claim 1, And a BER-based user selector for selecting a user terminal based on a minimum of a singular value of the precoding matrix after obtaining a precoding matrix using a channel matrix of each user terminal. Selection device. delete 3. The method according to claim 1 or 2, Further comprising a precoding matrix generator for generating a precoding matrix using the channel matrix by generating a channel matrix for all selected user terminals by combining channel vectors for the selected user terminals, Lt; / RTI > 5. The method of claim 4, Wherein the precoding matrix generator comprises: A first precoding matrix generator for generating a first precoding matrix for inter-user interference cancellation using the channel matrix; And And a second precoding matrix generator for generating a second precoding matrix for separating each user terminal into eigenmodes using the channel matrix and the first precoding matrix. . 5. The method of claim 4, And a modulation level assigning unit for assigning a modulation level to the selected user terminal. The method according to claim 6, Wherein the modulation level is set based on a number of selected user terminals, a bit per channel use (BPCU), and a symbol allocation function. The method according to claim 6, And a power allocator for allocating power to the selected user terminal according to the allocated modulation level. 9. The method of claim 8, Wherein the power is set based on a symbol allocation function, an equal minimum distance function, and an equal minimum distance level. 9. The method of claim 8, And a data transmission unit transmitting the corresponding data to the selected user terminal. A method for user selection in a multi-user MIMO system, A method comprising: a) receiving channel information from a plurality of user terminals; b) obtaining a precoding matrix for each user terminal using the received channel information; c) calculating a minimum of a singular value of the obtained precoding matrix for each user terminal; And d) selecting a user terminal having the largest minimum value among the calculated minimum values for each user terminal. 12. The method of claim 11, And repeating steps b) to d) for the remaining user terminals except for the user terminal selected in step d). 13. The method of claim 12, Further comprising: assigning a modulation level based on a number of user terminals selected for the selected user terminal, a bit per channel use (BPCU), and a symbol allocation function. 14. The method of claim 13, Further comprising allocating power for the selected user terminal based on a symbol allocation function, an equal minimum distance function, and an equal minimum distance level. ≪ RTI ID = 0.0 > Way. A method for user selection in a multi-user MIMO system, A method comprising: a) receiving channel information from a plurality of user terminals; b) obtaining a channel matrix set of already selected user terminals and preliminarily selected user terminals using the received channel information; c) calculating a minimum of a singular value of the obtained channel matrix set; And d) selecting a preliminarily selected user terminal corresponding to a set of channel matrixes having the largest minimum value among the calculated minimum values. 16. The method of claim 15, And repeating steps b) to d) for the remaining user terminals except for the user terminal selected in step d). 17. The method of claim 16, Further comprising: assigning a modulation level based on a number of user terminals selected for the selected user terminal, a bit per channel use (BPCU), and a symbol allocation function. 18. The method of claim 17, Further comprising allocating power for the selected user terminal based on a symbol allocation function, an equal minimum distance function, and an equal minimum distance level. ≪ RTI ID = 0.0 > Way.

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