KR20140042042A - Method and apparatus for selecting cell considering mimo precoding - Google Patents

Abstract

Disclosed are a method and a device for selecting a cell considering a multi-input multi-output (MIMO) precoding. According to the present invention, the device for selecting a serving cell in a MIMO communications system includes a candidate cell selection unit which selects multiple candidate base stations using a synchronization signal received from multiple base stations; a channel capacity calculation unit which calculates effective channel capacity of each candidate base station based on an effective signal to interference plus noise ratio (SINR) measured for each of the candidate base stations; and a serving cell selection unit which selects a cell corresponding to the candidate base station with the maximum effective channel capacity calculated as a serving cell.

Description

[0001] METHOD AND APPARATUS FOR SELECTING CELL CONSIDERING MIMO PRECODING [0002]

The present invention relates to a cell selection method and apparatus considering MIMO precoding, and more particularly, to a method and apparatus for initially selecting an optimal cell in a MIMO environment.

Recently, with the spread of smart phones, users who want to receive high quality data service in real time from everywhere are increasing rapidly. As a result, there is a growing need for a technique for supporting high data rate at the cell edge as well as the cell edge. At the cell center, it is possible to increase the transmission rate simply by increasing the number of antennas. However, the UEs at the edge of the cell experience a sudden performance degradation due to interference signals of neighboring cells. Therefore, It is difficult to increase.

Therefore, the CoMP (Coordinated Multi-Point Transmission) technique is called CoMP (Coordinated Multi-Point Transmission) in 3GPP. It is progressing.

Generally, CoMP is divided into Joint Processing (JP) and Coordinated Scheduling / Coordinated Beamforming (CS / CB) in the downlink.

JP does not receive signals of surrounding cells other than the serving cell as interference signals but constructs cells to be cooperated among the cells around the terminal to receive all the signals received from the cooperating cells to increase the transmission speed at the cell edge without interference signals And the CS / CB scheme increases the transmission rate by minimizing the interference signal received from neighbor cells differently from the JP.

The JP method can be further divided into JT (Joint Transmission) and DCS (Dynamic Cell Selection).

Joint Transmission transmits data by allocating cooperated cells, i.e. serving cells, as well as neighboring cells, to the same resource for one terminal. To do this, channel state information (CSI) and transmission data must be shared with each other through a backhaul between the base stations, which increases the amount of system overhead. However, since the interference signals received from neighboring cells are lost, the signal to interference plus noise power ratio (SINR) increases and the transmission rate at the cell edge is improved.

DCS (Dynamic Cell Selection) must share the data and CSI information through the backhaul in the same cell as JT. The difference from JT is that the UEs located at the edge of the cell do not transmit / receive simultaneously through the same resource to the cooperating cells and transmit / receive through the cells having the best reception channel among the cooperative cells. Also, dynamic blacking (DB) can be used to increase the performance at the edge of a cell by dynamically eliminating power at the transmission time determined to act as a large interference to the terminal, have. Therefore, DCS and DB schemes can maximize the SINR and improve the transmission speed.

In the CS / CB (Coordinated Scheduling / Coordinated Beamforming) scheme, a time or frequency resource for transmitting a signal is appropriately adjusted between transmission points in order to reduce interference to other transmission points, or a beam The direction is adjusted to minimize the interference signal.

In order to apply the above schemes, the cooperative cells must share information to be transmitted to the UE. Since the Joint Transmission and DCS methods all receive the data of the cooperated cell, the performance of the CS / CB scheme is higher than that of the CS / CB scheme. However, since the data and channel information to be transmitted to the UE must be shared, do. In the CS / CB scheme, only the information on the channel (CQI, RI, PMI, etc.) is shared, data need not be shared, and only the current serving cell needs to have data to be transmitted to the UE.

Therefore, the performance improvement is small because the interference signal is relatively larger than the joint transmission and DCS method. However, the amount of information to be shared is small, so the load of the backhaul is relatively less. Considering the constraints among the cooperated cells, the CS / CB scheme is effective in efficiently increasing the cell edge performance while effectively reducing the interference signal in the multi-cell environment.

In the CS / CB scheme, a base station basically uses a method of reducing interference by forming a beam only to a specific mobile station. In this process, it is most important to select a base station capable of maximizing a transmission rate.

In this case, when the CS / CB scheme is used, once the serving cell is selected, it is necessary to communicate with the initially selected cell until the cell reselection process is performed. Therefore, a cell selection process suitable for the CS / CB scheme is required in the initial selection process. However, when synchronization and cell selection are performed through the existing synchronization signal, the UE generally performs a synchronization and cell selection process using a single antenna, and thus can not consider the actual MIMO channel environment.

The base station and the mobile station transmit and receive data by applying an actual MIMO precoder. In the initial cell selection, the gain obtained through the precoder can not be considered. Therefore, the cell found in the existing synchronization process can not be regarded as an optimal cell.

Here, the CoMP technique is applied to the data area of the OFDM system. However, the synchronization and cell selection process, which is first performed when the UE is powered on, generally uses a synchronization signal. For example, in 3GPP LTE, CoMP is applied only in a PDSCH (Physical Downlink Shared Channel), and when a cell is initially selected, a selection is made through a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), and a Cell Specific Reference Signal do.

1 is a flowchart of a 3GPP LTE cell selection procedure.

Referring to FIG. 1, the mobile station synchronizes with a plurality of base stations (step 100). Then, a cell ID of one of index 0, 1, and 2 is searched through S (step 102) .

The PSS sequence is generated as follows.

The PSS generated through the Zadoff-chu sequence acquires the cell ID if the sequence corresponding to the three root indexes is cross-correlated and a root index with a large cross-correlation result is found. The formula for taking a cross-correlation is given in the time domain as:

는 시간영역 수신 신호를 나타내고,

는 루트인덱스

에 해당하는 PSS의 시간영역 시퀀스를 나타낸다. 또한

는 25, 29, 34를 나타내고 순서대로 각각의 루트인덱스에 따라

는 0,1,2로 나타낸다. 또한,

은 PSS의 시간영역 시퀀스의 샘플 수를 나타내고

은 타이밍 오프셋 샘플 수를 나타낸다. here,

Represents a time domain received signal,

Root index

And the time-domain sequence of the PSS corresponding to the PSS. Also

Represents 25, 29, 34, and in order according to each root index

Lt; / RTI > Also,

Represents the number of samples in the time domain sequence of the PSS

Represents the number of timing offset samples.

(Cell ID Group)을 검출한다(단계 104). SSS는 두 개의 m-시퀀스 조합으로 시퀀스를 생성한다. SSS는 총 168개의

(Cell ID Group)으로 구성되어 있으며, SSS 시퀀스의 상호상관을 이용하여 찾게 된다. Next, through the SSS

(Cell ID Group) (step 104). The SSS generates a sequence with two m-sequence combinations. The SSS has a total of 168

(Cell ID Group), which are searched using the cross correlation of SSS sequences.

The sequence formula of the SSS is as follows.

는 주파수영역 수신 신호를 나타내고,

는 168개 중 i번째 SSS 주파수영역 시퀀스를 나타내며,

은 코히어런트 방식 적용 시 PSS에서 추정한 채널 값을 나타낸다. 또한,

는 수신 신호와 i번째 SSS 시퀀스와 상호상관을 취한 결과를 나타낸다. PSS, SSS를 통해 찾은

와

를 통해

을 찾을 수 있다. here,

Domain received signal,

Represents the i-th SSS frequency domain sequence out of 168,

Represents the channel value estimated by the PSS when the coherent method is applied. Also,

Denotes the result of cross-correlation with the received signal and the i-th SSS sequence. PSS, found through SSS

Wow

Through the

Can be found.

을 찾는 수식은 다음과 같이 나타낼 수 있다.

Can be expressed as follows.

In 3GPP LTE, a total of 504 different cell IDs can be distinguished. PSS, and SSS, and then constructs a candidate cell (base station).

As a next step, a reference signal received power (RSRP) of a cell specific reference signal (CRS) corresponding to a candidate cell is measured, and the most suitable cell is found based on the RSRP and the cell selection process is completed (step 106).

The cell selection process selects a cell based on the strength of the synchronization signal and the cell specific reference signal. After the cell selection is completed, the BS and the UE transmit and receive data through the PDSCH region, and various techniques can be applied to improve performance.

As mentioned above, by using the CS / CB scheme, which is one of CoMP technologies, it is possible to improve the performance of the terminals located at the cell edge by performing interference control. In the CS / CB scheme, after the UEs access the serving cell through the cell selection process, the CS / CB scheme configures cells having large interference among adjacent cells as cooperating cells and controls the interference signals of cells in addition to the serving cells through scheduling and beamforming . In this case, when the UEs initially determine the serving cell, the CS / CB scheme can not flexibly select the cell as in the DCS scheme, and the CS / CB scheme can perform the reselection process to reconnect to another cell.

Therefore, it is important to select cells that can achieve optimal performance when using the CS / CB scheme at the initial cell selection. As described above, by maximizing the signal of the serving cell through the CS / CB and minimizing the interference signal, the SINR of the UE can be improved. However, in the present method, the CS / CB scheme is applied in the direction of increasing the SINR with respect to the selected serving cell after selecting the serving cell through the synchronization signal, which is an existing cell selection method. In this case, the SINR is improved and the performance is improved. However, when CS / CB scheme is applied to other cells among the cooperative cells, a better SINR environment can be obtained than the current serving cell. It is difficult to see cells as optimal.

Also, considering only the SINR when selecting the serving cell and not considering the channel rank, it is not the optimal cell in terms of the channel capacity.

In order to solve the problems of the conventional art as described above, MIMO precoding capable of selecting a cell capable of achieving optimal performance in consideration of an effective SINR applied with precoding of a data region and an effective channel capacity thereof, And a cell selection method and apparatus considered.

According to an aspect of the present invention, there is provided an apparatus for selecting serving cells in a multi input multiple output (MIMO) communication system, the apparatus comprising: A candidate cell selector for selecting a base station; A channel capacity calculation unit for calculating an effective channel capacity of each of the plurality of candidate base stations based on a measured SINR (Signal to Interference plus Noise Ratio) measured for each of the plurality of candidate base stations; And a serving cell selector for selecting, as a serving cell, a cell corresponding to a candidate base station having the maximum available channel capacity.

Wherein the channel capacity calculation unit selects a maximum precoding matrix index for maximizing one base station (first candidate base station) signal among the plurality of candidate base stations, and selects a minimum pre-coding matrix index for minimizing the remaining candidate base station signals except for the first candidate base station. A precoding matrix index selector for selecting a coding matrix index; An effective SINR calculator for calculating an effective SINR for each of the plurality of candidate base stations after the maximum precoding matrix index and the minimum precoding matrix index are applied; And an effective channel capacity calculation unit for calculating an effective channel capacity of the first candidate base station using the calculated effective SINR.

The channel capacity calculation unit may include a channel estimation unit that estimates a channel for each of the plurality of candidate base stations using a cell specific reference signal.

The precoding matrix index selector may select the maximum precoding matrix index and the minimum precoding matrix index using a channel rank for the estimated channel.

The effective channel capacity of the first candidate base station is determined based on the transmission power of the first candidate base station, the noise power, the transmission power of the remaining candidate base stations excluding the first candidate base station, the received power, A maximum precoding matrix for maximizing the signal of the first candidate base station, and a minimum precoding matrix for minimizing a signal of the remaining candidate base stations except for the first candidate base station, .

The synchronization signal includes a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), and a Cell Specific Reference Signal (CRS). The candidate cell selection unit acquires a cell ID via the PSS, Group, and the plurality of candidate base stations can be selected using the cell ID and the cell ID group.

The effective channel capacity can be calculated according to the following equation.

[Mathematical Expression]

,

는 각각 i 번째 후보 기지국의 송신 전력과 잡음 전력을 나타내고,

,

는 각각 간섭 셀의 송신 전력과 상기 제1 후보 기지국을 제외한 나머지 후보 기지국의 수신 전력을 나타내며, 상기 나머지 후보 기지국의 수신 전력은 후보 기지국들이 신호를 전송할 때 신호를 최소화시키는 프리코딩 행렬이 적용된 후 측정되며.

,

은 i 번째 후보 기지국의 송신 안테나의 수와 채널 랭크,

는 i 번째 후보 기지국의 채널 벡터를 나타내고,

,

는 신호를 최대화시키는 i 번째 후보 기지국의 프리코딩 행렬과 신호를 최소화시키는 i 번째 후보 기지국의 프리코딩 행렬임here

,

Represents the transmission power and the noise power of the i-th candidate base station, respectively,

,

Represents a transmission power of an interference cell and a reception power of a remaining candidate base station excluding the first candidate base station, and the reception power of the remaining candidate base station is measured after a precoding matrix for minimizing a signal when a candidate base station transmits a signal is applied .

,

The number of transmit antennas of the i < th > candidate base station, the channel rank,

Denotes the channel vector of the i < th > candidate base station,

,

Is a precoding matrix of an i-th candidate base station that maximizes a signal and a precoding matrix of an i-th candidate base station that minimizes a signal

The effective SINR may be calculated according to the following equation.

[Mathematical Expression]

는 i 번째 후보 기지국의 유효 채널 벡터를 나타내며,

,

는 각각 i번째 후보 기지국이 서빙 셀일 때 신호를 최대화시키는 프리코딩 행렬, j번째 간섭 신호를 최소화시키는 프리코딩 행렬이고,

는 평균이 0이고 분산이

인 가산성 백색 잡음임.here,

Denotes the effective channel vector of the i < th > candidate base station,

,

Is a precoding matrix for maximizing a signal when the i < th > candidate base station is a serving cell, and a precoding matrix for minimizing the j < th >

The average is 0 and the variance is

Additive white noise.

The serving cell may be selected according to the following equation.

[Mathematical Expression]

는 i 번째 후보 기지국의 유효 채널 용량,

는 SINR의 대한 수식으로 나타낼 수 있으며

은 i번째 후보 기지국의 유효 SINR임here

Is the effective channel capacity of the i-th candidate base station,

Can be expressed by the equation of SINR

Is the effective SINR of the i-th candidate base station

According to another aspect of the present invention, there is provided a method of selecting a serving cell in a mobile communication apparatus of a MIMO communication system, comprising: selecting a plurality of candidate base stations adjacent to each other using a synchronization signal received from a plurality of base stations; Calculating an effective channel capacity of each of the plurality of candidate base stations based on the measured effective SINR for each of the plurality of candidate base stations; And selecting a cell corresponding to a candidate base station having the maximum effective channel capacity as a serving cell.

According to another aspect of the present invention, there is provided a method of selecting a serving cell in a mobile communication device of a MIMO communication system, the method comprising: selecting a plurality of candidate base stations adjacent to each other using a synchronization signal received from a plurality of base stations; Estimating a channel for each of the plurality of candidate base stations through a cell specific reference signal; And selecting a serving cell using at least one of a channel rank, an effective channel capacity, and an effective SINR of a data region to be actually transmitted between the plurality of candidate base stations and the mobile communication apparatus through the estimated channel and codebook A serving cell selection method is provided.

According to the present invention, since the serving cell is initially selected using the effective SINR and the effective channel capacity in consideration of the MIMO environment, an optimal cell can be selected.

1 is a flowchart of a 3GPP LTE cell selection process.
2 is a diagram illustrating a cell selection apparatus according to a preferred embodiment of the present invention.
3 is a diagram illustrating a detailed configuration of a channel capacity calculation unit according to a preferred embodiment of the present invention.
4 is a flowchart of a cell selection process according to the present invention;
5 is a diagram comparing effective channel capacities according to channel ranks.
6 is a diagram illustrating the assumption that there are one terminal and two base stations and four antennas in the simulation according to the present invention.
FIG. 7 is a diagram illustrating an optimal BS selection probability when a channel rank of two BSs is 1 and both precoding is applied and applied; FIG.
FIG. 8 is a diagram showing a case where channel rank of two base stations is 4 and both precoding is applied and an optimal base station selection probability when applied. FIG.
FIG. 9 shows that there is almost no change when probability of selecting base station 1 increases when precoding is applied to base station 1 with channel rank 1 and when precoding is applied to base station 2 with channel rank 4.
FIG. 10 illustrates that the serving cell of the optimum performance can be selected through the cell selection process according to the present invention. FIG.

The present invention proposes a method and apparatus for increasing the probability of selecting a cell having superior performance at the edge of a cell when selecting a cell using codebook-based precoding among CS / CB schemes.

개의 송신 안테나를 가지고 있고 단말은

개의 수신 안테나를 가지고 있으며, 기지국과 단말간의 채널은 Cell Specific Reference Signal을 통해 각각에 대해 모두 추정할 수 있고, 단말 주변에 있는 기지국들과 동기가 모두 맞춰져 있는 환경을 가정한다. In a multi-cell environment,

Lt; RTI ID = 0.0 >

And the channel between the base station and the terminal can be estimated for each of the channels through the cell specific reference signal and it is assumed that the base station and the terminals are synchronized with the base stations around the terminal.

2 is a diagram illustrating a cell selection apparatus according to an exemplary embodiment of the present invention.

2, the cell selection apparatus according to the present invention may include a candidate cell selection unit 200, a channel capacity calculation unit 202, and a serving cell selection unit 204.

The cell selection device may be a mobile communication terminal that has a plurality of antennas and transmits and receives an OFDM signal with a base station having a plurality of antennas. Hereinafter, the cell selection device is assumed to be a mobile communication terminal.

The candidate cell selector 200 according to the present invention selects a plurality of cells that are candidates for a serving cell by using a synchronization signal transmitted from a plurality of neighboring base stations.

As described above, in the 3GPP LTE system, the candidate cell selector 200 selects a plurality of candidate base stations serving as candidates of the serving cell through the PSS, the SSS, and the CRS.

As shown in FIG. 4, the candidate cell selection process (steps 400 to 406) according to the present invention is the same as that of FIG. However, while step 106 selects the best cell as a serving cell based on the RSRP, the present invention selects only the candidate cell in step 406, and the serving cell is selected according to the channel rank, effective SINR, do.

The channel capacity calculation unit 202 measures the effective SINR for each of the plurality of candidate base stations, calculates the effective channel capacity of each of the plurality of candidate base stations based on the measured effective SINR, and the serving cell selection unit 204 calculates And selects the cell corresponding to the candidate base station having the maximum effective channel capacity as a serving cell.

Such a serving cell selection procedure is shown in steps 408 to 416 of FIG.

3 is a diagram illustrating a detailed configuration of a channel capacity calculation unit according to a preferred embodiment of the present invention.

3, the channel capacity calculation unit 202 according to the present invention includes a channel estimation unit 300, a precoding matrix index selection unit 302, an effective channel capacity calculation unit 304, an effective SINR calculation unit 304, (306).

The channel estimation unit 300 estimates a channel for each of a plurality of candidate base stations selected using a cell specific reference signal. The channel estimation information includes information on the channel rank.

The precoding matrix index selector 302 selects a maximum precoding matrix index that maximizes a signal of each base station and a minimum precoding matrix index that minimizes a signal using channel estimation information.

The selected index is input to the effective channel capacity calculation unit 304 and the effective SINR calculation unit 306.

The effective channel capacity calculation unit 304 and the effective SINR calculation unit 306 according to the present invention calculate effective channel capacity and effective SINR of each base station using channel estimation information and maximum / minimum precoding matrix indexes.

According to the present invention, effective channel capacity and effective SINR are calculated based on one of a plurality of candidate base stations.

That is, when one of the plurality of candidate base stations is selected as the first candidate base station, the remaining candidate base stations are regarded as interference cells and the effective channel capacity and the effective SINR for the first candidate base station are calculated. As shown in FIG. 4, The calculation process for all candidate base stations is repeatedly performed.

In more detail, the effective channel capacity of the first candidate base station is determined by minimizing the transmission power of the first candidate base station, the noise power, the transmission power of the remaining candidate base stations excluding the first candidate base station, and the signal of the first candidate base station A minimum precoding matrix for minimizing a signal of the first candidate base station and a maximum precoding matrix for maximizing a signal of the first candidate base station, May be calculated using at least one of the matrices.

At this time, the effective SINR is considered together with the effective channel capacity of each candidate base station.

After the effective channel capacity calculation determined based on the effective SINR is performed for all candidate base stations, the serving cell selection unit 204 selects the optimal serving cell.

According to the present invention, since the serving cell is initially selected considering the channel rank, the SINR, and the channel capacity, higher performance can be assured.

Hereinafter, the serving cell selection process according to the present invention will be described in detail with reference to the equations.

When the mobile communication terminal is at the cell edge, the OFDM signal received at the base station of the OFDM-based cellular system is given as follows.

는 i 번째 기지국에서 전송하는 k번째 서브케리어에 해당하는

송신 신호벡터이고,

는 i번째 기지국과 단말의 k번째 서브케리어에 해당하는

MIMO 채널벡터를 나타낸다. 또한

는 인접 기지국 수를 나타내며, 수학식 4에서 우변의 첫 번째 항은 서빙 셀의 기지국으로부터 수신되는 신호를 나타내며, 두 번째 항은 인접 기지국으로부터 수신되는 모든 간섭신호를 나타낸다. 또한

는 평균이 0이고 분산이

인 가산성 백색 잡음(Additive White Gaussian Noise, AWGN)을 나타낸다. here

Corresponding to the k < th > subcarrier transmitted from the i < th &

Is a transmission signal vector,

Corresponds to the k < th > subcarrier of the i < th >

Denotes a MIMO channel vector. Also

In Equation (4), the first term of the right side represents a signal received from the base station of the serving cell, and the second term represents all the interference signals received from the adjacent base station. Also

The average is 0 and the variance is

Additive White Gaussian Noise (AWGN).

The formula for selecting the precoding matrix index is given by:

는 각각 i 번째 후보 기지국의 신호를 최대화, 최소화시키는 M개 중 하나의 프리코딩 행렬 인덱스를 나타내고,

는 i 번째 후보 기지국의 프리코딩 행렬 인덱스에 해당하는 프리코딩 행렬을 나타낸다. 또한

은 i 번째 후보 기지국 채널의 랭크(rank)를 나타내며, C는 채널 용량을 나타낸다.here,

Represents one precoding matrix index out of M that maximizes and minimizes the signal of the ith candidate base station,

Represents a precoding matrix corresponding to a precoding matrix index of an i-th candidate base station. Also

Denotes a rank of an i-th candidate base station channel, and C denotes a channel capacity.

를 적용한 유효(effective) 채널의 채널 용량을 각 후보 기지국에 대해 구한다. 각 후보 기지국의 송신전력이 1이라고 가정하였고, 유효 채널 용량을 구하는 식은 다음과 같이 주어진다. Maximize channel capacity

The channel capacity of the effective channel to which each candidate base station is applied is obtained for each candidate base station. Assuming that the transmission power of each candidate base station is 1, the equation for obtaining the effective channel capacity is given as follows.

,

는 각각 i 번째 후보 기지국의 송신 전력과 잡음의 전력을 나타내고,

,

는 각각 간섭 셀의 송신 전력과 서빙 셀을 제외한 주변의 모든 간섭 셀의 수신 전력을 나타낸다. 서빙 셀을 제외한 후보 기지국들의 수신 전력은 후보 기지국들이 신호를 전송할 때 신호를 최소화시키는 프리코딩 행렬이 적용된 후 측정한다.

,

은 i 번째 후보 기지국의 송신 안테나의 수와 채널의 랭크를 나타낸다. 또한

는 i 번째 후보 기지국의 채널벡터를 나타내고,

,

는 신호를 최대화시키는 i 번째 후보 기지국의 프리코딩 행렬과 신호를 최소화시키는 i 번째 후보 기지국의 프리코딩 행렬을 나타낸다.

는 랜덤 변수이므로

는 순시적인 채널 용량을 나타내며, 에르고딕 채널 용량을 구하면

가 된다.here

,

Represents the transmission power and noise power of the i < th > candidate base station, respectively,

,

Represents the transmission power of the interference cell and the reception power of all surrounding interference cells excluding the serving cell. The received power of the candidate base stations excluding the serving cell is measured after a precoding matrix for minimizing the signal is applied when the candidate base stations transmit the signal.

,

Denotes the number of transmit antennas of the i-th candidate base station and the rank of the channel. Also

Denotes the channel vector of the i < th > candidate base station,

,

Represents a precoding matrix of an i-th candidate base station that maximizes a signal and a precoding matrix of an i-th candidate base station that minimizes a signal.

Is a random variable

Represents the instantaneous channel capacity, and the ergodic channel capacity is obtained

.

Therefore, when determining the effective channel capacity for each candidate base station, one of the surrounding cells is selected as the serving cell, the effective SINR is measured by measuring the remaining cell signal as interference, and the effective channel capacity is measured based on the measured effective SINR .

Assuming that the transmission power of all candidate base stations is 1, the SINR equation of Equation (6) can be rewritten as Equation (7), and the equations are given as follows.

는 i 번째 후보 기지국의 유효 채널 벡터를 나타내며,

,

는 각각 i번째 후보 기지국이 서빙 셀일 때 신호를 최대화시키는 프리코딩 행렬, j번째 간섭 신호를 최소화시키는 프리코딩 행렬을 나타낸다.

는 평균이 0이고 분산이

인 가산성 백색 잡음을 나타낸다.here,

Denotes the effective channel vector of the i < th > candidate base station,

,

Represents a precoding matrix for maximizing a signal when the i-th candidate base station is a serving cell, and a precoding matrix for minimizing the j-th interference signal.

The average is 0 and the variance is

Additive white noise.

The SINR is measured for each candidate base station and the cell corresponding to the candidate base station having the best available channel capacity is selected as a serving cell by using the effective channel capacity obtained by Equation (6).

In Equation (6), the effective channel capacity can be represented by an equivalent equation in which the variable is SINR, and a base station having the best available channel capacity among surrounding base stations is selected as a serving cell. The formula for selecting the serving cell can be expressed as:

는 i 번째 후보 기지국의 유효 채널 용량을 나타낸다.

는 SINR의 대한 수식으로 나타낼 수 있으며

은 i번째 후보 기지국의 유효 SINR을 나타낸다.here

Represents the effective channel capacity of the i-th candidate base station.

Can be expressed by the equation of SINR

Represents the effective SINR of the i-th candidate base station.

Conventional cell selection schemes use a strength of a synchronization signal and a reference signal to select a serving cell. If only the intensity of the synchronization signal and the reference signal is used at the time of cell selection, there is a probability that the cell is not the cell capable of achieving the optimum performance in view of the channel capacity but is selected.

5 compares the effective channel capacity according to the channel rank. It is assumed that there are four transmit antennas and four receive antennas, and a codebook of 3GPP LTE is used. The effective channel capacity is measured by applying a precoding matrix for maximizing the signal when the channel rank is 1 and a precoding matrix for maximizing the signal when the channel rank is 4.

When the channel rank is 1, when the effective channel capacity is seen, it can be seen that the channel rank becomes smaller than a certain SINR than when the channel rank is 4. When the channel rank is 4, the spatial multiplexing gain can be obtained from a specific SINR or higher, so that a large difference in channel capacity occurs.

Therefore, if the serving cell is selected based on the reception strength of the synchronization signal and the reference signal, considering the current SINR and the channel rank, there is a limit in selecting the optimal cell when determining the performance in terms of channel capacity.

Also, assuming that the terminal selects one of the neighboring base stations and the terminal is connected to the serving cell at the edge of the cell and performs transmission / reception in the CS / CB scheme of the CoMP scheme at the edge of the cell, Beamforming and scheduling to maximize the signal of the cell. During this process, beamforming and scheduling are performed such that signals of the remaining interference cells except the serving cell are minimized. Since the serving cell has already been determined, it is possible to perform better among other cooperating cells than the current serving cell, but it can transmit / receive only with the current serving cell.

As a result, if a cell is selected in consideration of the beamforming to be applied to the data region at the time of initial cell selection as in the present invention, precoding according to the channel rank is considered differently from the case of considering only the strength of the existing synchronous signal and reference signal When CS / CB scheme is used, it is possible to select a serving cell that can obtain better performance.

Although the present invention has been described in terms of initial cell selection, it can be applied as a cell selection method at handover as it is. In addition, although the present invention has been made in view of the CS / CB, even in the case of the non-cooperative communication that does not use the CS / CB, if only the noise term is considered while ignoring the interference term, it can be directly applied at the time of initial cell selection and handover.

The following simulations have been conducted for the present invention.

6, it is assumed that there are one terminal, two base stations, and four antennas, respectively. Also, it is assumed that the strengths of the signals transmitted from the neighboring base stations to the mobile stations are all the same, and the frequency synchronization and the timing synchronization between the mobile station and the two base stations are all aligned, and the cell IDs are all found around the synchronization signal.

First, we test the probability of choosing base station 1 when precoding on precoding of reference signal and precoding off when base station's channel rank is different from the same channel rank.

Referring to FIG. 7, if the channel rank of both base stations is 1, and if precoding is not applied to both base stations, it is confirmed that the probability of the UE selecting the base station 1 is about 50% when the signal-to-interference ratio (SIR) is 0 dB . However, when the probability of selecting base station 1 is the same after applying precoding to base station 1, it can be seen that base station 1 is selected at about 90% when SIR is 0 dB. Second, when the channel rank of the two base stations is 4, and the probability of the UE selecting the base station 1 in the same environment as the first is measured, almost the same results are obtained when the precoding is applied and when the base station is not applied have. Third, when the channel ranks of the two base stations are different from each other, the probability of selecting the base station 1 is measured. As shown in the above results, since the higher the channel rank, the greater the precoding gain, , It can be seen from FIG. 9 that there is almost no change when the precoding is applied to the base station 2 having the channel rank of 4 although the probability of selecting the base station 1 is increased.

From these results, it can be seen that the lower the rank of channel when precoding is applied, the more precoding gain can be obtained. However, there are limitations in selecting a base station with only signal strength. Measuring the channel capacity according to the channel rank shows that the precoding gain is larger when the channel rank is 1 but larger than when the channel rank is 4 from the specific SNR or higher to 1 when the channel rank is higher than 1. Therefore, when selecting the base station initially, the terminal must consider the effective SINR and effective channel capacity between each base station and the terminal.

In FIG. 10, a base station is selected based on SINR when there is one base station with two terminals, SNR is 10dB and SIR is 0dB, and precoding of a reference signal, which is an existing method, is not applied. When the base station is selected based on the effective channel capacity and the effective SINR considering precoding according to the channel condition at the initial stage and the available channel capacity (Proposed Approach), the effective channel capacity Respectively.

Considering the precoding matrix to be applied to the data region from the time when the UE initially selects the cell as shown in FIG. 10, when the UE is at the cell edge, performance is improved from the viewpoint of the channel capacity It can be confirmed that the probability of selecting a better cell is increased.

It will be apparent to those skilled in the relevant art that various modifications, additions and substitutions are possible, without departing from the spirit and scope of the invention as defined by the appended claims. The appended claims are to be considered as falling within the scope of the following claims.

Claims (11)

An apparatus for selecting a serving cell in a MIMO (Multi Input Multiple Output) communication system,
A candidate cell selector for selecting a plurality of candidate base stations using a synchronization signal received from a plurality of base stations;
A channel capacity calculation unit for calculating an effective channel capacity of each of the plurality of candidate base stations based on a measured SINR (Signal to Interference plus Noise Ratio) measured for each of the plurality of candidate base stations; And
And a serving cell selector for selecting a cell corresponding to the candidate base station having the maximum calculated effective channel capacity as a serving cell. The method of claim 1,
Wherein the channel capacity calculation unit comprises:
Selects a maximum precoding matrix index that maximizes one base station (first candidate base station) signal among the plurality of candidate base stations, and selects a minimum precoding matrix index that minimizes the remaining candidate base station signals excluding the first candidate base station A precoding matrix index selector;
An effective SINR calculator for calculating an effective SINR for each of the plurality of candidate base stations after the maximum precoding matrix index and the minimum precoding matrix index are applied; And
And an effective channel capacity calculator configured to calculate an effective channel capacity of the first candidate base station using the calculated effective SINR. 3. The method of claim 2,
Wherein the channel capacity calculation unit comprises:
And a channel estimator for estimating a channel for each of the plurality of candidate base stations using a cell specific reference signal. The method of claim 3,
Wherein the precoding matrix index selector comprises:
And selecting the maximum precoding matrix index and the minimum precoding matrix index using a channel rank for the estimated channel. 5. The method of claim 4,
Wherein the effective channel capacity of the first candidate base station
The first candidate base station and the first candidate base station, the transmission power of the first candidate base station, the noise power, the transmission power of the remaining candidate base stations excluding the first candidate base station, the received power, the number of transmit antennas of the first candidate base station, And a minimum precoding matrix for minimizing a signal of a remaining candidate base station excluding the first candidate base station. The method of claim 1,
The synchronization signal includes a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), and a Cell Specific Reference Signal (CRS)
Wherein the candidate cell selection unit acquires a cell ID through the PSS, acquires a cell ID group through the SSS, and selects the plurality of candidate base stations using the cell ID and the cell ID group. 3. The method of claim 2,
Wherein the effective channel capacity is calculated according to the following equation.
[Mathematical Expression]

here

,

Represents the transmission power and the noise power of the i-th candidate base station, respectively,

,

Represents a transmission power of an interference cell and a reception power of a remaining candidate base station excluding the first candidate base station, and the reception power of the remaining candidate base station is measured after a precoding matrix for minimizing a signal when a candidate base station transmits a signal is applied .

,

The number of transmit antennas of the i < th > candidate base station, the channel rank,

Denotes the channel vector of the i < th > candidate base station,

,

Is a precoding matrix of an i-th candidate base station that maximizes a signal and a precoding matrix of an i-th candidate base station that minimizes a signal 8. The method of claim 7,
Wherein the effective SINR is calculated according to the following equation.
[Mathematical Expression]

here,

Denotes the effective channel vector of the i < th > candidate base station,

,

Is a precoding matrix for maximizing a signal when the i < th > candidate base station is a serving cell, and a precoding matrix for minimizing the j < th >

The average is 0 and the variance is

Additive white noise. 9. The method of claim 8,
Wherein the serving cell is determined according to the following equation:
[Mathematical Expression]

here

Is the effective channel capacity of the i-th candidate base station,

Can be expressed by the equation of SINR

Is the effective SINR of the i-th candidate base station A method of selecting a serving cell in a mobile communication device of a MIMO communication system,
Selecting a plurality of candidate base stations adjacent to each other using a synchronization signal received from a plurality of base stations;
Calculating an effective channel capacity of each of the plurality of candidate base stations based on the measured effective SINR for each of the plurality of candidate base stations;
And selecting a cell corresponding to the candidate base station having the maximum calculated effective channel capacity as a serving cell. A method of selecting a serving cell in a mobile communication device of a MIMO communication system,
Selecting a plurality of candidate base stations adjacent to each other using a synchronization signal received from a plurality of base stations;
Estimating a channel for each of the plurality of candidate base stations through a Cell Specific Reference Signal; And
And selecting a serving cell using at least one of a channel rank, an effective channel capacity, and an effective SINR of a data area to be actually transmitted between the plurality of candidate base stations and the mobile communication device through the estimated channel and codebook. Cell selection method.

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