KR20100004853A - Inter-cell interference avoidance for downlink transmission - Google Patents

Abstract

The present invention relates to a wireless network system including a plurality of base stations capable of communicating with a plurality of user terminals, and includes potential interfering base stations that select different codebook vectors within a standardized codebook to avoid interference with each other. The base station of may select the codebook vectors or matrices such that its signal to be transmitted generates only minimal interference to other base stations of the same band, thereby reducing interference to the terminal of the neighboring cell and improving cell edge throughput. .

Description

Inter-cell interference avoidance for downlink transmission {INTER-CELL INTERFERENCE AVOIDANCE FOR DOWNLINK TRANSMISSION}

The present invention relates to the situation of multiple base stations using standard codebooks for precoding transmission for each user terminal. Here, each user terminal uses a multiple transmit antenna.

Typically, a feedback-based mechanism called a "closed-loop multiple input multiple output system" is called "3GPP TS25.814. V7.1.0., Physical Layer for evolved Universal Terrestrial Radio Access (UTRA)". As used in the " a " aspect of the " For example, the user terminal performs channel measurement using a training signal, and then maximizes the SNR of the received signal based on the channel from the serving base station with the expected signal to noise ratio (SNR) value. Provides a codebook vector or matrix to the base station. Thereafter, the serving base station configures data based on feedback information of a user terminal (intended) in a cell, and transmits the data to the user terminal. In this way the performance of the wireless system, i.e. throughput, is improved by standard codebooks.

The problem with this approach is that signal interference occurs between the designated user terminals of some base stations. This scenario is called 'inter-cell interference', and FIG. 1 shows a case where two base stations communicate with two user terminals in the same frequency band.

This situation occurs when two user terminals receive data in the same frequency band. In FIG. 1, base station 1 eNB1 is a serving base station of user terminal 1 UE 1, and base station 2 eNB2 is a serving base station of user terminal 2 UE2. The user terminal 1 (UE1) is a designated user of the base station 1 (eNB1) and the user terminal 2 (UE2) is a designated user of the base station 2 (eNB2). Here, the number of transmission antennas of each base station is referred to as N _T , the number of reception antennas of the user terminal is referred to as N _R , and each channel gain is referred to as H ₁₁ , H ₁₂ , H ₂₁ , and H ₂₂ . Then, the signal received at the user terminal 1 (UE 1) and the signal received at the user terminal 2 (UE2) is represented by the following equation (1).

Here, Y _i represents the (N _R × 1) vector of the signal received from the user terminal i, X _i represents the (N _T × 1) vector of the signal transmitted from the base station i, Ni is (N _R x1) Represents an Additive White Gaussian Noise (AWGN) vector.

Typically, user terminals report only information about a preferred codebook vector or matrix to their serving base station based on their channel with the serving base station. That is, the user terminal 1 (UE1) selects a codebook vector transmitted from the base station 1 (eNB1) based on H ₁₁ , and the user terminal 2 (UE2) selects the base station 2 (eNB2) based on H ₂₂ . Select the transmitted codebook vector. Thus, different user terminals may be subject to strong interference due to different cells using the same bandwidth. In particular, when the user terminals are users at a cell edge, the received power levels of the interference signal and the designated signal are similar to each other, so that the signal to interference and noise ratio (SINR) is very low at the user terminal. Will cause). In this particular situation, the signal X ₁ transmitted from the base station 1 UE1 to the user terminal 1 UE1 causes strong interference with respect to the received signal X2 of the user terminal 2 UE2. . In FIG. 1, when the user terminals UE1 and UE2 are at the cell edge, the throughput of the user terminal at the cell edge is largely lost due to the interference because the power levels of the specified signal and the interference signal are similar. . This is one of the reasons why average cell edge throughput is significantly lower than average cell throughput. Therefore, improving the cell edge throughput as indicated in "R1-082186, Physical Layer Requirements for LTE Advanced" is the biggest issue in the 3GPP LTE-Advanced standard.

In "IEEE C802.16m-08 / 429, Downlink Collaborative MIMO for Cell-edge Users in a Multi-Cell Environment", a technology called 'Precoder Matrix Index Restriction' by LGE This has been proposed. This means that each user terminal indirectly feeds back the codebook vector to the interfering base station causing the highest interference to its signal. The interfering base station then includes a codebook vector provided from the codebook and performs codebook vector selection in limited circumstances. In this way, cell-edge throughput can be improved. However, using this approach, since the user terminal only reports the codebook vector and the limited codebook that cause the strongest interference, the interference by the interfering base station may still appear very high, if not the highest. In “Frame Structure Supporting Inter-cell Interference Mitigation in Downlink Traffic Channels Using IEEE C80216m-08_017, Co-MIMO and FFR”, each user terminal indirectly codes the codebook vector of the interfering base station that has the least interference. Feedback. However, this method is too limited for interfering base stations. In general, the interfering base station should also consider the performance of the terminal it serves.

SUMMARY OF THE INVENTION The present invention is derived to solve the above problems, and an object of the present invention is to provide a technique for improving cell edge throughput through cooperation with interfering base stations in a wireless communication system.

Another object of the present invention is to provide a technique for interfering base stations in a wireless communication system to avoid mutual interference by selecting different codebook vectors from a standard codebook.

According to an aspect of the present invention for achieving the above object, a wireless communication system including a plurality of base stations capable of communicating with a plurality of user terminals, avoids interference with each other by selecting different codebook vectors in a standardized codebook Potential interference base stations, wherein each base station selects codebook vectors or matrices such that its signal to be transmitted generates only minimal interference to other base stations of the same band.

According to the present invention, by selecting different codebook vectors within a standardized codebook through cooperation between interfering base stations, the interference to the terminal of the neighboring cell can be reduced, and the cell edge throughput can be improved. It works.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

Hereinafter, a description will be given of a technique in which interfering base stations can avoid interference by selecting different codebook vectors from standard codebooks.

The present invention aims to improve cell edge throughput through cooperation with interfering base stations in a unified manner. Without loss of generality (WLOG), assuming that UE1 (UE1) is a cell-edge user, the throughput of UE1 (00) due to interference caused by BS 2 (112) at this time Will be lowered. However, this phenomenon can eliminate or greatly reduce the interference by performing an appropriate operation at base station 2 (112). This can be seen through the case where N _T = 4 and N _R = 2. The interference channel matrix H _{21 is represented} by independent values as follows.

Where U is a 2x2 unit matrix, Λ is a 2x4 matrix, and V is a 4x4 unit matrix. Here, Λ has a structure as shown in Equation 3 below.

Therefore, if the first two elements of VX ₂ converge to zero, the interference to signal Xi at user terminal 1 UE1 will also be eliminated. In other words, if N _T > N _R, there are several codebook vectors that, when used at base station 2 (eNB2), have no interference with the signal of base station 1 (eNB1) or the interference is very small. Under these observations, if UE 1 can estimate the channel matrix H ₂₁ , UE 1 UE 1 recommends a transmission direction that is recommended in a condition where there is no interference or low interference with its signal. It can be informed to the base station 2 (eNB2) through.

Such analysis provides the basis for "Docket No. 2008.06.005.WS0, Intercell Interference Prevention for Downlink Transmission" with the present invention.

According to an embodiment of the present invention, we propose a technique in which interfering base stations can avoid interference with each other by selecting different codebook vectors from a standard codebook. This is made possible by allowing the base stations to select themselves a codebook vector or matrix to use when transmitting their own signals in a space with little or no interference to user terminals of other cells in the same bandwidth.

The following describes the main procedures for achieving these goals based on standardized codebooks.

1. Channel Estimation: In each cell, cell edge user terminals estimate the channel matrices from the strong base stations and the channel matrices from the serving base station through the reference signal.

2. Interference Level Generation: The base stations send a message to the user terminals that includes a configurable threshold. The threshold indicates an interference level at the terminal. For example, the base stations may transmit a parameter ε, which causes a precoding vector or matrix report for avoiding interference between adjacent cells, to the user terminals. The parameter ε represents a target interference level for the user terminal. The base stations may also send a message to the user terminals indicating whether the terminal is allowed or not allowed to participate in interference avoidance. For example, the terminal may participate in the interference avoidance procedure at the request of the base station, or by default, all users may participate in the interference avoidance procedure.

3. Feedback Information Generation: The user terminal searches for a codebook vector or matrix that maximizes its received signal power based on the estimated channel matrix, or other code that maximizes the power of the received signal in addition to the codebook vector or matrix. Measure performance or other performance from interfering base stations. The terminal calculates a message ξ representing a recommended or limited precoding vector or matrix for the interfering base station from the interference level parameter obtained from the base station. If the base station and the user terminal agree to feed back any one of a precoding vector or a combination of matrices that produce interference equal to or less than ε or any precoding vector that the user terminal generates interference equal to or less than ε Or if the combination of the matrix is not found, the user terminal can only feed back the precoding vector or matrix for the serving base station. Similarly, if the base station and the user terminal feed back any one of a combination of precoding vectors or matrices that produce interference greater than ε or the combination of any precoding vector or matrix the user terminal generates interference greater than ε If not found, the user terminal can only feed back the precoding vector or matrix for the serving base station.

4. Feedback Information: There is some information that the interfering base station and the serving base station should receive feedback from.

Iii) The information relates to a codebook vector or matrix that maximizes the received signal power or some performance measure for the serving base station, and relates to a codebook vector or matrix that maximizes the received signal power or some performance measure for the interfering base station.

ii) The information relates to an average signal-to-interference-noise ratio (SINR) or some measure of performance. For example, the information represents an average SINR or some performance measure achievable when the serving base station uses a precoding vector or matrix fed back from the user terminal.

iii) The message ξ of each of the interfering base stations matches (calculated in step 3 using the configurable interference level).

iv) The information relates to SINR (or some performance measure) enhancement and ΔCQI for the case where the interfering base station uses a recommended set of precoding vectors or matrices.

For example, the ΔCQI is the SINR (or some performance measure) expected when the interfering base station uses the recommended set and the SINR (or some performance measure) expected when the interfering base station uses the recommended set. May be an error between

For example, the ΔCQI is the worst SINR (or some performance measure) for when the interfering base station uses the recommended set and the worst SINR for the case where the interfering base station does not use the recommended set. Or some measure of performance).

For example, the ΔCQI is the worst SINR (or some performance measure) for the interfering base station using the recommended set and the SINR (or expected) if the interfering base station does not use the recommended set. Some performance measure).

For example, the ΔCQI is the SINR (or some performance measure) expected when the interfering base station uses the recommended set and the worst SINR (or when the interfering base station does not use the recommended set). Some performance measure).

In the present invention, the user terminal to feed back information related to the interfering base station to the interfering base station. In particular, when the set is applied to a neighboring base station, the SINR (or some performance measure) together with information relating to a codebook vector or matrix for the interfering channel, a message ξ representing a set of recommended or restricted precoding vectors or matrices, is applied to that neighbor. Can be sent directly to the base station.

5. Information Forwarding: A serving base station forwards information related to a codebook vector or matrix or channel matrix for another interfering base station to the interfering base station. The message ξ indicating the recommended or restricted precoding vector or matrix set is reported to the interfering base station. Similarly, messages and SINR (or some performance measure) enhancements corresponding to the other base stations are forwarded to their scheduled base stations.

6. Base Station Codebook Selection: The interfering base station receives feedback information ΔCQI (ie, precoding codebook vector or matrix, message ξ and SINR (or some performance measure) improvement) when information is received from other base stations. A codebook vector or matrix is selected for transmission to a designated user terminal of the interfering base station.

For example, when a specific base station receives a request from another base station, the specific base station selects whether to follow the recommendation based on the SINR improvement report. If the particular base station decides to follow the recommendation, the base station will have to choose one precoding codebook vector or matrix from the set specified by message ξ. The specific base station can simply select the precoding codebook vector or matrix that maximizes the SINR from the user terminal it serves in the set.

When the specific base station receives a complex request from other base stations, the specific base station selects whether to follow the recommendation based on the SINR improvement report from the various base stations. The order of processing of the request may be determined based on the channel and ΔCQI between the base station and the terminal served by the specific base station itself.

According to an embodiment of the present invention, it is proposed to generate the feedback message ξ based on some distance measure that divides the precoding codebook vectors into two parts. Basically, the precoding codebook can be divided into two parts. One part (preferred set) includes a codebook vector or matrix that causes interference to a received signal smaller than ε, and the other part (limited set) contains the remaining codebook vector or matrix of the first set. The message ξ is a threshold for distinguishing the two sets according to different distance measurements as shown in FIG. 2.

In FIG. 2, f (H ₂₁ , P _i , ε) is a function for checking whether the precoding vector P _i satisfies a predetermined criterion. The P _i belongs to the preferred set S ₁ when the predetermined criterion is satisfied, but belongs to the limited set S ₂ when the predetermined criterion is not satisfied. For example, the predetermined criteria may be as follows.

Here, V is a filter of the user terminal.

When two sets are formed, the present invention may calculate the distance from one element in one particular set for a codebook vector or matrix that maximizes the interference power (or other performance measure) received at the user terminal. Here, the threshold value ξ can be used to distinguish the two sets.

For example, one of the distance measurements is in a correlation (in the device) between the precoding codebook vectors and the precoding codebook vector maximizing the interference received by the user terminal.

For example, the present invention may use a chordal distance to measure the distance between different precoding codebook matrices and determine the distance from the set of elements preferred in the precoding matrix that maximizes the interference power. You can set a threshold to maximize.

According to an embodiment of the present invention, the base station may configure the target value ε of the acceptable interference level and the SINR (or other performance measure) improvement target value ΔCQI during two steps (interference level generation). In step 2, under this situation, the base station transmits a message comprising a configurable threshold (indicating some interference level) and an SINR (or other performance measure) improvement target to user terminals. After receiving the information, the user terminal proceeds to step 3. In step 4 of feeding back information, user terminals whose SINR (or other performance measure) improvement is greater than the target value ΔCQI should report information related to interference coordination. UEs whose SINR enhancement does not satisfy the threshold will transmit a message indicating that they do not feed back information related to interference avoidance or that the target ΔCQI has not been satisfied.

Based on the information related to the average SINR or some other performance measure for these terminals, the base stations may decide to choose a different strategy to serve the terminals. For example, when the average SINR for the terminal is large, the base station may select nothing. In addition, when the average SINR for the terminal is small, the base station may select to reschedule the terminal in another resource block.

According to an embodiment of the present invention, the terminal may select the interference level and the SINR improvement target value by itself. That is, during the second step of generating the interference level, the base station transmits only a message indicating whether the terminals will participate in the interference avoidance procedure. The terminals construct the message ξ to be transmitted to the local interfering base station in step 3. The message ξ can be calculated in some way. For example, in the present invention, a value of ξ can be obtained based on the locally configured allowable interference level through other distance measurements. In this situation, each terminal may have a locally configured acceptable interference level ε. After measuring the channels for the interfering base station, the terminal divides the precoding codebook vectors and matrices into two sets, and calculates a threshold based on different distance measurements. Examples of distance measurements are in correlation with the chordal distance between different precoding codebook vectors and other precoding codebook matrices. Steps 4 through 6 remain unchanged.

According to an embodiment of the present invention, the base station may configure a set of user terminals participating in the interference avoidance procedure together with the SINR (or other performance measure) improvement target value. Under these circumstances, the base stations transmit a message to the user terminals including an SINR (or other performance measure) improvement target with an indicator of the user terminals to participate in the interference avoidance procedure in step 2. The user terminals generate a message ξ to be transmitted to the partially interfering base station in step 3. In step 4 of feeding back information, user terminals whose SINR (or other performance measure) improvement is greater than the target ΔCQI need to report information related to interference coordination. User terminals whose SINR (or other performance measure) enhancement does not meet the threshold will send a message indicating that they do not feed back the information related to interference avoidance or did not meet the target value? CQI. Based on the information related to the average SINR or some other performance measure for these terminals, the base stations may decide to choose a different strategy to serve the terminals. For example, if the average SINR (or some other performance measure) for the terminal is large, the base station may choose nothing. In addition, if the average SINR (or some other performance measure) for the terminal is small, the base station may choose to reschedule the terminal in another resource block.

3 illustrates a simulation result for a system for avoiding interference between adjacent cells based on a threshold value.

3 compares the performance between a system that does not adjust interference and a system that avoids interference between adjacent cells based on a PMI limiting system and a threshold. The performance measure used in the comparison is the average SINR value after adjustment. The X axis is the average SINR value of the channel and the Y axis is the average SINR value after the interference coordination procedure. Basically, the adjusted system is a system in which each base station selects a precoding codebook vector or matrix that maximizes the SNR of terminals it serves. Table 1 below shows the system parameters shown in the above results.

Parameters values System bandwidth 10 MHz Transmit antennas at eNBs 4 Receive antennas at UEs 1 (uncorrelated) Antenna Spacing at eNBs (wavelength) 0.5 Number of Interferer Cells One Channel Model SCM IoT model AWGN Channel Estimation Perfect Channel Estimation

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

1 is a diagram illustrating a configuration of a wireless communication system in which two base stations simultaneously communicate with two terminals;

2 is a diagram illustrating a procedure of a terminal for identifying a preferred set and a limited set in a wireless communication system according to the present invention; and

3 is a view showing a result of a performance comparison of a wireless communication system according to the present invention and the prior art.

Claims (1)

In the wireless communication system comprising a plurality of base stations that can communicate with a plurality of user terminals, Potential interference base stations that select different codebook vectors within a standardized codebook to avoid interference with each other, Wherein each base station selects codebook vectors or matrices such that its signal to transmit generates only minimal interference to other base stations of the same band.

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