KR20150074718A - Method and appratus for searching a cell based on interference cancellation - Google Patents

Abstract

The present invention relates to a method for searching a cell by using interference cancellation and a device for the same. The method of the present invention comprises the steps of: receiving a signal for searching a cell from a base station of an adjacent cell; estimating a channel of an interference cell based on at least one of a first primary synchronization signal (PSS) and a first secondary synchronization signal (SSS) received from a pre-detected cell, from the received signal; cancelling interference by removing a second PSS and a second SSS received through the estimated channel of the interference cell; and searching a cell based on the signal having the interference removed therefrom.

Description

TECHNICAL FIELD [0001] The present invention relates generally to a cell search based on interference cancellation,

The present invention relates to a method and an apparatus for searching for a cell in a wireless communication system, and more particularly, to a cell search method using interference cancellation to detect a cell even when the RSRP difference in a multi- And a device therefor.

In general, a frame structure of LTE can be classified into a type 1 and a type 2. The type 1 is a structure based on a Frequency Division Duplexing (FDD) scheme and the type 2 is a structure based on a time division duplexing (TDD) scheme.

The FDD scheme refers to transmission of a radio frame with different uplink and downlink frequencies, and the TDD scheme refers to transmission of a radio frame divided into an uplink subframe and a downlink subframe on a time axis.

In a 3GPP LTE based Frequency Division Duplexing (FDD) cellular system, a terminal is a cell having a good radio quality in order to establish a communication environment with a base station based on a synchronization signal upon intermittent reception during power-on, standby, communication, And perform a cell search for a radio link connection.

In the cell search process, when a cell that transmits a signal with a weak signal intensity as compared with a signal strength of a serving cell (for example, a pico cell) is weak due to a signal of a serving cell Cells of signal strength were difficult to detect. Therefore, there is a need for an efficient cell search technique for detecting cells with weak signal strength.

Accordingly, the present invention proposes a cell search method and apparatus in a cellular system for detecting cells with weak signal strength.

It is an object of the present invention to provide a method and apparatus for searching a cell in a wireless communication system.

It is another object of the present invention to provide a method and apparatus for searching for a cell using interference cancellation in a wireless communication system.

It is another object of the present invention to provide a method and apparatus for searching for a cell using interference cancellation in a multi-cell environment in which a difference in RSRP between adjacent cells occurs in a specific dB or more in a wireless communication system.

It is another object of the present invention to provide a method and apparatus for searching for a cell using interference cancellation that removes an interference cell by removing PSS and SSS of an adjacent cell to be removed in a wireless communication system.

It is yet another object of the present invention to provide a method and apparatus for searching a cell using interference cancellation that estimates a channel of an interference cell by circularly shifting a transmission PSS in a wireless communication system and cross-correlating the received PSS with the received PSS. It has its purpose.

It is still another object of the present invention to provide a method and apparatus for searching for a cell using interference cancellation that estimates a channel of an interference cell using at least one of PSS and SSS in a wireless communication system.

It is another object of the present invention to provide an interference cancellation-based cell search method and apparatus for performing cell search by eliminating a plurality of interference using multiple windows in a wireless communication system.

The technical objects to be achieved by the present invention are not limited to the technical matters mentioned above, and other technical subjects which are not mentioned can be clearly understood by those skilled in the art from the following description. There will be.

According to an aspect of the present invention, there is provided a method of detecting a cell, the method comprising: receiving a signal for cell search from a base station of a neighboring cell; Estimating a channel of an interference cell based on at least one of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) received from a cell already detected in the received signal; Removing interference by deleting a second PSS (Primary Synchronization Signal) and a second SSS (Secondary Synchronization Signal) received through the channel of the estimated interference cell; And searching for a cell based on the interference-canceled signal. The present invention also provides a method of performing cell search using interference cancellation in a wireless communication system.

Also, in the present invention, a channel of the interference cell is estimated by a sum of a first channel and a second channel, the first channel represents a channel estimated using the first PSS, A channel estimated by using the first SSS, and a weight is applied to the first channel and the second channel.

Also, in the present invention, the channel of the interference cell is estimated using a cross correlation between a primary synchronization signal (PSS) and a reception PSS, and the primary synchronization signal (PSS) is circularly shifted ). ≪ / RTI >

In the present invention, the received signal includes a main interference signal and a sub interfering signal, wherein the main interference signal indicates an interference signal having the strongest signal strength among the received interference signals, and the sub- Wherein the step of estimating a channel of the interference cell comprises the step of estimating a channel of the interference cell using at least one of PSS and SSS of the main interference signal and the sub- And removing the interference may remove the PSS and SSS received from the channel of the estimated main interference signal and the channel of the estimated sub-interference signal, respectively.

The method may further comprise measuring RSRP of a neighboring cell from the received signal, wherein the interference cell is selected based on RSRP of the measured neighboring cell have.

In addition, in the present invention, the step of searching for the cell may include: amplifying the interference canceled signal; And searching for a cell based on the amplified signal.

Also, a terminal according to the present invention includes: a receiver for receiving a signal for cell search from a base station of a neighboring cell; A channel estimator for estimating a channel of an interference cell based on at least one of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) received from a cell already detected in the received signal; An interference canceller for removing a second PSS (Primary Synchronization Signal) and a second SSS (Secondary Synchronization Signal) received through the channel of the estimated interference cell; A cell searcher for searching for a cell from the interference canceled signal; And a controller for determining whether the interference is removed from the received signal.

Also, in the terminal according to the present invention, the channel of the interference cell is estimated by a sum of a first channel and a second channel, the first channel indicates a channel estimated using the first PSS, The channel may represent a channel estimated using the first SSS, and the weights of the first channel and the second channel may be weighted.

Also, in the present invention, the channel of the interference cell is estimated using a cross correlation between a primary synchronization signal (PSS) and a primary synchronization signal (PSS), and the primary synchronization signal (PSS) And can be circularly shifted.

Also, in the terminal according to the present invention, the interference canceller is composed of a PSS remover and an SSS remover, and the channel estimator, the PSS remover, and the SSS remover may perform a parallel interference cancellation, a successive interference cancellation, ) Or an iterative interference cancellation (Iterative IC).

In addition, in the mobile station according to the present invention, the apparatus may further include a measurer for measuring a reference signal received power (RSRP) of the neighboring cell, wherein the interference cell is selected based on the measured neighboring cell's RSRP .

The cell search method and terminal according to the present invention have the following effects.

According to the present invention, when a terminal searches for a neighboring cell in a wireless communication system, cell search is possible even in a multi-cell environment in which RSRP is different by more than a specific dB (for example, 9 dB) between adjacent cells.

According to the present invention, in a wireless communication system, when a terminal estimates a channel of an interference cell, PSS and SSS can be used together to utilize the advantages of PSS and SSS (for example, excellent autocorrelation characteristics of PSS).

According to the present invention, when a mobile station estimates a channel of an interference cell in a wireless communication system, the effect of ISI (Inter Symbol Interference) can be reduced by circularly shifting the transmission PSS by a predetermined number of samples.

According to the present invention, in a wireless communication system, when a terminal searches for an adjacent cell, a cell signal having a weak RSRP is amplified, thereby enabling detection of a cell signal having a weak RSRP.

According to the present invention, when a mobile station searches for a cell in an asynchronous environment in a wireless communication system, a plurality of interference signals can be removed using multiple windows, thereby preventing unnecessary handover.

FIG. 1 illustrates a cellular system in a wireless communication system to which the present invention is applied.
FIGS. 2 to 4 show the internal structure of a terminal to which the present invention is applied. FIG. 2 shows a structure of a terminal inside the terminal to which the present invention is applied. FIG. Respectively.
FIG. 4 illustrates a specific structure for removing an interference signal in a terminal to which the present invention is applied.
FIG. 5 is a flowchart illustrating a process of searching for a neighboring cell according to a first embodiment of the present invention.
FIGS. 6 to 10 show a second embodiment to which the present invention is applied. FIG. 6 is a view briefly showing a case where cell search is required using interference cancellation in a cellular system to which the present invention is applied, Shows a flowchart for explaining a process of searching for a cell using interference cancellation.
FIG. 8 is a flowchart illustrating a process of using PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal) to estimate a channel of an interference cell. FIG. 9 illustrates a process of circularly shifting a transmission PSS, And FIG. 10 is a flowchart illustrating a method of amplifying a signal having a weak transmission power with interference removed. Referring to FIG.
11 to 12 illustrate a third embodiment to which the present invention is applied. FIG. 11 shows a method for avoiding unnecessary handover by using multiple windows. FIG. 12 shows a case where a plurality of interference A specific example of a method of removing a signal is shown by a signal graph.

The above objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. It is to be understood, however, that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and similarities. Like reference numerals designate like elements throughout the specification. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

Hereinafter, an electronic apparatus related to the present invention will be described in more detail with reference to the drawings. The suffix "module" and " part "for the components used in the following description are given or mixed in consideration of ease of specification, and do not have their own meaning or role.

The electronic devices described herein may include a mobile phone, a smart phone, a laptop computer, a digital broadcasting terminal, a PDA (Personal Digital Assistants), a PMP (Portable Multimedia Player), and navigation. However, it will be understood by those skilled in the art that the configuration according to the embodiments described herein may be applied to a fixed terminal such as a digital TV, a desktop computer, and the like, unless the configuration is applicable only to a mobile terminal.

For convenience of description, it is assumed that a 3GPP LTE based cellular system is used in the present invention.

FIG. 1 illustrates a cellular system in a wireless communication system to which the present invention is applied.

Referring to FIG. 1, one example of a layout structure of a macro cell and a pico cell in a cellular system can be examined (100).

The conventional network arrangement method has a typical homogeneous network type centered on a macro cell. However, the arrangement of homogeneous networks is too complex, iterative, and the incidence of shaded areas in urban areas increases. Therefore, a heterogeneous network (100) has been discussed as a method for solving such a problem.

The heterogeneous network generally includes a macro cell 110, a picocell 120, a femtocell, and the like. In FIG. 1, a heterogeneous network including a macro cell 110 and a picocell 120 is illustrated. In this heterogeneous network, the macro cell base station 110 transmits a signal at a high transmission power of about 40W at 5W, and the pico cell base station 120 transmits a signal at a low transmission power of about 100mW to about 2W.

In the heterogeneous network, when the picocell 120 is introduced, the cell capacity improving effect becomes large when a large number of terminals receive data from the picocell 120. [ Therefore, it is necessary to induce terminals 130 to connect to picocell 120 rather than macrocell 110 if possible. For this reason, even if the received signal strength of the macrocell 110 signal is stronger than the received signal strength of the picocell 120, there is an extended region of the picocell 120 defined to connect to the picocell 120 base station.

However, in order to handover to the piconet 120 after receiving the service from the macrocell 110, the terminals 130 search for or detect the adjacent piconet 120 according to the movement of the terminal 130, The picocell 130 is synchronized with the picocell 120. When the received signal of the picocell 130 is different from the macrocell 110 by more than a specific dB (for example, 9 dB) It becomes difficult to detect a cell, and a method different from a general cell search is required.

FIG. 2 shows the entire structure of a terminal to which the present invention is applied.

Referring to FIG. 2, a terminal 200 receiving a service from a base station 210 includes a transmission / reception antenna 220, an RF module 230, a channel estimator 240, A controller 250, a demodulator 260, a controller 270, a memory 290, and a modulator 280. The components shown in FIG. 2 are not essential, and may be implemented in a terminal 200 having more or fewer components.

2, the transmitting and receiving antenna 220 is shown as one in the terminal 200, but may also include a plurality of antennas. Accordingly, the terminal 200 according to the present invention can support a multiple input multiple output (MIMO) system.

The antenna 220 may receive a signal transmitted from a neighboring base station 210 or another terminal and provide the received signal to a receiver 231. The signal transmitted from the base station 210 may include a downlink synchronization signal. The antenna 220 receives the downlink synchronization signal from the base station 210 and transmits the downlink synchronization signal to the receiver 231 to perform downlink synchronization .

The RF module 230 of the terminal 200 may transmit or receive a radio signal from the neighboring base station 210 or another terminal and the RF module 230 may transmit and receive radio signals from the receiver 231 and the transmitter 232).

The receiver 231 receives a signal transmitted from the base station 210 or another terminal. Herein, the received signal includes a synchronization signal transmitted from another neighboring cell and an interference signal received from a base station of another cell for the handover of the terminal 200. The terminal 200 transmits the synchronization signal to the RF module 230 through a receiver 231 and perform downlink synchronization.

The transmitter 232 may transmit a modulated signal from the modulator 280 to the base station 210 or another terminal. The modulated signal may be transmitted to the base station 210 for uplink synchronization, and the base station 210 and the terminal 200 may perform uplink synchronization using the transmitted synchronization signal.

Demodulator 240 demodulates the received signals and the demodulated signals may be passed to channel estimator 250 for channel estimation.

The channel estimator 250 can estimate a channel using the demodulated signal when the demodulator 240 demodulates the signal received through the receiver 231 of the RF module 230. [ The estimated channel includes channel estimation in the cell search for handover and channel estimation of the interference cell to remove the interference cell.

The interference canceller 260 may remove the interference signal received from other neighboring cells or terminals received through the receiver 231. [ The interference signal includes an interference signal received from a channel of the interference cell estimated by the channel estimator 250, and the interference canceller 260 removes the interference signal estimated from the channel estimator 250 to facilitate cell search .

The controller 270 directs all operations of the terminal (e.g., control, adjust, and manage) and can operate in conjunction with the respective devices. The controller can determine whether the channel estimator 250 is performing channel estimation and whether interference cancellation by the interference canceller 260 is eliminated.

The controller 270 may be referred to as a controller, a microcontroller, a microprocessor, or the like, and the controller 270 may be hardware, firmware, Or a combination of these.

The modulator 280 can function as a counterpart to the demodulator, that is, when receiving data to be transmitted from the controller, the modulator 280 can transmit the modulated data to the transmitter. The modulator 280 exchanges a signal received from the controller 270 with a modulated signal suitable for transmission, and modulates the modulated signal with a carrier wave.

The memory 290 is a medium for storing various kinds of information of the terminal and is connected to the controller 270 to temporarily store a program, an application, a general file, and input / output data for operation of the controller 270 .

The memory unit 290 may be a flash memory type, a hard disk type, a multimedia card micro type, a card type memory (for example, SD or XD memory) , A random access memory (SRAM), a read only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a programmable read-only memory (PROM) And may include one type of storage medium. The terminal 200 may operate in association with a web storage that performs a storage function of the memory unit 290 on the Internet.

FIG. 3 illustrates a structure for canceling an interference signal in a terminal to which the present invention is applied (300).

3, a structure for performing cell search using interference cancellation is implemented through a receiver 310, a channel estimator 320, an interference canceller 330, a cell searcher 340, and a controller 350 .

Specifically, the receiver 310 may receive a signal for cell search from an adjacent cell through an antenna. The received signal may include a synchronization signal as well as a signal received from all adjacent cells, and the signal thus included may act as an interference signal. Therefore, in order to remove the interference signal, the channel estimator 320 may estimate the channel of the interference cell using the PSS or the SSS of the adjacent interference cell that has been detected, and the interference canceller 330 may estimate the interference channel It is possible to remove the interference signal received from the channel of < RTI ID = 0.0 >

In order to estimate a channel of an interference cell, the channel estimator 320 may use various channel estimation techniques. In addition to a conventional channel estimation method for searching for a new cell, Signal, as well as Secondary Synchronization (SSS).

A signal from which interference is removed from the interference canceller 330 is transmitted to the cell searcher 340 and the cell searcher 340 can search neighboring cells for handover using the transmitted signal. The cell searcher 340 may detect not only a macro cell but also a pico cell and a femto cell.

The controller 350 may control each function in conjunction with the channel estimator 320, the interference canceller 330, and the cell searcher 340. For example, the controller 350 may instruct the channel estimator 320 to estimate a channel of an interference cell, and may determine whether the interference is removed from the interference canceller 330.

FIG. 4 illustrates a specific structure for removing an interference signal in a terminal to which the present invention is applied.

Referring to FIG. 4, the interference canceller 330 of FIG. 3 is composed of a PSS interference cancellation and an SSS interference canceller. ) and (c), respectively.

The interference channel estimator 420 may perform the same function as the channel estimator 320 illustrated in FIG. Therefore, the interference channel estimator 420 may estimate a channel of an interference cell in a signal received from a receiver.

The PSS remover 430 and the SSS remover 440 embody the interference canceller 330 shown in FIG. 3. When a channel of an interference cell is estimated from the interference channel estimator 420, 430 and the SSS remover 440 remove the PSS and the SSS received from the estimated channel, thereby eliminating the interference signal transmitted from the interference cell. The PSS remover 430 and the SSS remover 440 can use the IC (Interference Cancellation) technique to actively extract and remove interference from the received signal.

The structure of FIG. 4 (a) shows a parallel interference cancellation (PIC). In the parallel interference cancellation structure, interference cancellation estimation, PSS, and SSS elimination processes are connected in parallel, so that a previously canceled signal may not affect the next interference cancellation.

(b) and (c) show Successive Interference Cancellation (SIC) and Iterative Interference Cancellation (SIC). Unlike the parallel interference cancellation structure (PIC) of (a), the structures of (b) and (c) above may cause performance deterioration due to error propagation according to the order of the interference cancellation cells. That is, when an error is generated by removing an incorrect cell or an erroneous cell at the time of removing the first interference cell, the following processes are all affected by such an error, and the error is gradually added, and performance deterioration may occur. Therefore, the performance degradation should be minimized. The ordering block may determine whether to remove the detected interference cells by using the reliability of the detected cells in the process of removing the detected cells. Also, the order of removing the interference cells can be minimized by using various other metrics instead of simply determining the reception power.

In addition, (c) the structure (Iterative IC) can perform channel estimation, PSS removal, and SSS removal of the interference cells repeatedly, unlike the structure (b). By taking such a repetitive structure (Iterative), the influence of residual interference after interference cancellation can be minimized.

FIG. 5 is a flowchart illustrating a process of searching for a neighboring cell according to a first embodiment of the present invention.

Referring to FIG. 5, a mobile station can detect a PSS and an SSS to detect a neighboring cell and synchronize with the neighboring cell. When the mobile station synchronizes with a neighboring cell, a physical layer ID, a CP (Cyclic Prefix) can confirm. Through this process, the UE can confirm the new cell by checking RSRP (Reference Signal Received Power).

More specifically, the terminal can detect a primary synchronization (PSS) as a synchronization signal in a signal received from a neighboring cell (S510). Using the detected PSS, One cell ID can be found. The terminal may allocate a Zadoff-chu sequence in the frequency domain and a time domain sequence through IFFT (Inverse Fast Fourier Transform) operation. The PSS acquires the time synchronization of the downlink 5 msec by taking a cross-correlation of the time domain sequence corresponding to the three root indices (S520). If time synchronization of the downlink 5 msec is obtained (S520), SSS detection is enabled (S530), and the cell ID group can be found through the SSS (S540). An SSS can generate a sequence with two m-sequence combinations. The SSS is composed of 168 cell group IDs, and the downlink 10 msec time synchronization is obtained using the cross-correlation of the frequency domain sequence of the SSS, and a CP (Cyclic Prefix) mode and a cell ID group can be found S550).

In step S560, RSRP (reference signal received power) may be measured and validated using the cell-specified reference signal (CRS) for the detected cell ID group. The terminal acquires 40 msec time synchronization and cell parameters using a PBCH (Physical Broadcast Channel) for a cell having the largest RSRP value using the measured RSRP (S660), thereby performing cell search The terminal can perform handover to the selected cell.

The performance of such a cell detection depends on a threshold value determined as a candidate cell by using PSS and SSS. In the case of using a low threshold value, the cell detection performance is increased, but the complexity problem of measuring RSRP for many candidate cells May occur.

FIG. 6 schematically shows a case in which a search for an adjacent cell is required (600) using interference cancellation in a cellular system to which the present invention is applied.

6, if the terminal 640 moves out of the macro cell 610 while receiving a service from the macro cell 610, the terminal 640 transmits the macro cell 610 from the macro cell 610 You will no longer be able to receive the service you received.

Accordingly, the MS 640 can continue to receive services that have been received only after handover from the macrocell 610 to another cell. In order to perform such a handover, a cell to be handed over must first be searched. However, when the MS searches for such a handover target cell, other macrocells 630 that do not significantly differ in RSRP from the macrocell 610 receiving the service (for example, within 6dB) A small cell (for example, picocell 620) having a difference of RSRP from the macrocell 610 by a certain dB or more (for example, 9 dB) may be covered by the signal of the macrocell, It is difficult to navigate. (Hereinafter, the small cell will be described as an example of the picocell 620.)

Therefore, in order to search for the picocell 620, an interference cancellation (IC) based on a cell having a high RSRP among the already detected cells, directly removing the received interference signal, We need a way to search the cell. A method for searching for an IC-based cell includes estimating a strongest cell to transmit the strongest signal, removing the PSS and SSS received from the detected cell, May be used.

Such a cell search method based on the IC (Interference Cancellation) has a high complexity and severe performance degradation due to error propagation may occur. In addition, since the analog signals received from the multiple cells are converted into digital signals through ADC (Analog to Digital Converter), they have different SQNR (Signal to Quantization Noise Ratio) , We must overcome this.

FIG. 7 shows a flowchart for explaining a process of searching for a cell using interference cancellation.

Referring to FIG. 7, a procedure for searching for a cell using interference cancellation is described. First, a signal for cell search from a neighbor cell is received (S710). The received signal includes adjacent macrocells, Cell, etc., and other interfering signals may also be included.

The received signal may be used to estimate the channel of the interference cell already detected by the channel estimator (S720). At this time, the channel estimator can use various methods for estimating the channel of the interference cell. Since the channel estimation of the already detected cell is estimated, both PSS and SSS can be used.

Therefore, the channel estimator estimates the channel using the advantages of the PSS and the SSS, or estimates the channel of the interference cell using the PSS in which the PSS is circularly shifted by a predetermined number of samples, Of the channel estimation unit.

By thus estimating the channel of the interference cell, the signal from the adjacent interference cell can be removed by removing the PSS and SSS received from the channel estimated by the interference canceller. Such an interference canceller is described in detail in FIG. 3 and FIG.

If the interference is removed from the received signal, the cell searcher can search for neighboring cells again based on the interference canceled signal. At this time, the cell searcher can search for a cell using the cell search method described in the first embodiment. The PSS of the second embodiment is circularly shifted by an arbitrary number of samples, So that the cell can be searched.

FIG. 8 is a flowchart illustrating a process of using PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal) to estimate a channel of an interference cell.

Referring to FIG. 8, since channel estimation of an interference cell estimates a channel from an already detected cell, a channel can be estimated using both PSS and SSS, and only PSS or SSS can be used.

Generally, a cell searcher performs channel estimation using PSS for coherent SSS detection. However, since the PSS is divided into only three root indices, if the interference signal is removed using the channel obtained based on the PSS, the PSS component received from other cells can be partially removed. This can then lead to severe performance degradation in coherent SSS detection. On the other hand, when the arrival times of signals received from the neighboring cells are different from each other even when the PSS codes overlap, the PSS can greatly reduce the channel estimation performance deterioration due to excellent autocorrelation characteristics.

On the other hand, the SSS can be divided into a larger number of codes than the PSS, and the probability of using the same SSS between adjacent cells is low, but the cross correlation characteristic and the autocorrelation characteristic are less preferable than the PSS. In addition, the SSS is transmitted twice in the frame like the PSS, where the PSS repeats while the SSS has a different code combination both times. Accordingly, the channel estimation performance using the SSS can be changed according to the SSS code, the arrival time difference of the received signal, the channel model, and the like.

As described above, since PSS and SSS each have advantages and disadvantages according to the cell detection environment, the following channel estimation method can be used to consider both advantages of the PSS and the SSS.

Referring to the channel estimation method of the interference cell, the channel estimator can estimate the second channel using the SSS using the first channel (S810) using the PSS (S820). The channel estimation using the PSS and the SSS is not related to the order, and the channel estimation can be performed using the SSS after using the PSS first. Then, the channel estimation is performed using the SSS first, Channel estimation can be performed.

When the first channel and the second channel are estimated, a weight to be added to each channel may be determined (S830). The weights determine a weight occupied by the first channel and the second channel for interference cell estimation and vary according to the accuracy of the first channel and the second channel, and the weights may add up to one.

When the weight is determined, a weight may be added to the first channel and the second channel (S840). The channel of the interference cell can be estimated by summing the weighted first channel and the second channel (S850).

This can be expressed as follows.

와

는 각각 PSS와 SSS를 이용하여 추정된 i번째 간섭 셀의 제 1채널과 제2 채널을 나타낸다. 또한, w는 제 1채널(

)과 제 2채널(

)의 가중치를 나타내는 파라메터이며, PSS와 SSS를 이용하여 추정된 상기 제1채널과 상기 제2 채널의 정확도를 유추할 수 있는 사전 정보에 따라 값을 조절할 수 있다. 이때, 가중치는 subframe 0과 5에서 다른 값을 가질 수 있으며, 그 값은 SNR(Signal to Noise Ratio)등에 따라 달라질 수 있다.here

Wow

Represents the first channel and the second channel of the ith interference cell estimated using PSS and SSS, respectively. Further, w denotes a first channel (

) And the second channel

), And it is possible to adjust the value according to the prior information that can estimate the accuracy of the first channel and the second channel estimated using the PSS and the SSS. At this time, the weights may have different values in subframes 0 and 5, and the values may vary depending on the SNR (Signal to Noise Ratio) and the like.

In the LTE system, since the PSS is selected from 0, 1, and 2, the probability of overlapping PSS IDs between adjacent cells is much larger than SSS. Therefore, when the PSSs overlap, W is set to 0 so that only the channel estimated in the SSS can be used. Also, even when the PSS IDs do not overlap, since the cross-correlation characteristics between the IDs are different, W can be set differently according to the equation.

FIG. 9 shows a flow for estimating a channel of an interference cell using a cross-correlation with a reception PSS by circularly shifting a transmission PSS.

Generally, a channel estimation scheme using PSS uses a cross correlation between a reception PSS signal and a transmission PSS signal in a time domain, and a cross-correlation at a d-th sample position with respect to an i-th cell ID (CellID) is given as follows.

은 각각 수신 신호와 i번째 셀 아이디(CellID)에 대한 송신 PSS 시퀸스(sequence)를 나타낸다. 이때, 다중 페이딩 환경에서는 첫 번째 채널 임펄스 응답을 제외한 나머지 성분들은 ISI(Inter Symbol Interference)의 영향을 받아 성능 열화가 발생할 수 있다. 이는 이후 간섭 신호 제거 과정에서 심각한 오차 전파를 발생시킬 수 있으며, 셀 검출 성능을 열화 시킨다. 이를 완화하기 위하여, 송신 PSS 신호를 CP(Cyclic Prefix) 길이보다 짧은 임의의 샘플 수 α 만큼 원형 쉬프트(circular shift) 하여 상호 상관을 취할 수 있다.Where N is the length of the PSS sequence, r (n)

Represents a transmission PSS sequence for a received signal and an i-th cell ID (CellID), respectively. At this time, in the multi-fading environment, components other than the first channel impulse response may be affected by ISI (Inter Symbol Interference), which may cause performance degradation. This can cause serious error propagation in the process of removing the interference signal thereafter and deteriorate cell detection performance. In order to mitigate this, the transmission PSS signal can be cross-correlated by circularly shifting by a random number of samples? That is shorter than the CP (Cyclic Prefix) length.

To be specific, the MS can shift the transmission PSS by a predetermined number of samples (S910). In this case, the arbitrary number of samples? Refers to the number of samples shorter than the CP (Cyclic Prefix) length.

The channel of the interference cell can be estimated using the cross-correlation between the transmission PSS and the reception PSS shifted in this manner (S920), and the estimated channel value is subjected to post-compensation by an arbitrary number of circularly shifted samples? (S930). This can be implemented in such a way as moving the number of samples in the time domain or rotating the phase in the frequency domain. The estimated channel of the interference cell is removed from the interference canceller by removing the PSS and the SSS.

By expressing this in a mathematical expression,

 This can be expressed as follows.

The channel estimation method illustrated in FIG. 9 can be used not only for channel estimation of an interference cell but also for channel estimation in a cell search.

FIG. 10 shows a flowchart of a method of amplifying a signal having low transmission power with interference removed.

Referring to FIG. 10, in the case of a further enhanced Inter-Cell Interference Coordination (FEICIC) environment in which the difference in RSRP between adjacent cells is greater than or equal to a certain dB (for example, 9 dB), there is a large difference in power between signals received from the macrocell and the picocell . Thus, the received signal of a base station (e.g., a picocell or a femtocell base station) having a weak RSRP may be transmitted to an adjacent base station (e.g., a base station of a macrocell) in the ADC (Analog to Digital Converter) Signal to quantization noise ratio (SQNR).

In addition, in the Orthogonal Frequency Division Multiplexing (OFDM) system, in order to lower the complexity of the multipliers and adders in the internal calculation process of the FFT / IFFT (Fast Fourier Transform / Inverse Fast Fourier Transform), the SQNR is not significantly attenuated at every stage The bit is selected according to the distribution of the signal within the range. Therefore, the low-power signal in which the interference is canceled may be degraded in signal search performance due to the attenuation of the signal during the FFT operation. In order to overcome this problem, a post gain controller and a method for amplifying the signal size before the FFT and IFFT operations are required considering the power distribution of the received signal from which the interference signal is removed.

The method for amplifying the signal may include amplifying the interference canceled signal by removing the PSS and the SSS from the channel of the interference cell to remove the interference (S1010) S1020). The amplified signal is transmitted to the cell searcher, and the cell searcher can search neighboring cells based on the transmitted signal (S1030). The FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) techniques are used to prevent signal loss in the bit selection process of the FFT or IFFT internal calculation process using the power of the interference- Fast Fourier Transform) of the input signal.

11 schematically shows a method for avoiding unnecessary handover by using multiple windows.

Referring to FIG. 11, a terminal 1150 performs handover from a macro cell 1 1110 to a piconet 2 1140 of an adjacent macro cell 2 1120 (1100).

Generally, a method of searching for a cell using interference cancellation using a single window is performed by the terminal 1150 in the macro cell 1 (1110) serving as the piconet 2 1140 of the adjacent macro cell 2 (1120) The terminal 1150 must perform tracking and monitoring of the macro cell 2 1120 adjacent to the macro cell 1 1110 that has already been detected. The terminal 1150 can detect the pico cell 1 1130 by removing the PSS and SSS signals of the macro cell 1 1110 and the terminal 1150 can detect the pico cell 1 1130 Over.

The terminal 1150 which has performed the handover to the picocell 1 1130 can again perform the handover to the macrocell 2 (1120). The MS 1120 having handed over to the macrocell 2 1120 detects the BS of the piconet 2 1140 by removing the PSS and the SSS of the macrocell 2 1120, To perform handover.

In this handover method, the piconet 2 1140 can be detected only by performing handover to the macrocell 2 1120. This is because, when a single window is used, PSS and SSS of a base station which is received at a time different from that of a serving cell can not be detected, and thus the piconel 2 1140 can not be detected in advance. When the terminal 1150 moves because the service range of the adjacent macro cell 2 1120 is wider than the service range of the second piconet 1140, It enters the service area of the second cell 1120 and can not detect the second cell 1120 before the second cell 1120. [

11, a method of searching interference cancellation based cells having two or more multiple windows removes PSS and SSS of the macro cell 1 1110 and the macro cell 2 1120 at a time, It is possible to perform handover to the piconet 2 1140 without unnecessary procedures.

Specifically,

(a) The terminal 1150 can perform tracking and monitoring of the macro cell 2 (1120) base station adjacent to the macro cell 1 1110 already detected.

(b) Removing both the PSS and SSS signals of the macro cell 1 1110 and the neighboring macro cell 2 1120 using multiple windows to remove the pico cell 1 1130 and the pico cell 2 1140 base station And the terminal 1150 can perform a handover to the piconet 1 1120 base station.

(c) The MS may perform a handover to the piconet 2 1140 base station by omitting handover to the macrocell 2 1120 base station.

When using such multiple windows, handover to the macro cell 2 (1120) is unnecessarily unnecessary, so that the process of handing over to the piconet 2 (1140) can be reduced.

FIG. 12 is a signal graph showing a specific embodiment of a method for removing a plurality of interference using multiple windows in a cell search method to which the third embodiment is applied.

A cell search method to which the present invention is applied can eliminate a plurality of interference by using multiple windows for a signal received from a base station of a neighboring cell. For example, in order to search for a plurality of small cells (for example, a picocell or a femtocell) at the time of handover, a plurality of windows can be applied when a plurality of interference signals of neighboring macro cells are to be removed.

Specifically, a plurality of interference cell channels can be estimated by applying the multiple windows, and a plurality of interference signals can be removed by removing PSS and SSS received through the estimated plurality of channels, respectively. Thus, the neighboring small cell can be searched without going through a separate handover. Meanwhile, interference can be removed even when the plurality of interference cells are found at different points in time.

12, it is assumed that a terminal receives a service in synchronization with a macro-A cell in which a power of a received signal is largest, and a black solid line graph And shows the cross-correlation result for the synchronization signal.

The interference signal having the greatest signal intensity may be a signal coming from a Serving Cell receiving service, or a signal coming from another adjacent macro cell (for example, Macro-B Macro-C). Hereinafter, an interference signal having the highest signal strength is referred to as a main interference signal, and an interference signal having a weaker intensity as a specific dB than the main interference signal is referred to as a sub-interference signal. For example, referring to FIG. 12, a signal received from the macro-A cell may be referred to as a main interference signal, and a signal received from the macro-B cell or the macro-C cell may be referred to as a sub- .

In a multi-cell environment, since a mobile station can acquire synchronization with a cell 6 dB weaker than a cell received with the largest power, the MS can obtain a macro-B and a macro-C cell, which are received at 6 dB less power than the macro- It is possible to acquire synchronization and manage adjacent cells.

In this case, the terminal needs to detect a pico-A cell received at a power 9 dB weaker than the macro-A cell in order to support feICIC, which is possible by cell search after interference cancellation within the window-A interval. However, it is impossible to detect the pico-B1 and pico-B2 cells included in the macro-B cell on a single window, which may cause the unnecessary handover described in Fig.

In this case, in order to prevent unnecessary handover, it is necessary to remove the sub interfering signal (macro-B interfering signal), and window-B can be used to eliminate the sub interfering signal. (Macro-B) using the PSS or SSS received from the interference cell (macro-B) in the window-B section and receives the interference channel The PSS and the SSS are removed, and the pico-B1 and pico-B2 cells can be detected by removing the sub interfering signal (macro-B signal).

If the picocell included in the macro-C cell is detected, the signal of macro-C can be removed by the same method using the window-C.

Here, the length of each window section for detecting the PSS and SSS can be set considering the length of the PSS or the SSS, the CP and the length of the multipath channel, and in the LTE system, Can be set to 5 to 6 usec each. Also, the number of multiple windows can be flexibly enabled or disabled by comparing measured values such as RSRP of a macro cell rather than being fixedly allocated for the power consumption efficiency of the terminal. In FIG. 12, xdB represents an RSRP difference between adjacent cells.

Activation of such multiple windows may be activated when there are other sub-interfering signals within a certain dB and the main interfering signal (macro-A signal). 12, the main interfering signal (macro-A signal) represents the strongest interfering signal, and macro-B and macro-C within a specific dB (for example, 6 dB) Signal exists. In this case, multiple windows can be activated, such as Windows-A, Windows-B, and Windows-C.

In the case of the specific dB, it may be varied according to the set value. If there is no interference signal of other interference cells within the specific dB, the multiple window is inactivated and a single window is applied to remove the interference.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. The present invention is not limited to the drawings.

110: macro cell 120: picocell
200: terminal 220: antenna
230: RF module 240: Demodulator
250: channel estimator 260: interference canceller
270: controller 280: modulator
290: Memory

Claims (11)

A method of searching for a cell using interference cancellation in a wireless communication system,
Receiving a signal for cell search from a base station of a neighboring cell;
Estimating a channel of an interference cell based on at least one of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) received from a cell already detected in the received signal;
Removing interference by deleting a second PSS (Primary Synchronization Signal) and a second SSS (Secondary Synchronization Signal) received through the channel of the estimated interference cell; And
Searching for a cell based on the interference canceled signal;
Gt; a < / RTI > cell. The method according to claim 1,
Wherein a channel of the interference cell is estimated by a sum of a first channel and a second channel,
Wherein the first channel represents an estimated channel using the first PSS, the second channel represents a channel estimated using the first SSS, the first channel and the second channel are weighted ≪ / RTI > The method according to claim 1,
The channel of the interference cell is estimated using a cross correlation between a primary synchronization signal (PSS) and a reception PSS, and the transmission primary synchronization signal (PSS) is circularly shifted by a specific number of samples How to navigate a cell. The method according to claim 1,
Wherein the received signal includes a main interference signal and a sub interfering signal,
Wherein the main interference signal indicates an interference signal having the strongest signal strength among the received interference signals and the sub interfering signal indicates an interference signal having a difference within a certain dB from the main interference signal,
Estimating a channel of the interference cell using at least one of a PSS and an SSS of the main interfering signal and the sub interfering signal,
Wherein removing the interference removes each PSS and SSS received from the channel of the estimated main interference signal and the channel of the estimated sub-interference signal. The method according to claim 1,
Measuring a reference signal received power (RSRP) of an adjacent cell from the received signal,
Wherein the interference cell is selected based on the measured RSRP of the neighboring cell. The method according to claim 1,
The step of searching for the cell comprises:
Amplifying the interference canceled signal; And
And searching for a cell based on the amplified signal. A receiver for receiving a signal for cell search from a base station of a neighboring cell;
A channel estimator for estimating a channel of an interference cell based on at least one of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) received from a cell already detected in the received signal;
An interference canceller for removing a second PSS (Primary Synchronization Signal) and a second SSS (Secondary Synchronization Signal) received through the channel of the estimated interference cell;
A cell searcher for searching for a cell from the interference canceled signal; And
And a controller for determining whether interference has been removed from the received signal. 8. The method of claim 7,
Wherein a channel of the interference cell is estimated by a sum of a first channel and a second channel,
Wherein the first channel represents an estimated channel using the first PSS, the second channel represents a channel estimated using the first SSS, the first channel and the second channel are weighted The cell search apparatus comprising: 8. The method of claim 7,
The channel of the interference cell is estimated using a cross correlation between a primary synchronization signal (PSS) and a primary synchronization signal (PSS), and the primary synchronization signal (PSS) is circularly shifted by a specific number of samples. The cell search apparatus comprising: 8. The method of claim 7,
Wherein the interference canceller comprises a PSS remover and an SSS remover,
Wherein the channel estimator, the PSS eliminator and the SSS eliminator are configured to have any one of a parallel interference cancellation, a successive interference cancellation, and an iterative interference cancellation. Device. 8. The method of claim 7,
Further comprising a meter for measuring a reference signal received power (RSRP) of the neighboring cell,
Wherein the interference cell is selected based on the measured RSRP of the neighboring cell.

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