JP2016213530A - Cell search method and user device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique in which a reception timing of a synchronization signal is appropriately detected.SOLUTION: In a radio communication system having first and second cells, a cell search method to be performed by a user device for communicating with a base station, includes: a reception step for receiving a first main synchronization signal used for a first cell and a second main synchronization signal used for a second cell, from a base station; a first estimation step for estimating a reception timing of the first main synchronization signal, based on a correlation between the first main synchronization signal and a replica signal of the first main synchronization signal; a second estimation step for estimating a frequency offset of the first cell, based on the reception timing of the first main synchronization signal; a third estimation step for estimating a frequency offset of the second cell, based on the frequency offset of the first cell; and a fourth estimation step for estimating a reception timing of the second main synchronization signal, using the frequency offset of the second cell.SELECTED DRAWING: Figure 11

Description

  The present invention relates to a cell search method and a user apparatus.

  In recent years, in order to efficiently accommodate traffic that dramatically increases in mobile communication networks, heterogeneous networks in which macro cells are overlaid on small cells have been studied.

  With heterogeneous networks, mobile communications networks use macrocells to guarantee coverage, while small cells installed in high traffic locations such as hotspots, large halls, shopping malls, etc., stay in place. However, it is possible to efficiently accommodate user devices that require high-speed communication (for example, see Non-Patent Document 1).

  Here, in the downlink of LTE (Long Term Evolution), an Orthogonal Frequency Division Multiple Access (OFDMA) scheme is adopted. The user apparatus performs a cell search for searching for a connection destination cell when power is turned on, that is, when a physical channel is set up. The user apparatus acquires a physical cell ID (PCI: physical Cell Identity) of the cell by cell search and performs radio frame timing synchronization.

  In LTE, a hierarchical synchronization signal suitable for OFDMA is defined so that cell search is performed efficiently. Specifically, the hierarchical synchronization signal includes a first synchronization signal (PSS: Primary Synchronization Signal) and a second synchronization signal (SSS: Secondary Synchronization Signal) (see, for example, Non-Patent Document 2).

  In the first step of the cell search, the user apparatus uses the PSS included in the radio signal transmitted from the base station to detect the PSS reception timing (that is, detect the symbol timing) and the physical cell ID. The physical cell ID in the group is detected. Next, in the second step, the user apparatus detects the radio frame timing and the physical cell ID group using the SSS included in the radio signal transmitted from the base station.

  FIG. 1 is a diagram showing a multiplexing structure of PSS and SSS in the time domain. The LTE radio interface is composed of a radio frame having a length of 10 ms and 10 subframes having a length of 1 ms in the time domain. Further, one subframe is composed of two slots having a length of 0.5 ms. The PSS is multiplexed on the last OFDM symbol of the first and eleventh slots in the radio frame. Also, the SSS is multiplexed with the penultimate OFDM symbol adjacent to the PSS. Further, the LTE radio interface has a plurality of subcarriers spaced at 15 kHz intervals in the frequency domain. For example, when the system bandwidth of the radio interface is 5 MHz, the radio interface is composed of 300 subcarriers.

  FIG. 2 is a diagram illustrating a multiplexed configuration of PSS and SSS in the frequency domain. PSS and SSS are transmitted in a 945 kHz band composed of 63 subcarriers existing in the center of the system bandwidth of the base station in the frequency domain. The PSS is continuously multiplexed on 62 subcarriers excluding the central DC subcarrier. On the other hand, the SSS is composed of two SSS sequences. The two SSS sequences are called SSC1 (Second Synchronization Code 1) and SSC2. SSC1 and SSC2 are alternately multiplexed for each subcarrier with respect to 62 subcarriers in which PSS is multiplexed.

3GPP TS 36.872 (V12.1.0) 3GPP TS 36.211 (V11.6.0)

  In the cell search processing procedure, the physical channel that the user apparatus first receives and demodulates is PSS. Therefore, it is desirable that the PSS sequence is basically a sequence common to all cells so that the user apparatus can easily demodulate the PSS. However, in a wireless communication system having a plurality of base stations (cell sites), when the same PSS sequence is used between different cells, the estimation of the PSS channel response used when the SSS sequence is in-phase combined in the frequency domain Accuracy will deteriorate. Therefore, in the LTE standard, three Zadoff-Chu sequences having a sequence length 62 represented by the following equation are used for PSS (see Non-Patent Document 2).

The variable M is a root value taking values 25, 29, and 34, and corresponds to three physical layer IDs (0, 1, 2), respectively. In LTE, 504 physical layer cell IDs are defined. The 504 physical layer cell IDs are grouped into 168 physical layer cell ID groups, and each group includes three physical layer IDs. The three PSS sequences represented by the above formulas correspond to three physical layer IDs in the physical cell ID group, respectively. The SSS sequence represents 168 physical cell ID groups.

  Normally, a macro cell having a three-cell (sector) configuration is often given three different types of PSS sequences. On the other hand, when a small cell is installed in a macro cell, several methods can be considered as a method of assigning a PSS sequence to the small cell. As an example, a method may be considered in which a PSS sequence different from that of an overlaid macro cell is assigned to a PSS sequence of a small cell. Moreover, when the said small cell is comprised from a some small cell (when a small cell group is comprised), the method by which a common PSS series is allocated to all the some small cells can be considered.

  Here, a heterogeneous network composed of a macro cell with a carrier frequency of 2 GHz and a small cell with a carrier frequency of 3.5 GHz is assumed. As the carrier frequency increases, the frequency offset due to the frequency error of the temperature compensated crystal oscillator (TCXO) of the user apparatus increases. Here, the frequency offset is a frequency generated based on a frequency of an actual radio signal transmitted from the base station and a synchronization signal output from the TCXO of the user apparatus (a frequency recognized by the user apparatus itself). It means a deviation. PSS and SSS are the first physical channels where the user equipment connects the radio link with the base station. That is, the user apparatus must perform a cell search before the TCXO oscillation frequency follows the frequency of the base station by automatic frequency control (AFC). That is, in a small cell using a carrier frequency of 3.5 GHz, the influence of the frequency offset on the initial cell search time in cell search is greater than that of a macro cell using a carrier frequency of 2 GHz.

  For example, assuming that the frequency error in the free-running mode of TCXO included in the user apparatus is 3 ppm, the frequency offset at the carrier frequency of 2 GHz is about 6 kHz. On the other hand, the frequency offset at the carrier frequency of 3.5 GHz increases to 10.5 kHz.

  FIG. 3 is a diagram illustrating frequency offset and correlation power in the PSS sequence. In addition, although the vertical axis | shaft in FIG. 3 is an autocorrelation, it replaces with correlation electric power here. Note that the route value of the PSS sequence shown in FIG. The correlation power is a square value of a signal obtained by integrating the time domain waveform of the received signal and the PSS sequence replica with a predetermined number of samples corresponding to a 1 FFT (Fast Fourier Transform) block section.

  When the frequency offset increases, the difference in chip timing between the received signal received from the base station and the PSS sequence replica increases, and the correlation power decreases. For example, as shown in FIG. 3, when the frequency offset is increased to about 8.5 kHz, it can be seen that the correlation power is reduced to about half compared to the case where the frequency offset is zero.

  FIG. 4 is a diagram illustrating the correlation power of the PSS sequence when there is a time shift due to the frequency offset (7.5 kHz). FIG. 5 is a diagram illustrating the correlation power of the PSS sequence when there is a time shift due to the frequency offset (8.5 kHz). FIG. 6 is a diagram illustrating the correlation power of the PSS sequence when there is a time shift due to the frequency offset (10.5 kHz). The route value of the PSS sequence shown in FIGS. 4 to 6, the correct reception timing (PSS reception timing to be detected by the user apparatus) is a position where the reception timing is 3324.

  As shown in FIG. 4, when the frequency offset is 7.5 kHz, the PSS correlation power peak corresponds to the correct reception timing. Assuming that the frequency error of the TCXO of the user equipment is 3 ppm, the frequency offset in the macro cell using the carrier frequency of 2 GHz is about 6 kHz. That is, it can be seen that even if the frequency offset is about 6 kHz, the user apparatus can correctly detect the reception timing (symbol timing) of the PSS.

  Similarly, as shown in FIG. 5, when the frequency offset is 8.5 kHz, it can be seen that the peak of the correlation power is reduced and is very close to the second correlation power. However, even in the case of FIG. 5, the peak of the correlation power corresponds to the correct reception timing.

  On the other hand, as shown in FIG. 6, when the frequency offset is 10.5 kHz, the peak of the correlation power does not correspond to the correct reception timing. That is, as is done in conventional LTE, in the method of detecting the correct reception timing using the maximum correlation power between the received signal and the PSS sequence replica, for example, like a small cell using a carrier frequency of 3.5 GHz It can be seen that, in a cell having a large frequency offset, the user apparatus has a low probability of correctly detecting the PSS reception timing.

  The disclosed technique has been made in view of the above, and an object thereof is to provide a technique that can appropriately detect the reception timing of a synchronization signal in a cell in which a high carrier frequency is used.

  The cell search method of the disclosed technology is a cell search method performed by a user apparatus that communicates with a base station in a wireless communication system having a first cell and a second cell. A receiving step for receiving a first main synchronization signal used in the cell and a second main synchronization signal used in the second cell; the first main synchronization signal and the first main synchronization signal; A first estimation step of estimating the reception timing of the first main synchronization signal from the correlation with the replica signal of the first, and the frequency of the first cell based on the reception timing of the first main synchronization signal A second estimating step for estimating an offset; a third estimating step for estimating a frequency offset of the second cell based on a frequency offset of the first cell; and a frequency of the second cell. off Using Tsu bets, estimates the reception timing of the second main synchronization signal has a fourth estimation step.

  Further, a user apparatus according to the disclosed technique is a user apparatus that communicates with a base station in a wireless communication system having a first cell and a second cell, and is used for the first cell from the base station. Receiving means for receiving the first main synchronization signal and the second main synchronization signal used for the second cell, a replica signal of the first main synchronization signal and the first main synchronization signal, From the correlation power of the first main synchronization signal, the reception timing of the first main synchronization signal is estimated, the frequency offset of the first cell is estimated based on the reception timing of the first main synchronization signal, and the first cell Estimating means for estimating the frequency offset of the second cell based on the frequency offset of the second cell and estimating the reception timing of the second main synchronization signal using the frequency offset of the second cell. .

  According to the disclosed technique, a technique capable of appropriately detecting the reception timing of a synchronization signal in a cell using a high carrier frequency is provided.

It is a figure which shows the multiplexing structure of PSS and SSS of a time domain. It is a figure which shows the multiplexing structure of PSS and SSS of a frequency domain. It is a figure which shows the correlation power in a frequency offset and a PSS sequence. It is a figure which shows the correlation electric power of a PSS series in case there exists a time shift resulting from a frequency offset (7.5 kHz). It is a figure which shows the correlation electric power of a PSS series in case there exists a time shift resulting from a frequency offset (8.5 kHz). It is a figure which shows the correlation electric power of a PSS series in case there exists a time shift resulting from a frequency offset (10.5kHz). It is a figure which shows the structure of the radio | wireless communications system which concerns on embodiment. It is a figure which shows the outline | summary of the cell search method which the user apparatus which concerns on embodiment performs. It is a figure which shows an example of a function structure of the user apparatus which concerns on embodiment. It is a figure which shows an example of a function structure of the base station which concerns on embodiment. It is a flowchart which shows an example of the process sequence (the 1) which concerns on embodiment. It is a figure which shows the object area of the partial correlation of the received signal received from the base station, and a PSS sequence replica. It is a flowchart which shows an example of the process sequence (the 2) which concerns on embodiment. It is a figure which shows the cumulative distribution function of an initial cell search time. It is a figure which shows the time characteristic of initial cell search time.

  Embodiments of the present invention will be described below with reference to the drawings. The embodiment described below is only an example, and the embodiment to which the present invention is applied is not limited to the following embodiment. For example, although the wireless communication system according to the present embodiment assumes a system based on LTE, the present invention is not limited to LTE and can be applied to other systems. In addition, in this specification and claims, “LTE” corresponds to not only a communication method corresponding to Release 8 or 9 of 3GPP but also Release 10, 11, 12, 13, or Release 14 or later of 3GPP. It is used in a broad sense that includes communication methods.

<Overview>
(Overall configuration of wireless communication system)
FIG. 7 is a diagram illustrating a configuration of the wireless communication system according to the embodiment. As shown in FIG. 7, the radio communication system according to the present embodiment is a radio communication system including a user apparatus 10, a base station 20 that forms a macro cell, and a base station 20 that forms a small cell. In the radio communication system according to the present embodiment, a small cell is overlaid on a macro cell. In the example of FIG. 7, one macro cell is shown, but this is also for convenience of illustration, and a plurality of macro cells may exist. In the example of FIG. 7, one small cell is shown, but this is also for convenience of illustration, and a plurality of small cells may exist. Further, the base station 20 that forms the small cell may be, for example, an RRH (Remote Radio Head) that is connected to the base station 20 that forms the macro cell by an optical fiber or the like.

  It is assumed that a carrier frequency higher than that of the macro cell is set for the small cell. In the radio communication system according to the present embodiment, the carrier frequency of the macro cell may be 2 GHz, for example. The carrier frequency of the small cell may be 3.5 GHz, for example.

  The user apparatus 10 has a function of communicating with the base station 20 and the core network and the like through radio. The user device 10 is, for example, a mobile phone, a smartphone, a tablet, a mobile router, or a wearable terminal. The user device 10 may be any user device 10 as long as the device has a communication function. The user device 10 includes a CPU such as a processor, a memory device such as a ROM, a RAM, or a flash memory, an antenna for communicating with the base station 20, an RF (Radio Frequency) device, and hardware resources such as a TCXO. Each function and process of the user device 10 may be realized by a processor processing or executing data or a program stored in the memory device. However, the user apparatus 10 is not limited to the hardware configuration described above, and may have any other appropriate hardware configuration.

  The base station 20 communicates with the user apparatus 10 through radio. The base station 20 includes a CPU such as a processor, a memory device such as a ROM, a RAM, or a flash memory, an antenna for communicating with the user device 10 and the like, a communication interface device for communicating with the adjacent base station 20 and the core network, and the like. It consists of hardware resources. Each function and process of the base station 20 may be realized by a processor processing or executing data or a program stored in a memory device. However, the base station 20 is not limited to the hardware configuration described above, and may have any other appropriate hardware configuration.

  FIG. 8 is a diagram illustrating an outline of a cell search method performed by the user apparatus according to the embodiment. 8 until user apparatus 10 according to the embodiment performs a cell search for a macro cell and a small cell, and estimates the radio frame timing and physical layer cell ID of the macro cell, and the radio frame timing and physical layer cell ID of the small cell, using FIG. An outline of the processing procedure will be described.

  In step S10, the user apparatus 10 estimates the PSS reception timing and the PSS sequence in the macro cell.

  In step S11, the user apparatus 10 estimates the frequency offset of the macro cell using the PSS reception timing estimated in the processing procedure of step S10. As described above, the frequency offset is a frequency generated based on the frequency of the actual radio signal transmitted from the base station 20 and the synchronization signal output from the TCXO of the user apparatus 10 (recognized by the user apparatus 10 itself). Frequency). That is, the user apparatus 10 estimates a deviation between the carrier frequency of the macro cell recognized by the user apparatus 10 itself and the carrier frequency of the radio signal transmitted from the base station 20. Hereinafter, the deviation between the carrier frequency of the macro cell recognized by the user apparatus 10 itself and the carrier frequency of the radio signal of the macro cell transmitted from the base station 20 is referred to as “macro cell frequency offset”. Similarly, the deviation between the carrier frequency of the small cell recognized by the user apparatus 10 itself and the carrier frequency of the radio signal of the small cell transmitted from the base station 20 is referred to as “small cell frequency offset”.

  In step S12, the user apparatus 10 estimates the small cell frequency offset based on the macro cell frequency offset estimated in the processing procedure of step S11.

  In step S13, the user apparatus 10 compensates the frequency offset estimated in the processing procedure of step S12 for the radio signal in the small cell, and uses the small cell radio signal in which the frequency offset is compensated. PSS reception timing and PSS sequence are estimated.

<Functional configuration>
(User device)
FIG. 9 is a diagram illustrating an example of a functional configuration of the user apparatus according to the embodiment. As illustrated in FIG. 9, the user apparatus 10 includes a signal reception unit 101, a signal transmission unit 102, a macro cell synchronization unit 103, a small cell synchronization unit 104, and a signal processing unit 105. FIG. 9 shows only functional units that are particularly related to the embodiment of the present invention in the user apparatus 10, and has at least a function (not shown) for performing an operation based on LTE. The functional configuration shown in FIG. 9 is only an example. As long as the operation according to the present embodiment can be performed, the function classification and the name of the function unit may be anything.

  The signal receiving unit 101 receives a radio signal transmitted from the base station 20. In addition, the signal reception unit 101 detects a carrier frequency of a macro cell or a small cell by searching each frequency band defined in LTE. In addition, the signal receiving unit 101 receives a radio signal including PSS and SSS by searching around the center frequency of the carrier frequency of the macro cell or the small cell. In the following description, a radio signal received by the signal receiving unit 101 may be referred to as a “received signal”.

  The signal transmission unit 102 generates a lower layer signal from the upper layer information and transmits it wirelessly.

  The macro cell synchronization unit 103 estimates the reception timing (symbol timing) using the PSS included in the radio signal of the macro cell and also estimates the PSS sequence (physical layer ID). In addition, the macro cell synchronization unit 103 estimates the frequency offset of the macro cell using the detected reception timing of the macro cell. In addition, the macro cell synchronization unit 103 estimates the radio frame timing using the SSS included in the radio signal of the macro cell and also estimates the SSS sequence (physical layer cell ID group).

  The small cell synchronization unit 104 estimates the small cell frequency offset using the macro cell frequency offset estimated by the macro cell synchronization unit 103. Further, the small cell synchronization unit 104 estimates the reception timing (symbol timing) using the estimated small cell frequency offset using the PSS included in the small cell radio signal, and also uses the PSS sequence (physical layer ID). ). Further, the small cell synchronization unit 104 estimates the radio frame timing using the SSS included in the radio signal of the small cell and also estimates the SSS sequence (physical layer cell ID group).

  The signal processing unit 105 performs various types of signal processing on signals from the lower layer to the upper layer received wirelessly. Various signal processing is, for example, a physical layer (PHY: Physical Layer), a MAC (Media Access Control) layer, an RLC (Radio Link Control) layer, a PDCP (Packet Data Convergence Protocol) layer, an RRC (Radio Resource Control) layer, etc. It is processing of.

(base station)
FIG. 10 is a diagram illustrating an example of a functional configuration of the base station according to the embodiment. As illustrated in FIG. 10, the base station 20 includes a synchronization signal generation unit 201 and a signal transmission unit 202. FIG. 10 shows only functional units that are particularly related to the embodiment of the present invention in the base station 20, and has at least a function (not shown) for performing an operation based on LTE. Further, the functional configuration shown in FIG. 10 is merely an example. As long as the operation according to the present embodiment can be performed, the function classification and the name of the function unit may be anything.

  The synchronization signal generation unit 201 generates a PSS signal and an SSS signal based on a PSS sequence and an SSS sequence set in the base station 20 in advance.

  The signal transmission unit 202 transmits a radio signal including the PSS signal and the SSS signal generated by the synchronization signal generation unit 201 based on the cell frequency and system band set in advance in the base station 20.

<Cell search procedure>
(Processing procedure (1)
FIG. 11 is a flowchart illustrating an example of a processing procedure (part 1) according to the embodiment. A processing procedure performed by the user device 10 according to the embodiment will be described with reference to FIG.

  In step S301, the signal reception unit 101 detects the carrier frequency of the macro cell by searching each frequency band defined in LTE.

  In the following processing procedure, it is assumed that the user apparatus 10 has two reception antennas, but the user apparatus 10 according to the embodiment may be the user apparatus 10 having one reception antenna. And the user apparatus 10 which has three or more receiving antennas may be sufficient.

  In step S302, the macro cell synchronization unit 103 estimates the PSS reception timing and PSS sequence of the macro cell from the PSS included in the macro cell reception signal received by the signal reception unit 101. For example, the macro cell synchronization unit 103 calculates the correlation power between the received signal received from the base station 20 and the PSS sequence replica using the following equation (2), and the reception timing and the PSS sequence that gives the largest correlation power are The macro cell PSS reception timing and the PSS sequence may be estimated.

  In equation (2), the sample index is an index for uniquely identifying each section obtained by equally dividing one OFDM symbol section by the number of FFT samples. In Expression (2), since the number of FFT samples is 512, a sample index of 0 to 511 is assigned to each of the sections obtained by equally dividing one OFDM symbol section.

  In Expression (2), the reception timing (μ) is, for example, a transmission timing shift (a symbol recognized by the user apparatus 10) from the transmission timing when viewed from the base station 20 with respect to the reception signal of the macro cell The time difference between the timing and the symbol timing recognized by the base station 20) and the propagation delay until the radio signal transmitted from the base station 20 is received by the user apparatus 10 Also good.

  In addition, the macro cell synchronization unit 103, for example, has a sample index as a first sample interval at an arbitrary timing (for example, a time when the user apparatus 10 starts a cell search) on the time axis in the received signal of the macro cell from the base station 20. It may be defined as a section corresponding to (n = 0), and the correlation power may be repeatedly calculated while increasing the value of the reception timing (μ).

  Macro cell synchronization section 103 uses equation (2) to generate power correlation profiles for three PSS sequences, and the reception timing and PSS that give the largest correlation power among the correlation power peaks that appear in the power correlation profile. The sequence may be estimated to be the PSS reception timing of the macro cell and the PSS sequence.

  The power correlation profile is a graph showing time (reception timing) and correlation power, for example, as shown in FIGS. For example, when there are a plurality of macro cells having the same PSS sequence, a plurality of correlation power peaks appear as shown in FIGS. In this case, in step S302, the macro cell synchronization unit 103 receives the reception signal and the PSS sequence that gives the largest correlation power from the base station 20 with the smallest path loss (that is, the reception signal to be detected by the user apparatus 10). It will be estimated that.

  In the following equation (2), it is assumed that the number of FFT samples is 512, but other FFT sample numbers (64, 128, 256, 1024, etc.) may be used.

In step S303, the macro cell synchronization unit 103 estimates the frequency offset of the macro cell from the PSS reception timing and the PSS sequence estimated in the processing procedure in step S302. For example, the macro cell synchronization unit 103 may estimate the frequency offset of the macro cell by the following equations (3) to (5).

  First, the macro cell synchronization unit 103 uses the following equation (3) to calculate each sample period and the latter half of the first half of the section corresponding to the PSS sequence replica (that is, one OFDM symbol section recognized by the user apparatus 10 based on TCXO). In each sample period, the partial correlation between the received signal received from the base station 20 and the PSS sequence replica is calculated. Subsequently, the macro cell synchronization unit 103 estimates the amount of phase rotation due to the frequency error of the TCXO from the calculated partial correlations in the first and second half sample sections according to the following equation (4). In Equation (3), it is assumed that the number of FFT samples is 512. Subsequently, the macro cell synchronization unit 103 estimates the frequency offset of the macro cell from the estimated phase rotation amount using the following equation (5).

The macro cell synchronization unit 103 calculates the correlation between the received signal received from the base station 20 and the PSS sequence replica by the following equation (6) instead of the equation (3) in the processing procedure of step S303. It may be. The following equation (6) is obtained by dividing a section corresponding to the PSS sequence replica (that is, one OFDM symbol section recognized by the user apparatus 10 based on TCXO) into Q blocks on the time axis, and then adding a block q (0 In each of the first sample period and the second sample period in ≦ q <Q, the partial correlation between the received signal received from the base station 20 and the PSS sequence replica may be calculated. In the following expression, an arbitrary value may be set as the number of FFT samples. For example, the number of FFT samples may be 512, 1024, or 2048.

Further, the macro cell synchronization unit 103 may estimate the amount of phase rotation caused by the frequency error of the TCXO by the following equation (7) instead of the equation (4).

By using Equation (6) and Equation (7), the amount of phase rotation in a short time interval can be estimated as the number (number of Qs) for dividing a section (1 FFT block) corresponding to a PSS sequence replica increases. The macro cell synchronization unit 103 can improve the accuracy (resolution) of the frequency offset of the measurable macro cell PSS.

  FIG. 12 is a diagram illustrating a target section for partial correlation between a received signal received from a base station and a PSS sequence replica. FIG. 12A shows calculation target sections when the macro cell synchronization unit 103 calculates partial correlations in the first and second half sample sections without dividing the section corresponding to the PSS sequence replica. FIG. 12B shows a partial correlation between the first half and the second half of the sample section when the macro cell synchronization unit 103 divides the section corresponding to the PSS sequence replica into two (ie, Q = 2) and q = 0 or 1. The calculation object section in the case of calculating is shown.

  In step S304, the macro cell synchronization unit 103 or the small cell synchronization unit 104 estimates the frequency error of the TCXO from the estimated frequency offset of the macro cell. For example, the small cell synchronization unit 104 may estimate the frequency error of the TCXO from the frequency offset of the macro cell using the following equation (8).

In step S305, the signal reception unit 101 detects the carrier frequency of the small cell by searching each frequency band defined in LTE.

  In step S306, the small cell synchronization unit 104 estimates the frequency offset of the small cell from the frequency of the small cell and the frequency error of the TCXO. The small cell synchronization unit may estimate the small cell frequency offset using, for example, the following equation (9).

In step S307, the small cell synchronization unit 104 estimates the PSS reception timing and the PSS sequence of the small cell from the PSS included in the small cell received signal received by the signal reception unit 101. The small cell synchronization unit 104 calculates the correlation between the small cell received signal compensated for the small cell frequency and the PSS sequence replica by the following equation (10), and detects the maximum peak of the correlation power. Thus, the PSS reception timing and the PSS sequence of the small cell may be estimated.

  Note that although the following equation (10) assumes that the number of FFT samples is 512, other FFT sample numbers (64, 128, 256, 1024, etc.) may be used.

In step S308, the macro cell synchronization unit 103 and / or the small cell synchronization unit 104 calculates the largest correlation power in the macro cell and the largest correlation power in the small cell calculated by the processing procedure in step S302 and the processing procedure in step S308. Which correlation power is larger is determined. When the correlation power of the macro cell is larger than the correlation power of the small cell, the macro cell synchronization unit 103 estimates the radio frame timing and the SSS sequence from the SSS included in the received signal of the macro cell received by the signal reception unit 101. . On the other hand, when the correlation power of the small cell is larger than the correlation power of the macro cell, the small cell synchronization unit 104 determines the radio frame timing and SSS from the SSS included in the reception signal of the small cell received by the signal reception unit 101. Estimate the series.

  For example, the macro cell synchronization unit 103 or the small cell synchronization unit 104 may estimate the radio frame timing and the SSS sequence by the following method.

  First, the macro cell synchronization unit 103 (or small cell synchronization unit 104) compensates the frequency offset of the macro cell (or small cell) for the received signal (time domain signal) of the macro cell (or small cell) including the PSS, The signal is converted into a frequency domain signal by fast Fourier transform (FFT).

  Subsequently, the macro cell synchronization unit 103 (or the small cell synchronization unit 104) generates an estimated value of the channel response at each subcarrier position by multiplying the frequency domain signal including the PSS by the complex conjugate of the PSS sequence replica. . At this time, the macro cell synchronization unit 103 (or the small cell synchronization unit 104) performs in-phase addition of the channel response estimation values of the constant subcarrier sections on both sides with the channel response at the subcarrier position of interest (generated) as the center. Then, an average value of the channel response at each subcarrier position may be generated. Thereby, the influence of a noise component can be reduced.

  Subsequently, the macro cell synchronization unit 103 (or the small cell synchronization unit 104) compensates for the small cell frequency offset with respect to the small cell reception signal (time domain signal) including the SSS, and performs fast Fourier transform (FFT). Convert to frequency domain signal.

  Subsequently, the macro cell synchronization unit 103 (or the small cell synchronization unit 104) multiplies the frequency domain signal including the SSS by the complex conjugate of the estimated value of the channel response at each subcarrier position, and further, the SSS sequence at each subcarrier position. After multiplying the complex conjugate of the replica (SSC1 sequence or SSC2 sequence replica), integration is performed for 31 subcarriers every other subcarrier for each subcarrier including SSC1 and interleaved SSC1. Thus, the correlation power of each of the subcarriers storing SSC1 and SSC2 is calculated, and these correlation powers are added in phase.

  The macro cell synchronization unit 103 (or the small cell synchronization unit 104) performs the above-described processing procedure with respect to the slot 1 and the slot 11, and adds the respective correlation powers in phase. As shown in FIG. 1, the slots 1 and 11 refer to the positions of the slots 1 and 11 among 20 slots existing in one radio frame.

  Subsequently, the macro cell synchronization unit 103 (or the small cell synchronization unit 104) obtains the correlation power of each SSS sequence by further adding in-phase the correlation powers of the two receiving antennas.

  The above procedure is performed for all SSS sequences, and the macro cell synchronization unit 103 (or small cell synchronization unit) is also performed by performing the above procedure for a received signal obtained by shifting the received signal from the base station 20 by 5 ms. 104) can estimate the SSS sequence with the highest correlation power (ie, estimate the physical layer cell ID group and radio frame timing).

  Note that the small cell synchronization unit 104 includes the PSS for the received signal from the base station 20 so that the above procedure is also performed for the received signal obtained by shifting the received signal from the base station 20 by 5 ms. This is because it cannot be estimated whether the slot to be recorded is slot 1 or slot 11 (only the symbol timing is estimated).

  The above procedure is expressed by the following formula (11).

The SSS sequence (SSC1 sequence and SSC2 sequence) is defined by the LTE standard and can be expressed by the following formula. A binary M sequence is used as the SSS sequence (see Non-Patent Document 2).

Note that the macro cell synchronization unit 103 and the small cell synchronization unit 104 perform the processing procedure of step S308 independently for each of the macro cell reception signal and the small cell reception signal, so that the user apparatus 10 performs macro cell synchronization. Of the correlation power of the SSS sequence of the macro cell and the correlation power of the SSS sequence of the small cell, which are independently calculated by the unit 103 and the small cell synchronization unit 104, the cell is connected to a cell having a large correlation power (the base station via the cell). 20). In this case, even when the magnitude of the received power from the base station 20 changes due to the change in the propagation path between the time when the PSS correlation power is calculated and the time when the SSS correlation power is calculated. The user equipment 10 can be connected to an appropriate cell.

  In addition, the macro cell synchronization unit 103 and the small cell synchronization unit 104 perform the processing procedure of step S308 independently for each of the macro cell reception signal and the small cell reception signal. You may make it connect to both (it communicates with the base station 20 via a macrocell and a small cell).

  As described above, according to the cell search processing procedure (part 1), even when the user apparatus 10 performs a cell search before the oscillation frequency of the TCXO follows the frequency of the base station 20 by automatic frequency control, The reception timing and PSS sequence of the small cell PSS can be estimated with high accuracy.

(Processing procedure (2)
FIG. 13 is a flowchart illustrating an example of a processing procedure (part 2) according to the embodiment. A processing procedure performed by the user device 10 according to the embodiment will be described with reference to FIG.

  The processing procedures in steps S401 to S406 are the same as the processing procedures in steps S301 to S306 in FIG.

  In step S407, the small cell synchronization unit 104 estimates a plurality (Λ) of PSS reception timings and PSS sequences of the small cells from the PSS included in the small cell reception signal received by the signal reception unit 101. The small cell synchronization unit 104 calculates, for example, the correlation between the small cell received signal compensated for the small cell frequency and the PSS sequence replica by the following equation (13), and the correlation power is increased in the descending order of the correlation power. May detect a plurality of (Λ) PSS reception timings and PSS sequences of a small cell.

  Note that the small cell synchronization unit 104 generates a power correlation profile for the three PSS sequences using Equation (13), and among the correlation power peaks appearing in the power correlation profile, the small cell synchronization unit 104 increases the correlation power in descending order. Plural (Λ) PSS reception timings and PSS sequences may be estimated.

  Note that the small cell synchronization unit 104 may generate a power correlation profile using the estimated Λ correlation powers. The power correlation profile is a graph representing time (reception timing) and correlation power, for example, as shown in FIGS.

In step S408, the small cell synchronization unit 104 determines the PSS reception timing of the Λ small cells, the small cell phase rotation amount corresponding to each of the PSS sequences, and the small cell phase rotation amount estimated from the TCXO frequency error. And the PSS reception timing and PSS sequence with the smallest phase rotation difference are estimated as the correct small cell PSS reception timing and PSS sequence.

  For example, the small cell synchronization unit 104 may estimate the correct PSS reception timing and PSS sequence of the small cell by the following procedure.

  First, the small cell synchronization unit 104 uses the equations (3) and (4) used in the processing procedure of step S403 (S303) to estimate the estimated PSS reception timing and PSS sequence of Λ small cells. For each, the phase rotation amount of the small cell is estimated.

  Subsequently, the small cell synchronization unit 104 estimates the amount of phase rotation of the small cell from the frequency error of the TCXO obtained in the processing procedure of step S404 (S304) by the following equation (14).

Subsequently, the small cell synchronization unit 104 estimates the reception timing and the PSS sequence of the PSS with the smallest difference in the amount of phase rotation by, for example, the following equation (15).

The phase rotation amount and the frequency error can be converted into each other by the following equation (16). That is, in step S408, the small cell synchronization unit 104 determines the PSS reception timing of the Λ small cells and the small cell frequency offset corresponding to each of the PSS sequences, and the small cell frequency offset estimated from the TCXO frequency error. And the PSS reception timing and PSS sequence closest to the small cell frequency offset estimated from the frequency error of the TCXO are estimated as the correct small cell PSS reception timing and PSS sequence. Good.

Since the processing procedure of step S409 is the same as the processing procedure of step S308, description thereof is omitted.

  As described above, according to the cell search processing procedure (part 2), even when the user apparatus 10 performs a cell search before the oscillation frequency of the TCXO follows the frequency of the base station 20 by automatic frequency control, The reception timing and PSS sequence of the small cell PSS can be estimated with high accuracy. Further, the user apparatus 10 can estimate the PSS reception timing and the PSS sequence of the small cell with high accuracy even when a plurality of PSS reception timings and PSS sequences of the small cell are detected.

(simulation result)
The result of having simulated the cell search time when the cell search processing procedure in this Embodiment is applied is shown.

  FIG. 14 is a diagram illustrating a cumulative distribution function (CDF) of the initial cell search time. FIG. 14 shows that even when the small cell frequency offset increases to about 18 kHz, the physical layer ID is detected with a high probability of 95% or more in the initial cell search time of 100 ms.

  FIG. 15 is a diagram illustrating time characteristics of the initial cell search time. In FIG. 15, the carrier frequency of the macro cell is 2 GHz, and the frequency error (ε) of TCXO is 3.0 ppm. FIG. 15 shows that the cell search time for realizing the physical layer ID detection probability with a probability of 96% in the range where the carrier frequency of the small cell is about 6 GHz is 100 ms or less.

<Effect>
As described above, according to the embodiment, in a radio communication system having a first cell and a second cell, a cell search method performed by a user apparatus that communicates with a base station, A receiving step for receiving a first main synchronization signal used in the cell and a second main synchronization signal used in the second cell; the first main synchronization signal and the first main synchronization signal; A first estimation step of estimating the reception timing of the first main synchronization signal from the correlation with the replica signal of the first, and the frequency of the first cell based on the reception timing of the first main synchronization signal A second estimating step for estimating an offset; a third estimating step for estimating a frequency offset of the second cell based on a frequency offset of the first cell; and a frequency of the second cell. Offset It is used to estimate the reception timing of the second primary synchronization signal, and the fourth estimation step cell search method with is provided.

  According to the user apparatus 10 that performs this cell search method, it is possible to appropriately detect the reception timing of the synchronization signal in a cell in which a high carrier frequency is used.

  The third estimating step estimates a frequency error of a reference oscillator included in the user apparatus based on the first frequency offset, and calculates the second frequency offset based on the estimated frequency error. You may make it estimate. In addition, the fourth estimation step includes the second main synchronization signal compensated for the frequency offset of the second cell and the correlation between the second main synchronization signal and the replica signal of the second main synchronization signal. The reception timing of the main synchronization signal may be estimated.

  As a result, the user apparatus 10 can accurately detect the PSS reception timing and the PSS sequence of the small cell even when the cell search is performed before the TCXO oscillation frequency follows the frequency of the base station by automatic frequency control. Can be estimated.

  The fourth estimation step includes a plurality of reception timings based on a correlation between the second main synchronization signal in which the frequency offset of the second cell is compensated and a replica signal of the second main synchronization signal. The second cell corresponding to each of the estimated plurality of reception timing candidates, and estimating the frequency offset of the second cell corresponding to each of the estimated plurality of reception timing candidates. A reception timing candidate having the smallest difference between the frequency offset of the second cell and the frequency offset of the second cell estimated in the third estimation step is selected, and the selected reception timing is Alternatively, the reception timing of the main synchronization signal may be estimated.

  Thereby, the user apparatus 10 can estimate the reception timing and the PSS sequence of the small cell with high accuracy even when a plurality of PSS reception timings and PSS sequences of the small cell are detected.

  In the second estimating step, the replica signal of the first main synchronization signal is divided into one or more sections on the time axis, and any one of the one or more sections is further divided in half. And calculating a partial correlation between the first main synchronization signal and the replica signal of the first main synchronization signal with respect to the first half portion and the second half portion, and calculating the first half portion correlation and the second half portion correlation. And calculating the phase rotation amount of the first main synchronization signal, and estimating the first frequency offset from the calculated phase rotation amount of the first main synchronization signal.

  Thereby, the user apparatus 10 can improve the accuracy (resolution) of the frequency offset of the PSS of the measurable macro cell.

  The cell search method further includes the largest correlation power among the correlation powers between the first main synchronization signal and the replica signal of the first main synchronization signal, the second main synchronization signal, and the second main synchronization signal. The largest correlation power among the correlation powers with the replica signal of the second main synchronization signal is compared, and the largest correlation among the correlations between the first main synchronization signal and the replica signal of the first main synchronization signal When the power is greater than the correlation power between the second main synchronization signal and the replica signal of the second main synchronization signal, the second sub-sync signal used for the first cell is received. A reception step; a time domain signal of the first main synchronization signal; and a time domain signal of the first sub synchronization signal; a frequency domain signal of the first main synchronization signal and the first sub synchronization signal, respectively. The first variable to convert to A frequency domain signal of the first subsynchronization signal multiplied by a complex conjugate of the channel response of each subcarrier estimated using the time domain signal of the first main synchronization signal, A first detection step for detecting a radio frame timing and a sequence of the first sub-synchronization signal from a correlation with the replica signal of the sub-synchronization signal; and the second main synchronization signal and the second main synchronization signal When the largest correlation power among the correlations with the replica signal is larger than the correlation power between the first main synchronization signal and the replica signal of the first main synchronization signal, the second power used for the second cell A third receiving step for receiving a sub-synchronization signal, a time-domain signal of the second main synchronization signal, and a time-domain signal of the second sub-synchronization signal, respectively, in the frequency domain of the second main synchronization signal Signal and said A second conversion step for converting the sub-synchronization signal to a frequency domain signal, and the second sub-multiplier multiplied by a complex conjugate of the channel response of each subcarrier estimated using the second main synchronization signal. And a second detection step of detecting a radio frame timing and a sequence of the second sub-synchronization signal from a correlation between the frequency domain signal of the synchronization signal and the replica signal of the second sub-synchronization signal. May be.

  The cell search method further includes a second reception step of receiving a first sub-synchronization signal used for the first cell, a time domain signal of the first main synchronization signal, and the first A first conversion step of converting the time domain signal of the sub-synchronization signal into the frequency domain signal of the first main synchronization signal and the frequency domain signal of the first sub-synchronization signal, respectively, A frequency domain signal of the first subsynchronization signal multiplied by a complex conjugate of the channel response of each subcarrier estimated using the time domain signal of the main synchronization signal; and a replica signal of the first subsynchronization signal; A first detection step for detecting a radio frame timing and a sequence of the first sub-synchronization signal, and a third reception step for receiving a second sub-synchronization signal used for the second cell, The second main synchronization The second time domain signal and the second sub-synchronization signal time domain signal are converted into the second main synchronization signal frequency domain signal and the second sub-synchronization signal frequency domain signal, respectively. The frequency domain signal of the second sub-synchronization signal multiplied by the complex conjugate of the channel response of each subcarrier estimated using the second main synchronization signal, and the second sub-synchronization You may make it have a 2nd detection step which detects a radio | wireless frame timing and a series of said 2nd sub synchronizing signal from the correlation with the replica signal of a signal.

  Thereby, the user apparatus 10 can detect the radio frame timing and physical cell ID of a macro cell or a small cell.

  As described above, according to the embodiment, in a radio communication system having a first cell and a second cell, a user apparatus that communicates with a base station, which is used by the base station for the first cell. Receiving means for receiving a first main synchronization signal and a second main synchronization signal used in the second cell; and a replica signal of the first main synchronization signal and the first main synchronization signal From the correlation power, estimate the reception timing of the first main synchronization signal, estimate the frequency offset of the first cell based on the reception timing of the first main synchronization signal, An estimation unit configured to estimate a frequency offset of the second cell based on a frequency offset, and to estimate a reception timing of the second main synchronization signal using the frequency offset of the second cell. Equipment provided It is.

  According to this user apparatus 10, it is possible to appropriately detect the reception timing of the synchronization signal in a cell in which a high carrier frequency is used.

<Supplement of embodiment>
Although the embodiments have been described above, the disclosed invention is not limited to such embodiments, and those skilled in the art will understand various variations, modifications, alternatives, substitutions, and the like. Although specific numerical examples have been described in order to facilitate understanding of the invention, these numerical values are merely examples and any appropriate values may be used unless otherwise specified. The classification of items in the above description is not essential to the present invention, and the items described in two or more items may be used in combination as necessary, or the items described in one item may be used in different items. It may be applied to the matters described in (if not inconsistent). The boundaries between functional units or processing units in the functional block diagram do not necessarily correspond to physical component boundaries. The operations of a plurality of functional units may be physically performed by one component, or the operations of one functional unit may be physically performed by a plurality of components. For convenience of explanation, the user apparatus and the base station have been described using functional block diagrams, but such an apparatus may be realized in hardware, software, or a combination thereof. Software operated by a processor included in a user apparatus and software operated by a processor included in a base station according to the embodiment of the present invention are random access memory (RAM), flash memory, read-only memory (ROM), EPROM, and EEPROM. , A register, a hard disk (HDD), a removable disk, a CD-ROM, a database, a server, or any other suitable storage medium. The present invention is not limited to the above embodiments, and various modifications, modifications, alternatives, substitutions, and the like are included in the present invention without departing from the spirit of the present invention.

  In the embodiment, PSS is an example of the first main synchronization signal or the second main synchronization signal. SSS is an example of a first sub-sync signal or a second sub-sync signal. The macro cell is an example of a first cell. The small cell is an example of a second cell. The signal receiving unit 101 is an example of a receiving unit. The macro cell synchronization unit 103 and the small cell synchronization unit 104 are examples of estimation means.

DESCRIPTION OF SYMBOLS 10 User apparatus 20 Base station 101 Signal receiving part 102 Signal transmitting part 103 Macro cell synchronizing part 104 Small cell synchronizing part 105 Signal processing part 201 Synchronization signal generating part 202 Signal transmitting part

Claims (8)

In a radio communication system having a first cell and a second cell, a cell search method performed by a user apparatus communicating with a base station,
Receiving from the base station a first main synchronization signal used for the first cell and a second main synchronization signal used for the second cell;
A first estimation step of estimating the reception timing of the first main synchronization signal from the correlation power between the first main synchronization signal and the replica signal of the first main synchronization signal;
A second estimation step of estimating a frequency offset of the first cell based on the reception timing of the first main synchronization signal;
A third estimation step, estimating a frequency offset of the second cell based on the frequency offset of the first cell;
Using the frequency offset of the second cell to estimate the reception timing of the second main synchronization signal; a fourth estimation step;
A cell search method comprising:   The third estimation step estimates a frequency error of a reference oscillator included in the user apparatus based on the frequency offset of the first cell, and based on the estimated frequency error, the frequency of the second cell The cell search method according to claim 1, wherein an offset is estimated.   The fourth estimation step includes calculating the second main synchronization signal from a correlation power between the second main synchronization signal in which the frequency offset of the second cell is compensated and a replica signal of the second main synchronization signal. The cell search method according to claim 1, wherein the reception timing of the synchronization signal is estimated. The fourth estimation step includes
From the correlation power between the second main synchronization signal in which the frequency offset of the second cell is compensated and the replica signal of the second main synchronization signal, a plurality of reception timing candidates are estimated,
Estimating a frequency offset of the second cell corresponding to each of the plurality of estimated reception timing candidates, and estimating a frequency offset of the second cell corresponding to each of the estimated plurality of reception timing candidates And a reception timing candidate having the smallest difference between the frequency offset of the second cell estimated by the third estimation step,
The cell search method according to claim 2, wherein the selected reception timing is estimated as the reception timing of the second main synchronization signal. The second estimation step includes
Dividing the replica signal of the first main synchronization signal into one or more sections on the time axis;
Partial correlation between the first main synchronization signal and the replica signal of the first main synchronization signal with respect to the first half portion and the second half portion obtained by further dividing any one of the one or more intervals into half To calculate
Calculate the amount of phase rotation using the calculated partial correlation of the first half and partial correlation of the second half,
The cell search method according to claim 1, wherein a frequency offset of the first cell is estimated from the calculated phase rotation amount. The cell search method according to any one of claims 1 to 5, further comprising:
Of the correlation power between the first main synchronization signal and the replica signal of the first main synchronization signal, the largest correlation power, and the second main synchronization signal and the replica signal of the second main synchronization signal Compare with the largest correlation power among the correlation power,
Among the correlations between the first main synchronization signal and the replica signal of the first main synchronization signal, the largest correlation power is the correlation between the second main synchronization signal and the replica signal of the second main synchronization signal. If greater than power,
A second receiving step of receiving a first sub-synchronization signal used in the first cell;
The time domain signal of the first main synchronization signal and the time domain signal of the first sub synchronization signal are respectively the frequency domain signal of the first main synchronization signal and the frequency domain signal of the first sub synchronization signal. A first conversion step to convert to
The frequency domain signal of the first sub-synchronization signal multiplied by the complex conjugate of the channel response of each subcarrier estimated using the time domain signal of the first main synchronization signal, and the first sub-synchronization signal A first detection step of detecting a radio frame timing and a sequence of the first sub-synchronization signal from the correlation with the replica signal of
Among the correlations between the second main synchronization signal and the replica signal of the second main synchronization signal, the largest correlation power is the correlation power between the first main synchronization signal and the replica signal of the first main synchronization signal If greater than
A third receiving step of receiving a second sub-synchronization signal used for the second cell;
The time domain signal of the second main synchronization signal and the time domain signal of the second sub synchronization signal are respectively the frequency domain signal of the second main synchronization signal and the frequency domain signal of the second sub synchronization signal. A second conversion step for converting to
A frequency domain signal of the second sub-synchronization signal multiplied by a complex conjugate of the channel response of each subcarrier estimated using the second main synchronization signal; and a replica signal of the second sub-synchronization signal; A second detection step of detecting a radio frame timing and a sequence of the second sub-synchronization signal from the correlation of:
A cell search method. The cell search method according to any one of claims 1 to 5, further comprising:
A second receiving step of receiving a first sub-synchronization signal used in the first cell;
The time domain signal of the first main synchronization signal and the time domain signal of the first sub synchronization signal are respectively the frequency domain signal of the first main synchronization signal and the frequency domain signal of the first sub synchronization signal. A first conversion step to convert to
The frequency domain signal of the first sub-synchronization signal multiplied by the complex conjugate of the channel response of each subcarrier estimated using the time domain signal of the first main synchronization signal, and the first sub-synchronization signal A first detection step of detecting a radio frame timing and a sequence of the first sub-synchronization signal from the correlation with the replica signal of
A third receiving step of receiving a second sub-synchronization signal used for the second cell;
The time domain signal of the second main synchronization signal and the time domain signal of the second sub synchronization signal are respectively the frequency domain signal of the second main synchronization signal and the frequency domain signal of the second sub synchronization signal. A second conversion step for converting to
A frequency domain signal of the second sub-synchronization signal multiplied by a complex conjugate of the channel response of each subcarrier estimated using the second main synchronization signal; and a replica signal of the second sub-synchronization signal; A second detection step of detecting a radio frame timing and a sequence of the second sub-synchronization signal from the correlation of:
A cell search method. In a wireless communication system having a first cell and a second cell, a user apparatus that communicates with a base station,
Receiving means for receiving, from the base station, a first main synchronization signal used for the first cell and a second main synchronization signal used for the second cell;
From the correlation power between the first main synchronization signal and the replica signal of the first main synchronization signal, the reception timing of the first main synchronization signal is estimated, and based on the reception timing of the first main synchronization signal Estimating the frequency offset of the first cell, based on the frequency offset of the first cell, estimating the frequency offset of the second cell, and using the frequency offset of the second cell, Estimating means for estimating the reception timing of the second main synchronization signal;
A user device.