Method and user equipment for receiving sidelink synchronisation signal for proximity service

A disclosure of the present specification provides a method for receiving a sidelink synchronisation signal (SLSS) for proximity service (ProSe). The method may comprise the steps of: receiving an SLSS from a neighbouring ProSe UE; and measuring a reference signal received power (RSRP) of the SLSS during a pre-determined measurement period. The measurement step can be executed assuming that the SLSS transmission from the neighbouring ProSe UE during the measurement period is not abandoned more than once. The measurement period can be extended if the SLSS transmission from the neighbouring ProSe UE is abandoned more than once.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to mobile communication.

Related Art

As disclosed in 3GPP TS 36.211 V10.4.0 (2011-12) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)”, a physical channel of LTE may be classified into a downlink channel, i.e., a PDSCH (Physical Downlink Shared Channel) and a PDCCH (Physical Downlink Control Channel), and an uplink channel, i.e., a PUSCH (Physical Uplink Shared Channel) and a PUCCH (Physical Uplink Control Channel).

Meanwhile, due to an increase user requirements for SNS (Social Network Service), communication among UEs physically close to each other, that is, D2D (Device to Device) communication is required.

Such D2D communication is also referred to as Proximity Service (ProSe). The LE performing the neighboring service is also referred to as a ProSe UE. A link between UEs used in the D2D communication is also referred to as a sidelink.

The ProSe UE transmits and receives a Sidelink Synchronization signal (SLSS). The ProSe UEs outside the coverage of the base station a ProSe UE within the coverage the base station as synchronization reference UEs. To this end, the ProSe UE outside the coverage of the base station measures the RSRP (reference signal received power) for the SLSS received from the ProSe UE within the coverage of the base station.

However, when the SLSS is not received in an interval for measuring the RSRP for the SLSS, there is a problem that the synchronization reference UE can not be selected.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to solve the above-mentioned problems.

To achieve the objection, according to the one embodiment of the present specification, there is provided a method for receiving a sidelink synchronization signal: (SLSS) for a proximity service (ProSe). The method may be performed by a ProSe user equipment (UE) and comprise: receiving a SLSS from a neighboring ProSe UE; and measuring a reference signal received power (RSRP) with respect to the SLSS. The measurement may be performed by regarding that the neighboring ProSe UE does not drop up to one a transmission of the SLSS during a measurement period. If the neighboring ProSe UE drops more than one the transmission of the SLSS during the measurement period, the measurement period may be extended.

The measurement period may be 400 ms, and the SLSS may be transmittable from the neighbor ProSe UE with a period of 40 ms.

The step of measuring may be performed by considering that the SLSS transmission from the neighboring ProSe UE is to be dropped up to 2% within 20 seconds.

The step of measuring may further comprise a step of determining whether the measured RSRP for the SLSS meets an accuracy of absolute reference and accuracy of relative reference.

The step of measuring may further comprise a step of selecting the neighboring ProSe UE as a synchronization reference UE, when the measured RSRP for the SLSS is greater than a predetermined minimum requirement by a predetermined hysteresis.

The step of selecting may further comprise a step of determining whether the neighboring ProSe UE is positioned within the coverage of a base station.

To achieve the objection, a disclosure of the present specification, may provide a User Equipment (UE) for receiving a Sidelink Synchronization signal (SLSS) for Proximity Service (ProSe). The UE may comprise a RF unit; and a processor configured to control the RF unit to receive a SLSS from a neighboring ProSe UE, and to measure a reference signal received power (RSRP) of the SLSS during a predetermined measurement period. The measurement may be performed by regarding that the neighboring ProSe UE does not drop up to one a transmission of the SLSS during a measurement period. The process may extend the measurement period, if the neighboring ProSe UE drops more than one the transmission of the SLSS during the measurement period.

According to the embodiment of the present disclosure, the aforementioned problem of the related art is solved.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, based on 3rd Generation Partnership Project (3GPP) long term evolution (LTE) or 3GPP LTE-advanced (LTE-A), the present invention will be applied. This is just an example, and the present invention may be applied to various wireless communication systems. Hereinafter, LTE includes LTE and/or LTE-A.

The expression of the singular number in the present invention includes the meaning of the plural number unless the meaning of the singular number is definitely different from that of the plural number in the context. In the following description, the term ‘include’ or ‘have’ may represent the existence of a feature, a number, a step, an operation, a component, a part or the combination thereof described in the present invention, and may not exclude the existence or addition of another feature, another number, another step, another operation, another component, another part or the combination thereof.

As used herein, ‘base station’ generally refers to a fixed station that communicates with a wireless device and may be denoted by other terms such as eNB (evolved-NodeB), BTS (base transceiver system), or access point.

As used herein, ‘user equipment (UE)’ may be stationary or mobile, and may be denoted by other terms such as device, wireless device, terminal, MS (mobile station), UT (user terminal), SS (subscriber station), MT (mobile terminal) and etc.

As seen with reference toFIG. 1, the wireless communication system includes at least one base station (BS)20. Each base station20provides a communication service to specific geographical areas (generally, referred to as cells)20a,20b,and20c.The cell can be further divided into a plurality of areas (sectors).

The UE generally belongs to one cell and the cell to which the UE belong is referred to as a serving cell. A base station that provides the communication service to the serving cell is referred to as a serving BS. Since the wireless communication system is a cellular system, another cell that neighbors to the serving cell is present. Another cell which neighbors to the serving cell is referred to a neighbor cell. A base station that provides the communication service to the neighbor cell is referred to as a neighbor BS. The serving cell and the neighbor cell are relatively decided based on the UE.

Hereinafter, a downlink means communication from the base station20to the UE10and an uplink means communication from the UE10to the base station20. In the downlink, a transmitter may be a part of the base station20and a receiver may be a part of the UE10. In the uplink, the transmitter may be a part of the UE10and the receiver may be a part of the base station20.

Meanwhile, the wireless communication system may be generally divided into a frequency division duplex (FDD) type and a time division duplex (TDD) type. According to the FDD type, uplink transmission and downlink transmission are achieved while occupying different frequency bands. According to the TDD type, the uplink transmission and the downlink transmission are achieved at different time while occupying the same frequency band. A channel response of the TDD type is substantially reciprocal. This means that a downlink channel response and an uplink channel response are approximately the same as each other in a given frequency area. Accordingly, in the TDD based wireless communication system, the downlink channel response may be acquired from the uplink channel response. In the TDD type, since an entire frequency band is time-divided in the uplink transmission and the downlink transmission, the downlink transmission by the base station and the uplink transmission by the terminal may not be performed simultaneously. In the TDD system in which the uplink transmission and the downlink transmission are divided by the unit of a sub-frame, the uplink transmission and the downlink transmission are performed in different sub-frames.

The operating band used in the above wireless communication system is as follows.

In this case, FUL_lowmeans the lowest frequency of an UL operating band. Furthermore, FUL_highmeans the highest frequency of an UL operating band. Furthermore, FDL_lowmeans the lowest frequency of a DL operating band. Furthermore, FDL_highmeans the highest frequency of a DL operating band.

Then, the band is grouped as follows.

Hereinafter, the LTE system will be described in detail.

FIG. 2Shows a Downlink Radio Frame Structure According to FDD of 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE).

The radio frame ofFIG. 2may be found in the section5of 3GPP TS 36.211 V10.4.0 (2011-12) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)”.

The radio frame includes 10 sub-frames indexed 0 to 9. One sub-frame includes two consecutive slots. Accordingly, the radio frame includes 20 slots. The time taken for one sub-frame to be transmitted is denoted TTI (transmission time interval). For example, the length of one sub-frame may be 1 ms, and the length of one slot may be 0.5 ms.

The structure of the radio frame is for exemplary purposes only, and thus the number of sub-frames included in the radio frame or the number of slots included in the sub-frame may change variously.

Meanwhile, one slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols. The number of OFDM symbols included in one slot may vary depending on a cyclic prefix (CP). One slot includes 7 OFDM symbols in case of a normal CP, and one slot includes 6 OFDM symbols in case of an extended CP. Herein, since the 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink (DL), the OFDM symbol is only for expressing one symbol period in a time domain, and there is no limitation in a multiple access scheme or terminologies. For example, the OFDM symbol may also be referred to as another terminology such as a single carrier frequency division multiple access (SC-FDMA) symbol, a symbol period, etc.

FIG. 3Illustrates the Architecture of a Downlink Radio Frame According to TDD in 3GPP LTE.

For this, 3GPP TS 36.211 V10.4.0 (2011-23) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, Ch. 4 may be referenced, and this is for TDD (time division duplex).

Sub-frames having index #1and index #6are denoted special sub-frames, and include a DwPTS (Downlink Pilot Time Slot: DwPTS), a GP (Guard Period) and an UpPTS (Uplink Pilot Time Slot). The DwPTS is used for initial cell search, synchronization, or channel estimation in a terminal. The UpPTS is used for channel estimation in the base station and for establishing uplink transmission sync of the terminal. The GP is a period for removing interference that arises on uplink due to a multi-path delay of a downlink signal between uplink and downlink.

In TDD, a DL (downlink) sub-frame and a UL (Uplink) co-exist in one radio frame. Table 1 shows an example of configuration of a radio frame.

‘D’ denotes a DL sub-frame, ‘U’ a UL sub-frame, and ‘S’ a special sub-frame. When receiving a UL-DL configuration from the base station, the terminal may be aware of whether a sub-frame is a DL sub-frame or a UL sub-frame according to the configuration of the radio frame.

FIG. 4Illustrates an Example Resource Grid for One Uplink or Downlink Slot in 3GPP LTE.

Referring toFIG. 4, the uplink slot includes a plurality of OFDM (orthogonal frequency division multiplexing) symbols in the time domain and NRB resource blocks (RBs) in the frequency domain. For example, in the LTE system, the number of resource blocks (RBs), i.e., NRB, may be one from 6 to 110.

The resource block is a unit of resource allocation and includes a plurality of sub-carriers in the frequency domain. For example, if one slot includes seven OFDM symbols in the time domain and the resource block includes 12 sub-carriers in the frequency domain, one resource block may include 7×12 resource elements (REs).

Meanwhile, the number of sub-carriers in one OFDM symbol may be one of 128, 256, 512, 1024, 1536, and 2048.

In 3GPP LTE, the resource grid for one uplink slot shown inFIG. 4may also apply to the resource grid for the downlink slot.

A carrier aggregation system is described hereinafter.

A carrier aggregation system aggregates a plurality of component carriers (CCs). A conventional definition of a cell is changed according to carrier aggregation. According to carrier aggregation, a cell may denote a combination of a downlink component carrier and an uplink component carrier or a downlink component carrier alone.

Further, in carrier aggregation, cells may be divided into a primary cell, a secondary cell, and a serving cell. A primary cell denotes a cell operating at a primary frequency, in which a UE performs an initial connection establishment procedure or a connection reestablishment procedure with a BS or which is designated as a primary cell in a handover procedure. A secondary cell denotes a cell operating at a secondary frequency, which is configured once RRC connection is established and is used to provide an additional radio resource.

As described above, the carrier aggregation system may support a plurality of component carriers (CCs), that is, a plurality of serving cells, unlike a single carrier system.

The carrier aggregation system may support cross-carrier scheduling. Cross-carrier scheduling is a scheduling method for performing resource allocation for a PDSCH transmitted through a different component carrier through a PDCCH transmitted through a specific component carrier and/or resource allocation for a PUSCH transmitted through a component carrier different from a component carrier basically linked with the specific component carrier.

On the other hand, a reference signal (RS) will be described below.

Generally, transmission information, e.g., data, is easily distorted or changed while being transmitted over a wireless channel Therefore, a reference signal is required to demodulate such transmission information without an error.

The downlink reference signal is divided into a cell-specific RS (CRS), a multimedia broadcast and multicast single frequency network (MBSFN) reference signal, a UE-specific RS (URS), a positioning RS (PRS), and a channel state information reference signal (CSI-RS). The CRS is a reference signal transmitted to all UEs in a cell and thus may be referred to as a common reference signal. The CRS may be used for channel estimation for CQI feedback and channel estimation for PDSCH. The MBSFN reference signal may be transmitted in a subframe allocated for MBSFN transmission. The URS may be referred to as a demodulation reference signal (DM-RS) with reference signals received by a specific UE or a specific UE group in the cell. The DM-RS is used mainly for data demodulation by a specific UE or a specific UE group. The PRS may be used to estimate a position of the UE. The CSI-RS is used for channel estimation for the PDSCH of the LTE-A UE. The CSI-RS is relatively sparsely placed in the frequency domain or time domain and may be punctured in the data domain of the normal subframe or MB SFN subframe.

FIG. 5Illustrates an Exemplary Pattern with which a CRS is Mapped to a RB, if a Base Station Uses a Single Antenna Port.

Referring toFIG. 5, R0denotes an RE to which the CRS transmitted by the antenna port number 0 of the base station is mapped.

The CRS is transmitted in all downlink subframes within the cell supporting PDSCH transmission. The CRS may be transmitted on antenna ports 0 through 3.

The resource element (RE) allocated to the CRS of one antenna port can not be used for transmission of another antenna port and should be set to zero. Also, in a multicast-broadcast single frequency network (MBSFN) subframe, the CRS is transmitted only in a non-MBSFN region.

In the mobile communication system, mobility support of the UE100is essential. Accordingly, the UE100continuously measures the quality of a serving cell currently providing services and the quality of a neighboring cell. The UE100reports the measurement results to the network at an appropriate time, and the network provides the best mobility to the UE through handover or the like. Often, measurements of this purpose are referred to as radio resource management (RRM).

Meanwhile, the UE100monitors a downlink quality of the primary cell (Pcell) based on the CRS. This is referred to as a Radio Link Monitoring (RLM). For the RLM, the UE100estimates the downlink quality and compares the estimated downlink quality with thresholds, e.g., Qout and Qin. The threshold value Qout is defined as a level at which the downlink cannot be stably received, which corresponds to a 10% error of the PDCCH transmission considering a PCFICH error. The threshold value Qin is defined as a level at which the downlink is too much reliable compared with the Qout, which corresponds to a 2% error of the PDCCH transmission considering the PCFICH error.

FIG. 6Illustrates a Measurement and a Measurement Report Procedure.

Referring toFIG. 6, when the serving cell200aand the neighboring cell200btransmit a cell-specific reference signal (CRS) to the UE100, respectively, the UE100performs measurement, through the CRS, and transmits an RRC measurement report message including the measurement result to the serving cell200a.

In this case, the UE100can perform measurement in the following three methods.

1) RSRP (reference signal received power): Indicates an average received power of all REs carrying CRSs transmitted over the entire band. It is also possible to measure the average received power of all REs carrying the CSI RS instead of the CRS.

2) RSSI (Received Signal Strength Indicator): Indicates a received power measured in the entire band. The RSSI includes all of signal, interference, and thermal noise.

3) Reference symbol received quality (RSRQ): Indicates the CQI and may be determined by an RSRP/RSSI depending on the measured bandwidth or subband. That is, the RSRQ means a signal-to-noise interference ratio (SINR). Since the RSRP does not provide sufficient mobility information, the RSRQ may be used instead of RSRP in handover or cell reselection.

The RSRQ may be calculated as RSRQ=RSSI/RSSP.

Meanwhile, the UE100receives a measurement configuration (hereinafter also referred to as “measconfig”) information element (IE) from the serving cell100afor the measurement. A message including the measurement configuration Information Element (IE) is referred to as a measurement configuration message. Here, the measurement configuration Information Element (IE) may be received via an RRC Connection Reconfiguration message. The UE reports the measurement result to the base station if the measurement result meets the reporting condition in the measurement configuration information. A message including a measurement result is referred to as a measurement report message.

The measurement configuration IE may include measurement object information. The measurement object information is information on an object to be measured by the UE. The measurement object includes at least one of an intra-frequency measurement object to be measured in a cell, an inter-frequency measurement object to be measured among cells, and an inter-RAT measurement object to be an inter-RAT measurement. For example, an intra-frequency measurement object indicates a neighboring cell having the same frequency band as a serving cell, an inter-frequency measurement object indicates an adjacent cell having a frequency band different from that of the serving cell, and an inter-RAT measurement object can indicate the adjacent cell of the RAT different from the RAT of the serving cell.

Specifically, the measurement configuration IE includes an IE (information element) as shown in the following table.

The Measurement objects IE includes measObjectToRemoveList indicating a list of measObjects to be removed and measObjectToAddModList indicating a list to be newly added or modified.

Meanwhile, the measGapConfig is used to configure or release a measurement gap (MG).

The measurement gap MG is an interval for performing cell identification and RSRP measurement on a different frequency from the serving cell.

Meanwhile, the UE100also receives a Radio Resource Configuration Information Element (IE) as shown.

The Radio Resource Configuration Dedicated Information Element (IE) is used for configuring/modifying/releasing a Radio Bearer, modifying a MAC configuration, and the like. The Radio Resource Configuration IE includes subframe pattern information. The subframe pattern information is information on a measurement resource restriction pattern on the time domain for measuring the RSRP and RSRQ for a serving cell (e.g., a primary cell).

On the other hand, the D2D communication expected to be introduced in the next generation communication system will be described below.

FIG. 7Illustrates a Concept of D2D (Device to Device) Communication Expected to be Introduced in the Next Generation Communication System.

Due to the increase in user requirements for SNS (Social Network Service), communication between UEs physically close to each other, that is, D2D (Device to Device) communication has been required.

In order to reflect the above-described requirements, as shown inFIG. 6, a method for directly enabling communication without intervention of the base station (eNodeB)200has been discussed, among UE #1100-1, the UE #2100-2, the UE #3100-3, UE #4100-4, or among UE #5100-5, UE #6100-6, UE #7100-7, and UE #8100-8. Of course, with the help of the base station (eNodeB)200, they can communicate directly between the UE #1100-1and the UE #5100-5. Meanwhile, the UE #3100-3or the UE #4100-4may serve as a repeater for the UE #1100-1and the UE #2100-2.

Meanwhile, D2D communication is also referred to as Proximity Service (ProSe). The UE performing the Proximity Service is also referred to as a ProSe UE. A link between UEs used in the D2D communication is also called a sidelink. The frequency band that can be used for the sidelink is as follows.

The physical channels used in the sidelink are as follows.

In addition, the physical signals used in the sidelink are as follows.

Demodulation Reference signal (DMRS)

The SLSS includes a primary sidelink synchronization signal (Primary SLSS) and a secondary sidelink synchronization signal (Secondary SLSS: SSLSS).

FIG. 8Illustrates a Process in which the UE #1100-1Shown inFIG. 7Selects a Synchronization Reference UE Based on a Sidelink Synchronization Signal (SLSS) from a Neighboring UE.

The UE #1100-1and the UE #2100-2shown inFIG. 8are positioned outside the coverage of the base station as shown inFIG. 8and the UE #3100-3and the UE #4100-4are positioned within the coverage of the base station.

The UE #3100-3and UE #4100-4positioned within the coverage receive SIB type 19 from the base station.

The SIB type 19 includes discSyncConfig as follows.

TABLE 7SIB Type 19discSyncConfigIndicates a configuration as to whether the UE is allowedto receive or transmit synchronization information. Thebase station (E-UTRAN) may configure discSyncConfigwhen it intends to allow for the UE to transmitsynchronization information using dedicated signaling.

The discSyncConfig includes SL-SyncConfig. The SL-SyncConfig includes configuration information for SLSS reception and SLSS transmission as shown in the following table.

TABLE 8SL-SyncConfig field descriptiondiscSyncWindowIndicates a synchronization window in whichvalue w1 the UE expects the SLSS. The valuecorresponds may be configured to w1 or w2. Therepresents 5 milliseconds, and the value w2to the length of the normal CP divided by 2.syncTxPeriodicIndicates whether the UE transmits the SLSSonce or periodically (e.g., every 40 ms)within each period of a detection signal transmittedby the UE. For periodic transmissions, the UEalso transmits a MasterInformationBlock-SL.syncTxThreshICIndicates the threshold used when in coverage.If the measured RSRP value for counterpartUE (recognized as a cell) selected forsidelink communication is lower than the threshold,then the UE may transmit the SLSS for sidelinkcommunication with the counterpart UE.txParametersIncludes parameters for configuration fortransmission

The UE #3(100-3) and the UE #4(100-4) receiving the SIB type 19 transmit the SLSS.

On the other hand, since the UE #2100-2is positioned outside the coverage of the base station, the UE #2100-2cannot receive the SIB type 19, and thus transmits the SLSS depending on a preset parameter.

The UE #1100-1detects and measures the SLSS from neighboring UEs for sidelink communication. Then, the UE #1100-1selects a synchronization reference UE (also referred to as a SyncRef UE). The synchronization reference UE is a ProSe synchronization source capable of transmitting a synchronization signal for ProSe.

In this manner, the UE #1100-1uses preset parameters for detection and measurement.

The parameters used in advance by the UE #1100-1and the UE #2100-2are SL-Preconfiguration as follows, and the SL-Preconfiguration includes information as shown in the following table.

TABLE 9SL-PreconfigurationcarrierFreqIndicates a carrier frequency to use for sidelinkoperationSL-PreconfigSyncPreset configuration for SLSS

The SL-PreconfigSync includes information as shown in the following table.

TABLE 10SL-PreconfigSyncsyncCP-LenCP length to be used for the SLSSsyncRefDiffHystIf the hysteresis value used for evaluating byrelatively comparing the synchronizationreference UE (SyncRef UE) is configured todB0, then it means 0 dB.syncRefMinHystIf the hysteresis value used for evaluating bycomparing the synchronization reference UE(SyncRef UE) with its absolute value isconfigured to dB0, then it means 0 dB.

Meanwhile, the UE #1100-1calculates the S-RSRP for the SLSS received from each UE, to select a synchronized reference UE (SyncRef UE), and determines whether the UE S-RSRP in coverage is selected as a candidate, even if the UE S-RSRP out of coverage is larger, when the UE S-RSRP in the coverage is above than the minimum value indicated in the syncRefMinHyst, and if the highest S-RSRP among the UE out of the coverage exceeds the minimum value indicated in the syncRefMinHyst, then the UE that has transmitted the highest S-RSRP is selected as the candidate of the synchronization reference UE, when the UE S-RSRP in the coverage is not above than the minimum value indicated in the syncRefMinHyst.

If the UE selected as the candidate further satisfies another condition, then the UE #1100-1finally selects the UE as a SyncRef UE.

Meanwhile, the UE #1100-1determines whether an SLSS having a higher S-RSRP is received. If the SLSS having a higher S-RSRP is received, then the UE #1100-1determines whether the higher S-RSRP is larger than the S-RSRP of the selected synchronization reference UE by the value indicated in the syncRefDiffHyst. If it is larger, then the UE that has transmitted the SLSS with the higher S-RSRP is reselected to the synchronization reference UE.

On the other hand, an Absolute S-RSRP Accuracy and Relative Accuracy of S-RSRP are required for the S-RSRP measurement on the intra-frequency.

The Absolute S-RSRP Accuracy is as follows.

The above Ês represents the received energy per RE in an effective part of symbol (i.e., excluding CP).

The Io represents the maximum power of the received signal including the signal and interference.

The Iot represents the power density of the total noise and interference in a specific RE.

Meanwhile, the Relative Accuracy of S-RSRP is defined as S-RSRP measured from another ProSe synchronization source compared to S-RSRP measured from one ProSe synchronization source. The Relative Accuracy of S-RSRP is as follows.

Absolute accuracy and relative accuracy for the S-RSRP measurements as described above are required. These measurement accuracies should all be satisfied within a predetermined measurement period (i.e., a predetermined number of measurements). According to the current standard document, the interval in which the measurement accuracy is to be satisfied is 5 subframes (i.e., the number of times for measurement accuracy is 5). For example, when there are 20 subframes receiving the SLSS, the S-RSRP measured in each of at least 5 subframes should satisfy the measurement accuracy. It uses 5 subframes of predetermined interval for the accuracy in RSRP measurement using existing CRS.

FIG. 9AIllustrates the Period in which the SLSS is Transmitted, andFIG. 9BIllustrates the Measurement of the SLSS.

Referring toFIG. 9A, the UE transmitting SLSS can transmit the SLSS every 40 ms (i.e., 4 frames or 40 subframes) indicated by syncTxPeriodic in the above table.

Referring toFIG. 9B, according to the current standard document, the UE #1100-1determines that a new UE can be detected within 20 seconds indicated by the Tdetect,SyncRef UE. At this time, the interval in which the UE #1100-1performs the measurement is 400 ms.

However, the current standard document allows that if it is difficult for the UE transmitting the SLSS to transmit, then the SLSS transmission can be dropped only up to 2% (20*2%=400 ms) within 20 seconds. However, the 400 ms in which the SLSS transmission is dropped, may overlap with the measurement period (400 ms). In this case, there is a problem that the measurement accuracy of the S-RSRP can not be satisfied.

<Description of the Present Invention>

Therefore, the present specification represents a solution for solving the above-mentioned problems.

Before solving the problems described above, it is necessary to first consider two items. One is the time for detecting the synchronization reference UE (i.e., the SyncRef UE), and the other is the interval for measuring the synchronization reference UE (i.e., the SyncRef UE).

First, the detection time will be described as follows.

First, when the UE #3(100-3), which is positioned within the coverage of the base station and can operate as a synchronization reference UE (SyncRef), does not perform uplink transmission to the base station, the time for detecting the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) by the UE #1100-1will be described as follows.

In general, the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) can transmit the SLSS every 40 ms. However, when the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) transmits the uplink to the base station, it can drop the SLSS transmission. This is because the transmission to the base station has a higher priority than the SLSS transmission. Considering that the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) can drop the SLSS transmission by a maximum of 2% within 20 seconds, opportunities for which the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) is detected by the UE #1100-1becomes 10 times (=20 seconds/40 ms×2%).

During the 10 times, there is a chance that the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) does not transmit the SLSS due to the uplink transmission to the base station. Considering the worst situation, i.e., during the 10 times of opportunities, in which the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) drops all transmissions of the SLSS due to the uplink being transmitted to the base station, the UE #1100-1may not have an opportunity to detect the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE). However, there may be a situation in which the UE #3100-3(that is, the synchronization reference UE or the SyncRef UE) transmits SLSS 5 times during the above 10 opportunities. At this time, it is necessary to consider whether 20 seconds indicated by the Tdetect,SyncRef UE, is still valid. In order to obtain answers, it is required to examine the possibility of this situation to be occurred. Considering a situation where the UE #3100-3(i.e., the synchronization reference UE or the SyncRef UE) does not perform uplink transmission to the base station within 20 seconds at all, the number of times that the UE #3100-3(that is, the synchronization reference UE or the SyncRef UE) transmits the SLSS, becomes 500 times. With respect to the UE #1100-1, assuming that UE #3(100-3) (i.e., synchronization reference UE or SyncRef UE)-1) drops to transmit up to 2%, then the UE #1100-1has 10 chances to measure the SLSS received every 40 ms in the measurement period (i.e., 400 ms). The possibility that the UE #1100-1selects the subframe in which the SLSS is not actually received, among the number of times 500 for which the SLL can be received, may be very low. On the other hand, consider the SLSS transmission of UE #2(100-2). Since the UE #2100-2is positioned outside the coverage of the base station, in order to receive the SLSS from the UE #2100-2operating as a synchronization reference UE (i.e., SyncRef UE), its SLSS transmission can be only drop up to 2% within 20 seconds. Therefore, the chance that the UE #2(100-2) does not transmit SLSS is very low.

Second, an interval for measuring the synchronization reference UE (i.e., SyncRef UE) will be described as follows. There are 10 subframes (=400 ms/40 ms) in which the UE #1100-1can measure the SLSS transmitted from the UE #3100-3capable of operating as the synchronization reference UE. Among the 10 subframes, it is required to examine the possibility that the UE #3100-3drops the transmission of the SLSS due to the uplink transmission to the base station. In the worst case, among the 10 subframes, the UE #3100-3may drop all transmissions of the SLSS. In this case, there is no possibility that the UE #1100-1succeeds in the measurement. Therefore, the UE #1(100-1) is required to prevent the UE #3(100-3) from performing the measurement in the subframe in which the SLSS transmission is dropped. That is, the UE #1100-1should perform measurement on the subframe in which the UE #3100-3transmits the SLSS transmission. Also, since the measurement accuracy should be satisfied in five subframes, the UE #1100should perform measurement on the subframe in which the SLSS is transmitted from the UE #3100-3at least five times. Otherwise, the measurement period should be increased. In other words, the UE #3100-3can drop or delay the SLSS transmission up to 5 times for 400 ms, which is the measurement period of the UE #1100-1. However, if averaging 10 opportunities (=20 sec/40 ms×2%) within 20 seconds, UE #3100-3will drop or delay the SLSS transmission for one opportunity (i.e., two seconds), then the UE #3100-3may drop or delay the SLSS transmission only once for 400 ms during the measurement period of the UE #1100-1.

Taken the above description together, the above can be suggested as follows.

Proposal 1: The ProSe UE can measure the S-RSRP on the subframe in which the SLSS is transmitted from the synchronization reference UE (i.e., SyncRef UE).

Proposal 2: The ProSe UE can skip the measurement of the S-RSRP on the subframe in which the synchronization reference UE (i.e., SyncRef UE) performs uplink transmission to the base station and drops the SLSS transmission.

Proposal 3: If the synchronization reference UE (i.e., SyncRef UE) is positioned within the coverage of the base station, then the synchronization reference UE should transmit SLSS at least 5 times within 400 ms of the measurement period of the ProSe UE, such that the UE can satisfy accuracy in measurement of the S-RSRP.

Proposal 4: If the synchronization reference UE (i.e., SyncRef UE) is positioned within the coverage of the base station and the synchronization reference UE (i.e., SyncRef UE) fails to perform SLSS transmission at least five times within the measurement period of the ProSe UE, then the measurement period can be increased to satisfy the measurement accuracy in the S-RSRP of the ProSe UE.

Proposal 5: In order to ensure that the UE receiving the SLSS satisfy the measurement accuracy of the S-RSRP, when the UE transmitting the SLSS is positioned outside the coverage, the UE transmitting the SLSS can drop or delay the SLSS transmission at least once within the measurement period.

Proposal 6: If a test case is defined for the measurement accuracy of the S-RSRP for the SLSS from the synchronization reference UE (i.e., SyncRef UE) positioned within the coverage of the base station, then the transmission to the base station for 400 ms during the measurement period, may be defined to be performed depending on a transmission pattern (e.g., 0101010101, where 1 means transmission to the base station).

Proposal 7: If a test case is defined for the measurement accuracy of the S-RSRP for the SLSS from the UE positioned outside the coverage of the base station, then the transmission to the base station during the measurement period of 400 ms, may be assumed to be only dropped or delayed at least once.

On the other hand, the following items are required to be additionally considered for proposals 3 and 4 above.

The interval in which the measurement accuracy of the S-RSRP is to be satisfied (i.e., 5 subframes) utilizes the predetermined interval for the RSRP measurement accuracy using the existing CRS. However, when the number of REs of CRS existed in one subframe is compared with the number of REs of SLSS (including DMRS), then it is SLSS/CRS=6*2/4=3 times. That is, the number of REs in the SLSS existed in one subframe is three times larger than the number of REs in the CRS. Therefore, considering 5 times/3 times=1.7=about 2 times, the proposal 3 can be modified as follows.

Proposal 3-1: If the SyncRef UE is positioned within the coverage of the base station, then the UE should transmit the SLSS at least twice, within the measurement period of 400 ms of the ProSe UE so that the ProSe UE can satisfy the measurement accuracy of the S-RSRP.

Accordingly, the proposal 4 can be modified as follows.

Proposal 4-1: If the synchronization reference UE (i.e., SyncRef UE) is positioned within the coverage of the base station, and the synchronization reference UE (i.e., SyncRef UE) do not perform the SLSS transmission at least twice within the measurement period of the ProSe UE, then the measurement period may be increased to satisfy the measurement accuracy of the S-RSRP of the ProSe UE.

The above-mentioned proposals are summarized in the drawings.

FIG. 10is an Exemplary Diagram Illustrating a Proposal According to the Disclosure of the Present Disclosure.

Referring toFIG. 10, the above-described proposals will be summarized as follows.

Proposal A: A UE capable of performing ProSe direct communication, such as UE #2100-2or UE #3100-3, should be able to transmit the SLSS at least 5 times during the measurement period of 400 ms. Otherwise, the measurement period of the UE receiving the SLSS, e.g., UE #1100-1, is increased. Therefore, the UE that transmits the SLSS, i.e., the UE #2100-2or the UE #3100-3, can drop or delay the SLSS transmission up to once during the measurement period.

Proposal A-1: It is assumed that a UE capable of performing ProSe direct communication, that is, UE #1100-1is a UE that transmits the SLSS during the measurement period of 400 ms, that is, UE #2100-2or UE #3100-3transmits the SLSS at least 5 times. Otherwise, the measurement period of the UE receiving the SLSS, i.e., UE #1100-1, is increased. Therefore, the UE receiving the SLSS, that is, the UE #1100-1, assumes that the UE transmitting the SLSS, i.e., the UE #2100-2, or the UE #3100-3can drop or delay the SLSS transmission within the measurement period.

Proposal B: The UE capable of performing ProSe direct communication, that is, UE #2100-2or UE #3100-3should be able to transmit the SLSS at least twice during the measurement period of 400 ms. Otherwise, the measurement period of the UE receiving the SLSS is increased. Therefore, the UE that transmits the SLSS, i.e., the UE #2100-2or the UE #3100-3, can drop or delay the SLSS transmission up to once during the measurement period.

The embodiments of the present invention described so far may be implemented by various means. For example, embodiments of the present invention may be implemented by hardware, firmware, software, or a combination thereof. More specifically, the description will be made with reference to the drawings.

FIG. 11is a block diagram illustrating a wireless communication system in which the present disclosure of specification is implemented.

The base station200includes a processor201, a memory202and an RF unit (radio frequency (RF) unit203). The memory202is coupled to the processor201and stores various information for operating the processor201. The RF unit203is coupled to the processor201to transmit and/or receive a radio signal. The processor201implements the proposed functionality, process and/or method. In the above-described embodiment, the operation of the base station can be implemented by the processor201.

The UE100includes a processor101, a memory102and an RF unit103. The memory102is connected to the processor101and stores various information for driving the processor101. The RF unit103is coupled to the processor101to transmit and/or receive a radio signal. The processor101implements the proposed functions, procedures and/or methods.

The processor may comprise an application-specific integrated circuit (ASIC), other chipset, logic circuitry and/or data processing device. The memory may include read-only memory (ROM), random access memory (RAM), flash memory, memory cards, storage media, and/or other storage devices. The RF unit may include a baseband circuit for processing the radio signal. When the embodiment is implemented in software, the above-described techniques may be implemented with modules (processes, functions, and so on) that perform the functions described above. The module is stored in memory and can be executed by the processor. The memory may be internal or external to the processor and may be coupled to the processor by any of a variety of well known means.

In the exemplary system described above, although the methods are described on the basis of a flowchart as a series of steps or blocks, the present invention is not limited to the order of the steps, and some steps may occur in different orders with different steps or simultaneously. It will also be appreciated by those skilled in the art that the steps shown in the flowchart are not exclusive and that other steps may be included or that one or more steps in the flowchart may be eliminated without affecting the scope of the invention.