METHOD AND DEVICE FOR CONTROLLING TRANSMISSION POWER OF TERMINAL IN D2D COMMUNICATION

Abstract

The present invention relates to a method for controlling the power of a terminal in device-to-device (D2D) communication and, particularly, to a device and a method for supporting Type 1 discovery or Mode 2 D2D communication. The present invention relates to a method for controlling the transmission power of a terminal performing D2D communication, and the terminal, the method comprising the steps of: selecting an arbitrary resource in a discovery period for D2D resource selection; determining whether the selected resource satisfies a preset transmission power control condition; and transmitting information for the D2D communication through the selected resource, by using the power determined according to the determination result. The present disclosure relates to a communication scheme for fusing IoT technology with a 5G communication system for supporting a data rate higher than that of a 4G system and subsequent systems thereafter. The present disclosure can be applied to intelligent services (for example, a smart home, a smart building, a smart city, a smart or connected car, healthcare, digital education, retail business, security and safety related services, and the like) on the basis of the 5G communication technology and IoT related technology.

Claims

1. A transmit power control method of a terminal performing Device-to-Device (D2D) communication, the method comprising: selecting a resource during a discovery period configured for D2D resource selection; determining whether the selected resource fulfils a predetermined transmit power control condition; and transmitting D2D communication information on the selected resource at a transmit power selected according to the determination result.

2. The method of claim 1, wherein the discovery period comprises at least one non-controlled area for transmitting the D2D communication information at a maximum transmit power and at least one controlled region for transmitting the D2D communication information at a transmit power less than the maximum transmit power, and the predetermined transmit power control condition is that the selected resource belongs to the controlled region.

3. The method of claim 1, further comprising determining whether a network is in a congested state, wherein the predetermined transmit power control condition is that the network is in the congested state.

4. The method of claim 3, wherein determining whether the network is in the congested state comprises: measuring power on the resource in the discovery period; and determining whether the network is in the congested state based on the power measurement result.

5. The method of claim 4, wherein determining whether the network is in the congested state comprises: performing a Cyclic Redundancy Check (CRC) test on the resource in the discovery period; and determining whether the network is in the congested state based on a CRC success rate.

6. The method of claim 1, wherein transmitting the D2D communication information comprises transmitting the D2D communication information on the selected resource at a predetermined transmit power when the predetermined transmit power control condition is fulfilled and at a maximum transmit power when the predetermined transmit power control condition is not fulfilled.

7. The method of claim 1, further comprising determining whether interference occurs between the selected resource and a cellular terminal uplink resource adjacent to the selected resource, wherein transmitting the D2D communication information comprises transmitting the D2D communication information on the selected resource at a transmit power determined based on at least one of the determination results on whether the predetermined power control condition is fulfilled or whether interference occurs between the resources.

8. The method of claim 1, further comprising requesting, when the selected transmit power is less than a predetermined threshold value, to a base station for changing a D2D communication resource allocation mode.

9. A terminal performing Device-to-Device (D2D) communication, the terminal comprising: a communication unit which transmits and receives D2D communication information; and a controller which selects a resource during a discovery period configured for D2D resource selection, determines whether the selected resource fulfils a predetermined transmit power control condition, and controls the communication unit to transmit D2D communication information on the selected resource at a transmit power selected according to the determination result.

10. The terminal of claim 9, wherein the discovery period comprises at least one non-controlled area for transmitting the D2D communication information at a maximum transmit power and at least one controlled region for transmitting the D2D communication information at a transmit power less than the maximum transmit power, and the predetermined transmit power control condition is that the selected resource belongs to the controlled region.

11. The terminal of claim 9, wherein the controller determines whether a network is in a congested state, and the predetermined transmit power control condition is that the network is in the congested state.

12. The terminal of claim 11, wherein the controller measures power on the resource in the discovery period and determines whether the network is in the congested state based on the power measurement result.

13. The terminal of claim 11, wherein the controller performs a Cyclic Redundancy Check (CRC) test on the resource in the discovery period and determines whether the network is in the congested state based on a CRC success rate.

14. The terminal of claim 9, wherein the controller controls transmitting the D2D communication information on the selected resource at a predetermined transmit power when the predetermined transmit power control condition is fulfilled and at a maximum transmit power when the predetermined transmit power control condition is not fulfilled and requests to a base station for changing a D2D communication resource allocation mode when the selected transmit power is less than a predetermined threshold value.

15. The terminal of claim 9, wherein the controller determines whether interference occurs between the selected resource and a cellular terminal uplink resource adjacent to the selected resource and controls the communication unit to transmit the D2D communication information on the selected resource at a transmit power determined based on at least one of the determination results on whether the predetermined power control condition is fulfilled or whether interference occurs between the resources.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0043] FIG. 1 is a diagram for explaining D2D resource allocation according to an embodiment of the present invention;

[0044] FIG. 2 is diagrams for explaining in-band emission occurring in a radio communication system;

[0045] FIG. 3 is a diagram for explaining in-band emission at a D2D Rx UE;

[0046] FIG. 4 is a diagram for explaining in-band emission at an eNB;

[0047] FIG. 5 is a diagram for explaining in-band emission among D2D Tx UEs;

[0048] FIG. 6 is a diagram illustrating a D2D resource structure according to an embodiment of the present invention;

[0049] FIG. 7 is a diagram illustrating a D2D resource structure with a plurality of discovery resource pools according to another embodiment of the present invention;

[0050] FIG. 8 is a diagram illustrating a D2D resource structure with a plurality of discovery pools having different discovery periods and discovery intervals according to another embodiment of the present invention;

[0051] FIG. 9 is a diagram illustrating a D2D resource structure in which a plurality of discovery resource pools is Time Division Multiplexed (TDMed) and Frequency Division Multiplexed (FDMed), according to another embodiment of the present invention.

[0052] FIG. 10 is a flowchart illustrating a power control method of a D2D Tx UE according to an embodiment of the present invention;

[0053] FIG. 11 is a flowchart illustrating a power control method of a D2D Tx UE according to another embodiment of the present invention;

[0054] FIG. 12 is a diagram illustrating an exemplary table listing predetermined transmit power values according to an embodiment of the present invention;

[0055] FIG. 13 is a flowchart illustrating a power control method of a D2D Tx UE according to still another embodiment of the present invention; and

[0056] FIG. 14 is a block diagram illustrating a configuration of a D2D UE according to the present invention.

MODE FOR THE INVENTION

[0057] The present invention can be applied to all the types of devices or services supporting D2D communication as well as normal electronic devices such as a smartphone, a portable terminal, a mobile terminal, a cellular phone, a Personal Digital Assistant (PDA), a Portable Multimedia Player (PMP), a handheld device, a laptop computer, a Wibro terminal, a smart TV, and a smart refrigerator.

[0058] Embodiments of the present invention are described in association with D2D transmitting devices and D2D receiving devices as UEs supporting D2D communication. The transmitting or receiving device may be referred to as a system, a subscriber unit, a subscriber station, a mobile station, a mobile terminal, a remote station, a remote terminal, a mobile device, a user terminal, a terminal, a D2D terminal, a radio communication device, a user agent, a user device, or a user equipment. The D2D transmitting device may be referred to as a D2D transmitter, a transmitter, a D2D transmitting terminal, a transmission terminal, a transmission node, or a transmitting party; the D2D receiving device may be referred to as a D2D receiver, a receiver, a D2D reception terminal, a reception terminal, a reception node, or a receiving party. A UE supporting D2D communication may operate as a transmitting device or a receiving device according to its operation mode.

[0059] A D2D UE performs data communication in one of the D2D discovery and D2D communication schemes. The D2D UE performs D2D discovery in one of Type 1 and Type 2 discovery. The Type 1 discovery is characterized in that a plurality D2D UEs select resources to use in a distributed manner in the resource pool allocated by the eNB, and it may be referred to as Type 1, Type 1 discovery, first discovery allocation scheme (method), distributed discovery resource allocation, and the like. The type 2 discovery is characterized in that an eNB allocate discovery resources to the D2D UEs explicitly, and it is referred to as Type 2, type 2 discovery, second discovery resource allocation scheme (method), explicit discovery resource allocation, and the like. Meanwhile, the D2D UE performs D2D communication in one of Mode 1 and Mode 2 communication schemes. In Mode 1 communication, the eNB allocates D2D communication resources to the D2D UEs, and this scheme is referred to as Mode 1, first communication resource allocation scheme (method), explicit communication resource allocation, and the like. Mode 2 communication is characterized in that a plurality of D2D UEs select resources in a distributed manner in the resource pool allocated by the eNB, and this scheme is referred to as Mode 2, second communication resource allocation scheme, distributed communication resource allocation, and the like. The embodiments of the present invention can be more effectively applied to (but are not limited to) the Type 1 discovery and Mode 2 communication among the D2D discovery and D2D communication schemes, and it is obvious that the present invention can be applied to various types of D2D discovery and D2D communication schemes including the aforementioned schemes without departing from the spirit and scope of the present invention.

[0060] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and they should be interpreted neither as having an excessively comprehensive meaning nor as having an excessively contracted meaning.

[0061] As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but they do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof.

[0062] Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. Further, the following terms are defined in consideration of the functionality in the present invention, and they may vary according to the intention of a user or an operator, usage, etc. Therefore, the definition should be made on the basis of the overall content of the present specification.

[0063] FIG. 1 is a diagram for explaining D2D resource allocation according to an embodiment of the present invention. In detail, FIG. 1 is a diagram illustrating a D2D resource allocation method for use in a radio communication system (e.g., LTE system) according to an embodiment of the present invention. The resource allocation method depicted in FIG. 1 may exemplify resource allocation for Type 1 discovery and Mode 2 communication.

[0064] Although the embodiment of FIG. 1 is directed to a Frequency Division Duplexing (FDD) system, the present invention is not limited to the FDD system.

[0065] In reference to FIG. 1, the FDD system uses different frequency bands for downlink (DL) and uplink (UL). For each frequency band, one radio frame 101 consists of 10 subframes 103. One subframe consists of two slots 105, and one slot consists of 6 or 7 symbols. The subframe 103 includes control channel regions 107a and 107b for transmitting control channels and data channel regions 109a and 109b for transmitting data channels, and the control and data channel regions are separated in the time domain on the DL frequency band and in the frequency domain on the UL frequency band.

[0066] A Primary Synchronization Signal (PSS) 111 and a Secondary Synchronization Signal (SSS) 113 are mapped to the last two symbols of the first slots of the first subframe (subframe 0) and the sixth subframe (subframe 5) of the DL frame. The PSS 111 and the SSS 113 are used for synchronization between a UE and an eNB.

[0067] A Physical Broadcast Channel (PBCH) 115 is mapped to the second slot of the first subframe (subframe 0) of the DL frame, and the UE acquires system information from the PBCH 115.

[0068] In the radio communication system of the present invention, the D2D resource allocation information is transmitted in a System Information Block (SIB) 117 which is broadcast at an interval, which can be designed to have a value in the range from 40 ms to 640 ms or higher.

[0069] The D2D UEs acquire downlink synchronization with the system based on the PSS 111 and SSS 113 and receive the information on their serving cells based on the Master Information Block (MIB). For example, the MIB may include essential parameters such as DL system bandwidth, system frame number, and Physical Hybrid-ARQ Indication Channel (PHICH). The UEs that have received the MIB may receive the Physical Downlink Control Channel (PDCCH) transmitted by the eNB at every subframe. Basically, the PDCCH carries DL/UL resource allocation information. Each UE decodes SIB resource allocation information contained in the PDCCH using a previously informed System Information-Radio Network Temporary Identifier (SI-RNTI) or D2D-RNTI. That is, the UE performs PDCCH decoding with the SI-RNTI to acquire the information on the time-frequency region to which the SIB 117 is mapped and then decodes the SIB 117 on the corresponding time-frequency region.

[0070] The UE that has succeeded decoding the SIB 117 acquires D2D resource allocation information (D2D UL subframe information) (this may be the information on the D2D resource pool for Type 1 discovery or Mode 2 communication) to check the subframe numbers and successive subframes for D2D purposes in the corresponding frame and the D2D subframe interval.

[0071] If the D2D subframe positions are changed in the corresponding frame (e.g., D2D subframe may be changed from subframe 3 to subframe 5 or the number of subframes may increase from 2 to 3), the eNB may notify the UEs of the change of the D2D resources through an SIB or a paging channel.

[0072] In the Type 1 discovery or Mode 2 communication, the transmitting UE selects the D2D resource in the D2D subframe(s) allocated through the SIB to transmit the D2D information (D2D signal and D2D data), and the receiving UE decodes the D2D subframe(s) allocated through the SIB.

[0073] FIG. 2 is diagrams for explaining in-band emission occurring in a radio communication system. The embodiments of FIGS. 2A and 2B are directed to the cases of using 50 Resource Blocks (RBs) in the bandwidth of 10 MHz.

[0074] FIG. 2 is directed to a case where the D2D Tx UE uses the 12.sup.th RB (1 RB), and to a case where the D2D Tx UE uses the 12.sup.th to 17.sup.th RBs (6 RBs).

[0075] In FIG. 2, the relative transmit power of the D2D Tx UE using the 12.sup.th RB is 0 dB. That is, if the transmit power of the D2D Tx UE is X dBm at the 12.sup.th RB, the relative transmit power at the 12.sup.th RB is 0 dB. Although the transmit power should be confined to the 12.sup.th RB under the assumption of no in-band emission, FIG. 2B shows leakage power caused by the in-band emission at the 9.sup.th, 10.sup.th, 11.sup.th, 13.sup.th, 14.sup.th, and 15.sup.th RBs because in-band emission exists in the real communication environment. This means that although the D2D Tx UE use only the 12.sup.th RB this may incur interference at the adjacent RBs. If the resource amount for use by the D2D Tx UE increases, the effect of in-band emission interference becomes worse as shown in FIG. 2.

[0076] FIG. 3 is a diagram for explaining in-band emission at a D2D Rx UE. In detail, FIG. 3 shows the interference problem caused by in-band emission of the D2D Tx UE (D2D transmitter) at the D2D Rx UE (D2D receiver)

[0077] In reference to FIG. 3, there are remote D2D transmitters (D2D UE-1 and D2D UE-2) located far from the D2D receiver and close D2D transmitters (D2D UE-3 and D2D UE-4) located close to the D2D receiver in the radio communication system. Each D2D transmitter transmits D2D discovery signal or D2D data at the maximum power on the D2D resource, i.e., PUSCH, allocated by the eNB or selected autonomously.

[0078] In this case, although all of the D2D transmitters use the orthogonal frequency resources (RBs), the RB-receiving power of PUSCH occupied by the D2D UE-3 or D2D UE-4 may cause interference to the RB-receiving power of PUSCH occupied by the D2D UE-1 and D2D UE-2 at the D2D receiver because of the AGC dynamic range limit of the D2D receiver as denoted by reference numbers 301 and 303.

[0079] FIG. 4 is a diagram for explaining in-band emission at an eNB. In detail, FIG. 4 shows an interference problem caused by in-band emission of a D2D Tx UE (D2D transmitter) at an enhanced node B (eNB).

[0080] In reference to FIG. 4, there is a cellular UE (UE-1) located close to the eNB receiver and a cellular UE (UE-2) located far from the eNB receiver in a radio communication system. Also, there are D2D transmitters (D2D UE-3 and D2D UE-4) located close to the eNB receiver in the radio communication system.

[0081] It is assumed that the cellular UEs are transmitting feedback such as HARQ ACK/NACK and CQI through PUCCH. At this time, in order for the eNB receiver to receive the PUCCH at a predetermined reception power, the eNB performs transmit power control on the cellular UEs (UE-1 and UE-2) transmitting PUCCH. For example, the eNB controls the cellular UEs (UE-1 and UE-2) such that the UE-2 located far from the eNB transmits PUCCH at a high transmit power and the UE-1 located close to the eNB transmits PUCCH at a low transmit power.

[0082] Meanwhile, it is assumed that the D2D transmitters (UE-3 and UE-4) located close to the eNB perform D2D discovery or D2D communication on the D2D resources, i.e., PUSCH, allocated by the eNB or selected autonomously.

[0083] At this time, although the PUCCH used by the cellular UEs (UE-1 and UE-2) and the PUSCH used by the D2D UE are mapped to different resources on the time axis, if the D2D transmitter does not perform power control (i.e., if the D2D UE transmits D2D discovery signal or D2D data at the maximum power), the PUCCH of the cellular UE undergoes in-band emission interference caused by the PUSCH of the D2D UE at the eNB as denoted by reference numbers 401 and 403.

[0084] In another example, the cellular resources allocated to the cellular UEs (UE-1 and UE-2) for UL data transmission and the D2D resources allocated to the D2D UEs (UE-3 and UE-4) may be frequency division-multiplexed (FDMed) into the same subframe. The transmit power control is performed on the PUSCH for use by the cellular UEs (UE-1 and UE-2) in a similar manner to PUCCH. That is, the UE-1 transmits the cellular PUSCH at a low transmit power while the UE-2 transmits the cellular PUSCH at a high transmit power. If the D2D transmitter does not control the transmit power, the PUSCH of the cellular UE undergoes interference caused by the PUSCH transmitted by the D2D UE at the eNB.

[0085] FIG. 5 is a diagram for explaining in-band emission among D2D Tx UEs. In detail, FIG. 5 shows interference caused by in-band emission among the D2D transmitters at a D2D receiver in a congested network environment.

[0086] In reference to FIG. 5, there are D2D transmitters (UE-1 and UE-2) located close to the D2D receiver and D2D transmitters (UE-3 and UE-4) located far from the D2D receiver in a radio communication system. In a congested network environment, a plurality of UEs exist around the UE-1 and UE-2. This situation occurs frequently at a hotspot such as a department store, an airport, and a shopping mall.

[0087] The D2D transmitters transmit D2D discovery signals or D2D data at their maximum powers. In this case, the in-band emissions of the D2D transmitters (UE-1 and UE-2) located close to the D2D receiver cause interference to the D2D discovery signals or D2D data of the D2D transmitters (UE-3 and UE-4) located far from the D2D receiver; thus, the D2D receiver cannot correctly receive the signals of the D2D transmitters (UE-3 and UE-4) located far from the D2D receiver.

[0088] D2D discovery aims to discover as many as possible of neighboring D2D UEs during a given time period. D2D communication aims to broadcast data such that as many as possible of the neighboring UEs receive the broadcast data. In order to meet such requirements, it is necessary to solve the in-band emission problem. That is, in the congested environment as shown in FIG. 5, the D2D signals transmitted by the UEs located close to the D2D receiver such as UE-1 and UE-2 may cause in-band emission problems to the signals transmitted by the UEs located far from the D2D receiver such as UE-3 and UE-4. There is therefore a need of a method for solving this problem.

[0089] FIG. 6 is a diagram illustrating a D2D resource structure according to an embodiment of the present invention.

[0090] A D2D UE performs D2D discovery to detect and select resources for D2D discovery or D2D communication. Here, D2D discovery can be used in the same or a different meaning as or from D2D discovery for discovering interested neighboring UEs as a kind of D2D communication.

[0091] The eNB broadcasts information on one discovery period comprised of N Transmission Time Intervals (TTIs) for D2D discovery using an SIB for the UEs located within the cell. The information on the one discovery period includes the size of the discovery period (number of TTIs) and the length of the discovery period (time).

[0092] In an embodiment of the present invention, one discovery period includes maximum transmit power resource regions 601 (or referred to as Max. region, maximum transmit power region, non-control region, or maximum region) and controlled transmit power resource regions 603 (or referred to as Ctrl. region, controlled region, and non-maximum region) for D2D Tx UEs. The Max. region 601 and Ctrl. region 603 may be distinguished from each other based on the maximum allowed transmit power or whether power control is required. According to various embodiments, the length of one discovery period and the numbers and positions of the Max. regions 601 and Ctrl. regions 603 are not limited.

[0093] The resource region information may be delivered from the eNB to the D2D UEs using an SIB. According to various embodiments, the D2D resource region information (type of D2D resource region and number and positions of arbitrary resource regions) may be transmitted in various message formats, without restriction, as well as SIB. For example, the D2D resource region information may be transmitted to the D2D UE through higher layer signaling such as RRC signaling. According to an embodiment of the present invention, the D2D UE controls transmit power for transmitting D2D information (D2D discovery signal or D2D data) based on the type of resource region and presence/absence of network congestion. The D2D UEs acquire the discovery period information and D2D resource region information from the SIB. The D2D UEs also determine whether the network is in a congested state based on the total power level of the received signals. As a consequence, the UE may control the transmit power for transmitting the corresponding D2D information based on the type of the D2D resource pool including the D2D resource to use and the presence/absence of network congestion.

[0094] According to an embodiment of the present invention, a D2D UE may operate differently in the D2D information transmission mode and the D2D information reception mode. In the D2D transmission mode, the D2D Tx UE may operate differently depending on the method for selecting D2D resources for transmitting the D2D information. The D2D UE operates as follows in the respective operation modes.

[0095] 1. Random resource selection [0096] (1) Tx UE operation [0097] 1) The UE selects the resources to use randomly in on discovery period. [0098] 2) The UE determines whether the type of the resource region including the resource it has selected is the Max. region or the Ctrl. region. [0099] 3) The UE determines, when the selected resource region is the Ctrl. region, whether the network is congested in use of the resource. At this time, the Tx UE can determine whether the network is congested in various ways. For example, the Tx UE may measure the power on the resources during the discovery period in the reception mode to determine the congested state when the measurement result is equal to or greater than (or only when greater than) a predetermined threshold value. Also, the Tx UE may perform Cyclic Redundancy Check (CRC) on the resources during the discovery period to determine the congested state when the success rate of CRC result is equal to or less than (or only when less than) a predetermined threshold value. Also, the Tx UE may determine that the network is not in a congested state when the success rate of the CRC result is equal to or greater than (or only when greater than) a predetermined threshold value. The threshold value may be stored according to an instruction of the eNB or preconfigured at the UE manufacturing stage. [0100] 4) If the selected resource region is the Ctrl. region and the network is currently in the congested state, the UE transmits the D2D information at a predetermined transmit power rather than the maximum transmit power. The predetermined transmit power may be stored according to an instruction of the eNB or preconfigured at the UE manufacturing stage. The predetermined transmit power may be mapped to the congestion degree (congestion level) of the network. In an embodiment, the network congestion may be divided into two levels to which predetermined transmit power values are mapped, and the transmit power may be set to one of a predetermined value and 0, i.e., transmission on/off, for each level. It is obvious that the transmit power can be set to 0 (off for indicating non-transmission of D2D information) in association with the high congestion level. [0101] 5) In all other cases, the UE performs transmission at the maximum transmit power. [0102] (2) Rx UE operation [0103] 1) The UE performs decoding on the whole resources during the discovery period with the exception of subframes including the resources for use in transmission. [0104] 2) After decoding, the UE calculates and stores the CRC success rate. This can be used for determining presence/absence of network congestion when it operates as a Tx UE. [0105] 3) The UE measures power on the whole resources during the discovery period with the exception of subframes including resources for use in transmission. The measurement result can be used for determining presence/absence of network congestion when it operates as a Tx UE.

[0106] 2. Energy sensing-based resource selection [0107] (1) Tx UE operation [0108] A. The UE performs energy scanning (sensing) on the Max. region during one discovery period for selecting resources. In this case, the energy scanning may be interpreted as measuring an energy level that can be calculated by averaging per-symbol Reference Signal Received Powers (RSRPs) over time. [0109] B. The UE selects resources based on the scanning result. For example, the UE selects the resource on which the sensed energy is equal to or less than a predetermined threshold energy level or one of the resources having the lowest z % energy level randomly after sorting the energy levels of all resources. [0110] C. The UE determines whether the type of the resource region including the selected resource is the Max. region or the Ctrl. region. [0111] D. If the selected resource region is the Ctrl. region, the UE determines whether the network is congested in use of the resource. At this time, the Tx UE can determine whether the network is congested in various ways. For example, the Tx UE may determine whether the network is congested based on the energy level on the resource during the discovery period predetermined in the energy sensing process. In this case, the Tx UE may determine that the network is in the congested state when the number of RBs having the energy equal to or greater than a predetermined threshold energy level (e.g., a dBm) is equal to or less than x % of the total number of RBs. The Tx UE may also determine that the network is not in a congested state when the number of RBs having the energy equal to or less than a predetermined threshold energy level (e.g., b dBm) is equal to or greater than y % of the total number of RBs. [0112] Also, the Tx UE may perform Cyclic Redundancy Check (CRC) on the resources during the discovery period to determine the congested state when the success rate of the CRC result is equal to or less than (or only when less than) a predetermined threshold value. Also, the Tx UE may determine that the network is not in a congested state when the success rate of the CRC result is equal to or greater than (or only when greater than) a predetermined threshold value. The threshold value may be stored according to an instruction of the eNB or preconfigured at the UE manufacturing stage. [0113] E. If the selected resource region is the Ctrl. region and if the network is currently in the congested state, the UE transmits the D2D information at a predetermined transmit power rather than the maximum transmit power. The predetermined transmit power may be stored according to an instruction of the eNB or preconfigured at the UE manufacturing stage. The predetermined transmit power may be mapped to the congestion degree (congestion level) of the network. In an embodiment, the network congestion may be divided into two levels to which predetermined transmit power values are mapped, and the transmit power may be set to one of a predetermined value and 0, i.e., transmission on/off, for each level. It is obvious that the transmit power can be set to 0 (off for indicating non-transmission of D2D information) in association with the high congestion level. [0114] F. In all other cases, the UE performs transmission at the maximum transmit power. [0115] (2) Rx UE operation [0116] The UE performs decoding on the whole resources during the discovery period with the exception of subframes including the resources for use in transmission.

[0117] According to an embodiment of the present invention, the eNB may calculate the number of UEs utilizing D2D service within a cell statistically through D2D capability negotiation procedure for the initial attach of D2D UE. The eNB may determine the congestion occurring in resource access (or resource occupancy) of the D2D UE based on the calculation result to control the resource access of the D2D UE. For example, if the number of D2D UEs located within the cell is statically large, the eNB may increases the amount of the D2D resources for use by the D2D UEs. In contrast, if the number of D2D UEs located within the cell is statically small, the eNB may reduce the amount of the D2D resources for use by the D2D UEs. In this way, it may be possible to avoid resource collision occurring when D2D UEs perform random resource selection or energy level-based resource selection. The D2D resource information (e.g., frequency-time domain positions of D2D resources and period and interval of D2D resources) delivered to the D2D UEs through an SIB or RRC signaling within the cell may be changed at a predetermined time or according to a predetermined event. For example, it is predicted that the D2D service utilization increases from the business closing time to AM 1˜2 in residential areas, and it may be preferred to increase resource amounts in business areas (company concentration areas) during the period from business open time to business closing time. Also, it is predicted that the D2D service utilization increases at a stadium such as a ball park or a soccer field on a game day, the eNB may increase the resource amount for use by the D2D UEs within the cell when such an event occurs. If the resource amount is not changed, the eNB may inform the UE whether there is any change in the resource information including resource amount and power control parameter. The UEs that have already attached may not receive the resource information and power control parameter being transmitted after recognizing no change in the resource information and power control parameter through such signaling.

[0118] According to an embodiment, the eNB may check the congestion degree without adjusting the resource amount to avoid resource collision occurring when the D2D UEs select resources. In detail, the eNB increases a D2D UE resource access control probability p for the case where the number of D2D UEs increases within a cell, and decreases the D2D UE resource access control probability p for the case where the number of D2D UEs decreases within the cell. The eNB transmits the D2D UE resource access control probability p to the D2D UE through an SIB or RRC signaling. For example, assuming p={0, 0.2, 0.4, 0.8, 1}, if the eNB sets p to 1, the UE may perform the random resource selection or energy level-based resource selection on one of the discovery resources during every discovery period when the total discovery period is set to L. In contrast, if the eNB sets p to one of the values with the exception of 1, the UE performs resource access at an interval of p*L (e.g., Assuming L=10 and p=0.5, p*L is 5; thus, the random resource selection or energy level-based resource selection is performed at every 5 discovery periods. If p*L is not an integer, truncation or rounding may be performed). For example, the UE resource access control probability may be applied during the discovery period. That is, the UE performs the random resource selection or energy level-based resource selection at the probability of p during one discovery period and performs resource access at the probability of p during every discovery period while the eNB does not change the probability p.

[0119] For example, the resource access control may be performed based on the combination of aforementioned eNB-predicted congestion (acquired through D2D capability negotiation procedure) and the UE-predicted congestion through CRC test. In detail, assuming the UE's resource occupancy probability based on the eNB-predicted congestion is p1 and the UE's resource access probability based on the CRC test-based congestion determination is p2, the resource access probability of the UEs within the cell may be determined based on the various combinations of p1 and p2. For example, the weighted sum of p1 and p2 can be used. That is, the resource access probability of a UE may be determined by c*p1+(1−c)*p2. Here, c denotes a value which the eNB transmits to the UE through an SIB or RRC signaling and is set to a value between 0 and 1. The information on whether to use p1 and p2 for determining the congestion degree of the UE may be transmitted to the UE through the SIB or RRC signaling. For example, if p2 is set to off, only p1 is used to determine the congestion degree. FIG. 7 is a diagram illustrating a D2D resource structure according to another embodiment of the present invention. In the embodiment of FIG. 6, one discovery period corresponding to N TTIs (N subframes) is split into a maximum transmit power resource region (Max) and a controlled transmit power resource region (Ctrl). Unlike the embodiment of FIG. 6, FIG. 7 exemplifies use of equal transit power in one discovery period consisting of N TTIs. That is, it may be possible to use the maximum transmit power in one discovery period while the controlled transmit power is used in another discovery period. Also, it is exemplified that different transmit power levels may be applied per discovery period. Here, the transmit power for each discovery period may be determined by i) the eNB, ii) the UE with the assistance of the eNB, or iii) by the UE without assistance of the eNB. [0120] i) Case of being determined by eNB: The eNB may transmit the transmit power value per discovery period along with the discovery resource information (on N TTIs constituting the discovery period and M TTIs constituting an interval) to the D2D eNBs through the SIB or RRC signaling within the cell. The eNB may inform to use the maximum transmit power during the discovery period K, transmit power P1 during the discovery period K+1, and transmit power P2 during the discovery period K+2. Such transmit power values may be mapped to the discovery periods implicitly (in this case, only the resource information, without the transmit power values, is transmitted to the UE through the SIB or RRC signaling) or transmitted explicitly (in this case, the transmit power values to be used for respective discovery periods are transmitted to the UE through the SIB or RRC signaling along with the resource information). The UE operates as described with reference to FIG. 6 when the eNB determines the transmit power values. [0121] ii) Case of being determined by UE with assistance of eNB: The eNB transmits to the UE a transmit power parameter rather than the transmit power values allowed for the UEs. For example, in an LTE cellular system, P.sub.0 PUSCH and α are used for transmit power control as shown in equation (1).

[00001]$\begin{array}{cc}{P}_{\mathrm{PUSCH}}\left(i\right)=\min .Math.\left\{\begin{array}{c}{P}_{\mathrm{CMAX}}\left(i\right),\\ 10.Math..Math.{\log }_{10}\left(M_{\mathrm{PUSCH}}\left(i\right)\right)+P_{0.Math.\mathrm{\_PUSCH}}\left(j\right)+\alpha \left(j\right).Math.\mathrm{PL}+{\Delta }_{\mathrm{TF}}\left(i\right)+f\left(i\right)\end{array}\right\}.Math.\hspace{1em}\left[\mathrm{dBm}\right]& \left(1\right)\end{array}$

[0122] In equation (1), P.sub.0 PUSCH denotes the transmit power of Physical Uplink Shared Channel at the i.sup.th subframe of the UE.

[0123] The P.sub.0 PUSCH parameter indicating P.sub.0 NOMINAL PUSCH+P.sub.0 UE PUSCH and transmitted from the eNB to the UE through higher layer signaling. In particular, P.sub.0 NOMINAL PUSCH is an 8-bit cell-specific value in the range of [−126, 24] dB. Meanwhile, P.sub.0 UE PUSCH is a 4-bit UE-specific value in the range of [−8, 7] dB. The eNB broadcasts the cell-specific value through an SIB and transmits the UE-specific value to the UE through dedicated RRC signaling. Meanwhile, α denotes a cell-specific value for compensating for the path loss and is 3-bit information carrying one of {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}.

[0124] If P.sub.0 PUSCH and α are received from the eNB, the UE may calculate the transmit power based thereon. That is, P.sub.0 PUSCH and α values for use during the discovery period K and the P.sub.0 PUSCH and α values for use during the discovery period K+1 that are transmitted by the eNB through the SIB or RRC signaling may differ from each other. Upon receipt of these values, the UE may calculate its transmit power based on the parameter values corresponding to the selected resource region and the path loss measured between the eNB and the UE.

[0125] In order to apply the parameter values being used in the legacy LTE cellular system to the D2D system, it is necessary to modify the parameter values. For example, the UE-specific parameter P.sub.0 UE PUSCH may not be necessary in the D2D discovery supporting transmission of RRC_Idle UEs. This is because the UEs in the RRC_Idle state cannot receive any dedicated RRC signal from the eNB.

[0126] Also, in order to support Public Service (public safety and disaster) recovery purpose, it is necessary to extend the range of the cell specific P.sub.0 NOMINAL PUSCH from [−126, 24] dB to [−126, 31] dB. This is because the UE class for use in Public Service is higher than that of the normal UE and thus supports a maximum transmit power (31 dB) higher than that of the normal UE (23 dB). The D2D Tx UE and D2D Rx UE operate as follows in the case where the UE determines the transmit power with the assistance of the eNB.

[0127] 1. Random resource selection [0128] (1) Tx UE operation [0129] A. The UE receives discovery resource pool information (purpose of resource pool, positions on time and frequency axes of resource pool, resource access probability, and transmit power control parameters per resource pool (P.sub.0.sub._.sub.PUSCH and α)). [0130] B. The UE selects resources randomly during the discovery period in one or more resource pools that the eNB has allowed for use among a plurality resource pools. [0131] C. If the eNB commands a D2D Tx UE to use the maximum transmit power through dedicated RRC signaling or D2D DCI or commands all D2D UEs located within the cell to use the maximum transmit power in a specific resource pool through an SIB or RRC signaling, the UE ignores the transmit power control parameters received previously through the SIB or RRC signaling and uses the maximum transmit power according to the command from the eNB. If such a command is not received, the D2D Tx UE operates as follows. [0132] D. The UE calculates the transmit power using the path loss value estimated based on the Reference Signal Received Power (RSRP) from the eNB and the transmit power control parameters (at this time, in order for the UE operating in a cellular DRX mode to measure RSRP for D2D transmission power control, the UE has to go out of the DRX mode and performs RSRP measurement using the Cell-specific Reference Signal (CRS) carried in downlink subframes. Before RSRP measurement, the D2D Tx UE may achieve downlink synchronization with the eNB using the downlink synchronization signals (Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS)) of the eNB). [0133] E. The UE determines whether to perform transmission on the resources selected in the discovery period based on parameters for use in determining whether congestion exists in use of the resource. At this time, the parameters for use in determining whether congestion exists may be received from the eNB or determined through CRC test performed when the D2D Tx UE operates in the reception mode. These two methods may be used in a combined manner. [0134] F. The UE transmits the discovery signal at a given resource access probability using the transmit power value calculated for the selected resource. [0135] (2) Rx UE operation [0136] A. The UE receives resource pool information for receiving D2D data (e.g., resource pool information, resource configuration information on time and frequency axes of resource pool). [0137] B. The UE performs decoding on the whole resources during the discovery period with the exception of the subframes including the resources for use in transmission in one or more resource pools designated by the eNB for reception. [0138] C. The UE determines whether the eNB controls the resource access probability p2 according to the presence/absence of the congestion degree measured by the D2D UE, resource access probability p1 according to the congestion degree predicted by the eNB through a D2D capability negotiation procedure, or resource access at a probability of a combination of p1 and p2 (the UE receives p1=on/off or p2=on/off through an SIB or RRC signaling. If p1=on and p2=on, the UE also receives a c value determining c*p1+(1−c)*p2 through the SIB or RRC signaling). [0139] D. If p I=off and p2=on or p1=on and p2=on, the UE performs decoding and calculates and stores the CRC success rate. The CRC success rate may be used to determine presence/absence of congestion in use of the resources when the UE operates in the transmission mode. Otherwise if p1=off and p2=off or if p1=on and p2=off, the UE performs decoding only and does not calculate the CRC success rate.

[0140] 2. Energy sensing-based resource selection [0141] (1) Tx UE operation [0142] A. The UE receives the discovery resource pool information, resource access probability, and transmit power control parameters per resource pool in a way similar to the process A of the random resource selection. [0143] B. The UE performs scanning on the resources during the discovery period in one or more resource pools that the eNB allowed for use among a plurality of resource pools in a way similar to the process B of the random resource selection. [0144] C. The UE selects resources based on the scanning result. For example, the UE may select the resource on which the sensed energy is equal to or less than a predetermined threshold energy level or one of the resources having the lowest z % energy level randomly after sorting the energy levels of all resources. [0145] D. Identical with process C of the random resource selection [0146] E. Identical with process D of the random resource selection [0147] F. The UE may determine whether to perform transmission on the resource selected during the discovery period based on the parameters for use in determining presence/absence of congestion in use of resource. At this time, the parameter for use in determining presence/absence of congestion may be received from the eNB or determined through energy sensing when the D2D UE is operating in the reception mode. These two methods may be used in a combined manner. Particularly in the case of determining the congestion degree in use of the resource through energy sensing, the Tx UE may determine that the network is in the congested state when the number of RBs having the energy equal to or greater than a predetermined threshold energy level (e.g., a dBm) is equal to or less than x % of the total number of RBs. The Tx UE may also determine that the network is not in a congested state when the number of RBs having the energy equal to or less than a predetermined threshold energy level (e.g., b dBm) is equal to or greater than y % of the total number of RBs. [0148] G. If the selected resource region is the Ctrl. region, the UE determines that the network is in the congested state in use of the resource. The Tx UE may determine whether the network is in the congested state in various ways. For example, the Tx UE may determine whether the network is in the congested state based on the energy value on the resource in the discovery period that is predetermined in the energy sensing process. In this case, the Tx UE may determine that the network is in the congested state when the number of RBs having the energy equal to or greater than a predetermined threshold energy level (e.g., a dBm) is equal to or less than x % of the total number of RBs. The Tx UE may also determine that the network is not in a congested state when the number of RBs having the energy equal to or less than a predetermined threshold energy level (e.g., b dBm) is equal to or greater than y % of the total number of RBs. [0149] H. The UE transmits the discovery signal at a given resource access probability using the transmit power value calculated on the selected resource. [0150] (2) Rx UE operation [0151] A. The UE receives resource pool information for receiving D2D data (e.g., resource pool information, resource configuration information on time and frequency axes of resource pool). [0152] B. The UE performs decoding on the whole resources during the discovery period with the exception of the subframes including the resources for use in transmission in one or more resource pools designated by the eNB for reception. [0153] C. The UE determines whether the eNB controls the resource access probability p2 according to the presence/absence of the congestion degree measured by the D2D UE, resource access probability p1 according to the congestion degree predicted by the eNB through a D2D capability negotiation procedure, or resource access at a probability of a combination of p1 and p2 (the UE receives p1=on/off or p2=on/off through an SIB or RRC signaling. If p1=on and p2=on, the UE also receives a c value determining c*p1+(1−c)*p2 through the SIB or RRC signaling). [0154] D. If p1=off and p2=on or p1=on and p2=on, the UE performs decoding and calculates and stores the CRC success rate. The CRC success rate may be used to determine presence/absence of congestion in use of the resources when the UE operates in the transmission mode. Otherwise if p1=off and p2=off or if p1=on and p2=off, the UE performs only decoding and does not store the power value measured on the resource. [0155] iii) Case of being determined by UE without assistance of eNB: In a disaster situation such as a fire and earthquake, an emergency situation such as a terror incident or a building collapse, or a public safety situation, it is difficult to guarantee communication between an eNB and a UE because of a breakdown of the eNB. In such a situation, the UE may perform transmit power control based on the previously stored transmit power parameters and resource information. That is, the UE may use the transmit power P1 in the resource region K and the transmit power P2 in the resource region K+1. At this time, P1 and P2 may be the maximum transmit powers.

[0156] FIG. 8 is a diagram illustrating a D2D resource structure according to another embodiment of the present invention. Unlike FIGS. 6 and 7, FIG. 8 exemplifies a case of multiple discovery resources. In FIGS. 6 and 7, the discovery resource is configured such that a discovery period consisting of N TTIs appears repeatedly at an interval equivalent to M TTIs; however, in FIG. 8 a discovery period consisting of N1 TTIs appears repeatedly at an interval of M1 TTIs and another discovery period consisting of N2 TTIs appears repeatedly at an interval of M2 TTIs. The D2D resource structure may be configured such that one discovery period consisting of N1 or N2 TTIs includes resource regions with different transmit powers as shown in FIG. 6 or that each of the discovery periods includes the resource regions with the same transmit power as shown in FIG. 7 but the resource regions of the different discovery periods have different transmit powers. In order to accomplish this, the eNB transmits to the UE the information on the purpose of each discovery resource pool (e.g., commercial/Public Safety purpose, Type 1 discovery/Type 2B discovery, and short/medium/long range classes), time axis resource information of the discovery resource pool (e.g., discovery period and discovery interval), frequency axis resource information of the discovery pool (e.g., start and end points of the discovery resource pool on the frequency axis), and transmit power control parameters for each resource pool through an SIB or RRC signaling. At this time, as described with reference to FIG. 7, the transmit power may be determined by the eNB according to the type of the transmit power control parameters or by the UE autonomously with or without assistance of the eNB.

[0157] FIG. 9 is a diagram illustrating a D2D resource structure according to another embodiment. The embodiment of FIG. 9 differs from the embodiment of FIG. 8 in that a plurality of discovery resource pools can exist on the frequency axis. In detail, a discovery resource pool consisting of N1 TTIs and having a discovery interval of M1 TTIs may include two or more different resource pools with different transmit power control parameters. Also, there may be another discovery resource pool consisting of N2 TTIs and having a discovery interval of M2 TTIs. As described with reference to FIG. 8, the eNB transmits the time and frequency resource information of the discovery resource pool and the transmit power control parameters for the respective resource pools to the UE through the SIB or RRC signaling. Although not shown in FIG. 9, two or more different resource pools may be frequency-division-multiplexed (FDMed) into a Physical Uplink Shared Channel (PUSCH) for cellular (Wide Area network (WAN)) uplink transmission during one discovery period.

[0158] FIG. 10 is a flowchart illustrating a power control method of a D2D Tx UE according to an embodiment of the present invention.

[0159] In reference to FIG. 10, the Tx UE receives a D2D power control parameter at step 1001. The Tx UE receives the D2D power control information from the eNB through an SIB. The D2D power control information may include D2D discovery information and D2D resource selection information. For example, the D2D power control information may include the D2D resource region information, discovery period information, D2D resource pool type information, and preconfigured transmit power information. The D2D power control parameter may be transmitted through, but not limited to, the SIB.

[0160] The Tx UE may select D2D resource at step 1003 using the above described random resource selection method or energy sensing-based resource selection method.

[0161] The Tx UE determines whether the selected D2D resource fulfils a predetermined condition, i.e., transmit power control condition, at step 1005. That is, the Tx UE may determine whether the selected D2D resource belongs to a Ctrl. region and whether the condition that the network is currently congested is fulfilled.

[0162] In detail, the Tx UE determines the type of the resource region including the selected D2D resource and whether the network is congested. The Tx UE may determine whether the selected D2D resource belongs to a Max. region or a Ctrl. region. The Tx UE may also determine whether the network is congested based on one of the power or energy level measured on the selected D2D resource and CRC test result on the selected D2D resource.

[0163] If the selected D2D resource fulfils the transmit power control condition, the Tx UE controls transmit power based on the D2D power control parameter to transmit a D2D discovery signal or D2D communication data at step 1007.

[0164] Otherwise if the selected D2D resource does not fulfil the transmit power control condition, the Tx UE transmits the D2D discovery signal or D2D communication data at the maximum transmit power at step 1009.

[0165] FIG. 11 is a flowchart illustrating a power control method of a D2D Tx UE according to another embodiment of the present invention.

[0166] Unlike the embodiment of FIG. 10, which is directed to the power control in consideration of solving the in-band emission problem between D2D UEs, the embodiment of FIG. 11 is directed to the power control in consideration of solving the in-band emission problem being incurred also to the eNB.

[0167] The power control for solving the in-band emission problem between D2D UEs is performed in consideration of the D2D receiver, but the power control for solving an in-band emission problem being incurred to the eNB is performed in consideration of the eNB receiver. Accordingly, it is necessary for two different power control schemes operating under different conditions to interwork, and the power control method designed in consideration of such interworking is proposed in the embodiment of FIG. 8

[0168] In order to solve the in-band emission problem being incurred to the eNB, the D2D Tx UE located close to the eNB has to perform power control to avoid in-band emission on the PUCCH transmitted to the eNB. In a situation where the D2D Tx UE located close to the eNB causes in-band emission to a D2D Rx UE, the D2D Tx UE has to perform power control to cancel the in-band emission interference to the D2D Rx UE.

[0169] In reference to FIG. 11, the D2D Tx UE receives power control parameters from the eNB through an SIB at step 1101. The power control parameters may include a transmit power value P.sub.eNB for solving the in-band emission occurring at the eNB and another transmit power value P.sub.D2D for solving the in-band emission occurring at the D2D receiver. Although one transmit power value per each of two transmit power control schemes is provided in an embodiment of the present invention, the eNB may transmit the transmit power values to the UE in the form of a table including a plurality of levels through an SIB. The transmit power values are formatted in a table shown in FIG. 12 according to whether the Tx UE causes interference to the PUCCH at the eNB and whether the network is congested, and then the table is transmitted to the UE.

[0170] Upon receipt of the power control parameters, the D2D Tx UE measures downlink quality on the link established with the eNB for estimating a distance from the eNB at step 1103. The downlink quality may be measured based on various types of reference signals transmitted from the eNB to the UE such as Cell-specific Reference Signal (CRS) and Demodulation Reference Signal (DMRS). The downlink quality measurement may be performed before or after transmission resource selection.

[0171] Next, the Tx UE selects D2D resource using the above-described random resource selection method or energy sensing-based resource selection method at step 1105.

[0172] In the case of transmitting signals at the maximum transmit power on the selected D2D resource, the Tx UE determines at step 1107 whether the transmission affects PUCCH reception at the eNB and whether the network is currently congested.

[0173] The Tx UE may determine whether the transmission affects PUCCH reception at the eNB according to the result of a comparison between the measured downlink quality and P.sub.eNB value received through the SIB. This can be determined based on whether the Tx UE is located close to the eNB and whether the PUCCH is mapped to the frequency adjacent to the resource selected by the Tx UE.

[0174] The Tx UE may also determine whether the network is congested based on at least one of the power or energy level measured on the selected D2D resource and the result of CRC test performed on the selected D2D resource.

[0175] If it is determined that the transmission affects the PUCCH reception at the eNB or the network is congested, the Tx UE controls the transmit power based on the received D2D power control parameters to transmit the D2D discovery signal or the D2D communication data at step 1109. For example, if it is determined that the transmission affects PUCCH reception at the eNB, the Tx UE transmits the D2D information at the transmit power P.sub.eNB; if it is determined that the network is congested, the Tx UE transmits the D2D information at the transmit power P.sub.D2D.

[0176] Otherwise, if it is determined that neither the transmission affects PUCCH reception at the eNB nor the network is congested, the Tx UE transmits the D2D discovery signal or the D2D communication data at the maximum transmit power at step 1111.

[0177] In the case of both the event triggering transmission at the transmit power P.sub.D2D and the event triggering transmission at the transmit power P.sub.eNB on the selected D2D resource, i.e., if the transmission affects PUCCH reception at the eNB and the network is congested, the Tx UE may adjust the transmit power to the smallest one of the two values (i.e., min{P.sub.eNB, P.sub.D2D}). If one of the two events occurs, the Tx UE performs D2D transmission at the transmit power matching the corresponding event as described above. If none of the events occurs, the Tx UE transmits the D2D discovery signal or data at the maximum transmit power as described above.

[0178] FIG. 13 is a flowchart illustrating a power control method of a D2D Tx UE according to still another embodiment of the present invention.

[0179] Unlike the embodiment of FIG. 11, the embodiment of FIG. 12 is directed to the case of using a minimum D2D transmit power P.sub.TH to guarantee D2D performance; thus, the D2D Tx UE requests to the eNB for resource changed when the min{P.sub.eNB, P.sub.D2D} becomes less than P.sub.TH.

[0180] Steps 1301 to 1309 are equivalent to the corresponding steps of FIG. 11.

[0181] In reference to FIG. 13, the Tx UE determines at step 1311 whether the transmit power determined according to the determination result at step 1307 is less than the minimum threshold value.

[0182] If the transmit power is not less than the minimum threshold value, the Tx UE transmits the D2D information at the transmit power determined finally, as in the embodiment of FIG. 11, at step 1315.

[0183] If the transmit power is less than the minimum threshold value, the Tx UE request to the eNB for resource change at step 1313.

[0184] In the case of D2D discovery, if the transmit power of the UE operating in Type 1 is less than P.sub.TH, the Tx UE requests to the eNB for switching the discovery method to Type 2. In the case of D2D communication, if the transmit power of the UE operating in Mode 2 is less than P.sub.TH, the Tx UE requests to the eNB for switching the communication method to Mode 1.

[0185] In the case that the UE to be switched to Type 2/Mode 1 is in the RRC_Idle state, the Tx UE performs random access to be allocated resources for transmitting a resource request. In the case that the UE to be switched to Type 2/Mode 1 is in the RRC_Connected state, the Tx UE performs random access even when there is no resource for transmitting the resource request. In the case of being allocated resources for transmitting the resource request in the RRC_Connected state, the Tx UE transmits the resource request on the corresponding resources (i.e., If the resource for cellular PUSCH transmission is allocated, the resource request is piggybacked on the cellular data). If the resource request is received from the D2D Tx UE, the eNB may accept the resource request and transmit to the UE the information on the changed time/frequency resource through Physical Downlink Control Channel (PDCCH).

[0186] Meanwhile, the UEs that have performed D2D transmit power control may report the transmit power control results to the eNB. If the reports are received, the eNB performs monitoring during a predetermined time and then determines whether to increase or decrease the ratio of the Ctrl. regions. If the ratio of the Ctrl. regions is changed, the eNB broadcasts the change through an SIB in order for receipt by all D2D UEs within the cell.

[0187] FIG. 14 is a block diagram illustrating a configuration of a D2D UE according to the present invention.

[0188] In reference to FIG. 14, the D2D UE 1400 according to the present invention includes a communication unit 1401, a controller 1403, and storage 1405. The D2D UE 1400 may operate as a Tx UE or an Rx UE according to an embodiment of the present invention.

[0189] The communication unit 1401 may transmit or receive data to or from another UE. For this purpose, the communication unit 1401 may include at least one communication module and at least one antenna.

[0190] The controller 1403 may control the components of the UE 1400 for the power control according to the present invention.

[0191] The storage 1405 may store various parameters for use in the operations according to the present invention.

[0192] It is to be appreciated that those skilled in the art can change or modify the embodiments without departing the technical concept of this invention. Accordingly, it should be understood that the above-described embodiments are essentially for illustrative purpose only and not in any way for restriction thereto. Thus the scope of the invention should be determined by the appended claims and their legal equivalents rather than the specification, and various alterations and modifications within the definition and scope of the claims are included in the claims.