WO2015066916A1

WO2015066916A1 - A method and apparatus for registering user equipment for device-to-device discovery

Info

Publication number: WO2015066916A1
Application number: PCT/CN2013/086845
Authority: WO
Inventors: Juha Korhonen; Yixue Lei; Zexian Li; Kodo Shu; Zhi Zhang
Original assignee: Nokia Technologies Oy; Nokia (China) Investment Co., Ltd.
Priority date: 2013-11-11
Filing date: 2013-11-11
Publication date: 2015-05-14

Abstract

A method and apparatus may be configured to request registration for device-to-device discovery. The method may also include receiving a confirmation of the registration. The method may also include performing discovery-signal transmissions based on the confirmation.

Description

A METHOD AND APPARATUS FOR REGISTERING USER EQUIPMENT FOR DEVICE-TO-DEVICE DISCOVERY

BACKGROUND:

Field:

[0001] Embodiments of the invention relate to registering user equipment for device-to-device discovery.

Description of the Related Art:

[0002] Long-term Evolution (LTE) is a standard for wireless communication that seeks to provide improved speed and capacity for wireless communications by using new modulation/signal processing techniques. The standard was proposed by the 3^rd Generation Partnership Project (3GPP), and is based upon previous network technologies. Since its inception, LTE has seen extensive deployment in a wide variety of contexts involving the communication of data.

SUMMARY:

[0003] According to a first embodiment, a method may include requesting, by a user equipment, registration for device-to-device discovery. The method may also include receiving a confirmation of the registration. The method may also include performing discovery-signal transmissions based on the confirmation.

[0004] In the method of the first embodiment, the method further includes transmitting, by the user equipment, a random-access preamble. The method also includes, after receiving a response for the transmitted preamble, transmitting a registration request message.

[0005] In the method of the first embodiment, the registration request message includes Medium-Access-Control level signaling.

[0006] In the method of the first embodiment, the preamble is transmitted using a sequence, frequency and time resource combination that is reserved only for the registration for device-to-device discovery. [0007] In the method of the first embodiment, the requesting includes transmitting an identity of the user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

[0008] In the method of the first embodiment, the receiving the confirmation of the registration includes receiving an identity that was included in the registration request message.

[0009] According to a second embodiment, an apparatus includes at least one processor. The apparatus may also include at least one memory including computer program code. The at least one memory and the computer program code may be configured, with the at least one processor, to cause the apparatus at least to request registration for device-to-device discovery. The apparatus may also receive a confirmation of the registration. The apparatus may also perform discovery-signal transmissions based on the confirmation.

[0010] In the apparatus of the second embodiment, the apparatus is further caused to transmit a random-access preamble. The apparatus may also, after receiving a response for the transmitted preamble, transmit a registration request message.

[0011] In the apparatus of the second embodiment, the registration request message includes Medium-Access-Control level signaling.

[0012] In the apparatus of the second embodiment, the preamble is transmitted using a sequence, frequency and time resource combination that is reserved only for the registration for device-to-device discovery.

[0013] In the apparatus of the second embodiment, the requesting includes transmitting an identity of the apparatus for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity. [0014] In the apparatus of the second embodiment, the receiving the confirmation of the registration includes receiving an identity that was included in the registration request message.

[0015] According to a third embodiment, a computer program product may be embodied on a non-transitory computer readable medium. The computer program product may be configured to control a processor to perform a process including requesting registration for device-to-device discovery. The process also includes receiving a confirmation of the registration. The process also includes performing discovery-signal transmissions based on the confirmation.

[0016] In the computer program product of the third embodiment, the process may also include transmitting a random-access preamble. The process also includes, after receiving a response for the transmitted preamble, transmitting a registration request message.

[0017] In the computer program product of the third embodiment, the registration request message includes Medium-Access-Control level signaling.

[0018] In the computer program product of the third embodiment, the preamble is transmitted using a sequence, frequency and time resource combination that is reserved only for the registration for device-to-device discovery.

[0019] In the computer program product of the third embodiment, the requesting includes transmitting an identity of the user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

[0020] In the computer program product of the third embodiment, the receiving the confirmation of the registration includes receiving an identity that was included in the registration request message.

[0021] According to a fourth embodiment, a method may include receiving, by a network node, a request. The request may include a request for registration for device-to-device discovery. The method may also include transmitting a confirmation of the registration.

[0022] In the method of the fourth embodiment, the network node includes an evolved Node B.

[0023] In the method of the fourth embodiment, the method may also include receiving, by the network node, a random-access preamble. The method may also include transmitting a preamble response. The preamble response includes an identity for addressing user equipment.

[0024] In the method of the fourth embodiment, the receiving the request includes receiving the request via Medium-Access-Control level signaling.

[0025] In the method of the fourth embodiment, the receiving the request includes receiving an identity of user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

[0026] According to a fifth embodiment, an apparatus may include at least one processor. The apparatus may include at least one memory including computer program code. The at least one memory and the computer program code may be configured, with the at least one processor, to cause the apparatus at least to receive a request. The request includes a request for registration for device-to-device discovery. The apparatus may also transmit a confirmation of the registration.

[0027] In the apparatus of the fifth embodiment, the apparatus includes an evolved Node B.

[0028] In the apparatus of the fifth embodiment, the apparatus is further caused to receive a random-access preamble. The apparatus may also transmit a preamble response. The preamble response includes an identity for addressing user equipment. [0029] In the apparatus of the fifth embodiment, the receiving the request includes receiving the request via Medium-Access-Control level signaling.

[0030] In the apparatus of the fifth embodiment, the receiving the request includes receiving an identity of user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

[0031] According to a sixth embodiment, a computer program product may be embodied on a non-transitory computer readable medium. The computer program product may be configured to control a processor to perform a process including receiving, by a network node, a request. The request includes a request for registration for device-to-device discovery. The process may also include transmitting a confirmation of the registration.

[0032] In the computer program product of the sixth embodiment, the process may include receiving, by the network node, a random-access preamble. The process may also include transmitting a preamble response. The preamble response includes an identity for addressing user equipment.

[0033] In the computer program product of the sixth embodiment, the receiving the request includes receiving the request via Medium-Access-Control level signaling.

[0034] In the computer program product of the sixth embodiment, the receiving the request includes receiving an identity of user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

[0035] According to a seventh embodiment, an apparatus includes means for requesting, by a user equipment, registration for device-to-device discovery. The apparatus may also include means for receiving a confirmation of the registration. The apparatus may also include means for performing discovery-signal transmissions based on the confirmation. [0036] According to an eighth embodiment, an apparatus includes means for receiving, by a network node, a request. The request includes a request for registration for device-to-device discovery. The apparatus also includes means for transmitting a confirmation of the registration.

BRIEF DESCRIPTION OF THE DRAWINGS:

[0037] For proper understanding of the invention, reference should be made to the accompanying drawings, wherein:

[0038] Fig. 1 illustrates registering and deregistering procedures in accordance with embodiments of the invention.

[0039] Fig. 2 illustrates a flowchart of a method in accordance with embodiments of the invention.

[0040] Fig. 3 illustrates a flowchart of a method in accordance with embodiments of the invention.

[0041] Fig. 4 illustrates an apparatus in accordance with embodiments of the invention.

[0042] Fig. 5 illustrates an apparatus in accordance with embodiments of the invention.

[0043] Fig. 6 illustrates an apparatus in accordance with embodiments of the invention.

DETAILED DESCRIPTION:

[0044] Embodiments of the invention relate to registering user equipment for device-to-device (D2D) discovery. Device-to-device proximity detection is part of a 3GPP LTE RAN study item (See Study Item RP- 122009). Proximity detection methods may be performed on user equipment (UE) based on the transmission and the detection of discovery signals. In one use case, the UEs are in the coverage area of a cellular network, and the network assigns an overall discovery-resource pool from where a UE obtains resources for its discovery signal transmission. The overall resources may be assigned only for simplifying signaling related to UE-specific resource allocation or they may be assigned for contention-based usage by the UEs. Contention-based usage means that UEs may have freedom to select a resource from multiple choices and different UEs may end up selecting the same resource. Although no agreement on a method of sharing resources between the UEs has yet been reached, a method that includes contention-based resource sharing is likely to be specified. The contention-based resource sharing may become the only method of discovery-resource access or it may be specified along with UE-specific allocation. The reason that contention-based resource sharing is important is that it may be especially suitable for resource sharing among Radio-Resource-Control (RRC) idle-mode UEs. UEs are, most of the time, in the RRC idle-mode and thus it will be important to also allow idle-mode UEs to transmit discovery signals.

[0045] As described by RAN 2 (specifically, RAN2-83bis Chairman Notes), both idle-mode UEs and RRC-connected-mode UEs may participate in the discovery signal transmissions and/or reception. However, a problem with having idle-mode UEs participate in the discovery of signal transmissions is that the network generally does not track the location of these UEs on the cell basis, but rather knows only the so-called tracking area of each UE. A tracking area is typically composed of multiple cells. Therefore, the network cannot predict the number of idle -mode UEs transmitting discovery signals on the resources shared in the contention-based manner. The network thus cannot (1) adapt to the load by controlling the amount of available resources for the UEs, nor (2) control the load that the UEs are allowed to cause. An evolved Node B (eNB) (of the network) may not be able to directly observe the discovery signals because the transmission power of the discovery signals (as transmitted by the UEs) could be limited. Therefore, in order to provide the network with better means for resource control, a procedure by which the idle-mode UEs may register for performing discovery-signal transmissions may be desirable.

[0046] One embodiment of the present invention relates to methods for UEs to register and deregister themselves as device-to-device (D2D) users. The UEs may perform their registration and deregistration using a random-access procedure. In one embodiment of the present invention, the (D2D-discovery related) registering/deregistering is a new cause for starting a random-access procedure. In one embodiment of the present invention, the registration and deregistration of UEs is performed by using only Medium- Access-Control-level (MAC-level) signaling, and idle-mode UEs are maintained in an RRC-idle state even after the random access procedure.

[0047] In one embodiment of the present invention, the previous approaches for performing random-access procedures may be modified. The modifications may include changes in the partitioning of preamble signatures. The modifications may also include changes in the content of the message that a UE transmits during the random access procedure. In the LTE standard, such a UE message is called "Message 3", and it is the first uplink (UL) transmission on a Physical-Uplink-Shared-Channel (PUSCH), transmitted on a resource that is indicated by an earlier Message 2. The Message 2 is transmitted by an eNB as a response to an observed preamble. The modifications may also include changes in the content of the message that the network sends as a response to the UE's random access message. In LTE, such a message is called the random access "Message 4". According to present specification, Messsage 4 is transmitted for contention resolution as its contents identify which Message 3 it is a response to. In a procedure, modified for supporting registration for discovery, Message 4 may be generally considered to be an acknowledgement of the registration of a UE for transmitting discovery signals. Message 4 may signal the allowed resource utilization rate and the timer value for re -registration to the UE (from an eNB, for example). Messages 2-4 are discussed in greater detail below. Embodiments of the present invention also introduce methods of indicating the registration of UEs between different cells. In the following discussion, we use LTE terminology of the random access procedure but it should be understood that corresponding signals and messages appear in the random access procedures of different systems, although they may be named differently in the other systems.

[0048] As discussed above, in embodiments of the present invention, before UEs are allowed to start the transmissions of discovery signals, the UEs may need to register as D2D users of a network. The UEs do not necessarily initiate the registration procedure before every discovery signal transmission, but then the registration could continue over a longer period, during which multiple discovery signal transmissions may occur. The registration over the longer period may even continue until a deregistration procedure occurs.

[0049] A connected-mode UE may register itself as a D2D user by sending a Media-Access-Control (MAC) control element or other indication on a PUSCH or on a Physical Uplink Control Channel (PUCCH). By sending the MAC control element or the indication on the PUSCH or PUCCH, the UE may ask for resources via a normal scheduling request procedure. Embodiments of the present invention may be directed to UEs that are in idle mode that register through a new modified random-access procedure. For efficient control of a load on the resources for discovery signal transmission, deregistration of a UE (when the UE does not need the resources any more) may be important. Also, re-registering the UE may be performed in accordance with periodic renewal or may be performed in accordance with a change in the registration. Embodiments of the present invention may also be directed to UEs in RRC connected mode that do not have resources on PUCCH for sending scheduling or registration requests nor have ongoing data transmissions on PUSCH. Such UEs may also register for discovery through the new modified random access procedure because that is a more straightforward method than first asking PUSCH resources by normal random access procedure and only after that sending the registration MAC control element or indication on PUSCH.

[0050] Fig. 1 illustrates registering and deregistering procedures in accordance with embodiments of the present invention. With regard to the transmitting/receiving of the random-access (RA) preamble 100, embodiments of the present invention have two alternative ways for an eNB to determine a new cause of a random access. According to a first alternative, a signature of a preamble, like the preamble sequence or the frequency or time resource or certain combinations of sequence, frequency and time, may be used to indicate the new cause of a random access to the eNB. As discussed above, in embodiments of the present invention, new causes of a random access may include the registration and/or the deregistration of a UE for D2D discovery. If the preamble indicates the cause to the eNB, the content of subsequent random-access Message 3 may then be specified for the new cause without necessarily retaining, for example, the MAC header structure of the present Message 3. Messages 3 and 4 may be needed only for contention resolution with this alternative or they could also carry information for D2D discovery, like the UE indicating to the network how frequently the UE wants to transmit discovery signals or the reason why the UE wants to transmit discovery signals or the network indicating to the UE how frequently the UE is allowed to transmit discovery signals. The preamble signature indicating the new cause of random access should not be used by UEs of the earlier releases with the existing causes of random access. With the LTE system, a simple way is to specify that a preamble sequence used for indicating the new cause is from the pool of the preambles the UEs of the earlier releases regard as so called dedicated preambles that they may utilize only when requested by eNB.

[0051] According to a second alternative, the cause of the random access may be indicated in random access Message 3 to the eNB. This indicating may be performed by defining/using a MAC header Logical-Channel-ID (LCID) value for such indication and the corresponding MAC message. [0052] With regard to the transmitting/receiving of a preamble response 101, in embodiments of the present invention, a preamble response may be sent by the eNB to the UE in a manner similar to the manner of a normal random access procedure. The preamble response may specify a resource allocation for Message 3 and a temporary identity T-CRNTI for addressing the UE on a Physical-Downlink-Control Channel (PDCCH) during the rest of the procedure.

[0053] With regard to the transmitting/receiving of a request for registration 102 (and the sending of requests for deregistration 1 10), besides indicating an identity of the UE used for contention resolution, the information fields in the random-access Message 3 may indicate (1) the cause of the random access, (2) the desired utilization rate of discovery signal resources, and/or (3) a registration identity.

[0054] With regard to the desired utilization rate, the desired utilization rate of discovery signal resources may correspond to the frequency (or the duty cycle) of discovery signal transmissions. Depending on the service or application for which the discovery signals are transmitted for, the UE may request a different utilization rate.

[0055] With regard to indicating a registration identity, a reason for defining a registration identity may be to allow more accurate load estimation as compared to merely counting the number of registered UEs, if different utilization rates of the discovery resources are allowed for different UEs. The UE may receive its registration identity, assigned by the eNB, during the registering procedure. The UE would include such an assigned identity in the deregistering message so that the eNB may link the indicated deregistration with the corresponding earlier registration. In one embodiment, in order to mitigate the effect of some error cases where UEs leave the cell without deregistering, the UEs need to re -register at certain intervals in order to be allowed to continue using the resource to transmit discovery signals. Re-registering would generally follow the same procedure as deregistering, including the sending of the registration identity. Alternatively, if the eNB does not receive periodic registration for a UE, the eNB would assume that the UE has left the cell (and that the UE has possibly camped to another cell or has been switched off). The validity time of the registration could be a parameter broadcasted in the system information, or the validity time could be signalled separately for each UE in Message 4 if more flexibility is needed.

[0056] With regard to receiving confirmation of registration 103 (or receiving confirmation of deregistration 1 13), in embodiments of the present invention, the registration / deregistration / re-registration with the network may be confirmed by reception of the random access Message 4 by the UE. In the event of de-registration with the network, only acknowledgement of the reception of the deregistration message may be necessary. In case of registration or re-registration, the eNB could additionally signal the allowed resource-utilization rate to the UE, or general or UE-specific information on discovery-signal resources. In case of registration, the eNB could include a registration identity. One option is that a temporary, cell-specific, identity, such as a temporary-cell-radio-network-temporary identifier (T-CRNTI), which was previously assigned to the UE in the preamble response, may be used as the registration identity. A pre-existing cell-radio-network-temporary identifier (CRNTI) may also be used as the registration identity. In such cases, no registration identity would need to be sent in the Message 4. Although an RNTI, valid on a cell basis, would be sufficient for registration- status handling and would be compatible with present identity handling in the network, tracking area-specific registration identities could also be considered. For instance, the identity SAE-Temporary Mobile Subscriber Identity (S-TMSI), which is included in Message 3 by idle-mode UEs for contention resolution, may be used as the registration identity.

[0057] Using the S-TMSI as the registration identity for the UEs would allow the network to track discovery-signal transmitting UEs of a camping cell. [0058] During and after the registration procedure, UE may remain in the idle mode, while when an idle mode UE is performing the normal random access, it intends to establish or re-establish RRC connection.

[0059] Embodiments of the present invention may also change registration from one cell to another cell. In embodiments of the present invention, idle -mode UEs may autonomously select the camping cell. Some modifications to the normal cell selection procedures might be needed for the transmitting of discovery signals by UEs in order to ensure that the UEs use the discovery resources of the right cell. In view of these modifications, the measurement requirements may be similar measurement requirements as used by connected-mode UEs but without the burden of reporting the results to the network.

[0060] In embodiments of the present invention, when a registered UE moves from an original cell to the coverage area of another cell, the registration of the registered UE should be changed from the original cell to the new cell. In one method, the registered UE may de-register from the original cell and then register with the new cell. This method may save the network from performing communication between the eNBs (corresponding to the cells) due to the cell change. Another possibility is that, when the UE registers with the new cell, the UE would indicate that it has registered in another cell. The network would then indicate to the original cell that registration may be terminated at the original cell. Such indication may happen with access stratum signaling. Such a method may save a random access procedure per each cell change. In yet another possibility, the UE may register with the new cell, and the registration in the old cell may terminate when re-registration does not occur in due time.

[0061] According to Release 8 random access procedures, a UE may receive a T-CRNTI in a preamble response. The T-CRNTI may be used for addressing the UE on the PDCCH during the random access procedure. If the UE does not have a CRNTI when the UE starts the procedure (for example, idle mode UEs do not have a CRNTI) and the UE wins the contention resolution, the T-CRNTI becomes the UE's CRNTI.

[0062] In Message 3, the UE sends its identity that is used for contention resolution. If the UE has CRNTI, the UE includes the CRNTI with Message 3. If the UE does not have CRNTI, but has an S-TMSI, the UE includes the S-TMSI. If the UE has neither CRNTI nor S-TMSI, the UE includes a random identity. The eNB includes the random identity or S-TMSI in Message 4, the random identity or S-TMSI having been sent by the UE via Message 3. If the UE observes a matching identity in Message 4, the UE knows that the UE has won the contention resolution. If the UE has included CRNTI in Message 3, the PDCCH transmission addressed to the CRNTI and containing a UL grant for a new transmission indicates a successful contention resolution for the UE. In this case, no Message 4 needs to be transmitted.

[0063] In embodiments of the present invention, after successful contention resolution, one possibility is to define that the T-CRNTI becomes the UE's registration identity. If a longer identity is needed, the eNB could give an additional identity byte in Message 4 that, when added to the T-CRNTI, would give a sufficient number of registration identities.

[0064] Another possibility is to use S-TMSI as the registration identity if it is thought to be useful to be able to track the camping cells of registered UEs.

[0065] In view of the above, embodiments of the present invention provide means for the network to accurately estimate a load and to control the load on discovery signal resources. In certain embodiments of the present invention, no signaling higher than MAC level signaling is needed.

[0066] Fig. 2 illustrates a flowchart of a method in accordance with embodiments of the invention. The method illustrated in Fig. 2 includes, at 200, requesting registration for device-to-device discovery. The method includes, at 201, receiving a confirmation of the registration. The method includes, at 202, performing discovery-signal transmissions based on the confirmation. [0067] Fig. 3 illustrates a flowchart of a method in accordance with embodiments of the invention. The method illustrated in Fig. 3 includes, at 300, receiving, by a network node, a request. The request includes a request for registration for device-to-device discovery. The method includes, at 301, transmitting a confirmation of the registration.

[0068] In another embodiment, an apparatus is disclosed comprising means to perform any of the described methods.

[0069] Fig. 4 illustrates an apparatus in accordance with an embodiment of the invention. In one embodiment, the apparatus may be a user equipment. In another embodiment, the apparatus may be an eNB/base station. Apparatus 10 may include a processor 22 for processing information and executing instructions or operations. Processor 22 may be any type of general or specific purpose processor. While a single processor 22 is shown in Fig. 4, multiple processors may be utilized according to other embodiments. Processor 22 may also include one or more of general-purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), and processors based on a multi-core processor architecture, as examples.

[0070] Apparatus 10 may further include a memory 14, coupled to processor 22, for storing information and instructions that may be executed by processor 22. Memory 14 may be one or more memories and of any type suitable to the local application environment, and may be implemented using any suitable volatile or nonvolatile data storage technology such as a semiconductor-based memory device, a magnetic memory device and system, an optical memory device and system, fixed memory, and removable memory. For example, memory 14 include any combination of random access memory (RAM), read only memory (ROM), static storage such as a magnetic or optical disk, or any other type of non-transitory machine or computer readable media. The instructions stored in memory 14 may include program instructions or computer program code that, when executed by processor 22, enable the apparatus 10 to perform tasks as described herein.

[0071] Apparatus 10 may also include one or more antennas (not shown) for transmitting and receiving signals and/or data to and from apparatus 10. Apparatus 10 may further include a transceiver 28 that modulates information on to a carrier waveform for transmission by the antenna(s) and demodulates information received via the antenna(s) for further processing by other elements of apparatus 10. In other embodiments, transceiver 28 may be capable of transmitting and receiving signals or data directly.

[0072] Processor 22 may perform functions associated with the operation of apparatus 10 including, without limitation, precoding of antenna gain/phase parameters, encoding and decoding of individual bits forming a communication message, formatting of information, and overall control of the apparatus 10, including processes related to management of communication resources.

[0073] In an embodiment, memory 14 may store software modules that provide functionality when executed by processor 22. The modules may include an operating system 15 that provides operating system functionality for apparatus 10. The memory may also store one or more functional modules 18, such as an application or program, to provide additional functionality for apparatus 10. The components of apparatus 10 maybe implemented in hardware, or as any suitable combination of hardware and software.

[0074] Fig. 5 illustrates an apparatus in accordance with another embodiment. Apparatus 500 may be a user equipment, for example. Apparatus 500 may include a requesting unit 501 that requests registration for device-to-device discovery. Apparatus 500 may also include a receiving unit 502 that receives a confirmation of the registration. Apparatus 500 may also include a performing unit 503 that performs discovery-signal transmissions based on the confirmation.

[0075] Fig. 6 illustrates an apparatus in accordance with another embodiment. Apparatus 600 may be an eNB/base station, for example. Apparatus 600 may include a receiving unit 601 that receives a request. The request may include a request for registration for device-to-device discovery. Apparatus 600 may also include a transmitting unit 602 that transmits a confirmation of the registration.

[0076] The described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and/or with hardware elements in configurations which are different than those which are disclosed. Therefore, although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method, comprising:

requesting, by a user equipment, registration for device-to-device discovery;

receiving a confirmation of the registration; and

performing discovery-signal transmissions based on the confirmation.

2. The method according to claim 1, the method further comprising: transmitting, by the user equipment, a random-access preamble; and after receiving a response for the transmitted preamble, transmitting a registration request message.

3. The method according to claim 1 or 2, wherein the registration request message comprises Medium-Access-Control level signaling.

4. The method of any of the claims 1-3, wherein the preamble is transmitted using a sequence, frequency and time resource combination that is reserved only for the registration for device-to-device discovery.

5. The method according to any of claims 1-4, wherein the requesting comprises transmitting an identity of the user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

6. The method according to any of the claims 1-5, wherein the receiving the confirmation of the registration comprises receiving an identity that was included in the registration request message.

7. An apparatus, comprising: at least one processor; and

at least one memory including computer program code,

the at least one memory and the computer program code configured, with the at least one processor, to cause the apparatus at least to

request registration for device-to-device discovery;

receive a confirmation of the registration; and

perform discovery-signal transmissions based on the confirmation.

8. The apparatus according to claim 7, wherein the apparatus is further caused to:

transmit a random-access preamble; and

after receiving a response for the transmitted preamble, transmit a registration request message.

9. The apparatus according to claim 7 or 8, wherein the registration request message comprises Medium-Access-Control level signaling.

10. The apparatus according to claims 7-9, wherein the preamble is transmitted using a sequence, frequency and time resource combination that is reserved only for the registration for device-to-device discovery.

11. The apparatus according to any of claims 7-10, wherein the requesting comprises transmitting an identity of the apparatus for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

12. The apparatus according to any of claims 7-11, wherein the receiving the confirmation of the registration comprises receiving an identity that was included in the registration request message.

13. A computer program product, embodied on a non-transitory computer readable medium, the computer program product configured to control a processor to perform a process, comprising:

requesting registration for device-to-device discovery;

receiving a confirmation of the registration; and

performing discovery-signal transmissions based on the confirmation.

14. The computer program product according to claim 13, the process further comprising:

transmitting a random-access preamble; and

after receiving a response for the transmitted preamble, transmitting a registration request message.

15. The computer program product according to claim 13 or 14, wherein the registration request message comprises Medium-Access-Control level signaling.

16. A method, comprising:

o receiving, by a network node, a request, wherein the request comprises a request for registration for device-to-device discovery; and

transmitting a confirmation of the registration.

17. The method according to claim 16, wherein the network node 5 comprises an evolved Node B .

18. The method according to claim 16 or 17, further comprising:

receiving, by the network node, a random-access preamble; and transmitting a preamble response, wherein the preamble response o comprises an identity for addressing user equipment.

19. The method according to any of claims 16-18, wherein the receiving the request comprises receiving the request via Medium-Access-Control level signaling.

20. The method according to any of claims 16-19, wherein the receiving the request comprises receiving an identity of user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

21. An apparatus, comprising:

at least one processor; and

at least one memory including computer program code,

receive a request, wherein the request comprises a request for registration for device-to-device discovery; and

transmit a confirmation of the registration.

22. The apparatus according to claim 21, wherein the apparatus comprises an evolved Node B.

23. The apparatus according to claim 21 or 22, wherein the apparatus is further caused to:

receive a random-access preamble; and

transmit a preamble response, wherein the preamble response comprises an identity for addressing user equipment.

24. The apparatus according to any of claims 21-23, wherein the receiving the request comprises receiving the request via Medium- Access-Control level signaling.

25. The apparatus according to any of claims 21-24, wherein the receiving the request comprises receiving an identity of user equipment for contention resolution, a cause of a random access, a desired utilization rate of discovery-signal resources, or a registration identity.

26. A computer program product, embodied on a non-transitory computer readable medium, the computer program product configured to control a processor to perform a process, comprising:

receiving, by a network node, a request, wherein the request comprises a request for registration for device-to-device discovery; and

transmitting a confirmation of the registration.

27. The computer program product according to claim 26, wherein the receiving the request comprises receiving the request via Medium-Access-Control level signaling.

28. An apparatus, comprising:

means for requesting, by a user equipment, registration for device-to-device discovery;

means for receiving a confirmation of the registration; and

means for performing discovery-signal transmissions based on the confirmation.

29. An apparatus, comprising:

means for receiving, by a network node, a request, wherein the request comprises a request for registration for device-to-device discovery; and

means for transmitting a confirmation of the registration.