IN A WIRELESS NETWORK

Technical field

Examples of the present disclosure relate to methods and apparatus in a wireless network, and particularly relate to radio resource management for device-to-device transmissions in a wireless network.

Background

Release 12 of the LTE standard provided for device to device (D2D) communications (also known as "sidelink" and "LTE Direct") targeting both commercial and public safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated applications by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.

One of the potential extensions for the D2D specifications consists of support for vehicle-to-x (V2x) communication, which includes any combination of direct communication between vehicles, pedestrians and infrastructure. V2x communication may take advantage of a network infrastructure, when available, but at least basic V2x connectivity should be possible even in case of lack of coverage. Providing an LTE- based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the network infrastructure (vehicle-to-infrastructure, or V2I), vehicle-to-person (V2P) and vehicle-to-vehicle (V2V) communications, as compared to using a dedicated V2x technology.

V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc.

The European Telecommunications Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM). CAM: The CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM message also serves as active assistance to safety driving for normal traffic. The availability of a CAM message is indicatively checked for every 100ms, yielding a maximum detection latency requirement of <=100ms for most messages.

DENM: The DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100ms.

Depending on the use case latency requirements for CAM and DENM may vary significantly. As an example, latency may vary from 20ms for pre-crash warnings, to 100ms for emergency stop, or queue warnings, or 1000ms for non-safety related use cases such as traffic flow optimization, curve speed warnings etc.

The package size of CAM and DENM message varies from 100+ to 800+ bytes and the typical size is around 300 bytes depending on the specific V2X use case, message type (e.g. DENM is supposed to be larger than CAM) and on the security format included in the packet (i.e. full certificate or certificate digest). The message is supposed to be detected by all vehicles in proximity.

The SAE (Society of the Automotive Engineers) also defined the Basic Safety Message (BSM) for DSRC with various messages sizes defined. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.

Figure 1 shows a wireless network 10 comprising a wireless network node 12 and a plurality of terminal devices. The terminal devices include, as examples, a mobile phone 14, terminal devices employed in a plurality of different vehicles 16a, 16b, 16c, and a terminal device carried by a user 18.

As is conventional, the wireless network 10 provides for wireless communications from the terminal devices to the wireless network node 12 ("uplink"), and wireless communications from the wireless network node 12 to the terminal devices ("downlink"). In addition, the network 10 provides for direct D2D communications between the terminal devices themselves. Such D2D communications may provide for one or more of vehicle-to- vehicle (V2V) communications, vehicle-to-person (V2P) communications and vehicle-to-infrastructure (V2I) communications.

Summary

For periodic traffic, it is usually desirable to have scheduling methods that reduce the associated overhead. Moreover, for systems using distributed resource allocation, it is useful that UEs inform each other about their intentions with respect to using resources for future transmissions. In this way, UEs can predict future usage of the resources and find good allocations for their transmissions.

One way of improving resource allocation is to avoid continuous reselection of the resources. That is, a UE uses the same resources (e.g., in a periodically repeating time-frequency grid) for transmission and only changes them occasionally (e.g., when a collision is detected, etc.). In this way, a UE can predict the utilization of future resources by assuming it will be similar to the current one. However, UEs changing their resources (e.g., because they detect a collision, or because their allocation expires, etc.) still pose a problem. Failure to predict these changes results in an increased number of collisions and resources being utilized less efficiently. For some algorithms this may also result in increased interference or half-duplex issues.

Another problem is related to the behaviour of V2x traffic. V2x traffic is characterized by messages with a certain typical predefined ranges for message sizes and frequency of generation. As a result, one method to relieve the aforementioned problem of unpredictability of resource utilization and to meet the characteristic of V2x traffic is to pre-provision resources for message transmissions. However, the actual generation frequency and the packet size at a given time instance are not easily predictable as they depend on a number of external factors (e.g. UE trajectory, speed and geographical position), on the specific V2x use case, on network congestion and other higher layer aspects (such as the periodic presence of larger packet headers including for instance security components) that cannot be known a priori by lower layers. This makes the applicability of the above pre-provisioning method or of any other delay- aware scheduler for V2x traffic inefficient, since the periodic resources need to be provisioned in a conservative way. It is an aim of the present disclosure to provide methods and apparatus which obviate or reduce at least one or more of the disadvantages mentioned above. In one aspect, the present disclosure provides a method in a wireless network, the method comprising, in a first terminal device: transmitting a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, transmitting a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission.

In another aspect, the present disclosure provides a method in a wireless network, the method comprising, in a second terminal device: receiving a control message from a first terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, listening for a confirmation message from the first terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission. In a further aspect, the present disclosure provides a terminal device for a wireless network, adapted to: transmit a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, transmit a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the terminal device for a device-to-device transmission. In another aspect, the present disclosure provides a terminal device for a wireless network, the terminal device being adapted to: receive a control message from a remote terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the remote terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, listen for a confirmation message from the remote terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the remote terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the remote terminal device for a device-to-device transmission.

In another aspect, the present disclosure provides a terminal device for a wireless network, comprising a processor and a memory, said memory containing instructions executable by said processor, whereby said terminal device is operative to: transmit a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, transmit a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the terminal device for a device-to-device transmission. In another aspect, the present disclosure provides a terminal device for a wireless network, the terminal device comprising a processor and a memory, said memory containing instructions executable by said processor, whereby said terminal device is operative to: receive a control message from a remote terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the remote terminal device for a plurality of device-to-device transmissions; and prior to a first time slot of the plurality of time slots, listen for a confirmation message from the remote terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the remote terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the remote terminal device for a device-to- device transmission. In another aspect, the present disclosure provides a terminal device for a wireless network, comprising: a first module configured to transmit a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the terminal device for a plurality of device-to-device transmissions; and a second module configured to, prior to a first time slot of the plurality of time slots, transmit a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the terminal device for a device-to-device transmission.

In another aspect, the present disclosure provides a terminal device for a wireless network, comprising: a first module configured to receive a control message from a remote terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the remote terminal device for a plurality of device-to-device transmissions; and a second module configured to, prior to a first time slot of the plurality of time slots, listen for a confirmation message from the remote terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the remote terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the remote terminal device for a device-to- device transmission.

Brief description of the drawings

For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Figure 1 shows an example of a wireless network in which examples of the present disclosure may be employed.

Figures 2 to 5 show flowcharts of methods according to examples of the present disclosure. Figures 6 to 9 show terminal devices according to examples of the present disclosure.

Figures 10 and 11 are signalling diagrams of terminal devices according to examples of the present disclosure.

Detailed description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer- readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions. Although the term "wireless device" may be used in the description, it is noted that this term encompasses other terms used to denote wireless devices, such as user equipment (UE). It should be understood by the person skilled in the art that "UE" is a non-limiting term comprising any mobile or wireless device or node equipped with a radio interface allowing for at least one of: transmitting signals in uplink (UL), receiving and/or measuring signals in downlink (DL), and transmitting and/or receiving signals in a D2D/sidelink mode. A wireless device herein may comprise a UE (in its general sense) capable of operating or at least performing measurements in one or more frequencies, carrier frequencies, component carriers or frequency bands. It may be a "UE" operating in single- or multi-radio access technology (RAT) or multi-standard mode. As well as "wireless device" or "UE", the terms "mobile device" and "terminal device" may be used interchangeably in the description, and it will be appreciated that such a device does not necessarily have to be 'mobile' in the sense that it is carried by a user. Instead, the term "mobile device" encompasses any device that is capable of communicating with communication networks that operate according to one or more mobile communication standards, such as the Global System for Mobile communications, GSM, Universal Mobile Telecommunications System (UMTS), Long- Term Evolution, LTE, etc.

It should be noted that use of the term "wireless network node" as used herein can refer to a base station, such as an eNodeB, a network node in the RAN responsible for resource management, such as a radio network controller (RNC), or, in some cases, a core network node, such as a mobility management entity (MME), a ProSe function (ProSe-F) node or a ProSe Application Server.

The disclosure proposes a solution that allows a terminal device to reserve resources for its coming transmissions while also allowing the possibility of temporarily or permanently ceasing the use of such reserved resources. In the latter case, other terminal devices may be informed, such that they can take this information into account when selecting resources for their own transmissions. Figure 2 is a flowchart of a method in accordance with an example of the present disclosure. The method may be carried out in a wireless first terminal device.

In step 100, the first terminal device transmits or broadcasts a control message to one or more second terminal devices over a direct communications link. That is, the control message is transmitted or broadcast from the first terminal device using resources that may be reserved for device-to-device (D2D) communication. The control message may be detected and received directly by any other terminal devices capable of D2D communication (hereinafter, "second terminal devices") within range of the transmission, without having travelled via an intervening network node such as a base station or similar. The control message contains information identifying wireless resources reserved by the first terminal device for a plurality of future D2D transmissions. The wireless resources identified in the control message thus relate to a plurality of time slots in which those future D2D transmissions are intended to be transmitted, but may also define codes, frequencies or other resources upon which the D2D transmissions are intended to be transmitted.

The plurality of time slots identified in the control message may be periodic in nature, i.e. occurring at substantially equal intervals. For example, the time slots may be separated by periods of 100 ms. The time slots may continue indefinitely, or be limited to a finite number.

The D2D transmissions for which resources are reserved may be one or more of: control plane messages (e.g. SA) and data plane messages (e.g. data transmissions). The D2D transmissions may comprise vehicle-to-X transmissions, such as one or more of co-operative awareness messages (CAMs), and decentralized environmental notification messages (DENMs). The first terminal device may therefore be comprised within a vehicle. The resources identified in the control message may be scheduled in a number of different ways. For example, the terminal device itself may determine appropriate resources for the intended D2D transmissions. Alternatively, a wireless network node, such as a base station, may specify appropriate resources for the intended D2D transmissions. The present disclosure is not limited in this respect.

The control message may include priority information associated with the transmissions intended on such resources. For example, the priority information may include a parameter indicating that the transmission for a particular resource (e.g. for a particular time slot) is relatively high priority or relatively low priority, i.e. is relatively important, or relatively unimportant. The priority information may take the form of a single bit indicating high or low priority, or more than one bit indicating a range of values for the priority. Priority information may be specified separately for each intended transmission, for multiple intended transmissions, or for all intended transmissions to which the control message relates. The control message may take the form of a scheduling assignment (SA) message or any other signalling protocol.

In step 102, prior to a first time slot of the plurality of time slots identified in the control message, the first terminal device transmits or broadcasts a confirmation message to the one or more second terminal devices conveying that the resources for at least the first time slot will be used, or transmits or broadcasts a confirmation message to the one or more second terminal devices conveying that the resources for at least the first time slot will not be used. In an alternative example, if the resources for at least the first time slot are to be used by the first terminal device, no confirmation message is transmitted and the one or second terminal devices can infer that the resources are to be used.

Note that the "first time slot" is not necessarily the time slot of the plurality of time slots occurring first in time, but may be any of the time slots identified in the control message.

Thus the disclosure provides for the resources reserved by the first terminal device in step 100 being released prior to the time slot at which the resources occur. This may allow for the resources to be used by one or more of the second terminal devices, thus increasing efficiency in the network.

A confirmation message may be sent separately prior to each of the time slots identified in the control message, indicating whether or not the resources for that time slot are to be used by the first terminal device or, alternatively, a confirmation message may indicate whether or not multiple time slots of those identified in the control message are to be used.

The confirmation message may take the form of a scheduling assignment (SA) message or any other signalling protocol. Figure 3 is a flowchart of a method in accordance with another example of the present disclosure. The method may be carried out by a first terminal device.

Step 200 is similar to step 100, described above, in which the first terminal device transmits or broadcasts a control message to one or more second terminal devices over a direct communications link. That is, the control message is transmitted or broadcast from the first terminal device using resources that may be reserved for D2D communication. The control message may be detected and received directly by any second terminal devices capable of D2D communication within range of the transmission, without having travelled via an intervening network node such as a base station or similar.

The control message contains information identifying wireless resources reserved by the first terminal device for a plurality of future D2D transmissions. The wireless resources identified in the control message thus relate to a plurality of time slots in which those future D2D transmissions are intended to be transmitted, but may also define codes, frequencies or other resources upon which the D2D transmissions are intended to be transmitted.

The plurality of time slots identified in the control message may be periodic in nature, i.e. occurring at substantially equal intervals. For example, the time slots may be separated by periods of 100 ms. The time slots may continue indefinitely, or be limited to a finite number.

The D2D transmissions for which resources are reserved may be one or more of: control plane messages (e.g. SA) and data plane messages (e.g. data transmissions). The D2D transmissions may comprise vehicle-to-X transmissions, such as one or more of co-operative awareness messages (CAMs), and decentralized environmental notification messages (DENMs). In one particular example, the D2D transmissions are CAMs. The control message may include priority information associated with the transmissions intended on such resources. For example, the priority information may include a parameter indicating that the transmission for a particular resource (e.g. for a particular time slot) is relatively high priority or relatively low priority, i.e. is relatively important, or relatively unimportant. The priority information may take the form of a single bit indicating high or low priority, or more than one bit indicating a range of values for the priority. Priority information may be specified separately for each intended transmission, for multiple intended transmissions, or for all intended transmissions to which the control message relates. The control message may take the form of a scheduling assignment (SA) message or any other signalling protocol. In step 202, prior to a first time slot of the time slots identified in the control message, the first terminal device determines whether a D2D transmission is required for the first time slot. Note that the first time slot may be the time slot occurring first in time, or any other of the plurality of time slots identified in the control message.

Step 202 may comprise determining whether or not the first terminal device has one or more data packets stored in a memory for transmission. If the memory stores one or more data packets for transmission, the first terminal device may decide that a D2D transmission is required for the first time slot. If the memory does not store any data packets for transmission, the first terminal device may decide that a D2D transmission is not required for the first time slot.

Alternative methods for determining whether a D2D transmission is required are also possible. For example, the first terminal device may determine whether a message is required to be sent on the control plane.

If a D2D transmission is required for the first time slot, the method proceeds to step 204 in which the first terminal device transmits or broadcasts a confirmation message that resources previously reserved for one or more time slots, including the first time slot, will be used. Alternatively, the first terminal device may transmit no such confirmation message, with other terminal devices inferring from the absence of a confirmation message that the resources will be used. Although not illustrated in the flowchart, one or more D2D transmissions are subsequently made using the resources identified for at least the first time slot.

If it is determined in step 202 that a D2D transmission is not required for the first time slot, the method proceeds to step 206 in which the first terminal device transmits or broadcasts a confirmation message that resources previously reserved for one or more time slots, including the first time slot, will not be used. These resources are thus released for potential use by one or more second terminal devices.

The confirmation message may further include an indication of one or more transmission formats suitable for transmission on the resources for at least the first time slot. Therefore second terminal devices receiving the confirmation message are informed that certain, previously reserved resources have become available, and are also informed of the transmission formats suitable for use of those resources. Subsequently, therefore, the second terminal devices may use the released resources for one or more D2D transmissions of their own, using one of the one or more transmission formats identified in the confirmation message. Alternatively, the transmission formats may be known to terminal devices in the network (e.g. through some other form of control signalling, such as from a wireless network node), such that they may not be included within the confirmation message.

The confirmation messages sent in step 204 or 206 may be sent separately prior to each of the time slots identified in the control message, indicating whether or not the resources for that time slot are to be used by the first terminal device. In such an embodiment, prior to each time slot identified in the control message, the first terminal device may transmit a respective confirmation message conveying that resources for a respective time will be used or will not be used. Alternatively, the confirmation message may indicate whether or not multiple time slots of those identified in the control message are to be used.

Optionally, the method may proceed to step 208, in which the first terminal device may determine that the resources identified in the control message are no longer required, and indicate to the one or more second terminal devices whether or not the resources will continue to be used.

In one example, this step may comprise determining that resources for a number of the plurality of time slots exceeding a threshold number of time slots have not been used by the first terminal device for D2D transmissions. That is, the first terminal device may maintain a count of the time slots identified in the control message for which no D2D transmission is subsequently sent. If that count exceeds a threshold number, then the first terminal device may determine that the resources identified in the control message are no longer required. The count may be specified as an absolute number, a ratio of utilized time slots to reserved time slots, or any other suitable metric. The count may be made over a finite, rolling time window, i.e. the count may relate to reserved time slots in a time window immediately preceding the current time.

In another example, step 208 may comprise determining that the resources identified in the control message have not been used for at least a threshold period of time. For example, the first terminal device may determine that no D2D transmissions have been made for a threshold number of consecutive time slots.

The step of indicating to the one or more second terminal devices whether or not the resources will continue to be used may comprise transmitting a flag (e.g. 1 bit) to the one or more second terminal devices indicating whether or not the first terminal device will continue using the resources identified in the control message. The flag may be contained in one or more of the confirmation message and a device-to-device transmission of the plurality of device-to-device transmissions.

Alternatively, the step of indicating to the one or more second terminal devices whether or not the resources will continue to be used may comprise changing a transmission behaviour of the first terminal device that indicates whether or not the first terminal device will continue using the resources identified in the control message. Changing a transmission behaviour of the first terminal device may comprise changing a transmission format of the device-to-device transmissions. Upon detecting a change in the transmission format of the device-to-device transmissions, second terminal devices may infer that the resources previously reserved for the first terminal are no longer required.

Alternative methods of indicating that reserved resources are no longer required will be immediately apparent to those skilled in the art, and do not depart from the scope of the present disclosure. In a further example, the first terminal device may determine in step 208 that a certain number of time slots of those identified in the control message are required, but that remaining time slots identified in the control message are not required for D2D transmissions. In this example, the first terminal device may transmit or broadcast an index indicating a number of remaining time slots for which the terminal device wishes to transmit D2D transmissions. The index may be transmitted once, periodically, or with each D2D transmission to indicate to second terminal devices that the resources previously reserved for the first terminal device are about to be released. For example, the index may be transmitted as part of a confirmation message or as part of the D2D transmission itself. Figures 2 and 3 set out flowcharts of methods in which a first terminal device transmits a control message identifying resources which are to be reserved for D2D transmissions by the first terminal device. Figures 4 and 5 show flowcharts of methods in terminal devices, termed second terminal devices, that receive the control messages described above.

The flowchart of Figure 4 begins in step 300, where a control message is received by a second terminal device from a first terminal device. The control message may be as described above with respect to steps 100 and 200, and therefore specifies resources, including a plurality of time slots, that are intended for use by the first terminal device for one or more D2D transmissions. The control message may be detected and received directly by the second terminal device when it is within range of the transmission, without having travelled via an intervening network node such as a base station or similar.

In step 302, prior to a first time slot of the plurality of time slots, the second terminal device listens for a confirmation message from the first terminal device. The confirmation device may be as specified in any of steps 102, 204 or 206, and therefore conveys that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission. Note that, once again, the first time slot may be the time slot occurring first in time, or any other of the plurality of time slots identified in the control message. In one example, if no confirmation message is received in step 302, the second terminal device may determine that the first terminal device will use the resources for at least the first time slot.

The second terminal device is therefore informed initially that certain wireless resources are reserved for the first terminal device for D2D transmissions. Therefore the second terminal device is able to prioritize transmissions on other resources to avoid conflict and interference. Subsequently, the usage of those resources is confirmed by the receipt or otherwise of a confirmation message from the first terminal device. If the confirmation message conveys that resources, including at least resources for the first time slot, are not to be used by the first terminal device, then the second terminal device is free to use them. Further detail regarding this aspect can be found below with respect to Figure 5.

Figure 5 begins in step 400, in which a control message is received from a first terminal device, by the second terminal device. Step 400 is substantially similar to step 300.

In one example, the second terminal device uses the information in the control message (e.g. the resources identified for use by the first terminal device) to prioritize other resources for its own transmissions. The second terminal device may prioritize other resources regardless of whether or not a subsequent confirmation message is received conveying that the resources will or will not be used. Thus confirmation messages are not taken into account in this example. Since such distributed protocols benefit from stable resource allocations by neighbouring devices in order to converge, it may be advantageous to consider only the reserved resources rather than those actually used. See Figure 10 for further description of this example.

In other examples, the method proceeds to step 402 in which, prior to a first time slot of the plurality of time slots identified in the control message, the second terminal device listens for confirmation messages conveying whether or not resources for at least the first time slot will be used by the first terminal device for D2D transmissions. This step is substantially similar to step 302, described above.

In step 404, prior to the first time slot, the second terminal device determines whether or not a confirmation message has been received in step 402. Step 404 may be carried out prior to each of the plurality of time slots identified in the control message, or only when the second terminal device has data to transmit. Additionally, a previous confirmation message may have identified multiple time slots which the first terminal device it will or won't use for D2D communications (thus making separate confirmation messages for these time slots unnecessary).

If no confirmation message is received, the method proceeds to step 406 in which the resources identified for the first time slot are not used by the second terminal device for its own transmissions. For example, the second terminal device may refrain from transmitting at all, or may prioritize transmissions using other resources (e.g. other transmission codes and/or frequencies). The absence of a confirmation message in step 404 is thus treated by the second terminal device as a confirmation that the resources for the first time slot will be used by the first terminal device. By taking this approach, the signalling protocol is made robust against confirmation messages that are accidentally missed or cannot be decoded, for example, due to poor reception. If it is determined that a confirmation message was received in step 404, the method proceeds to step 408 in which the second terminal device determines whether the resources identified for at least the first time slot are to be used by the first terminal device for D2D transmissions. For example, the received confirmation message may convey that the resources are going to be used, in which case the method proceeds to step 412 and the second terminal device may, for example, refrain from transmitting at all during the first time slot, or may prioritize transmissions using other resources (e.g. other transmission codes and/or frequencies).

Alternatively, the received confirmation message may convey that the resources for at least the first time slot are not going to be used by the first terminal device. In one example, the mere fact that a confirmation message is received may convey that the resources for the first time slot are not going to be used (cf step 204 above, in which optionally no confirmation message is transmitted if the resources are to be used by the first terminal device). The method proceeds to step 410, in which the second terminal device may use the resources for at least the first time slot, previously reserved by the first terminal device, for its own transmissions.

Step 410 may additionally comprise determining whether or not the second terminal device has a transmission to make (and therefore can make use of the now-released resources). For example, the second terminal device may determine whether or not a D2D transmission is required. The second terminal device may determine whether there is data, or control information stored in a memory of the second terminal device for transmission as an uplink transmission or as a D2D transmission. The second terminal device may transmit one or more of control plane and data plane messages (in sequence or simultaneously) on the resources for at least the first time slot identified in the confirmation message. In one example, the second terminal device may transmit its own control message identifying resources, including at least one and potentially all of the plurality of time slots previously identified in the control message received from the first terminal device, for its own plurality of D2D transmissions. Subsequently the second terminal device may transmit its own confirmation messages in respect of those time slots, in a similar manner to that set out above in Figures 2 and 3. In another example, the second terminal device may transmit an aperiodic event-triggered reporting message, such as a DENM message. In the latter case, making use of recently released resources may help to reduce the latency of delivering such messages from the second terminal device, which is useful since this kind of message often has low latency requirements.

In one example, the confirmation message conveying that resources for the first time slot will not be used by the first terminal device for a D2D transmission may further comprise an indication of one or more transmission formats suitable for transmission on the resources for at least the first time slot. In that case, the second terminal device may transmit one or more messages over the resources for at least the first time slot using one of the transmission formats identified in the confirmation message. Alternatively the transmission formats may be known by all terminal devices in the network.

In further examples, step 408 may comprise determining that resources for a number of the time slots specified in the control message have not been used by the first terminal device for D2D transmissions (whether the first terminal device signalled that fact in a confirmation message or not). That is, the second terminal device may maintain a count of the time slots identified in the control message for which no D2D transmission is subsequently sent. If that count exceeds a threshold number, then the second terminal device may determine that the resources identified in the control message are no longer required, and consider them to be "released" and available for its own use. In that case, the second terminal device may decide, in step 410, to use any of the resources identified in the control message for its own transmissions. The count may be specified as an absolute number, a ratio of utilized time slots to reserved time slots, or any other suitable metric. The count may be made over a finite, rolling time window, i.e. the count may relate to reserved time slots in a time window immediately preceding the current time.

In another example, step 408 may comprise determining that the resources identified in the received control message have not been used by the first terminal device for at least a threshold period of time. In that case, the second terminal device may determine that the resources identified in the control message are no longer required, and consider them to be "released" and available for its own use. In that case, the second terminal device may decide, in step 410, to use any of the resources identified in the control message for its own transmissions.

In a further example, step 408 may comprise determining that that one or more device- to-device transmissions, received on resources reserved for the first terminal device in the control message and for which no confirmation message conveying that the resources will not be used has been received, contains no useful data. For example, it may be that the first terminal device attempts to maintain reserved resources by transmitting a D2D transmission on the reserved resources containing only padding bits, or null bits. In the absence of a mechanism for dealing with such devices, resources may be reserved and not used effectively for the efficiency of the network as a whole. If the second terminal device detects D2D transmissions on reserved resources containing no useful data, then the second terminal device may determine that the resources identified in the control message are no longer required, and consider them to be "released" and available for its own use. The second terminal device may then go on to transmit one or more messages using the resources identified in the received control message.

The second terminal device may receive in step 408 an indication from the first terminal device that it no longer intends to use the resources identified in the control message. For example, the first terminal device may transmit a flag (e.g. 1 bit) indicating that the resources identified in the control message are no longer required. Additionally or alternatively, particularly where the resources identified in the control message are periodic in nature, the first terminal device may transmit an index setting out the number of remaining time slots which the first terminal device intends to use for D2D transmissions. The flag and/or the index may be included with or within the confirmation message, or in one of the D2D messages transmitted by the first terminal device on the reserved resources. Alternatively, the first terminal device may change a transmission behaviour, thus indicating to the second terminal device that the first terminal device will not continue using the resources identified in the control message. For example, the first terminal device may change a transmission format of the D2D transmissions. Upon detecting the change in the transmission format, the second terminal device can then infer that the resources previously reserved for the first terminal device are no longer required. Thus the methods set out in Figures 4 and 5 allow for a second terminal device to make use of resources that become available at short notice, increasing the efficient use of resources across the network. Figure 6 shows a wireless terminal device 500 according to an example of the present disclosure. The terminal device 500 may be suitable for use as the first terminal device described above. The terminal device 500 comprises a processor 502 and a memory 504. The memory 504 contains instructions executable by the processor 502. The terminal device 500 is operative to transmit a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to-device transmissions. The terminal device 500 is further operative to, prior to a first time slot of the plurality of time slots, transmit a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission. Figure 7 shows a wireless terminal device 600 according to a further example of the present disclosure. The terminal device 600 may be suitable for use as the second terminal device described above. However, it will be apparent to those skilled in the art that in general a terminal device may perform either or both the functions of the first and second terminal devices described above. That is, at different times of its operation, a terminal device may transmit a control message identifying resources for its own use, and receive one or more control messages identifying resources for other terminal devices' use.

The terminal device 600 comprises a processor 602 and a memory 604, said memory 604 containing instructions executable by said processor 602, such that the terminal device 600 is operative to: receive a control message from a first terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to- device transmissions; and prior to a first time slot of the plurality of time slots, listen for a confirmation message from the first terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission. Figure 8 shows a wireless terminal device 700 according to another example of the present disclosure. The terminal device 700 comprises a first module 702 configured to transmit a control message to one or more second terminal devices over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to-device transmissions. The wireless device comprises a second module 704 configured to, prior to a first time slot of the plurality of time slots, transmit a confirmation message to the one or more second terminal devices over the direct communication link, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission.

Figure 9 shows a wireless terminal device 800 according to another example of the present disclosure. The terminal device 800 comprises a first module 802 configured to receive a control message from a first terminal device over a direct communication link, the control message identifying resources, including a plurality of time slots, reserved by the first terminal device for a plurality of device-to-device transmissions; and a second module 804 configured to, prior to a first time slot of the plurality of time slots, listen for a confirmation message from the first terminal device, the confirmation message conveying that resources for at least the first time slot will be used by the first terminal device for a device-to-device transmission, or conveying that resources for at least the first time slot will not be used by the first terminal device for a device-to-device transmission. Figures 10 and 11 are signalling diagrams showing the behaviour of terminal devices (also called UEs) according to various examples of the present disclosure.

Figure 10 shows one example in which a first terminal device, UE1 informs second terminal devices UE2 and UE3 about its reservation of resources for multiple transmissions via a control message as described above. In the example, the reserved resources are periodic in nature. Prior to each reserved time slot, UE1 transmits a confirmation message to UE2 (and any other terminal devices that are listening), conveying whether or not the resources reserved for that time slot are required by UE1 for D2D transmissions. Alternative embodiments have been given above in which the absence of a confirmation message for a time slot is treated as confirmation that UE1 intends to use the reserved resources for that time slot. Shortly after each confirmation message is transmitted, a sidelink transmission is made by UE1 (or not, as the case may be). After three such transmissions (although in practice any number of transmissions may occur), the resources reserved by UE1 in its original control message are released. This fact may be signalled to UE2 and UE3 by UE1 , using one of the mechanisms described above, or UE2 and/or UE3 may infer that the resources are released, again by one of the mechanisms described above.

In this example, UE3 detects the control message from UE1 and excludes the resources reserved by UE1 from its own list of selectable resources, regardless of the receipt of further confirmation messages. Before each periodic transmission UE1 sends a confirmation of whether it will actually transmit or not (hence whether it will use the reserved resources or not, respectively). Since UE3 has excluded the resources reserved by UE1 from its own list of selectable resources, UE3 does not take into account the confirmation messages sent by UE1 when performing resource selection. That is, UE3 refrains from selecting the resources booked by UE1 , even if UE1 announces that it will not use them. As mentioned above, this mechanism helps maintain a certain level of stability in resource allocation of both UE1 and UE3, meaning they do not change their selected resources too often. It also prevents UE3 from selecting a resource that is actually used by UE1 , which can happen due to limitations in the sensing operation of UE3. Figure 11 shows another example of a signalling diagram. This scenario is similar to that shown in Figure 10. However, in this example UE3 does not exclude the resources reserved by UE1 from its list of selectable resources. When UE1 transmits a confirmation message conveying that it will not use a reserved resource for its transmission, UE3 opportunistically uses that resource for its own transmission to UE2. The present disclosure thus provides methods and apparatus defining protocols that allow a terminal device to reserve desirable resources for its transmissions, with the flexibility to cease the use of such resources if required, freeing up the resources for other devices.

It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, "first", "second" etc are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Any reference signs in the statements shall not be construed so as to limit their scope.