WO2020248997A1

WO2020248997A1 - Sidelink retransmission overbooking

Info

Publication number: WO2020248997A1
Application number: PCT/CN2020/095313
Authority: WO
Inventors: Virgile Garcia; Bruno Jechoux; Umer Salim
Original assignee: JRD Communication (Shenzhen) Ltd.
Priority date: 2019-06-11
Filing date: 2020-06-10
Publication date: 2020-12-17
Also published as: CN113196852A

Abstract

A method of overbooking transmission resources for sidelink communication retransmissions. After a first UE has reserved transmission & retransmission resources for a sidelink transmission, a second UE may also reserve the retransmission resources for a sidelink communication transmission from the second UE. Such overbooking may be enabled by system configuration, or by RRC configuration messages. The permitted overbooking may vary by retransmission number such that different overbooking is permitted depending on how the retransmission number.

Description

Sidelink Retransmission Overbooking

Technical Field

The following disclosure relates to the use of resources for sidelink communications between mobile devices of a cellular communications system, and in particular to overbooking of such resources.

Background

Wireless communication systems, such as the third-generation (3G) of mobile telephone standards and technology are well known. Such 3G standards and technology have been developed by the Third Generation Partnership Project (3GPP) . The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Communication systems and networks have developed towards a broadband and mobile system.

In cellular wireless communication systems User Equipment (UE) is connected by a wireless link to a Radio Access Network (RAN) . The RAN comprises a set of base stations which provide wireless links to the UEs located in cells covered by the base station, and an interface to a Core Network (CN) which provides overall network control. As will be appreciated the RAN and CN each conduct respective functions in relation to the overall network. For convenience the term cellular network will be used to refer to the combined RAN &CN, and it will be understood that the term is used to refer to the respective system for performing the disclosed function.

The 3rd Generation Partnership Project has developed the so-called Long Term Evolution (LTE) system, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN) , for a mobile access network where one or more macro-cells are supported by a base station known as an eNodeB or eNB (evolved NodeB) . More recently, LTE is evolving further towards the so-called 5G or NR (new radio) systems where one or more cells are supported by a base station known as a gNB. NR is proposed to utilise an Orthogonal Frequency Division Multiplexed (OFDM) physical transmission format.

In conventional cellular communication networks, all signalling is between each mobile device and a base station rather than directly between mobile devices, even if the mobile devices are within wireless communication range of each other. This may lead to inefficient use of wireless transmission resources and may increase base station resource utilisation. Sidelink communications allow mobile devices to communicate directly rather than via a base station, potentially improving wireless and base station resource utilisation. Sidelink communications are considered particularly interesting for Machine to Machine communications, particularly Vehicle to Vehicle (V2V) and Vehicle to Everything/Anything (V2X) communications.

The disclosure below relates to various improvements to cellular wireless communications systems, and in particular sidelink communications in such systems.

Summary

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

There is provided a method of overbooking transmission resources for sidelink communication retransmissions. After a first UE has reserved transmission &retransmission resources for a sidelink transmission, a second UE may also reserve the retransmission resources for a sidelink communication transmission from the second UE. Such overbooking may be enabled by system configuration, or by RRC configuration messages. Furthermore, SCI messaging may be used for aspects of the configuration. The permitted overbooking may vary by retransmission number such that different overbooking is permitted depending on the retransmission number, or may vary for each retransmission resource within a multiple retransmission reservation.

There is also provided a UE which receives an indication of sidelink transmission resource reservations from a second UE, and which reserves resources for a sidelink transmission, wherein the reserved resources include resources reserved by the second UE for retransmissions.

The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory.

Brief description of the drawings

Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

Figure 1 shows a schematic diagram of selected components of a cellular communications network;

Figure 2 shows a flow chart of a method of resource overbooking;

Figures 3 &4 show a representation of resource availability;

Figure 5 shows an example of configuration options; and

Figure 6 shows a flow chart of a method of resource sharing.

Detailed description of the preferred embodiments

Those skilled in the art will recognise and appreciate that the specifics of the examples described are merely illustrative of some embodiments and that the teachings set forth herein are applicable in a variety of alternative settings.

Figure 1 shows a schematic diagram of three base stations (for example, eNB or gNBs depending on the particular cellular standard and terminology) forming a cellular network. Typically, each of the base stations will be deployed by one cellular network operator to provide geographic coverage for UEs in the area. The base stations form a Radio Area Network (RAN) . Each base station provides wireless coverage for UEs in its area or cell. The base stations are interconnected via the X2 interface and are connected to the core network via the S1 interface. As will be appreciated only basic details are shown for the purposes of exemplifying the key features of a cellular network. In the proposed NR protocols, the Uu interface is between the base station and UEs. A PC5 interface is provided between UEs for SideLink (SL) communications. The interface and component names mentioned in relation to Figure 1 are used for example only and different systems, operating to the same principles, may use different nomenclature.

The base stations each comprise hardware and software to implement the RAN’s functionality, including communications with the core network and other base stations, carriage of control and data signals between the core network and UEs, and maintaining wireless communications with UEs associated with each base station. The core network comprises hardware and software to implement the network functionality, such as overall network management and control, and routing of calls and data.

Resources for SL communications are discussed in TR 38.885. Particularly relevant sections for the current disclosure are: -

(A) 5.1.1 Physical layer structures

In this section, the design of a physical SL control channel (PSCCH) , a physical SL shared channel (PSSCH) , a physical SL feedback channel (PSFCH) and other matters related to physical layer structures are studied. In addition to what is discussed in this TR, at least aspects related to modulation, scrambling, RE mapping and rate matching would be included in normative work. For design of the physical SL broadcast channel (PSBCH) , refer to Section 5.2.

The waveform supported in the study is CP-OFDM.

[…]

(1) 5.1.1.3 SL bandwidth parts and resource pools

BWP is defined for SL, and the same SL BWP is used for transmission and reception. In specification terms, in a licensed carrier, SL BWP would be defined separately, and have separate configuration signalling, from Uu BWP. One SL BWP is (pre-) configured for RRC IDLE and out-of-coverage NR V2X UEs in a carrier. For UEs in RRC_CONNECTED mode, one SL BWP is active in a carrier. No signalling is exchanged over SL for the activation or deactivation of a SL BWP.

In a carrier, only one SL BWP is configured for a UE, and the UE is not expected to use at the same time a different numerology in the SL BWP than an active UL BWP.

A resource pool is a set of time-frequency resources that can be used for SL transmission and/or reception. From the UE point of view, a resource pool is inside the UE's bandwidth, within a SL BWP and has a single numerology. Time domain resources in a resource pool can be non-contiguous. Multiple resource pools can be (pre-) configured to a UE in a carrier.

(2) 5.1.1.4 Resource arrangements

NR V2X may be deployed in a carrier dedicated to ITS services, or a carrier shared with cellular services. Therefore, resource arrangements where all the symbols in a slot are available for SL, and where only a subset of consecutive symbols in a slot (which are not dynamically indicated) are available for SL are supported. The latter case is not intended for use in ITS spectrum, if normative specification work does not find a forward compatibility issue.

Resource allocation for PSSCH is based on the concept of sub-channels in the frequency domain, and a UE performs either transmission or reception in a slot on a carrier.

[…]

Two modes for resource allocation for SL communications are defined. In a first mode (Mode 1) the base station schedules SL resources to be used by UEs (the term UE will be utilised as a convenient descriptor of any mobile device) for SL communications. In a second mode (Mode 2) the base station allocates resources which can be used for SL communications and UEs select which resources to use for each SL transmission. In this second mode UEs operate autonomously to select resources for transmissions. Resource selection is based on sensing available resources and then reserving appropriate available resources. To enable this process UEs reserve their resources by transmitting a Sidelink Control Information (SCI) message. This message can be sensed by other UEs such that they are aware of which resources have already been reserved. In the second mode, a UE may also utilise measurements to determine which resources are available.

A HARQ transmission protocol is proposed for SL communications, meaning that in the case of a failed transmission the transport block is retransmitted. Transmission resources are thus required for both the initial transmission and any potential retransmissions. Resources for retransmissions may be reserved upfront, for example with the initial transmission, or on-demand if a retransmission is required. A disadvantage of the upfront approach is that resources may be reserved but not used if the initial transmission was successful and are hence wasted (retransmissions are typically expected in 10%of transmissions) , but an on-demand approach introduces additional latency for any retransmission. For many proposed SL applications, the additional latency is too high and the on-demand approach cannot be utilised.

As set out in detail below, the current disclosure permits over-booking of resources for re-transmissions. That is, more than one UE may schedule retransmissions on the same resources. It is anticipated that the initial transmission will be very reliable and hence the probability more than one UE utilising the over-booked resources is low. A target of a 10%failure rate for the first transmission has been proposed, which if met means two UEs will only both make a first retransmission (and hence collide) in 1%of cases. A collision between transmissions leads to interference between the signals which may degrade the error rate of one or both transmissions, and may to the failure of one or both transmissions.

Over-booking thus permits improved resource utilisation (since fewer resources are allocated in total for a set of UEs) with a small risk of causing failure of a retransmission due to a collision.

Figure 2 shows a flow chart of a method for overbooking resources for SL communications. The term overbooking is used herein to indicate the reservation of the same resources for more than one potential transmission. For example, two different UEs may reserve the same resources for a transmission.

At step 200 the cellular network configures UEs to enable SL resource overbooking. As set out in more detail below, the configuration may be performed in any appropriate manner, for example utilising default configurations or RRC communications. A range of options for overbooking configurations are set out below.

At step 201 a first UE, UE A, has an SL transmission to make and reserves resources for the initial transmission and any retransmissions. The reservation may be indicated by transmission of an SCI message. At step 202 a second UE, UE B, receives &decodes UE A’s SCI message to determine the resources which have been reserved. At step 203 UE B has an SL transmission to make, and hence selects and reserves resources. When selecting its resources UE B can select, according to the configuration at step 200, resources already reserved by UE A for its retransmissions. That is, the resources reserved by UE B for its retransmissions can be overbooked. At step 204 UEs A &B make their transmissions, and any required retransmissions, on the reserved resources. If both UEs need to make a retransmission a collision may occur.

The method of Figure 2 thus permits the reservation of the same resources by UEs A &B for retransmissions of their SL transmission. Resource utilisation is thus reduced, but there is a risk of a collision between retransmissions which utilise the same resources.

The resources reserved for the initial transmission by UE A cannot also be reserved by UE B since it is effectively certain that those resources will be utilised by UE A. That is, in general, resources reserved by a UE for an initial transmission are no longer available and cannot be overbooked. However, resources reserved for a retransmission can be booked by another UE for an initial transmission. However, resources reserved for a retransmission may be considered a lower priority for selection since a collision is more likely due to the certainty that the initial transmission will be made, and it is only one of the reservations for the resources that may not be utilised.

Figure 3 shows a graphical representation of the process of Figure 2. UE A transmits an SCI message 300 which falls within the sensing period of UE B for detecting which resources are available for UE B for an SL transmission. UE A reserves resources 301 for the initial transmission and

resources

302, 303, 304 are reserved for three potential retransmissions. Based on receipt of UE A’s SCI message 300, UE B can update the resources it considers available for an SL transmission as shown in chart 305. Resources 301 are not available because they are utilised by UE A for an initial transmission, but

resources

302, 303, 304 are available for reservation by UE B since they are reserved for retransmissions and hence can be overbooked if permitted by the active configuration.

In order to balance the resource saving from overbooking against the increased risk of collisions the details of how overbooking can occur can be configured and adapted for particular circumstances. In a basic approach overbooking is a binary configuration and is enabled or disabled for all retransmission resources, with all retransmission resources being treated the same. The activation of overbooked may be indicated using a particular true/false indication in an appropriate control signal, for example in an RRC signal.

Alternatively, retransmissions may be treated differently in relation to overbooking. For example, each retransmission opportunity is less likely to be utilised than the previous one. Hence greater overbooking may be permitted for the later retransmissions as the prospects of collisions are lower. For example, overbooking may only be permitted for the last N transmissions, where N is defined by the system in the range 0 (no overbooking) to the maximum number of retransmissions (which would indicate all retransmission resources can be overbooked) . A value of 1 would indicate only the last retransmission can be overbooked. This may be configured by a control message (for example RRC) indicating the value of N.

Overbooking may also be only permitted for transmission resources to be used for Redundancy Versions (RVs) which are not self-decodable (i.e. not for RV0 and RV3) . This may be configured in conjunction with other options discussed herein.

In a further configuration option, the amount of overbooking between two specific pairs of devices (a pair being a transmitter and the intended destination device for the data to be transmitted by the transmitter (which may be a single UE in a unicast transmission, or a group for groupcast) ) can be restricted. This may avoid successive collisions which could create repetitive NACKs on both sides (at the receivers of the respective SCI transmissions) . Example options are that only a subset, M, of overbookable retransmission resource are overbookable by the two pairs of devices.

The number of times each reserved resource can be overbooked may be defined in the configuration. In order to implement this a UE must listen for reservations (SCI messages) and accumulate the reservations made. The total reservations can then be utilised to indicate which resources can be reserved for retransmissions. If the value is set to 0, overbooking is not permitted of those resources.

The number of overbookings can be made dependent on the index of retransmission (or RV index) , the retransmission coding scheme, and/or the total number of retransmissions. For example, the value may be set to 0 on RVs that are self-decodable (RV0 and RV3 for a 4-transmission example) and set to a value greater than 0 for other retransmissions (for example, 8) . Furthermore, the value can vary through the sequence of retransmissions since, as discussed above, the probability of a later retransmission being utilised is lower. Assuming a 10%error rate target, the first retransmission has a 10%chance of being used, the second has a 1%chance, &the third a 0.1%chance. In an example the number of overbookings may be set as 4 for the first transmission, 8 for the second, and 16 for the third etc. The number of available (or remaining) overbookings may be utilised by a UE when selecting resources to reserve. For example, resources with a higher available number of overbookings may be selected.

Figure 4 shows a further example of resource reservation according to the principles described herein, and also an extension to a third UE. As described with reference to Figure 3, UE A transmits an SCI message which is received by UE B. UE B updates its available resources, but includes the number of available overbookings in relation to each resource. Thus, resource 400 has 4 overbookings available, resource 401 has 8, and resource 402 has 16.

UE B selects resources for its retransmission and at 403 transmits an SCI reserving resource 404 for its initial transmission, and resource 405 for one retransmission. UE C receives both UE A &B’s SCI and updates its available resources appropriately. Thus resources 406 & 407 are not available (previously used for initial transmissions) , and resources 400 &401 still have 4 &8 overbookings available but resource 408 now has only 15 overbookings available.

UE C thus determines which resources to reserve for its SL transmission based on the cumulative reservations from UE A &B, and the previously configured available overbookings.

The principles described with relation to Figure 4 can be extended to any number of UEs and any pattern and number of resource overbookings.

Any appropriate configuration method can be utilised to configure UEs regarding overbooking of resources. RRC configuration may be preferred since it allows semi-static configuration, but with flexibility to adjust the configuration dependent on circumstances. SCI messaging could be utilised for configuration which may provide a more dynamic configuration process. A predefined set of configurations may be defined to reduce the RRC signalling overhead. For example, an integer of 3 bits would allow selection of a configuration from 8 options.

An SCI message reserving resources may also include specific overbooking information for the indicated reservation to override or supplement the currently active configuration. UEs receiving the SCI can update their configuration appropriately. For example, SCI may be used to override a configuration if a UE has a high reliability and/or low latency packet to deliver. The UE may disable overbooking, or reduce the amount of overbooking, for the specific resources reserved for the transmission to avoid the possibility of collisions and help ensure reliability and/or latency targets are met. For example, a flag can be included in the SCI disabling overbooking for resources. In a further example, the system may allow reserved retransmission resources to be reused by the same UE for other TBs if the initial transmission is successful and the retransmission resources are not required. If the UE has data in its buffer then it knows the retransmission resources will be used (either for a retransmission, or the pending TBs) and hence may disable overbooking.

The SCI information may include the number of overbookings remaining, or the original number available (which each UE can update based on received SCIs) . Alternatively the cumulative number of overbookings could be indicated, with a UE calculating the remaining number based on the configuration. A single value can be sent if all retransmissions are treated the same, or a set/array of values if the available overbookings vary by resource/retransmissions. The parameters may be transmitted as a binary or integer value as appropriate, either giving a direct indication of a value or a reference to a set of predefined values.

The overbooking rules to be applied by a UE can be traffic/QoS/priority dependent. Several rule sets can be predefined (as standard configurations in static set-up or via RRC configuration) and then which rule to be applied to a given (re) transmission is chosen based on the data it transmits (e.g. (re) using some QoS information, traffic class information, priority information that is usually included in the SCI) . Activation of overbooking can also be dependent on system load so that under a certain threshold overbooking is not utilised (thus avoiding collision risk) , but it is activated over the threshold to reduce resource utilisation. For example, the Channel Busy Ratio (CBR) can be utilised as an indicator of system load.

Figure 5 shows an example of a hierarchy of configurations. A range of possible options are provided by enabling overbooking at a system level, and the RRC configuration may then define a subset of those which are available. SCI messaging may then specify further details from the RRC set of configurations.

Examples of parameters utilised to configure overbooking configuration include: -

● Enabled : yes/no/loadBased

● Type : Systematic, PriorityBased

● Limit : OverbookingCounterMax

● Mode : lastN, lastN-1, lastN-2, NonSelfDecodable

Figure 6 shows an example method in which a UE which has overbooked resources can utilise feedback from other UEs sharing the resources to indicate whether to utilise the resources. At step 600 a UE B has scheduled resources for retransmission in the same resources on which another UE A has scheduled retransmissions. At step 601 UE B identifies the previous retransmission, or initial transmission, from UE A for the overbooked resources, and the corresponding feedback channel.

At step 602 the UE B listens to the identified feedback channel and decodes the transmitted feedback. If at step 602 the feedback is a NACK this indicates that the UE A will utilise the shared resources for a retransmission. At step 603 the UE B may thus not make its transmission to avoid a collision. If at step 602 no feedback, or positive feedback, is detected this suggests that UE A will not use the shared resources, and UE B can utilise the resources for a retransmission at step 604 if required. If no retransmission is made at step 603 the UE B may transmit an indication of this, for example by flushing the HARQ buffer, corruption indication, or a further retransmission reservation) .

The method of Figure 6 thus permits overbooking of resources as described hereinbefore, but may also allow collisions to be avoided where more than one UE needs to make use of the overbooked resources.

Although not shown in detail any of the devices or apparatus that form part of the network may include at least a processor, a storage unit and a communications interface, wherein the processor unit, storage unit, and communications interface are configured to perform the method of any aspect of the present invention. Further options and choices are described below.

The signal processing functionality of the embodiments of the invention especially the gNB and the UE may be achieved using computing systems or architectures known to those who are skilled in the relevant art. Computing systems such as, a desktop, laptop or notebook computer, hand-held computing device (PDA, cell phone, palmtop, etc. ) , mainframe, server, client, or any other type of special or general purpose computing device as may be desirable or appropriate for a given application or environment can be used. The computing system can include one or more processors which can be implemented using a general or special-purpose processing engine such as, for example, a microprocessor, microcontroller or other control module.

The computing system can also include a main memory, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by a processor. Such a main memory also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor. The computing system may likewise include a read only memory (ROM) or other static storage device for storing static information and instructions for a processor.

The computing system may also include an information storage system which may include, for example, a media drive and a removable storage interface. The media drive may include a drive or other mechanism to support fixed or removable storage media, such as a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a compact disc (CD) or digital video drive (DVD) read or write drive (R or RW) , or other removable or fixed media drive. Storage media may include, for example, a hard disk, floppy disk, magnetic tape, optical disk, CD or DVD, or other fixed or removable medium that is read by and written to by media drive. The storage media may include a computer-readable storage medium having particular computer software or data stored therein.

In alternative embodiments, an information storage system may include other similar components for allowing computer programs or other instructions or data to be loaded into the computing system. Such components may include, for example, a removable storage unit and an interface , such as a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, and other removable storage units and interfaces that allow software and data to be transferred from the removable storage unit to computing system.

The computing system can also include a communications interface. Such a communications interface can be used to allow software and data to be transferred between a computing system and external devices. Examples of communications interfaces can include a modem, a network interface (such as an Ethernet or other NIC card) , a communications port (such as for example, a universal serial bus (USB) port) , a PCMCIA slot and card, etc. Software and data transferred via a communications interface are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by a communications interface medium.

In this document, the terms ‘computer program product’ , ‘computer-readable medium’ and the like may be used generally to refer to tangible media such as, for example, a memory, storage device, or storage unit. These and other forms of computer-readable media may store one or more instructions for use by the processor comprising the computer system to cause the processor to perform specified operations. Such instructions, generally 45 referred to as ‘computer program code’ (which may be grouped in the form of computer programs or other groupings) , when executed, enable the computing system to perform functions of embodiments of the present invention. Note that the code may directly cause a processor to perform specified operations, be compiled to do so, and/or be combined with other software, hardware, and/or firmware elements (e.g., libraries for performing standard functions) to do so.

The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory. In an embodiment where the elements are implemented using software, the software may be stored in a computer-readable medium and loaded into computing system using, for example, removable storage drive. A control module (in this example, software instructions or executable computer program code) , when executed by the processor in the computer system, causes a processor to perform the functions of the invention as described herein.

Furthermore, the inventive concept can be applied to any circuit for performing signal processing functionality within a network element. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, such as a microcontroller of a digital signal processor (DSP) , or application-specific integrated circuit (ASIC) and/or any other sub-system element.

It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to a single processing logic. However, the inventive concept may equally be implemented by way of a plurality of different functional units and processors to provide the signal processing functionality. Thus, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organisation.

Aspects of the invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented, at least partly, as computer software running on one or more data processors and/or digital signal processors or configurable module components such as FPGA devices.

Thus, the elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate.

Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to ‘a’ , ‘an’ , ‘first’ , ‘second’ , etc. do not preclude a plurality.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ or “including” does not exclude the presence of other elements.

Claims

A method for reserving transmission resources for sidelink communications between UEs in a cellular communications network, the method comprising the steps of

at a first UE transmitting a message reserving first transmission resources for a first sidelink transmission, the first transmission resources comprising first initial transmission resources and first retransmission resources;

at a second UE receiving the message and decoding at least the first transmission resources which are reserved; and

at the second UE selecting second transmission resources for a second sidelink transmission, wherein the second transmission resources comprise at least part of the first retransmission resources.
A method according to claim 1, wherein the second transmission resources comprise second initial transmission resources and second retransmission resources, and the second retransmission resources comprise the at least part of the first retransmission resources.
A method according to claim 1 or claim 2, wherein during selection of the second transmission resources, the first retransmission resources are given a lower priority than unreserved transmission resources.
A method according to any preceding claim, wherein the message is an SCI message.
A method according to any preceding claim, further comprising the step of receiving a configuration message at the second UE permitting that UE to select resources previously reserved by another UE.
A method according to claim 5 wherein the configuration message is an RRC message or an SCI message.
A method according to claim 5, wherein the configuration message indicates that only a subset of resources reserved for retransmission by another UE may be selected by the UE.
A method according to claim 5, wherein the configuration message indicates that only retransmission resources reserved for redundancy versions which are not self-decodable may be selected by the second UE.
A method according to claim 5, wherein the configuration message indicates how many UEs may select retransmission resources reserved by a first UE.
A method according to claim 5, wherein the configuration message indicates that the second UE is permitted to select resources previous reserved by another UE depending on the priority of messages to be transmitted by the first or second UE.
A method according to any preceding claim, further comprising the steps of

at the second receiving a feedback message relating a transmission on the first initial transmission resources, and

if the feedback message is an ACK utilising the at least part of the first retransmission resources selected by the second UE if required by the second UE, and if the feedback message is a NACK not utilising the at least part of the first retransmission resources selected by the second UE.
A UE configured to perform the method of any of claims 1 to 11.