Method and apparatus for sidelink transmissions

Abstract

A technique, comprising: selecting at a user equipment one of a plurality of predefined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equ

Description

27 08 24 TECHNICAL FIELD The present disclosure relates to apparatus, a method, and a computer program and in particular but not exclusively to apparatus, methods and computer programs for sidelink transmissions. BACKGROUND Sidelink transmissions involve the transfer of data directly between user equipments (UEs) without the data going through the network. One mode of making sidelink transmissions may involve UEs autonomously selecting radio resources for sidelink transmissions. SUMMARY A method, comprising: selecting at a user equipment one of a plurality of predefined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the 27 08 24 following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. The plurality of pre-defined transmission procedures may comprise at least: (i) a first transmission procedure comprising the user equipment transmitting a first transport block without also reserving radio resources for transmission of a further transport block; and (ii) a second transmission procedure comprising the user equipment transmitting a first transport block and associated control information, wherein the associated control information includes a reservation of radio resources for transmission of a further transport block. The estimate of the degree of congestion may be a measurement of how much of at least part of the resource pool was occupied by sidelink transmissions over a measurement period. The method may comprise receiving the estimate of the degree of congestion at the user equipment from another user equipment. The blindly selected resources may be resources selected randomly without an operation to sense reservation of resources by other user equipments. The method may comprise comparing the estimate of the degree of congestion against first and second congestion threshold values. The method may comprise: in response to determining that the estimate of the degree of congestion is below the first congestion threshold, operating according to a first rule; wherein the first rule allows the user equipment to (i) select the first transmission procedure, unconditional on a priority value for the first transport block, and unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure unconditional 27 08 24 on a priority value for the first transport block, but conditional on the possibility for feedback from one or more receivers. The method may comprise: in response to determining that the estimate of the degree of congestion is between the first and second congestion threshold values, operating according to a second rule; wherein the second rule allows the user equipment to (i) select the first transmission procedure conditional on the first transport block having a priority value higher than a first priority threshold, but unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure, conditional on the first transport block having a priority value higher than a first priority threshold and conditional on the possibility for feedback from one or more receivers. The method may comprise: in response to determining that the estimate of the degree of congestion is above the second congestion threshold, operating according to a third rule; wherein the third rule allows the user equipment to select the second transmission procedure, conditional on the first transport block having a priority value higher than a second priority threshold, and conditional on the possibility for feedback from one or more receivers. Apparatus comprising: means for selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or 27 08 24 more of the following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. The plurality of pre-defined transmission procedures may comprise at least: (i) a first transmission procedure comprising the user equipment transmitting a first transport block without also reserving radio resources for transmission of a further transport block; and (ii) a second transmission procedure comprising the user equipment transmitting a first transport block and associated control information, wherein the associated control information includes a reservation of radio resources for transmission of a further transport block. The estimate of the degree of congestion may be a measurement of how much of at least part of the resource pool was occupied by sidelink transmissions over a measurement period. The apparatus may comprise means for receiving the estimate of the degree of congestion at the user equipment from another user equipment. The blindly selected resources may be resources selected randomly without an operation to sense reservation of resources by other user equipments. The apparatus may comprise means for comparing the estimate of the degree of congestion against first and second congestion threshold values. The apparatus may comprise: means for, in response to determining that the estimate of the degree of congestion is below the first congestion threshold, operating according to a first rule; wherein the first rule allows the user equipment to (i) select the first transmission procedure, unconditional on a priority value for the first transport block, and unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission 27 08 24 procedure unconditional on a priority value for the first transport block, but conditional on the possibility for feedback from one or more receivers. The apparatus may comprise: means for, in response to determining that the estimate of the degree of congestion is between the first and second congestion threshold values, operating according to a second rule; wherein the second rule allows the user equipment to (i) select the first transmission procedure conditional on the first transport block having a priority value higher than a first priority threshold, but unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure, conditional on the first transport block having a priority value higher than a first priority threshold and conditional on the possibility for feedback from one or more receivers. The apparatus may comprise: means for, in response to determining that the estimate of the degree of congestion is above the second congestion threshold, operating according to a third rule; wherein the third rule allows the user equipment to select the second transmission procedure, conditional on the first transport block having a priority value higher than a second priority threshold, and conditional on the possibility for feedback from one or more receivers. An apparatus comprising: at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the apparatus to perform: selecting at a user equipment one of a plurality of predefined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising 27 08 24 sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. The plurality of pre-defined transmission procedures may comprise at least: (i) a first transmission procedure comprising the user equipment transmitting a first transport block without also reserving radio resources for transmission of a further transport block; and (ii) a second transmission procedure comprising the user equipment transmitting a first transport block and associated control information, wherein the associated control information includes a reservation of radio resources for transmission of a further transport block. The estimate of the degree of congestion may be a measurement of how much of at least part of the resource pool was occupied by sidelink transmissions over a measurement period. The at least one memory and computer program code may be configured to, with the at least one processor, cause the apparatus to: receive the estimate of the degree of congestion at the user equipment from another user equipment. The blindly selected resources may be resources selected randomly without an operation to sense reservation of resources by other user equipments. The at least one memory and computer program code may be configured to, with the at least one processor, cause the apparatus to: compare the estimate of the degree of congestion against first and second congestion threshold values. 27 08 24 The at least one memory and computer program code may be configured to, with the at least one processor, cause the apparatus to: in response to determining that the estimate of the degree of congestion is below the first congestion threshold, operate according to a first rule; wherein the first rule allows the user equipment to (i) select the first transmission procedure, unconditional on a priority value for the first transport block, and unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure unconditional on a priority value for the first transport block, but conditional on the possibility for feedback from one or more receivers. The at least one memory and computer program code may be configured to, with the at least one processor, cause the apparatus to: in response to determining that the estimate of the degree of congestion is between the first and second congestion threshold values, operate according to a second rule; wherein the second rule allows the user equipment to (i) select the first transmission procedure conditional on the first transport block having a priority value higher than a first priority threshold, but unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure, conditional on the first transport block having a priority value higher than a first priority threshold and conditional on the possibility for feedback from one or more receivers. The at least one memory and computer program code may be configured to, with the at least one processor, cause the apparatus to: in response to determining that the estimate of the degree of congestion is above the second congestion threshold, operate according to a third rule; wherein the third rule allows the user equipment to select the second transmission procedure, conditional on the first transport block having a priority value higher than a 27 08 24 second priority threshold, and conditional on the possibility for feedback from one or more receivers. Apparatus, comprising: selecting circuitry for selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern ; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. A computer readable medium comprising program instructions stored thereon for performing: selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a predefined hopping pattern; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following: an estimate of the degree of congestion of at least part of the resource pool; a 27 08 24 priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. A non-transitory computer readable medium comprising program instructions stored thereon for performing: selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. A computer program comprising computer executable code which when run on at least one processor is configured to cause an apparatus at least to: select at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a pre-defined hopping pattern ; wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources; wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and wherein the selecting of a transmission procedure at the user equipment is 27 08 24 based at least partly on one or more of the following: an estimate of the degree of congestion of at least part of the resource pool; a priority value of the data or the user equipment; a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers. In the above, many different aspects have been described. It should be appreciated that further aspects may be provided by the combination of any two or more of the aspects described above. Various other aspects are also described in the following detailed description and in the attached claims. BRIEF DESCRIPTION OF THE FIGURES Some example embodiments will now be described in further detail, by way of example only, with reference to the following examples and accompanying drawings, in which: Figure 1 shows a representation of sidelink transmissions among a group of UEs according to some example embodiments; Figure 2 shows a representation of selecting radio resources for sidelink transmissions according to some example embodiments; Figure 3 shows a representation of an example of operations at a UE according to some example embodiments; Figure 4 shows a representation of apparatus for implementing operations according to some example embodiments; and Figure 5 shows a representation of non-volatile memory media. The following description focuses, by way of non-limiting example only, on sidelink transmissions via radio resources shared by UEs operating according to 27 08 24 3GPP Release 16 and UEs operating according to Release 17. However, the techniques are also applicable to other types of UE with comparable capabilities. Sidelink transmission operation at Release 16 and 17 UEs may comprise Mode 2: UE autonomous resource selection, or Mode 1: gNB scheduled resource allocation. Autonomous resource selection Mode 2 may comprise Option-1 operation or Option-2 operation. Option-2 operation involves a semi-persistent scheduling procedure according to which, for example, the UE autonomously selects radio resources for a set of consecutive transport blocks (TBs), and transmits the first of the TBs with sidelink control information (SCI) also indicating the radio resources reserved by the UE for sidelink transmission of the remaining one or more TBs. The remaining one or more TBs for which the UE reserves radio resources may be TBs that are ready for transmission at the time of selecting radio resources, or may be TBs that are not ready for transmission at the time of selecting radio resources. The UE may reserve radio resources for TBs that the UE knows or expects will become ready for transmission (e.g. at a rate of one TB (packet) per 40ms). Option-1 operation involves a dynamic scheduling procedure according to which the UE autonomously selects radio resources for the sidelink transmission of a single TB, without reserving radio resources for the sidelink transmission of one or more later TBs. The autonomous selection of radio resources for sidelink transmissions by UEs operating in Mode 2 according to 3GPP Releases 16 and 17 may comprise a sensing operation involving the UE sensing the reservation of radio resources by other UEs. The UE may search a sensing window for SCI about reservations by other UEs. The UE selects radio resources for its own sidelink transmission(s) partly on the result of this sensing operation. The selection operation may involve the UE randomly selecting radio resources from the sub-set of radio 27 08 24 resources that the UE determines not to be already reserved by other UEs based on the sensing operation. UEs performing a sensing operation as part of the selection of sidelink transmission resources are referred to below as UE2 (Release 17) and UE3 (Release 16). Some UEs (referred to below as UE1) operating according to Release 17 may adopt a different process for selecting radio resources for sidelink transmissions. Some UEs (UE1) operating according to Release 17 may instead blindly select radio resources for the sidelink transmission of one or more TBs, without performing the sensing operation mentioned above. UE1 may randomly select radio resources for the sidelink transmission of a first TB without performing any sensing operation. The random selection operation may involve UE1 following a pre-defined frequency hopping pattern known to other Release 17 UEs, but information about the frequency hopping pattern used by Release 17 UE1 may not be available to Release 16 UEs. This alternative "blind resource selection" mode of selecting radio resources for sidelink transmissions may, for example, be useful for UEs for which low power consumption is an important requirement. With reference to Figures 1 to 3, operations of UE1 according to some example embodiments are as follows. Release 17 UE1 operating according to "blind resource selection" mode has one or more transport blocks (TBs) for transmitting to one or more other (Release 16 or 17) UEs (by unicast or group cast), and proceeds to select a procedure for transmitting the TBs via a shared resource pool (RP). The RP may be wholly or partly shared with sidelink transmissions by UE2s and UE3s performing sensing operations as part of their sidelink transmission operations. The RP may partly 27 08 24 or wholly overlap with a resource pool for sidelink transmissions by UE2s and UE3s selecting sidelink transmission resources based on sensing operations. UE1 first obtains an estimate of the degree of congestion of the resource pool RP (STEP 300). According to one example, UE1 measures a channel busy ratio (CBR) for all or part of the resource pool RP over a configured time period. According to one example, the UE1 may measure the CBR for the whole of the RP. According to other examples, UE1 measures CBR only for a sub-set of the RP, or only for the part of the RP randomly selected by UE1 for the sidelink transmission according to the frequency hopping pattern mentioned above. The CBR measurement indicates the degree to which the resource pool RP is used / occupied in the most recent time period 6, and provides an estimate of the degree of congestion of the resource pool RP at the time that the UE1 may make a SL transmission. The CBR measurement time period 6 may be defined in terms of a number of slots immediately preceding the time T1 at which UE1 starts the process of selecting radio resources and a transmission procedure for the sidelink transmission(s). For example, the number of slots may be e.g. 10, 20 or 50 slots. A slot is a unit of time comprising e.g. fourteen sidelink symbols. UE1 may be configured such that the length of the CBR measurement time period 6 used by UE1 to estimate the degree of congestion for the resource pool RP varies according to e.g. a priority value of the TB(s) to be transmitted and / or whether or not there is the possibility for UE1 to receive HARQ (Hybrid Automatic Repeat Request) feedback from one or more intended recipients of the TB(s). According to one example, measurement of the CBR for the CBR measurement time period 6 involves measuring the proportion of sub-channels within the resource pool RP for which a received signal strength indicator (RSSI) exceeds a pre-defined threshold. A sub-channel is a group of a pre-configured number of physical resource blocks (PRBs). This measurement of CBR does not 27 08 24 involve the more energy-consuming operation of decoding sidelink control information (SCI) to obtain information about reservation of resources in the sensing operation. The CBR measurement period for UE1 may be much shorter than the sensing window used by UE2 and UE3 for the operation of sensing reservation of radio resources by other UEs. Thus, the CBR measurement of UE1 consumes much less energy than the sensing operation of UE2 and UE3. According to another example, UE1 uses one or more CBR measurements provided by one or more nearby / peer UEs. For example, one or more nearby / peer UEs may make sidelink transmissions including a CBR measurement in the payload. As described below, UE1 proceeds to select Option 1 or Option 2 for the sidelink transmission based on the CBR measurement and other parameters. The UE1 may proceed with the sidelink transmission as soon as the CBR measurement is available and a selection of Option 1 or Option 2 has been made at UE1. UE1 compares the CBR value against a lower congestion threshold value Th1 and an upper congestion threshold value Th2. Th1 has a lower value than Th2. For example, UE1 determines whether the CBR value is less than Th1 (STEP 302). If the CBR value is not less than Th1, UE1 determines whether the CBR value is less than Th2 (STEP 304). In the event that the CBR value is less than Th1, UE1 follows Rule A for the sidelink transmission (STEP 306). According to Rule A: UE1 may adopt Option-1 or Option-2 for sidelink transmissions via the radio resources blindly selected from the resource pool RP; and UE1 may make sidelink transmissions irrespective of the priority value for the one or more TBs to be transmitted. UE1 27 08 24 may use Option 1 with or without feedback-enabled HARQ, or may use Option-2 with feedback-enabled HARQ. In the event that the CBR value is greater than or equal to Th1 and less than Th2, UE1 follows Rule B for the sidelink transmission (STEP 308). According to Rule B: UE1 may adopt Option-1 or Option-2 for sidelink transmissions via the radio resources blindly selected from the resource pool RP; but the UE1 may make only sidelink transmissions for TBs having a priority value higher than a first priority threshold P1. The higher the priority value for a TB, the greater the priority for transmitting the TB. UE1 may use Option 1 with or without feedback-enabled HARQ for TBs having a priority value higher than P1, or may use Option-2 with feedback-enabled HARQ for TBs having a priority value higher than P1. In the event that the CBR value is greater than or equal to Th2, UE1 follows Rule C for the sidelink transmission (STEP 310). According to Rule C: UE1 may only adopt Option-2 for sidelink transmissions via the radio resources blindly selected from the resource pool RP; and UE1 may only make sidelink transmissions for TBs having a priority value higher than a second priority threshold P2 (wherein P2 is higher than P1 mentioned above for Rule B). UE1 may use Option-2 with feedback-enabled HARQ for TBs having a priority value higher than P2. As mentioned above, Option-2 involves UE1 blindly selecting radio resources 8 for the transmission of a first TB , and including in the SCI for the first TB transmission an indication of the reservation of radio resources 10 for the transmission of one or more further TBs. 27 08 24 The use of feedback-enabled HARQ involves the RP for sidelink transmissions being equipped with physical sidelink feedback channel (PSFCH) resources for HARQ feedback. In the event that UE1 uses Option-2 with feedback-enabled HARQ, UE1 may determine to reselect sidelink transmission radio resources after a number N consecutive NACKs are received at UE1. Since the sidelink transmissions by UE1 may be a unicast transmission or a groupcast transmission, number N may be a function of the number of UEs addressed by the UE1 sidelink transmissions. The congestion and priority thresholds mentioned above may be static or dynamic thresholds. According to one example, UE1 is configured to adapt the congestion and / or priority threshold values (e.g. Th1, Th2, P1, P2) based, for example, on the received HARQ feedback. For example: for a given CBR value, a decrease in negative acknowledgments (NACK) may indicate that the other UEs with which UE1 shares the resource pool RP currently have an increased proportion of UEs having information about the frequency hopping pattern used by UE1, and having better capability to avoid collisions with UE1 sidelink transmissions. A decrease in NACKs may indicate that UE1 currently shares the resource pool RP with a decreased proportion of other UE1 s (other Release 17 UEs operating in "random selection" mode), and / or a decreased proportion of UE3s (Release 16 UEs having no information about the pre-defined frequency hopping pattern used by UE1 to randomly select sidelink transmission resources). A change in the number of NACKs may trigger the UE1 to change the priority threshold values P1 and P2 and / or congestion threshold values Th1 and Th2. A decrease in priority threshold values P1 and P2 has the effect that UE1 may transmit data with a greater range of priority values when operating under Rules B and C. 27 08 24 The use of HARQ feedback allows UE1 to more effectively determine a need for resource reselection, since (as mentioned above) a negative acknowledgment may indicate a collision in the selection of resources by UE1 and one or more competing UEs (UEs sharing the same RP for sidelink transmissions). In the examples described above, CBR is used to select Option 1 or Option 2 for making a sidelink transmission. In other example embodiments, the selection of a transmission procedure may be made without using CBR, and may be dependent on the priority value of the TB, and / or the possibility to receive HARQ feedback from one or more UEs receiving the TB. In the examples described above, two congestion threshold values (CBR threshold values) and two priority threshold values are used. In other examples, more than two congestion threshold values and / or more than two priority threshold values may be used, to create a greater range of rules and transmission procedure options. The above-described techniques facilitate the avoidance of sidelink transmission collisions between UE1 and other UEs, and better avoid the risk of persistent collisions when UE1 is operating according to Option-2 involving selecting resources for a plurality of TBs in a single selection operation. The abovedescribed techniques can also achieve a fairer share of the resource pool between Release 17 UEs operating in "blind resource selection" mode and Release 16 and 17 UEs actively checking for resource reservations by other UEs, and thus better facilitate their co-existence. 27 08 24 Figure 4 illustrates an example of an apparatus for implementing the operations of a UE in the embodiments described above. The apparatus may comprise at least one processor 602 coupled to one or more interfaces 608 to e.g. other equipment for which the UE functionality provides radio communications. The at least one processor 602 is also coupled to a radio unit 604 including one or more antennas etc. for making and receiving radio transmissions. The at least one processor 602 may also be coupled to at least one memory 606. The at least one processor 602 may be configured to execute an appropriate software code to perform the operations described above. The software code may be stored in the memory 606. Figure 5 shows a schematic representation of non-volatile memory media 1100a (e.g. computer disc (CD) or digital versatile disc (DVD)) and 1100b (e.g. universal serial bus (USB) memory stick) storing instructions and / or parameters 1102 which when executed by a processor allow the processor to perform one or more of the steps of the methods described previously. It is to be noted that embodiments of the present invention may be implemented as circuitry, in software, hardware, application logic or a combination of software, hardware and application logic. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as the base stations or user equipment of the above-described embodiments. 27 08 24 As used in this application, the term "circuitry" refers to all of the following: (a) hardware- only circuit implementations (such as implementations in only analog and / or digital circuitry) and (b) to combinations of circuits and software (and / or firmware), such as (as applicable): (i) to a combination of processor(s) or (ii) to portions of processor(s) / software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as the user equipment or base stations of the above-described embodiments, to perform various functions) and (c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of 'circuitry' applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and / or firmware. The term "circuitry" would also cover, for example and if applicable to the particular claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in server, a cellular network device, or other network device. The described features, advantages, and characteristics of the invention can be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages can be recognized in certain embodiments that may not be present in all embodiments of the invention. One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and / or with hardware elements in configurations which are different than those which are disclosed. Therefore, although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. 27 08 24

Claims

1. A method, comprising:selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a predefined hopping pattern;wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources;wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; andwherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following:an estimate of the degree of congestion of at least part of the resource pool;a priority value of the data or the user equipment;a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers.

2. The method according to claim 1, wherein the plurality of pre-defined transmission procedures comprise at least: (i) a first transmission procedure comprising the user equipment transmitting a first transport block without also reserving radio resources for transmission of a further transport block; and (ii) a second transmission procedure comprising the user equipment transmitting a27 08 24first transport block and associated control information, wherein the associated control information includes a reservation of radio resources for transmission of a further transport block.

3. The method according to any of claims 1 or 2, wherein the estimate of the degree of congestion is a measurement of how much of at least part of the resource pool was occupied by sidelink transmissions over a measurement period.

4. The method according to any of claims 1 to 3, comprising receiving the estimate of the degree of congestion at the user equipment from another user equipment.

5. The method according to any preceding claim, wherein the blindly selected resources are resources selected randomly without an operation to sense reservation of resources by other user equipments.

6. The method according to any preceding claim, comprising comparing the estimate of the degree of congestion against firstand second congestion threshold values.

7. The method according to claim 6, comprising: in response to determiningthat the estimate of the degree of congestion is below the first congestion threshold, operating according to a first rule; wherein the first rule allows the user equipment to (i) select the first transmission procedure, unconditional on a priority value for the first transport block, and unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure unconditional on a priority value for the first transport block, but conditional on the possibility for feedback from one or more receivers.

8. The method according to claim 6, comprising: in response to determining that the estimate of the degree of congestion is between the first and second27 08 24congestion threshold values, operating according to a second rule; wherein the second rule allows the user equipment to (i) select the first transmission procedure conditional on the first transport block having a priority value higher than a first priority threshold, but unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure, conditional on the first transport block having a priority value higher than a first priority threshold and conditional on the possibility for feedback from one or more receivers.

9. The method according to claim 6, comprising: in response to determining that the estimate of the degree of congestion is above the second congestion threshold, operating according to a third rule; wherein the third rule allows the user equipment to select the second transmission procedure, conditional on the first transport block having a priority value higher than a second priority threshold, and conditional on the possibility for feedback from one or more receivers.

10. Apparatus comprising:means for selecting at a user equipment one of a plurality of pre-defined transmission procedures for a sidelink transmission of data via radio resources blindly selected at the user equipment from a resource pool according to a predefined hopping pattern;wherein the resource pool is at least partly shared with sidelink transmissions by other user equipments selecting radio resources for sidelink transmissions according to an operation comprising sensing the reservation of radio resources;wherein the other user equipments sharing the resource pool include one or more user equipments without information of the hopping pattern; and27 08 24wherein the selecting of a transmission procedure at the user equipment is based at least partly on one or more of the following:an estimate of the degree of congestion of at least part of the resource pool;a priority value of the data or the user equipment;a possibility for Hybrid Automatic Repeat Request (HARQ) feedback from one or more receivers.

11. The apparatus according to claim 10, wherein the plurality of pre-defined transmission procedures comprise at least: (i) a first transmission procedure comprising the user equipment transmitting a first transport block without also reserving radio resources for transmission of a further transport block; and (ii) a second transmission procedure comprising the user equipment transmitting a first transport block and associated control information, wherein the associated control information includes a reservation of radio resources for transmission of a further transport block.

12. The apparatus according to any of claims 10 to 11, wherein the estimate of the degree of congestion is a measurement of how much of at least part of the resource pool was occupied by sidelink transmissions over a measurement period.

13. The apparatus according to any of claims 10to 12, comprising means for receiving the estimate of the degree of congestion at the user equipment from another user equipment.

14. The apparatus according to any of claims 10 to 13, wherein the blindly selected resources are resources selected randomly without an operation to sense reservation of resources by other user equipments.27 08 2415. The apparatus according to any of claims 10 to 14, comprising means for comparing the estimate of the degree of congestion against first and second congestion threshold values.

16. The apparatus according to claim 15, comprising: means for, in response to determining that the estimate of the degree of congestion is below the first congestion threshold, operating according to a first rule; wherein the first rule allows the user equipment to (i) select the first transmission procedure, unconditional on a priority value for the first transport block, and unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure unconditional on a priority value for the first transport block, but conditional on the possibility for feedback from one or more receivers.

17. The apparatus according to claim 15 or claim 16, comprising: means for, in response to determining that the estimate of the degree of congestion is between the first and second congestion threshold values, operating according to a second rule; wherein the second rule allows the user equipment to (i) select the first transmission procedure conditional on the first transport block having a priority value higher than a first priority threshold, but unconditional on the possibility for feedback from one or more receivers, or (ii) select the second transmission procedure, conditional on the first transport block having a priority value higher than a first priority threshold and conditional on the possibility for feedback from one or more receivers. .

18. The apparatus according to any of claims 15 to 17, comprising: means for, in response to determining that the estimate of the degree of congestion is above the second congestion threshold, operating according to a third rule; wherein the third rule allows the user equipment to select the second transmission procedure, conditional on the first transport block having a priorityvalue higher than a second priority threshold, and conditional on the possibility for feedback from one or more receivers.

19. A computer program comprising computer executable code which when run on at least one processor is configured to cause an apparatus at least to cause the method of any one of claims 1 to 9 to be performed.27 08 24

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