FIRST DEVICE, METHOD FOR OPERATING THE FIRST DEVICE, SECOND DEVICE AND METHOD FOR OPERATING THE SECOND DEVICE

Description

State of the art

[0001] The invention relates to a first terminal device, a method for operating the first terminal device, a second terminal device and a method for operating the second terminal device.

[0002] From EP 2 146 515 A1 and EP 1 879 403 A1, communication systems comprising a base station and several terminal devices are known, wherein the base station can transmit data to the terminal devices simultaneously using Multimedia Broadcast Multicast Service (MBMS). Disclosure of the invention

[0003] The problem underlying the invention is solved by a first terminal device according to claim 1, by a method for operating the first terminal device according to a dependent claim, by a second terminal device according to a dependent claim and by a method for operating the second terminal device according to a dependent claim.

[0004] According to a first aspect of this description, a first terminal device of a radio communication network is provided, wherein the first terminal device comprises at least one processor, at least one memory containing computer program code, at least one communication module, and at least one antenna, wherein the computer program code is configured such that, in conjunction with the at least one processor, the at least one communication module, and the antenna, it causes the first terminal device to send initial messages on a first radio resource via a sidelink channel to a group of further terminal devices, to receive at least one response after the transmission of the respective initial message from at least one of the further terminal devices of the group via the sidelink channel, and to determine a resource change decision depending on the at least one response.and sends second messages, depending on the resource change decision, on a second radio resource via the sidelink channel to the group of further terminal devices, where the second radio resource differs from the first radio resource.

[0005] The first endpoint can decide, based on the received responses, whether to retain or switch the radio resource used for a broadcast or multicast message, which has been assigned semi-persistently or persistently. By triggering this resource change decision, the first endpoint switches to the second radio resource, thus avoiding consecutive collisions and ensuring a high reception rate for the transmitted messages as quickly as possible. This allows for a rapid response to collisions and simultaneously creates a distributed collision avoidance strategy that operates without a central scheduling unit such as a base station.

[0006] The provided decentralized collision avoidance strategy benefits applications where the loss of individual messages is acceptable, but the loss of multiple consecutive messages is problematic. One example of such an application is platooning, where, for instance, trucks traveling in convoy communicate with each other and coordinate acceleration and braking.

[0007] An advantageous embodiment is characterized in that the first terminal receives an initial number of responses after the transmission of the respective message via the sidelink channel, identifies a second number of terminals in the group, compares the first number of responses with the second number of terminals in the group, and determines the resource change decision based on this comparison. For example, if responses are reduced (i.e., the initial number of acknowledgments is lower) compared to the second number of terminals belonging to the group, the first terminal assumes an increased collision rate and changes the radio resource used to mitigate this situation. Advantageously, the number of terminals in the group is utilized to improve the collision response of the first terminal in terms of the resource change decision, thereby improving spectral efficiency.

[0008] An advantageous embodiment is characterized in that the first terminal receives an initial number of responses after the respective message is sent via the sidelink channel, determines a second number of terminals in the group, determines a third number by multiplying the first number by a scaling factor, compares the third number with the second number of terminals in the group, and determines the resource change decision based on this comparison. Advantageously, only a subset of the terminals in the group sends a response. This subset is determined by the scaling factor. The first terminal then calculates, from the number of received responses and the scaling factor, the number of terminals that would confirm receipt / non-receipt. Due to the scaling factor, the number of sent responses decreases, thereby increasing the spectral efficiency of the provided collision avoidance strategy.

[0009] An advantageous embodiment is characterized in that the response is a negative acknowledgment. The advantage of a negative acknowledgment is that—assuming a large number of the participating terminals correctly receive the transmitted messages—a smaller number of terminals send a response to the message. The smaller proportion of terminals that acknowledge non-reception with a negative acknowledgment then generates less data traffic on the radio channel, which increases spectral efficiency.

[0010] An advantageous embodiment is characterized in that the response includes a resource indicator that uniquely identifies the relevant radio resource. For example, the resource indicator can include a timestamp or an identification identifier of the first radio resource. The resource indicator thus enables the first terminal to uniquely and easily assign the received response to the radio resource, thereby simplifying the determination of the resource change decision. Collision detection is improved.

[0011] An advantageous embodiment is characterized in that the first terminal receives positive acknowledgments in response to each transmitted message via the sidelink channel, receives negative acknowledgments in response to each transmitted message, determines a collision indicator depending on the number of positive acknowledgments and depending on the number of negative acknowledgments, and determines the resource change decision based on a comparison of the collision indicator with a collision threshold. Advantageously, the collision indicator provides reliable information about the penetration rate of the transmitted message within the group of other terminals. The collision detection accuracy increases, thereby also improving the remediation measures in terms of switching radio resources.

[0012] An advantageous embodiment is characterized in that the first terminal receives at least one movement and / or position information from terminals via a further message, determines the group of further terminals based on the received movement and / or position information, and determines the resource change decision based on responses from the terminals of the determined group. Advantageously, by considering the movement and / or position information, only those further terminals of the group—as a subset of the terminals directly reachable via radio—that actually benefit from switching a radio resource are taken into account. Advantageously, the other terminals, which potentially only experience brief collisions, are excluded from determining the resource change decision.

[0013] For example, vehicles traveling in the opposite lane on a highway benefit less from a change in radio resource than vehicles traveling in the same direction. Consequently, movement information related to direction of travel offers advantages in making resource change decisions.

[0014] Similarly, distant devices can be ignored, thereby increasing the relevance of devices closer to the first device when deciding on resource changes. Furthermore, the repetition interval of the radio resource used for sending messages can be reduced, as collisions for only a subset of the other devices now need to be monitored and resolved.

[0015] An advantageous embodiment is characterized in that the first terminal determines the scaling factor depending on the number of other terminals in the group. Advantageously, by considering the number of terminals in the group, the scaling factor takes into account the network load, which scales with the number of terminals. Consequently, the number of responses can be set inversely proportional to the network load using the scaling factor, thus maintaining the distributed collision avoidance strategy even under increased radio channel load. This allows the first terminal to decide, based on the received responses, whether to retain or switch to the current radio resource. Simultaneously, decentralized collision detection and resolution can be used even with increasing network load.

[0016] An advantageous embodiment is characterized in that the first terminal receives a first number of responses after the transmission of the respective message sent on the first radio resource from terminals of the group via the sidelink channel, receives a second number of responses after the transmission of the respective message sent on the second radio resource from terminals of the group via the sidelink channel, determines a further resource change decision when an absolute difference between the first and second number of responses exceeds a threshold, and sends further messages on a third radio resource to the group of further terminals depending on the resource change decision, wherein the third radio resource differs from the first and second radio resources.This provides a way for the first device to immediately trigger another radio resource change upon receiving a response. In this decentralized manner, a radio resource with as few collisions as possible is found as quickly as possible.

[0017] An advantageous embodiment is characterized in that the messages, in particular the first, second and third messages as well as the further message, are Cooperative Awareness Messages, CAM, in particular according to ETSI TS 102 637-2 V1.2.1 (2011-03), and / or Decentralized Environmental Notification Messages, DENM, in particular according to ETSI TS 102 637-3 V1.1.1 (2010-09).

[0018] A second aspect of this description provides a method for operating a first terminal device of a radio communication network, wherein the method comprises: sending first messages on a first radio resource via a sidelink channel to a group of further terminal devices, receiving via the sidelink channel at least one reply after sending the respective first message from at least one of the further terminal devices of the group, determining a resource change decision depending on the at least one reply, and sending second messages depending on the resource change decision on a second radio resource via the sidelink channel to the group of further terminal devices, wherein the second radio resource is different from the first radio resource.

[0019] A third aspect of this description concerns a second terminal device of a radio communication network, wherein the second terminal device comprises at least one processor, at least one memory containing computer program code, at least one communication module, and at least one antenna, wherein the computer program code is configured to cause the second terminal device, in conjunction with the at least one processor, the at least one communication module, and the antenna, to receive first messages from a first terminal device on a first radio resource via the sidelink channel, determine a response depending on whether the first message has been successfully decoded, send the response to the first terminal device via the sidelink channel, and receive second messages from the first terminal device on a second radio resource via the sidelink channel, wherein the second radio resource is different from the first radio resource.

[0020] An advantageous embodiment is characterized in that the second terminal determines a random number between two values, compares the random number with a threshold value, and sends the response to the first terminal depending on the comparison.

[0021] An advantageous embodiment is characterized in that the second terminal device sends the response to the first terminal device via the sidelink channel only if the determined random number is less than or equal to the threshold, where the threshold is the inverse of a scaling factor.

[0022] An advantageous embodiment is characterized in that the second terminal receives at least one movement and / or position information from other terminals via a further message over the sidelink channel and determines the scaling factor depending on the number of other terminals. Advantageously, by taking the number of terminals into account, the scaling factor ensures that the network load scales with the number of terminals. Consequently, the number of responses can be set inversely proportional to the network load using the scaling factor, thus maintaining the distributed collision detection and prevention strategy even under increased load on the radio channel. This allows the first terminal to decide, based on the received responses, whether to maintain or switch to the currently used radio resource.At the same time, decentralized collision detection and resolution can also be used with increasing network load.

[0023] In an advantageous embodiment, the received signal strength is determined during the reception of the first message, and the response is calculated based on this signal strength and on whether the first message was successfully decoded. This advantageously takes into account the transmission range of the first message when determining the response.

[0024] An advantageous embodiment is characterized in that the response is a negative acknowledgment. The advantage of a negative acknowledgment is that—assuming a large number of the participating terminals correctly receive the transmitted messages—a smaller number of terminals send a response to the message. The smaller proportion of terminals that acknowledge non-reception with a negative acknowledgment then generates less data traffic on the radio channel, which increases spectral efficiency.

[0025] An advantageous embodiment is characterized by the fact that the response includes a resource indicator that uniquely identifies the relevant radio resource. For example, the resource indicator can include a timestamp or an identification identifier of the first radio resource. The resource indicator thus enables the first terminal to uniquely and easily assign the received responses, thereby simplifying the determination of the resource change decision. This reduces computational effort and simultaneously improves collision detection.

[0026] An advantageous embodiment is characterized in that the messages, in particular the first, second and subsequent messages, are Cooperative Awareness Messages, CAM, in particular according to ETSI TS 102 637-2 V1.2.1 (2011-03), and / or Decentralized Environmental Notification Messages, DENM, in particular according to ETSI TS 102 637-3 V1.1.1 (2010-09).

[0027] A fourth aspect of this description concerns a method for operating a second terminal device of a radio communication network, wherein the method comprises: receiving first messages from a first terminal device on a first radio resource via a sidelink channel, determining a response depending on whether the first message was successfully decoded or not, sending the response to the first terminal device via the sidelink channel, and receiving second messages from the first terminal device on a second radio resource via the sidelink channel, wherein the second radio resource is different from the first radio resource.

[0028] Further features and advantages can be found in the following description of exemplary embodiments and the drawing. The drawing shows: Figure 1 is a schematic representation of a radio communication network; Figure 2 is a schematic representation of a terminal device; Figures 3, 4, 8 and 11 are each a schematic representation of a flowchart; and Figures 5, 6, 7, 9, 10 and 12 are each a schematic representation of a sequence diagram.

[0029] Figure 1Figure 1 shows a schematic representation of a wireless communication network. Terminals A to L are located, for example, in a vehicle or on roadside infrastructure. These terminals are configured, for example, according to the IEEE standard "802.11p-2010 - IEEE Standard for Information Technology - Local and Metropolitan Area Networks-" Specific Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 6: Wireless Access in Vehicular Environments". IEEE 802.11p is a standard that extends the WLAN standard IEEE 802.11. The goal of IEEE 802.11p is to establish wireless technology in passenger vehicles and to provide a reliable interface for Intelligent Transport Systems (ITS) applications. IEEE 802.11p is also the basis for Dedicated Short Range Communication (DSRC) in the 5.85 to 5.925 GHz range.In another example, devices A to L are configured according to the LTE-V2X standard, specifically 3GPP TS 36.300 V14.2.0 (2017-03), with devices A to L communicating directly with each other via a sidelink channel. In radio communication network 2, data is transmitted between devices A to L via a radio resource that is not specified or scheduled by a central unit. In the example shown, devices A and J use this radio resource to send messages to a respective group of devices. Device A sends its message to the group of devices B, C, D, E, F, and G, which are within a radio distance 4A of device A. Device J sends its message to the group of devices B, G, H, and K, which are within a radio distance 4J of device J, excluding device L.Consequently, the transmitted messages address either all or a subset of the directly reachable devices in a broadcast or multicast manner. Devices B and G are located within the two radio distances 4A and 4J, respectively, and are also part of the respective groups addressed by the messages. Simultaneous transmission of messages leads to a collision between devices B and G.

[0030] Figure 2 Figure 1 schematically shows an end device 200. The end device 200 comprises a processor 202, a communication module 204, an antenna 205, and a memory 206. The memory 206 is equipped with computer program code 208, which is configured to execute the steps described in this specification using the at least one processor 202, the at least one communication module 204, and the at least one antenna 205. The end devices A to L from Figure 1are trained, for example, according to the terminal 200.

[0031] Figure 3 shows a schematic flowchart for operating a first terminal device A, J from Figure 1 According to step 302, the first terminal sends initial messages on a first radio resource to a group of other terminals. According to step 304, the first terminal receives at least one response after sending the respective initial message from at least one of the other terminals in the group. According to step 306, the first terminal determines a resource change decision based on the at least one response. According to step 308, the first terminal sends second messages based on the resource change decision on a second radio resource to the group of other terminals.

[0032] The first and second radio resources are scheduled independently by the respective end devices A and J, thus providing distributed scheduling and eliminating the need for a central unit to determine the allocation of radio resources. The first radio resource is used for a first period of time, and the second radio resource is used for a second period of time. Therefore, the radio resources are scheduled decentrally and semi-persistently or persistently.

[0033] Each of the radio resources used is characterized by the periodically recurring use of at least one time-frequency resource, which can also be referred to as a resource block. Different radio resources differ, for example, in their periodicity and / or the spacing of the respective time-frequency resource used and / or in the subchannel used and / or the modulation and coding scheme (MCS).

[0034] In one example, the first device monitors available radio resources. The first device counts radio resource switches performed by other devices, with these switches being initiated by the respective other device based on one of the described methods. As soon as the number of switches per unit of time exceeds a threshold, the first device will refrain from switching radio resources, even though it has determined that a resource change is necessary. This is advantageous in high-traffic areas, as the probability of collisions increases with excessively frequent radio resource switches.

[0035] Figure 4 shows a schematic flowchart for operating a second terminal device C, D, E, B, G, F, H, K from Figure 1According to step 402, the second terminal receives initial messages from a first terminal on the first radio resource. According to step 404, the second terminal determines a response depending on whether the initial message was successfully decoded. According to step 406, the second terminal sends the response to the first terminal. According to step 408, the second terminal receives subsequent messages from the first terminal on the second radio resource. The first and second radio resources are, for example, each identified by an assigned time-frequency resource, which is used periodically.

[0036] The primary and secondary radio resources used are either negotiated between the end devices themselves or determined by a single end device. There is no other central unit, such as a base station, meaning that communication between the end devices is coordinated by the end devices themselves.

[0037] Figure 5 This shows a schematic sequence diagram. In step 502, terminal J determines an initial message N1 and sends it via an initial radio resource R1, specifically to terminals K, G, and B. Terminals G and B do not receive the initial message N1 or cannot successfully decode it. Terminal K, however, receives the message N1 and, in step 504, determines a response A1, indicating successful receipt, in order to forward it to terminal J. In the example of... Figure 5Given that devices K, G, and B are among the devices addressed by message N1, the number of three devices is compared with the first number of a single response received by device J in step 506. The number of responses can also be zero. Thus, in step 506, it is determined that three devices K, G, and B should have received message N1, but only one of them, namely device K, actually received the message. Therefore, in step 506, device J can assume that the collisions occurred when receiving message N1 at devices G and B. Consequently, the resource change decision is determined based on the comparison performed in step 506.The resource change decision, in step 508, results in the selection of a second radio resource, R2, different from the first radio resource, R1, for a second message N2. This second radio resource is then used to send the second message N2 to terminals K, G, and B. In steps 510, 512, and 514 respectively, a response A2, A3, and A4 is determined and subsequently sent to terminal J. The number of terminals belonging to the group is determined by terminal J, for example, based on received position messages, and maintained in a corresponding list. Figure 5 For example, only positive acknowledgments of receipt (ACKs) are sent.

[0038] The respective responses A1, A2, etc., can be transmitted either as a separate message or embedded within another message. This applies to the entire description. Each response A1, A2, etc., must include at least one of the following pieces of information: a positive or negative acknowledgment; the sender of the message to which the respective response A1, A2 is determined and sent; a timestamp identifying the time the associated message was received; and an identification of the radio resources used for the message.

[0039] Figure 6The diagram shows a schematic sequence diagram. Both terminals J and A attempt to transmit a message N1, N5 to terminal G on the same resource R1 at the same time. Terminal G determines a negative acknowledgment A5, A6 in step 602 and transmits this negative acknowledgment A5, A6 to the respective terminal J, A. Of course, only a single negative acknowledgment can also be sent, which indicates the terminals J, A or the radio resource being used.

[0040] The respective terminal J, A infers from the respective responses A5, A6 in steps 604, 606 that a collision occurred during the transmission of messages N1, N5. Accordingly, in steps 604, 606, a resource change decision is made to send subsequent messages N2, N6 on radio resources R2, R3, which differ from the first radio resource R1. Radio resources R3, R2 differ, for example, in that a different phase is selected, resulting in the messages being transmitted at different times. Radio resources R2, R3 are selected randomly in steps 604, 606. It is also conceivable that the selection of radio resources R2, R3 is carried out according to a predetermined scheme to avoid a further collision or to reduce the risk of a collision.In steps 608 and 610, the respective messages N1 and N6 are successfully received. In this example, successful reception is not reported back to terminals J and A; instead, only unsuccessful reception is acknowledged with responses A5 and A6, indicating a negative receipt confirmation.

[0041] Figure 7Figure 1 shows a schematic sequence diagram. In the example shown, only terminal K receives a message N1 from node J via radio resource R1, which node J determines in step 706. Terminals G and B each detect a collision between the data transmission of message N1 on radio resource R1 and a data transmission originating from node A in steps 702 and 704, respectively. Terminal J receives corresponding responses A7, A8, and A9, where response A7 is a positive acknowledgment and responses A8 and A9 are negative acknowledgments. In step 710, the received responses A7, A8, and A9 are combined to determine the resource change decision. In step 712, depending on the resource change decision, radio resource R2 is selected to transmit message N2 to terminals K, G, and B.In each step 714, 716, 718, the message N2 is successfully received, and subsequently, according to a respective response A10, A11, A12, it is sent to the terminal J as a positive confirmation of receipt.

[0042] Figure 8 shows a schematic flowchart of step 710 from Figure 7In step 802, both positive and negative delivery acknowledgments are received in response to each sent message. In step 804, a collision indicator is determined based on the number of positive and negative delivery acknowledgments. For example, the number of positive delivery acknowledgments can be divided by the number of negative delivery acknowledgments. Alternatively, a relative collision indicator can be calculated by dividing the number of positive delivery acknowledgments by the sum of the number of positive delivery acknowledgments and the number of negative delivery acknowledgments. In step 806, the collision indicator is compared to a collision threshold.If, in an example, the collision indicator is a ratio of positive to negative delivery acknowledgments, then in step 810 the resource change decision is determined if the ratio is below the collision threshold of, for example, 1, which means that more than half of the addressed terminals are not successfully receiving the messages. However, if the collision indicator is above or at the collision threshold, then in step 812 no resource change decision is determined in order to retain the selected first radio resource R1.

[0043] Figure 9Figure 1 shows a schematic sequence diagram. In step 902, terminal G determines its movement and / or position information and transmits it to terminal J via a further message Nx. Terminal J stores the movement and / or position information of terminal G in step 904. Messages N1 and N11, each originating from a different terminal J and A but transmitted on the same radio resource R1, collide, which terminal G detects in step 906. The registered collision of message N1 with message N11 is transmitted to terminal J as a response A13 to message N1.

[0044] Upon receiving response A13, terminal J executes step 908, comprising steps 910 and 912. In step 910, a distance between terminals J and G is determined based on movement and / or position information from these terminals. Alternatively or additionally, the system determines whether terminals J and G are moving in the same or a similar direction. By comparing this distance with a distance threshold and / or considering the movement directions of terminals J and G, a group of additional terminals, including terminal G, is identified. These additional terminals are located, for example, within a radius defined by the distance threshold around terminal J and / or exhibit the same or a similar direction of movement as terminal J.

[0045] If terminal G is assigned to the group to which message N1 was intended in step 912, then in step 914 the resource change decisions are determined for the entire group of terminals, including terminal G, in order to transmit the second message N2 via a second radio resource R2 that is different from the first. In step 916, terminal G successfully receives the second message N2.

[0046] Figure 10The diagram shows a schematic sequence diagram. Terminal J sends message N1 on the first radio resource R1. In step 1020, the distance and / or direction of travel relative to terminal J is determined. In step 1022, the distance and / or direction of travel is compared with a corresponding threshold value. Depending on the comparison, the process proceeds to step 1002. This ensures that if terminal J is too far from terminal G or if terminal J is moving in the opposite direction to terminal G, no response A1 is transmitted to terminal J.

[0047] Response A1 is randomly generated by device G, so that only a certain number of devices G receiving message N1 generate a response A1, A20. In step 1002, device G generates a random number Z1, Z2 between two predefined fixed values ​​W1, W2. This random number Z1, Z2 is compared to a threshold S. If the random number Z1 is less than the threshold S, message A1 is sent. If, however, the random number Z2 is greater than or equal to the threshold S, message A1 is not sent. The threshold is, for example, the inverse of the scaling factor described below, which is determined, for instance, based on the number of devices in a group. Of course, the scaling factor can also be fixed. Responses A1, A20 can be either positive or negative acknowledgments of receipt.

[0048] If responses A1 and A20 are negative acknowledgments, steps 1020 and 1022 can be omitted. In the event of a collision during the reception of message N1, and with a sufficient SINR, terminal G successfully decodes message N1. Based on the ratio to the measured power / energy, terminal G detects a high probability of a message collision, i.e., simultaneous transmission of data on the radio resource R1. Consequently, a second message could not be decoded. In this case, terminal G sends a negative acknowledgment to the terminal whose message was successfully decoded. To prevent future collisions, terminal G performs steps 1020 and 1022 if interference from other terminals on the same radio resource R1 increases.

[0049] Terminal J receives responses A1 and A20 and, in step 1004, determines an initial number of received responses A1 and A20 after sending message N1. In step 1006, a second number of terminals in the group is determined, assuming that the terminals in the group received message N1. In step 1008, a third number is determined by multiplying the first number by the scaling factor. This estimates the number of terminals that would have sent a response similar to A1 and A20, but did not do so due to step 1002.

[0050] In step 1010, the third and second numbers are compared. If, in the case of negative reception acknowledgments, the quotient resulting from dividing the third and second numbers is above a threshold, then in step 1012, a resource change decision is determined to switch from the previously used radio resource R1 to the second radio resource R2 in step 1014. However, if the aforementioned quotient is below the threshold in the case of negative reception acknowledgments, then in step 1016 no resource change decision is determined, and the terminal J continues to use the radio resource R1.

[0051] In another example, if, in the case of positive reception acknowledgments, the quotient determined as described in the preceding paragraph is below a threshold, the resource change decision is determined. Should the quotient be above the threshold, the terminal J continues to use the radio resource R1.

[0052] Figure 11The diagram shows a schematic flowchart. In step 1102, an initial number of responses are received by devices in the group after the respective message has been sent on the first radio resource. In step 1104, a second number of responses are received by devices in the group after the respective message has been sent on the second radio resource. In step 1106, a further resource change decision is determined if the absolute difference between the first and second number of responses exceeds a threshold. In step 1108, further messages are sent to the group by additional devices on a third radio resource, depending on the resource change decision. This third radio resource differs from the first and second radio resources.Therefore, if it is detected that the second radio resource is heavily affected by collisions, the system immediately switches to the third radio resource. Consequently, this switching between radio resources leads to a rapid reduction in collisions. Figure 12Figure 1 shows a schematic sequence diagram. In step 1202, the terminal device G determines the received power. Alternatively, another quantity, such as the received energy (i.e., received power integrated over time), can be compared with a corresponding threshold value in step 1204. In step 1204, it is checked whether the received power is above a threshold value, i.e., whether the received power was sufficiently high when message N1 was received. If this is the case, terminal device G attempts to decode message N1 in step 1206. If it is determined in step 1208 that the decoding was successful, the response A1 to the receipt of message N1 is determined and sent in step 1210.Consequently, the response A1 is determined depending on the received power and depending on whether the respective first message N1 was successfully decoded or not.

[0053] Evaluating the received power is not only used to consider the range of the first message when sending a reply. Rather, this additional parameter is essential for distinguishing between a collision and a SINR (signal-to-interference plus noise ratio) that is too low for successful decoding, for example, due to natural background noise. Without evaluating the received power, second devices that cannot successfully decode the message from the first device solely due to background noise might send corresponding negative replies, even though no "classic" collision has occurred. Depending on the characteristics of the first device's receiver or the transmission parameters of the message, the first device might then correctly receive the negative reply and switch to a second radio resource, but this second resource might not offer any improvement in transmission quality, as, for example,Background noise affects all radio resources equally. Therefore, the first device would constantly switch between radio resources, negating the fundamental advantages of the lower collision probability with (semi-)persistent scheduling and thus proving counterproductive.

Claims

1. First terminal (A; J) for a radio communications network (2), the first terminal (A; J) comprising at least one processor, at least one memory containing computer program code, and at least one communication module and at least one antenna, the computer program code being configured such that it uses the at least one processor, the at least one communication module and the at least one antenna to cause the first terminal (A; J) - to send first messages to a group of further terminals on a first radio resource via a sidelink channel, - to receive at least one response from at least one of the further terminals in the group via the sidelink channel after the respective first message has been sent, - to determine a resource change decision on the basis of the at least one response, and - to send second messages to the group of further terminals on a second radio resource via the sidelink channel on the basis of the resource change decision, the second radio resource being different from the first radio resource.

2. First terminal (A; J) according to Claim 1, the first terminal (A; J) - receiving a first number of responses via the sidelink channel after the respective message has been sent, - determining a second number of terminals in the group, - comparing the first number of responses with the second number of terminals in the group, and - determining the resource change decision on the basis of the comparison.

3. First terminal (A; J) according to Claim 1, the first terminal (A; J) - receiving a first number of responses via the sidelink channel after the respective message has been sent, - determining a second number of terminals in the group, - determining a third number by multiplying the first number by a scaling factor, - comparing the third number with the second number of terminals in the group, and - determining the resource change decision on the basis of the comparison.

4. First terminal (A; J) according to one of the preceding claims, the response being a negative acknowledgement of receipt.

5. First terminal (A; J) according to one of the preceding claims, the response comprising a resource indicator that uniquely identifies the relevant radio resource.

6. First terminal (A; J) according to one of the preceding claims, the first terminal (A; J) - receiving positive acknowledgements of receipt in response to the respective sent message via the sidelink channel, - receiving negative acknowledgements of receipt in response to the respective sent message via the sidelink channel, - determining a collision indicator on the basis of the number of positive acknowledgements of receipt and on the basis of the number of negative acknowledgements of receipt, and - determining the resource change decision on the basis of a comparison of the collision indicator with a collision threshold value.

7. First terminal (A; J) according to one of the preceding claims, the first terminal (A; J) - receiving at least one movement and / or position information item from terminals by means of a further message via the sidelink channel, - determining the group of further terminals on the basis of the received movement and / or position information, - determining the resource change decision on the basis of responses from the terminals in the determined group.

8. First terminal (A; J) according to Claim 3, the first terminal (A; J) - determining the scaling factor on the basis of the second number of further terminals in the group.

9. First terminal (A; J) according to one of the preceding claims, the first terminal (A; J) - receiving a first number of responses from terminals in the group after the respective message sent on the first radio resource has been sent, - receiving a second number of responses from terminals in the group after the respective message sent on the second radio resource has been sent, - determining a further resource change decision when an absolute difference between the first and second numbers of responses exceeds a threshold value, and - sending further messages to the group of further terminals on a third radio resource on the basis of the resource change decision, the third radio resource being different from the first and second radio resources.

10. First terminal (A; J) according to one of the preceding claims, the messages, in particular the first and second messages and also the further message, being Cooperative Awareness Messages, CAM, in particular according to ETSI TS 102 637-2 V1.2.1, 2011-03, and / or Decentralized Environmental Notification Messages, DENM, in particular according to ETSI TS 102 637-3 V1.1.1, 2010-09.

11. Method for operating a first terminal (A; J) of a radio communications network (2), the method comprising: - sending first messages to a group of further terminals on a first radio resource via a sidelink channel, - receiving at least one response from at least one of the further terminals in the group via the sidelink channel after the respective first message has been sent, - determining a resource change decision on the basis of the at least one response, and - sending second messages to the group of further terminals on a second radio resource via the sidelink channel on the basis of the resource change decision, the second radio resource being different from the first radio resource.

12. Second terminal (B-H; K) for a radio communications network (2), the second terminal (B-H; K) comprising at least one processor, at least one memory containing computer program code, and at least one communication module and at least one antenna, the computer program code being configured such that it uses the at least one processor, the at least one communication module and the at least one antenna to cause the second terminal (B-H; K) - to receive first messages from a first terminal on a first radio resource via a sidelink channel, - to determine a response on the basis of a successful or unsuccessful decoding of the respective first message, - to send the response to the first terminal via the sidelink channel, and - to receive second messages from the first terminal on a second radio resource, the second radio resource being different from the first radio resource.

13. Second terminal (B-H; K) according to Claim 12, the second terminal (B-H; K) - determining a random number between two values, - comparing the random number with a threshold value, and - sending the response to the first terminal on the basis of the comparison.

14. Second terminal (B-H; K) according to Claim 13, the second terminal (B-H; K) - sending the response to the first terminal via the sidelink channel only if the determined random number is less than or equal to the threshold value, the threshold value being a reciprocal value of a scaling factor.

15. Second terminal (B-H; K) according to Claim 14, the second terminal (B-H; K) - receiving at least one movement and / or position information item from further terminals by means of a further message, and - determining the scaling factor on the basis of the number of further terminals.

16. Second terminal (B-H; K) according to one of Claims 12 to 15, the second terminal (B-H; K) - determining a reception power during reception of the first message via the sidelink channel, and - determining the response on the basis of the reception power and on the basis of the successful or unsuccessful decoding of the respective first message.

17. Second terminal (B-H; K) according to one of Claims 12 to 16, the response being a negative acknowledgement of receipt.

18. Second terminal (B-H; K) according to one of Claims 12 to 17, the response comprising a resource indicator that uniquely identifies the relevant radio resource of the sidelink channel.

19. Second terminal (B-H; K) according to one of Claims 12 to 18, the messages, in particular the first and second messages and also the further message, being Cooperative Awareness Messages, CAM, in particular according to ETSI TS 102 637-2 V1.2.1, 2011-03, and / or Decentralized Environmental Notification Messages, DENM, in particular according to ETSI TS 102 637-3 V1.1.1, 2010-09.

20. Method for operating a second terminal (B-H; K) of a radio communications network (2), the method comprising: - receiving first messages from a first terminal on a first radio resource via a sidelink channel, - determining a response on the basis of a successful or unsuccessful decoding of the respective first message, - sending the response to the first terminal via the sidelink channel, and - receiving second messages from the first terminal on a second radio resource via the sidelink channel, the second radio resource being different from the first radio resource.