WO2014161159A1

WO2014161159A1 - Group communication establishment

Info

Publication number: WO2014161159A1
Application number: PCT/CN2013/073665
Authority: WO
Inventors: Zhenhong Li; Fei YIN; Sami-Jukka Hakola; Samuli Turtinen
Original assignee: Broadcom Corporation
Priority date: 2013-04-02
Filing date: 2013-04-02
Publication date: 2014-10-09

Abstract

There is provided a method, comprising: deciding, by a first user terminal, to perform a group call to a group comprising a plurality of second user terminals; causing a transmission of a connection setup request to the group, wherein the connection setup request carries a first resource indication for determining a radio resource for a transmission of a connection setup response by each of the plurality of the second user terminals; causing a reception of at least one connection setup response carrying an identifier of the second user terminal that transmitted the connection setup response; and cause a transmission of a connection setup complete message to the group.

Description

GROUP COMMUNICATION ESTABLISHMENT

Field

The invention relates generally to mobile communication networks. More particularly, the invention relates to establishing connection and performing communication among user terminals belonging to a public safety group.

Background

The long term evolution (LTE) has been selected for the future of public safety (PS) mobile broadband. A PS LTE aims at improving provision of PS communications and the safety of the citizens during crisis and emergency situations. The PS communications may be of use, for example, to police, fire departments and other public safety agencies as they typically enter first to an emergency location, for example. For such first responders, the PS communications may provide means for enabling communication in situations with and without network coverage. As one problem the first responders may face is that the PS user terminals at the location need to be able to join a trusted group communication in an efficient manner.

Brief description of the invention

According to an aspect of the invention, there are provided methods as specified in claims 1 and 15.

According to an aspect of the invention, there are provided apparatuses as specified in claims 24, 40, and 51 .

According to an aspect of the invention, there is provided a computer program product as specified in claim 52.

According to an aspect of the invention, there is provided a computer-readable distribution medium carrying the above-mentioned computer program product.

According to an aspect of the invention, there is provided an apparatus comprising a processing system configured to cause the apparatus to perform any of the embodiments as described in the appended claims.

According to an aspect of the invention, there is provided an apparatus comprising means for performing any of the embodiments as described in the appended claims.

Embodiments of the invention are defined in the dependent claims. List of drawings

In the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which

Figures 1A and 1 B present networks according to some embodiments;

Figures 2 to 7B, and 8 show methods, according to some embodiments;

Figure 7C illustrates use of radio resources, according to an embodiment;

Figure 9 depicts a signaling flow diagram, according to an embodiment; and

Figures 10 and 11 illustrate apparatuses, according to some embodiments.

Description of embodiments

The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

To improve operational effectiveness of first responders in their day to day mission or in case of a major incident or disaster relief, by using advanced multimedia services and applications, there is a need to use a different technology than existing narrow-band professional mobile radio (PMR) technologies like the terrestrial trunked radio (TETRA), the TETRA enhanced data services (TEDS) or the P25. As an option, the long term evolution (LTE) may be used as the technology for the public safety (PS) network. Additionally, a variety of PS proximity services (ProSe) requirements have been defined. The requirements raise many points in developing the ProSe for the PS use. A PS user equipment (UE) may need to operate both in the PS spectrum for PS services and in the commercial spectrum of the mobile network operator for general services (e.g. voice call). However, as specified in the 3GPP TR 22.803, only the PS spectrum is used for the public safety ProSe. Further requirements for the public safety may include a PS UE discovery within or without the network coverage, enabling one-to-one and one-to-multiple communications (i.e. a ProSe group) communications.

One of the issues related to the LTE is whether the LTE supports other types of voice services, specifically the Push-To-Talk voice with and without network coverage, e.g. when the UEs or other ProSe devices are out- of-coverage of the network. It should be noted that in today's LTE, the legacy UEs and devices are fully controlled by the network and not at the device level. Thus, although the LTE has been widely used for commercial purposes, the use of the LTE for public safety is somewhat different from the commercial use of the LTE. As illustrated in Figure 1 , the public safety communication may occur with or without the network coverage. In Figure 1 , all the UEs 102 to 108 are cellular UEs and ProSe capable (i.e. capable of ProSe functionalities including direct device-to-device, D2D, communication without routing the data via the eNB). For the public safety communications, the D2D ProSe capable UEs may need to discover each other in proximity in a discovery process by themselves or under the assistance of the network.

In Figure 1 A, the UE 102 is within the coverage of an eNB 100 while in Figure 1 B, the UE 102 is out-of-coverage of the eNB 100. In case the UE 102 is in the coverage of the network, the UE 102 may help the PS UEs which are out-of-coverage to discover each other. In the following, it is assumed that a dedicated channel resource is reserved for extension link communication from the UE 102 towards the UEs 104 to 108. It is further assumed that there are no collisions in the connection setup requests so that the UEs 102 to 108 are not simultaneously transmitting the connection setup request. In fact, in the following it is assume that the UE 102 serves as a master UE for a group while the UEs 104 to 108 serve as slaves in the group.

As said, there are may be two types of public safety calls, an individual call or a group call. The individual call is a complete sequence of related call transactions between two devices. A group call is a complete sequence of related call transactions involving two or more devices. The number of participants in a group call may be configured, e.g. the UEs 102 to 108 may form a public safety group. Let us assume that the number of UEs in one group is not very large, e.g., less than 15 UEs, for the sake of connection setup latency. Participants may join and leave an ongoing group call. Regarding the group call, a PS UE may need to be capable of receiving a ProSe group communications transmission regardless of whether or not the PS UE has been discovered by the transmitting UE. Further, an authorized PS UE may need to be capable of sending a broadcast message to all authorized PS UEs within transmission range, regardless of group membership, using ProSe broadcast communications in a single transmission.

Although transmission of an acknowledgment (ACK) or a response from the receiving side is not compulsory in case of the group call in order to avoid collision of responses, the transmissions of the responses may be advantageous in some cases, such as for confirming the number of the receiving UEs, for example. For a dedicated one-to-one PS call, on the other hand, the receiving side may be required to send the acknowledgement back to the transmitting side prior to data traffic transmission.

Before looking further at the proposals for the public safety group communication establishment, let us take a quick look at how connection setup is performed in the current LTE on the commercial spectrum. In the LTE system, when a UE intends to connect to the network (to the eNB), a preamble is sent from the UE to the eNB in a random access channel (RACH) procedure. Thereafter, the eNB may respond by indicating the resources reserved for this UE along with the timing advance (TA) information, the preamble ID and a temporary cell radio network temporary identifier (T- CRNTI), for example. Then, the UE may send a connection setup request by using the scheduled resources given by the eNB. The connection setup request may be transferred from the UE to the eNB by using a signaling radio bearer (SRB) #0 on a common control channel (CCCH). This may be because neither the SRB #1 nor a dedicated control channel (DCCH) has been setup at this point. A connection setup response message may then be transferred to the UE. After this, the UEs which have received the connection setup response message may continue to data transmission, etc., while other UE's may try the RACH procedure again.

However, the above described connection setup process used in the commercial LTE represents a centralized solution in a cellular system and it is not applicable as such to the PS discovery in which two UEs may need to perform the setup without the network. In addition, the current TETRA specification focuses on one-to-one communications instead of group communications. Thus, for an LTE based PS group communication, how to efficiently perform the connection setup remains a problem to be solved. It should be noted that in the public safety communication, each slave device may only need to response to the master device that the connection setup request has been correctly received and the receiving slave device is willing to be discovered by the master device. This may be beneficial from the fast setup and resource efficiency point of view for the out-of-coverage PS UEs.

Therefore, there is provided an efficient resource allocation for the

PS group connection setup. Assume the situation as depicted in either of Figures 1A and 1 B. In such situations, the UE 102 may be called as a first UE or as the master UE, whereas the UEs 104 to 108 may be called as second UEs or as the slave UEs. The master UE 100 may be the one that controls the connection setup phase of the PS group. In the case of Figures 1A and 1 B, the group may be seen to comprise the UEs 102 to 108. As shown in Figure 2, the first UE 102 decides in step 200 to perform a direct group call to the group comprising the plurality of second UEs 104 to 108. The second UEs 104 to 108 may be currently locating out-of-network coverage in proximity of the first UE 102. The proximity may here be considered to represent such a distance that the UEs 102 to 108 may directly communicate with each other in a D2D fashion.

In an embodiment, prior to deciding to perform the group call, the first UE 102 may have discovered the plurality of second UEs 104 to 108 in proximity of the first UE 102. This may have taken place during an initial discovery phase by listening to specific discovery signals, advertisements or beacons from the second UEs 104 to 108. In such a manner, the first UE 102 may acquire knowledge of the second UEs 104 to 108 which are locating in proximity and which are thus able to participate in the group call. For example, the first UE 102 may acquire knowledge of the number of the second UEs 104 to 108 locating in proximity and consider this as the number of second UEs 104 to 108 from which responses in the connection setup process is expected to be received.

In an embodiment, the discovery applies a restricted ProSe discovery in which case, subject to user and operator settings, a ProSe enabled UE 104 to 108 may be discovered only by the first UE 102 in proximity which is explicitly permitted by the discoverable UE 104 to 108. Both the user and operator may have capabilities to restrict/accept the prose discoverability. The explicit permission from the second UE 104 to 108 being discovered may be required as well. For example, if some of the second UEs 104 to 108 were a member of a different PS group and/or not a member of this PS group controlled by the master UE 102, that second UE might not allow to be discovered by the master UE 102.

In another embodiment, no discovery process takes place and the fist UE 102 "blindly" decides to pursue for the group call. In such case, information of the members of the group, such as the number of the members in the group, may be obtained from the network or that information may be preconfigured in a subscriber identity module (SIM) of the first UE 102, for example. In such case the number of the members in the group may be considered as the number of second UEs 104 to 108 from which responses in the connection setup process is expected to be received

It should be noted that the group may comprise more second UEs than detected to be in the proximity of the first UE 102. For example, the group may comprise 15 second UEs out of which only the second UES 104 to 108 are in the proximity.

In step 202, the first UE 102 may transmit a connection setup request directly to the group (out of which all or only a subset is in the proximity of the first UE 102). The connection setup request may carry a first resource indication for determining a radio resource for a transmission of a connection setup response by/from each of the plurality of the second UEs 104 to 108. It is assumed that the detected radio frequency carrier for the PS communication is free, i.e., the public safety channel is free. In an embodiment, the first resource indication may comprise an implicit indication in which no radio resources are explicitly allocated for a given second UE 104 to 108. In another embodiment, the first resource indication may comprise an explicit allocation of radio resources in which dedicated radio resources are explicitly allocated for a given second UE 104 to 108. Let us later take a look at the way in which the radio resources may be indicated to the second UEs 104 to 108.

In an embodiment, the slave devices 104 to 108 monitor the radio frequency (RF) carrier for a period of one frame at least in order to detect the potential connection setup request from the master device 102. For reasons of battery economy, the slave devices 104 to 108 may not keep the receiver permanently active while performing channel surveillance. Thus, in an embodiment, the second UE 104 to 108 periodically tries to detect the connection setup request by activating the receiver at specific intervals for monitoring the potential connection request from the master device 102. In an embodiment, the connection setup request may carry at least one of the following information elements presented in Table 1 .

Table 1 : Information elements possibly carried in the connection setup request

However, there may not be need to send all the information elements (IE) in the connection setup request. In an embodiment, the indications of the lEs of Table 1 are distributed in a predetermined manner between the connection setup request and a connection setup complete message. In one embodiment, only the master UE ID, the group ID, the service type ID, the call type, the restriction indication, and the number of expected target UEs 104 to 108 are transmitted in the connection setup request. The rest of the lEs mentioned in Table 1 may, in an embodiment, be delivered later on in the connection setup complete message.

In an embodiment, the proposed efficient resource allocation for the PS group connection setup may include the restriction indication transmitted to the second UEs 104 to 108 over dedicated channels in the extension links (i.e., to the out-of-coverage UEs 104 to 108), as implied in Table 1 . In case the receiving second UE's 104 to 108 response is not requested in a group call, no radio resources is reserved for the connection setup responses and the second UE 104 to 108 may directly after receiving the connection setup request enter into a slave mode at least in case where this second UE 104 to 108 accepts the connection setup request (e.g., the target UE ID matches) and decides to receive messages from the master device 102. However, in one embodiment, the restriction indication does require the second UEs 104 to 108 receiving the connection setup request to transmit the connection setup response back to the first UE 102.

In step 204, the first UE 102 may cause a reception of at least one connection setup response carrying an identifier of the second UE 104 to 108 that transmitted the connection setup response. The connection setup response may be transmitted on at least one of the radio resources reserved for the connection setup response transmissions. The second UEs 104 to 108 may determine the radio resource to be used, as will be described later.

Thereafter, in step 206, the first UE 102 may cause a transmission of the connection setup complete message to the group. This is shown in more detail in Figure 3. In an embodiment, the first UE 102 first detects in step 300 that the connection setup response has been received from each of the each of the second UEs 102 to 108. After such detection, the first UE 102 knows that it may proceed to the transmission of the connection setup complete message to the group in step 206.

In an embodiment, the first UE 102 detects in step 302 that a predetermined time duration reserved for the reception of the connection setup responses has elapsed. Such predetermined time duration may be preconfigured, possibly on the basis of the expected number of second UEs 104 to 108, for example. After such detection, the first UE 102 may proceed to the transmission of the connection setup complete message to the group in step 206.

In other words, when all the expected UE IDs (e.g. known from the preconfigured information or on the basis of the initial discovery phase) are resolved or the master UE 102 otherwise decides to complete the "restriction checking" due to, e.g., time limitation, the master UE 102 may send the connection setup complete message to the group in step 206. In an embodiment, the connection setup complete message carries an indication of the reserved radio resources for data traffic (i.e. a traffic slot reservation). In an embodiment, the connection setup complete message may carry at least one of the following: the carrier ID, the priority indication, the QoS information, the data transmission mode indication, the frame/slot indication, and power control information, if not already transmitted in the connection setup request.

From the point of view of one of the second devices 104 to 108 locating out-of- network coverage in proximity of a first UE 102, the proposal may comprise, as shown in Figure 4, receiving in step 400 the connection setup request from the first UE 102. As said, the connection setup request may be targeted to the group comprising the plurality of second UEs 104 to 108 (and possibly even more UEs). Further, as said, the connection setup request carries a first resource indication for determining radio resources for the transmissions of the connection setup responses by the plurality of second UEs 104 to 108. The first resource indication may be an implicit or an explicit indication, as will be described later. Here it is assumed that the second UEs 104 to 108 need to respond to the request as specified by the restriction indication. It is further assumed that the second UEs 104 to 108 accept the connection setup request from the first UE 102.

In step 402, the second UE 104, 106 or 108 determines the radio resource for transmitting the connection setup response on the basis of the received first resource indication. This may be advantageous as then possible collisions of transmissions of the connection responses from each of the plurality of second UES 104 to108 may be at least reduced. The details for this step will be described later.

In step 404, the second UE 104, 106 or 108 may then transmit the connection setup response to the first UE 102 on the determined radio resource. As the target UE ID of the connection setup request may in an embodiment be a group ID, the receiving target UE 104 to 108 may know that it needs to send its own UE ID in the connection setup response. Therefore, the connection setup response may carry the ID of the second UE 104, 106, 108, respectively. This may be in order to enable the first UE 102 to know which second UEs 104 to 108 have transmitted the connection setup response. It should be noted that even though the amount of radio resources reserved for the response transmissions may be known by the first UE 102, the first UE 102 may not know which second UE 104 to 108 transmits at which of the reserved radio resources.

In step 406, the second UE 104, 106 or 108 then receives the connections setup complete message, after which the user data transmissions within the group may be started.

As shown in Figure 5, there may be cases in which the first UE 102 detects, after step 204 in step 500, that the connection setup response has not yet been received from each of the plurality of second UEs 104 to 108. This may be due to collisions, for example. Therefore, the first UE 102 may initiate iterations for the connection setup responses. As shown in step 502, the first UE 102 may transmit a connection setup status message to the group. The status message may also be called as a confirmation message. The status message may be transmitted periodically so as to allow appropriate time periods for the second UEs 104 to 108 to transmit the remaining responses. These time periods may be configured by the system, for example.

The connection setup status message may carry an indication about those at least one second UE from which the connection setup response has been received. In an embodiment, this may be done by indicating the IDs of the at least one second UE 104 to 108 from which the connection setup response has already been received. That is, by broadcasting the resolved UE IDs to the group the second UEs 104 to 108 implicitly acquire knowledge of collisions that have taken place in the earlier transmissions. In another embodiment, this may be done by indicating the IDs of the at least one second UE from which the connection setup response has not yet been received, at least in a case in which the IDs of the second UEs 104 to 108 are known by the first UE 102 beforehand. Then the second UEs 104 to 108 receiving the status message may determine whether or not that second UE 104 to 108 still needs to transmit the response. In step 504, the first UE 102 may then receive at least one connection setup response from at least one second UE 104 to 108 that still needed to transmit the connection setup response to the first UE 102. Here again, the corresponding second UE 104 to 108 may itself determine the radio resource to be used for the response transmission, as will be described.

In step 506, the first UE 102 may determine whether the connection setup response has been received from each of the second UEs 104 to 108, or alternatively or in addition to, whether the predetermined time duration has elapsed. Upon determining the positive, the method may proceed to step 206 in which the UE 102 sends the connection setup complete message to the group. Upon determining the negative, the first UE 102 may transmit yet another connection setup status message to the group in step 502.

In an embodiment, the connection setup status message further carries a second resource indication for determining radio resources for the transmissions of the connection setup responses from those at least one second UE 104 to 108 from which the connection setup response has not yet been received. That is, the same rules for the resource indications as for the connection setup request may be applied. However, it should be noted that now the amount of resources reserved for the response transmissions may be less than in connection of the step 202 (i.e. in the connection setup request) because at least some of the plurality of second UEs 104 to 108 may have already successfully transmitted the connection setup response.

From the point of view of one of the second UEs 104 to 108, as shown in Figure 6, an example method may comprise in step 600 receiving the connection setup status message directly from the first UE 102. Thereafter, the second UE 104, 106 or 108 may in step 602 detect on the basis of the received connection setup status message whether or not the connection setup response still needs to be transmitted to the first UE 102. This may take place by checking if the own UE ID has yet been resolved. Let us assume first that the connection setup response still needs to be transmitted. In step 604, the second UE 104, 106, 108 may (re-)determine a radio resource for transmitting the connection setup response on the basis of the received second resource indication. Let us later look at how the radio resource is determined. In step 606, the UE 104, 106 108 may then (re)transmit the connection setup response to the first UE 102 on the determined new radio resource. Alternatively, if the second UE 104, 106, 108 detects that the connection setup response from that second UE is already received by the first UE 102 in step 608, that second UE 104, 106, 108 may continue waiting for the connection setup complete message in step 610.

Let us now take a look at how the resources may be indicated and how the second UEs 104 to 108 determine which resources to apply for the transmission of the connection setup response. In an embodiment, the response messages from the second UEs 104 to 108 are transmitted in a predefined time period and in a predefined resource. It may be advantageous to transmit the responses immediately after the connection setup request. The resources for the connection setup responses need not be consecutive resources. In an embodiment, the radio resources (e.g. frames/slots) reserved for the for the connection setup requests is configured by the network. In an embodiment, the connection setup request may include synchronization signals. In an embodiment, the radio resources (e.g. frames/slots) reserved for the connection setup requests are configured by the applied system, such as the public safety system. In an embodiment, the radio resources (e.g. frames/slots) reserved for the connection setup responses are configured by the applied system, such as the public safety system, or by the connection setup request. In an embodiment, the radio resources (e.g. frames/slots) reserved for the connection setup status messages are configured by the applied system, such as the public safety system, or by the connection setup request. In an embodiment, repetitive connection setup responses may be applied in order to keep the reliable link quality.

Figures 7A and 7B depict an embodiment in which the first UE 102 determines in step 700 the number of the plurality of second UEs from which the connection setup responses are expected to be received. This may be possible due to the initial discovery phase or due to preconfigured information related to the members of the group. Such information may be stored in the SIM of the first UE 102, for example. In step 702, the first UE 102 then transmits an indication of the determined number of the plurality of the second UEs 104 to 108 to the group in the first and/or second resource indication. In other words, assuming the master UE 102 is aware of the number of the second UEs 104 to 108 from which the connection setup responses are to be received, the corresponding amount of radio resources may be allocated for the connection setup responses beforehand. For example, if there are four receiving UEs, then four time or frequency slots may be implicitly indicated in the connection setup request, assuming that one connection setup response occupies one radio resource slot.

Thus, the connection setup request of step 202 may carry this information or the connection setup status message of step 502 may carry this information. In the first resource indication the number of the plurality of second UEs from which the connection setup responses are expected to be received may correspond to the size of the group and/or to the number of detected second UEs 104 to 108 in the initial discovery phase. In the second resource indication the number of the plurality of second UEs from which the connection setup responses are expected to be received may correspond to the number of those at least one second UE from which the connection setup response is still to be received.

From the receiving second UE's 104 to 108 point of view as shown in Figure 7B, the second UE 104, 106 and/or 108 may receive an indication of the number of the second UEs from which the connection setup responses are expected to be received in step 710. As said, this number may be comprised in the first and/or second resource indication. Further, the second UEs 104 to 108 receive the restriction indication according to which each receiving UE 104, 106, 108 may send its own ID back to the first UE 102 in the implicitly allocated resources, if the own UE ID has not been resolved yet. Note that when the receiving second UE 104 to 108 receives the connection setup request, the second UE 104 to 108 may acquire knowledge about the number of time slots allocated for the connection setup responses. However, the second UE 104 to 108 may not yet know which exact time slot is to be used by any given second UE, that is, which one of the radio resources is to be used by the receiving second UE 104, 106 or 108.

Therefore, in step 712, the second UE 104, 106 and/or 108 may determine the radio resource R for transmitting the connection setup response on the basis of the received number of the second UEs and at least part of the identifier of the second UE. Therefore, among the total amount of resources reserved for the connection setup responses, the explicit time/frequency slot for each of the second UEs 104 to 108 is determined at least partially by the ID. In an embodiment, this is done by applying a modulo-function as R=Mod (A, B) for determining the own time/frequency slot for the connection setup response transmission. As known, Mod (A, B) returns a remainder after a number A is divided by a divisor B. In an embodiment, A represents at least some predefined digits from the ID of the receiving second UE 104, 106 or 108, whereas B represents the number of time slots for the connection setup responses (as indicated above and given implicitly in the first and/or second resource indication). As one example, assuming four second UEs (=> B=4) and that the identifier of the second UE (e.g. last two digits of the ID) results in ,4=18, R=Mod (18, 4) = 2. In this case that second UE may transmit the connection setup response in the second radio resource (e.g. second frame/slot).

Such implicit indication of to-be-used radio resources may be beneficial as the collisions may be reduced without consuming lots of radio resources for the transmission of explicit radio resource allocations. Further, this may ensure low latency for the public safety communication without the eNB (network) assistance. Let us take a look at the use of the radio resources closer with respect to Figure 7C. As shown, the transmission of the UE 102 is shown in the first horizontal line of blocks of radio resources, whereas the second horizontal line of blocks of radio resources corresponds to actions performed by the second UEs 104 to 108. The blocks marked with right leaning diagonal lines denote that the corresponding devices listen to the channel during those blocks. As shown, the first UE 102 sends the connection setup requests in two radio resources (e.g. frame/slots). The second devices 104 to 108 listen to the channel during those radio resources. The radio resources may be, e.g. sub-frames, or resource block (RBs) as defined in the specifications of the LTE/LTE-A.

After this, the second devices 104 to 108 determine the radio resource to be used for the connection setup response transmissions. This may take place by applying the modulo-function, for example. As it happens, the second UEs 104 and 106 select the same radio resource for the connection setup response transmissions. As a consequence, these connection setup responses are not successfully received by the first UE 102. E.g. the transmission may collide. The second UE 108 determines to transmit in the third reserved radio resource without problems.

Thereafter, as the master UE 102 receives the response messages only from the second UE 108, the master UE 102 sends the connection setup status message indicating the resolved (received) UE IDs. The information send in the status message may be scrambled by the group ID, for example. As indicated earlier, the status message may implicitly indicate 1 ) the number of time slots allocated for the connection setup responses from those second UEs 104, 106 which have not yet successfully transmitted the response, and 2) which UE IDs have been received by the master UE 102.

Thereafter, the collision involving UEs 104 and106 will re-send their connection setup responses carrying their UE IDs in radio resources determined by the modulo-function, for example. This time, the determination of the radio resources results in selection of different radio resources. For example, it may be that different digits of the UE ID are used and/or that the changed number of radio resources affects the radio resource selection. Thereafter, the first UE 102 may send the connection setup complete message to the group. In this way the master UE 102 has the right to end the restriction indication iterations by transmitting a connection setup complete message, possibly equipped with an ending indication. Thereafter, data transfer within the group may begin.

In an embodiment, the first and/or second resource indication comprises an explicit indication of the radio resources allocated to the second UEs 104 to 108. As shown in Figure 8, the first UE 102 may in step 800 acquire knowledge of the IDs of the second UEs 104 to 108 from which the connection setup responses are still to be received. As said, the second UEs 104 to 108 from which the connection setup responses are still to be received may vary depending on the phase of the connection setup process. For this embodiment, it may be assumed that the first UE 102 knows the IDs of the group members on the basis of preconfigured information, for example.

In step 802, the first UE 102 may allocate radio resources for each of the plurality of second UEs 104 to 108 from which the connection setup responses are expected to be received on the basis of the IDs. These allocated resources may be dedicated resources. For example, the radio resources may be allocated on the basis of the order of the IDs, e.g., on the basis of the increasing or decreasing order of the IDs. Thereafter, the first UE 102 may in step 804 transmit information of the allocation to the group in the first and/or second resource indication. The second UEs 104 to 108 may then apply the allocated (dedicated) resources directly for the transmission of the connection setup responses to the first UE 102.

In an embodiment, the radio resources reserved for the transmissions of the connection setup responses is predefined by the network or the system, such as the public safety system. However, in an embodiment, the first UE 102 may reserve radio resources for the transmissions of the connection setup responses on the basis of the number of the second UEs 104 to 108 from which the connection setup responses are still to be or expected to be received. Then, the first UE 102 may indicate the reserved radio resources to the group of the second UEs 104 to 108 implicitly by indicating the number the second UEs 104 to 108. The second UEs 104 to 108 may then consider the received number of the plurality of second UEs 104 to 108 from which the connection setup responses are to be received as the number/amount of radio resources available for the transmissions of the connection setup responses. Again, it should be noted that the number of the plurality of second UEs 104 to 108 from which the connection setup responses are to be received may vary depending on how many connection setup responses the first UE 102 has already received.

As illustrated in Figure 1A, the first UE 102 locates in the coverage of the cellular network while the second UEs 104 to 108 are out-of-coverage of the network. The first UE 102 may connect, for example, to the public safety ProSe server in the network side for potential signaling assistance. Therefore, in an embodiment, the first UE 102 may cause a transmission of information related to the connected plurality of second UEs 104 to 108 to a network. For example, after the PS group communication via the PS spectrum is setup between the UEs 102 to 108, the first UE 102 may forward the information of the seconds UEs 104 to 108 to the ProSe server via the cellular link in the network side. This may advantageously allow the ProSe to maintain up-to-date information of the PS UEs 104 to 108 which are out of the network coverage. The potential information elements transmitted to the ProSe server comprise the IDs of the UEs 104 to 108, their location information, for example

It may also be worth noting that in case of Figure 1A, the first UE 102 only needs to monitor the downlink control signaling periodically if the dedicated PS channel for the extension links is configured by eNB. If the dedicated resources for the extension links are configured by the network/PS system, like in the figure 1 B, then no assistance from the network side may be needed.

Let us take one more look at the proposal by referring to Figure 9 which shows a signaling flow diagram. In step 900 the initial discovery may take place from which the first UE 102 detects the number of second UEs 104 to 108 in the proximity, i.e. determines the number of second UEs from which the connection setup responses are expected to be received. Optionally, the number of second UEs 104 to 108 in the PS group may be preconfigured information for the first UE 102 or received from the network (e.g. from the eNB 100 of Figure 1 A).

In step 902 the first UE 102 decides to perform a group call to the group or to a part of the group locating in proximity. As shown, the group call applies direct D2D communication between the UEs 102 to 108. As a first action, the first, master UE 102 transmits in step 904 the connection setup request to the second, slave UEs 104 to 108. In the request, the first UE 102 may ask the receiving second UEs 104 to 108 to reply (i.e. a restriction indication may be given). The request may also carry, e.g., an implicit resource indication in the form of the number of expected receiving UEs 104 to 108.

In step 906, the second UE 108 successfully transmits the connection setup response to the first UE 102. Before doing so, the second UEs 104 to 108 may have determined which radio resource each of them is going to use for the response transmission. As shown, the selection of the radio resource from the point of view of the second UE 108 is successful, while the second UEs 104 and 106 select the same resource. As a result, a collision of the responses from the UEs 104 and 106 takes place in step 908.

As the first UE 102 has not received a response from each of the expected second UEs 104 to 108, the first UE 102 may transmit the connection setup status/confirmation message in step 910. This indicates to the group that only the second UE 108 has successfully transmitted the response. Further, another implicit indication of resources may be given. This may be the number of the second UEs 104 and 106 from which the response is still expected to be received so as to enable those second UEs 104, 106 to use this information as the amount of resources allocated for the transmissions of the responses. The second UEs 104, 106 may then apply, e.g. the modulo-function in determining which one of the allocated/reserved resources to use.

In steps 912 and 914, the responses from the second UEs 104, 106 are then successfully received. It should be noted that the master UE 102 may perform the iterations of step 910 as long as needed to receive the responses from each of the group members locating in proximity of the first UE 102. Optionally, there may be a predetermined time duration for stopping the iterations. In step 916, the first UE 102 may then transmit the connection setup complete message to the group members in the proximity to indicate that the connection setup is complete and data traffic may start in step 918.

Even though the second UEs 104 to 108 are described to serve as slave UEs in the group mastered by the first UE 102, in an embodiment, at least one of the second UEs 104 to 108 serve as a master UE for another group comprising other UEs in proximity of that second UE 104 to 108.

In an embodiment, the first and second UEs 102 to 108 are public safety UEs capable of operating on a public safety spectrum and the transmissions take place on the public safety spectrum. The first and second UEs 102 to 108 may also be capable to perform proximity services (defined in the 3GPP). However, even though the embodiments have been described from the point of view of the public safety procedure, the embodiments are applicable to non-PS use, such as for use in commercial LTE spectrum.

Figures 10 to 1 1 provide apparatuses 1000, 1 100 comprising a control circuitry (CTRL) 1002, 1 102, such as at least one processor, and at least one memory 1004, 1 104 including a computer program code (PROG), wherein the at least one memory and the computer program code (PROG), are configured, with the at least one processor, to cause the respective apparatus 1000, 1 100 to carry out any one of the embodiments described. The memory 1004, 1 104 may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory.

The apparatuses 1000, 1 100 may further comprise communication interfaces (TRX) 1006, 1 106 comprising hardware and/or software for realizing communication connectivity according to one or more communication protocols. The TRX may provide the apparatus with communication capabilities to access the radio access network, for example.

The apparatuses 1000, 1 100, 1200 may also comprise user interfaces 1008, 1 108 comprising, for example, at least one keypad, a microphone, a touch display, a display, a speaker, etc. Each user interface may be used to control the respective apparatus by the user.

The control circuitry 1002 may comprise a group connection setup control circuitry 1010 for performing the functionalities related to the connection setup establishment and control, such as transmission of the setup requests, status messages, complete messages, performing discovery phases, etc. A PS circuitry 1012 may be, e.g., for determining the public safety spectrum to be used, for monitoring common rules of the public safety communication, for example.

The control circuitry 1 102 may comprise a group connection setup response circuitry 1 1 10 for performing the functionalities related to the connection setup, such as transmission of the setup responses, performing discovery phases, etc. A PS circuitry 1 1 12 may be, e.g., for determining the public safety spectrum to be used, for monitoring common rules of the public safety communication, for example.

In an embodiment, the apparatuses 1000, 1 100 may comprise the terminal device of a cellular communication system, e.g. a user equipment (UE), a user terminal (UT), a computer (PC), a laptop, a tabloid computer, a cellular phone, a mobile phone, a communicator, a smart phone, a palm computer, or any other communication apparatus. Alternatively, the apparatuses may be comprised in such a terminal device. Further, the apparatuses may be or comprise a module (to be attached to the apparatus) providing connectivity, such as a plug-in unit, an "USB dongle", or any other kind of unit. The unit may be installed either inside the apparatus or attached to the apparatus with a connector or even wirelessly. In an embodiment, the apparatus 1000 may be, comprise or be comprised in a mobile phone, such as the first, master UE 102, operating according to the long term evolution or according to the long term evolution advanced. In an embodiment, the apparatus 1 100 may be, comprise or be comprised in a mobile phone, such as the second, slave UE 104, 106 or 108, operating according to the long term evolution or according to the long term evolution advanced.

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) combinations of circuits and software (and/or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s)/software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) 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. 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 a 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 element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus(es) of embodiments may be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chip set (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by processors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and/or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with regard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art.

Embodiments as described may also be carried out in the form of a computer process defined by a computer program. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art.

Although, the embodiments have been described such that the LTE or the LTE-A is applied as the radio access technology, the embodiments may be applied in a plurality of communication networks regardless of the applied radio access technology. For example, at least one of the following radio access technologies (RATs) may be applied: Worldwide Interoperability for Microwave Access (WiMAX), Global System for Mobile communications (GSM, 2G), GSM EDGE radio access Network (GERAN), General Packet Radio Service (GRPS), Universal Mobile Telecommunication System (UMTS, 3G) based on basic wideband-code division multiple access (W-CDMA), highspeed packet access (HSPA), LTE, and/or LTE-A. The present embodiments are not, however, limited to these protocols.

Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.

Claims

WHAT IS CLAIMED IS:

1 . A method, comprising:

deciding, by a first user terminal, to perform a direct group call to a group comprising a plurality of second user terminals;

causing a transmission of a connection setup request directly to the group, wherein the connection setup request carries a first resource indication for determining a radio resource for a transmission of a connection setup response by each of the plurality of the second user terminals;

causing a reception of at least one connection setup response carrying an identifier of the second user terminal that transmitted the connection setup response; and

causing a transmission of a connection setup complete message to the group.

2. The method of claim 1 , further comprising: detecting that the second user terminals are locating out-of-network coverage in proximity of the first user terminal.

3. The method of any of claims 1 to 2, further comprising: determining whether the connection setup response has been received from each of the second user terminals;

upon determining that the connection setup response has been received from each of the second user terminals, proceeding to the transmission of the connection setup complete message; and

upon determining that the connection setup response has not been received from each of the second user terminals, causing a transmission of a connection setup status message.

4. The method of any of claims 1 to 3, further comprising: detecting that the connection setup response has not yet been received from each of the second user terminals;

causing a transmission of the connection setup status message to the group, wherein connection setup status message carries an indication about those at least one second user terminal from which the connection setup response has been received; and causing a reception of at least one connection setup response from at least one second user terminal that still needed to transmit the connection setup response.

5. The method of any of claims 3 to 4, wherein the connection setup status message further carries a second resource indication for determining a radio resource for a transmission of the connection setup response by each of those at least one second user terminal from which the connection setup response has not yet been received.

6. The method of any of claims 1 to 5, further comprising: determining the number of at least one second user terminal from which the connection setup response is expected to be received; and

including the determined number of the at least one second user terminal in the first and/or second resource indication.

7. The method of claim 6, further comprising:

considering the determined number of the at least one second user terminal as the amount of radio resources allocated for the transmissions of the connection setup responses.

8. The method of any of claims 1 to 5, further comprising: acquiring knowledge of the identifiers of the at least one second user terminal from which the connection setup response is to be received; and allocating radio resources for each of the at least one second user terminal from which the connection setup response is expected to be received on the basis of the identifiers; and

causing a transmission of information related to the allocation in the first and/or second resource indication.

9. The method of any of claims 1 to 8, further comprising: detecting that the connection setup response has been received from each of the second user terminals; and

causing the transmission of the connection setup complete message.

10. The method of any of claims 1 to 9, further comprising:

detecting that the a predetermined time duration reserved for the reception of the connection setup responses has elapsed; and

causing the transmission of the connection setup complete message.

1 1. The method of any of claims 1 to 10, wherein the connection setup complete message carries an indication of the reserved radio resources for data traffic.

12. The method of any of claims 1 to 1 1 , further comprising:

upon locating in an area having network coverage, causing a transmission of information related to the connected second user terminals to a network.

13. The method of any of claims 1 to 12, wherein the connection setup request carries a restriction indication according to which the second user terminals receiving the connection setup request are requested to send the connection setup response.

14. The method of any of claims 1 to 13, wherein the first user terminal is a public safety user equipment capable of operating on a public safety spectrum and the transmissions take place on the public safety spectrum.

15. A method, comprising:

causing, by a second user terminal, a reception of a connection setup request directly from a first user terminal, wherein the connection setup request is targeted to a group comprising a plurality of second user terminals and wherein the connection setup request carries a first resource indication for determining a radio resource for a transmission of a connection setup response by the second user terminal;

determining the radio resource for transmitting the connection setup response on the basis of the received first resource indication; causing a transmission of the connection setup response carrying an identifier of the second user terminal to the first user terminal on the determined radio resource; and

cause a reception of a connection setup complete message from the first user terminal.

16. The method of claim 15, further comprising:

causing a reception of a connection setup status message from the first user terminal, wherein the connection setup status message carries an indication about those at least one second user terminal from which the connection setup response has not yet been received by the first user terminal and a second resource indication for determining a radio resource for a transmission of the connection setup response by each of those at least one second user terminal.

17. The method of claim 16, further comprising:

detecting on the basis of the received connection setup status message that the connection setup response still needs to be transmitted to the first user terminal;

determining a radio resource for transmitting the connection setup response on the basis of the received second resource indication; and

causing a transmission of the connection setup response to the first user terminal on the determined radio resource.

18. The method of any of claims 15 to 17, wherein the received first and/or second resource indication comprises an indication of the number of at least one second user terminal from which the connection setup response is expected to be received, the method further comprising:

determining the radio resource for transmitting the connection setup response on the basis of the number of the at least one second user terminal and at least part of the identifier of the second user terminal.

19. The method of claim 18, further comprising:

considering the received number of second user terminals from which the connection setup responses are expected to be received as the amount of radio resources allocated for the transmissions of the connection setup responses.

20. The method of any of claims 15 to 17, wherein the first and/or second resource indication comprises an indication of a dedicated radio resource for the transmission of the connection setup response.

21 . The method of any of claims 15 to 20, further comprising:

detecting that the connection setup response from the second user terminal is already received by the first user terminal; and

continue waiting for the connection setup complete message.

22. The method of any of claims 15 to 21 , wherein the connection setup complete message carries an indication of the reserved radio resources for data traffic.

23. The method of any of claims 15 to 22, wherein the second user terminal is a public safety user equipment capable of operating on a public safety spectrum and the transmissions take place on the public safety spectrum.

24. An apparatus, comprising:

at least one processor and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus at least to:

decide to perform a direct group call to a group comprising a plurality of user terminals;

cause a transmission of a connection setup request directly to the group, wherein the connection setup request carries a first resource indication for determining a radio resource for a transmission of a connection setup response by each of the plurality of the user terminals;

cause a reception of at least one connection setup response carrying an identifier of the user terminal that transmitted the connection setup response; and cause a transmission of a connection setup complete message to the group.

25. The apparatus of claim 24, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

detect that the user terminals are locating out-of-network coverage in proximity of the apparatus.

26. The apparatus of any of claims 24 to 25, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

determine whether the connection setup response has been received from each of the user terminals;

upon determining that the connection setup response has been received from each of the user terminals, proceed to the transmission of the connection setup complete message; and

upon determining that the connection setup response has not been received from each of the user terminals, cause a transmission of a connection setup status message.

27. The apparatus of any of claims 24 to 26, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

detect that the connection setup response has not yet been received from each of the user terminals;

cause a transmission of the connection setup status message to the group, wherein connection setup status message carries an indication about those at least one user terminal from which the connection setup response has been received; and

cause a reception of at least one connection setup response from at least one user terminal that still needed to transmit the connection setup response.

28. The apparatus of any of claims 26 to 27, wherein the connection setup status message further carries a second resource indication for determining a radio resource for a transmission of the connection setup response by each of those at least one user terminal from which the connection setup response has not yet been received.

29. The apparatus of any of claims 24 to 28, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

determine the number of at least one user terminal from which the connection setup response is expected to be received; and

include the determined number of the at least one user terminal in the first and/or second resource indication.

30. The apparatus of claim 29, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

consider the determined number of the at least one user terminal as the amount of radio resources allocated for the transmissions of the connection setup responses.

31 . The apparatus of any of claims 24 to 28, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

acquire knowledge of the identifiers of the at least one user terminal from which the connection setup response is to be received; and

allocate radio resources for each of the at least one user terminal from which the connection setup response is expected to be received on the basis of the identifiers; and

cause a transmission of information related to the allocation in the first and/or second resource indication.

32. The apparatus of any of claims 24 to 31 , wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

detect that the connection setup response has been received from each of the user terminals; and

cause the transmission of the connection setup complete message.

33. The apparatus of any of claims 24 to 32, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

detect that the a predetermined time duration reserved for the reception of the connection setup responses has elapsed; and

cause the transmission of the connection setup complete message.

34. The apparatus of any of claims 24 to 33, wherein the connection setup complete message carries an indication of the reserved radio resources for data traffic.

35. The apparatus of any of claims 24 to 34, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus further to:

upon locating in an area having network coverage, cause a transmission of information related to the connected user terminals to a network.

36. The apparatus of any of claims 24 to 35, wherein the connection setup request carries a restriction indication according to which the user terminals receiving the connection setup request are requested to send the connection setup response.

37. The apparatus of any of claims 24 to 36, wherein the apparatus is a public safety user equipment capable of operating on a public safety spectrum and the transmissions take place on the public safety spectrum.

38. The apparatus of any of claims 24 to 37, wherein the apparatus is or is comprised in a user equipment operating according to the long term evolution or according to the long term evolution advanced.

39. The apparatus of claim any of claims 37 to 38, wherein the user equipment is a mobile phone.

40. An apparatus, comprising: at least one processor and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause a second user terminal coupled to the apparatus at least to:

cause a reception of a connection setup request directly from a first user terminal, wherein the connection setup request is targeted to a group comprising a plurality of second user terminals and wherein the connection setup request carries a first resource indication for determining a radio resource for a transmission of a connection setup response by the second user terminal;

determine the radio resource for transmitting the connection setup response on the basis of the received first resource indication;

cause a transmission of the connection setup response carrying an identifier of the second user terminal to the first user terminal on the determined radio resource; and

41 . The apparatus of claim 40, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the second user terminal further to:

cause a reception of a connection setup status message from the first user terminal, wherein the connection setup status message carries an indication about those at least one second user terminal from which the connection setup response has not yet been received by the first user terminal and a second resource indication for determining a radio resource for a transmission of the connection setup response by each of those at least one second user terminal.

42. The apparatus of claim 41 , wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the second user terminal further to:

detect on the basis of the received connection setup status message that the connection setup response still needs to be transmitted to the first user terminal; determine a radio resource for transmitting the connection setup response on the basis of the received second resource indication; and

cause a transmission of the connection setup response to the first user terminal on the determined radio resource.

43. The apparatus of any of claims 40 to 42, wherein the received first and/or second resource indication comprises an indication of the number of at least one second user terminal from which the connection setup response is expected to be received, and wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the second user terminal further to:

determine the radio resource for transmitting the connection setup response on the basis of the number of the at least one second user terminal and at least part of the identifier of the second user terminal.

44. The apparatus of claim 43, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the second user terminal further to:

consider the received number of second user terminals from which the connection setup responses are expected to be received as the amount of radio resources allocated for the transmissions of the connection setup responses.

45. The apparatus of any of claims 40 to 42, wherein the first and/or second resource indication comprises an indication of a dedicated radio resource for the transmission of the connection setup response.

46. The apparatus of any of claims 40 to 45, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the second user terminal further to:

detect that the connection setup response from the apparatus is already received by the first user terminal; and

continue waiting for the connection setup complete message.

47. The apparatus of any of claims 40 to 46, wherein the connection setup complete message carries an indication of the reserved radio resources for data traffic.

48. The apparatus of any of claims 40 to 47, wherein the second user terminal is a public safety user equipment capable of operating on a public safety spectrum and the transmissions take place on the public safety spectrum.

49. The apparatus of any of claims 40 to 48, wherein the apparatus is or is comprised in the second user terminal operating according to the long term evolution or according to the long term evolution advanced.

50. The apparatus of any of claims 40 to 49, wherein the second user terminal is a mobile phone.

51 . An apparatus, comprising processing means configured to cause the apparatus to perform the method according to any of claims 1 to 23.

52. A computer program product embodied on a distribution medium readable by a computer and comprising program instructions which, when loaded into an apparatus, execute the method according to any of claims 1 to 23.