WO2021164883A1

WO2021164883A1 - Implicit resource allocation for contention free group based rach access

Info

Publication number: WO2021164883A1
Application number: PCT/EP2020/054625
Authority: WO
Inventors: Jeroen Wigard; Mads LAURIDSEN; István Zsolt KOVÁCS; Rafhael MEDEIROS DE AMORIM
Original assignee: Nokia Technologies Oy
Priority date: 2020-02-21
Filing date: 2020-02-21
Publication date: 2021-08-26

Abstract

An apparatus is provided which comprises at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: forming at least one group of mobile devices from a plurality of mobile devices,allocating a group identifier to each group,preparing at least one factor for each group, preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending one handover command prepared for each group to each group.

Description

IMPLICIT RESOURCE ALLOCATION FOR CONTENTION FREE GROUP

BASED RACH ACCESS

Field of the Invention

The present invention relates to an apparatus, a method and a computer program product for providing an implicit resource allocation for contention free group based RACH access.

Related background Art

The following meanings for the abbreviations used in this specification apply:

CBRA: Contention based random access

CFRA: Contention free random access

CP: Cyclic prefix

CSI : Channel state information

GEO Geostationary Earth Orbiting

HAPS High Altitude Platform Station

HO: Handover

ID: Identifier

LEO: Low earth orbit

MEO Medium Earth Orbiting

NR: New Radio

NTN : Non-terrestrial networks

PRACH Physical random access channel

RA Random access

RACH Random access channel

RAN Radio access network

RRC Radio resource control

RS Reference signal

SSB Synchronization signal block UE User equipment

Example embodiments, although not limited to this, relate to handovers in case of specific networks such as a non-terrestrial networks, but are not limited thereon.

The 3GPP has concluded a study item on non-terrestrial networks for the fifth generation New Radio (5G NR) in release 16. The outcome, including recommendations on future work is provided in the technical report 38.821. The target is to provide 5G NR service to users on Earth through air and spaceborne networks, e.g. through Low-Earth Orbit (LEO) satellites. Other examples include Geostationary Earth Orbiting (GEO) and Medium Earth Orbiting (MEO) satellites and High Altitude Platform Station (HAPS).

In Non-Terrestrial Networks (NTN), two concepts are considered, namely the concept of fixed cells and the concept of moving cells. In the concept of fixed cells, the satellite continuously steers its beam(s) and thus NR cell (s) towards fixed points on Earth. In the concept of moving cells, the beam pointing direction is fixed on the satellite and thus the NR cell (s) move on Earth with satellite movement.

In NTN, the concept of fixed cells will lead to the requirement that all UEs in a cell need to be handed over to a new cell in short time, when one satellite can no longer provide coverage to a specific point on Earth and the next satellite in the constellation instead initiates service in the area. At the same time also with moving cells in NTN, one can expect a large number of UEs to be handed over in a short time, as all users on a cruise ship or all UEs in a small village/hotspot will need to be handed over almost at once. This is not only limited to NTN as also in terrestrial networks in case of trains/subways there are very similar requirements.

Hence, in such scenarios, a handover procedure for a large number of UEs may be challenging. Summary of the Invention

Example embodiments of the present invention address this situation aim to provide measures for facilitating handovers in case a large number of mobile devices (such as user equipments) are involved.

According to a first aspect, an apparatus is provided which comprises at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: forming at least one group of mobile devices from a plurality of mobile devices, allocating a group identifier to each group, preparing at least one factor for each group, preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending one handover command prepared for each group to each group.

According to a second aspect, a method is provided which comprises: forming at least one group of mobile devices from a plurality of mobile devices, allocating a group identifier to each group, preparing at least one factor for each group, preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending one handover command prepared for each group to each group.

The first and second aspects may be modified as follows:

A mobile device identifier may be allocated to each mobile device. The at least one group of mobile devices may formed from active mobile devices from the plurality of mobile devices.

The at least one factor may be adjusted in order to determine unique indexes for each mobile device within a group of mobile devices using the same opportunity and/or resources to access a random access channel.

The at least one factor may contain a first factor and a second factor, wherein the second factor is dependent on the group identifier.

The group identifier and/or the mobile device identifier may be transmitted to each mobile device before sending the handover command.

The handover command for each group may be transmitted to each group by groupcasting transmission.

According to a third aspect, an apparatus, in a mobile device, is provided, the apparatus comprising: at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receiving a handover command, the handover command comprising at least one factor and at least one indication of a dedicated opportunity to access a random access channel, calculating an index based on the at least one factor contained in the handover command and at least one identifier related to the mobile device, mapping the calculated index to a random access preamble, and performing a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

According to a fourth aspect, a method, in a mobile device, is provided, the method comprising: receiving a handover command, the handover command comprising at least one factor and at least one indication of a dedicated opportunity to access a random access channel, calculating an index based on the at least one factor contained in the handover command and at least one identifier related to the mobile device, mapping the calculated index to a random access preamble, and performing a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

The third and fourth aspects may be modified as follows:

The at least one identifier may be allocated to the mobile device by the network.

The at least one identifier may comprise a group identifier allocated to a group of mobile devices.

The at least one identifier may further comprise a mobile device identifier.

The at least one factor may contain a first factor and a second factor, the second factor being dependent on the group identifier.

The index may be calculated based on the following equation:

Index= (GR_ID-a + UE_ID-b(GR)) modulo max_RA_preambles, wherein GR_ID is the group identifier, UE_ID is the user equipment identifier, a is the first factor, b(GR) is the second factor depending on the group, and max_RA_preambles is the maximum number of random access preambles available for the group.

According to a fifth aspect of the present invention a computer program product is provided which comprises code means for performing a method according to any one of the second and fourth aspects and/or their modifications when run on a processing means or module. The computer program product may be embodied on a computer-readable medium, and/or the computer program product may be directly loadable into the internal memory of the computer and/or transmittable via a network by means of at least one of upload, download and push procedures.

According to a sixth aspect an apparatus is provided, which comprises means for forming at least one group of mobile devices from a plurality of mobile devices, means for allocating a group identifier to each group, means for preparing at least one factor for each group, means for preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and means for sending one handover command prepared for each group to each group.

According to a seventh aspect an apparatus, in a mobile device, is provided, which comprises means for receiving a handover command, the handover command comprising at least one factor and at least one indication of a dedicated opportunity to access a random access channel, means for calculating an index based on the at least one factor contained in the handover command and at least one identifier related to the mobile device, means for mapping the calculated index to a random access preamble, and means for performing a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

Brief Description of the Drawings

These and other objects, features, details and advantages will become more fully apparent from the following detailed description of example embodiments of the present invention which is to be taken in conjunction with the appended drawings, in which:

Fig. 1A shows a gNB 1 according to an example embodiment, Fig. IB shows a method carried out by the gNB 1 according to the example embodiment,

Fig. 2A shows a source UE 2 according to an example embodiment,

Fig. 2B shows a method carried out by the source UE 2 according to the example embodiment,

Fig. 3 shows a signalling chart between network and UE of an example embodiment,

Fig. 4A to 4C show a concrete example how groups of UE can be combined on factors a and b according to an example embodiment,

Fig. 5 illustrates RACH access possibilities according to prior art, and

Fig. 6 shows a table indicating RACH-ConfigDedicated.

Detailed Description of example embodiments

In the following, description will be made to example embodiments of the present invention. It is to be understood, however, that the description is given by way of example only, and that the described example embodiments are by no means to be understood as limiting the present invention thereto.

As mentioned above, in NTN, it can be expected that a large number of UEs have to be handed over in a short time (e.g., in case of a cruise ship or a small village/hotspot). It is again emphasized that this is not only limited to NTN, but may also an issue in terrestrial networks in case of trains/subways. Instead of signalling a handover command (through the RRCReconfiguration message cf. TS 38.300) to all users separately, according to embodiments of the present invention, a single command is sent to a group of users and thereby lower the signalling overhead.

The regular per UE handover involves accessing the new cell through the Random Access procedure using the Random Access Channel (RACH). Fig. 5 shows a flow chart of the possibilities for RACH access. In general, there are two RACH procedures: contention based random access (CBRA) and contention free random access (CFRA). For handover the contention based random access can be avoided, thereby improving the performance. CBRA is illustrated on the right side in the rectangle enclosed by a dashed-dotted line. Pure CFRA is reflected on the left side in the rectangle enclosed by a dashed- dotted line in Fig. 5 and it means the network provides a preamble for the selected SSB and the RA occasion mask, which indicates the dedicated resources the UE can access (and only this UE can access those).

Similar to LTE, in 5G NR there are 64 preambles defined in each time- frequency PRACH occasion. The preamble consists of two parts cyclic prefix (CP) and Preamble Sequence.

When the RACH-ConfigDedicated, which is shown in the Table listed in Fig. 6 (from TS 38.331) for the CFRA part and which indicates the resources, is assigned, there will be a sequence of CFRA-SSB-Resources defined. Each entry there defines a SSB index and a Preamble index, in other words, the UE has one Preamble index assigned for each SSB. Also this combination (SSB + preamble) shall follow a SSB Occasion mask Index (see Fig. 6).

The following parameters are used in the table of Fig. 6: cfra: Resources for contention free random access to a given target cell ra-ssb-OccasionMasklndex: Explicitly signalled PRACH Mask Index for RA Resource selection. The mask is valid for all SSB resources signalled in ssb- ResourceList ssb: The ID of an SSB transmitted by this serving cell. ra-Preamblelndex: The preamble index that the UE shall use when performing CF-RA upon selecting the candidate beams identified by this SSB. csi-RS: The ID of a CSI-RS resource defined in the measurement object associated with this serving cell. ra-OccasionList: RA occasions that the UE shall use when performing CF-RA upon selecting the candidate beam identified by this CSI-RS. ra-Preamblelndex: The RA preamble index to use in the RA occasions associated with this CSI-RS.

The UE will use the ra-OccasionList and the ra-Preamblelndex to access the resources which are reserved for its use.

As mentioned above, the contention free access procedure requires that the network sends a dedicated (RRC) message to every UE. In case a lot of UEs need to be handed over in a short time, this creates significant signalling overhead during that short period of time.

In NTN scenarios the size of the cells results in a very large number of UEs camping or being served. This number is significantly higher than in terrestrial networks scenarios where group HO mechanisms have been studied in the past, e.g. for trains, buses, ships. In NTN LEO scenarios (fixed or moving beams) large number of UEs will need to execute HO in relatively short time, especially when there are traffic 'hot-spots' within the NTN beam coverage footprint. Furthermore, the gNB serving a certain area can change due to moving beams on Earth and/or feeder link switch, and this means that the same gNB has to be able to reconfigure mobility factors very flexibly and 'on the fly'.

The NTN specific mobility scenarios have not been addressed so far in the group HO studies, thus there are no solutions available which can provide flexible resource reconfiguration options for handovers, and specifically not for group handovers.

In the following, a general overview of some example embodiments is described by referring to Figs. 1A, IB, 2A and 2B.

Fig. 1A shows a gNB 1 as an example for a network control device according to the present example embodiment. The network control device may be a device such as a gNB, but can be any kind of network control device capable of carrying out the functions described in the following. A procedure carried out by the gNB 1 is illustrated in Fig. IB.

The gNB 1 comprises at least one processor 11 and at least one memory 12 including computer program code. The at least one processor 11, with the at least one memory 12 and the computer program code, is configured to cause the apparatus to perform: forming (as shown in Sll of Fig. IB) at least one group of mobile devices (e.g. UEs) from a plurality of mobile devices, allocating (S12) a group identifier to each group, preparing (S13) at least one factor (e.g., a and b as described later) for each group, preparing (S14) a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending (S15) one handover command prepared for each group to each group.

In other words, in S15 one single handover command, which has been prepared in S14 for each group, is sent to each group. Fig. 2A shows a user equipment (UE) 2 as an example for a mobile device according to the present example embodiment. The mobile device is not limited to a user equipment but may be any suitable network element capable of carrying out the corresponding functions. A procedure carried out by the UE 2 is illustrated in Fig. 2B.

The UE 2 comprises at least one processor 21 and at least one memory 22 including computer program code. The at least one processor 21, with the at least one memory 22 and the computer program code, is configured to cause the apparatus to perform: receiving (as shown in S21 of Fig. 2B) a handover command, the handover command comprising at least one factor and a at least one indication of a dedicated opportunity to access a random access channel, calculating (S22) an index based on the at least one factor contained in the handover command and at least one identifier related to the user equipment, mapping (S23) the calculated index to a random access preamble, and performing (S24) a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

The gNB 1 may further comprise an I/O unit 13, which is capable of transmitting to and receiving from other network elements, and the UE 2 2 may further comprise an I/O unit 23, which is capable of transmitting to and receiving from other network elements.

Thus, according to some example embodiments as described above, a common HO command for a group of UEs is sent in common to the UEs of the group. Each UE is able to compute an index for its RA_Preamble based on the at least one factor sent to the UE with the HO command and the identifier related to the UE. Therefore, each UE may perform a contention free random access even though it did not receive an individual message beforehand, as it would be necessary according to prior art procedures. Hence, signalling overhead in connection with a handover can be reduced, which is in particular advantageous in case a large number of UEs will have to perform a handover almost simultaneously, such as in case of NTN.

In the following, this is further described by referring to some further detailed embodiments.

As mentioned above, some embodiments introduce a way to lower the overhead for the HO signalling while utilising the RACH opportunities effectively for contention free access. This is illustrated in Fig. 3 and explained below.

At the top of Fig. 3, a mapping between an index and an RA-preamble is shown. This may be an existing mapping from preamble index (e.g. 1 to 64) to RA_Preambles, which is available for all UEs.

In signalling SI, the network (RAN, in this case the serving gNB) sends, via dedicated signalling, a GR-ID and optionally a UE-ID to the UE. This is performed before a HO command, for example with a regular RRC message. In S2, it is assumed that a handover is necessary, and the network sends the HO command to the group of UEs. With the HO command, RACH opportunities and the factors a and b are also sent. In S3, each UE in the group calculates an index based on the received information and maps the index to the RA- preamble. An example for the index calculation is explained later in detail. In S4, the UE performs a contention free RACH access as part of a handover. After the successful handover, optionally, the new gNB sends in S5, via dedicated signalling, a new GR-ID and optionally a new UE-ID to the UE, so that these identifiers may be used for a later handover.

This is described in the following in some more detail. In particular, for those UEs which are likely to be part of a handover at the same (similar) time instance as other UEs in the same cell, the network allocates a group ID (GR_ID) and optionally a UE ID (UE_ID).

- It is noted that the UE_ID is optional, as in principle the network can reuse the last X (e.g. 6) bits of the existing C-RNTI. If this the case, RAN needs to ensure that UEs with the same last bits do not end up in the same group GR_ID.

- Each group can have maximum a number of UEs equal to the maximum number of preambles times the number of RACH opportunities per group, e.g. 64 in case of one dedicated RACH opportunity.

- The GR_ID and potentially the UE_ID can be allocated e.g. as part of the RRC Connection Reconfiguration or RRC Connection Release message. Therefore, the UE may receive the IDs at the time of connection setup (as part of a larger RRC message) or it may receive it after completing a handover.

When the UEs, which have been allocated a GR_ID, and potentially UE_ID, need to perform a handover (cell change), one HO command from the serving gNB is transmitted per group or groups of UEs. This can be done through groupcasting transmission (groupcast signaling).

- In this groupcast signaling, the handover command is given to all UEs in the group(s), and the RACH opportunity or opportunities which is/are available for this.

- Besides this, also two sets of factors (a and b) are allocated by the command sent by the serving gNB. They can be determined by the serving cell, based on the configuration of the target cell or by the target cell. These are needed if the network wants to improve the effectiveness of the RACH opportunity usage and change/modify or combine UE groups. The network needs to find the factors a and b which lead to unique indexes, and thus unique RA preambles within the RA opportunity, for all UEs in the combined groups. For example, the network (the serving gNB) may adjust the factors and and/or b in order to determine unique indexes for each mobile device within a group of UEs (mobile devices) using the same opportunity (RACH opportunity) and/or resources to access a random access channel.

- As the factor sets a and b are used for access in the target cell, signaling between source and target cell need to be happening, such RACH opportunities and preambles can be sent to the UEs by the source cell in the group handover command. This can be done through a rather static configuration, which will require little information exchange or a dynamic configuration where signaling between the cells needs to be taking place more frequent.

When the handover command is received by the UE, the UE calculates its index from its UE_ID, GR_ID, and the factors a and b and then map this index to a RA_Preamble.

- One example of how the UE can do this would be to use the function:

Index = (GR_ID a + UE_ID b(GR)) modulo max_RA_preambles, where b(GR) is a set of b factors depending on the group number. It is noted that the operator "modulo" may also be referred to as "mod".

- In case the handover command is only send to one group, a = 0 and b = 1 can be used.

- GR_IDs are mapped to integers; e.g. GR_0 = 0, GR_1 = 1;

- UE_IDs are mapped to integers; e.g. UE_ID1 = 1, UE_ID2 = 2;

- The factor b depends on the number of GR_IDs, i.e. one entry in b per GR_ID. In other words, for different groups, different factor b's can be used.

Now the UE can use the RA_Preamble to access the indicated RA opportunity or opportunities in the new cell, i.e. perform the handover.

When the handover is completed, naturally a RRC complete message is sent by the UE to the target cell, implying the handover succeeded. If a cell decides to update the GR_ID and/or UE_ID it can do so by including this in the next RRCReconfigu ration message (e.g. simultaneously with confirming the successful handover).

It is noted that the basic idea as described in some example embodiments can be further expanded by having similar mapping to RACH opportunities, thereby making the size of the groups larger.

Advantages of example embodiments are that the signaling overhead is lowered through avoiding individual handover messages. Moreover, a high efficiency of RACH opportunities can be achieved, as groups can be combined, which is particularly important for NTN.

In the following, a more detailed example of embodiments as described above is given by referring to Fig. 4. Fig. 4 illustrates examples of how groups can be combined based on the factors a and b. The different (a, b) pairs are assigned in a mapping table to the corresponding RA_Preamble indexes to be used by the UEs.

In this example, it is assumed that only 4 RA_preambles are used for simplicity reasons.

The index to RA_preamble mapping becomes as shown in Fig. 4A.

It is further assumed that the cell has 8 UEs, which are divided into 2 groups: GR_ID=0 and GR_ID = 1. As explained earlier, the same UE_IDs (UE_ID1 ... UE_ID4) can be used in each group, e.g. last x bits of the C-RNTI. The 2 groups are shown in Fig. 4B.

Now if all UEs are active, or more than 4 across both groups, two RACH opportunities at least are needed, wherein the UE_ID maps directly to the RA_preamble, i.e. a=0 and b=l. The more interesting case is if only half of the UEs are active (RRC Connected) and need to perform HO. Some of these cases are illustrated in Fig. 4(c), where an italic font means a UE is inactive and a normal font means they are active. In this example, the equation "index = (GR_ID a + UE_ID b(GR)) mod max_RA_preambles" is applied, where b(GR) = [bl b2].

Thus, an efficient use of the RACH resources is enabled through combining groups of users voiding waste of RACH resources for inactive users.

Summarizing, example embodiments allow an efficient use of the RACH resources by combining groups of users. Some embodiments show the following procedures:

- Grouping the UEs which are likely part of a handover at a similar time instance.

- A GR-ID and optionally a UE-ID are allocated to each active UE.

- Further two parameters, a and b, are provided by the network to enable the combination of groups.

- At every group handover decision, the a and b factors are adjusted, in order to reflect the changes in users who have become active/inactive to always ensure high utilization.

- To inactive UEs no resource is assigned. In more detail, in principle they still have a group and UE ID assigned, they just don't use it because they are inactive (the IDs were assigned when the UE was still active).

- One HO command from the serving gNB is transmitted per group or groups of UEs e.g. via groupcasting transmission. In this groupcast signaling, the handover command is given to all UEs in the group(s), and the RACH opportunity or opportunities which is/are available for this.

- A UE receiving this handover command calculates an index (via GR-ID, UE- ID and factors a and b) and to map this index to a RA-Preamble via following rule:

Index= (GR_ID a + UE_ID b(GR)) mod max_RA_preambles.

- The UE uses the mapped RA-Preamble to access the indicated RA opportunity in the new cell. The above-described example embodiments are only examples and may be modified.

Names of network elements, protocols, and methods are based on current standards. In other versions or other technologies, the names of these network elements and/or protocols and/or methods may be different, as long as they provide a corresponding functionality.

In general, the example embodiments may be implemented by computer software stored in the memory (memory resources, memory circuitry) 12, 22 and executable by the processor (processing resources, processing circuitry) 11, 21 or by hardware, or by a combination of software and/or firmware and hardware.

As used in this application, the term "circuitry" refers to all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) to combinations of circuits and software (and/or firmware), such as (as applicable): (i) to a combination of processor(s) or (ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

(c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of "circuitry" applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term "circuitry" would also cover, for example and if applicable to the particular claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in server, a cellular network device, or other network device.

The terms "connected," "coupled," or any variant thereof, mean any connection or coupling, either direct or indirect, between two or more elements, and may encompass the presence of one or more intermediate elements between two elements that are "connected" or "coupled" together. The coupling or connection between the elements can be physical, logical, or a combination thereof. As employed herein two elements may be considered to be "connected" or "coupled" together by the use of one or more wires, cables and printed electrical connections, as well as by the use of electromagnetic energy, such as electromagnetic energy having wavelengths in the radio frequency region, the microwave region and the optical (both visible and invisible) region, as non-limiting examples.

The memory (memory resources, memory circuitry) 12, 22 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, and non- transitory computer-readable media. The processor (processing resources, processing circuitry) 11, 21 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on a multi core processor architecture, as non-limiting examples.

It is to be understood that the above description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.

Claims

1. An apparatus, comprising: at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform : forming at least one group of mobile devices from a plurality of mobile devices, allocating a group identifier to each group, preparing at least one factor for each group, preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending one handover command prepared for each group to each group.

2. The apparatus according to claim 1, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform: allocating a mobile device identifier to each mobile device.

3. The apparatus according to claim 2, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform : forming the at least one group of mobile devices from active mobile devices from the plurality of mobile devices.

4. The apparatus according to any one of the claims 1 to 3, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform: adjusting the at least one factor in order to determine unique indexes for each mobile device within a group of mobile devices using the same opportunity and/or resources to access a random access channel.

5. The apparatus according to any one of the claims 1 to 4, wherein the at least one factor contains a first factor and a second factor, the second factor being dependent on the group identifier.

6. The apparatus according to any one of the claims 1 to 5, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform: transmitting the group identifier and/or the mobile device identifier to each mobile device before sending the handover command.

7. The apparatus according to any one of the claims 1 to 6, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform: sending the handover command for each group to each group by groupcasting transmission.

8. Apparatus, in a mobile device, comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform : receiving a handover command, the handover command comprising at least one factor and at least one indication of a dedicated opportunity to access a random access channel, calculating an index based on the at least one factor contained in the handover command and at least one identifier related to the mobile device, mapping the calculated index to a random access preamble, and performing a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

9. The apparatus according to claim 8, wherein the at least one identifier is allocated to the mobile device by the network.

10. The apparatus according to claim 9, wherein the at least one identifier comprises a group identifier allocated to a group of mobile devices.

11. The apparatus according to claim 9 or 10, wherein the at least one identifier further comprises a mobile device identifier.

12. The apparatus according to claim 11, wherein the at least one factor contains a first factor and a second factor, the second factor being dependent on the group identifier.

13. The apparatus according to claim 12, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to further perform: calculating the index based on the following equation:

14. A method comprising: forming at least one group of mobile devices from a plurality of mobile devices, allocating a group identifier to each group, preparing at least one factor for each group, preparing a handover command for each group, the handover command including the at least one factor and at least one indication of a dedicated opportunity to access a random access channel, and sending one handover command prepared for each group to each group.

15. The method according to claim 14, further comprising: allocating a mobile device identifier to each mobile device.

16. The method according to claim 15, further comprising: forming the at least one group of mobile devices from active mobile devices from the plurality of mobile devices.

17. The method according to any one of the claims 14 to 16, further comprising: adjusting the at least one factor in order to determine unique indexes for each mobile device within a group of mobile devices using the same opportunity and/or resources to access a random access channel

18. The method according to any one of the claims 14 to 17, wherein the at least one factor contains a first factor and a second factor, the second factor being dependent on the group identifier.

19. The method according to any one of the claims 14 to 18, further comprising: transmitting the group identifier and/or the mobile device identifier to each mobile device before sending the handover command.

20. The method according to any one of the claims 14 to 19, further comprising: sending the handover command for each group to each group by groupcasting transmission.

21. A method, in a mobile device, comprising receiving a handover command, the handover command comprising at least one factor and at least one indication of a dedicated opportunity to access a random access channel, calculating an index based on the at least one factor contained in the handover command and at least one identifier related to the mobile device, mapping the calculated index to a random access preamble, and performing a contention free random access at the dedicated opportunity to access the random access channel using the random access preamble.

22. The method according to claim 21, wherein the at least one identifier is allocated to the mobile device by the network.

23. The method according to claim 22, wherein the at least one identifier comprises a group identifier allocated to a group of mobile devices.

24. The method according to claim 22 or 23, wherein the at least one identifier further comprises a mobile device identifier.

25. The method according to claim 24, wherein the at least one factor contains a first factor and a second factor, the second factor being dependent on the group identifier.

26. The method according to claim 25, further comprising: calculating the index based on the following equation:

27. A computer program product comprising code means for performing a method according to any one of the claims 14 to 26 when run on a processing means or module.

28. The computer program product according to claim 27, wherein the computer program product is embodied on a computer-readable medium, and/or the computer program product is directly loadable into the internal memory of the computer and/or transmittable via a network by means of at least one of upload, download and push procedures.