WO2023209454A1

WO2023209454A1 - Mobility load balancing for multicast/broadcast services

Info

Publication number: WO2023209454A1
Application number: PCT/IB2023/052584
Authority: WO
Inventors: Ugur Baran ELMALI; Guillaume DECARREAU; Philippe Godin; David NAVRÁTIL; Athul Prasad
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2022-04-25
Filing date: 2023-03-16
Publication date: 2023-11-02

Abstract

According to an aspect, there is provided an apparatus configured to perform the follow- ing. The apparatus determines a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS service. The apparatus transmits, to the second access node, a request re- questing at least load information for the MBS session in the second cell and receives, from the second access node, a response comprising the at least the load information. The apparatus hands over, based at least on the at least the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node to the second cell for the MBS session.

Description

MOBILITY LOAD BALANCING FOR MULTICAST/BROADCAST SERVICES

TECHNICAL FIELD

Various example embodiments relate to wireless communications.

BACKGROUND

Support for Multicast and Broadcast Services (MBS) is to be implemented in New Radio (NR) communication systems in the near future. Therein, point-to-mul- tipoint (PTM) transmissions are to be employed to efficiently provision MBS services to multiple users by using the same radio framework as used for unicast transmissions. In general, terminal devices operating in a Radio Resource control (RRC) connected mode are able receive MBS transmissions using delivery mode 1 (corresponding to multicast) while terminal device operating in any RRC mode are able to receive MBS transmissions using delivery mode 2 (corresponding to broadcast). Delivery mode 1 can provide transport blocks (TBs) both in PTM or point-to-point (PTP) manner to the terminal device. In order to be able to efficiently utilize resources reserved for MBS, a new mobility load balancing (MLB) scheme taking into account the specific features of MBS as implemented in NR communication systems is needed.

SUMMARY

According to an aspect, there is provided 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, with the at least one processor, to cause the apparatus at least to perform: determining a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, from the second access node, a response comprising the load information; and causing handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node from the first cell to the second cell for the MBS session.

According to an aspect, there is provided 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, with the at least one processor, to cause the apparatus at least to perform: receiving, from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of a second access node; transmitting, to the first access node, a response comprising said load information; and in response to receiving a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out a handover procedure based on the handover request.

According to an aspect, there is provided a method comprising: determining, by a first access node, a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, from the first access node to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, at the first access node from the second access node, a response comprising the load information; and causing, by the first access node, handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session.

According to an aspect, there is provided a method comprising: receiving, at a second access node from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of the second access node; transmitting, from the second access node to the first access node, a response comprising said load information; and in response to receiving, at the second access node, a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out, by the second access node, a handover procedure based on the handover request. According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: determining a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, from the second access node, a response comprising the load information; and causing handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node from the first cell to the second cell for the MBS session.

According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: receiving, from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of a second access node; transmitting, to the first access node, a response comprising said load information; and in response to receiving a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out a handover procedure based on the handover request.

According to an aspect, there is provided the subject matter of the independent claims. Embodiments are defined in the dependent claims.

One or more examples of implementations are set forth in more detail in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

In the following, example embodiments will be described in greater detail with reference to the attached drawings, in which

Figure 1 illustrates an exemplified wireless communication system; Figures 2 to 4 illustrate exemplary processes according to embodiments; and Figure 5 illustrates an apparatus according to embodiments.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

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

In the following, different exemplifying embodiments will be described using, as an example of an access architecture to which the embodiments may be applied, a radio access architecture based on long term evolution advanced (LTE Advanced, LTE-A) or new radio (NR, 5G), without restricting the embodiments to such an architecture, however. It is obvious for a person skilled in the art that the embodiments may also be applied to other kinds of communications networks having suitable means by adjusting parameters and procedures appropriately. Some examples of other options for suitable systems are the universal mobile telecommunications system (UMTS) radio access network (UTRAN or E-UTRAN), long term evolution (LTE, the substantially same as E-UTRA), wireless local area network (WLAN or WiFi), worldwide interoperability for microwave access (WiMAX), Bluetooth®, personal communications services (PCS), ZigBee®, wideband code division multiple access (WCDMA), systems using ultra-wideband (UWB) technology, sensor networks, mobile ad-hoc networks (MA- NETs) and Internet Protocol multimedia subsystems (IMS) or any combination thereof.

Figure 1 depicts examples of simplified system architectures showing some elements and functional entities, all being logical units, whose implementation may differ from what is shown. The connections shown in Figure 1 are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the system typically comprises also other functions and structures than those shown in Figure 1.

The embodiments are not, however, restricted to the system given as an example but a person skilled in the art may apply the solution to other communication systems provided with necessary properties.

The example of Figure 1 shows a part of an exemplifying radio access net- work. Figure 1 shows user devices 100 and 102 configured to be in a wireless connection on one or more communication channels in a cell with an access node (such as (eZg)NodeB) 104 providing the cell. The physical link from a user device to a (eZg)NodeB is called uplink or reverse link and the physical link from the (eZg)NodeB to the user device is called downlink or forward link. It should be appreciated that (eZg)NodeBs or their functionalities may be implemented by using any node, host, server or access point etc. entity suitable for such a usage.

A communications system typically comprises more than one (eZg)NodeB in which case the (eZg)NodeBs may also be configured to communicate with one another over links, wired or wireless, designed for the purpose. These links may be used for signaling purposes. The (eZg)NodeB is a computing device configured to control the radio resources of communication system it is coupled to. The NodeB may also be referred to as a base station, an access point or any other type of interfacing device including a relay station capable of operating in a wireless environment. The (eZg)NodeB includes or is coupled to transceivers. From the transceivers of the (eZg)NodeB, a connection is provided to an antenna unit that establishes bi-directional radio links to user devices. The antenna unit may comprise a plurality of antennas or antenna elements. The (eZg)NodeB is further connected to core network 110 (CN or next generation core NGC). Depending on the system, the counterpart on the CN side can be a serving gateway (S- GW, routing and forwarding user data packets), packet data network gateway (P-GW), for providing connectivity of user devices (UEs) to external packet data networks, or mobile management entity (MME), etc.

The user device (also called UE, user equipment, user terminal, terminal device, etc.) illustrates one type of an apparatus to which resources on the air interface are allocated and assigned, and thus any feature described herein with a user device may be implemented with a corresponding apparatus, such as a relay node. An example of such a relay node is a layer 3 relay (self-backhauling relay) towards the base station.

The user device typically refers to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (mobile phone), smartphone, personal digital assistant (PDA), handset, device using a wireless modem (alarm or measurement device, etc.), laptop andZor touch screen computer, tablet, game console, notebook, and multimedia device. It should be appreciated that a user device may also be a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network. A user device may also be a device having capability to operate in Internet of Things (loT) network which is a scenario in which objects are provided with the ability to transfer data over a network without requiring human-to-human or human-to-com- puter interaction. The user device (or in some embodiments a layer 3 relay node) is configured to perform one or more of user equipment functionalities. The user device may also be called a subscriber unit, mobile station, remote terminal, access terminal, user terminal or user equipment (UE) just to mention but a few names or apparatuses.

Various techniques described herein may also be applied to a cyber-physical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the implementation and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors microcontrollers, etc.) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyberphysical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals.

It should be understood that, in Figure 1, user devices are depicted to include 2 antennas only for the sake of clarity. The number of reception and/or transmission antennas may naturally vary according to a current implementation.

Additionally, although the apparatuses have been depicted as single entities, different units, processors and/or memory units (not all shown in Figure 1) may be implemented.

5G enables using multiple input - multiple output (MIMO) antennas, many more base stations or nodes than the LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller stations and employing a variety of radio technologies depending on service needs, use cases and/or spectrum available. 5G mobile communications supports a wide range of use cases and related applications including video streaming, augmented reality, different ways of data sharing and various forms of machine type applications, including vehicular safety, different sensors and real-time control. 5G is expected to have multiple radio interfaces, namely below 6GHz, cmWave and mmWave, and also being integratable with existing legacy radio access technologies, such as the LTE. Integration with the LTE may be implemented, at least in the early phase, as a system, where macro coverage is provided by the LTE and 5G radio interface access comes from small cells by aggregation to the LTE. In other words, 5G is planned to support both inter-RAT operability (such as LTE-5G) and inter-RI operability (inter-radio interface operability, such as below 6GHz - cmWave, below 6GHz - cmWave - mmWave). One of the concepts considered to be used in 5G networks is network slicing in which multiple independent and dedicated virtual sub-networks (network instances) may be created within the substantially same infrastructure to run services that have different requirements on latency, reliability, throughput and mobility.

The current architecture in LTE networks is fully distributed in the radio and fully centralized in the core network. The low latency applications and services in 5G require to bring the content close to the radio which leads to local break out and multiaccess edge computing (MEC). 5G enables analytics and knowledge generation to occur at the source of the data. This approach requires leveraging resources that may not be continuously connected to a network such as laptops, smartphones, tablets and sensors. MEC provides a distributed computing environment for application and service hosting. It also has the ability to store and process content in close proximity to cellular subscribers for faster response time. Edge computing covers a wide range of technologies such as wireless sensor networks, mobile data acquisition, mobile signature analysis, cooperative distributed peer-to-peer ad hoc networking and processing also classifiable as local cloud/fog computing and grid/mesh computing, dew computing, mobile edge computing, cloudlet, distributed data storage and retrieval, autonomic self-healing networks, remote cloud services, augmented and virtual reality, data caching, Internet of Things (massive connectivity and/or latency critical), critical communications (autonomous vehicles, traffic safety, real-time analytics, time-critical control, healthcare applications).

The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet 112, or utilize services provided by them. The communication network may also be able to support the usage of cloud services, for example at least part of core network operations may be carried out as a cloud service (this is depicted in Figure 1 by “cloud” 114). The communication system may also comprise a central control entity, or a like, providing facilities for networks of different operators to cooperate for example in spectrum sharing.

Edge cloud may be brought into radio access network (RAN) by utilizing network function virtualization (NVF) and software defined networking (SDN). Using edge cloud may mean access node operations to be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head or base station comprising radio parts. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. Application of cloudRAN architecture enables RAN real time functions being carried out at the RAN side (in a distributed unit, DU 104) and non-real time functions being carried out in a centralized manner (in a central or centralized unit, CU 108). It should also be understood that the distribution of labor between core network operations and base station operations may differ from that of the LTE or even be non-existent. Some other technology advancements probably to be used are Big Data and all-IP, which may change the way networks are being constructed and managed. 5G (or new radio, NR) networks are being designed to support multiple hierarchies, where MEC servers can be placed between the core and the base station or nodeB (gNB). It should be appreciated that MEC can be applied in 4G networks as well.

5G may also utilize satellite communication to enhance or complement the coverage of 5G service, for example by providing backhauling. Possible use cases are providing service continuity for machine -to-machine (M2M) or Internet of Things (loT) devices or for passengers on board of vehicles, or ensuring service availability for critical communications, and future railway/maritime/aeronautical communications. Satellite communication may utilize geostationary earth orbit (GEO) satellite systems, but also low earth orbit (LEO) satellite systems, in particular mega-constellations (systems in which hundreds of (nano)satellites are deployed). At least one satellite 106 in the mega-constellation may cover several satellite -enabled network entities that create on- ground cells. The on-ground cells may be created through an on-ground relay node 104 or by a gNB located on-ground or in a satellite.

It is obvious for a person skilled in the art that the depicted system is only an example of a part of a radio access system and in practice, the system may comprise a plurality of (eZg)NodeBs, the user device may have an access to a plurality of radio cells and the system may comprise also other apparatuses, such as physical layer relay nodes or other network elements, etc. At least one of the (eZg)NodeBs or may be a Home(eZg)nodeB. Additionally, in a geographical area of a radio communication system a plurality of different kinds of radio cells as well as a plurality of radio cells may be provided. Radio cells may be macro cells (or umbrella cells) which are large cells, usually having a diameter of up to tens of kilometers, or smaller cells such as micro-, femto- or picocells. The (eZg)NodeBs of Figure 1 may provide any kind of these cells. A cellular radio system may be implemented as a multilayer network including several kinds of cells. Typically, in multilayer networks, one access node provides one kind of a cell or cells, and thus a plurality of (eZg)NodeBs are needed to provide such a network structure.

For fulfilling the need for improving the deployment and performance of communication systems, the concept of “plug-and-play” (eZg)NodeBs has been introduced. Typically, a network which is able to use “plug-and-play” (eZg)Node Bs, includes, in addition to Home (eZg)NodeBs (H(eZg)nodeBs), a home node B gateway, or HNB-GW (not shown in Figure 1). A HNB Gateway (HNB-GW), which is typically installed within an operator’s network may aggregate traffic from a large number of HNBs back to a core network.

6G networks are expected to adopt flexible decentralized and/or distributed computing systems and architecture and ubiquitous computing, with local spectrum licensing, spectrum sharing, infrastructure sharing, and intelligent automated management underpinned by mobile edge computing, artificial intelligence, short-packet communication and blockchain technologies. Key features of 6G will include intelligent connected management and control functions, programmability, integrated sensing and communication, reduction of energy footprint, trustworthy infrastructure, scalability and affordability. In addition to these, 6G is also targeting new use cases covering the integration of localization and sensing capabilities into system definition to unifying user experience across physical and digital worlds.

The system of Figure 1 may be a (5G NR or 6G) communication system configured to support Multicast and Broadcast Services (MBSs). The MBS mechanism as used in the system may be based on Single-Cell Point-To-Multipoint (SC-PTM) transmissions. In SC-PTM, MBS transmissions are performed cell specifically. In such a system, any of the terminal devices 100, 102 operating in RRC connected mode may be able to receive messages transmitted using a first MBS delivery mode (or MBS delivery mode 1) corresponding to multicast while all terminal device 100, 102 operating in any RRC operating mode may be able to receive messages transmitted using a second MBS delivery mode (or MBS delivery mode 2) corresponding to broadcast. MBS delivery mode 1 may provide transport blocks (TBs) both in point-to-multipoint (PTM) or point- to-point (PTP) manner to the terminal devices 100, 102. Here the use of PTM may imply that the terminal devices are scheduled with a single downlink control information (DCI) using a group-common physical downlink control channel (PDCCH), which is used to signal the group-common physical downlink shared channel (PDSCH). Such a system may lack hybrid automatic repeat request (HARQ) or any other feedback mechanism for broadcast transmissions (though such a feedback mechanism may be available for multicast transmissions). The broadcast transmissions may be made using a fixed modulation and coding scheme (MCS) without dynamic link adaptation. However, (blind) retransmissions or repetitions may be possible for broadcast transmissions.

In such systems, terminal devices operating in RRC connected mode may be able to utilize ACK/NACK (acknowledgment/negative acknowledgment) -based HARQ feedback while NACK-only based HARQ feedback may be supported for PTM transmissions for delivery mode 1. ACK/NACK-based HARQ feedback means here that the terminal device is configured to transmit an ACK message in response to successfully receiving a data packet (or a data signal) and a NACK message in response to failing to successfully receive a data packet (or a data signal). Correspondingly, NACK-only based HARQ feedback means here that the terminal device is only configured to transmit a NACK message in response to failing to successfully receive a data packet (or a data signal) (i.e., the terminal device is configured not to transmit an ACK message in response to a successful reception).

In case of group-common NACK-only feedback, the terminal devices receiving the same MBS may be assigned via DCI the same (group-common) Physical Uplink Control Channel (PUCCH) resource on which they are to transmit a signal indicating a NACK if the corresponding Physical Downlink Shared Channel (PDSCH) TB transmission was not decoded successfully. As all terminal devices transmit the same signal, the (serving) access node may carry out energy detection to determine if any terminal device has failed to decode the PDSCH transport block (TB) and thus requires a retransmission. Accordingly, only a single NACK bit can be transmitted with this scheme using a single PUCCH resource. ACK/NACK feedback mechanism, on the other hand, is similar to unicast operation, where the terminal devices transmit HARQ feedback on orthogonal PUCCH resources to reflect the error status of the received group- common PDSCH TB transmission.

The ACK/NACK feedback may be assumed to be supported by all terminal devices while NACK-only feedback may be supported only by some terminal devices (based on their capability). Since a group-common resource is used for NACK-only feedback, the main benefit is in the reduced PUCCH overhead.

Mobility load balancing (MUB) is a self-organizing network (SON) use case that comprises two functionalities: load information exchange and mobility setting change to enforce load balancing. MUB may be used for offloading (i.e., handing over) some selected terminal devices of a cell to a suitable access node providing a neighbouring cell in an overload situation. In NR, MUB may be implemented on an X2 interface between a master access node (master eNB, MeNB) and a secondary access node (secondary gNB, SgNB), on an Xn interface between two access nodes (e.g., gNBs or ng- eNBs), on an Fl interface between centralized and distributed units of an access node and/or on an El interface between two centralized units (e.g., control-plane and userplane centralized units) of an access node.

It would be advantageous if MUB could be used also in connection with MBSs for offloading some selected terminal devices at a first cell served by a first access node for providing a certain MBS to a suitable second (neighboring) cell provided by a second access node. However, certain problems need to be overcome to enable this. Consider a scenario where N terminal devices are served by a first cell provided by a first access node in a PTP manner for a certain MBS (where N is a positive integer). The threshold (in terms of number of terminal devices) for switching from PTP communication to PTM communication may be defined to be M terminal devices, where M is a positive integer with M> N. Dedicated downlink (DL) PDCCH and/or PDSCH and uplink (UL) PUCCH resources are defined and used for each of the N terminal devices. Thus, benefits of using PTM, e.g., using a common PDCCH and/or PDSCH for multiple terminal devices to reduce DL overhead, cannot be utilized here.

It is further assumed, in this scenario, that there exists a second access node providing a second cell which is a neighboring cell to the first cell and may provide the MBS in a PTM manner. The second cell supports the same MBS as the first cell. To optimize resource usage in the first and second cells, it might be, in such a case, beneficial if some of the N terminal devices could be offloaded to the second cell associated PTM transmissions, even if there is no overload as such in the first cell. Offloading could also be beneficial if the second access node (which may currently be using PTP communication) has a lower threshold for using PTM communication compared to the first access node.

Handing over some of the PTP load of the first cell for a certain MBS to a second cell using PTM communication (before or at least following the handover) for the same MBS may provide at least the following two benefits:

1. The first cell would save power by not having to perform PTP transmissions to N terminal devices (note that power saving is one of the important targets of MLB), whereas the second cell may require less power compared to an initial state of the first cell due to it using the PTM framework of MBS.

2. The first cell would have more spare control and data channel capacity for other uses, whereas the second cell may have less overhead compared to an initial state of the first cell due to it using the PTM framework of MBS.

However, the current MLB framework does not contain any MBS specific information. Thus, the first access node would be unable to assess whether offloading to a neighbor cell would be beneficial for a particular MBS. Therefore, current means are insufficient/inefficient to perform MLB at least for some MBS scenarios.

Figure 2 illustrates signaling in one or more wireless communication networks according to embodiments for determining that there exists a neighboring access node suitable for a handover (i.e., offloading) from a first cell to a second cell for a given MBS and for carrying out said handover (i.e., for performing mobility load balancing between the first and second cells). Specifically, Figure 2 illustrates signaling between a first access node (i.e., access node 1) providing the first cell and a second access node (i.e., access node 2) providing the second cell. The first and/or second access node may correspond to the access node 104 of Figure 1.

It may be initially assumed that the first access node is serving, via the first cell, a first set of one or more terminal devices for providing an MBS (being associated with a particular MBS session) and that the second access node is serving, via the second cell, a second set of one or more terminal devices for providing the same MBS. In other words, the first and second sets have joined the same MBS session for the same MBS, respectively, in the first and second cells.

In some embodiments, the first access node is an access node of a first wireless communication network, and the second access node is an access node of a second wireless communication network (different from the first wireless communication network). In other embodiments, the first and second access nodes are both access nodes of a first wireless communication network (i.e., of the same wireless communication network).

In some embodiments, the first access node may be serving the first set of one or more terminal devices via the first cell using PTP communication and/or the second access node may be serving the second set of one or more terminal devices via the second cell using PTM communication.

The first access node determines, in block 201, a second access node that is capable of providing, via a second cell of the second access node, an MBS session for an MBS. Here, said MBS session for the MBS may be an MBS session which one or more terminal devices have j oined in the first cell of the first access node . In other words, the first access node determines, in block 202, that there exists a possibility for offloading or handing over at least one terminal device currently receiving the MBS session in the first cell of the first access node to the second cell of the second access node. Based on the procedure to be discussed below in connection with elements 202 to 206, it is determined whether a handover should be performed for all of the one or more terminal devices receiving the MBS session in the first cell, only for some of them (i.e., a subset) or for none of them.

The determining in block 201 may be based, e.g., on one or more measurement reports received from one or more terminal devices in the first cell and/or the properties and/or status of the first and/or second access node(s) (e.g., PTP-to-PTM switching thresholds and/or the number of terminal devices receiving said MBS), as will be described in detail in connection with Figures 3 & 4. The first access node transmits, in message 202, to the second access node, a request requesting at least load information for the MBS session (i.e., for the MBS) in the second cell provided by the second access node.

The request 202 may be a resource status request (as defined in 3GPP Release 16 specification). Thus, the request may comprise any information commonly included in a resource status request transmitted between two access nodes (namely, between two gNBs or ng-eNBs).

The load information associated with said MBS (or MBS session) at the second cell (comprised in the request 202) may at least comprise information on the load at uplink. Specifically, the information on the load associated with said MBS (or MBS session) at the second cell may comprise information on an absolute or relative amount of available and/or used physical uplink control channel (PUCCH) resources at the second cell (for said MBS specifically or for all MBSs). The relative amount of available PUCCH resources at the second cell may correspond, e.g., to a ratio of available PUCCH resources at the second cell for said MBS to all PUCCH resources (i.e., both used and available PUCCH resources) for said MBS at the second cell. Similarly, the relative amount of used PUCCH resources at the second cell may correspond, e.g., to a ratio of used PUCCH resources at the second cell to all PUCCH resources for said MBS at the second cell. In either case, the ratio may be given, e.g., as a percent value.

The available and/or used PUCCH resources as discussed above may correspond specifically to available and/or used PUCCH resources (reserved) for transmission of ACK and NACK messages at the second cell for at least one HARQ process associated with the MBS.

In some embodiments, the request 202 may comprise a request also for overall (non-MBS -specific) PUCCH load at the second cell.

In some embodiments, the request 202 may comprise a request for capabilities of one or more terminal devices served by the second access node (via the second cell) for transmitting NACK-only HARQ feedback for one or more MBSs. Here, the capabilities of the one or more terminal device for transmitting NACK-only HARQ feedback may indicate whether the one or more terminal devices are capable of transmitting a NACK message in response to failing to successfully receive a transmission of the MBSs and of not transmitting an ACK message (i.e., omitting transmission of an ACK message) in response to successfully receiving a transmission of the one or more MBSs. Said information on the capabilities may be requested least for terminal device(s) receiving said (aforementioned) MBS at the second cell. In some embodiments, the request 202 may comprise a request for a capability of the second access node for receiving NACK-only HARQ feedback for one or more MBSs. Here, the capability of the second access node for receiving NACK-only HARQ feedback indicates whether the second access node is capable, following a transmission of the one or more MBSs to a terminal device, of either receiving a NACK message indicating an unsuccessful reception by the terminal device or of receiving no ACK or NACK message (i.e., of not waiting for reception of any ACK message if no NACK is received) indicating a successful reception by the terminal device.

In some embodiments, the request 202 may comprise information identifying said MBS (or equally identifying said MBS session) such as an MBS service identifier for the MBS.

In some embodiments, the request 202 may further comprise a request for information on capacity of the second cell associated with said MBS (or MBS session). Additionally or alternatively, the request 202 may comprise a request for information on capacity of the second cell associated with (all) MBSs being provided by the second access node via the second cell (comprising said MBS and zero or more further MBSs). The information on the capacity of the second cell associated with said MBS and/or the capacity of the second cell associated with all MBSs being provided by the second access node via the second cell may comprise, for example, composite available capacity (CAC) information, physical resource usage information and/or radio resource control (RRC) connection information. The physical resource usage information may comprise, for example, downlink and/or uplink physical resource block (PRB) usage information.

In some embodiments, the request 202 may comprise a request for one or more measurement reports for one or more terminal devices receiving one or more MBSs (via one or more MBS sessions) provided by the second access node via the second cell. Here, the one or more MBSs may comprise at least said (aforementioned) MBS. The one or more measurements reports associated with the second cell may be defined similar to as will be described for the one or more measurement reports associated with the first cell in connection with block 301 of Figure 3.

In some embodiments, the request 202 may further comprise a request for information on an absolute or relative amount of available and/or used physical downlink control channel (PDCCH) control channel element (CCE) resources for said MBS (or all MBS(s) provided by the second access node) at the second cell.

The second access node receives, in block 203, the request requesting at least the load information for said MBS session in the second cell provided by the second access node. The second access node transmits, in message 204, to the first access node, a response comprising at least said load information for said MBS session in the second cell.

The response 204 may be a resource status response (as defined in 3GPP Release 16 specification). Thus, the request may comprise any information commonly included in a resource status response transmitted between two access nodes (namely, between two gNBs or ng-eNBs).

The response 204 may further comprise any other (explicitly) requested information (as listed above in connection with element 202). Thus, for example, if the capabilities of the one or more terminal devices served by the second access node and/or the second access node for transmitting and/or receiving NACK-only HARQ feedback for MBS(s) was requested in message 202, the response 204 may further comprise information on said capabilities of the one or more terminal devices and/or the second access node for transmitting and/or receiving NACK-only HARQ feedback.

The second access node may be assumed to maintain said load information and/or any other requested information in a memory. This information may be collected or acquired regularly or periodically by the second access node.

The first access node receives, in block 205, the response (as defined above) from the second access node. The first access node may store any information comprised in the response to a memory.

Then, the first access node (in co-operation/communication with the second access node) hands over, in block 206, based at least on the load information, at least one terminal device of the one or more terminal devices receiving the MBS session for the MBS in the first cell of the first access node from the first cell to the second cell for the MBS session. Namely, at least the load information may be used for determining whether or not any handover should be performed and optionally also for determining the number of said at least one terminal device to be handed over. In addition to the load information, the handover 206 may be based on any other information comprised in the response 204 (and requested in the request 202). In other words, the first access node makes, in block 206, a mobility load balancing decision by selecting at least one terminal device for handover to the second cell for said MBS session based on the response. If N terminal devices are receiving the MBS session in the first cell, the handover in block 206 may involve M terminal devices, where N and M are positive integers with M< N (dependent on available downlink and/or uplink capacity at the second cell). The handover in block 206 may be initiated by the first access node by transmitting a handover request to the second access node. The handover request may comprise at least information on the at least one terminal device selected for handover (and possibly information identifying the first cell, the second cell, the first access node, the second access node and/or the MBS session).

As mentioned above, in some embodiments, the first and second cells may be assumed to be associated with PTP and PTM communication, respectively. In such cases, there is an extra incentive (e.g., due to power savings and efficient use of con- trol/data channel capacity) to hand over terminal devices from the first cell to the second cell, as was described above.

The decision-making regarding the handover in block 206 may comprise evaluating whether offloading at least one (or all) of the one or more terminal devices to the second cell would cause uplink capacity issues in the second cell (i.e., whether offloading is viable or sensible in view of the uplink capacity at the second cell). This evaluation may be performed based, e.g., on said load information for the MBS session in the second cell, the capabilities of the one or more terminal devices served by the second access node (via the second cell) for transmitting NACK-only HARQ feedback for MBS(s) and/or the information on capacity of the second cell associated with the MBS (or MBS session) and/or with all MBSs (or all MBS sessions). One or more predefined uplink evaluation criteria for some or all of the listed quantities or combinations thereof may be defined and employed in the uplink-based evaluation.

In some embodiments, the first access node may determine, in block 206, to offload at least one (or all) of the one or more terminal devices to the second cell when the first access node received information about relatively high uplink load if the terminal device is capable of transmitting NACK-only HARQ feedback and the second access node is capable of receiving NACK-only feedback (via the second cell). In another embodiment, the first access node may determine to offload at least one (or all) of the one or more terminal devices to the second cell when the first access node received information about relatively low uplink load if the terminal device is not capable of transmitting NACK-only HARQ feedback.

The decision-making regarding the handover in block 206 may comprise evaluating also whether offloading at least one (or all) of said one or more terminal devices to the second cell would cause downlink capacity issues in the second cell. This evaluation may be performed based, e.g., on PDCCH CCE load for said MBS (or equally said MBS session) in the second cell and/or the one or more measurement reports associated with the first cell (and said MBS) and/or with the second cell (and said MBS). One or more pre-defined downlink evaluation criteria for some or all of the listed quantities or combinations thereof may be defined and employed in the downlink-based evaluation.

To give an example, the first access node may compare the CSI-RS measurements of terminal device(s) (possibly using PTM communication) in the second cell with the corresponding measurements of terminal device(s) (possibly using PTP communication) in the first cell and estimate additional DL PRB usage in the second cell following a handover of at least one terminal device. For example, a worse modulation and coding scheme (MCS) may need to be utilized due to possibly worsened channel conditions of the offloaded terminal device(s) which should be taken into account in the estimation.

In conclusion, following the completion of the procedure of Figure 2, the first access node may be serving, via the first cell, a reduced first set of zero or more terminal devices for providing said MBS session for the said MBS, and the second access node may be serving, via the second cell, an enlarged second set of plurality of terminal devices for providing said MBS session for said MBS.

While Figure 2 shows only a single response 204 to the request 202, in other embodiments, the request 202 may comprise a configuration for configuring the second access node to transmit responses periodically (based on a periodicity timer) or according to a pre-defined schedule. These further responses (i.e., responses following the response 204) may be resource status updates (as defined 3GPP Release 16 specification).

Figure 3 illustrates alternative signaling in one or more wireless communication networks according to embodiments for determining whether handover (i.e., offloading) from a first cell to a second cell is feasible for a given MBS (or MBS session) and for carrying out said handover (i.e., for performing mobility load balancing between the first and second cells). Similar to Figure 3, Figure 4 illustrates signaling between a first access node providing the first cell and a second access node providing the second cell. The first and/or second access node may correspond to the access node 104 of Figure 1.

The initial assumptions regarding the first and second access nodes described in connection with Figure 2 (before block 201) apply equally here.

Referring to Figure 3, the first access node receives, in block 301, one or more measurement reports from one or more terminal devices (of the first set) receiving an MBS session via the first cell of the first access node. The one or more terminal devices from which the one or more measurement reports are received may comprise all or at least some of the terminal devices receiving (i.e., having joined) said MBS session for said MBS in the first cell. The one or more measurement reports comprise information on one or more (radio) measurements relating to one or more (radio) channels. The one or more measurement reports (i.e., the measurements defined therein) may relate to the first cell of the first access node and/or the second cell of the second access node. Each of the one or more measurement reports may comprise, for example, results of channel state information reference signal (CSI-RS) measurements for one or more (propagation) channels (namely, at least for channels between the terminal device and the first access node and between the terminal device and the second access node). The results of CSI-RS measurements may comprise, e.g., channel state information reference signal received power (CSI-RSRP), channel state information reference signal received quality (CSI-RSRQ) and/or channel state information signal-interference-plus-noise ratio (CSI-SINR) values.

Then, the first access node determines, in block 302, a second access node that is capable of providing, using the second cell of the second access node, said MBS session for said MBS (i.e., the same MBS/MBS session that the one or more terminal devices in the first cell are receiving or have joined). The determining in block 302 may be based at least on the one or more measurement reports received from the one or more terminal devices receiving the MBS session in the first cell. Namely, the one or more measurement reports may be used, for example, for evaluating that sufficiently high signal quality or power (i.e., signal quality or power level exceeding a certain pre-defined threshold) is receivable from the second access node (via the second cell) by at least some of the one or more terminal devices.

The determining in block 302 may be further based, e.g., on the properties and/or status of the first and/or second access node(s) (e.g., PTP-to-PTM switching thresholds and/or the number of terminal devices receiving said MBS), as will be described in detail in connection with Figure 4.

The following elements 303 to 306 of Figure 3 may correspond, mutatis mutandis, to elements 202 to 205 of Figure 2.

In Figure 3, the MLB-related functionalities are illustrated with two separate elements 307, 308. Nevertheless, the operation relating to elements 307, 308 may correspond, unless otherwise stated, to the operation discussed in connection with block 206 of Figure 2.

First, the first access node determines, in block 307, a handover procedure (i.e., makes an MLB decision) for handing over at least one terminal device of the one or more terminal device (currently) receiving the MBS session in the first cell based on the response (i.e., based on any information comprised in the response). The determining of the handover procedure in block 307 may comprise at least selecting said at least one terminal device from the one or more terminal devices for handover to the second cell for said MBS session based on the response.

In some embodiments, the determining of the handover procedure in block 307 may be further based on the one or more measurement reports received in block 301.

Second, the first access node executes, in messages 308, the determined handover procedure for handing over said at least one terminal device from the first cell to the second cell for the MBS session. Similar to as discussed in connection with block 206 of Figure 2, the handover in block 308 may be initiated by the first access node by transmitting a handover request to the second access node. The handover request may comprise at least information on said at least one terminal device selected for handover (and possibly information identifying the first cell, the second cell, the first access node, the second access node and/or the MBS session).

Figure 4 illustrates alternative signaling in one or more wireless communication network according to embodiments for determining whether handover (i.e., offloading) from a first cell to a second cell is feasible for a given MBS and for carrying out said handover (i.e., for performing mobility load balancing between the first and second cells). Similar to Figures 2 and 3, Figure 4 illustrates signaling between a first access node providing the first cell and a second access node providing the second cell. The first and/or second access node may correspond to the access node 104 of Figure 1.

In some embodiments, the first access node is an access node of a first wireless communication network, and the second access node is an access node of a second wireless communication network (different from the first wireless communication network). In other embodiments, the first and second access nodes are both access nodes of a first wireless communication network.

In some embodiments, the first access node may be serving the first set of one or more terminal devices via the first cell using PTP communication and/or the second access node may be serving the second set of one or more terminal device via the second cell using PTM communication. Moreover, it may be assumed that the second access node is configured to operate according to a PTP-to-PTM switching threshold defined for (at least) the second cell. The PTP-to-PTM switching threshold for the second cell may define a maximum number of terminal devices servable by the second access node via the second cell using PTP communication before the second access node switches to using PTM communication in the second cell. In other words, the second access node may be configured to serve, via the second cell, at most M- 1 terminal devices using PTP communication (M being a positive integer). If at least M terminal devices need to be served in the second cell, the second access node is configured to switch to using PTM communication for serving said M terminal devices, where M corresponds to the PTP-to-PTM switching threshold.

Similar operating principle (in regard to the PTP/PTM operation) as described in the previous paragraph for the second access node and the second cell may apply also for the first access node and the first cell (though ' may be different). Thus, the first access node may maintain, in a memory, information on a PTP-to-PTM switching threshold for the first cell.

The PTP-to-PTM switching threshold configured to the second access node may be defined to be specific to the second cell and/or to said MBS (or equally to said MBS session for said MBS). Alternatively, a single PTP-to-PTM threshold may be defined for all cells and all MBSs provided by the second access node.

The procedure of Figure 4 is initiated by the first access node transmitting, in message 401, to the second access node, a first request for information on PTP/PTM communication status (at least in the second cell) and/or configuration (or capability) of the second access node relating to said MBS (or equally to said MBS session). For example, said first request 401 may comprise a request for information on a number of terminal devices receiving said MBS (or MBS session) provided by the second access node via the second cell using PTM communication and/or for the PTP-to-PTM switching threshold (at least) for the second cell (as defined above). The first request 401 may concern only the second cell (as described in the previous sentence) or any cells provided by the second access node (in which case the information may be assumed to be provided cell-specifically).

In some embodiments, the first access node may, additionally or alternatively, request, in message 401, information on a number of terminal devices receiving said MBS (or said MBS session for the MBS) provided by the second access node (via the second cell and/or via any cell) using PTP communication. The second access node receives, in block 402, the first request from the first access node. Subsequently, the second access node transmits, in message 403, a first response comprising the requested information. The requested information may be assumed to be maintained in a memory of the second access node. Thus, the second access node may simply retrieve the requested from the memory before transmission in message 403.

The first access node receives, in block 404, the first response from the second access node. The first access node may store any information comprised in the first response to a memory.

Thereafter, similar to block 301 of Figure 3, the first access node receives, in block 405, one or more measurement reports from one or more terminal devices (currently) receiving said MBS (via the MBS session) via the first cell of the first access node.

The first access node determines, in block 406, a second access node that is capable of providing, via a second cell of the second access node, said MBS session for said MBS. Here, the determining in block 406 is based on (at least some) information received in block 404 as a part of the first response 403 and optionally on the one or more measurement reports received in block 405. Namely, the determining in block 406 is based at least on the number of terminal devices receiving said MBS (or equally said MBS session for the MBS) provided by the second access (via the second cell) node using PTM communication and/or the PTP-to-PTM switching threshold for the second cell.

For example, the first access node may determine, in block 406, that the number of terminal devices receiving said MBS (or said MBS session for said MBS) provided by the second access (via the second cell) node using PTM communication is currently zero but by offloading a certain (small) number of terminal devices from the first cell to the second cell, the second access node can be made to switch from using PTP communication to using PTM communication (and thus communication in the wireless communication network as a whole is made more efficient). Thus, the first access node may determine, in block 406, that at least a certain number of terminal devices served in the first cell should be offloaded. In some embodiments, the first access node may select, in block 406, said certain number of terminal devices served in the first cell as candidates for handover (which specific terminal devices are selected being dependent on the one or more measurements reports).

To give another example, the first access node may determine in block 406 that the number of terminal devices receiving said MBS (or equally said MBS session for said MBS) provided by the second access node (via the second cell) using PTM communication is currently below a certain pre-defined limit indicating that offloading at least one terminal device from the first cell to the second cell (for providing said MBS) may potentially be sensible at least for a certain number of terminal devices.

Additionally, the determining in block 406 may be based on the PTP-to- PTM switching threshold for the first cell (maintained in a memory of the first access node). For example, the first access node may compare the PTP-to-PTM switching thresholds for the first and second cells. Said at least one (candidate) terminal device for potential handover may be determined in block 305, for example, if the PTP-to-PTM switching threshold for the second cell is determined to be lower than for the first cell.

The following elements 407 to 412 of Figure 4 may correspond, mutatis mutandis, to elements 303 to 308 of Figure 3. In Figure 4, the request 407 and the response 409 are called, respectively, a second request and a second response merely for differentiating them from the earlier first request 401 and first response 403. The second request 407 and the second response 409 may correspond (fully) to the request 303 and response 305 of Figure 3.

In some embodiments, the decision-making regarding the handover in block 411 may be also based on (at least some of) the information received in block 404 as a part of the first response 403 and/or the PTP-to-PTM switching threshold for the first cell.

While Figures 2 to 4 show the illustrated processes as being performed by the first and second access node, in some embodiments, said processes may be carried out by a specific element, unit or part (e.g., a computing device) comprised in the first and second access nodes. For example, the processes carried out in any of Figures 2 to 4 by the first access node may be carried out by a centralized unit of the first access node (in communication with at least one distributed unit of the first access node). Similarly, the processes carried out in any of Figures 2 to 4 by the second access node may be carried out by a centralized unit of the second access node (in communication with at least one distributed unit of the second access node).

It should be noted that, in the case of a CU-DU split architecture at the first access node (e.g., a first gNB) and/or the second access node (e.g., a second gNB), signaling enhancements for Fl interface may be implemented for carrying out the process according to embodiments. Namely, the first gNB-CU shall learn about the load (e.g., PUCCH load) associated with an MBS (or an MBS session) under its own first gNB- DU. In addition, the second gNB-CU may learn about the status in the second gNB-DU. As described in TS 38.473, a similar resource status request/response/update procedure exists between a gNB-CU and gNB-DU for acquiring MLB information as defined above for the first and second access nodes (or first and second gNB). Thus, similar information elements to the above signaling procedures according to embodiments at Xn interface may be implemented between a gNB-CU and a gNB-DU over the Fl interface.

The blocks, related functions, and information exchanges (messages) described above by means of Figures 2 to 4 in no absolute chronological order, and some of them may be performed simultaneously or in an order differing from the given one. Other functions can also be executed between them or within them, and other information may be sent, and/or other rules applied. Some of the blocks or part of the blocks or one or more pieces of information can also be left out or replaced by a corresponding block or part of the block or one or more pieces of information.

Figure 5 provides an apparatus 501 according to some embodiments. Specifically, Figure 5 may illustrate an apparatus 501 configured to carry out at least the functions described above in connection with performing mobility load balancing between first and second cells provided by first and second access node for a particular MBS (or a particular MBS session for the particular MBS). The apparatus may be or form a part of an access node (e.g., of the first or second access node as described above), a part of a distributed unit of a distributed access node or a part of a centralized unit of a distributed access node. The access node may be the access node 104 of Figure 1.

The apparatus 501 may comprise one or more control circuitry 520, such as at least one processor, and at least one memory 530, including one or more algorithms 531, such as a computer program code (software) wherein the at least one memory and the computer program code (software) are configured, with the at least one processor, to cause the apparatus 501 to carry out any one of the exemplified functionalities of the first and/or second access nodes described above. Said at least one memory 530 may also comprise at least one database 532.

Referring to Figure 5, the one or more communication control circuitry 520 of the apparatus 501 comprise at least MBS MLB circuitry 521 which is configured to perform the mobility load balancing for a multicast/broadcast service. The MBS MLB circuitry 521 may be configured to perform functionalities described in connection with the first access node or the second access node in the previously described embodiments. To this end, the MBS MLB circuitry 521 of the apparatus 501 is configured to carry out at least some of the functionalities of the first and/or second access nodes described above, e.g., by means of any of elements of any of Figures 2 to 4, using one or more individual circuitries. Referring to Figure 5, the memory 530 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.

Referring to Figure 5, the apparatus 501 may further comprise different interfaces 510 such as one or more communication interfaces (TX/RX) comprising hardware and/or software for realizing communication connectivity according to one or more communication protocols. Specifically, the one or more communication interfaces 510 may comprise, for example, interfaces providing a connection to the Internet and a core network of a wireless communications network. The one or more communication interface 510 may provide the apparatus with communication capabilities to communicate in a cellular communication system and enable communication with user devices (terminal devices) and different network nodes or elements and/or a communication interface to enable communication between different network nodes or elements, for example. The one or more communication interfaces 510 may comprise, for example, an Xn interface and/or an Fl interface. The one or more communication interfaces 510 may comprise standard well-known components such as an amplifier, filter, frequency-converter, (de)modulator, and encoder/decoder circuitries, controlled by the corresponding controlling units, and one or more antennas.

As used in this application, the term ‘circuitry’ may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of hardware circuits and software (and/or firmware), such as (as applicable): (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processors) with software, including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus, such as a terminal device or an access node, to perform various functions, and (c) hardware circuit(s) and processor(s), such as a microprocessors) or a portion of a microprocessor(s), that requires software (e.g. firmware) for operation, but the software may not be present when it is not needed for operation. This definition of ‘circuitry’ applies to all uses of this term in this application, including any claims. As a further example, as used in this application, the term ‘circuitry’ also covers an implementation of merely a hardware circuit or processor (or multiple processors) or a portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term ‘circuitry’ also covers, for example and if applicable to the particular claim element, a baseband integrated circuit for an access node or a terminal device or other computing or network device. In an embodiment, at least some of the processes described in connection with Figures 2 to 4 may be carried out by an apparatus comprising corresponding means for carrying out at least some of the described processes. Some example means for carrying out the processes may include at least one of the following: detector, processor (including dual-core and multiple-core processors), digital signal processor, controller, receiver, transmitter, encoder, decoder, memory, RAM, ROM, software, firmware, display, user interface, display circuitry, user interface circuitry, user interface software, display software, circuit, antenna, antenna circuitry, and circuitry. In an embodiment, the at least one processor, the memory, and the computer program code form processing means or comprises one or more computer program code portions for carrying out one or more operations according to any one of the embodiments of Figures 2 to 4 or operations thereof.

According to an embodiment, there is provided an apparatus (e.g., a first access node or a part thereof) comprising means for performing: determining a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, from the second access node, a response comprising the load information; and causing handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node from the first cell to the second cell for the MBS session.

According to an embodiment, there is provided an apparatus (e.g., a second access node or a part thereof) comprising means for performing: receiving, from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of a second access node; transmitting, to the first access node, a response comprising said load information; and in response to receiving a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out a handover procedure based on the handover request. Embodiments as described may also be carried out in the form of a computer process defined by a computer program or portions thereof. Embodiments of the methods described in connection with Figures 2 to 4 may be carried out by executing at least one portion of a computer program comprising corresponding instructions. The computer program may be provided as a computer readable medium comprising program instructions stored thereon or as a non-transitory computer readable medium comprising program instructions stored thereon. 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. The computer program medium may be a non-transitory medium. 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.

Even though the embodiments have been described above with reference to examples according to the accompanying drawings, it is clear that the embodiments are 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

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, with the at least one processor, to cause the apparatus at least to perform: determining a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, from the second access node, a response comprising the load information; and causing handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node from the first cell to the second cell for the MBS session.

2. The apparatus according to claim 1, wherein the first access node is an access node of a first wireless communication network and the second access node is an access node of a second wireless communication network.

3. The apparatus according to claim 1, wherein the first and second access nodes are access nodes of a first wireless communication network.

4. The apparatus according to any preceding claim, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to further perform: receiving, from the one or more terminal devices, one or more measurement reports on at least the first cell of the first access node and/or the second cell of the second access node; and causing the handing over of the at least one terminal device from the first cell to the second cell for the MBS session further based on the one or more measurement reports.

5. The apparatus of claim 4, the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to perform the determining of the second access node based at least on the one or more measurement reports.

6. The apparatus according to any preceding claim, wherein the one or more terminal devices are receiving the MBS via the first cell using point-to-point, PTP, communication and the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to determine, before the handing over of said at least one terminal device, that the second access node is capable of providing the MBS using the MBS session at least for said at least one terminal device of the one or more terminal devices via the second cell using point-to-multipoint, PTM, communication.

7. The apparatus according to any preceding claim, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to further perform: requesting and receiving, from the second access node, information on a number of terminal devices receiving the MBS in the second cell of the second access node using PTM communication and/or a PTP-to-PTM switching threshold for the second cell, wherein the PTP-to-PTM switching threshold defines a maximum number of terminal devices servable using PTP communication before switching to use PTM communication; and determining the second access node that is capable of providing the MBS session in the second cell at least based on the number of terminal devices receiving the MBS in the second cell of the second access node using PTM communication and/or the PTP-to-PTM switching threshold for the second cell.

8. The apparatus according to any preceding claim, wherein the request and the response comprise information identifying the MBS.

9. The apparatus according to any preceding claim, wherein the load information comprises information on an absolute or relative amount of available and/or used physical uplink control channel, PUCCH, resources at the second cell.

10. The apparatus according to any preceding claim, wherein the load information further comprises information on an absolute or relative amount of available and/or used physical downlink control channel, PDCCH, control channel element, CCE, resources for the MBS at the second cell.

11. The apparatus according to any preceding claim, wherein the request comprises a request for capabilities of one or more terminal devices served by the second access node in the second cell for transmitting NACK-only HARQ feedback for one or more MBSs comprising at least the MBS and the response comprises information on said capabilities of the one or more terminal devices, wherein said capabilities of the one or more terminal devices for transmitting NACK-only HARQ feedback indicate whether the one or more terminal devices are capable of transmitting a NACK message in response to failing to successfully receive a transmission of the one or more MBSs and of not transmitting an ACK message in response to successfully receiving the transmission of the one or more MBSs, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to causing the handing over of said at least one terminal device further based on the information on said capabilities of the one or more terminal devices.

12. The apparatus according to any preceding claim, wherein the request comprises a request for a capability of the second access node for receiving NACK-only HARQ feedback for one or more MBSs comprising at least the MBS and the response comprises information on said capability of the second access node, wherein the capability of the second access node for receiving NACK-only HARQ feedback indicates whether the second access node is capable, following a transmission of the one or more MBSs to a terminal device, of either receiving a NACK message indicating an unsuccessful reception by the terminal device or receiving no ACK or NACK message indicating a successful reception by the terminal device, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to causing the handing over of said at least one terminal device further based on the information on said capability of the second access node.

13. The apparatus according to any preceding claim, wherein the response further comprises information on capacity of the second cell associated with the MBS and/or information on capacity of the second cell associated with MBSs being provided by the second access node via the second cell, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to causing the handing over of said at least one terminal device further based on the information on capacity of the second cell associated with the MBS and/or the information on capacity of the second cell associated with the MBSs being provided by the second access node via the second cell.

14. The apparatus according to claim 13, wherein the information on the capacity of the second cell associated with the MBS and/or the capacity of the second cell associated with the MBSs being provided by the second access node via the second cell comprises composite available capacity, CAC, information, physical resource usage information and/or radio resource control, RRC, connection information.

15. The apparatus according to any preceding claim, wherein the response comprises one or more measurement reports for one or more terminal devices receiving one or more MBSs provided by the second access node via the second cell, the one or more MBSs comprising at least the MBS, the at least one memory and the computer program code being configured, with the at least one processor, to cause the apparatus to causing the handing over of said at least one terminal device further based on the one or more measurement reports for the one or more terminal devices receiving the one or more MBSs provided by the second access node via the second cell.

16. The apparatus according to any preceding claim, wherein the request comprises a configuration for configuring the second access node to transmit responses periodically or according to a pre -defined schedule.

17. The apparatus according to any preceding claim, wherein the apparatus is the first access node or a part thereof.

18. 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, with the at least one processor, to cause the apparatus at least to perform: receiving, from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of a second access node; transmitting, to the first access node, a response comprising said load information; and in response to receiving a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out a handover procedure based on the handover request.

19. The apparatus according to claim 18, wherein the first access node is an access node of a first wireless communication network and the second access node is an access node of a second wireless communication network.

20. The apparatus according to claim 18, wherein the first and second access nodes are access nodes of a first wireless communication network.

21. The apparatus according to any of claims 18 to 20, wherein, before said handing over, said at least one terminal device is receiving the MBS in the first cell of the first access node using PTP communication and the second access node is serving, via the second cell, one or more terminal devices providing the MBS using PTM communication.

22. The apparatus according to any of claims 18 to 21, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to further perform: receiving, from the first access node, a request for information on a number of terminal devices receiving the MBS in the second cell of the second access node using PTM communication and/or a PTP-to-PTM switching threshold for the second cell, wherein the PTP-to-PTM switching threshold defines a maximum number of terminal devices servable using PTP communication before switching to use PTM communication; and transmitting, to the first access node, the number of terminal devices receiving the MBS in the second cell of the second access node using PTM communication and/or the PTP-to-PTM switching threshold for the second cell.

23. The apparatus according to any of claims 18 to 22, wherein the request and the response comprise information identifying the MBS.

24. The apparatus according to any of claims 18 to 23, wherein the load information comprises information on an absolute or relative amount of available and/or used PUCCH resources at the second cell.

25. The apparatus according to any of claims 18 to 24, wherein the load information further comprises information on an absolute or relative amount of available and/or used PDCCH CCE resources for the MBS at the second cell.

26. The apparatus according to any of claims 18 to 25, wherein the request comprises a request for capabilities of one or more terminal devices served by the second access node in the second cell for transmitting NACK-only HARQ feedback for one or more MBSs comprising at least the MBS and the response comprises information on said capabilities of the one or more terminal devices, wherein said capabilities of the one or more terminal devices for transmitting NACK-only HARQ feedback indicate whether the one or more terminal devices are capable of transmitting a NACK message in response to failing to successfully receive a transmission of the one or more MBSs and of not transmitting an ACK message in response to successfully receiving the transmission of the one or more MBSs.

27. The apparatus according to any of claims 18 to 26, wherein the request comprises a request for a capability of the second access node for receiving NACK-only HARQ feedback for one or more MBSs comprising at least the MBS and the response comprises information on said capability of the second access node, wherein the capability of the second access node for receiving NACK-only HARQ feedback indicates whether the second access node is capable, following a transmission of the one or more MBSs to a terminal device, of either receiving a NACK message indicating an unsuccessful reception by the terminal device or to receiving no ACK or NACK message indicating a successful reception by the terminal device.

28. The apparatus according to any of claims 18 to 27, wherein the request further comprises a request for information on capacity of the second cell associated with the MBS and/or information on capacity of the second cell associated with MBSs being provided by the second access node via the second cell and the response further comprises said information on the capacity of the second cell associated with the MBS and/or said information on the capacity of the second cell associated with the MBSs being provided by the second access node via the second cell.

29. The apparatus according to claim 28, wherein the information on the capacity of the second cell associated with the MBS and/or the capacity of the second cell associated with the MBSs being provided by the second access node via the second cell comprises CAC information, physical resource usage information and/or RRC connection information.

30. The apparatus according to any of claims 18 to 29, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to further perform: receiving one or more measurement reports from one or more terminal devices served by the second access node in the second cell for providing one or more MBSs, wherein the one or more MBSs comprise at least said MBS; and including the one or more measurement reports in the response.

31. The apparatus according to any of claims 18 to 30, wherein the request comprises a configuration for configuring the second access node to transmit responses periodically or according to a pre-defined schedule and the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to further perform: configuring the second access node according to the configuration.

32. The apparatus according to any of claims 18 to 31, wherein the apparatus is the second access node or a part thereof.

33. A method comprising: determining, by a first access node, a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, from the first access node to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, at the first access node from the second access node, a response comprising the load information; and causing, by the first access node, handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session.

34. A method comprising: receiving, at a second access node from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of the second access node; transmitting, from the second access node to the first access node, a response comprising said load information; and in response to receiving, at the second access node, a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out, by the second access node, a handover procedure based on the handover request.

35. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: determining a second access node that is capable of providing, via a second cell of the second access node, a multicast and broadcast service, MBS, session for an MBS; transmitting, to the second access node, a request requesting at least load information for the MBS session in the second cell; receiving, from the second access node, a response comprising the load information; and causing handing over, based at least on the load information, at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of a first access node from the first cell to the second cell for the MBS session.

36. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following: receiving, from a first access node, a request requesting at least load information for an MBS session for an MBS in a second cell of a second access node; transmitting, to the first access node, a response comprising said load information; and in response to receiving a handover request requesting a handover of at least one terminal device of one or more terminal devices receiving the MBS session for the MBS in a first cell of the first access node from the first cell to the second cell for the MBS session, carrying out a handover procedure based on the handover request.