Empowering public safety services using multicast in 5G-Advanced
At Nokia, we are committed to ensuring the safety of our society by developing innovative technologies for public safety. We help build critical communication networks that cater to the demanding requirements of public safety scenarios such as push-to-talk group radio communications used by safety personnel during emergency events. Other examples include fire brigades using drones to stream real-time video, and live video streaming through body cams or vehicle cameras for better situational awareness. In these example use cases and many other like them, the network needs to provide a group of users with reliable communications delivering the same content simultaneously to multiple users, while ensuring efficiency in terms of energy and radio resources.
Point-to-multipoint (PTM) communications is one of the key enablers of these kinds of group communications. PTM is already possible in 4G networks, but it is targeted at best-effort services that require high bandwidth such as TV broadcast over LTE. In 5G-Advanced, functionalities for PTM were standardized to enable mission-critical services requiring low latency, reliable and resource-efficient communications. This blog will guide you through the challenges and requirements of public safety scenarios and explain the latest enhancements, which Nokia helped to promote in the 5G-Advanced standards workgroups.
Multicast and broadcast services (MBS), which were standardized in 5G-Advanced, utilize PTM communications to support public safety use cases. Whereas broadcast services provide the same content simultaneously to all users in a geographical area, multicast services deliver the same data simultaneously to only a selected subset of users in the area and dynamically optimize the PTM transmission based on users’ feedback.
The following functionalities were targeted for 5G-Advanced MBS to enable mission-critical services [TS23.774]:
a) Fast MBS session setup/modification/release times down to 100ms, e.g., which is below human perception of “delay” in a conversation
b) Mobility of the users while avoiding packet loss, e.g., communication without interruption even though user connects to a different base station while receiving a service
c) Rapid and spectrum-efficient operation for quick adaptation to changing network conditions, e.g., being able to adapt to changing radio conditions and audience size in a timely manner
d) Support to provide service scalable to many users, which may even exceed limits for a base station in case of incidents.
5G-Advanced MBS standardization introduced many innovative multicast capabilities which were promoted by Nokia as explained later in the blog. Applying these capabilities enables operators to meet the stringent requirements for reliable public safety communication. The standard mechanisms for multicast ensure that PTM achieves similar performance (e.g., packet loss rate) to conventional unicast communication in point-to-point (PTP) transmissions.
Figure 1. Illustrative example of MBS transmissions using feedback to ensure reliability
Figure 1 illustrates a scenario where a group of users join a multicast session and receive a common data service using a multicast transmission mechanism from a base station. Irrespective of the PTM/PTP transmission type, users can be configured to provide feedback to improve resource utilization efficiency and decrease the packet loss rate. The base station responds to the feedback by applying mechanisms such as retransmission. When the feedback indicates that PTM is insufficient, users can receive retransmissions via PTP that can be adapted based on the user’s radio conditions.
Following Nokia’s efforts, the 5G core network is designed around the session-management function, an entity that controls sessions used to deliver data in 5G. This enables multicast sessions to be dynamically delivered only in geographical areas where at least one user of interest resides, paving the way to satisfy requirements (a) and (c) above. This architecture also allows a lean technology design; it uses well-known data session relevant procedures to facilitate communications between users and core network entities, while requiring minimum changes to existing 5G procedures. With this lean design, promoted by Nokia, even a network involving a mixture of non-MBS and MBS-supporting base stations can provide the multicast data. Non-MBS supporting base stations use PTP transmissions to reach the user(s) in their coverage areas.
In addition, other multicast features, such as dynamic switching between PTP and PTM transmissions, enable operators to exploit the spectrum resources more efficiently based on the demand and the audience size. They can do this without introducing any significant end-to-end changes in the 5G system, ensuring satisfaction of requirements (a) and (c). In our view, efficient switching between PTP and PTM transmissions is of great importance and can increase resource utilization efficiency for the operators.
Figure 2. Spectral efficiency of PTP vs PTM for a scenario where multiple users receive a multicast session.
Figure 2 illustrates the cumulative distribution functions (CDFs) for resource utilization efficiency per cell. It compares PTM and PTP transmissions for the same numbers of users using two scenarios of 10 and 15 users per cell. Notably, the difference between the CDF curves for the same number of users in PTM and PTP transmissions significantly depends on the number of users receiving an MBS session and results indicate that a minimum amount of users are needed to see benefit of PTM over PTP. For the studied scenarios, the gain in efficiency is much greater with 15 users as compared to 10 users. The observation of our simulation work shows that the higher the number of users, the higher the gains realized by PTM in terms of resource utilization efficiency while meeting the reliability requirements (see further simulation assumptions and explanation of the scenario in our paper and further physical layer details of MBS in this recent paper).
The multicast feature allows lossless communications while users move from the coverage of one base station to the coverage of another base station, as required by (b). For this purpose, Nokia promoted a “sequence number synchronization” method among base stations in 5G-Advanced. This approach enables different base stations to assign the same sequence number for the same data packet based on the sequence numbers received from the core network. In this way a base station can identify the duplicates of the packets when a user is moving among base stations and can identify the gaps. For example, the data packets that have already been transmitted by a target base station via PTM to other users can be retransmitted to the incoming user via PTP, as illustrated in Figure 3.
Figure 3. Example illustration of sequence number synchronization for ensuring mobility without interruptions for multicast
In addition to sequence number synchronization, there are further enhanced unicast techniques that can be employed in a target base station to ensure delivery of missed packets during a user’s mobility.
Finally, to meet requirement (d), which addresses situations of base station congestion scenarios, requires enhancements to the way UE inactive and connected states are treated in MBS. Some public safety incidents such as earthquakes, fires and tsunamis require authorities to communicate to hundreds of users who have requested the same transmission. Conventionally, in 5G an “inactive state” is defined to save power at the user side, while ensuring minimum reconnection transition time when needed for the so-called “connected state” — i.e., where the user is fully active and able to transmit or receive all the data. 5G-Advanced multicast enhancements enable even users in inactive state to be able to receive the same multicast transmission that users receive in a connected state. This allows operators to be able to serve hundreds of users, while requiring minimum additional network resources.
5G-Advanced MBS enables many functionalities that cater to demanding requirements related to public safety scenarios. Nokia truly believes in the power of communications technology to mitigate disastrous situations. Therefore, Nokia has joined forces with other companies in 3GPP to promote and standardize MBS, which will allow critical communication networks to serve hundreds of affected users during emergency events, while keeping the network and spectrum costs to a minimum.
