Title: Push-To-Talk (PTT) Mission Critical Services (MCS) over Regenerative NTN LEO sat
The existing terrestrial wireless communication technologies for Mission Critical Services (MCS) can be classified into two mainstream categories: professional mobile radios (PMR) and cellular mobile radios (CMR). Also other less used options exists. In the following, we briefly discuss some main features of these technologies.
The older 2G and 3G commercial CMR systems have not been developed to satisfy MCS requirements. However, some organizations, offering MCS, use them if there are no other options for proper communications. The recent development is taking place within 3GPP specified 4G LTE and 5G to be modified towards MCS requirements and first deployments have been made. Essentially, the First Responder Network Authority (FirstNet), operated in USA by AT&T, is one of the most important ones. Currently, it is not intended to replace legacy PMR as it does not possess all the required features and there is need to support the continuing use of the legacy systems. Nevertheless, it provides significant path towards higher data multimedia and sensing applications supporting MCS requirements. New spectrum is taken into use. Nokia and Nordic Telecom reported recently the launch of world’s first MCS ready LTE network in 400 MHz band in Czech Republic (Nokia 2019). Other authoritative cellular networks under development include Emergency Service Network in UK.
On the other hand Aerial technologies encompass low altitude platforms (LAP) with unmanned aerial vehicles as well as high altitude platforms (HAP). There is no consensus on the threshold where low altitude changes to high altitude but typically low altitude is below few kilometers and high altitude is between few kilometers and few tens of kilometers. For instance, ITU defines HAP to be at an altitude of 20 to 50 km (ITU 2019). In the following, we look at these technologies from a communication point of view when applied to enhance MCS.
Our project studies the feasibility study for PTT MCS over 5G LEO NTN regenerative sat connectivity. The most important performance metrics of satellite networks include:
- End-to-end delay: There are a number of delay components between the source and destination node of a satellite communication system that contribute to the overall time delay. The most significant ones include i) propagation delay (time for the signal to travel from one node to another inherent to particular transmission media such as radio waves, optical fibre, etc.), ii) transmission delay (time to transmit all the bits of a single data packet), iii) processing delay (time to perform all computation operations of the protocol stack for a single packet), and iv) queueing delay (time to wait at a buffer so that access is granted to use transmission media with scheduled resources). Note that the above components exist for each link between the source and destination, which are typically connected via base stations, gateways, or other relays nodes. In integrated networks, the specific characteristics of terrestrial, aerial, and satellite links must be studied.
- Coverage area: The satellite’s coverage area identifies the geographic area where communications is possible via a satellite. For a global coverage, for LEO constellations typical global coverage require tens of satellites, as in practice, some overlapping between the adjacent satellites is required to ensure service continuity with practical handover mechanisms. The coverage area can be fractioned into cells (multibeam spots) using antenna arrays similarly to terrestrial systems.
- Data rate: The data rate affects the transmission delay and should be sufficient so that the transmission delay is not too much dominating the overall packet delay. The requirement is application dependent where the packet size varies, i.e. the amount of data bits that need to be transmitted during a packet time duration varies. Link budget analysis can be used as a first order to estimate the achievable data rate for given constellation geometry.
- Visibility time: The visibility time of the satellite for a given geographical position on Earth is an important factor, if the position of the satellite changes with respect to a given Earth location. For LEO satellites the visibility time can be in the order of few minutes that makes the visibility time a fundamentally important factor.
- Mobility support: The relative orbital movement of satellites causes several challenges that are related to sufficient mobility support of a communication service. Two major mobility measures are handover frequency, connection-dropping probability, and Doppler frequency. In essence, it is desirable to keep the handover frequency and connection dropping probability due to handover low while compensating the frequency shifts caused by the Doppler effect.
- Security: Satellites are vulnerable to many attacks, including passive eavesdropping and active cybersecurity attacks. Especially, the satellite handover process can be fragile. Space debris can harm satellites physically.
Part of our analysis is also the architecture of satellite system. Softwarization (also software defined networking, SDN) and virtualization (also network virtual function, NVF) techniques will be studied for NTN slicing communication systems. The increasing need for the service specialization and network integration has led the network design paradigm shift towards more holistic softwarization of network functions as well as network resource slicing to adapt to specific service need e.g. within MCS and different subnetworks. We analyze the architecture parts into three segments as:
- Space segment: Consists of one or more satellites organized in different type of formations: a constellation (global coverage), a cluster (a group of closely spaced satellites covering a restricted Earth surface in different orbits), and a trailing (a group of closely spaced satellites covering a restricted Earth surface in the same orbit). The satellites can be connected via inter-satellite links as well as terrestrial/aerial uplinks (towards satellite) and downlinks (away from satellite). A communication satellite includes a payload (radio transceivers and electronics) and platform subsystems supporting the payload (orbit control, propulsion, power supply, on-board data processing, thermal control).
- Ground segment: Includes all earth stations (or aerial stations in case of integration to aerial systems) that encompass interface stations or gateways to terrestrial networks, fixed or mobile user terminals that are directly connected to satellite, and service or hub stations that are used to collect and distribute data.
- Control segment: Includes all control and management functionalities with tracking, telemetry and station-keeping commands as well as traffic control and resource management.
https://www.firstnet.com/mission-critical.html
Space Safety Coalition website. Available: https://spacesafety.org/
https://www.researchgate.net/publication/340173872_Emerging_5G_satellite-aided_networks_for_mission-critical_services_A_survey_and_feasibility_study
https://www.esa.int/Applications/Telecommunications_Integrated_A%20pplications/Reprogrammable_satellite_shipped_to_launch_site
https://arxiv.org/ftp/arxiv/papers/2104/2104.10533.pdf
https://www.defmin.fi/files/2378/Finland_s_%20Cyber_Security_Strategy.pdf
