First of all, we have to establish whether the mobile emergency call button is to be used locally in a building, on a company campus, or nationwide in a European or a non-European country. In this article, we will limit the mobile emergency button to use in Germany. The design for Germany can easily be transferred to other regions.

A Nationwide mobile emergency call button for Germany

For a nationwide mobile emergency call button in Germany, NB-IoT with a fallback to GSM is currently the best for future-proofing. Since April 2020 national roaming for NB-IoT has been available in Germany from Deutsche Telekom and Vodafone. The emergency call device therefore has access to two parallel but independent NB-IoT networks. Furthermore, there are now very inexpensive NB-IoT modules offering GSM operation in parallel to NB-IoT. With the addition of GSM such an emergency call system can access three independent GSM networks and as a result an NB-IoT/GSM combination module can achieve a fivefold network redundancy in Germany.

Emergency call button providing location on a company campus

If a mobile emergency call system is needed locally on a company campus, LoRaWAN is a suitable solution. LoRaWAN offers us inexpensive gateways including an antenna for outdoor installation for about € 500. The frequency range for LoRaWAN in Germany is license-free.

Using the Volkswagen plant in Wolfsburg as a worked example, fewer than ten gateways are needed to cover the entire plant area including inside the buildings.

Another use case is a planned private radio network on LoRaWAN for the community of Kirchheim in Hessen. Here, twelve villages are spread over 57 km² in eight valleys. The villages have between 80 and 1800 inhabitants. Twelve LoRaWAN gateways are needed to provide coverage inside the houses. Twelve gateways worth € 500 installed by the voluntary fire brigade would be economical if enough participants in Kirchheim wanted to use the private LPWAN. However, NB-IoT and GSM already offer five-fold redundancy at most locations in the area, including indoors without the need to install a new network.

Indoor emergency call button with localisation

If you need a mobile emergency call button which also offers location information inside a building using the field strength, Neocortec with its SubGHz Meshnet is a suitable option. With Neocortec the radio network is built up completely independently. Each node is also a router. Due to the high link budget and the good penetration through walls, only a few permanently installed nodes are necessary to cover a building completely. These permanently installed nodes are called anchors. Each mobile node synchronizes every second up to every 30 seconds with 3-12 neighbouring nodes. The field strength is also transmitted to the neighbours during synchronization. The gateways in such a SubGHz Meshnet cyclically receive the field strength values of the nodes to their neighbours. Since the locations of the anchors are known, the approximate location of the mobile nodes can be calculated.

During a test in our office in Munich, we established a radio link from the meeting room on the left side of the building to the offices on the next higher floor on the right side of the building. In doing so, we passed through the floor/ceiling and four walls and still had plenty of link margin-left. For a mobile emergency call system in a building or on an oil platform I would recommend SubGHz Meshnet. If you limit the number of neighbours to three and set the synchronization time to 30 seconds, a radio module from Neocortec works on two standard AA cells with 2500 mAh for 7 years including 170 messages per day with 21-byte user data per message.

Each message from a node to a neighbour is acknowledged at the node. This means that an alarm message is acknowledged from end-to-end. In addition, cyclic synchronization means that the gateway knows which nodes are located in the network. If a node fails to appear during synchronisation several times, the node has been lost or has left the network. If you compare this with NB-IoT and LoRaWAN, you will find that there is no cyclical synchronisation of the stations in the network. A missing subscriber is not detected and a failed emergency call button does not work anymore and cannot transmit a message in case of emergency.

This unique SubGHz Meshnet changes the radio channel with every connection and hops across the whole 868 MHz bands in Europe. If a packet is lost during transmission to a neighbour because the radio channel used is interfered with, the message is repeated on another radio channel. The radio module selects the 3-12 neighbours randomly. If 50 participants are visible in a radio network with 500 participants, then the participants with the highest field strength are not selected but a random selection is made. This means that two or three participants in the same room may see the same 50 neighbours but select their route to the gateway completely differently. Since all 500 participants in the network do this, the radio network is very evenly balanced.

In conclusion, a combination of SubGHz Meshnet and a NB-IoT/GSM combination module provides perfect indoor coverage including automatic detection that the subscriber is no longer present in the SubGHz network plus perfect fivefold redundant coverage outside the building or company campus.

For our village community example, a SubGHz Meshnet distributed over all twelve villages for 3600 inhabitants in many houses is probably not economical. In order to guarantee this, every second or even third streetlight would have to be equipped with an anchor. However If all streetlights were equipped with a SubGHz Meshnet from Neocortec to monitor the function of the streetlights and to send control commands, then this existing Meshnet could be used for other tasks like an emergency call system.

Emergency call button with other LPWAN technologies

The problem with an LPWAN solution is the coverage that is available from the private operator that you select for the service.  In this example we look at Sigfox as an alternative to the implementation of a private LPWAN discussed earlier. The results would be similar for other networks (were they available in the Kirchheim area).

The diagram above shows the Sigfox coverage in blue over the 12 villages of the community of Kirchheim. You will note that none of them has complete indoor coverage.  In all twelve villages, on the other hand you can make a phone call with your GSM phone. In addition, in the area, both NB-IoT network operators offer more or less good network coverage.

NB-IoT, GSM and also LoRaWAN offer the possibility to acknowledge an emergency call as often as you like per day. Sigfox works in the upload without automatic acknowledgement and offers in the download only four messages with 8-byte acknowledgement per day. Sigfox, therefore, offers the worst network coverage for the community of Kirchheim and, on top of that, the most insecure connection quality, because you have to do without the acknowledgement of the message after four messages per day.

Network modelling

The deployment of the LPWAN can be simulated using various levels of software.  The most basic is free of charge Beyond this there is software which costs only $500 and offers more functionality such as corrections for the diffraction and refraction of the radio waves at the edges, and the reflection of the radio waves at the surfaces of the houses.  The software also supports the import of 3D models from OpenStreetMap.

Furthermore, this software offers you the capability of computing buildings automatically. The software recognizes buildings by their rectangular floor plans and independently calculates building structures. In this case, to map the Wolfsburg factory you only have to enter the height of the buildings.