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 use of the mobile emergency button to Germany however, this local design can easily be transferred to other countries.
A Nationwide mobile emergency call button for Germany
For a nationwide mobile emergency call button for 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 fallback. 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 locally on a company campus
If a mobile emergency call system is needed locally on a company campus or in a building, LoRaWAN is a suitable solution. LoRaWAN offers us inexpensive gateways including an antenna for outdoor installation for about € 500. The frequency range for LoRaWAN is license-free in Germany. For the Volkswagen plant in Wolfsburg, fewer than ten gateways are needed to cover the entire plant area including inside the buildings.
Indoor emergency call button with localisation
If you need a mobile emergency call button which also offers a localization via the field strength in the building, Neocortec with its SubGHz Meshnet is suitable. 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 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 all the European 868 MHz bands. 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.
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.
Emergency call button with other LPWAN technologies
In the LPWAN Cookbook, the author compares the well-known LPWAN technologies NB-IoT, LoRaWAN and in addition to SubGHz Meshnet and comes to the conclusion that in many cases SubGHz Meshnet from Neocortec needs less energy and has a much higher operational reliability.
In the community of Kirchheim with its twelve villages, none of the villages is completely covered by Sigfox indoors (blue area). But in all twelve villages you can make a phone call with your GSM phone. In addition, in the community of Kirchheim, both NB-IoT network operators offer more or less good network coverage. A SubGHz Meshnet distributed over all twelve villages for 3600 inhabitants in many houses is probably not economical. In order to guarantee a connection, every second or even third streetlight would have to be equipped with an base station. If all streetlights are equipped with a SubGHz Meshnet from Neocortec to monitor the function of the streetlights and to send control commands, this existing Meshnet could be used for other tasks like an emergency call system.
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 without the need to install a new network. 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. I can therefore not recommend Sigfox for emergency call applications.