08 April 2022

Hauke Holm, VP solutions & product management, DAMM Cellular Systems A/S

For many generations the design of PMR networks have been built in the same way, continuing to this day. Network coverage is achieved with traditional indoor base stations consisting of an antenna system (including cables), transceiver, base station controller, power supply and a battery backup system (UPS). All components are typically doubled to provide maximum availability for mission and business critical users.

The drawback of such a solution is that the complexity of different components in a base station requires a lot of space and therefore a shelter is a requisite to host all the equipment. In most cases also an air conditioning unit is required to create a comfortable environment for the electronic equipment. Along with such a solution, spare part management becomes a challenge as every single component must be locally available so in case of a failure the component can be replaced immediately. With component scarcity in mind an operator needs to have enough spare parts at hand to maintain the availability of the radio network – a difficult and expensive exercise.

But not only is it expensive it is also not the ideal solution. Users are expecting the network to be available at all times. If a component fails, the redundant part is expected to take over to provide the same service. Users expect seamless operation, however in reality, this is not provided. In most cases a fail-over will cause down time and therefore service interruption. If for example, a base station controller fails, the system will need time to detect the failure. A typical value for detecting this kind of failure is 30 seconds. After this period the second base station controller will take over which usually requires keying down the carriers. Radio users will therefore lose the connection and have no service for quite a while.

With that in mind how could this be improved?
If we compare commercial networks developed over time, starting with 1G (analog) and 2G (GSM) to today’s 4G and 5G networks, we find that up to 3G (partially) they were built in a very similar way to PMR networks. Depending on the operator the requirements for battery backup systems and redundancy in general were different but otherwise we find the same kind of shelters, air conditioning etc. required to build a site. But with 3G a significant change is noticeable. The amount and size of equipment were significantly reduced. First to a size where much smaller and simpler outdoor shelters were useable and later to outdoor base stations, to a point where they replaced indoor base stations completely.

Obviously, the requirements for commercial networks are quite different from private networks. But on the other hand, also a commercial operator wants to provide 24/7 operation and service to their customers. So, in some way the requirements are overlapping as availability is key for both.

But how can that be achieved?
First the radio cell can be reduced to the bare minimum of components, basically to a single box and an antenna. As the Mean Time Between Failure (MTBF) is degraded the more components are put in series, reducing the number of components therefore improves the MTBF and so the overall availability.

Second, the complex indoor base stations with all those extra components have a much more inferior MTBF compared to outdoor base stations. In short, less complexity means higher MTBF. On top of that the quality of the parts used and today’s manufacturing provides a much higher accuracy and control over the manufacturing process which further improves the quality and so the MTBF.

With all this we can see the MTBF of a modern outdoor site is superior to the classical way of building radio coverage with indoor base stations.

Now you might say that in case of a failure there will be no coverage. But it is not as simple as that. In modern commercial networks all services are IP based and the throughput achievable depends mainly on the signal quality or in other words the distance between base station and radio user.

Commercial networks are built with cell overlap. In case of a cell failure, coverage will remain and data throughput will be slightly reduced, most of the time not even noticeable for the users. However, for the operator there are great savings on maintenance and operation:

• no batteries
• no air conditioning
• very small spare parts stock
• much less energy cost

Most PMR (/LMR) vendors have outdoor base stations but they are not as commonly used as indoor base stations and often only used to fill coverage gaps.

But how can PMR networks leverage from the developments happening in commercial networks? How can this approach be used in a narrowband network?
Very simple: Build your network with outdoor base stations.

Seriously, with a decent cell overlap the requirement for local redundancy becomes redundant. And even better you will get a georedundant system with a much higher availability compared to the traditional approach and lower cost at the same time.

Furthermore, a georedundant system will perform better in case of transmission failure since there is also implicit redundancy on the transmission path. And last but not least, cell overlapping will provide significantly better protection against complete base station outage e.g. due to natural disasters, terrorist attacks or any other force majeure incidents.

Unfortunately, public tenders for PMR networks are often explicitly demanding indoor base stations and redundancy at the component level. It would be much better to change this requirement into something that is relevant for the users – AVAILABILITY.

All those redundancy requirements have been designed more than 20 years ago, at a time when outdoor base stations did not exist. But a lot has changed since then. A variety of different outdoor base stations (single carrier, multicarrier, high TX power, radio head only, all in one etc.) have been developed by different vendors and they have been successfully proven under various conditions in hot, in cold, under salt spray etc. to name but a few.

So next time you are planning a PMR network you might want to consider this approach and benefit from the advantages this brings. It is the right time to re-think network design.