System design is the next article taken from the new guide on how to protect and strengthen your LMR system. If you missed the first chapter you can read that here: Technology Choice. The guide investigates every aspect of wireless communications, and considers how operators might make their LMR systems more resilient. For more information read the overview and download all ten articles.
Carefully considered and robust system design choices will protect your organization’s communications on a daily basis, and prepare you to meet future communication needs.
Coverage dictates the number and location of your radio sites. Together with frequency availability and traffic patterns, coverage decisions will determine where, when and how well your people can communicate, so they are fundamental to the strength and reliability of your system.
Defining coverage needs
For most systems, it is insufficient to define uniform coverage requirements across your entire service area. Typically, organizations have areas that require special consideration:
- critical use – key roads, prisons, courthouses, hospitals, refineries, critical infrastructure
- high population density – urban areas,
- challenging terrain – mountains, canyons, forests, shorelines
- challenging construction – significant buildings with “dead spots”, urban canyons, tunnels
Map your entire geographical area to identify the locations with specific coverage level challenges or requirements.
At the system design stage, you will identify where in-building coverage is needed. You can specify a uniform signal level in the most critical areas, assuming it will be sufficient in all buildings. Test signal strength from inside the buildings, using the signal from desired existing sites or temporary reference transmitters.
Alternatively, you can identify specific “must cover” buildings and place the burden of engineering on the vendors to provide this performance.
There are many ways to measure coverage performance. For example, Delivered Audio Quality (DAQ) is the most common signal quality measure, together with predicted reliability – the percentage of coverage area where the signal quality meets (or exceeds) the DAQ.
A widely-accepted coverage objective is DAQ 3.4 over the entire service area. This is defined as “speech understandable without repetition, some noise or distortion present”. To specify a lower DAQ may require excessive speech repetition, while a higher value may require a prohibitively high level of infrastructure investment.
A common reliability standard is 95%. This means that you can expect a signal of DAQ 3.4, 95% of the time across 95% of your coverage area.
Coverage requirements and coverage acceptance test plans deserve great care and respect. The processes, tests, parameters and vocabulary are well defined in the TSB 88 standards, maintained and updated by Telecommunications Industry Association (TIA). Reference to this standard will minimize the risk of misinterpretation of either your RFP or vendors’ responses.
The backhaul network interconnects your sites and control centers, and is the “big ticket item” on any communication system list. Include protection measures such as multi-path routing, ring structures and duplicate bearers to guarantee high system availability.
- Consider resilience in natural disasters. MiMo (multiple input, multiple output) linking is an alternative to wireless backhaul that has proven stability when microwave may be knocked out of alignment.
- If you are using your own microwave or fiber network, set up built-in monitoring to trigger alarms/switchovers when transmission parameters are compromised.
If your backhaul is provided by a third party, you need to include monitoring as part of your service agreement, with clearly identified performance parameters.
Paradoxically, even though backhaul service providers often use sophisticated technologies, they do not operate in a mission-critical environment, and backhaul failures commonly affect mission-critical radio systems, especially dispatch.
Your regular LAN isn’t mission-critical; you need dedicated links whether fibre or wireless. Probably the best argument for a mixed topology solution – typically fiber and microwave – is to provide the redundancy that ensures the greatest LMR system resilience.
Redundancy refers to duplication of system components (or their functions) to strengthen the system in the face of failure or threat. It is an important aspect of system design, balancing your needs with a complex mix of geography, perceived threat, regulation, and budget.
Virtually any component of your LMR system can be duplicated, but here are two common examples.
1. Duplicated/decentralized control center
Duplicating your control center function can protect against the loss of a dispatch facility due to fire, attack or a need to evacuate. You can specify a fully-redundant back-up center, or spread control between two (or more) centers in different locations. With good design, any center can take up the load from a disabled central control center when needed.
2. High availability
Protection from server failure is critical, and will impact on the number and location of servers included in your system design.
Server redundancy in IP-based radio comms systems may involve basic main/standby arrangements, but multiple servers in a geographically-diverse configuration can further protect your communications against failure or total loss of facilities. In this configuration all servers share the day-to-day handling of radio traffic. If a server fails, or is isolated due to link loss, the remaining servers can step in to provide continuity of communications.
This article is taken from the 10 part guide to Tougher LMR Systems.
If you would like to download this article and the other articles in the series you can do that on the Tait website.