The decision this past summer by the Federal Communications Commission (FCC) to allow TV White Space (TVWS) devices to share Channel 37 with Wireless Medical Telemetry Service (WMTS) systems changes the spectrum environment in an important way. Hospitals and other healthcare organizations have a three-to-five-year planning horizon to prepare for this change. While that may sound like a lot of time, as we all know time passes quickly. If a WMTS system needs significant redesign, it’s crucial to understand the implications of the FCC ruling now so that the needed changes can be made before there are problems. (It’s important to note that there are two other WMTS spectrum bands, at 1.4 GHz, and those WMTS systems are unaffected by the FCC decision.)
Running a coexistence test following the standard ANSI C63.27 will give organizations the information they need for their planning. A coexistence test helps to determine the sensitivity of a WMTS system to TVWS transmissions and also to digital television (DTV) and other WMTS systems that may be nearby. The result of the test will be a set of desired‑to‑undesired (D/U) signal ratios. There will be a D/U ratio for co‑channel transmissions and other D/U ratios for adjacent channel transmissions.
D/U ratios are then translated into threat distances. What is the greatest distance and therefore lowest intended signal level expected of a WMTS device? It is not advisable to operate any device at the absolute maximum range because other factors will result in erratic signal reception when the intended signal is very weak. A shorter, maximum recommended operating distance is needed to ensure reliable operation. From the maximum recommended operating distance, the D/U ratio will tell you how close a TVWS devices can come before it will cause a problem.
The new FCC rules require that TVWS devices stay 380 meters away from WMTS systems. If the threat distance is less than that, the risk of interference is greatly reduced, but not entirely eliminated. There is still the chance that a TVWS device might violate the FCC rules. WMTS systems should have a defense-in-depth risk management plan. There should be more than one layer of protection. Healthcare organizations will want to make sure that their WMTS systems provide additional layers of protection and ensure safe, reliable operation.
Stephen Berger is president of TEM Consulting. He also served on AAMI’s Wireless Strategy Task Force and is co-chair of the AAMI Wireless Eoexistence Work Group.