An increasing number of interference complaints has arisen in situations where analog and digital technologies are operating on VHF and UHF shared channels.
In last month's column ["Crystal ball says 2013 will be busy year for LMR industry"], I discussed those items which would occupy much of 2013. One of those items was the increased sharing of spectrum between analog and digital technologies on VHF and UHF shared channels. There have been an increasing number of interference complaints regarding this mixture of operations on a channel, enough to cause the Land Mobile Communications Council (LMCC) to create a working group to establish guidelines for channels where analog and digital systems are sharing. The LMCC is an umbrella group of the frequency coordinators, and manufacturer members.
Complaints can arise in several situations. First, there have been instances where trunked and/or digital systems have begun operations without implementing the channel-monitoring provisions of Section 90.187(b) of the's rules. Second, the differing technical nature of digital technologies means that co-channel separation for systems that need exclusive use (such as a continuous-duty control channel) need to be rethought.
These first two types of interference are fairly straightforward. It is a third type of potential interference that causes folks to stay up at night. What should be the coordination and use criteria, if any, for an analog and digital system to share a channel, where both systems are implemented correctly?
Here we are trying to answer a question which the FCC has thus far never addressed in these radio services: What is "harmful" interference? Obviously, an analog user hearing a 50-millisecond digital blip once every half hour (instead of the usual two-way voice conversation) isn't harmful. Equally obviously, a continuous data stream that prevents the analog user from accessing the channel is very harmful interference.
Somewhere between these two extremes is something that is: (1) acceptable to users; and (2) consistent with the commission's rules. Finding a proper middle ground will be difficult.
Any proposed solution also must consider the differing types of digital technologies, the different ways in which users will occupy the channel — control channel, digital voice or digital data — and the different types of users (public safety, taxi company, etc.).
The LMCC's goal during early 2013 will be to create a set of guidelines for these different types of situations. The guidelines are needed as quickly as possible, because the mandatorydeadline has passed, and more users will be looking to implement digital technologies.
Late in 2011, we filed on behalf ofa request for the commission to specify whether short tone bursts would be considered to be harmful interference. The FCC declined. Thus, it is now up to the industry to address the issue. Input will be solicited from various quarters, and the hope is to have direction soon.
Clearly, the future of land-mobile-radio communications (in most instances) is digital. As the industry goes through a change from one technology to the other, it must be done in a way that enables digital systems to flourish, but without negating the ability of analog systems to continue to serve. Finding the right mix to enable a smooth transition is the key to the future.
What do you think? Tell us in the comment box below.
Alan Tilles is counsel to hundreds of entities in the wireless industry. He is a partner in the law firm of Shulman Rogers Gandal Pordy & Ecker and can be reached at email@example.com or on Twitter @landmobilelaw.