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Analog/digital issue needs quick answer

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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 FCC'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 mandatory narrowbanding deadline has passed, and more users will be looking to implement digital technologies.

Late in 2011, we filed on behalf of Icom a 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 atilles@srgpe.com or on Twitter @landmobilelaw.

Discuss this Blog Entry 9

on Jan 10, 2013

Local PSAP is supposed to monitor State Net frequency but are turning the volume down to escape the constant burps of digital data (they thought it was just squelch noise, but there are discernible pulses in it) and users elsewhere report the same noise, so it's not local bleed off of desk equipment. Clearly a safety issue. Googled to see if anyone had commented on this but article above is first word on it.
Please fix this comment box, too small and when resized the right end of text is cut off.

Joe Leikhim (not verified)
on Jan 24, 2013

STAN
Is your State Net frequency exclusively for state interoperability? If so, it is likely no one is supposed to be using other than analog equipment. If you are near another state border and the frequency is used by local agencies of the other state, all bets are off. The big question is "why isn't CTCSS tone squelch being used? That would eliminate the noise and allow your dispatchers to monitor the channel. Talk to your technical folks and local frequency coordinators and get this sorted out.

on Jan 31, 2013

This is Alabama Mutual Aid Law Enforcement frequency (previously known as State Net) 155.010 carrier squelch. I find some towns licensed to this frequency in Mississippi west of Tupelo, or about 80+ miles away. That should be weak enough that CTCSS would solve the problem, but that'll call for reprogramming every radio in the state. Who to contact?

on Jan 11, 2013

Alan has managed to describe in 250 words or less what is a problem that has plagued the coordinators for several years. While Alan couches it in the terms of whether interference is harmful or not, we coordinators struggle with mixing incompatible technologies into an increasingly crowded slice of spectrum.

On the most basic level both analog and digital systems accomplish the same task, that of relaying information. The technology used is what sets them apart. Perhaps the recent conversion of television broadcasts from analog to digital would be a fair comparison.

There are a great many analog systems in use with no discernible end-of-life date operating in the same spectrum as digital systems which are also incompatible with each other. The advantage the "old" technology has over its newer sibling is compatibility in that two analog systems can exchange information with virtually no loss of intelligibility. However,the various digital systems being used are fortunate to detect the other systems presence let alone exchange information. The question of how to handle both types of systems within the same spectrum is (to borrow a metaphor) a can that's been kicked down road for others to resolve. One question still requiring resolution is should digital systems be treated differently depending on their emissions or should the standards regarding interference and service contours be the same regardless of emission type?

Anonymous (not verified)
on Jan 11, 2013

Does this sound suspiciously like the Nextel problem? Incompatible technology/usage causing interference?

Karl WA8NVW (not verified)
on Mar 12, 2013

Sounds more like 1987 and NPSPAC de ja vu with 20 kHz bandwidth on channel centers spaced only 12.5 apart. Now in HB the FCC has created narrow channels +/- 3.125 kHz from the 155.010 channel center, but then they authorized emissions of 11.25 kHz occupied bandwidth which overlap the bottom half of the 155.010 channel window. Several decades ago Winston Churchill said, "Those that fail to learn from history are doomed to repeat it."

Anonymous (not verified)
on Jan 11, 2013

The ongoing problem. Bandwidth is not the same as a channel. Some digital technologies seem to use channels - like P25, others use bandwidth like iDEN and MotoTRBO, which have been proven not to be compatable with channel users on adjacent frequencies. Nextel proved that.

Anonymous (not verified)
on Jan 11, 2013

we also need to refrain from selling our customers technology "upgrades" that are incompatible with the way the system is used. In the stone age, when VHF LMR changed from AM to FM, there was a service that analyzed the impact and made an intelligent choice not to go FM. That service is still AM to this day; the aviation band. The rationale is that a receiver (Pilot or ATC) will better be able to detect and possibly understand a "stepped on" transmission because of the lack of a feature of FM, the capture effect. Aircraft communications are being looked at as far as upgrades now, but the relative improvement in the next generation aircraft system will necessarily need to be well tested and proven before it gets accepted.

Anonymous (not verified)
on Jan 30, 2013

I am so glad to see that"Armstrong's Revenge" has arrived!
That just leaves the FCC's Narrowbanding fraud left to go.
Recall- just reduce to 2.5 Khv dev. and at the last minute,no
you have to buy new radios". Not to mention the VHF 12.5 fiasco.

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Alan Tilles is counsel to numerous entities in the private radio and Internet industries. He is a partner in the law firm of Shulman Rogers Gandal Pordy & Ecker and can be reached at atilles@...
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