This article first appeared in the June 2001 issue of Monitoring Times.
As we've discussed in previous columns, numerous public safety agencies are transitioning from older radios to new systems in the 800 Megahertz (MHz) frequency range. These new trunked radio systems promise digital clarity, interoperability with other jurisdictions, and the ability to handle a greater number of users. However, many agencies have also experienced a number of significant and potentially life-threatening problems with the reliability and usability of these more complex radios, as we can see in this letter from the mailbag:
Besides Washington, D.C., municipalities in California, Delaware, Georgia, Missouri, New York, and Oregon have had their officers' lives put at risk due to radio system problems.
The primary complaint from users of these new 800 MHz systems is that there are gaps, or "dead zones," where there is no service. If you've ever tried to use a cellular telephone in a remote or rural area you may have experienced the NO SERVICE warning on your phone because it wasn't close enough to a cell tower to receive a signal. The same kind of phenomenon is happening with 800 MHz digital radios, where the signal from the repeater tower is too weak, too distorted, or too far away to reach.
Many older public safety radio systems operate at much lower frequencies, primarily in the 400 MHz and 150 MHz bands. One characteristic of 800 MHz radio signals is that they do not penetrate buildings and other structures as well as those lower frequencies. So, in order to have the same level of coverage with an 800 MHz system as you would with a lower frequency system, you end up needing more towers. This increases the expense and effort involved in fielding a new system, and a number of cities have been reluctant to spend additional money to fill in these gaps.
Public safety users are not alone in the 800 MHz band. Other users include cellular telephone systems and Specialized Mobile Radio (SMR) operators. The largest and most pervasive SMR operator is Nextel Communications, Inc., which has built numerous radio towers across the country to provide coverage for their subscribers. Unfortunately, for historical reasons the radio frequencies used by Nextel are adjacent to public safety channels, and there is often a significant amount of interference where they coexist.
The task of establishing rules and procedures to eliminate this kind of interference ultimately falls to the Federal Communications Commission (FCC). Many years ago, when the FCC originally granted the Nextel frequencies, the SMR business was basically limited to trucking and taxicab dispatch operations, which required relatively few towers. Public safety systems, too, were designed with the expectation that adjacent frequencies would not be heavily used.
Now, with Nextel selling large numbers of handheld radios, SMR towers are popping up everywhere. Although Nextel insists they are operating within FCC guidelines, new 800 MHz public safety radio systems in many parts of the country are overwhelmed by these signals.
Nextel is not the only culprit, since cellular telephone systems are also widespread and operate very close to public safety frequencies.
Last year a number of parties brought together by the FCC formed a working group to study ways of reducing or eliminating interference between public safety systems and the cellular and SMR networks. Members of the working group include the Association of Public Safety Communications Officials International, Inc. (APCO), Motorola, and Nextel.
In parallel, the FCC is currently considering the rules for use of the 700 MHz frequency band, soon to be vacated by UHF television broadcasters. Public safety agencies are looking forward to 700 MHz as a way to ease overcrowding in the 800 MHz band and greatly reduce potential interference. However, despite a promise to protect public safety radio users, the FCC is under pressure from Congress to auction off as much 700 MHz spectrum as they can to commercial users. In doing so they may once again create rules that foster the interference occurring today.
Another type of interference occurs because of the nature of 800 MHz signals, which have a tendency to bounce off large flat surfaces like billboards and the sides of buildings. All of the resulting reflections combine with the unreflected signal to create a condition at the radio known as multipath. Each copy of the signal takes a different path to reach the radio (that is, multiple paths) and therefore arrives at the radio at a slightly different time than all the rest. These multiple overlapping signals interfere with each other and many times the original signal is so distorted that it cannot be recovered.
Multipath is highly dependent upon the exact location and orientation of the receiver as well as the relative locations of reflective surfaces. This makes it a significant challenge to predict exactly where such a condition may occur.
These new systems are almost always operated in digital mode, which means that the voice and message information are transmitted as series of binary digits (bits) rather than a continuous analog signal. When a digital signal encounters interference, the ones and zeroes of the transmission are overwritten or distorted and the radio that is receiving the signal may not be able to accurately reconstruct the original message. If the transmission is so badly garbled that the receiver cannot make sense of it, the typical action is to mute the speaker, meaning the user hears nothing.
With an analog system, interference results in irritating noises and other difficulties, but often the human ear can pick out the voice amid all the audio clutter. Shortwave listeners are especially good at this, since many times the signal from half way around the world has a lot of noise that comes along with it! A digital system, in contrast, will simply blank the audio and provide no information to the user, leaving them wondering whether the system is working at all. This also makes it difficult for a user to determine the source of the interference.
These new systems are much more complicated than their predecessors, with all of the new features and capabilities that a digital trunked system can bring. These features require a good deal of computer software, both inside the mobile radios (sometimes referred to as firmware) and at repeaters and dispatch centers. Any software this complex will have bugs, and sometimes these bugs only manifest themselves during unusual conditions. Since Murphy's Law ("anything that can go wrong will go wrong, and at the worst possible time") holds true for software, these bugs often appear only during very busy or critical times that are hard to reproduce in a manufacturer's development laboratory. Nashville's new trunked system, for example, experienced a serious problem on Election Night last November and shut itself down just before the evening festivities were to begin.
To be fair, some problems can be chalked up to lack of training and user inexperience. The simple "push-to-talk" microphone has been enhanced with a number of additional features and capabilities, some of which can be confusing. These radios must also be properly programmed before being put into service, and mistakes in programming have been known to happen.
When operating in digital mode, the radios must perform a conversion between the analog voice coming into the microphone and the digital bits being transmitted out the antenna. This conversion takes a certain amount of time, creating a momentary delay that takes some getting used to. If the user is not comfortable and confident in the way a radio operates, the effectiveness and usefulness of that radio is greatly reduced.
Thanks, Al, but don't hold out on us -- send in the frequencies you're monitoring as well! Here are the talkgroups mentioned in the letter, along with their hexadecimal and binary equivalents:
The pattern that I see in the talkgroups is apparent in the last two digits of the hex representation. Without knowing anything else about the system, I'd have to say that this is a Motorola Type II system and these are normal talkgroups. Recall that in a Type II system the last hex digit (the last four bits of the 16-bit talkgroup value) represent special conditions for the talkgroup.
In general, the assignment of actual talkgroup numbers typically depends on how the system is shared. A designer has to take into account all of the agencies and organizations that may use the system, how many talk groups each organization will need, and make some guesses as to how the users may access the system. Also, many systems start out small and gradually add more users, so the original talkgroup plan may have to be modified as the system grows.
That's all for this month. Get out there and enjoy the summertime, and be sure to send me the frequencies and talkgroups for the agencies you're monitoring. I can be reached via electronic mail at firstname.lastname@example.org, and you're welcome to visit my website at www.signalharbor.com. Until next month, happy monitoring!
Click here for the index page.
Click here for the main page.