This article first appeared in the February 2002 issue of Monitoring Times.
Public Safety radio systems are experiencing increasing levels of interference from commercial mobile radio systems (CMRS) such as cellular and specialized mobile radio (SMR). More than 20 cities have reported significant interference from Nextel, the largest SMR operator, or a local cellular provider.
Public safety and commercial users both use frequencies in 800 MHz band, an arrangement that began in the 1970's when the Federal Communications Commission (FCC) did away with UHF television stations 70 through 83 and reallocated much of that space to land mobile radio (LMR) users. These users include public safety, cellular telephone and SMR operators.
Cellular telephone service started in 1983 and has grown dramatically since then. SMR service in the 800 MHz band has also increased, although not to the same degree. To accommodate all of this growth, operators have built thousands of towers across the country with the goal of serving as many users as possible. Meanwhile, public safety agencies have also moved into the 800 MHz band, building their towers to maximize coverage area while attempting to minimize cost.
So today we have commercial and public safety users in the 800 MHz band, each operating with conflicting goals on adjacent, intermixed radio frequencies. The end result is interference and poor performance for public safety radios when they are near commercial towers.
Several organizations and manufacturers have been working to reduce this interference, including APCO (Association of Public Safety Communications Officials) with their Project 39 effort. Some measures being tried by commercial operators include reducing transmitter power, limiting use of certain frequencies at certain times of the day, and reorienting tower antennas. This is a slow and tedious process, and is not always successful.
Most public safety radio activity in the 800 MHz band currently takes place in two blocks of spectrum, one running from 806 MHz to 824 MHz and the other from 851 MHz to 869 MHz. Each of these blocks is split into 25 kHz wide channels, where one channel from the lower block is paired with a channel from the upper block. The channel from the lower block is used by mobile radios to transmit to a base station and the channel from the other block, exactly 45 MHz higher, is used by the base station to transmit out to the mobile radios. (Because transmissions from the base station are so much stronger than from mobiles, most scanner listeners monitor frequencies in the higher block.)
This past November, Nextel submitted a white paper to the FCC proposing a significant restructuring of the 800 MHz band. The stated purpose of the proposal is to reduce the interference currently experienced by public safety users from Nextel and cellular telephone operators, and to provide somewhat more spectrum to the public safety community.
The core of the Nextel proposal is for the FCC to scrap the current arrangement and create two separate blocks of channels in the 800 MHz band, one for public safety and one for commercial operations. The public safety block would run from 806 MHz to 816 MHz and from 851 MHz to 861 MHz (20 MHz of spectrum) and have a total of 400 paired channels. Commercial operators would have 16 MHz of continuous spectrum just above that, from 816 MHz to 824 MHz and from 861 MHz to 869 MHz, comprising a total of 320 channels.
Nextel would surrender 16 MHz of their existing licensed frequencies (they are currently allocated a total of 18 MHz in the 800 MHz band), and in exchange would receive two new allocations: 6 MHz at 821 to 824 MHz and 866 to 869 MHz, and a 10 MHz block up in the Mobile Satellite Service (MSS) band at 2.1 GHz.
Nextel's pitch is that by splitting up the users into their own contiguous bands, the interference problems would be greatly reduced. The proposal also highlights the fact that public safety is currently allocated less than 10 MHz in the 800 MHz band; their proposal would more than double public safety's allocation. Nextel also offers to relocate their licenses to other frequencies at their own expense and promises to contribute a significant amount of money to help other users relocate if their requests are granted.
Some of these relocation efforts may be easier than others. For instance, there are 50 channels set aside for business and industrial users between 809 MHz and 816 MHz (with their corresponding base station frequencies between 854 MHz and 861 MHz) that Nextel suggests be allowed to operate on a secondary, non-interfering basis until they eventually move on their own.
More problematic are the NPSPAC (National Public Safety Planning Advisory Commitee) channels between 821 MHz and 824 MHz and between 866 MHz and 869 MHz. Nextel suggests moving these channels into the proposed public safety block at 806 MHz, and acknowledges that this will be expensive.
NPSPAC, with the blessing of the FCC, has specified several nationwide frequencies to be set aside for mutual aid, in order to allow multiple agencies to communicate with each other in time of disaster or other emergency. The channels currently in place are:
The standard for transmissions on these frequencies includes a CTCSS tone squelch of 156.7 Hz.
The request for spectrum up in the 2 GHz is more of a regulatory challenge for the FCC. Although the slice of spectrum Nextel is asking for isn't currently being used by any primary license holder, it sits in a band that is designated for Mobile Satellite Service (MSS). The FCC would have to reallocate at least the 10 MHz of spectrum Nextel wants (2020 MHz to 2025 MHz and 2170 MHz to 2175 MHz) away from MSS and dedicate it to terrestrial mobile services. Secondary users in these frequencies, such as Broadcast Auxiliary Service and Fixed Point-to-Point Microwave, would have to be relocated sooner than the FCC currently requires.
Other parts of the proposal include Nextel relinquishing licenses for spectrum in the 700 MHz and 900 MHz bands.
A number of organizations support the Nextel proposal, at least in principal, and are urging the FCC to begin a rule-making process for the 800 MHz band. These organizations include the Association of Public Safety Communication Officials, the International Association of Fire Chiefs, the International Association of Chiefs of Police, Major Cities Chiefs Association, National Sheriff's Association, Major County Sheriff's Association and the National Public Safety Telecommunications Council.
Nextel is the fifth largest cellular mobile carrier in the United States with more than 8 million subscribers. Nextel is unique among cellular providers because they grew out of the two-way dispatch business rather than the telephone industry. One of the most popular features of their radios is the "walkie-talkie" capability where one Nextel subscriber can immediately communicate with another just by using a push-to-talk button. Nextel is also different because they use frequencies set aside for Specialized Mobile Radio (SMR) rather than cellular mobile telephone or personal communications services (PCS).
Nextel uses a technology developed by Motorola called integrated Digital Enhanced Network (iDEN). iDEN was formerly known as Motorola Integrated Radio System (MIRS) and provides digital voice and data services including enhanced dispatch (two-way talk group communications, similar to walkie-talkies), telephone interconnect (place and receive regular telephone calls) and messaging (alphanumeric messages similar to paging).
iDEN uses time division multiple access (TDMA) to fit six conversations into a single radio channel. This channel is divided up into timeslots, each slot lasting 15 milliseconds. This means each iDEN user can transmit for 15 milliseconds in each 90-millisecond period. By taking turns in this way, six users can share one radio channel.
Voice traffic on an iDEN is digital. A voice codec (encoder/decoder) takes the analog input from the microphone and produces a digital representation according to a VSELP (Vector Sum Excited Linear Prediction) algorithm. This representation is protected by error correction information, which allows the receiver to repair bit errors that the representation may have experienced during transmission. Signaling information is added, along with any low-speed data the user may wish to send. This whole package is transmitted when the next timeslot comes around, and the process is repeated every 90 milliseconds for as long as the user holds down the push-to-talk button.
The iDEN transmitter uses Quadrature Amplitude Modulation (QAM), which is a fairly complicated modulation scheme but provides for an overall data rate of 64 kilobits per second (kbps) in a 25 kHz radio channel. QAM was chosen, in part, because it doesn't require adaptive equalization or other relatively expensive methods to correct for transmission path delays and rapidly changing signal strength that are common in mobile radio environments.
So far there are no commercially available scanners that can follow iDEN transmissions.
Pro-92 Channel Lock Out
On my Pro 92 scanner when I turn it on it says all channels locked out. It will not pick up any channels when this feature is on and I want to turn it off. The Owner's Manual has not helped much so I was wondering if you could help me.
The "All Channels Locked Out" message appears on the PRO-92 and PRO-2067 scanners when there is an active control channel programmed into the radio but no available voice channels. This is a confusing message because it may appear even if you don't actually have any channels locked out!
What is happening is that the control channel is reporting activity on a voice channel, but the scanner either doesn't have that proper voice channel programmed into it or all of the voice channels are locked out.
First be sure that you really don't have voice channels locked out. You can do this using the instructions on page 50 of the Owner's Manual, which describes how to review and clear locked out frequencies. To review, press [SEARCH] then [FUNC] then [L/OUT]. Use [FUNC] and an arrow key to move through the banks. To clear all locked-out frequencies in a bank, press [SEARCH] and then turn on the bank you want to clear. Press [FUNC] then . The scanner will ask you to confirm the operation.
If you don't have any locked out frequencies, go back through all of the programmed frequencies in the bank and see if there is a control channel with missing or incorrect voice frequencies. Either add the appropriate voice frequencies or remove the entry for that control channel and you should no longer see the "All Channels Locked Out" message.
Keep in mind this isn't really a bug -- the scanner is doing the right thing by informing you that it cannot find a programmed voice channel to use with an active control channel.
That's all for this month. Take some time out to watch the Winter Olympics, if you can, and let me know what you're monitoring via electronic mail at firstname.lastname@example.org. As always, my website at http://www.signalharbor.com has additional information and links. Until next month, happy monitoring!
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