Where it all begins
In this article we will explore the most important topic regarding land-mobile radio systems: spectrum. In doing so we will go beyond the basics to discuss the advantages and disadvantages of certain bands, provide a cursory look at the FCC auctioning of airwaves, and examine ongoing events — such as narrowbanding and rebanding — that are affecting spectrum resource management.
Specifically we will address when and where to use each frequency band, as well as the impact each band has on the following: system architectures; power levels; subscriber-unit utilization, antenna systems, transmission lines, radio-frequency interference, link budgets, radio coverage, noise floor, and data and other non-voice communications.
Let’s start by identifying each of the primary frequency bands covered by the FCC’s Part 90 rules for use in private LMR systems:
- HF (3–30 MHz)
- Low-band VHF (30–50 MHz)
- Mid-band VHF (72 MHz and 75 MHz)
- Very-high-band VHF (136–174 MHz)
- 220 MHz (216–220 MHz)
- UHF (420–470 MHz)
- T-Band (470–512 MHz)
- 700 MHz
- 800 MHz and 900 MHz
- 900 MHz, 2.4 GHz and 5.8 GHz
- 3.6 GHz
- 4.9 GHz
Now let’s examine each one.
High frequency (HF). This band, also known as the “short wave” band, has been in use since the earliest days of radio. The commercial and public-safety users depend upon these frequencies when communications must take place over hundreds, if not thousands, of miles. Some of the applications include transcontinental airlines, oil pipeline companies, and statewide communications for public safety personnel and other agencies.
The problem with the short-wave bands are that a given frequency may work one day but not the next. In addition, the paths are dependent upon the 11-year sunspot cycle. In the off years, the paths sometimes are unreliable. In the peak years, sunspot interference is very high and may disrupt communications.
Most short-wave systems are set up to communicate with similarly equipped stations. The power levels can vary but, in most cases, the FCC expects that the units will use the minimum power necessary to establish and maintain communications.
Antenna systems for HF radio are large. Because of the frequencies, the ¼ wavelength radiator can be on the order of 100 feet. The vertical polarized Marconi antennas can be very tall, and the horizontal polarized Hertz antennas can extend out many feet away from the boom support. These antennas are not easy to move and need to have resonance on multiple frequencies. Moreover, the cost of a multi-frequency rotatable antenna can exceed $250,000. Transmission lines have very low loss at the HF frequencies, so the only parameter that is of major concern is the power-handling capability of the lines.
Radio-frequency interference can travel over very great distances on HF, so it usually is quite a chore to track down and cure such interference. In addition, link budgets are difficult to predict, as there is no set formula for the propagation, because the skip distance and reflection coefficient changes daily.
Radio coverage is spotty and unpredictable on a day-to-day basis in this band. Noise floor and atmospheric environments will vary and can be heard hundreds or thousands of miles away from the source. Licensing does have quite a few restrictions and the need for these channels must be established. Data and non-voice communications are restricted in this band.
Low-band VHF. This band is used when a very large geographic area must be covered with a minimum use of tower sites. This signal can travel up to 100 miles and still be useful.
The down side of low band is when “skip” is present during the high portions of the 11-year sunspot cycle. Similar to what occurs in the HF band, this atmospheric phenomenon can result in low-band stations a thousand miles away being stronger than a station only 10 miles away.
Almost every system in this band is a conventional radio system. The power level of low-band radios usually is in the 100-watt range. Portable walkie-talkie radios rarely are in use, as the antennas for low-band devices are approximately 6-feet long, unless a loading coil is used to shorten the antenna. The transmission lines used in low-band have very little loss, so smaller lines can be utilized.
RF interference normally is in the form of impulse-type noise caused by electrical contacts making or breaking — which causes arcing — by automotive distributors and spark plugs from nearby vehicles, or by thunderstorms within 100 miles from a system.
The link budgets can be calculated and radio range can be predicted, except when skip is present and the interfering signal overpowers the normal signal of units in the system.
Because of the unpredictability of the interference, very few new low-band systems are designed or implemented. In most situations, the FCC licensing is relatively easy, as many systems have ceased operation because of the skip conditions that occur every 11 years. Data and non-voice communications are limited in low-band VHF.
Mid-band VHF. This band, where signals travel up to 100 miles, generally is used to connect fixed sites. The mid-band rules once restricted use of such systems when TV channels 4 and 5 are neighbors. However, after the implementation of digital TV in 2009, these rules became irrelevant.
In most situations, the effective radiated power levels should range from 25 watts to 100 watts. The antennas on towers can be either omnidirectional or directional depending upon the topology of the system. The transmission lines can be relatively small as there is not much loss in the cables used in this band.
Because of the limited users in this band, and all of the protections that must be in place for the old television rules, there is little interference except when the skip occurs during the high-sunspot periods.
The link budgets are predictable and the range can approach 100 miles and still be usable. Licensing is relatively easy due to the low demand for the frequencies in most localities. Data and non-voice communications are allowed on these channels, besides the normal voice traffic.
Very-high-band VHF. These frequencies are the most popular for LMR use due to their excellent range, length of antennas, propagation characteristics and availability of low-cost equipment.
Besides conventional voice traffic on individual channels, the advanced controllers and radios in the marketplace also allow for trunking, data and other non-voice traffic. The radios come in all power levels, the antennas come in all types of configurations, the signal loss in the transmission lines is acceptable, and many choices of lines are available in the marketplace.
However, one of the biggest problems with this band is the potential for RF interference. Besides the normal problems from lightning, static discharge, and man-made issues, the very large number of VHF stations in most large and medium-size cities causes the noise floor to be high. Add the proliferation of desktop computers, other wireless devices, intermodulation from nearby users and the electrical noise found on many building tops, and RFI has a major impact on the range of many VHF systems.
The licensing of VHF systems is difficult in many areas, because the long coverage range and crowding — even in the rural areas — makes it difficult to add new channels to systems. The FCC’s narrowbanding requirement approaching in 2013 adds another level of complexity. This mandate requires virtually all systems operating below 512 MHz to switch to 12.5 kHz-wide channels, from the current 25 kHz-wide channels, by Jan. 1, 2013, in order to achieve greater spectral efficiency.
220 MHz. While this band is available as a spectrum resource, there are power, height, and other restrictions that make it unpopular for most users. The lack of low-priced equipment, an inability to put base antennas above 500 feet, and the marketplace’s reluctance to manufacture other peripheral equipment has added to that unpopularity.
UHF and T bands. The UHF band is a very popular for all types of communications systems, and it is absolutely perfect for use in urban environments. Besides conventional radio systems, trunking and networked systems allow for very large areas of communication coverage for system users.
The variety of equipment and the good range makes this band well-suited for almost every application that requires radio coverage. The system designer has a very broad range of choices for antennas, transmission lines, mounting configurations and other design criteria.
The RF interference experienced in this band usually comes from other radio systems, as natural noise and electrical impulse noise does not bother this band in most circumstances. The radio link budgets and radio coverage prediction programs are very accurate and building penetration is exceptional. The signal for UHF is mostly line-of-sight, but because of reflections, refractions, and the use good antennas with lots of gain, the signal can be sent further than normal LOS would predict. The narrowbanding requirement approaching in 2013 adds complexity similar to the VHF band.
The T-Band is an extension of the normal UHF band that provides additional frequencies that were carved out of the UHF television band, hence the name. In only 14 cities can two-way radio users leverage vacant UHF TV channels. The range and other characteristics of T-Band radios are the same as the normal UHF band, and narrowbanding of these frequencies also must be executed before 2013.
700 MHz. The 700 MHz spectrum was allocated by the FCC in 1998 as a new band for public safety. In order for the band to be utilized, the UHF TV broadcasters had to vacate channels 62 through 69. The television broadcasters also were required to use digital technologies in order to maximize their new spectrum. RF coverage in this band is excellent in most situations.
There are two separate types of channels for 700 MHz. There is a broadband allocation consisting of 10 MHz of spectrum that allows public-safety agencies to operate their own 3G or 4G networks utilizing a cellular technology known as LTE, which stands for Long-Term Evolution of GSM. Last month Congress allocated an additional 10 MHz of spectrum, known as the D Block, which originally was to be auctioned to commercial interests. This allocation, paired with the spectrum that public safety previously was given, creates a 20 MHz block of contiguous spectrum for first-responder broadband operations. Meanwhile, the second type of allocation is for narrowband voice channels that allow public-safety users to have interoperability throughout the U.S.
The equipment manufacturers have responded to this new band with an abundance of equipment with advanced features. Last month, Motorola introduced at IWCE 2012 the first LTE subscriber device.
800 MHz and 900 MHz. When the FCC opened up the 800 MHz band in the early 1980s, there was little thought given to the impact of how the specialized mobile radio (SMR) and the newly allocated A and B block cellular systems would interfere with each other.
Once the 800 MHz SMR systems were available, many of the medium and large city public-safety agencies realized how effective these new systems could be and almost all of the cities moved their radio operations to these channels. The cellular companies and the private LMR 800 MHz operators figured out how to mitigate the RF interference and for most of the 1980s the 800 MHz private and 800 MHz cellular systems operated very well together.
In the 1990s, Motorola developed the iDEN system, and many of the private SMR systems were sold to Nextel, which immediately switched those systems to iDEN so that they would be part of the carrier’s nationwide network. The problem with that change was that iDEN uses 45 KHz of bandwidth, but the channel allocations are 25 KHz; moreover, many public-safety channels were adjacent to the Nextel iDEN channels because the FCC decided to interleave spectrum in the band.
The result was that interference from the iDEN system was overwhelming in many situations. After much negotiation, Sprint, which had acquired Nextel, agreed to pay billions of dollars to move the iDEN systems and the public-safety and commercial 800 MHz systems to different parts of the band. This reconfiguration was popularly referred to as ”rebanding.” This solution does fix the problem where it has been completed, but there are still many 800 MHz systems that have not completed their rebanding, particularly in areas near the Mexican and Canadian borders.
When there is no interference within the 800 MHz band, the systems work very well. The equipment has many features and there are plenty of choices for engineers to leverage. In addition, the planning tools available today allow design engineers to pick locations, model the coverage and provide excellent results for 800 MHz system users.
The licensed portion of the 900 MHz band is an extension of the 800 MHz band and the coverage, operation, and features of the equipment are similar.
900 MHZ, 2.4 GHz and 5.8 GHz. The 2.450–2.500 GHz band was created to allow broadcasters, public safety and other users to have access to wireless private television service. This is a shared-use, licensed band that cannot be used without FCC authorization.
The operation is line-of-sight and is not designed for general broadcasting. As with any frequency above VHF, transmission line loss, antenna selection and other pertinent engineering design criteria are factors in the selection of components.
Meanwhile, the portion of the 900 MHZ and 2.4 GHz bands used for industrial, scientific and medical purposes, as well as the 5.8 GHz UNII bands, are unlicensed. They currently are used for broadband operations.
3.6 GHZ. This band is shared with satellite services, and only can be operated on a secondary basis if an earth station satellite operator gives its permission to operate within 150 km from its operation.
There is equipment available for private operators and good engineering practice is required for systems to operate properly. In most situations, this band will be used for backhaul of data and voice traffic from other radio services. It is a licensed band and there are quite a few restrictions regarding its use.
4.9 GHZ. In 2003, the 4.9 GHz band was set aside for public safety, although other applications are allowed. Agencies can use these systems for base, mobile or portable operations anywhere within the authorized boundary of their licenses. Anyone who operates in this band must coordinate with the other licensed users as the FCC has not set up individual channels.
The FCC typically sells or auctions spectrum to entities that use it for profit-making ventures, and gives it to entities that use it for nonprofit business, government operations or to protect citizens and their property.
For the profit-driven entities, the auction process requires companies or individuals to submit applications, financial deposits and bids. Winning bidders are then required to go through a license-application process. Non-profit entities that have been awarded spectrum also must submit a license application to the FCC directly or through a defined intermediary. License applications can be complicated, but at a minimum they must properly describe the system that will be deployed — listing each piece of equipment and its output power — and indicate that the system will meet the FCC’s rules regarding equipment and power levels, and that all tower approvals have been obtained.
The selection of the proper spectrum band is crucial when preparing to design a radio system. The next article in this series will focus on how to ensure that the system’s architecture matches the spectrum decision.
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at email@example.com.
Robert C. Shapiro, P.E, is a consulting engineer who has been in Land Mobile Radio since 1984. He serves on the TIA TR8 committee (TSB-88) as vice chair and is a senior member of the IEEE. He can be reached at firstname.lastname@example.org.
LMR 200 Series
- Part 1: School’s back in session