Spectrum redux

Last month we began to examine the most important element in wireless systems: spectrum. We took a look at the advantages and disadvantages of each and touched upon some special topics, such as narrowbanding and rebanding. Now we look at the infrastructure and system architecture implications.

Generally, systems in lower frequency bands communicate over longer distances, with less noise and interference, compared with systems that operate in the higher frequency bands. Equipment availability, cost and size are additional factors to be considered. Similar to last month’s discussion, here is a band-by-band review of the attributes of system architecture and design with a focus on equipment availability, cost and any size or electro-mechanical constraints. Trade-offs for link budgets, subscriber power levels and base station locations also will be reviewed.

High frequency (HF). Use of the HF band for land mobile radio operations traditionally requires specialized equipment. Whether single- or dual-side banded, there are a limited number of manufacturers of HF equipment. This limited supply has forced cost premiums. Antennas and filtering equipment are large given the long wavelengths. Mobiles and portables are not readily available.

The HF band is dependent upon reflection of the signals on the D, E, and F layers of the ionosphere. The amount of reflection is based upon solar activity, particularly sunspots, which follow an 11-year cycle from low activity to high activity to low activity. 2012 is expected to be a peak year, so HF activity should be good for the next year or so, because the more radiation that is emitted by the sun, the better the propagation. When the sunspot activity is low, the signals travel over shorter distances.

In most HF systems, numerous channels are assigned to different frequencies to accommodate the different bands required to have a path at different times of the day and different seasons of the year. Some of the modern HF equipment has circuits, called automatic linking equipment, which allow the radios to automatically pick the best band to use for the conditions of that hour.

After many recent tragedies, when most forms of communications failed, HF systems remained intact and were the only medium available. Over the past 10 years, HF systems have become more predominant among first responders; as a result, more manufacturers have developed products and the prices have decreased. There are still limitations on the size of the equipment and antennas, which may limit the communications sites that can be used. However, because the HF band has the best range of all, fewer sites are required. On the other hand, because sunspots and other interference and noise can limit range and quality, HF systems often are deployed as a backup to other means of communications.

Low-band VHF. While low-band VHF accommodates long-range communications, similar to HF, there also is limited equipment availability. In addition, the interference caused by power lines, alternating current motors, machinery, auto ignitions and magnetos creates difficulties for deployment in the field; as a result, this band is not commonly used. The effective receiver sensitivities are similar to those of Wi-Fi at –99 dBm, and link budgets are balanced by higher-power base stations and subscriber units to offset the degraded receivers.

The equipment size, for base stations and subscriber units, are large and may limit mounting. Similar to HF, low-band VHF offers excellent coverage and range, so fewer sites are required. But sunspots and other interference and noise can limit range and quality.

Mid-band VHF. The mid-band VHF frequencies are in the 72 MHz and 75 MHz range. Only base station transmitters are authorized for these channels — no subscriber devices are allowed. The antennas are fairly large, similar to low-band VHF, but slightly shorter. This band is utilized for backhaul links between stations and other fixed-location equipment.

The advantage of mid-band VHF is that skip is not too bad compared with HF and low-band VHF, and signals in the band can be heard by the intended receiving stations 60 to 100 miles away. In addition, the noise floor in this band usually is low, so the predicted link budgets are fairly accurate and reliable.

It should be noted that while television channels 4 and 5 used to be located just below and just above the mid-band VHF channels used for LMR operations. They were moved to UHF spectrum as part of the digital television transition. Nevertheless, the FCC’s rules and regulations for this band still assume that the TV channels are where they were prior to the move.

High-band VHF. Since the 1950s, high-band VHF has been very popular throughout the world. Currently it is still the predominant band used by public safety for systems that require a large area of coverage. There are plenty of manufacturers making myriad products for this band. The pricing is reasonable for most of the equipment because of the extreme amount of competition that exists in the marketplace. The newer systems include conventional and trunked radios, analog and digital formats, and radios in all ranges of power levels and price levels.

The good signal range, as well as the ability of the signal to go beyond line of sight, makes this the ideal band for urban and rural environments. The antenna systems are still rather large and this may limit some deployments.

This band is divided into segments: 138 MHz to 144 MHz is reserved for the U.S. military; 144 MHz to 148 MHz is set aside for amateur radio; 148 MHz to 150 MHz is used by the federal government; 150 MHz to 162 MHz is used by commercial entities and local government; 162 to 172 MHz is mainly set aside for the U.S. government; and 172 MHz has quite a bit of SCADA systems used by utilities. There are some other systems and agencies intermixed within these bands.

220 MHz: This band has minimal exposure in the LMR industry and as such there is limited equipment availability. Use of the 220 MHz band for LMR operations traditionally requires specialized equipment similar to that used in the HF band. This limited supply has forced cost premiums.

Antennas and filtering equipment are large given the long wavelengths. Mobiles and portables are not readily available. The band is used primarily for telemetry by the utility and transportation industries.

UHF and UHF T bands: The UHF band is at the threshold of the lower and upper frequencies and actually represents the best of both worlds. Long-range communications are possible — though not over the horizon — that penetrate tunnels and buildings. Equipment availability is common and prices are reasonable and stable. Antenna systems are smaller than in the lower frequencies, which allows for easier and less expensive installations.

Interference and intermodulation are concerns, however. The band is so popular that many systems are deployed and many of these systems are trunked, which creates potential intermodulation pairings. Third-order and fifth-order products within the band mixing with VHF are common. However, this potential intermodulation can be mitigated with proper filtering.

700, 800 and 900 MHz. These bands are similar in range, antenna designs and operations. Within each of these bands are services that emanate from different types of systems and modulation schemes including the following: analog, TDMA, GSM, CDMA, Project 25, QAM, CQPSK, C4FM, W-CQPSK and iDEN.

Because of the line-of-sight nature of these bands, systems that operate in them usually are located in urban and suburban areas, and rarely are used in rural areas unless there are many towers involved, such as in county-wide or statewide systems.

In the 700 MHz band, narrowband radios and trunking systems operate, as does a feature-rich, high-speed broadband system dubbed Long-Term Evolution, or LTE. The major cellphone carriers and the public-safety sector are each building LTE systems in this band. Recent federal legislation reallocated a major portion of the band, known as the D Block, to public safety for the purpose of building a nationwide broadband communications network for first responders.

In the 800 MHz band, there are wideband conventional and trunking radio systems, along with cellular systems that have been in existence since 1984. The 800 MHz trunking systems were interspersed with channels that were purchased by Nextel for its iDEN network. The iDEN system caused serious interference to the adjoining public-safety channels in urban and suburban areas, and the solution to that problem was to reconfigure the band, a process known as rebanding, which is ongoing. In simple terms, all of the iDEN systems were relocated to one part of the band and all of the remaining systems were moved to another part of the band. Most of the cost of the reconfiguration was covered by Sprint, which merged with Nextel in 2005.

The 900 MHz band is a mixture of narrowband trunking channels, amateur radio, wideband paging, cordless telephones and link radio systems. The 900 MHz trunking channels can be intermixed with the 700 MHz and 800 MHz channels to provide for more channels to accommodate large fleets of radios.

Unlicensed 900 MHz, 2.4 GHz and 5.8 GHz. The unlicensed bands offer many challenges and opportunities for deployment and operations. There is an abundant amount of equipment available and the locations for installation are plentiful since the equipment size is small and operates using low power.

The challenge is the short-range nature of the frequencies, and low-power levels require many base station locations, which increases operational costs. The receiver sensitivities are in the area of –99 dBm for all common unlicensed bands. Since the transmitting power levels are limited by the FCC for proactive interference protection, the link budgets are limited and many sites are required.

Another challenge is that the band is unlicensed. As such, there are many users in it. The result is that interference can be a problem one day but not the next, which makes system planning very difficult.

3.6 GHz. The 3.6 GHz band originally was allocated for satellite earth uplink and downlink systems. Because of sparse use of this band for this purpose, the FCC did allow backhaul point-to-point microwave systems on a secondary basis to use this band, as long as the earth stations are protected and have given consent for this purpose.

There are a limited number of vendors providing equipment for this band. This band is good in the respect that there are few radio systems close in frequency, so the noise floor is very low.

4.9 GHz. The 4.9 GHz band is reserved for public-safety agencies. The band does not have restrictions or a channel plan, and if there is more than one group trying to use this band, then the participants must work together to keep from interfering with each other.

This band — which originally was allocated to public safety for broadband operations — is versatile in that it can be used to transmit video and other high-bandwidth data, and for backhaul.

As the frequencies increase, the size of the radios decreases. There are limitations at the lower frequencies where base stations and antennas can be mounted due to this size limitation. Also, interference and noise potential at the lower frequencies sometimes outweighs the exceptional radio-coverage characteristics that they exhibit. As a result, there is less selection of base stations, ancillary equipment and subscriber units. This drives up cost.

Conversely, as the frequencies increase there are more options for mounting the radios. Thus, smaller base stations and antennas allow for more and better site availability, and subscriber units are more accessible. This drives down costs.

Radio link budget balancing is important in deciding which frequency band to use, and to manage limited resources. In other words, every RF watt saved is a power watt saved. The lower frequency bands have higher base station and subscriber power levels, but also inefficient receivers due to potential noise and interference. The increased range and coverage of the radio waves at the lower frequencies balance the link budget and allow less radio sites — but the cost to power systems operating at lower frequencies may offset the cost-savings that result from operating fewer sites.

At higher frequencies, typically UHF and above, the link budget balancing is less complicated. Receivers have effective sensitivities to –119 dBm and the base station and subscriber power levels can be reduced for balancing. The range and coverage is not as prolific as in the lower frequency bands, but the quality and power efficiencies are much higher.

Finally, the best balanced band for spectrum and economic efficiency is UHF.

In the next installment, we continue on the theme of managing limited resources. Before a deep dive into the components of the radio systems there will be a discussion of energy system options and “green” radio.

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 protected].

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 [email protected].

LMR 200 Series

LMR 100 Series