Municipalities and public safety departments have long relied on the 4.9-GHz band in the United States to build local wireless networks. The U.S. government has set aside this band for the delivery of public safety services. The band offers 50 megahertz of radio-frequency spectrum, and it is lightly licensed. Several benefits have motivated local government agencies to use this spectrum.
First, there is no cost to use it. Eligible government agencies pay no license fees.
Second, the frequency band typically has less interference than the 2.4-GHz and 5.8-GHz unlicensed bands. Licenses for transmitters in the 4.9-GHz band identify the users, and the FCC database makes available details about them. In addition, unlike the unlicensed bands that can be used for any of the consumer applications, only public safety applications can use the 4.9-GHz band.
Third, the comparatively low frequency of the 4.9-GHz band means signals have fairly good propagation characteristics that allow line-of-sight and non-line-of-sight connectivity.
Nevertheless, some drawbacks make it more difficult to deploy backhaul systems in the crowded and overused 4.9-GHz band. Several reasons motivate organizations to move their backhaul to higher frequency bands.
The band’s limited capacity restricts most backhaul and access links in channel size where many users operate near one another. Although the maximum channel size is 20 megahertz, most links are limited to 5 megahertz, which limits full-duplex capacities to about 20 Mbps.
Access Demand and Spectrum
Sometimes it’s impossible for a public safety agency to acquire any 4.9-GHz spectrum for a desired link because the frequency band may be too highly congested where it wants to operate, especially if the access demand has grown.
Its overuse, reduced channel width and self-interference with access systems make 4.9-GHz backhaul availability more and more difficult to predict. All too often, the delivered capacity cannot be engineered in advance, making service levels impossible to define.
The nature of time-division duplexing and the 4.9-GHz band’s thin slices of spectrum cause backhaul delays (latency) to be quite high, often in the range of 2 milliseconds to 5 milliseconds. Links with that much delay adequately provided Ethernet services, but for many of LTE’s desired services, that much delay often is too high.
These limitations give many government agencies enough reason to evolve their backhaul networks to point-to-point licensed spectrum at 6 GHz and higher. These frequency bands are readily available in the United States. A 10-year license for a channel costs less than $1,500. In addition, with channels as wide as 80-megahertz in duplex pairs available, operators can deploy Gigabit backhaul capacities. And, the spectrum is guaranteed and separate from any access spectrum, so an operator can scale a system independently of what happens in the access network.
There are five major considerations for an agency to take into account when moving operations to licensed bands at 6–23 GHz.
Each link must be individually coordinated and licensed. The FCC collects no licensing fee for government applications. Instead, typically a third party that handles the process charges a fee on the order of $500. The process usually takes about 30 days.
Rain attenuates radio signals more at frequencies above 6 GHz than it does with signals in the 4.9-GHz band, but the higher bands offer wider spectrum. The tradeoff is that despite the greater rain fade, sometimes lower modulations can be used. However, to determine the required antenna sizes, it is important to perform a detailed link engineering exercise, which takes into account the rain rate, link capacity, path length, mounting heights and desired availability. Many value-added resellers can perform this task, and some microwave vendors will also provide this service.
The bands from 6–23 GHz offer channel sizes from 30 megahertz to 80 megahertz wide, with 40 megahertz being a common channel size. In these licensed point-to-point bands, the FCC always assigns channels in pairs, allowing for full-duplex frequency-domain duplex (FDD) transmissions. With 2048 quadrature amplitude modulation and a 40-megahertz-wide channel, 400 Mbps of full-duplex throughput can be achieved, enabling significant scale versus a 4.9-GHz system.
Line of Sight
With frequencies in the bands from 6–23 GHz, it is important to have a clear line-of-sight path between the two radios. An electronic path analysis sometimes is enough to determine the line-of-sight path, but with longer paths, a two claim and path verification from someone at both ends may be required.
Historically, systems at frequencies from 6–23 GHz have been more expensive than 4.9-GHz systems. However, next-generation systems have come down significantly in cost and are offered at only a small premium to 4.9-GHz systems. It also is important to look at total cost of ownership when engineering a licensed microwave system. An all-outdoor system should be considered because it allows spending less money on equipment indoors for power and for heating, ventilation and air conditioning. An outdoor system can be connected with power and with a switch or base station indoors using outdoor-rated, low-cost CAT5E cables. It is also important to understand any costs related to the size of antenna on a tower. By selecting a system with an improved link budget, a public safety agency can reduce its antenna sizes and the associated tower rental cost.
Public safety backhaul networks have been built on 4.9-GHz frequencies for a long time, and they have historically fit the bill. However, overuse of the frequency band and increased network capacity requirements have pushed backhaul needs past the capabilities of the 4.9-GHz band. Fortunately, the bands from 6–23 GHz provide a cost-effective and scalable evolution path for existing and new networks. Operators can quickly upgrade their networks, as long as they pay appropriate attention to the few unique requirements of the higher bands.
Greg Friesen is vice president of product management at DragonWave.
Major Considerations for Moving to Licensed Bands at 6–23 GHz