August 2012

Public Safety Networks Mean Tight Skirts for Cavity Filters
By Sam Benzacar, President
Anatech Electronics, Inc.

After a failed spectrum auction, years of hand-wringing, and bureaucratic sloth, it shouldn’t be surprising that the law actually delivering a nationwide 700 MHz public safety broadband network would be sandwiched into a piece of legislation called the Middle Class Tax Relief and Job Creation Act of 2012. Public Law 112-96 was signed into law by the President on February 22, 2012, and essentially reallocates 10 MHz of spectrum in the infamous “D-block” to public safety, which along with an existing 10 MHz of spectrum currently held by the Public Safety Spectrum Trust (PSST) will create 20 MHz of spectrum dedicated exclusively to public safety data communications throughout the nation. LTE will be the transmission technology of choice.

The figure on pg X shows how this will work, and on closer inspection shows how close these bands are to each other, with only a 1 MHz guard band separating broadband and narrowband sectors as well as between the public safety bands and the Verizon Wireless “4G” LTE spectrum. Some people participating in the law’s creation stated that guard bands weren’t really necessary at all, as wireless carriers sometimes do not use their full channel allocations, thus creating guard bands by default. In fact, the final wording of the law states that “such guard bands shall be no larger than is technically reasonable to prevent harmful interference between licensed services outside the guard bands.”

This stern language is followed by a paragraph that might be summarized as “using everything, including the beaks”. After begrudgingly throwing a bone to the need for guard bands, the law makes sure to mention that “the Commission may permit the use of such guard bands for unlicensed use”. Like the plan to use “white spaces” (snippets of spectrum between licensed services) this would according to the Law rely on a geographic database of (in this case) public safety users to ensure users’ signal do not “collide”.

Such is the state of the spectrum today -- sardine-line sandwiching of services into an unrelenting spectrum, which provides a convenient segue to a discussion about the importance of filters, and in particular the high performance cavity type.

Thanks to their exceptional selectivity, cavity filters have been a vital part of RF and microwave wireless communications system infrastructure since the earliest days of land mobile radio. Their ability to keep signals from extending beyond their channels and into other services’ allocations and vice versa make cavity filters the only solution that can meet the needs of wireless infrastructure applications, from the public safety LTE network to any other system, whether it operates at VHF or 6 GHz or delivers 5 W or 50 kW.

Like commercial LTE services being deployed at 700 MHz, the public safety networks will require cavity filters with signal rejection of -40 dB or more at 700 kHz from the edges of the narrowband and broadband segments (see the figure) and between the public safety allocations and Verizon’s LTE network. Anything less will result in degraded performance of one or both systems.

Fortunately, this level of cavity filter performance is achievable at 700 MHz along with low VSWR and insertion loss, high isolation, very low passive intermodulation distortion, and substantial power levels. The filters are inherently rugged and can easily be made to withstand the punishments of the environment for very long periods.

The new 700 MHz public safety networks will once again validate the basic usefulness of high performance cavity filters, but it most assuredly will not be the last. The aforementioned white space systems when implemented will require cavity filters too. Another part of the law creating the public safety network mandates the “give back” within the next 11 years by public safety users in 13 major metro areas of spectrum between 470 and 512 MHz they have used since 1978 and shared with TV channels 14 through 20. The spectrum to be vacated will be auctioned and the proceeds used to help defray costs for relocation of the public safety systems formerly using the band, the remaining costs likely to be the responsibility of the winning bidder. Sounds like a job for high-performance cavity filters…

Anatech Electronics, Inc.
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