The Arduous Process of Finding a Home for 5G at C-Band
by Justin G. Pollock, PH.D., KP Performance Antennas
Achieving wide-area coverage requires huge amounts of expensive infrastructure and still may result in dead spots. As wireless carriers deploy millimeter-wave 5G infrastructure, the limitations of this spectral region are revealing themselves: Achieving wide-area coverage requires huge amounts of expensive infrastructure and still may result in dead spots, and that’s just outdoors; indoors is equally vexing. Thus the mad rush downward in frequency to squeeze every bit of spectrum from the so-called mid-band or “sub-6 GHz” region. As analysts are frequently noting, the mid-band region is the sweet spot for 5G, even though the industry is feverishly building out and promoting its millimeter-wave capabilities.
That said, as there is precious little spectrum available below 6 GHz that isn’t already occupied by other services, so the Federal Communication Commission has stepped up its efforts to find solutions. Its “Facilitate America’s Superiority in 5G Technology (FAST) Plan”, in addition to millimeter-wave bands, targets 600, 800, and 900 MHz as well as 2.5, 3.5, and 3.7 to 4.2 GHz (C-band). This, the commission has noted, could unleash up to 844 MHz for 5G. It’s a complex and massively comprehensive initiative as every spectral region under scru.tiny has its own challenges when attempting to satisfy everyone.
The C-band region is an excellent example of just how difficult this will be. The 500 MHz between 3.7 and 4.2 GHz has been used for more than 40 years by satellite operators as the downlink path for sending television programming back to cable head-ends on Earth. Every provider uses this distribution method, from cable, fiber, and streaming (Figure 1).
Figure 1 • A C-band earth station antenna, typical of what’s used at cable head-ends. (Source: SES)
Keep in mind during this discussion that in play is a total of 500 MHz, which although substantial isn’t as much as it would have been before the need to stream 4K (and ultimately 8K) video, virtual reality, and other demanding applications. Realistically, all the users of this spectrum will not be “refarmed” to provide additional bandwidth for 5G. Instead, they’re likely to be sharing it with 5G and possibly Wireless Internet Service Providers (WISPs) while most likely losing some of it as well.
WISPs, of which there are thousands in the U.S. and Canada, have since 2000 been providing Internet access mostly to rural areas that cable and wire.less providers have generally ignored, as it would not be profitable to serve them. Most are small companies that since 2000 have been using whatever unlicensed spectrum available as licensed spectrum is either unavailable or far too expensive to acquire. Nevertheless, they have been increasing their capabilities every year as new technology is introduced.
Antennas must be able to support this performance, as well. A good exam.ple is KP Performance Antennas’ four-port KP-3SX4-33, a 33-deg. sector antenna (Figure 2) designed for 3.5 GHz to 4.2 GHz operation. It has gain of 18.8 dBi, uses ±45 deg. slant polarization, and is compatible with 4 x 4 MIMO C-band radios. Its highly directional pattern and suppressed azimuth side-lobes allows for multiple sectors utilizing the same channels on each cell, sav.ing precious spectrum.
A Long Road...
The history of this initiative began well before the FCC’s FAST plan. In 2016, the Fixed Wireless Communications Coalition (FWCC), consisting of pro.viders, users, and manufacturers serving the WISP industry, filed a petition with the FCC. The coalition proposed modifications to the coordination procedures in C-band that govern the Fixed Satellite Service (FSS) and asked for it to be shared with WISPs. The FWCC stated that current procedures governing C-band are spectrally inefficient, with significant amounts of unused spectrum, and that the policy inherently makes it impossible for point-to-point services like WISPs to operate there.
Not surprisingly, satellite operators and content pro.viders opposed the FWCC’s proposals, although most other commenters supported them or at least called for a reexamination of the coordination procedures. They also supported a broad inquiry about rule changes that could support more intensive terrestrial fixed wireless use through shared access while also protecting incum.bent operations from harmful interference.
In 2017, the Broadband Access Coalition (BAC) peti.tioned the FCC to authorize the use of the entire 500 MHz for licensed point-to-multipoint broadband services on a shared basis. The coalition consists of the Wireless Internet Providers Association (WISPA) and individual WISPs such as Rise Broadband, and manufacturers such as Mimosa Networks and Cambium Networks, and others representing the interests of rural areas.
It required C-band licensees to refresh their list of earth stations to remove those not actually registered or operating. The WISPs would be allocated 20-MHz chan.nels in their coverage areas and no more than 40 GHz in a specific area. If at least three competitors are licensed in an area, licensees could obtain up to 160 MHz. The proposal relies on the use of automated frequency coordination (AFC) to provide protection of incumbents, a technology that bears discussion later in this article.
Then last year, the FCC issued an Order and Notice of Proposed Rulemaking (NPRM) calling for “flexible use” of the C-band spectrum to provide additional frequencies for 5G. The plan incorporates the idea of shar.ing the spectrum with WISPs as well.
Although the NPRM is densely packed with technical, regulatory, and other issues, the gist is that it pro.poses to add mobile applications to the band, possibly transitioning transition some or all the band to wireless broadband services, using band on a shared basis, determining whether the incumbent C-band operators actually need the entire 500 MHz to provide coverage (and thus reducing their allocated spectrum), and a variety of others. Regardless of the approach or approaches ultimately approved, existing satellite operations would be protected from interference as the band is used more extensively by other services.
As with all NPRM’s, the FCC called for comments on the plan’s 100 pages of possible solutions, and the response has been substantial, as wireless carriers, the satellite providers, content providers, the Wireless Internet Service Provider Association (WISPA), and many others have proposed various solutions. In each case, funding of the required modifications or realloca.tions would be provided by spectrum auctioned to wire.less carriers. The major commenters include the following coalitions:
- The C-band Alliance: A coalition formed last year by Intelsat, SES. Telesat, and Eutelsat, all major com.mercial satellite operators. They were later joined by content providers CBS, Discovery, FOX, The Walt Disney Company, Univision Communications Inc., and Viacom. Their plan is to reallocate current downlink frequencies in the lower 200 MHz of the band, leaving 300 MHz available for auction to wireless carriers. The alliance is adamant that co-channel sharing with fixed wireless access as proposed by WISPA, Microsoft, and Google is undesirable because of the use of automatic frequency coordination. They believe the approach could be com.pleted in 3 years.
- America’s Communications Association (“ACA Connects). This coalition consists of the Competitive Carriers Association and Charter Communications, Inc., and members representing specific interests. Their plan aims to realize at least 370 MHz of spectrum for 5G by replacing satellite distribution to fiber on a nationwide basis. This, according to the coalition, could be achieved in 18 months in urban areas, 3 years in most other areas, and 5 years in a few (no doubt rural) “select areas”. Costs incurred in achieving this (assumed accord.ing to ACA-funded a study) would be about $7 billion and would be derived from the proceeds of the auction of the 370 MHz to wireless carriers.
- WISPA, Google, and Microsoft: This proposal relies on frequency sharing of the entire 500 MHz between the incumbents and point-to-point and point-to-multipoint microwave used by WISPs. This would give WISPs access to more spectrum needed to increase data rates and provide better overall service that what their most.ly unlicensed current frequencies can offer. The plan would rely extensively on coordinated frequency sharing, like what is being used in the Citizens Broadband Radio Service (CBRS) and TV white spaces.
Automatic frequency coordination (AFC) is appear.ing more and more frequently in discussions about creating more bandwidth, whether for wireless carriers or other services. It requires real-time coordination of incumbent users and potentially thou.sands of unlicensed devices, typically relies on sensors at the incumbent loca.tions to detect potential interferers, and is managed by a very sophisticated cloud-based management system. An example of the decision-making process is shown in Figure 3.
Figure 3 • This example of AFC from the Dynamic Spectrum Alliance shows the link registration, automatic decision-making, and database coordination processes required to protect incumbents.
There are several versions of the approach, such as the three-tiered Citizens Broadband Radio Service (CBRS), which is managed by a dynamic database and shares the 3550 to the 3700-MHz region with incumbent U.S. Navy radars. Its two lower tiers include both licensed and lightly-licensed use.
Another approach referred to as opportunistic, unlicensed use of unused spectrum by frequency and location, is used in the TV white spaces, and an upcoming FCC plan is to use it for unlicensed sharing in the C-band uplink frequencies between 5.925 and 6.425 GHz and frequencies between 5925 and 7125 MHz. The method proposed for 3.7 to 4.2 GHz is coordinated, licensed sharing using database coordination. None of these systems are simple either in concept or deployment, but they will feature prominently in the future. Whether or not they are used in the downlink C-band scenario is uncertain as various commenters are either wary of it or simply refuse to consider it.
And the Winners Will Be…
As this is written, there is no consensus among the numerous organiza.tions and their coalitions as to the outcome of this often contentious situa.tion. What’s certain, however, is that wireless carriers will gain benefits from the ultimate FCC ruling, as it’s their need for more spectrum to support 5G that’s driving the initiative. The incumbents, including the satellite operators, content providers, and content distributors (mostly the cable compa.nies), will need to make changes, which range from moderate to extensive.
The WISP industry and future of rural broadband are, as usual, the least likely to benefit, based on what’s occurred before, and their proposal for frequency sharing based on AFC isn’t sitting well with other coalitions. None of these proposals will be easy or inexpensive to implement, including a full transition from satellite to fiber for content distribution.
The takeaway from all this is that finding “new” mid-band spectrum is extraordinarily difficult and that the process now unfolding at C-band will be repeated at every other band in question below 6 GHz, and even at millime.ter-wave frequencies as high as 94 GHz. Wi-Fi is also expanding, as even though Wi-Fi 6 (formerly called IEEE 802.11ax) is just now rolling out in routers and soon in smartphones, the next version (presumably called Wi-Fi 7) is in development.
In July, a consortium including Apple, Google, Facebook, Qualcomm, and many others met with the FCC’s technical staff to propose the use of frequencies around 6 GHz, including the use of AFC to protect incumbent users in the same band. Users should expect downstream data rates of a blistering 30 Gb/s when it arrives three or four years from now.
About the Author
Justin Pollock is senior antenna engineer at KP Performance Antennas in Edmonton, Alberta, Canada. He is responsible for the design, prototyping, and characterization of antennas for point-to-point and point-to-multipoint applications used in backhaul, client premise, and access point radio equip.ment. Justin received his BSEE and PhD degrees from the University of Alberta.
This article was originally featured in High Frequency Electronics.