Shared Spectrum strategies, like the USA Citizens Broadband Radio Service “CBRS”, generally will have incumbent usages within the spectrum that will be either technically accommodated or moved in the spectrum planning and implementation process. The accommodation process is to prevent negative effects on both the incumbents, and the new residents.
In the context of CBRS, the US Navy has radar systems that operate across a big part of Band 48. From this US Navy presentation:
The AN/SPN-43C Radar Set is an Air Search Radar (ASR) used for Aircraft Carrier and Amphibious Assault Ship Air Traffic Control, used on LHA, LHD, and CVN type ships.
Because these systems are woven into the electronic war fighting fabric of the US military, they will not be altered. But, the good news is CBRS enabled RAN equipment, as a new resident of the spectrum, is designed to avoid conflict with radars in coastal areas, and some inland areas where they may be used. There are identified coastal port areas in the CBRS spectrum plans that clearly identify where these radars are expected to regularly operate.
What’s the Monster Under the Bed? It’s this: the mis-information on these radars, and their “disastrous effects” have caused some in the RF planning community to think CBRS is not viable as a national strategy due to the risk of service outages when ships fitted with these radars arrive in a coastal port.
So, let’s grab our flashlight, peek under the bed, and hunt for the Monster. As part of the hunt, we’ll document radar technical capabilities, and shipboard operations.
Relevant technical capabilities:
- Operating frequency range: 3500-3650 MHz
- Transmission bandwidth: 6 MHz (Google presentation)
- Operates at 10 Mhz increments across spectrum
- At irregular intervals, will move to different frequency
- Radar is subject to mechanical wear (the antenna weighs 1.66 tons, and rotates at 15 RPM) so it’s idle when there are no flight operations
- Flight operations, that require this radar to be active, will occur within range of coast
- Radars will occasionally transmit in port as part of routine maintenance activity
With these technical and operational details defined, it’s apparent that things are not bleak for CBRS in coastal areas where naval radar systems will operate.
This is a brief comparison of spectrum of naval radar to CBRS:
- Only 100 MHz of the 150 MHz assigned to CBRS overlaps with radar
- When the radar operates in it’s lower 50 MHz range, it does not overlap CBRS
- CBRS channels are 10MHz wide with PAL range of 3550-3650 MHz and GAA range of 3550-3700 MHz
- GAA channels in 3650-3700 MHz range will not be disturbed by radar activity
- PAL channels (3550-3650 MHz) completely overlap with radar range
- The radar’s 1.6 MHz transmission bandwidth on a 10 MHz increment can result in an “incumbent protection event” affecting up to two CBRS channels
After chatting with a number of engineers about the above summary, it was explained that while up to two channels could be affected by each active radar, the Spectrum Access System “SAS”/ Environment Sensing Capability “ESC” does not just blanket move each Citizens Band Service Device “CBSD” (CBRS radio) to a different channel. The SAS/ESC does heavy lifting by determining appropriate individual changes for each CBSD that is in-scope for the incumbent protection event.
For CBSDs on impacted channels, the SAS will run interference calculations to determine how each one must be affected to protect the incumbent. The SAS could send commands to the CBSD to have it move to a different channel, or reduce its power. Or, the SAS could determine that no action is necessary. This implies that in a given port, many CBSD’s that are farther from the active radar or shielded from it behind other buildings will operate undisturbed even though they are occupying the same channel(s).
In summary, we think that the “Monster Under the Bed” of naval radar that has caused some angst for RF people disappears when we shine the light of decent information on it.
Sleep well, my friends.
– Art King, SpiderCloud Wireless, Director of Enterprise Services & Technologies