Atmospheric conditions can carry CBRS 3.5 GHz signals far beyond their intended range. When that energy arrives at your network, it presents as sector degradation with no identifiable local source. This happens more than most operators realize. The diagnostic path that finds it is different from every other interference scenario.
The call pattern looks like this: multiple subscribers on the same sector report degraded speeds. SINR is down across the board. RSSI is unchanged. You pull a spectrum scan and see elevated noise on your operating channel. You check for new local operators in your coverage area. Nothing shows up. The interference appears intermittently, worse in the evening, sometimes absent in the morning. No hardware faults. No new construction nearby. The degradation comes and goes on its own schedule over three to seven days and then largely disappears.
This is a tropospheric ducting event. The interference source is not in your market. It is not in the adjacent market. It may be operating 80 to 150 miles away, and under the right atmospheric conditions its signal is arriving at your sector with enough power to materially degrade your SINR floor.
Under normal atmospheric conditions, radio signals at 3.5 GHz propagate along a relatively predictable path. The atmosphere bends and absorbs signals in ways that are well-modeled and well-understood. Normal propagation follows the standard free-space path loss curves that link budget calculations are built around.
Tropospheric ducting occurs when temperature inversions in the lower troposphere, typically 0 to 2 km altitude, creating a layer where the refractive index of the atmosphere changes sharply with altitude. This layer acts as a waveguide, trapping radio signals and allowing them to propagate far beyond their normal range with dramatically less attenuation than free-space path loss would predict.
At 3.5 GHz, ducting events are most common in coastal and near-coastal regions during late spring, summer, and early fall, particularly in the evening hours when temperature inversions are strongest. Inland regions experience ducting less frequently but are not immune, particularly during stable high-pressure periods.
The signals that duct are not weak. A CBRS CBSD operating at full Category B power (47 dBm EIRP) that normally produces -85 dBm at 10 km can arrive at 100 km during a strong ducting event at power levels that are competitive with your own signal at the subscriber's CPE. When that happens to a subscriber sitting on the edge of your sector's coverage footprint, the arriving interference may be only 6 to 10 dB below your signal, enough to collapse the SINR from acceptable to marginal or below.
Every other interference scenario in CBRS has an identifiable local cause. A new co-channel operator. A CPE in an overlap zone. A SAS registration error producing unexpected frequency assignments. You can identify each of these by looking at what changed in your immediate operating environment.
Tropospheric ducting has no local cause. The interfering signal is coming from a CBSD that is operating legitimately in its own market, causing no problems there, and accidentally degrading your network as a byproduct of atmospheric physics. There is no regulatory mechanism that prevents this. The SAS does not manage inter-market interference at these propagation distances because under normal conditions no inter-market interference exists.
Operators in coastal and near-coastal markets should treat tropospheric ducting as a baseline operational risk, not an edge case. The combination of marine layer temperature inversions, coastal geography that concentrates ducting channels along the coastline, and the high density of CBRS deployments in coastal metropolitan areas creates conditions where ducting events occur multiple times per year in some markets.
Operators within 50 miles of a major coastline operating on GAA spectrum channels with no PAL protection should consider this risk when evaluating their frequency planning strategy. A ducting event affecting a GAA-only operation has no regulatory remedy. The interfering signal is arriving legally, from a legal source, and neither the SAS nor any regulatory process provides rapid relief.
The diagnostic process for a suspected ducting event is different from standard interference investigation. You are not looking for a local source. You are correlating your network behavior with atmospheric conditions.
The honest assessment of tropospheric ducting interference is that the atmospheric mechanism is not controllable. You cannot prevent a temperature inversion from forming over the ocean. You cannot prevent a distant CBSD from operating on your channel. You cannot file an interference complaint against a source that is operating legally and causing harm as a byproduct of atmospheric conditions that neither you nor they created.
What you can control is your network's resilience to it:
Most operators who experience a ducting event do not identify it as such. They experience a period of degraded performance, dispatch field resources that find nothing wrong at the CPE level, watch the interference resolve on its own, and file it as an unexplained anomaly. If it happens again, they repeat the process.
The operators who identify ducting as a pattern are the ones who track their interference events against atmospheric conditions over multiple occurrences. This takes no additional technology. It requires correlating your NMS data against publicly available weather records after the fact. Three correlated events is enough to establish the pattern with confidence.
Once the pattern is identified, the network management response changes. Unnecessary truck rolls during identified events are avoided. Subscriber communication is prepared and deployed. Channel strategy during events is pre-planned. The interference still arrives. The operational response to it costs less and produces better subscriber outcomes.
CBRS spectrum strategy, GAA vs PAL analysis, interference identification, and the failure pattern documentation for co-channel and adjacent-operator scenarios are in the NGWA Standards.