Incident broadband load becomes the receive path problem

Incident scenes today have more transmitters than there are frequency slots in the static LMR plans. In addition to P25 portable repeaters and interoperability gateways, incident scenes have vehicular routers, body camera docks, unmanned aircraft controllers, broadband modems, satellite terminals, temporary LTE or 5g cells, etc. In a small geographic space. So it’s not just increased bandwidth. There is a new type of radio frequency (rf) environment. As well as the increasing data capacity, the rf environment has a changing uplink duty cycle, antenna geometry and receiver exposure.
Ai video does not create another radio waveform. Ai video creates traffic behavior on existing radio waves. Real time video analytics, aerial imagery, sensor fusion increase sustained demand on upstream resources from broadband user equipment. Telemetry from drones adds command and control data, position information, payload video, return link status data and other traffic. When those streams are concentrated around an incident command vehicle or deployable cell then the area surrounding the command vehicle or deployable cell can become a short duration high density rf site. Narrowband receivers designed for voice reliability will be located near broadband sources whose activity is event driven and difficult to predict during planning prior to the incident.

700 MHz adjacency and Command Post Geometry

The 700 MHz public safety band plan presents a specific co-existence issue with respect to narrowband P25 receive paths. The FCC reallocated the Upper 700 MHz public safety spectrum such that narrowband operations occupy 769 – 775 MHz & 799 – 805 MHz while broadband occupies the lower adjacent blocks with guard bands between broadband & narrowband assignments. From a passive filter perspective, the broadband uplink region below 799 MHz is close enough to the P25 repeater input region at 799 – 805 MHz such that selectivity, antenna separation, & passive filtering determine receiver performance.
The FCC did not intend to say a Band 14 device automatically causes interference to a P25 receiver. Rather the margin depends on signal levels, physical distance, front end linearity of the P25 receiver, filter rejection characteristics, total power level of all devices active on the composite site & number of transmitters operated simultaneously. At an incident scene, many of these variables may change minute by minute based upon movement of vehicles, launching of drones, starting of Video Streams & repositioning of portable network assets.

Video Streams as uplink duty cycle drivers

Broadband Video Streams can remain active for long periods. The combination of a drone sending thermal video, mast camera streaming perimeter images and multiple body cameras feeds can keep schedulers busy even if P25 channels are quiet. From the perspective of an adjacent P25 channel, this changes the profile of undesirable energy from infrequent bursts to longer exposure intervals.
The resultant impact on the receiver depends upon whether the undesirable signals enter as noise within the desired channel, noise outside the desired channel but within adjacent channels or strong off channel signals which cause front end compression. A P25 receiver does not need an interferer present on its exact channel to lose performance. A nearby broadband uplink can reduce available dynamic range before the demodulator sees the desired P25 signal. Symptoms may appear as reduced talk-in range, intermittent decode failure or degraded gateway audio instead of continuous carrier.

Deployable cells and temporarily changing source location

Deployable broadband cells are useful in moving network capability closer to the demand created by incidents. However, they also change the local map of rf sources. A cell-on-wheels, rapidly deployable LTE system, agency-owned private broadband node establishes a known serving cell encouraging nearby devices to transmit toward it. When that point is nearby a command vehicle, antenna mast or shared shelter then the nearby broadband user equipment may represent the dominant temporary rf exposure for nearby LMR receivers.
The cell downlink power contribution comes from elevated or temporary antennas. More important than downlink power contributions from the deployable cell is often the combination of downlink power from the deployable cell and uplink power from many nearby devices. Together these two components can stress preselectors, multicouplers, receiver splitters, low noise amplifiers and junctions of both active and passive type in ways which were not indicated by single transmitter isolation calculations.

P25 receive path stress mechanisms

Protection of P25 receivers depends upon preserving usable signal-to-noise ratio (snr), linear headroom.
Thermal noise at room temperature is approximately minus 174 dbm/hz. Therefore in a 12.5 khz channel, thermal noise floor is approximately minus 133 dbm prior to addition of noise figure of the receiver.
A receiver having several decibels of noise figure has limited margin at edge-of-coverage. A few decibels of additional noise introduced by broadband desensitizes receivers materially reducing their ability to decode correctly.
Blocking of a receiver occurs when strong undesired signals cause a front end stage to depart its linear operating region.
Intermodulation products are produced when multiple strong signals mix through either active or passive nonlinear junctions and fall near or inside the passband of the receiver.
Adjacent channel desensitization occurs when filter selectivity and/or signal level do not prevent broadband signals from increasing effective noise floor seen by the receiver.
These mechanisms typically exist separately but are frequently observed together in compact command environments.

Passive Infrastructure provides receive path protection

Passive rf infrastructure carries most of the receive path protection responsibility wherein temporary broadband assets operate near narrowband P25 command receivers.
Preselectors define how much adjacent energy enters the receiver input.
Multicouplers distribute weak signals while maintaining noise figure & isolation characteristics. Cavity filters, duplexers & combiners determine what channel environment the receiver sees; either only the intended narrowband channel or a broader composition of incident scene energy.
There is still significant trade-off between insertion loss and improved rejection characteristics. Additional filtering improves rejection characteristics but increases receive path loss which raises system noise figure. At edge of coverage, a 1-degree-bel loss increase can be operationally meaningful.
Engineers must balance selectivity, insertion loss, dynamic range, antenna isolation and maintainability under high load conditions associated with temporary incidents.

Systems that survive these conditions are those with sufficient passive selectivity, linearity & isolation margin to survive the temporary density created modern communications use at incident sites.

Drone Telemetry and command link separation

Drone systems introduce multiple rf paths which may share no common band or modulation type. Control links, telemetry links, video payload links, cellular modem connections, wifi-based payload systems & remote controller backhaul can each be used at an incident.
Some drone systems place video transport on commercial or public safety broadband while others use separate command links and use broadband for video feed into a command application.
The primary rf planning concern is not just with the aircraft. Equally important is where ground support equipment is placed. Remote pilot stations, receive antennae, video downlinks receivers, broadband routers & command post gateways can sit close to narrowband P25 receive antennae. Short distances allow strong local signals to leach into receiver systems via antenna coupling, cable leakage poor grounding & insufficient front end filtering.

Site acceptance testing rarely captures the same amount of incident generated broadband loading as standard test methods. A clean P25 receiver test during commissioning does not demonstrate that same receiver path will remain stable when drone video is being sent to the command post and there are deployable cell traffic streams and many activated broadband router streams active in the immediate vicinity of the command post. The standard test condition lack the operational traffic mix.
Useful verification requires measuring the receive environment during realistic levels of broadband activity. Spectral monitoring should capture both narrowband inputs received by narrowband receivers and uplinks in nearby broadband bands. Drive testing should include areas surrounding staging locations for command posts; areas surrounding gateways; and likely drone launch zones. Logs of events should correlate P25 decode failures with uplink loading from broadband; movement of antenna masts; activation/deactivation times of deployable cells. Without making these types of correlations, problems experienced with P25 decoder functionality may be misinterpreted as poor coverage; subscriber radio behavior; dispatch audio impairment.

Margin for temporary rf density

Temporary incident generated broadband does not replace LMR technology. It increases requirements for protecting LMR receive paths from adjacent and off channel energy while broadband conveys video; telemetry; mapping; and situational awareness data. Hybrid public safety communication is now characterized by voice reliability remaining anchored in P25 for many agencies while increasing amounts of data are being conveyed using broadband technologies and shaping the rf environment at incident commands.
The design solution is not to conceptualize each technology as separate entities. The design solution is to consider an incident command center as a temporary multi-technology rf site. Receiver protection must take into account physical deployment practices; concentration of uplinking broadband energy at an incident scene; placement of deployable cells; procedures used for drone launch activities; filter selection characteristics; age related decline of legacy rf infrastructure. Systems which remains stable under these conditions are those with sufficient passive selectivity; linearity & isolation margin to survive the temporary density which modern incident communications produce.