Satellite Access Moves to a Public Safety Coverage Layer

The new FCC framework for supplemental coverage from space (SCS) allows satellite and terrestrial operators to utilize terrestrial mobile spectrum via defined authorizations and interference protection. This framework utilizes FirstNet licensed 700 MHz public safety BroadBand spectrum, often referred to as Band 14, for SCS purposes provided that the required submissions and demonstration have been met. That regulatory modification supports public safety since it regards satellite access as a coverage extension for regions where terrestrial BroadBand is unavailable due to damage, overloading, geographical limitations, etc.

That being said, this change does not obviate the requirement for LMR. Many public safety organizations will continue to require LMR (i.e., P25 and others), as these narrow-band voice layers offer determinate group voice; direct operational procedure; hardened site design; and coverage behavior which agencies can trust. Satellite-to-cellular coverage will merely add another transport/access layer around an incident. The radio frequency (rf) engineering aspect of this concern is that the additional layer will alter the assumption regarding ground-based gateway sites that currently provide sensitive LMR receive paths.

Gateway Sites Provide the RF Boundaries

An LMR gateway site could be a tower shelter, dispatch interface point, regional interoperability node, transport hub, or an incident command communications kit wherein P25, conventional LMR, BroadBand push-to-talk, IP backhaul, deployable BroadBand, and donor antenna systems come together. While direct satellite access might bypass a terrestrial cell site for the handset air interface, public safety operations require terrestrial integration points for dispatch, talk-group interoperability, logging, authentication and routing between the BroadBand and LMR realms.

In essence, the gateway site becomes the physical rf boundary between the narrowband mission voice (e.g., P25) and the broader BroadBand coverage extension. When satellite-enabled BroadBand services (LTE/5g), equipment, donor antennas, transport radios, and LMR receivers reside within the same shelter/tower structure(s), the local interference environment becomes more complicated than just the satellite link budget. In fact, the highest probability of failure is not a single satellite signal overpowering an LMR receiver. Rather, the far more probable cause of failure would be cumulative local rf energy degrading the headroom of passive infrastructure and receiving systems that are already operating at their design limits.

Protecting Receivers Begins with the Noise Budget

LMR receive systems rely upon small margins. Thermal noise at room temperature is approximately –174 dBm/hz. Across a 12.5 khz wide narrowband channel, the thermal noise floor would be approximately –133 dBm prior to considering receiver RF and distribution losses. Adding a 5 dB system noise figure results in placing the practical noise reference near –128 dBm prior to accounting for signal quality margin.

This explanation illustrates how small changes matter. One decibel (dB) of added loss across a pre-selector or One dB of effective noise rise will reduce One dB of available link margin. For example, at the edge of a P25 coverage contour; in a simulcast overlap area; or inside a building where penetration loss dictates coverage — that margin can differentiate between steady-state digital decode capability and intermittent frame loss. Because BroadBand equipment brought onto a site for satellite-to-cellular continuity do not necessarily have to share an LMR channel to compromise reliability — it can simply elevate the apparent noise floor by virtue of receiver blocking; intermodulation; BroadBand leakage; insufficient antenna isolation; or nonlinear passive junctions.

Local Coexistence Risks are as Relevant as Orbital Coexistence Risks

Although the FCC SCS order addresses satellite emissions through technical requirements (including aggregate oob power flux density limits to protect adjacent terrestrial operations in bands 600 MHz, 700 MHz, 800 MHz & PCS), compliance with such limits at the satellite system level does not negate the need to design locally. Public safety gateways contain amplifiers; filters; multicouplers; duplexers; combiners; tower top devices; grounding systems; and shared transmission lines that can all contribute to local failure modes regardless of what occurs on the space segment.

Ultimately, the coexistence question is whether or not the LMR receive path possesses sufficient selectivity; isolation; and linearity to reject BroadBand energy now surrounding the site. A 700 MHz or 800 MHz public safety receiver does not recognize regulatory categories. Instead, it recognizes total rf energy presented to its input. If strong BroadBand signals; donor antenna leakage; high-power ue uplinks; or deployable BroadBand cells introduce energy into the receive chain at levels greater than the blocking tolerance of the receiver, the LMR channel can experience degradation despite each individual transmitter being compliant.

Band 14 Proximity Increases Filter Selectivity Requirements

Since Band 14 SCS relevance is particularly significant because the FCC specifies 758-769 MHz & 788-799 MHz as the 700 MHz public safety BroadBand spectrum being considered for the SCS framework, Proximity issues related to adjacent channel rejection become critical. Narrowband public safety operations and other 700 MHz public safety receive paths exist in close enough Proximity that filter skirt behavior; duplexer isolation; antenna placement; and site-specific IM analysis become major considerations in design.

Adjacent Channel Rejection is not Just a Specification on a Data Sheet

Adjacent channel rejection is not only a specification on a data sheet. Adjacent channel rejection also depends on the entire passive chain. A cavity filter whose tuning has shifted; a combiner with increased insertion loss; a compromised connector; or a shared antenna configuration with reduced isolation can transform a manageable BroadBand neighbor into a receiver protection problem. The introduction of satellite enabled BroadBand coverage increases the Risk associated with existing weaknesses since agencies are more likely to rely on both LMR and BroadBand simultaneously during the same incident.

Non-Terrestrial Network Mobility Changes Coverage Expectations

As 3gpp release 17 formally initiated work on supporting non terrestrial networks (ntns), including satellite access in frequency range 1 (fr1) for handheld devices as well as supporting machine type communications (mtc), releases 18 & subsequent releases will continue addressing NTN service continuity; mobility; coverage enhancements; and satellite access in additional bands. These standardization efforts are relevant to public safety since they transition satellite connectivity away from a specialized terminal model toward integrating satellite connectivity with typical cellular operations.
However, satellite connectivity continues to exhibit rf characteristics that distinguish itself from terrestrial BroadBand. Movement of beams; changing satellite visibility; Doppler correction; propagation delay; handover behavior; and gateway routing create a coverage layer that does not correlate exactly with P25 sites or terrestrial LTE sectors. Therefore, at a public safety gateway site, this creates operational stressors to bridge two disparate networks whose physical coverage boundaries and timing behaviors are different. Thusly, designing rf systems so that LMR receives continued stability while BroadBand access transitions between terrestrial and non-terrestrial paths represents an engineering challenge.

Passive Infrastructure Bears Stability Responsibility

While engineers have little control over what transpires on the space segment, engineers do bear responsibility for maintaining passive rf system behavior. Whether or not hybrid coverage functions properly depends upon receiving pre-selectors rejecting unwanted BroadBand energy without adding too much insertion loss. Multicouplers must maintain noise figures and dynamic ranges when subjected to higher composite rf loads. Combiners and duplexers must preserve isolation as site temperatures change; vibrations occur; and duty cycles vary. Feedlines and connectors must prevent nonlinear junctions that generate passive intermodulation products within active receive channels.

Therefore, passive rf component manufacturers are responsible for providing manufacturing precision and long-term stability. TX RX systems manufactures filters, combiners and related rf components designed specifically for public safety applications where low-loss/high-isolation/stable mechanical interfaces Impacts long-term reliability of receivers. Value-added is not achieved through obvious network features. Instead, it is achieved through maintaining predictable passive behavior as rf densities increase around the site.

Verification Under Incident Loading Conditions Required

Commissioning tests evaluating each subsystem individually may fail to capture failure modes generated by simultaneous operation. A site may test satisfactory in terms of sweeps, return loss measurements and isolated receiver sensitivity tests yet fail under load when LMR repeaters, BroadBand gateways, deployable cells, satellite-enabled devices and/or donor antenna systems operate contemporaneously.

To accurately verify a site’s performance under composite rf conditions measure the receive path under concurrent composite rf conditions. Spectrum monitoring should observe BroadBand noise rise; discrete IM products; desensitize during LTE/5g uplink bursts; and P25 receiver BER changes. Compare overlap zones, building penetration areas and gateway transition points where satellite-assisted BroadBand & LMR are expected to function concurrently during field testing. The objective is to demonstrate not only regulatory compliance but also receiver protection under load conditions representative of public safety usage.

Hybrid Public Safety Site Design Considerations

Satellite-to-cellular coverage has evolved from an experiment toward becoming a viable extension of public safety BroadBand capabilities. Consequently, the Implications on public safety rf design represent an increasingly congested gateway environment where continuous BroadBand availability; stable P25 voice services; interoperability routing; and passive infrastructure must be treated as an integrated system.

Designs that are most reliable will treat satellite connectivity as merely another rf density source rather than as an independent transport solution. Reliability of designs will ultimately be determined by factors such as receiver protection & antenna isolation; filter selectivity & passive IM control; grounding quality & long-term environmental stability in order to ensure hybrid coverage enhances operational resiliency without undermining agency reliance on traditional LMR.