MCX Maturity Moving Toward Operational Interoperability
Broadband for mission critical applications is transitioning from standards discussions into actual operational interoperation. ETSI’s tenth Mission Critical Services Plugtests, which tested mission critical services (mc) in 4g and 5g networks, indicated an interoperability success rate of approximately 92% of the overall tests conducted. The tests evaluated mission critical Push to talk (MCPTT), Mission Critical Data (MCData), and Mission Critical Video (MCVideo). The focus was placed upon the ability to interoperate with other public safety communication systems including those utilizing P25 and TETRA.
It doesn’t necessarily mean that every public safety agency has completed their deployments. Rather, the Broadband service layer is maturing rapidly, so legacy LMR infrastructure is facing increased opportunities for operational coexistence with Broadband-based voice, video, data, dispatch and situational awareness systems.
The 3GPP mission critical services were initiated with MCPTT in Release 13 and have since been extended to include common mission critical functions, MCData, MCVideo, interconnects between mission critical systems, and interfaces to LMR systems. Therefore, the path forward is clear; Broadband services are becoming more robust, more standardized and thus more likely to appear next to existing P25 infrastructure. Consequently, coexistence Risk is shifting away from future planning and into the rf shelter, tower, donor antenna path, and receive multicoupler rack.
Coexistence Risks Will Move Beyond Legacy LMR Sites
Public safety Broadband does not eliminate the requirement for protecting legacy LMR Sites. As noted previously, nist’s continued research into public safety communications emphasizes the transition and coexistence nature of integrating LMR and Broadband rather than simply replacing one technology with another. Similarly, the public safety Broadband authority’s investment strategy outlines expanding the public safety Broadband network via a stand-alone 5g core, band 14 evolution, deployable capabilities, and mission-critical voice, video, data, and location services that augment local radio systems.
That augmentation model further increases the burden on legacy LMR Sites. Regional P25 systems may share a tower compound, rooftop, shelter, battery plant, grounding system, transmission line corridor or building coverage interface with Broadband cells, donor antennas, DAS paths, BDA infrastructure, microwave links, gps timing antennas, command post gateways, and mobile Broadband assets. While the gateway may be digital in nature, the failure mechanisms continue to be physically based. Thusly, receiver blocking, raised noise floor, passive intermodulation, transmit noise leakage, inadequate isolation and poor filter skirt management can degrade a narrowband receiver before any application-layer interworking occurs.
Gateway Site Risk Evolves Beyond Coverage
MCX/P25 interworking shifts emphasis towards gateway locations. Gateway Sites typically provide local P25 coverage, rssi connected traffic, dispatch integration with Broadband Push-to-talk data exchange video sessions and incident command connectivity. Henceforth, there exists a distinct Risk profile associated with gateway Sites as opposed to standalone repeaters. Gateway Sites are no longer merely a coverage asset; they serve as an interface between Broadband and narrowband systems. Any degradation at these points can potentially impact multiple agencies or service types simultaneously.
Simply referring to coverage maps does not address this issue. A P25 user may achieve a strong control channel while experiencing a reduced head room in the base station receiver due to nearby Broadband uplink activity. Conversely, a dispatch console may display normal registration while RF defense impacts incoming portable performance. Further still, a Broadband command application may transport traffic while a co-located LMR receiver experiences elevated levels of unwanted energy entering ahead of its pre-selector. For this reason, hybrid operations require RF protection analysis at the port level; i.e., not simply logical interoperability testing.
RF Receivers Require Isolation In Hybrid Broadband Environments
Narrowband receivers are specifically engineered to extract weak desired signals from surrounding ambient energy. However, dense Broadband environments decrease this margin by increasing the total amount of energy seen by the receiver front-end. High-power Broadband uplinks near command areas, high-powered Broadband downlinks near shared infrastructure and numerous public-safety transmitters within the same shelter can each decrease the useful receiver dynamic range.
Of greater importance are selectivity, insertion loss, isolation, noise figure, power-handling and thermal stability. Although a filter that improves rejection characteristics can improve overall system performance; however, if the filter introduces unacceptable amounts of loss into the system chain then it can actually decrease the overall system noise figure. Furthermore, although tower-top amplifiers can recover losses introduced into the feed-line by providing active gain; this active gain only recovers losses when out-of-band energy is properly managed prior to applying active gain. Finally, while a multicoupler can divide receiver signals among various connected receivers with efficiency; however it can also carry unwanted Broadband noise and overload products into each connected receiver. An additional 3 dB of noise-floor increase equates to an additional 3 dB reduction in link-margin and in many cases represents the difference between receiving a usable inbound-audio signal versus periodically losing inbound-decode functionality.
Systems Engineering Focuses Upon The Active/Passive Rf Layer
Systems engineering applies to TX RX systems regarding mcx coexistence issues since standards-compliance alone cannot ensure adequate protection for the antenna port. Texas-based TX RX systems provides rf-conditioning equipment throughout north america for VHF/UHF/700mhz/800mhz/900mhz LMR applications. TX RX systems’ product/service portfolio encompasses rf-filters, duplexer/filter combinations, transmitter combiners/receivers-multicouplers/tower-top-amplifier/DAS/BDA-support/passive-interference-mitigation/noise-analysis/and rf technical services.
In addition to supporting Broadband-enabled applications projects related to hybrid public-safety modernizations; TX RX systems views hybrid-public-safety modernization as primarily a site-protection project. Before additional Broadband services are layered above it; the physical receive-path must be characterized at each site. In addition, passive-components must exhibit sufficient stability to maintain consistent impedance values, low-loss insertion characteristics while maintaining high-isolation characteristics and predictable filter behavior for extended durations of service. At shared-site locations where Broadband P25 donor antenna mobile command assets occupy adjacent space; the passive layer serves as a control surface determining whether clean interoperability is achievable or degraded.
Characterizing Impairment Pathways
As stated earlier; solutions exist for mcx/LMR coexistence problems when viewed as measurable RF impairments rather than vague interoperability concerns. TX RX systems can assist in identifying undesirable pathways for energy flow; quantifying site-noise; selecting optimal filtering methods; preserving receiver sensitivity; maintaining isolation between transmit and receive infrastructure; etc.
Band-pass filters/pre-selectors reduce undesired energy flow into active-devices. Duplexers/transmitter/combiner systems minimize cross-channel interaction when shared antenna usage is necessary. Multicouplers enable efficient distribution of receiver-signals while maintaining active-gain/loss/noise contributions under engineer-control. Tower-top amplifiers can amplify weak-signal levels provided that proper filtering-overload protection techniques are implemented. Filtering DAS/BDA systems facilitate preventing unwanted-energy returns to donor-site or shared-receive-paths.
These are not software remedies; they are rf-controls allowing application-level interoperability to function on a “clean” physical layer.
Verify Hybrid Operations Under Realistic Traffic Conditions
Realistic traffic conditions must be modeled during hybrid-site verification activities. Sweep-test results/standalone frequency-checks often fail to detect temporary overloads/temporary inter-modulations/Broadband noise coupling between ports. MCX use cases incorporating video-telemetry-group calling-and/or deployable-Broadband-capabilities can produce operational conditions that differ significantly from typical LMR voice-loading scenarios. Therefore, realistic traffic models should comprise spectrum-observation, noise-floor-measurement, receiver-desensitize checks, port-isolation reviews, filter response validations, performance-correlation tests under representative Broadband activity.
This is where manufacturers possessing RF conditioning/experience in field-service activities add value. TX RX systems can provide support for evaluating individual Sites using an approach focused on measuring the rf-path from antenna input port to receiver input port. The objective here is not to impede Broadband-adoption; rather it is to ensure that Broadband-adoption does not consume the remaining receive-margin that P25 systems continue to require for reliable mission-critical inbound voice operations.