Public Safety Sites Transition From Individual Network Systems to Shared Radio Frequency (RF) Environments

Public Safety Sites Transition From Individual Network Systems to Shared Radio Frequency (RF) Environments

Public safety communications have transitioned away from stand-alone local mobile radio (LMR) networks. Many agencies continue to rely on P25, analog mutual aid channels, conventional LMR channels, trunked LMR systems and dispatch center RF infrastructures for mission critical voice. At the same time, FirstNet and other mission critical Broadband systems are expanding the usage of LTE/5g based services for data, situational awareness, video, location, telemetry, and push-to-talk. The results of these operations will be Coexistence of each other’s systems, and not replacement.

Therefore, receiver protection has become a current engineering issue rather than a legacy maintenance concerns. Same towers/shelters/buildings/rooftops etc., could now contain narrowband receivers, Broadband base station sectors, donor antennas, bidirectional amplifier paths, microwave links, GPS timing antennas, and common passive RF distribution. The receiver protection issue exists due to the total RF environment created by multiple transmitters and not just the single transmitter operating within its licensed emission mask.

700 MHz Coexistence creates a very thin physical margin

The 700 MHz public safety band plan puts narrowband and Broadband communications very close together and requires that receiver protection be viewed as a site level design function. The federal band plan identifies 758-768 MHz and 788-798 MHz for Broadband communications. Also identified in the same rule section is the narrowband public safety segments at 769-775 MHz and 799-805 MHz the narrowband channels are based on 6.25 khz channel spacing. Guard bands were identified at 768-769 MHz and 798-799 MHz.

These allocations create practical RF Reality at co-locating Sites. High power Broadband downlinks, nearby Broadband uplinks activity, or high duty cycle narrowband transmitter do not need to exceed their regulatory limits to stress an adjacent receiver chain. Receiver Blocking, desensitization, pre-selector overload, and intermod depend on the amount of power which reaches the receiver front End after Antenna isolation, filtering, duplexing, multicoupling, cabling losses are taken into consideration.

Receiver Blocking is a front End linearity issue

Receiver Blocking occurs when a strong undesirable signal drives the receiver input stage, pre-selector low noise amplifier mixer or active multicoupler device out of the region in which weak desired signals are processed normally. The undesirable signal may be outside of the assigned receive channel. Operational symptom is loss of usable sensitivity even though the desired channel is still present and correctly licensed.

In P25 systems this can manifest as increased bit error rate, intermittent audio recovery, unstable control channel decoding or apparent coverage loss areas which previously measured strong only using received signal strength indicators. The failure mechanism is not insufficient field strength. It is reduced dynamic range of the receiver due to excessive off channel energy entering the receive path.

Mission critical Broadband introduces modulation and traffic behavior different than traditional LMR. LTE and 5g systems utilize Broadband emissions, dynamic resource scheduling, and higher peak to average power conditions. The traffic load at incident scenes can change rapidly as responders transmit video mapping data telemetry & application traffic. These conditions alter the composite RF environment at a shared public safety site.

3gpp standardized mission critical push to talk in release 13 and expanded mission critical services in later releases to include mission critical data, mission critical video, and inter working with non-LTE systems such as P25. This standards direction supports industry trend toward hybrid operation. And means RF Sites must protect narrowband receiver performance while Broadband systems increasing support operational traffic in the same geographic and spectral environment.

Receiver protection begins prior to the receiver

Effectively receiver protection begins with passive infrastructure. Antenna separation, cavity preselection, duplex isolation, transmitter combiner performance, multicoupler input filter selectivity, low loss feed lines, and controlled grounding all determined amount of undesired signal energy that will reach the receiver input. Even with a receiver with published specifications that would be expected to meet demands of an operating environment may perform poorly if the site allows excessive off channel energy into the front end.
Filter selectivity should be evaluated against real co-location energy rather than nominal channel assignments. In dense 700 & 800 MHz Sites the most important question is whether complete passive path maintains enough rejection isolation & linearity during simultaneous LMR & Broadband operation. This includes high duty cycle transmits & Broadband sector activity along with adjacent agency systems & temporary deployable resources used during events.

Adjacency channel rejection is necessary but incomplete as a metric for receiver protection. A receiver can meet adjacency channel performance requirements in a laboratory configuration while still being vulnerable to site-level overload. The field condition includes coupled Antenna energy; high out-of-band carrier levels; passive intermodulation products; cable loss changes; thermal drift; and compression of multicouplers.

System-level margin is required for receiver protection since filter networks that protect quiet environments may not protect when LTE Broadband carriers, interoperability channels and in-building coverage links are included. The margin must include expected carrier levels; worst-case Antenna-coupled energy; aging; and potential combinations of unwanted signals at the receiver input.

Passive intermodulation become more relevant under hybrid loading

Hybrid LMR/Broadband Sites introduce more strong signals into the passive infrastructure. Corrosion inside connectors; mechanical stress on cables; loose hardware; oxidized interfaces between antennas; or aging filter components can produce passive intermodulation products. If these products fall inside a public-safety receive channel, the receiver considers them as in-band interference.
Passive intermodulation is particularly difficult to track down because it may occur only under heavy composite RF loading conditions. Regular sweeps tests can demonstrate good return loss while hiding non-linear behavior. A site performing well under normal working hours can degrade when Broadband carriers and LMR channels operate simultaneously during an event scene.

Emergency responder radio coverage systems and distributed Antenna systems add another layer of complexity to receiver protection. Placement of donor antennas; gain settings of BDAs; contribution of uplink noise to public-safety receivers; isolation between service and donor antennas; and quality of filters can affect noise and interference presented to public-safety receivers. When Broadband services; LMR channels; and Building Coverage Systems exist in close proximity, receiver protection must consider both conducted & radiated coupling paths.

The main technical risk is up-link noise and off-channel energy being transported through coverage enhancement infrastructure designed for improved coverage.

A modern receiver protection design should start with measured site energy rather than assumed channel spacing. Broadband carrier levels; LMR transmit duty cycles; Antenna coupling; multicoupler headroom; filter skirt response; and passive intermodulation performance will all affect the final receiver input conditions. Broadband Coexistence testing should include operational loading conditions instead of simply two-carrier static assumptions.

Cavity high-selectivity filters & low-loss passive RF infrastructures are crucial since they maintain the receiver dynamic range prior to active-device compression. Insertion loss must be managed as it negatively affects sensitivity while selectivity must be sufficient to prevent strong-off-channel energy from reaching the receiver. The useful design target is not solely maximal rejection. Stable receiver input conditions over the life of the site is what we want.

TX RX  Systems’ relevance in hybrid public-safety infrastructure

TX RX  Systems operates in the portion of the network where receiver protection is physically defined. Passive RF infrastructure does not dictate how Broadband is scheduled nor dictates P25 protocol behavior; however, it determines isolation; combining; filtering; distribution loss; and long-term linearity in the RF path. Those are the factors determining whether a narrowband receiver is protected from increasing amounts of energy surrounding it.

US-based manufacturer TX RX Systems is involved in designing low-failure rate designs with high-precision filtering along with long-term stability in Hybrid LMR/mission-critical Broadband deployment. Since Coexistence performance depends on passive components exhibiting predictable behavior under simultaneous multi-carrier loading.
Operational verification should accompany the changing RF environment

Routine maintenance needs to move beyond stand-alone return loss checks & power checks. We need to verify receiver protection by observing spectrum activity; measuring Antenna isolation; confirming filter responses; assessing uplink noise; monitoring during representative traffic conditions during events.

The best engineering position currently is conservative. Hybrid public-safe Sites should assume increasing RF density; greater Broadband traffic; continued dependence upon LMR & tightened Coexistence margins. Receiver protection is no longer just a final accessory in design. It is a primary requirement for preserving mission-critical voice reliability while public safety Broadband continues to expand.