Co-located (and) Broadband Pressure Inside Public Safety RF Sites

In today’s growing public safety communications environment, many densely populated public safety communications sites will house both P25 LMR transmit and receive infrastructure and mission-critical broadband systems. For example, there could be commercial LTE carriers; public safety broadband equipment; microwave backhaul systems; and in-building coverage interfaces. According to the Federal Communications Commission (FCC), public safety agencies operate across several bands including VHF, UHF, 700 MHz, 800 MHz, 4.9 GHz, and microwave bands using either Part 90 and/or Part 101 license framework with certain public safety frequencies reserved exclusively for interoperability and mutual aid purposes. As such, these allocations allow public safety agencies to engage in their mission-critical activities. However, due to the increased likelihood that strong off-channel energy will reach sensitive LMR receiver front-end components, the potential exists for increased interference.

While a front-end overload is essentially a type of interference event; it is distinct from other types of interference events. Specifically, a strong undesired signal, outside of its designated receive channel, can force one or more components of an LMR receiver’s front-end (i.e., low-noise amplifier(s), mixer(s), pre-selector(s), etc.) into compression. Once a component is forced into compression, the linear amplification of the desired signal is compromised. Consequently, the usable sensitivity of the receiver is diminished regardless of whether the wanted P25 carrier is present at a level which would typically permit reliable decoding.

Blocking Characteristics of 700/800 MHz Receive Pathways

There exists a relatively small “co-existence” margin in the 700/800 MHz public safety environment due to the fact that LMR receive channels; broadband carriers; inter-op channels; and emissions from neighboring sites can all occupy very similar spectral blocks. While receiver selectivity is used to reject unwanted off-channel energy; selectivity is not limitless. Therefore, it is possible for high-level broadband uplink/down-link energy to exceed the linear operating range of the front-end prior to the channel filter rejects the unwanted signal.

Unlike inter-modulation distortion (IMD); in the case of a front-end overload, the unwanted broadband signal does not need to generate a specific product within the pass-band. Rather, the overload process reduces the gain across the entire front-end portion of the receiver and thus reduces the effective signal-to-noise ratio (SNR) of the desired channel. This phenomenon can manifest itself as poor-quality talk-in; rising Bit Error Rates (BERs); intermittent failure of the control channel decoder; and missed inbound-subscriber-transmissions during periods of high broadband usage.

Coexistence of Mission-Critical-Broadband and LMR Services

As noted earlier; recent efforts by various organizations indicate that public-safety broadband will not supplant existing LMR capabilities. Rather, LMR will remain a primary means for public-safety agencies to communicate via voice in order to facilitate interoperability among responding agencies. CISA has published guidance regarding the integration of LMR and LTE services. NPSTC has also called for studies examining the effects of simultaneous LMR-LTE/5G service use so that interoperability continues to exist on-network/off-network. Therefore, based on previous comments; there will be increased RF-density at shared communications sites as a result of this coexistence model. Additionally, mission-critical push-to-talk (MCPTT) integration; ISSI-linked P25 systems; and broadband inter-working functions will create operational traffic flow across multiple Radio Access Technologies (RAT). Industry deployments demonstrating connections between P25 LMR systems and MCPTT broadband platforms illustrate the trend towards future public-safety communications. Nevertheless, the RF-site must continue to maintain receiver linearity while simultaneously carrying multiple-carrier loads.

LMR receiver sensitivity is commonly evaluated under controlled conditions where the desired carrier signal is separated from excess off-channel energy. Such conditions do not prevail at most dense public-safety sites. Strong adjacent/broadband emissions can cause front-end compressions that diminish the receivers’ ability to amplify weak inbound subscriber signals.
Degradation of receive sensitivity caused by front-end compressions may not produce traditional interference signatures on the intended receive channel. Degradations may be evidenced by: degradation of inbound coverage area; reduction of talk-back ranges for portable units; voting receiver unbalance; intermittent P25 frame-decoding errors; etc. TIA-TSB-88 provides guidelines for assessing performance in noise-limited and interference-limited situations, as well as procedures for quantifying how undesired signals affect system reliability. Blocking represents a form of interference limited behavior since the operating impairment is determined by the ratio between received signal strength, undesired signal power, and receiver dynamic range.

Shared passive infrastructure significantly affects block exposure. The extent to which undesired energy reaches the receiver-input is primarily influenced by multicouplers; pre-selectors; receiver-filters; duplexers; transmission-lines; and antenna-combining networks. Low-loss RF-systems are essential for maintaining receiver sensitivity. However, rejection/isolation characteristics are similarly important when broadband carriers share physical space with LMR receive systems.

In dense 700/800 MHz sites, filter-skirt performance assumes increasing importance due to proximity of broadband equipment to LMR receive paths. An older filter designed for lower-density LMR applications may not provide sufficient rejection once broadband equipment is installed in the same shelter/radio-room/tower/DA environment. Aging connectors/drifting cavities/jumper-layout changes can further erode isolation-margins. Thus, even if individual-component performance does not appear impaired, the protection-margin between the broadband environment and LMR-receiver may be compromised.

Tower-loading limitations; roof-top geometry constraints; shelter-design/layout issues; or multi-agency site-sharing practices all contribute to constrained antenna-separation distances. Reduced antenna separation results in greater broadband-isolation provided by physical separation compared to what might be achieved by passive-filtering alone. When antennas are placed in extremely close proximity, strong nearby emissions can couple into an LMR receive-antenna system at sufficiently high levels to compromise front-end-linearity.

Urban public-safety sites are particularly susceptible to this type of issue as they often include: regional interoperability communications; utility company communications; municipal-service communications; broadband-systems; and redundant/back-up systems at the same general location. While coordination-of-spectrum allows public-safety agencies to assign compatible channels; it cannot completely prevent Near-Field coupling; Cable-leakage; Shelter-ingress; or interactions-through-passive-paths. Ultimately, these site-specific mechanisms determine whether a theoretically-compatible channel-plan remains viable under actual RF-loading conditions.

Symptoms Caused By Blocked Receivers In P25 Networks

Blocked receivers often display symptoms that resemble coverage weaknesses/subscriber-equipment inconsistencies. Portables may fail to connect to a system from areas that were previously verified as being clear. Voting systems may choose a different receive-site even though the impacted site has superior expected coverage. Only during times when nearby broadband users are actively transmitting may control-channel or traffic-channel behaviors begin to degrade.

Due to blocking’s generally activity-dependent nature, short-duration maintenance-tests are unlikely to detect impairments. A receiver may perform nominally during low-broadband loading conditions and fail during high-load conditions at a public-safety site. Longer-term monitoring/Event-correlated-Spectrum Capture/Comparison-of-Receive Signal Strength with BER-performance provide better insight into the root causes of impairments.

Protective Margins For Hybrid Communication Sites

Successful mitigation begins with protecting receiver-dynamic-range. High-selectivity pre-selectors; properly-aligned cavity-filters; adequate antenna separation; proper grounding; high-quality transmission-lines; and carefully-designed multicoupler systems all serve to minimize undesirable-energy reaching the receiver-input. Protective margins must be assessed as part of the overall system functionality rather than simply as an attribute of an individual component.

TX RX Systems designs/manufactures passive-RF-infrastructure in the United States for public-safety/government/critical-communications networks where long-term-protection-of-the-front-end-for-receivers is paramount to overall system reliability. Precision-cavity-filtering; low-loss receive-distribution; and mechanical-stability-in-rf-assemblies all support predictable-performance as hybrid-LMR-and-broadband-environments grow more congested. The value of passive-infrastructure is directly measurable based upon how well it maintains receiver-linearization across time spent operating at dense communication-sites.

Realistic Performance Verification Under Simultaneous Loading Conditions

Commissioning-test methodologies should simulate simultaneous-broadband/LMR-loads rather than solely relying on isolated-narrowband-measurements. Analysis-of-blocking-phenomena requires measurement of undesired-signal-levels-at-the-receiver-input; assessment-of-pre-selector-rejection-characteristics; correlation-of-broadband-activity-with-block-events; and confirmation-of-P25-receiver-performance-under-realistic-traffic-conditions during commissioning-testing.