Hidden Failures of the P25 Trunked System Control Channel
Public Safety Communications (PSC) agencies rely upon reliable P25 trunked system control channels for successful registration, affiliation, assignment of channel grants, and attachment to a given system resource. While the failure point is typically identified as poor voice audio quality, the first indication of a “hidden” control channel decode margin failure is frequently marginal control channel decode performance which can occur when the received signal strength (RSSI) is considered adequate on either a service monitor or subscriber unit. Densely populated broadband (BDB), Broadband Data Access (BDA), and emergency operations centers utilizing mobile command vehicles will likely exacerbate this situation due to their influence upon the interference environment surrounding the receive path of each subscriber unit without altering the nominal level of the desired P25 carrier.
The degree to which control channel margin is compromised by environmental interference is directly related to the usable ratio between the desirable control channel carrier frequency and all sources of interference and noise located within the receiver’s passband. While a public safety receiver may exhibit strong RSSI values, if its ability to perform symbol recovery and frame decoding has been degraded by interference that lies within the receiver’s passband or causes receiver compression, then coverage area maps generated using only RSSI values are inaccurate representations of true operational reliability.
Proximity of Narrowband and Broadband Services Within the 700 MHz Band
Due to the fact that both narrowband and broadband services are used within the 700 MHz public safety spectrum, there exists significant potential for interference problems arising from shared use of this band. According to FCC Part 90.531, narrowband public safety frequencies lie between 769-775 MHz and 799-805 MHz with a channel spacing of 6.25 kHz; whereas broadband allocations lie within the bands 758-768 MHz and 788-798 MHz. Although a six megahertz internal guard band separates these two sets of allocations, this separation does not completely protect against interference issues in an actual RF site environment.
Practically speaking, this issue arises from site behavior. At an incident site location, a combination of narrowband P25 receivers, broadband user equipment, broadband donor links, temporary deployable cells, in-building radio systems, and command vehicle antennas may exist within close proximity to each other. Although frequency separation represents one aspect of a protection strategy, other aspects such as antenna isolation, filter selectivity, receiver linearity, passive intermodulation resistance, cable integrity, and site noise temperature determine whether a sufficient amount of decode margin will remain available for control channel functions during full operational loading at the site.
Uplink Loading Effects Due to Mobile Command Post Operation
It is becoming increasingly common for mobile command posts to support a variety of communication platforms through the use of cellular routers, Long-Term Evolution (LTE) and Fifth Generation (5G) devices, video terminals, mapping systems, Body Worn Camera Gateway systems, Unmanned Aircraft Vehicle (drone) control links, telemetry receivers, and Portable Data Devices (PDUs). FirstNet has publicly stated that public safety broadband will enhance existing LMR capabilities and provide deployable broadband resources for use at incidents, planned events, disasters, and remote areas. While the addition of these technologies enhances incident-related data gathering capabilities for public safety organizations, they also introduce additional levels of local RF activity in the vicinity of LMR receivers and gateway equipment supported by command vehicles and staging areas.
The greatest threat to the P25 control channel is not merely co-channel interference. Interference from strong nearby broadband uplinks can increase the effective noise floor at a receiver location, cause receiver front-end stages to become overloaded (i.e., compressed), or generate intermodulation products when they interact with other high-power carriers in either passive or active RF paths. A receiver experiencing blocking stress may experience a reduction in decode margin for the control channel even though the interfering carrier(s) is/are located outside the nominal P25 channel bandwidth.
BDA Noise Impact Upon Control Channels
Emergency Responder Radio Coverage Systems (ERRCS) and Broadband Data Access (BDA) systems have become widespread in structures where external public safety coverage is deemed inadequate. Section 90.219 of the Federal Communications Commission’s Rules describes requirements regarding signal booster operation for Private Land Mobile Radio Service Licensees and acknowledges both the engineering implications of intermodulation products and noise as well as establishing specific limits for noise emissions within a 10 kHz measurement bandwidth, including passband noise, noise greater than 1 MHz removed from the passband, and intermodulation product emissions.
In terms of the overall impact of BDA upon a region-wide P25 control channel, several factors come into play including BDA gain characteristics, BDA passband characteristics, donor antenna isolation characteristics, uplink path characteristics, and the number of systems being retransmitted by the BDA. If a poorly designed BDA configuration results in excessive amounts of wideband noise added to a receiver’s environment at a donor site or retransmits energy that was not intended to be part of the original system design, then BDA could potentially compromise a region-wide P25 control channel. Generally speaking, Class A BDA configurations employing narrow passbands and properly engineered filtering tend to establish a more defined RF boundary than wide-bandpass systems; however, each BDA configuration will ultimately be dependent upon measurable antenna isolation characteristics, stable gain characteristics, and coordination among licensees who are impacted by the BDA.
Loss of Control Channel Decoding Functionality Prior to Voice Failure
Prior to failing voice communications at a subscriber unit location, subscribers may experience difficulties associated with decoding functionality loss of their P25 control channels. These symptoms can include subscribers’ inability to consistently affiliate with preferred sites; failure to obtain required channel grants; remaining assigned to a less optimal site; or regaining acceptable communication function only after relocating a few meters. Symptoms indicative of impaired decoder functionality are often misinterpreted as software anomalies or high-talk group assignments rather than actual RF-based failures.
While analog-type noise tends to manifest itself immediately prior to audible coverage failures in voice communications systems, digital impairments to control channel decoding functionality can remain undetected until the errors exceed the capabilities of error-correcting/synchronization recovery algorithms. The threshold behavior exhibited by small increases in interference coupled with large jumps in message losses as soon as the demodulator transitions beyond the usable carrier-to-interference ratio performance is why measuring received signal strengths alone cannot effectively identify degradation to P25 control channel decoding functionality in dense environments.
Temporary RF Loading Conditions Resulting from Incident Staging Areas
Incident staging areas can bring together an assortment of transmitters that were never envisioned during initial site-planning efforts. For example, a single command vehicle may deploy multiple antennas on a small rooftop platform; numerous portable radios operating in high-duty-cycle modes may be positioned near the command vehicle; a temporary broadband cell may be activated; a drone video downlink transmitter may be operating; and/or an indoor BDA system may be retransmitting the same regional public-safety system. As a result of this increased concentration of transmitters within a relatively confined area, a short-duration RF environment develops that contains substantially higher concentrations of carriers than would normally be expected under regular day-to-day operations.
These conditions subject previously unforeseen limitations in passive infrastructure components such as duplexers, pre-selectors, multi-couplers, transmitter combiners, cavity filters, and coaxial interfaces to stresses caused by varying numbers of nearby carriers. As age takes its toll on connector material properties; moisture damage degrades feedline properties; oxidation affects junction properties; and thermal drift effects alter filter properties; the aforementioned component types will degrade performance characteristics that negatively affect the margin available to each active receiver stage. Since the control channel must continue to be decodable throughout this time period while all other RF carriers change position relative to the receiving subscriber unit’s location; this creates additional challenges for engineers responsible for ensuring reliable operations.
Measurements That Will Quantify Signal Integrity vs. Reliability Performance
Engineers assessing control channel performance must consider more than simply quantifying received field strength. The engineer should assess whether variations in adjacent channel energy levels; broadband uplink activities; BDA uplink noise; site ambient noise floors; receiver input powers; passband responses; and/or message integrity levels correlated with control channel decode performance. Spectrum monitoring performed during actual incident-style loading conditions will provide more meaningful assessments than tests conducted under quiet-hour conditions.
Using carrier-to-interference ratios; bit-error rates; symbol-quality metrics; noise-floor variability; and receiver desensitization metrics will allow engineers to distinguish between reliable coverage resulting from strong signals and unreliable decode margins.
Testing should incorporate active broadband devices; operational BDA paths; router functions contained within command-post operations; and realistic antenna installations. Therefore, testing should quantify the RF environment experienced by each P25 receiver during operational stresses rather than quantify the environment when all adjacent carriers are inactive.
Role of Passive RF Infrastructure in Receiver Protection
Protection afforded receivers in this type of environment relies heavily upon the performance attributes of passive RF infrastructure components. Utilizing passive components possessing high-passband selectivity pre-selectors; cavity-filters exhibiting stable-frequency response characteristics; low-loss receive-distribution feeds; multi-coupler designs offering high-isolation characteristics; and mechanically-stable connectors preserves margin by suppressing unwanted carrier energies before they reach active receiver stages. Passive component performance also eliminates introducing additional sources of noise into an already crowded shelter-environment.
TX-RX Systems manufactures passive RF infrastructure components specifically designed for public safety/government/critical communications applications where long-term stability is essential. Where maintaining narrowband receiver protection becomes necessary due to increasing levels of broadband/BDA/mobile-command activity in hybrid public-safety networks; engineers recognize that low-insertion-loss characteristics; predictable frequency-response behavior across time; high-isolation characteristics; and mechanical precision provide means to minimize likelihood of hidden-path degradation mechanisms affecting receive performance.