The following questions and answers address many key issues related to public safety radio uplink performance and testing. They were submitted by attendees at the webinar, “Public Safety Uplink Radio Performance: Addressing Interference, BDAs, and Other Challenges” hosted by IWCE’s Urgent Communications on September 7, 2023. Answers are by the webinar panelists: David Adams, PCTEL’s Director of Market Development and President of the Board of Directors at the Safer Buildings Coalition, and Mathew Theisz, an RF engineer who has been working with the DC Office of Unified Communications (OUC) for the last six years.
PCTEL offers test and measurement solutions for many of the issues addressed below. Please contact us for more information or to receive a free product demonstration.
Q) In systems that employ TTA/TMA (Tower Top Amplifier/Tower Mounted Amplifier), uplink is almost always less than downlink RSSI. What would be the benefit of UPLINK testing for such a system?
A) To ensure the correct power level and good quality for active channels, measure the noise contribution to the site, etc. Because uplink signals tend to be lower power, objective uplink signal quality measurements can be particularly useful for verifying network quality and reliability.
Q) Doesn’t uplink testing require a second test system at the AHJ’s site?
A) That is one method; however, PCTEL provides a more efficient solution that utilizes a SeeHawk™ Monitor Remote Test Unit (RTU) at the radio site to automatically provide uplink power and signal quality measurements. Other, less efficient uplink testing methods require the involvement of a second person during testing. This is either someone at the site or someone with privileges to use the network management system. Both of these methods usually provide power measurements only, are more expensive, and are prone to human error.
Q) When Uplink tests (DAQ and RSSI) were carried out and are very good, what factors may account for poor downlink signals, sometimes such that the downlink user’s voice call could not be heard at all?
A) Answer differs with and without BDAs (bidirectional amplifiers). Local inference around the DL (downlink) frequencies, differing gains on BDAs, and DL multipath can all contribute to a poor downlink signal without affecting the uplink.
Q) Are you aware of any BDA manufacturers that have conducted a detailed test report showing that the BDA meets the P25 Performance requirements for TDMA and FDMA trunking systems? A BDA has the potential to impact the noise requirements, adjacent channel & timeslot requirements and other gain and noise related performance items.
A) Most BDAs can be used in a compliant or non-compliant manner. Without a network protect standard built into P25 (or equivalent) it is impossible for a BDA to be safe without proper Field Engineering and installation. That said, manufacturers could do a lot to make it easier to understand their BDA’s performance, and easier to properly commission.
Q) How does an improperly installed donor antenna (i.e., pointed at incorrect macro antenna) get approved?
A) When radio shops are not properly involved AND someone simply reviews a coverage test would be one example.
Q) Does the FCC Licensee assist with providing SINR (Signal and Interference to Noise Ratio) readings at radio sites if integrator is not allowed [access]?
A) Yes, the FCC Licensee and the associated radio shop personnel (or someone they designate) must be involved since they are the only ones that have access to the radio sites. DC and PSERN provide this assistance. Other major markets hire companies to do this. However, they are usually able to provide power measurements only. PCTEL provides equipment to measure power, SINR and BER automatically.
Q) What solutions are proposed with this congestion [caused by many BDAs operating in the same area], especially significant height differences in buildings?
A) Careful BDA design in tight conjunction with knowledgeable radio system personnel.
Q) Is this issue [of interference due to intermodulation] common where numerous channels are enhanced (i.e., UHF, VHF, 700, 800) where mutual aid is required?
A) While adding more bands increases the intermodulation risk, I have not heard that it is significant.
Q) How bad is the noise background sans BDAs? Are BDAs the predominate cause of network noise background?
A) Many densely deployed areas have very bad noise. Yes, BDAs are a main contributor but there are other sources of noise contribution.
Q) What’s been the usual reasoning for a midnight to 6am [time period] for these events?
A) BDAs with Automatic Level control will increase their gain to meet the “target” output level. At night when there is no noise or other radio activity, the BDAs will go to max gain until they “ring”; or there is input making them “breathe.” The nature of these characteristics makes them extremely challenging to find.
Q) Would an auto Uplink Off timer not prevent that [type of nighttime interference issue] from happening, so that the BDA isn’t trying to amplify the noise?
A) No, while this can mitigate the impacts, it makes locating an interferer much harder, and unless the uplink off is per channel and fast enough to be Phase 2 compliant, it does not solve the problems.
Q) Is there a standard or regulating body that oversees placement of BDAs?
A) The FCC and the Licensees.
Q) Would cellphones that constantly search for public WiFi hotspots be a contributor to interference?
A) Possibly, but this may be localized so it is not as serious; it may mix with radio signal causing a mollified intermodulation which could be serious.
Q) When a BDA is installed is the orientation controllable without physically touching?
A) I have not heard of anyone using rotors or steerable arrays. This would exacerbate the uplink calculation problem.
Q) Are new brick and mortar facilities accounting for interference during design/blueprinting?
A) I am not aware of it and it is doubtful; hopefully they are at least preparing for a DAS.
Q) I recently ran into a situation where the AHJ was requiring townhomes to be tested for coverage, [even though they] don’t have common areas. Is there something in NFPA code to that regard or that was just that jurisdiction requirement?
A) Both national model codes and local codes often have limitations (size, floors, type, etc.) that do not require a test or a system, but AHJs can often make their own rules. (Some states such as Florida govern at that level.)
Q) Have you found any areas of a building where inside antennas cannot be placed? Some local AHJs may have a problem in stairways and elevator shafts.
A) DC has limitations around SCIFS (secure rooms) and the fire code official settled on placing them near the doors and using a relay operator inside. But this is an issue that the local building code official and fire code official need to work out.
Q) Do Class B BDAs create the majority of the uplink interference? What contribution is [made] by Class A BDAs?
A) Type As can also have problems, which can actually be more subtle and harder to determine.
Q) Can you provide the test requirements defined for Washington DC third party testing?
A) https://ouc.dc.gov/page/oucs-public-safety-building-radio-systems-requirements
Q) For SINR and BER, why are you not featuring what NFPA 1225 identifies that are “passing”?
A) 1225 picked numbers based on the TIA (Telecommunications Industry Association)’s Wireless Communications Systems Performance Standards, but we believe the NFPA’s numbers should be adjusted based on a closer examination of TIA TSB-88. Each jurisdiction can make adjustments as they see fit.
Q) Best strategies to test uplink?
A) Automated instrument test is the best for establishing a high-quality process with consistent, accurate, repeatable measurements. It also has the lowest long-term cost.
Q) Do fiber-based campus wide designs contribute to higher risks of interference vs standalone BDA solutions per building?
A) Concerning BDA related problems, it is better. Numerous tightly spaced BDAs are very likely to cause problems.
Q) Assuming that [PCTEL’s SeeHawk™ Monitor-based automated uplink testing] solutions tap into the uplink feedline to the P25 RCVR, do any departments/agencies express concerns over the 3db loss due to splitter added to RCVR port?
A) Most radio sites already have a monitor port or multi-coupler in line; those that do not add one. Never heard a concern about the loss from these.
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