Case Study 1: Near-Far Effect
Consider two emergency responders transmitting at the same time. Person A is standing directly under a DAS antenna, while Person B is at the edge of coverage with a much weaker signal. In a wideband (non-channelized) system, the uplink automatic gain control (AGC) reduces overall gain to accommodate Person A’s strong signal. As a result, Person B’s weaker transmission is not amplified enough to reach the public safety tower. This is the near–far effect.
To prevent this, channelized/Class A amplification is required. With per-channel AGC, each frequency is managed independently, ensuring that strong signals do not suppress weaker ones. Another mitigation is to improve coverage uniformity by adding more DAS antennas, reducing the signal strength gap between users.
Case Study 2: Equipment Placement and Choice of Coax
While designing a high-rise building DAS, the AHJ specified that public safety coverage must be provided in the UHF frequency range. Since only a limited number of OEMs manufacture UHF-band BDAs, we selected an approved solution. However, due to the large footprint of the building, a single BDA was not sufficient to overcome system losses and provide full coverage.
To address this, we redesigned the distribution system using high-quality plenum-rated coax with only 0.6 dB loss per 100 feet, compared to ~3.5 dB loss for standard coax. In addition, we optimized the donor antenna placement and headend location to minimize cable runs and maximize efficiency.
These design changes ensured that the system achieved both code compliance and reliable in-building coverage performance for first responders.
Case Study 3: Uplink Noise
It is a common misconception that higher BDA gain automatically equals better performance. In in-building RF systems, excessive uplink gain actually raises the noise floor delivered to the donor site. A higher noise floor reduces the signal-to-noise ratio (SINR) at the base station, which directly impacts audio quality for all first responders on the network.
For example, assume a base station has a 6 dB noise figure, giving a receiver noise floor of ~–127 dBm in 12.5 kHz. To maintain a SINR of 17 dB (required for DAQ ≥ 3.0), a portable needs to arrive at ~–110 dBm. If a new BDA raises the donor site noise floor by 6 dB, the effective uplink budget shrinks by 6 dB. In real terms, the maximum portable-to-site distance is effectively cut in half — a user 10 miles away may no longer be heard.
In dense metro areas, where multiple BDAs feed a single public safety base station, this noise aggregation effect can quickly degrade system performance. Even if each DAS uplink is “optimized,” the combined noise floor at the site may climb above acceptable thresholds.
The best mitigation is to deploy Class A, channelized BDAs with sharp filters and uplink squelch. These units only open a channel when a valid signal is present, preventing continuous noise transmission. By contrast, Class B BDAs amplify the entire passband at all times, including noise, which compounds the near-far effect and uplink receiver overload problem.
