Impacts on Radio Signals
Cleaned Lesson Transcript / Study Notes
In this lesson, we review the factors that impact radio signals, especially inside buildings.
Indoor public safety radio coverage comes with several challenges. Jurisdictions are generally responsible for the outdoor public safety radio networks, but those signals can weaken significantly once they enter a building.
Radio signals degrade with every layer of material they pass through. This is especially true in modern energy-efficient buildings.
1. Indoor Signal Coverage Challenges
Outdoor RF systems are designed to support emergency responders and public safety agencies. These systems are required to help keep responders safe.
However, when the radio signal reaches a building, the building itself can attenuate or weaken that signal.
Building materials reduce signal strength with every layer.
Common materials that affect signal include:
- Concrete
- Steel
- Low-E glass
- Energy-efficient windows
- Stairwell construction
- Elevator lobby construction
- Interior walls and barriers
New energy-efficient materials, such as low-E glass, are designed to block heat and sunlight. However, they also block RF signals.
Stairwells and elevator lobbies are often made of concrete, which can also have a major impact on coverage.
2. Material Attenuation
Different building materials create different levels of signal loss.
Low-E glass can cause approximately:
20 to 38 dB of attenuation
This means that less than 1% of the original RF signal may pass through the glass.
That level of loss can make a strong outdoor signal unusable inside the building.
3. Fresnel Zone
When a radio wave is transmitted between two points, it travels along a direct line-of-sight path. However, the signal also spreads out as it travels through the atmosphere.
The Fresnel zone is the area around the direct line-of-sight path where the radio wave spreads and can be affected by obstacles such as:
- Buildings
- Trees
- Terrain
- Other obstructions
This zone is important because the radio wave needs a clear path to maintain successful communication.
The Fresnel zone tapers at each end and is widest in the center.
Ideally, the first Fresnel zone should be at least:
80% clear of obstructions
At 800 MHz over a distance of one mile, the Fresnel zone has approximately a:
40-foot radius
4. Multipath and Fading
Several RF effects can affect signal quality, including:
- Scattering
- Reflections
- Diffraction
- Refraction
- Multipath
- Rayleigh fading
- Free-space path loss
- Absorption
Free-space path loss and absorption primarily reduce signal power.
Multipath and Rayleigh fading can cause signal variation even when the test instrument is not moving. This happens because the same signal can arrive at the instrument from multiple paths, and each path may be affected differently.
This is one reason averaging should be used when measuring control channels.
5. Power Is Not the Only Concern
Radio performance is not determined by power alone.
Other factors can impact signal quality, including:
- Noise
- Interference
- Time delay interference
- Overlapping signals
- Multipath distortion
A signal can have acceptable power but still perform poorly if the noise or interference level is too high.
6. SINR / SONAR
In the examples shown, power is represented by the RSSI measurement.
The blue trace represents signal power.
The red trace represents noise.
When the noise level is low, the signal-to-interference-plus-noise ratio is good.
This measurement is commonly referred to as:
SINR or SONAR
In one example, the noise level rises, possibly due to a BDA. The signal power remains the same, but the SINR drops from:
45 dB to 15 dB
That is a major reduction in signal quality and can cause delivered audio quality problems.
In another example, the signal power stays the same, but an overlapping signal causes poor quality measurements.
This shows why RSSI alone is not enough.
7. Time Delay Interference
Sometimes the same signal can arrive at different times.
This may happen because:
- Signals are coming from different towers in a simulcast system
- Signals have passed through filtering in a BDA
- Enhanced and natural signals are both present
Filtering inside a BDA takes time and causes signal delay.
The narrower the filter, the greater the delay.
The frequency does not change, but the delayed enhanced signal and the natural signal can interfere with each other. In effect, they can become noise to one another.
This is why one signal must dominate the other.
8. Digital Cliff
SINR is especially important in digital radio systems.
Analog systems gradually fade into noise as the signal degrades.
Digital systems behave differently. They work very well until quality suddenly drops off. This is known as the:
Digital cliff
SINR helps determine how close the system is to the edge of the digital cliff, even when the audio still sounds good.
9. Exterior Leakage
Exterior leakage occurs when the enhanced signal from inside the building leaks outside.
This leaked signal has passed through the BDA and may be delayed. If radios outside the building receive both the direct tower signal and the delayed leaked signal, the result can be time delay interference, or TDI.
This can create major problems during an emergency.
Examples include:
- Firefighters operating on the street near the building
- Police teams staging near an entrance
- Responders operating around the building perimeter
In these situations, radio users may receive both the outdoor macro signal and the leaked in-building signal. If the leaked signal is strong enough, communications can degrade.
BDA commissioning tools include an exterior leakage test to identify this problem.
10. BDA Congestion
BDA congestion can occur when the uplink signal from one enhanced building enters the DAS of a neighboring system.
This can cause both systems to activate and transmit to the tower.
At the tower, both signals may arrive:
- Delayed from each other
- At strong power levels
- On the same or related channel
When this happens, neither transmission may work properly.
BDAs usually include squelch or muting circuitry to help reduce this problem.
This issue is especially common in campus environments where multiple buildings have standalone BDAs instead of a shared fiber distribution system.
Final Summary
Many factors affect voice service by degrading RF signal quality in both the uplink and downlink directions.
Major causes include:
- Building material attenuation
- Low-E glass
- Concrete construction
- Free-space path loss
- Absorption
- Multipath
- Rayleigh fading
- Noise
- Interference
- Time delay interference
- Exterior leakage
- BDA congestion
Measuring power alone is not enough.
Delivered audio quality alone is also not enough.
SINR / SONAR is key to understanding the real quality of radio signals, especially in digital systems where performance can fall off quickly at the digital cliff.