Building Penetration Loss
Building penetration loss (BPL) is the difference in signal strength between the outdoor reference point immediately outside a building and a measurement point inside. It is expressed in decibels (dB). A loss of 10 dB means the indoor signal power is 10 times weaker than the outdoor value. A loss of 20 dB represents 100 times weaker.
3GPP standardizes indoor penetration loss models for use in coverage planning. The ITU-R P.2109 recommendation provides statistical models based on frequency, building type, and whether the model is for a traditional or thermally-efficient building.
Material Attenuation Values
Different construction materials cause different levels of signal attenuation. These values are frequency-dependent and increase at higher frequencies:
| Material | Attenuation @ 900 MHz | Attenuation @ 1800 MHz | Attenuation @ 3.5 GHz |
|---|---|---|---|
| Wooden wall (interior) | 3–5 dB | 4–6 dB | 6–8 dB |
| Brick wall (1 layer) | 5–8 dB | 7–10 dB | 12–18 dB |
| Concrete wall (15 cm) | 10–15 dB | 12–18 dB | 20–30 dB |
| Reinforced concrete | 15–25 dB | 20–30 dB | 30–40+ dB |
| Standard glass window | 2–3 dB | 2–4 dB | 4–6 dB |
| Low-E coated glass | 15–25 dB | 20–30 dB | 30–40 dB |
| Metal door / frame | 25–35 dB | 25–35 dB | 35–45 dB |
Values are approximate ranges from published ITU-R and 3GPP material databases. Actual attenuation depends on material thickness, composition, and moisture content.
Effect of Frequency on Indoor Penetration
Lower frequency signals penetrate buildings more effectively than higher frequency signals. This is the primary reason why German operators use 700–900 MHz spectrum for wide-area coverage and indoor penetration, while 3.5 GHz 5G NR has very poor building penetration and requires local small cells or indoor distributed antenna systems (DAS) for adequate indoor 5G service.
A device in a building may therefore connect to the operator's 4G LTE network on 800 MHz even when 5G NR at 3.5 GHz is available outside, because the lower frequency provides a more reliable indoor link.
Energy-Efficient Glazing: A Growing Problem
Modern buildings in Germany increasingly use low-emissivity (Low-E) glass with metallic coatings to meet energy efficiency standards under the EU Energy Performance of Buildings Directive. These coatings, while thermally effective, act as effective radio frequency shields, attenuating mobile signals by 20–30 dB or more.
Buildings constructed or renovated after 2010 in Germany frequently have this glazing type, explaining why indoor coverage in newer structures is often worse than in older buildings, even when the outdoor signal is strong.
Femtocells and Small Cells
A femtocell (also called a Home Node B or Residential Gateway) is a small base station that connects to the internet via broadband and provides local cellular coverage. It communicates with devices using standard LTE protocols, making it transparent to the device — the phone sees it as a normal cell tower.
Femtocells are available from some German operators for residential use, though availability has declined as Wi-Fi calling became widespread. For enterprise buildings with persistent indoor coverage issues, larger picocells or distributed antenna systems (DAS) are the standard solutions, though these require operator coordination and significant installation effort.
Wi-Fi Calling (VoWiFi)
Voice over Wi-Fi (VoWiFi) allows voice calls and SMS to be routed over a Wi-Fi network instead of the cellular radio. When a device detects poor cellular signal, it can automatically switch to VoWiFi if a Wi-Fi connection is available. The call quality is comparable to VoLTE (Voice over LTE).
All three German MNOs support VoWiFi for qualifying devices and plans. It must typically be enabled in both the handset settings and the operator account. Emergency calls (112) via VoWiFi are supported on German networks in accordance with the EU regulatory requirement.
Passive Antenna Repeaters
Passive signal boosters use a directional outdoor antenna to capture the strongest available outdoor signal and re-radiate it indoors via a coaxial cable and indoor antenna. Unlike active amplifiers, they do not require power and introduce no network signaling changes.
In Germany, the use of active signal amplifiers (active repeaters or signal boosters) requires Bundesnetzagentur approval and operator coordination, as unauthorized amplifiers can cause interference to the network. Passive systems using only cable and antenna elements have fewer regulatory constraints, though installation must still comply with building regulations.
Frequently Asked Questions
Why is my signal worse in newer buildings?
Modern energy-efficient buildings use metallic coated glazing and denser insulation materials that significantly attenuate mobile signals. This is particularly pronounced for high-frequency bands (1800 MHz and above).
Does going near a window improve signal?
Standard glass attenuates signals by only 2–4 dB, so positioning near a window can improve signal if the window faces a nearby base station. However, low-E coated glass (common in modern buildings) attenuates by 15–30 dB — nearly as much as a concrete wall — so window proximity may provide no benefit.
Can I legally use a signal booster in Germany?
Active signal amplifiers require type approval and, in practice, operator permission to avoid causing interference. The Bundesnetzagentur can issue fines for unauthorized use of active boosters. Passive systems (outdoor antenna + cable + indoor antenna, no amplification) generally do not require approval but should be reviewed against the specific product's regulatory documentation.