Why is mobile signal weak indoors?
By SignalFix · Facts checked 2026-10-02
When the balcony shows four bars and the bedroom shows one, it's tempting to blame the network. Usually the network is fine. The walls are the problem.
Every wall takes a share
Mobile signal is radio. Radio passes through building materials, but loses strength as it does. Engineers call the loss from outside to inside building entry loss. Then every internal wall and floor takes a little more. TRAI noted in 2018 that roughly 80% of mobile traffic starts or ends inside buildings, which is why this matters so much.
The ITU's building entry loss model gives a median of roughly 15 dB for conventional buildings and around 30 dB for thermally efficient ones with metallised glass and thick concrete. Every 10 dB is a tenfold drop in power, so 30 dB means about a thousandth of the outdoor signal gets in.
What each material costs
The 3GPP channel model, used to design 4G and 5G networks, gives formulas for how much signal common materials absorb. At two frequencies used in India:
| Material | ~700 MHz (4G/5G n28) | ~3.5 GHz (5G n78) |
|---|---|---|
| Standard glass | ~2 dB | ~3 dB |
| Concrete | ~8 dB | ~19 dB |
| Coated (IRR) glass | ~23 dB | ~24 dB |
| Wood | ~5 dB | ~5 dB |
Calculated from the TR 38.901 material formulas. Real walls vary; measured studies have found the model can overstate concrete loss at higher frequencies.
Why new buildings can be worse
Energy-efficient glass is coated with a thin metal layer to reflect heat. It reflects radio signal too. US government field measurements at 1.7 GHz found entry loss of roughly 11–15 dB on the first floor of a modern building but 24–29 dB on a floor with low-e metallised glass. A glass office tower can be harder to get signal into than an old brick house.
Lifts, basements and the middle of the building
- Lifts: a lift car is a metal box inside a concrete shaft. It behaves much like a shield. More on lifts.
- Basements: signal can't come in sideways through soil, only down ramps and shafts. More on basements.
- The middle of the building: rooms several walls from any outside face get what's left after every wall has taken its share.
We don't publish a single dB figure for lifts or basements: they vary too much from building to building. That's what measuring is for.
What this means for a fix
If the problem is the walls, more signal outside won't reach those rooms. The fix is to bring usable signal past the walls, or to carry calls another way, such as Wi-Fi calling. A SignalCheck tells you which rooms are affected and by how much, so the fix fits.
Sources
- ITU-R P.2109-2 (2023): Prediction of building entry loss
- 3GPP TR 38.901 (ETSI TR 138 901 v16.1.0): channel model incl. material penetration loss
- NTIA/ITS (2024): in-building entry loss measurements
- Ulster University (EuCAP 2018): building entry loss for traditional and thermally-efficient houses
- TRAI (2018): Recommendations on in-building access (indoor share of mobile traffic)