Definition
DAS (Distributed Antenna System) distributes RF signals from the source (RRU, base station) to multiple remote antennas (RAU) via optical fiber or coaxial cable, achieving uniform coverage in scenarios like indoor areas or tunnels. Compared with the independent deployment of small cells, DAS shares the signal source, supports multi-standard (2G/3G/4G/5G) co-feeding, and is suitable for large venues, subways, airports, and other wide-area indoor coverage, reducing the cost of repeated construction for multiple systems.
Types and Architectures
- Passive DAS: composed of power splitters, couplers, feeders, and antennas, with no active components; low cost but high loss and limited coverage range
- Active DAS: remote units contain amplifiers; baseband or intermediate frequency is transmitted via optical fiber; large coverage range; supports long distances and multiple floors
- Digital DAS: transmits digital IQ or CPRI via optical fiber; remote DAC/ADC converts to RF; supports multiple frequency bands, easy to expand, mainstream for 5G indoor coverage
Selection between DAS and Small Cell
DAS is suitable for large-scale, multi-standard, and uniform coverage; Small Cell is suitable for local hotspots and high capacity. They can be deployed in a mixed manner: DAS is responsible for overall coverage, while Small Cells supplement capacity in hotspots. The coverage quality of DAS directly affects the access success rate and speed of indoor IoT devices.
Typical Use Cases
Indoor coverage for subways, airports, shopping malls, office buildings, stadiums, hospitals, tunnels, etc. SimtoLink's IoT SIM cards can operate normally in DAS coverage areas, allowing devices to maintain stable connections indoors.
DAS systems improve cellular network coverage in indoor scenarios, enabling IoT devices to maintain stable connections in subways, shopping malls, and other indoor environments. SimtoLink's IoT SIM cards function normally in DAS coverage areas, and the SimtoLink cloud platform can monitor connection quality as devices transition between indoor and outdoor environments, providing connectivity assurance for IoT projects deployed in complex indoor scenarios.
After combining, the power each radio standard receives has to be counted on its own
A distributed antenna system often combines several radio standards onto one path and then sends them onward. Every combiner port has insertion loss, and every split in the downstream divider takes another cut from the power at a single antenna. One standard can look strong enough at the source output and already sit below the device’s minimum receive requirement at the antenna at the end of a corridor. Do not judge from the power meter on the source cabinet alone. Subtract combiner loss, feeder loss, and splitter loss per standard, and get the power at each antenna port.
If a new radio standard is patched straight into the old combiner, the port match of the existing standards changes, standing wave gets worse, and coverage of the original service shrinks with it. If IoT uses the narrower of those standards, keep a power budget for it on its own. Do not assume it shares evenly with the broadband standard used by phones.
Judge trunk loss with a link budget
A passive distribution sends the signal out through feeders and splitters. Once the distance is long, loss hits the limit before the coverage area does. Full bars on a phone in the lobby do not prove that the antenna port in the elevator shaft or the underground garage still has margin, because the lobby may sit very close to the source. The link budget should list every loss from the source to the farthest antenna, and leave margin for aging connectors and temperature. When that margin is used up, the device can still find the cell, but upload fails, because uplink power hits the ceiling first.
An active distribution puts amplification at the far end and runs the trunk over fiber, so loss no longer grows in a straight line with copper length. The remote unit still needs power. A remote that loses power takes out a whole floor at once, while a neighboring floor covered by another remote stays normal. That is different from one IoT device failing. See first whether every card on that floor disappeared together.
A source fault silences the whole antenna run together
When many antennas share one source, and the source drops, every antenna on that trunk loses the radio standard together. On site this is easy to misread as “every card in this building is bad.” If a fiber span in a digital distribution breaks, the affected area is the remotes behind that fiber, not the whole building. Draw the shared scope first, then decide whether to replace equipment or replace cards. If there is signal at the antenna port and the source alarm is still on, the monitor port and the service port may not be the same path. Use the service port.
A SimtoLink IoT SIM card uses the same attach procedure under indoor distribution as it does outdoors. The card does not know whether the signal comes from a distributed antenna or a macro site. If cards on the same floor drop together and a comparison card outdoors stays normal, point the cause at the source or the remote in that building, not at a batch suspend-and-restore. Metal ceilings and shafts indoors can make uplink fail first while downlink can still receive broadcast. Explain that asymmetry with the uplink budget. Do not conclude from downlink level alone.
When uplink noise rises, the power reading at the antenna port can still look normal
Downlink power of a distributed antenna can be measured at the antenna port. Uplink is what the device transmits, traveling back along the same antenna system to the source. When interference lifts the noise floor, the downlink power meter is still normal, the device still shows full bars, and uplink fails because the signal-to-noise ratio is not enough. The interferer may be an active device on some floor oscillating, or leakage after another system was combined in. Judge from the noise floor at the source’s uplink port, not only from phone bars in the corridor. If the noise floor rises on only one remote span, that remote is the failure domain. If every remote rises together, the cause is closer to the source or the combiner.
If new IoT devices use a band the indoor system does not carry, and the combiner has no port for that band, the devices grab a weak outdoor macro cell. The person is indoors and the cell is an outdoor site. A SimtoLink card does not switch onto the indoor system just because it walked into the building. It attaches to the most suitable cell it measures. If indoor cards on the platform keep an outdoor macro cell identity, this distributed antenna does not cover the standard or the frequency the card uses. Add it on the combiner and the source, rather than replacing cards in a batch.