Cell Phone Repeaters - What They Are & How They Work

What is a Cell Phone Repeater?

A cell phone repeater (also known as cellular repeater, amplifier, or cell signal booster) is a device used to improve cell phone reception in an indoor outdoor environment. A cell phone repeater is typically made up of three primary components:

  • A "donor" antenna - also called a "reception" antenna
  • A bi-directional signal amplifier
  • One or more rebroadcast antennas

Cell phone repeaters are most often installed inside buildings, and are used to bring signal from outdoors into the structure, negating attenuation of RF signal caused by the building materials. In-building cell phone repeater systems typically use an outdoor donor antenna to transmit and receive signal from the nearest cellular signal. The outdoor donor antenna is connected via coaxial cable to an amplifier unit which amplifies the signal, and retransmits it locally via one or more indoor antennas, providing improved signal strength inside the building.

Smaller cell phone repeaters are also used inside vehicles, boats, and recreational vehicles to improve reception for users. For more information on available cell phone booster kits and antennas, read our cell phone booster guide.

How Cell Phone Repeaters Work

The active component of cell phone repeaters is the bi-directional signal amplifier. A bi-directional amplifier amplifies signal in both the uplink and downlink directions. This allows cell phones served by the rebroadcast antennas to both send and receive data from nearby cell towers. 

A bi-directional amplifier typically contains the following components:

  • Diplexers or power splitters to split signal into multiple bands.
  • Duplexer filters to separate the uplink and downlink blocks within bands.
  • Amplifiers

The block diagram below shows a typical layout of a dual-band bi-directional amplifier:Cellular Bi-Directional Amplifier (BDA) Block Diagram

This diagram is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Requirements for Operation

In order to use a cell phone repeater to improve signal coverage, two primary requirements must be met:

  • Adequate donor signal
  • Adequate isolation between donor and rebroadcast antennas

Adequate donor signal

All RF amplifiers add noise to the existing noise floor from the donor signal. The additional noise added by an amplifier is referred to as its "noise figure." For most amplifiers, this is between 6 dB and 8 dB. In order for a cellular repeater to operate correctly, the signal-to-noise ratio (SNR) of the donor signal must be greater than the noise figure of the amplifier. Without adequate signal at the donor antenna location, no improved signal will be experienced by users.

Isolation between antennas

For correct operation of a cellular repeater, the isolation between the donor and indoor antennas must be greater than the gain of the amplifier. If adequate isolation does not exist, amplifier feedback effects will cause "oscillation."

In the US, the FCC requires that all consumer-rated boosters include automatic gain control (AGC). AGC reduces the gain of bi-directional amplifiers to ensure that the amplification is always lower than the path loss isolation between the outdoor an indoor antenna. The FCC requires that If a device is unable to control oscillation effects by utilizing AGC, it must detect the oscillation and automatically shut down.