How CEL-FI Works – The Inside Story


We’re often asked what it is that makes our smart signal booster “smart”. The fact is, when you want to design something that’s as easy to set up and use as CEL-FI, there’s actually a lot of complex thinking that goes on inside the system. So here’s a short explanation of some of the many important ingredients that go into making CEL-FI as smart as it is in optimizing your cellular signal indoors.

A big one is echo cancellation. It’s a continuous process (or conversation if you will) that goes on between the window unit (also known as the network unit) and the coverage unit. It enables CEL-FI to have higher gain throughout the coverage “bubble” without causing network interference.

Here’s how it plays out:

  1. When starting its setup, the coverage unit streams the signal from the window unit at its lowest strength. Within milliseconds, the window unit increases its power until the moment it receives the echo of the signal coming from the coverage unit.
  2. The processor within the window unit then ramps up its dynamic echo cancellation algorithms to cancel out the echo from the coverage unit and continues to instruct the coverage unit to increase power.
  3. When a maximum echo level is reached, the window unit will “ask” the coverage unit to stop increasing its power.

This dynamic three-step echo cancellation process repeats itself continuously in real time to ensure you always get the best coverage possible..

But there’s much more to CEL-FI’s inner workings than that:

  • Carrier specific functionality: CEL-FI focuses all its power on the network it is designed and approved for, which is very important as the new FCC regulation comes into play. CEL-FI is authorized to achieve a 100dB maximum gain (more than 1,000 times the gain of traditional wideband analog boosters) by only boosting the frequencies carrying the carrier’s signal.
  • Uplink path loss calculation: This is used for automatic gain control and ensures that the system will never transmit too much signal back to the carrier’s macro network. Traditional boosters will often overdrive the base station which raises the “noise floor” for every other user on the cell. This in turn shrinks the effective size of the cell leaving subscribers on the edge of the cell stranded without signal.
  • Downlink path loss calculations: These make sure that the coverage unit adds enough gain to cover the space where the system is used – making it lower for smaller spaces and higher for larger spaces.
  • Automatic channel selection on the 5GHz link: This feature makes sure there’s no interference with Wi-Fi routers, cordless phones, baby monitors etc.
  • Uplink gating: CEL-FI automatically turns off the signal from the window unit to the base station macro network when there is no active phone call or data session in progress. What that means is less power consumption and network noise, since it prevents unnecessary transmission between the phone, CEL-FI and the carrier network.
  • Adaptive antenna system: This system, which is made up of four directional antennas, further enhances signal strength by automatically selecting the best signal.


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