Cel-Fi LTE signal booster

T-mobile works fine for me, except in my house. I think that the nearest T-mobile cell site is about a half-mile away, but the signal path is filled with old growth (signal absorbent) trees. I can’t use my cell phone on the ground floor, and it’s usable in only a few spots on the second floor.

Neither T-mobile’s tech support people nor its store personnel have helped. I read that T-mobile is now providing “signal boosters” to customers with weak signals in their homes. Apparently AT&T and other carriers offer similar systems.

CelFi LTE booster from T-mobileOn Friday, I fetched a new T-mobile “Personal CellSpot 4G LTE Signal Booster” from the T-mobile store, after paying a $25 deposit.

My booster is the “Cel-Fi” model RS3 or DUO, manufactured by San Diego-based Nextivity. It consists of two small boxes — the window unit and the coverage unit — and their wall wart power supplies. The window unit receives T-mobile’s LTE or HSPA signal (presumably at 1700 MHz), demodulates it, and transports the data via a 5 GHz unlicensed UNI link to the coverage unit. I placed the window unit on the second floor and the coverage unit on the ground floor.

cel-fi system schematic in house

What is its theory of operation?

Apparently the system is essentially a repeater. I have no idea how completely it demodulates the tower’s signal before creating the in-house signal. Is the in-house signal that’s transmitted by the coverage unit on the same frequency as the tower’s signal that’s received by the window unit? I don’t know, but I doubt it. Nextivity merely states that the coverage unit “cleans up” (whatever that means) the signal. Neither unit has any user interface other than some front panel LEDs.

Does it work?

Placement of both units is critical. I needed about an hour to get the system working throughout my house. Without field strength measurement instruments, I relied upon the limited information that’s provided by the units’ front panel LEDs. It works.

I’ve found almost no technical information about this system except a bit in a thread on Howardforums and a press release regarding Nextivity’s use of 1/4 and 1/2 Watt output power amplifiers in this product. If you have technical information — especially antenna radiation patterns — on this product, please let us know.

What if it quits working?

Occasionally (maybe once a week) the received signal from the Coverage Unit drops to one bar and/or my phone reverts to a slow EDGE connection. I’ve found that resetting the Cel-Fi system restores signal strength and LTE speeds at my phone. Follow these steps, in sequence:

  1. Remove power to the Coverage Unit
  2. Remove power to the Window Unit
  3. Wait 30 seconds
  4. Restore power to the Window Unit
  5. Restore power to the Coverage Unit
  6. Wait a few minutes while the two units establish a good wireless link and the Coverage Unit adjusts its output level

You should eventually see a full 5 bars received signal strength at the phone.

Visit my website: http://russbellew.com
© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695

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