Power Plays – How To Get Out Of The Coverage Hole

22122010303_1 Over Christmas, I was staying at a place away from home that had predictably little network coverage. Right at the place where I was staying the network operator's coverage map had a peculiar hole. So I took a UMTS power amplifier with me that is available, for example at these places, for a less than 200 Euros.

And indeed, at that place I had no 2100 MHz UMTS coverage at all that could be detected by my devices and even 900 MHz GSM coverage was very weak. The power amplifier I brought along has a directional antenna with an opening angle of 60 degrees and a total gain of 7 dbi. Pointed towards the next city with a UMTS base station, my coverage jumped from nothing to -89 dbm and an EcNo of -4. Throughput reached was beyond 6 MBit/s in the downlink direction and 2 MBit/s in the downlink direction. Incredible!

The picture on the left shows the setup. For local distribution I used my Huawei D100 I bought a couple of years ago, basically a 3G to Wi-Fi bridge that uses a 3G dongle for UMTS connectivity. The dongle itself is in the cradle of the power amplifier that induces the signal it picks up via the antenna (not shown) into the antenna of the stick and vice versa. The roll of cable on the right of the picture is the extension cable to the antenna, which I put on the cabinet above. No need to even put it outside the house, just directing it to the nearest UMTS base station was enough.

Over the days I've been at the place, the setup has proven its worth as the Wi-Fi signal of the D100 just perfectly received everywhere. Next time, I should bring a femto along as well so voice telephony also benefits from the setup 🙂

7 thoughts on “Power Plays – How To Get Out Of The Coverage Hole”

  1. That is fantastic Martin!
    I bet you bring a whole arsenal of mobile goodies when you travel, but 6Mbps at a remote location is a godsend.

  2. Interesting. It would seem that the “total gain” you refer to is that of the antenna alone, not the antenna plus the amplifier, since a rise of “nothing” (say -115 dBm) to -89 dBm implies over 25 dB of end-to-end gain, net of 15 m of high-loss coax. It’s hard to tell from the power amp’s technical specs, but I’m guessing that there’s amplification in both the downlink and the uplink: it would be extraordinarily unusual for the uplink to be closable while the downlink RSSI is negligible. In other words, it would seem that this setup is functioning as an RF repeater. Given the antenna’s 60-degree beamwidth and 25 dB of gain, there’s a good chance that you were desensitizing the uplink of every cell in the nearby city. (Uplink desensitization is one reason why RF repeaters are used so sparingly in CDMA-based networks.) So while you were enjoying the outstanding performance, the poor folks of that neighboring city might have noticed an unusual rise in their call drop rate. 🙂

    Note also that this setup might technically have been illegal since private citizens aren’t normally permitted to radiate in spectrum owned by the carrier, except when utilizing type-approved mobile devices.

  3. Hi David,

    yes, interesting indeed! I am also a bit puzzled between the value of 7 dbi given with the product and the rise in reception signal level. But perhaps the 7 dbi refers to the total system amplification of the uplink power.

    Note that the product is sold officially and has a certificate from the German regulator that it complies with EU law concerning such products and is allowed for use by individuals. It is not a repeater, which are, as you point out quite correctly not allowed to be used by the public.

    So taken those two things together I don’t think there was a risk of desensitizing the uplink for others (i.e. it is an approved product and the uplink gain is likely the 7 dbi given). I wonder, however, of how close the product could be used to a base station without that happening. To a certain degree the base station can control the uplink power so it can tune the device down, but there are certainly limits.


  4. An interesting posting. I was so intrigued I pointed it out to a few of my former regulatory colleagues (and we had quite a debate); one of whom is far more informed than I, he said:

    Not only is there a risk of upsetting the cell balance but all amplifiers add noise to the signal, the limiting factor in a cell is the total noise on the uplink. Anyone using a device of this type is therefore taking more than his fair share of the noise budget, operators would therefore be entitled to charge more.

    However, the problem of cell balance is less serious than one might think because the base station normalises the cumulative signal from each incoming code such that they all remain relatively equally readable by instructing the mobile to turn down the transmitted energy. Less signal would go into the amplifier and therefore less would come out of it. There would only be a problem if the device was used in an area of good coverage and the mobile was unable to reduce power sufficiently.

    The effect of the antenna is also interesting, theoretically it is
    directional and pointing at the base station and therefore helps the
    neighbouring cells but think about the cells beyond the one being
    pointed at. Normally, at the edge of a cell, the distance to the cell
    beyond is three times the distance to the cell being worked, but if you are working a cell from outside it then he might easily be only twice
    as distant, a few minutes with your calculator will reveal that you will be eating up the limiting noise of the cell beyond too. If I were a network operator I would be tempted to charge a lot more for amplified users – however detecting such users is going to require a serious software change. Remember that the power at the base station being worked has to be the same, Log(3) is 0.4771 and Log(2) is 0.3010. 1/0.4771 = 2.096, 1/0.301 = 3.322 and 3.322/2.096 = 1.585. That is definitely an undesirable increase in noise from a distant terminal.

    The conclusion from my regulator friends is that such devices are undesirable from an operator’s perspective and could potentially impact more than one cell. And that’s before considering the impact of blocking interference on nearby adjacent channel operators.

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