Understanding GSM BTS Tx power levels

[Caleb Pal]

Good Evening,

I'd look at a GSM link budget planning tool to get some ideas of where
losses/gains are. Something like:
https://home.zhaw.ch/~kunr/NTM1/tools/simplified_gsm_link_budget.xls

The most influential factor is the MS has a 0dB gain omni antenna and is
fairly deaf. The BTS is much more sensitive, with a high gain sector
antenna (somewhere around 15-17dB gain). You may add diversity gain to
the BTS RX as well. So the BTS has to pump the power out to reach the
deaf MS on the downlink, but due to the BTS being more sensitive on RX,
the small MS power is OK.

Other factors on older BTS config (non-towertop) radios is coax line
loss, especially at 1800/1900Mhz. More power needed to overcome line
loss. Some have TMA's on the uplink side of the BTS to amplify the
received MS signal before going down to the BTS radio.

Hope that helps,

c

On 11/19/2020 19:41, Mychaela Falconia wrote:
> Hello GSM network side community,
>
> I am doing some research in preparation for possibly setting up my own
> GSM/2G BTS in my rural town of Middle-of-Nowhere, California to serve
> as a replacement for our current GSM/2G service provided by T-Mobile
> USA, a contingency plan to be activated if and when the evil corporate
> owners of T-Mobile shut down their 2G service.  I haven't bought my
> BTS hardware yet - I am still doing research and comparing options,
> and I also need to do some other preparatory work without which the
> BTS won't be of any use.
>
> I have a question about GSM BTS Tx power levels.  Some BTS vendors
> advertise models with up to 10 W power output, but I don't understand
> how such high power output from a BTS can be useful when MS power
> output in the other direction is limited to 1 W in high bands or to
> 2 W in low bands.  The whole point of a BTS is to provide service to
> subscribers, and in order for a subscriber to be able to make a call
> or to engage in a text conversation, there has to be two-way
> communication between the BTS and the MS: the MS needs to hear the
> downlink signal from the BTS, and the BTS needs to hear the uplink
> signal sent by the MS.  If you have a BTS that puts out 10 W on the
> downlink, wouldn't the effective range be limited at the point where
> the BTS can no longer receive the 1 W (high bands) or 2 W (low bands)
> uplink put out by the MS?  And if the range limit is set by the Tx
> power limit of standard GSM MS units, what is the point of having the
> BTS put out more power on the downlink than the very same 1 W or 2 W
> limit that applies to uplink Tx power?
>
> I can only reason that I must be missing something big here, as those
> BTS manufacturers who offer models with 10 W output surely make such
> hardware for some very valid use cases - but I would like to understand
> it better.  Can someone perhaps explain this mystery to me?
>
> TIA,
> Mychaela