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
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
Hi Caleb,
Thanks for the explanation regarding "proper" BTS setups having much more sensitive antennas for receiving uplink.
In my case, I will be operating my BTS indoors inside my apartment (no tower), it will be a sort of clandestine operation squatting on an unused frequency *without* the usual billion dollar license, and the antennas hooked up to the Tx & Rx ports on the indoor BTS will be the ordinary omnidirectional kind. At first I was thinking about using the same kind of 5 cm antennas as these ones sold by Sysmocom:
http://shop.sysmocom.de/products/ant-sma-quad
I have a big bag of SPK ECL antennas similar to the above, except that mine are the straight version - I use them with all of my GSM MS development boards - but now that I think about it, there is another antenna which probably has higher gain which I can probably use for the BTS:
https://www.sparkfun.com/products/8347
If I use either of the above, the Rx and Tx antennas on my BTS will probably be a little better than the internal antenna built into most cellphones, but it will be comparable to the MS antenna if the MS is one of my development boards. :)
The two BTS models I am currently considering as my top choices are either the PCS band version of ip.access nanoBTS (used from ebay) or sysmoBTS 1002, and both of these seem to have the same maximum Tx power output of 23 dBm. Am I correct in assuming that if my BTS puts out 23 dBm, then I will hit my range limit when the MS stops being able to hear the BTS, before I hit the limit in the uplink direction? Any guesses as to what kind of range I should expect?
I don't expect that my illegal BTS will ever serve any users other than just me - as I understand it, the total number of people *on the entire planet* who would voluntarily wish to use a 2G phone (as opposed to 5G/whatever) will never exceed maybe 10 users max planetwide, and I am pretty certain that right now I am the only active 2G user in all of California, if not in all of USA. As of today, T-Mobile USA still has active 2G service across all parts of California which I frequent these days - my travels nowadays are much less than pre-Covid, but I still occasionally drive as far south as downtown San Diego and as far north as almost hitting LA, and so far I have working service almost everywhere. But I can only reason that all those GSM cells must sit there 100% idle with exactly zero traffic almost all of the time, except during those brief moments when I happen to drive by that cell in my car - I don't see how it can be any other way when absolutely everyone else has standard sheeple 4G/5G phones. The only T-Mobile 2G cell that has nearly constant activity on it is the cell that serves my apartment, as that is where I stay 99% of the time in this Covid-driven reality.
Thus I can only reason that T-Mobile USA currently maintains a GSM/2G network that has *only one active user* perhaps in the entire country, so it is understandable that they are threatening to shut it down. If they shut it down before I kick the bucket, I am thinking of setting up my own illegal BTS (on the very same ARFCN on which wrongfully- shutdown T-Mobile service used to be) inside my apartment, connected to a VoIP tunnel on the back end. It won't help me at times when I have to drive somewhere, but it would be neat if it can cover areas within walking distance of my apartment - the two grocery stores in our little rural town both happen to be within walking distance.
Sorry for ranting, Mychaela
Hi Mychaela,
Your concerns are understandable given what happened e.g. in Australia earlier (they switched of 2G completely). However the current trend amongst operators seems to be keeping 2G and 4G while switching off 3G. 3G turned out to be the worst technology for carriers, because it does not have good coverage for simple voice calls and also it is quite cumbersome to operate while data rates are inadequate for today's subscribers. So this means 2G is kept for good coverage, basic voice service and 4G is kept for high data rates, assisting 5G in coverage etc. Some coverage about this in the news: https://www.vodafoneziggo.nl/en/verhalen/end-3g-era/ https://www.telekom.com/en/media/media-information/archive/bye-bye-3g-now-lt...
Of course in the US the situation might be different because of frequency allocation differences, nevertheless I'd still place my bets on 2G staying with us for a long time. Naturally you are free to do whatever you wish, and having a BTS at home is great in many cases even if commercial service is available as well. I just wanted to chime in on the discussion about 2Gs possible future not trying to change your mind. I only have limited knowledge about BTS ranges, one time I tried to operate a BTS with an SDR board (USRP B200) and you could probably guess how bad it was :) - inaccurate clock, almost no TX power, resulting in a mighty 5m or ~6yards :). With a good clock source and an amplifier though I think even with an SDR one could have good ranges, however the burden of operating a complete PC just for handling the SDR makes buying a nano/sysmoBTS much more attractive.
Kind regards, Domi
On Fri, Nov 20, 2020 at 6:25 AM Mychaela Falconia mychaela.falconia@gmail.com wrote:
Hi Caleb,
Thanks for the explanation regarding "proper" BTS setups having much more sensitive antennas for receiving uplink.
In my case, I will be operating my BTS indoors inside my apartment (no tower), it will be a sort of clandestine operation squatting on an unused frequency *without* the usual billion dollar license, and the antennas hooked up to the Tx & Rx ports on the indoor BTS will be the ordinary omnidirectional kind. At first I was thinking about using the same kind of 5 cm antennas as these ones sold by Sysmocom:
http://shop.sysmocom.de/products/ant-sma-quad
I have a big bag of SPK ECL antennas similar to the above, except that mine are the straight version - I use them with all of my GSM MS development boards - but now that I think about it, there is another antenna which probably has higher gain which I can probably use for the BTS:
https://www.sparkfun.com/products/8347
If I use either of the above, the Rx and Tx antennas on my BTS will probably be a little better than the internal antenna built into most cellphones, but it will be comparable to the MS antenna if the MS is one of my development boards. :)
The two BTS models I am currently considering as my top choices are either the PCS band version of ip.access nanoBTS (used from ebay) or sysmoBTS 1002, and both of these seem to have the same maximum Tx power output of 23 dBm. Am I correct in assuming that if my BTS puts out 23 dBm, then I will hit my range limit when the MS stops being able to hear the BTS, before I hit the limit in the uplink direction? Any guesses as to what kind of range I should expect?
I don't expect that my illegal BTS will ever serve any users other than just me - as I understand it, the total number of people *on the entire planet* who would voluntarily wish to use a 2G phone (as opposed to 5G/whatever) will never exceed maybe 10 users max planetwide, and I am pretty certain that right now I am the only active 2G user in all of California, if not in all of USA. As of today, T-Mobile USA still has active 2G service across all parts of California which I frequent these days - my travels nowadays are much less than pre-Covid, but I still occasionally drive as far south as downtown San Diego and as far north as almost hitting LA, and so far I have working service almost everywhere. But I can only reason that all those GSM cells must sit there 100% idle with exactly zero traffic almost all of the time, except during those brief moments when I happen to drive by that cell in my car - I don't see how it can be any other way when absolutely everyone else has standard sheeple 4G/5G phones. The only T-Mobile 2G cell that has nearly constant activity on it is the cell that serves my apartment, as that is where I stay 99% of the time in this Covid-driven reality.
Thus I can only reason that T-Mobile USA currently maintains a GSM/2G network that has *only one active user* perhaps in the entire country, so it is understandable that they are threatening to shut it down. If they shut it down before I kick the bucket, I am thinking of setting up my own illegal BTS (on the very same ARFCN on which wrongfully- shutdown T-Mobile service used to be) inside my apartment, connected to a VoIP tunnel on the back end. It won't help me at times when I have to drive somewhere, but it would be neat if it can cover areas within walking distance of my apartment - the two grocery stores in our little rural town both happen to be within walking distance.
Sorry for ranting, Mychaela
-
Caleb Pal -
Mychaela Falconia -
Tomcsányi, Domonkos