Hi Enzo, > Hello. I am working on setting up an osmoTRX based base station. Where I > am in the US, between 902 and 928 MHz is part of the ISM spectrum, and > unregulated. Therefore, I would like to configure my base station to use > both an uplink and down link frequency within this range.
Being likewise located in USA, I have given much thought to the same issue. > However, none > of the pre-existing ARFCN options have both the uplink and downlink > frequencies within this band. This situation holds because ARFCNs are defined by GSM specs, not by Osmocom or any other FOSS community body, and there does not exist any standard GSM band that puts both DL and UL in the 902 to 928 MHz range. > Is it possible to manually shift the > frequency used by the base station, or manually select the up/down > frequencies so that both remain within this range? The base station side is easy - on an SDR-based BTS we can do whatever we like, just add the code to support the new unofficial pseudostandard of our own invention. The part that will bite is the phones - no standard, unmodified GSM phone can ever be made to change its duplex spacing; if the DL (transmitted by the BTS) is somewhere in the 925 to 928 MHz range, then the phone will transmit its UL in 880 to 883 MHz range, obeying 45 MHz duplex spacing of the GSM900 standard. <LAWBREAKING -- please skip this part if you don't like such> One possibility is to be a naughty boy / naughty girl (we are talking USA here, hence American English slang expressions ought to be OK) and just let phones transmit at a law-breaking UL frequency while your BTS (the thing you actually operate) transmits at an unlicensed frequency in the ISM band. For example, if you set your band to EGSM900 and set your ARFCN to 975, your BTS will transmit its DL at 925.2 MHz - the frequency on which you transmit is within the ISM band, so you are not breaking laws with the equipment you actually operate and with your continuously-transmitted signal. Phones will transmit their UL at 880.2 MHz, which clashes with LTE Band 5 DL - but those are just people's phones, not equipment which *you* operate, and their transmissions are bursty, not continuous, hence much harder to catch. It is also worth noting that LTE Band 5 (formerly GSM850, and simply called "cellular" prior to that) is divvied up between carriers by FCC in a peculiar way: in the DL direction, 869 to 880 MHz go to carrier A and 880 to 890 MHz go to carrier B. Thus if phones transmitting their UL back to your semi-illegal pirate BTS transmit at 880.2 MHz, those transmissions will typically fall in the guard band between the two wideband LTE signals of the two carriers (typically AT&T and Verizon in most areas), hence no actual interference with anyone's service will occur most of the time. </LAWBREAKING - end of potentially-problematic section> Don't like this idea, want to be fully legal by putting both DL and UL of your GSM cell into the 902 to 928 MHz range? In that case would you be willing to forego the ability to use standard unmodified GSM phones and set up an almost-GSM cell that works only with phones whose firmware you are able to modify? If you are willing to limit your operations to phones on which you modify the fw, then you are in luck: we can pick frequencies such that only phone fw will need to be modified, without changing physical hw of the same phones. Choose frequencies as follows: * Put your DL somewhere in the 3 MHz sliver between 925 and 928 MHz. This way you are within the ISM band, but also within the passband of the SAW filter at the Rx input of standard phones with EGSM900 band support. * For the UL frequency, pick somewhere between 902 and 915 MHz - within ISM band, but also within the range of frequencies which the phone's Tx tract is designed to support. ARFCNs: we as a community (if indeed there is a community of people who desire the just-outlined hack) will have to come up with our own numbering scheme, our own community pseudostandard. Call it UGSM900 perhaps, with 'U' standing for unlicensed? HTH, Your naughty girl Mychaela (read this post reply to the tune of Naughty Girl by Beyonce, 2003)
