I have the impression that not a lot of people are developing for Layer-1 in OsmocomBB. Is it because GSM in that depth is too complicated ? Of course you need some GSM knowledge to start, but be assured you will learn a lot if you work on this level.
Besides the really cheap phones supported by OsmocomBB you also need a BTS or GSM Testset for doing serious work. You can use a BS-11 or nanoBTS with OpenBSC or an USRP-1 with OpenBTS. I understand that the price for this hardware might be too expensive for some of you.
However GSM Testsets are not that expensive any more. If you have the room for a large and heavy unit (around 30 Kg) the HP 8922 might be a good solution. You can buy them for a few hundred Euros and this unit is really nice: Besides the GSM Testset it can be used as a very accurate Signal Generator (10 MHz to 1 GHz) and it also comes with a simple Spectrum Analyzer (10 MHz to 1 GHz) as a hardware option. The unit is for GSM-900 but there is an additional converter available for GSM-1800 support. The converter doubles the size of the unit and you usually don't need it. One drawback is that the HP 8922 does not support Encryption. As far as I know this was a Hardware Option but I have never seen a unit for sale which has this option installed. Actually it would only require a different DSP code in one of the EEPROMs which includes the A5/1 or A5/2 encryption code, the rest (Layer-3 support and passing Kc to the DSP) is already there. As usual with most HP units, you can download the documentation from the HP/Agilent web site.
Another GSM Testset I use is the Racal 6103. Its smaller than the HP 8922 and has more Layer-3 options but is not that versatile on Layer-1. The "E" Variant of the unit also supports Encryption. You can sometimes find those units for a few hundred Euros, but be aware that it is not the "Anite" or "Aime" variant, those units require a special PC software to use them, without this software you can't do much with them. So far I have not seen this software, I only know that it is quite expensive.
If you use a GSM Testset, you have one big benefit: You can connect the phone to the Testset with a cable (there is no need for an attenuator) and don't have to care that much about emitting RF on the licensed spectrum.