>the hardware available for SDR MIMO development leaves much to be desired.
>  ... the blocks and implementations for OFDM coding in software are OK :)


>USRP N2X0 can do it, other setups require external reference clock
>with high fidelity.  any recommendations for non-GPS clocks performing
>at 50-100ppb or better?

Other than a good ovenized crystal oscillator (OCXO), there is something called a "DCXO" (Digitally-Compensated Crystal Oscillator) that is probably as good as 50 ppb over a moderate temperature range.  See, for example, :     http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1546247&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4%2F33000%2F01546247.pdf%3Farnumber%3D1546247  

http://electronicdesign.com/components/dcxo-combines-stability-low-power

Jim Bell