t y j t h j i t u03c3 h k j s k t u2200 j u2260 1 now since the channel of the interferer is given by 1 h 1 h m it is sufficient to find the h i s to do this the receiver correlates all the equations above with y 1 t and taking the expectation e y 1 t y 1 t p i u03c3h k 1 h k 1 p k e y j t y 1 t h j p i u03c3h k j h k 1 p k where p 39 s are the corresponding powers since the only unknowns in the above equations are p i and h j 39 s they can be easily computed thus even in the presence of k concurrent transmissions a 802 11 receiver can estimate the channel of the interferer without knowing the preamble a prototype of an embodiment of the approach described above has been built using the usrp2 radio platform and the gnuradio software package a 2 u00d7 2 mimo system is built using two usrp2 radio boards connected via an external clock each usrp2 is configured to span a 10 mhz channel by setting both the interpolation rate and decimation rate to 10 the resulting mimo node runs a phy layer similar to that of 802 11n i e it has 64 ofdm subcarriers a modulation choice of bpsk 4 qam 16 qam or 64.