  Electron Paramagnetic Resonance experiments show that the decay of Rabi
oscillations of ensembles of spin qubits depends noticeably on the microwave
power and more precisely on the Rabi frequency, an effect recently called
"driven decoherence". By direct numerical solution of the time-dependent
Schr\"odinger equation of the associated many-body system, we scrutinize the
different mechanisms that may lead to this type of decoherence. Assuming the
effects of dissipation to be negligible ($T_1=\infty$), it is shown that a
system of dipolar-coupled spins with -- even weak-- random inhomogeneities is
sufficient to explain the salient features of the experimental observations.
Some experimental examples are given to illustrate the potential of the
numerical simulation approach.
