Exploiting the degree of freedom in the frequency domain and the
near-far effect among different access points (APs), this paper proposes
an opportunistic transmission scheme in cell-free massive MIMO-OFDM
systems. The key idea is to orthogonally assign subcarriers among
different users, so that there is only one user on each subcarrier.
Then, a user is only served by its near APs through opportunistic
selection, while the far APs are deactivated to avoid wasting power over
their channels with severe propagation losses. Moreover, the number of
active APs per subcarrier becomes small due to the opportunistic
selection, making the use of downlink pilots and coherent detection
feasible. As corroborated by numerical results, the proposed scheme can
bring a significant performance boost in terms of both power efficiency
and spectral efficiency.