  Free-space communication links are severely affected by atmospheric
turbulence, which causes degradation in the transmitted signal. One of the most
common solutions to overcome this is to exploit diversity. In this approach,
information is sent in parallel using two or more transmitters that are
spatially separated, with each beam therefore experiencing different
atmospheric turbulence, lowering the probability of a receive error. In this
work we propose and experimentally demonstrate a generalization of diversity
based on spatial modes of light, which we have termed $\textit{modal
diversity}$. We remove the need for a physical separation of the transmitters
by exploiting the fact that spatial modes of light experience different
perturbations, even when travelling along the same path. For this
proof-of-principle we selected modes from the Hermite-Gaussian and
Laguerre-Gaussian basis sets and demonstrate an improvement in Bit Error Rate
by up to 54\%. We outline that modal diversity enables physically compact and
longer distance free space optical links without increasing the total transmit
power.
