  Coherence has been used as a resource for optical communications since its
earliest days. It is widely used for multiplexing of data, but not for encoding
of data. Here we introduce a coding scheme, which we call \textit{mutual
coherence coding}, to encode information in the mutual coherence of spatially
separated light beams. We describe its implementation and analyze its
performance by deriving the relevant figures of merit (signal-to-noise ratio,
maximum bit-rate, and spectral efficiency) with respect to the number of
transmitted beams. Mutual coherence coding yields a quadratic scaling of the
number of transmitted signals with the number of employed light beams, which
might have benefits for cryptography and data security.
