  The diagonalization of a conventional multiple-input multiple-output (MIMO)
channel into parallel and independent subchannels via singular value
decomposition (SVD) is a fundamental strategy that allows the MIMO channel
capacity to be achieved using scalar channel codes. This letter establishes a
similar channel diagonalization result for the uplink and the downlink of a
cloud radio access network (C-RAN), in which a central processor (CP) is
connected to a remote radio head (RRH) serving a single user via rate-limit
digital fronthaul carrying the compressed baseband signal. Specifically, we
show that the diagonalization of the MIMO channel between the RRH and the user
via SVD and the subsequent independent and parallel quantization of scalar
signals and channel coding in each of the subchannels is optimal. This letter
establishes this fact using the majorization theory. Further, an
uplink-downlink duality for the multiple-antenna C-RAN is identified for this
single-user case.
