  We study the transmission over a network in which users send information to a
remote destination through relay nodes that are connected to the destination
via finite-capacity error-free links, i.e., a cloud radio access network. The
relays are constrained to operate without knowledge of the users' codebooks,
i.e., they perform oblivious processing. The destination, or central processor,
however, is informed about the users' codebooks. We establish a single-letter
characterization of the capacity region of this model for a class of discrete
memoryless channels in which the outputs at the relay nodes are independent
given the users' inputs. We show that both relaying \`a-la Cover-El Gamal,
i.e., compress-and-forward with joint decompression and decoding, and "noisy
network coding", are optimal. The proof of the converse part establishes, and
utilizes, connections with the Chief Executive Officer (CEO) source coding
problem under logarithmic loss distortion measure. Extensions to general
discrete memoryless channels are also investigated. In this case, we establish
inner and outer bounds on the capacity region. For memoryless Gaussian channels
within the studied class of channels, we characterize the capacity region when
the users are constrained to time-share among Gaussian codebooks. Furthermore,
we also discuss the suboptimality of separate decompression-decoding and the
role of time-sharing.
