  Cloud radio-access networks (C-RAN) have been proposed as an enabling
technology for keeping up with the requirements of next-generation wireless
networks. Most existing works on C-RAN consider the uplink or the downlink
separately. However, designing the uplink and the downlink jointly may bring
additional advantage, especially if message source-destination information is
taken into account. In this paper, this idea is demonstrated by considering
pairwise message exchange between users in a C-RAN. A multi-pair two-way
transmission scheme is proposed which targets maximizing the end-to-end user
data rates. In the proposed scheme, a lattice-based computation strategy is
used, where the baseband processing unit (BBU) pool decodes integer linear
combinations of paired users' codewords instead of decoding linear combinations
of individual codewords. The BBU pool then compresses the computed signals and
forwards them to the remote radio heads (RRHs), which decompress the signals
and send them to the users. Finally, each user decodes its desired message
using its own message as side information. The achievable rate of this scheme
is derived, optimized, and evaluated numerically. Results reveal that
significant end-to-end rate improvement can be achieved using the proposed
scheme compared to existing schemes.
