  In cloud radio access networks (C-RANs), the baseband processing of the radio
units (RUs) is migrated to remote control units (CUs). This is made possible by
a network of backhaul links that connects RUs and CUs and that carries
compressed baseband signals. While prior work has focused mostly on single-hop
backhaul networks, this paper investigates efficient backhaul compression
strategies for the uplink of C-RANs with a general multihop backhaul topology.
A baseline multiplex-and-forward (MF) scheme is first studied in which each RU
forwards the bit streams received from the connected RUs without any
processing. It is observed that this strategy may cause significant performance
degradation in the presence of a dense deployment of RUs with a well connected
backhaul network. To obviate this problem, a scheme is proposed in which each
RU decompresses the received bit streams and performs linear in-network
processing of the decompressed signals. For both the MF and the
decompress-process-and-recompress (DPR) backhaul schemes, the optimal design is
addressed with the aim of maximizing the sum-rate under the backhaul capacity
constraints. Recognizing the significant demands of the optimal solution of the
DPR scheme in terms of channel state information (CSI) at the RUs,
decentralized optimization algorithms are proposed under the assumption of
limited CSI at the RUs. Numerical results are provided to compare the
performance of the MF and DPR schemes, highlighting the potential advantage of
in-network processing and the impact of CSI limitations.
