  This work investigates the joint design of fronthaul compression and
precoding for the downlink of Cloud Radio Access Networks (C-RANs). In a C-RAN,
a central unit (CU) performs the baseband processing for a cluster of radio
units (RUs) that receive compressed baseband samples from the CU through
low-latency fronthaul links. Most previous works on the design of fronthaul
compression and precoding assume constant channels and instantaneous channel
state information (CSI) at the CU. This work, in contrast, concentrates on a
more practical scenario with block-ergodic channels and considers either
instantaneous or stochastic CSI at the CU. Moreover, the analysis encompasses
both the Compression-After-Precoding (CAP) and the Compression-Before-Precoding
(CBP) schemes. With the CAP approach, which is the standard C-RAN solution, the
CU performs channel coding and precoding and then the CU compresses and
forwards the resulting baseband signals on the fronthaul links to the RUs. With
the CBP scheme, instead, the CU does not perform precoding but rather forwards
separately the information messages of a subset of mobile stations (MSs) along
with the compressed precoding matrices to the each RU, which then performs
precoding. Optimization algorithms over fronthaul compression and precoding for
both CAP and CBP are proposed that are based on a stochastic successive
upper-bound minimization approach. Via numerical results, the relative merits
of the two strategies under either instantaneous or stochastic CSI are
evaluated as a function of system parameters such as fronthaul capacity and
channel coherence time.
