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The implementation of uplink HARQ in a Cloud- Radio Access Network RAN(C-RAN) architecture is constrained by the two-way latency on the fronthaullinks connecting the Remote Radio Heads (RRHs) with the Baseband Units (BBUs)that perform decoding. To overcome this limitation, this work considers anarchitecture based on the separation of control and data planes, in whichretransmission control decisions are made at the edge of the network, that is,by the RRHs or User Equipments (UEs), while data decoding is carried outremotely at the BBUs. This solution enables low-latency local retransmissiondecisions to be made at the RRHs or UEs, which are not subject to the fronthaullatency constraints, while at the same time leveraging the decoding capabilityof the BBUs.  A system with BBU Hoteling system is considered first in which each RRH has adedicated BBU in the cloud. For this system, the control-data separationleverages low-latency local feedback from an RRH to drive the HARQ process of agiven UE. Throughput and probability of error of this solution are analyzed forthe three standard HARQ modes of Type-I, Chase Combining and IncrementalRedundancy over a general fading MIMO link. Then, novel user-centriclow-latency feedback strategies are proposed and analyzed for the C-RANarchitecture, with a single centralized BBU, based on limited "hard" or "soft"local feedback from the RRHs to the UE and on retransmission decisions taken atthe UE. The analysis presented in this work allows the optimization of theconsidered schemes, as well as the investigation of the impact of systemparameters such as HARQ protocol type, blocklength and number of antennas onthe performance of low-latency local HARQ decisions in BBU Hoteling and C-RANarchitectures.

Uplink HARQ for Distributed and Cloud RAN via Separation of Control and Data Planes