Two-Timescale Hybrid Compression and Forward for Massive MIMO Aided C-RAN

We consider the uplink of a cloud radio access network (C-RAN), where massive multiple-input multiple-output (MIMO) remote radio heads (RRHs) serve as relays between users and a centralized baseband unit (BBU). Although employing massive MIMO at RRHs can improve the spectral efficiency, it also significantly increases the amount of data that need to be transported over the fronthaul links between RRHs and BBU. Existing fronthaul compression methods for conventional C-RAN are not suitable in the massive MIMO regime because they require fully digital processing and/or real-time full channel state information (CSI), incurring high implementation cost for massive MIMO RRHs. To overcome this challenge, we propose to perform a two-timescale hybrid analog-and-digital spatial filtering at each RRH to reduce the fronthaul consumption. Specifically, the analog filter is adapted to the channel statistics to achieve the massive MIMO array gain, and the digital filter is adapted to the instantaneous effective CSI to achieve the spatial multiplexing gain. Such a design can alleviate the bottleneck of limited fronthaul while achieving reduced hardware cost and power consumption, and is more robust to the CSI delay. We propose an online algorithm for the two-timescale non-convex optimization of analog and digital filters, and establish its convergence to stationary solutions. Finally, simulations verify the advantages of the proposed scheme over the state-of-the-art baseline schemes.