Cloud radio access networks: Uplink channel estimation and downlink precoding

This chapter explores the problem of optimal functional split between remote radio heads (RRHs) and baseband unit (BBU) at the physical (PHY) layer by focusing on the two key baseband operations of channel estimation and channel encoding/precoding. It recalls that alternative functional splits are envisaged to be potentially advantageous in the presence of significant fronthaul capacity constraints. For the uplink, the chapter compares the standard implementation in which all baseband processing, including channel estimation, is performed at the BBU, with an alternative architecture in which channel estimation, along with the necessary frame synchronization and resource demapping, is instead implemented at the RRHs. The chapter discusses the downlink, where one can contrast the standard cloud radio access network (C-RAN) implementation with an alternative one in which channel encoding and precoding are applied at the RRHs, while the BBU retains the function of designing the precoding matrices based on the available channel state information.