I. Introduction
Cloud radio access networks (C-RANs) offer a flexible and more energy efficient cellular architecture [1]. In C-RANs, distributed RF units called remote radio head (RRH) receive signals, perform limited signal processing, and send them to a centralized baseband unit (BBU) via a fronthaul pipe. Unfortunately, the capacity of the fronthaul link is limited in practice [2], so that the bit rate has an upper bound. To satisfy the constraint, the conventional approach is to quantize the signals using high resolution ADCs followed by compression [2].