Introduction

The C-RAN architecture relies on fronthaul links to connect each remote radio head (RRH) to the managing baseband unit (BBU). In particular, for the uplink, the fronthaul links allow the RRHs to convey their respective received signals, either in analog format or in the form of digitized baseband samples, to the BBU. For the downlink, the BBU transfers the radio signal that each RRH is to transmit on the radio interface, in analog or digital format, on the fronthaul links to the RRHs. It is this transfer of radio or baseband signals that makes possible the virtualization of the baseband and higherlayer functions of the (RRHs) at the BBU, which defines the C-RAN architecture. The analog transport solution is typically implemented by means of radio-over-fiber (see, e.g., [1]) but solutions based on copper LAN cables are also available [2]. In contrast, the digital transmission of baseband, or IQ, samples is currently carried out by following the common public radio interface (CPRI) specification [3]. This ideally requires fiber optic fronthaul links, although practical constraints motivate the development of wireless-based digital fronthauling [4]. The digital approach seems to have attracted the most interest owing to the traditional advantages of digital solutions, their including resilience to noise and to hardware impairments as well as flexibility in the transport options (see, e.g., [5]). Furthermore, the connection between an RRH and the BBU may be direct, i.e., single-hop, or it may take place over a cascade of fronthaul links, i.e., be multi-hop, as illustrated in Fig. 8.1.

In this chapter we provide an overview of the state of the art on the problem of transporting digitized IQ baseband signals on the fronthaul links. As mentioned, the current de facto standard that defines analog-to-digital processing and transport options is provided by the common public radio interface (CPRI) specification [3]. This specification is widely understood to be unsuitable for the large-scale implementation of C-RAN owing to its significant fronthaul bit rate requirements under common operating conditions. As an example, as reported in [5], the bit rate needed for an LTE base station that serves three cell sectors with carrier aggregation over five carriers and two receive antennas exceeds even the 10 Gbits/s provided by standard fiber optic links.