The Cloud RAN (C-RAN)
The C-RAN (Centralised or Cloud Radio Access Network) is a cellular network architecture designed for efficiency and lower costs when deploying public access small cells. The C-RAN architecture separates the BBU (Base Band Unit) from the RRH (Remote Radio Head) using the CPRI (Common Public Radio Interface). The CPRI link between the BBU and the RRH is known as the ‘fronthaul’ whereas the backhaul network connects the BBU with the core network. The BBU is responsible for the signal processing functions and the RRH transmits the signal over the radio interface.
Placing BBUs at central locations enables more precise control of the radio spectrum between RRHs and interference coordination – it also reduces power consumption, the size of remote radio equipment and complexity of small metro cell deployment. The C-RAN has been implemented in dense environments such as sports stadiums, shopping centres and city centres. The increased flexibility and efficiency on the radio access side, however, comes at the expense of challenges in the fronthaul connections.
CPRI is a digital interface with various capacities up to 9.6 Gbit/s which is best carried over optical fibre. In addition, CPRI has very strict requirements on transmission delay (latency) and delay variation (jitter) in order to guarantee reliable system operation. For LTE operation, the delay over the CPRI interface must be less than 200 micro seconds. CPRI can be used between one BBU and a single RRH or it can be between one BBU and multiple RRHs.
In its basic form, optical fibre connects tower-mounted RRHs to BBUs at the bottom of the tower. In contrast to using conventional copper coax feeder cables to the antenna, the fibre carries a digital representation of the radio signal which is then converted to a radio signal directly at the RRH. An electrical power supply is required to be connected to the RRH. This process helps provide higher power and bandwidth to mobile devices and is referred to as FTTA (Fibre To The Antenna).
With CPRI, the link between the BBU and RRH can be extended to many kilometres. This opens up new opportunities for deploying small cells to offload data from macro cells and DAS (Distributed Antenna Systems). In densely populated areas, the demand for mobile broadband services is overloading the macro cells and network congestion can be reduced by positioning additional RRH in these traffic hotspots.