5.5 RAN functional split

The RAN functional split is another innovative and practical scheme for alleviating the imposed fronthaul requirements by the C-RAN architecture [11, 54]. For instance, to address the drawbacks of CPRI-based fronthaul solutions, an eCPRI specification presents additional physical layer functional split options and a packet-based solution. Consequently, unlike the conventional constant data rate CPRI in which the stream significantly depends on the carrier bandwidth, as well as the number of antennas, the eCPRI stream does not depend on either of the factors but on the actual traffic load. In essence, apart from being able to alleviate the stringent bandwidth demands, multiple eCPRI stream can also be multiplexed onto a wavelength for onward transmission over the fronthaul network [12].

In addition, with recent network architecture development, the traditional BBU and RRU have been reformed into different functional entities which are the CU, DU, and RRU/active antenna unit (AAU). With the configuration, the CU majorly focuses on non-real time and part of the traditional Evolved Packet Core functionalities. This involves high-level protocol processing like dual connectivity and radio resource management. In addition, the DU is responsible for the real-time media

Figure 7. Functional split options between CU and DU with emphasized PHY layer.

Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks DOI: http://dx.doi.org/10.5772/intechopen.85858

access control layer functions like HARQ flow and physical layer function processing. Also, when massive MIMO antennas are to be employed, certain parts of the physical layer functions can also be shifted to the RRU/AAU. The implementation will not only aid in lessening the associated transmission bandwidth between the RRU/AAU and DUs but will also help in reducing the transmission cost considerably. Therefore, a number of functional split options have been presented in order to reduce the processing and network resource cost considerably. As shown in Figure 7, each of the option is categorized according to the demarcation point between the CU and the DU. Therefore, depending on the deployment scenarios and use cases, each option offers different degrees of flexibility regarding resource allocation for different service requirements [12, 29].
