Given the mass volume of wireless cells that will be deployed in the 5G network, transporting huge amounts of data between thousands of cells and network core with low latency in a cost effective manner is a major challenge. To address the aforementioned challenges, the Xhaul architecture, aimed at developing the next generation of 5G integrated backhaul and fronthaul networks enabling a flexible
Figure 5.1: Scheme of the Xhaul network.
and software defined reconfiguration of all networking elements in a multi tenant and service oriented unified management environment. The envisioned Xhaul trans- port network will consist of high capacity switches and heterogeneous transmission links (e.g., fiber or wireless optics, high-capacity copper, or millimeter wave) in- terconnecting remote radio units, pooled processing units (mini data centers), and Points of Presence (PoPs) of the core networks of one or multiple service providers. This requires completely new physical layer technologies or a radical evolution of existing ones, such that the challenging 5G performance requirements can be met. The Xhaul architecture will use a novel unified data plane protocol able to transport both backhaul and fronthaul traffic, regardless of the functional RAN split [64].
The methodology presented here is referred to as the 5G network architecture as defined by 3GPP [65]. This architecture consists of two parts: the radio access network and the core network. The radio access network is expected to be based on the Xhaul concept which differs from current implementation in many ways. First, it extends between the user and the base station, which is called “gNodeB” (gNB). The gNB consists of three logical entities: Central Unit (CU), Distributed Unit (DU) and Remote Unit (RU). One gNB could contain one CU and multiple DUs and several RUs. In this sense, a gNB is a kind of mini-C-RAN. Each split option comes with different requirements such as latency, bandwidth, and usage of Processing Units (PU).
Figure 5.1 shows a 5G logical network architecture as divided into 3 parts. Fron- thaul is the network segment from RU till the corresponding DU. The distance of these two entities can not be more than 20 km due to the delay-sensitive function- alities which will be executed in DU. Normally the bandwidth in this segment is the highest because of the low layers splits. The network segment between DU and responsible CU, where upper layer BBU functionalities are performed, is called mid- haul. Several DUs can reside in this part of the network which is connected to the same CU. The distance in this segment is more relaxed (80-100 km), compared to the fronthaul, due to more relaxed delay requirements of upper layers splits. The third part is the backhaul which is extended between gNB and the core network.
In order to relax the stringent fronthaul requirements, functional splits between the DU and CU are defined [66], [67]. The functional split refers to a division of signal processing functionalities between the DU and CU. 3GPP has identified eight functional splits with different suboptions. Besides, CPRI released a new version of CPRI called eCPRI [68], which already uses new splits. However, CPRI and eCPRI do not deliver a full interface standardization that would allow true interoperability among different vendors. On the other hand, the recently formed xRAN fronthaul working group supports an open, interoperable and efficient fronthaul interface.
Figure 5.2: 3GPP functional splits.
Table 5.1: Functional splits analysis.
Use case One-way latency DL bandwidth UL bandwidth
RRC-PDCP 30 ms 151 Mbps 48 Mbps PDCP-RLC 30 ms 151 Mbps 48 Mbps RLC-MAC 6 ms 151 Mbps 48 Mbps Split MAC 6 ms 151 Mbps 48 Mbps MAC-PHY 250 µs 152 Mbps 452 Mbps PHY-RF 250 µs 1966 Mbps 1966Mbps
Several different functional splits are currently being investigated to be used for a New Radio access network (NR). In NR the radio processing and baseband functions from 3GPP protocol stack are split up into a DU and a CU. Figure 5.2 illustrates the LTE protocol stack for reference, as the NR protocol stack has not yet been announced. In figure 5.2, the processing functions closest to the antenna ports are located in the bottom, and moving upwards the signal is going through more and more processing before it is sent into the fronthaul network. 3GPP has proposed eight functional split options including several sub options. The arrows within figure 5.2 illustrate different options for functional splits, and the functions below arrow will be the functions implemented in the DU, where the functions above the arrow will be performed in the CU. The functions left in the DU are very close to the users as they will be located at the antenna mast, the functions located in the CU will benefit from processing centralization, and high processing powers within a data center referred to as the CU pool. The more functions located in the DU, the more processing has already been done before data is transmitted on the fronthaul network and the lower bit rate on the fronthaul network.
Table 5.1 illustrates the trade-off between (qualitative) gains and (quantitative) network requirements for different splits in LTE. Option 8 is equivalent to pure C-RAN, i.e., all functions are centralized enabling maximum gain, namely, interfer- ence coordination mechanisms such as Coordinated MultiPoint (CoMP) are enabled, computational resources are pooled and can be scaled based on demand, etc., at the cost of the toughest network requirements [69].