Radio Unit (RU) handles the digital front end (DFE) and the parts of the PHY layer. Among all of these differences, out of the most outstanding one would be that the gNB internal structure is split into two parts called CU (Central Unit) and DU (Distributed Unit) as shown below and these two entities are connected by a new interface called F1 (For the details of F1 Intreface, refer to 38.473). In the DL, iFFT and CP addition functions reside in the DU, the rest of PHY functions reside in the CU.
The challenge of C-RAN deployments is that such a functional split requires these two elements to be connected through a high-speed, low-latency, and accurately synchronized network, the so-called fronthaul.These service points are handled by the eCPRI protocol stack over IP/Ethernet.5G networks will move towards centralizing the radio function using an eCPRI based fronthaul. Architecture Change of 5G RAN eCPRI interface (DU-RRU) L2 internal division L1 internal division NGFI interface (CU-DU) CU DU RRU Change in architecture：function split of 3GPP 5G RAN defines CU and DU two-stages architecture CU：non-real-time processing, centralized deployment, general hardware For the initial phase, 3GPP has taken LTE protocol stack as a basis for the discussion, until RAN2 defines and freezes the protocol stack for New Radio (NR). This lowers cost as air conditioning is not needed at the antenna site.The network is split into three parts:The link between the cloud-hosted C-RAN and REC is via a traditional IP based backhaul. Fs-C and Fs-U provide control plane and user plane connectivity over Fs interface.Therefore, by splitting the MAC layer into 2 entities (e.g. The C-RAN approach advocates for the separation of the radio elements of the base station (called remote radio heads, RRHs) from the elements processing the baseband signal (called baseband units, BBUs), which are centralized in a single location or even virtualized into the cloud. This option requires some kind of compression technique to reduce transport bandwidth requirements between the DU and CU.Option 7-2 In this option the UL, FFT, CP removal, resource de-mapping and possibly pre-filtering functions reside in the DU, the rest of PHY functions reside in the CU. An example of this kind distribution given below.This option splits the RLC sublayer into High RLC and Low RLC sublayers such that for RLC Acknowledge Mode operation, all RLC functions may be performed at the High RLC sublayer residing in the central unit, while the segmentation may be performed at the Low RLC sublayer residing in the distributed unit.
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