Function Splits in ORAN/5G NR:
Centralized baseband processing was introduced several years ago to ease installation of base stations in large buildings. Digital radio interfaces and remote radio heads (RRHs) both was enabled by it, which allowed the connection between RRHs and digital baseband units (BBUs) to be carried over fiber.
The concept introduced to span larger areas involving many radio sites while still using a central BBU. With the increase in deployment fiber and availability of required fronthauls (FHs) became a major problem. In recent years, due to new 5G requirements, 3GPP and other standards bodies started different activities to address this issue.
By distributing protocol stacks between different components (different splits between CU & DU ), solution providers focus on addressing the tight requirements for a near perfect fronthauls between RRHs and BBUs. This splitting is basically known as function split in 5G NR.
Below figure represents functional split in 5G NR:
- Option 1 (RRC/PCDP 1A-like split)
- Option 2 (PDCP/RLC Split 3C-like split)
- Option 3 (High RLC/Low RLC split, Intra RLC split)
- Option 4 (RLC-MAC split)
- Option 5 (Intra MAC split)
- Option 6 (MAC-PHY split)
- Option 7 (Intra PHY split)
- Option 8 (PHY-RF split)
Option 1 (RRC/PDCP, 1A-like split): RRC is in the central unit while PDCP, RLC, MAC, physical layer and RF are kept in the distributed unit. Thus the entire user plane is in the distributed unit.
Option 2 (PDCP/RLC split): Option 2 may be a base for an X2-like design due to similarity on U-plane, but some functionality may be different e.g. C-plane since some new procedures may be needed. There are two possible variants available in this option.
- Option 2-1: Split U-plane only (3C like split): In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit.
- Option 2-2: In this split option, RRC, PDCP are in the central unit. RLC, MAC, physical layer and RF are in the distributed unit. In addition, this option can be achieved by separating the RRC and PDCP for the CP stack and the PDCP for the UP stack into different central entities.
Option 3 (High RLC/Low RLC Split): In this option, two approaches are taken based on Real time/Non-Real time functions split which are as follows:
- Option 3-1 Split based on ARQ
- Option 3-2 Split based on TX RLC and RX RLC
Option 3-1 Split based on ARQ
- Low RLC may be composed of segmentation functions;
- High RLC may be composed of ARQ and other RLC functions;
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.
Option 3-2 Split based on TX RLC and RX RLC
- Low RLC may be composed of transmitting TM RLC entity, transmitting UM RLC entity, a transmitting side of AM and the routing function of a receiving side of AM, which are related to downlink transmission.
- High RLC may be composed of receiving TM RLC entity, receiving UM RLC entity and a receiving side of AM except for the routing function and reception of RLC status reports, which are related to uplink transmission.
Option 4 (RLC-MAC split): In this split option, RRC, PDCP, and RLC are in the central unit. MAC, physical layer, and RF are in the distributed unit.
Option 5 (Intra MAC split):
- Central Unit- Higher part of the MAC layer (High-MAC), RLC and PDCP.
- Distributed Unit – RF, physical layer and lower part of the MAC layer (Low-MAC).
High-MAC sub layer : the centralized scheduling in the High-MAC sub layer will be in charge of the control of multiple Low-MAC sub layers. It takes high-level centralized scheduling decision.
Low-MAC Sublayer the time-critical functions with stringent delay requirements (e.g. HARQ) or the functions where performance is proportional to latency (e.g. radio channel and signal measurements from PHY, random access control). It reduces the delay requirements on the fronthaul interface.
Option 6 (MAC-PHY split): The MAC,RLC, PDCP & RRC are in the central unit (CU). PHY layer and RF are in the DU.
Option 7 (Intra PHY split): We have 3 types of Sub splits in option 7 as given below:
Option 7-1 UL: FFT, CP removal and possibly PRACH filtering functions reside in the DU, the rest of PHY functions reside in the CU.
Option 7-1 DL : iFFT and CP addition functions reside in the DU, the rest of PHY functions reside in the CU.
Option 7-2 UL: FFT, CP removal, resource de-mapping and possibly pre-filtering functions reside in the DU and rest of PHY functions reside in the CU.
Option 7-2 DL: iFFT, CP addition, resource mapping and precoding functions reside in the DU, the rest of PHY functions reside in the CU.
Option 8 (PHY-RF split): This option allows to separate the RF and the PHY layer. Legacy C-RAN. Option 7.2 is called as ORAN split option. For more updates on types of splits and advantages stay tuned on ORAN page of site.