Types of Splits:
We have three categories of splits
1) High Layer Splits
2) Low Layer Splits
3) Double Splits
✓ High Layer Split: It consist of split 2,3,4 & 5.The benefits of opting this functional
splits are drastically reduced bandwidth, latency tolerant i.e for long distances.
✓ Low Layer Split: It consists of split 6,7 &8. The benefits of opting this functional
split are cost effective RRH, Ideal for COMP. It has High Bandwidth requirements
and very tight latency requirements.
✓ Double Split: Double splits are the splits in which two splits have been considered
between RAN i.e CU,DU & RU. Practically we have two double splits that are :
a) Option 2 & 6 , b) option 2 & 7.2
Below figure represents the functional split:
Choosing a Functional Split:
When considering the functional split defining a fronthaul interface there are two competing
interests:
a) There is a benefit in keeping an O-RU as simple as possible because size, weight, and
power draw are primary deciding considerations and the more complex an O-RU, the larger,
heavier and more power-hungry the O-RU tends to be.
b) There is a benefit in having the interface at a higher level which tends to reduce the
interface throughput relative to a lower-level interface – but the higher-level the interface, the
more complex the O-RU tends to be.
✓ To resolve this, O-RAN has selected a single split point, known as “7-2x” but allows a
variation, with the precoding function to be located either “above” the interface in the O-DU or “below” the interface in the O-RU.
✓ O-RUs within which the precoding is not done are called “Category A” O-RUs while O-RUs within which the precoding is done are called 27 “Category B” O-RUs.
✓ When O-RU Category A is supported by O-DU it is mandatory to support a total number
of pre-coded streams of up to 8. Support for more than 8 pre-coded streams is optional.
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:
Functional Splits:
▪ 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.