In ORAN what are functional splits?
Source: https://www.5gtechnologyworld.com/functional-splits-the-foundation-of-an-open-5g-ran/
- Functional splits refer to how the RF and Baseband functions are distributed between the Radio Unit and the Network Functions (CU/DU).
- Functional splits are dictated by the capability of the front-haul and the services they provide.
What are the various layers of the RAN protocol stack?
RF -> Low PHY -> High PHY -> Low MAC -> High MAC -> Low RLC -> High RLC -> RRC & Data.
Layer 1 = RF + Low PHY + High PHY
Layer 2 = Low MAC + High MAC + Low RLC + High RLC
Layer 3 = PDCP + RRC
What is Split Option 8?
- This is one extreme
- Only the RF function is placed in the RU
- All the other BB function (Low PHY to RRC) is placed in a CU/DU.
- This imposes very high requirements on the Front-haul which has to be fiber.
- But it also allows for better use of virtualization (i.e. running the stack on COTS HW).
What is Split Option 1?
- This is the other extreme.
- All RF and BB functions are in the RU.
- This complicates the RU which becomes very bulky
- But Fronthaul requirements are much simpler.
What are the other split options 2 through 7?
Splits occur in different parts of the RAN protocol stack.
The higher up in the protocol stack the split occurs, the lower is the demand on the front haul.
Thus option 8 demanded the highest level of bit rate and low latency on front haul because the split occurs very low in the stack - i.e. between the RF and Low PHY.
Why is option 7 emerging as the lead horse?
In option 7, the split occurs between Low and High PHY. i.e. RF+Low Phy is the RU, and everything else in the DU/CU.
Option 7, or PHY-level functional splits offers a good balance between RU complexity, fronthaul bandwidth, and inter-cell cooperation.
Functions like CoMP (UE conntected to multiple gNBs for better cell edge coverage) is still possible.
Within Option 7, what is 7.2?
Option 7 has three sub-categories.
All three options leave the Fast Fourier Transformation, (FFT) functionality in the RU, significantly reducing the fronthaul bit rate. By allocating slightly more functionality, (pre-coding and resource element mapping) to the RU, Option 7.2 further reduces the fronthaul bit rate, when compared to Option 7.1. Because the resource element mapping detects unused subcarriers from the RF link, locating this function in the RU leads to a variable bit rate on the fronthaul network — Option 7.1 gives a constant bit rate. Option 7.3 is a downlink-only option and achieves further fronthaul bit rate reduction by allocating even more functionality to the RU, which becomes more complex.
Option 7.2, however, has increasingly gained traction within the industry with its lower and variable fronthaul bit rate being compatible with the eCPRI protocol.
What is the eCPRI Interface?
The protocol used on the front haul (RH to DU).
Can run over Ethernet. Fronthaul latency constrained to 100 microsec (but 50 musec for URLLC).
The earlier CPRI was not open - i.e. RUs and BBUs had to be from the same vendor.
It also required constant bit rate which meant that expensive point-to-point fiber was the option -e.g. connect one or more RUs to a BBU located 20 KMs away.
eCPRI uses Ethernet - so multiple RUs can be multiplexed on a single fiber backhaul to the BBU (DU/CU) - no need for p2p links. Cost reduction.
Also, with the 7.2 split, the fronthaul does not have to be fixed rate (can be variable rate).
What do Baseband Units do?
- Baseband units (BBUs) sit at the base of radio towers.
- BBUs demodulate the RF signal, converting the output into digital data streams for transport on the backhaul to the core network.
What is a baseband signal?
Baseband signal is the original frequency of the carrier wave before it is modulated by information.
How is the protocol stack split between Open RAN components?
RU - handles Digital Front End (DFE) and lower PHY.
DU - close to the RU. Runs RLC, MAC and parts of the PHY.
CU - RRC and PDCP.
A single CU may control multiple DUs.
Both CU and DU are deployable on COTS HW.
A gNB = 1 CU + 1 DU.
How does the DU and CU split protocol functions?
- While many splits are being considered for RU/DU, the defacto split for DU CU is Option 2.
- CU = PDCP + RRC
- DU = RLC + MAC + PHY
What is Split 8?
Split 8 is what is already in place in the older generation - with CPRI.
Everything except the RF function in the DU/CU (BBU).
Very demanding requirements on the front-haul.
Split 8 using eCPRI is in the works enabling older LTE to transition to 5G.
What are the HW requirements for DU?
DUs handle DSP that are real time sensitive.
Hardware acceleration via FPGA and GPUs.
What is dark fiber?
https://enterprise.spectrum.com/support/faq/internet/what-is-dark-fiber.html
- Dark fiber is fiber-optic infrastructure that is not yet “lit” or put into use by a service provider.
- A dark fiber lease requires the customer rather than the service provider to maintain and operate the equipment required to “light” the fiber and use it for Internet access and communications.
- A dark fiber lease can provide cost savings and greater flexibility, but also comes with an increased responsibility for maintaining and monitoring another layer of technology.
- Lit fiber services are delivered by a service provider that owns and maintains the equipment to light the fiber.
What is the CUS plane in ORAN?
CUS Plane is the set of functions on the Front Haul (RU-DU) interface. There are three protocol sets supporting the different functions.
- C/U Plane = Control /User Plane (eCPRI/Ethernet)
- Sync - for time sync between RU and DU (PTP, Sync E/Ethernet)
- Management Plane (NETCONF/SNMP)
