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understand 5G RAN

understand 5G RAN 

Overview
We are covering basic understanding on RAN element and details from 5G technology/ specs. The content is organized, to provide consumable information for reader to help increase their understanding on RAN. The page covers basic terminology, concepts and 3gpp roadmaps and features for RAN functionality. Hope my attempt to explain things as simple as possible and help you leave with acquisition and understanding of some concepts or least with touch points. This (#RAN) has been and remains to be crucial/ complex part of telecom network as rest of network is getting simplified by moving from more sophisticated protocols/ framework to generic and open standards by 3GPP body.

1. Terminology

#  Radio -------- Element Name ----------- Core
1/  LTE      -----     eNodeB/ eNB ------- LTE
2/  LTE      -----     ng-eNB       ------- 5G NR
3/  5G NR  -----    en-gNB        ------- LTE
4/  5G NR  -----    gNB             ------- 5G NR

The RAN traditionally used to consist of RRH and BBU units. The interface between UE to RRH was Air interface or RAT based on 3GPP technology (2G/3G/4G) and interface between BBU to Core is backhaul interface (S1-C/ S1-U for LTE with MME/ SGW) and (NG-C/ NG-U for 5G with AMF/ UPF)

RRH - Remote Radio Unit or Radio Head
BBU - Base Band Unit or (DU+CU)
NSA - 5G Non-Standalone (5G system operating along with LTE)
SA - 5G Standalone (Only 5G system)

Interface between RRH and BBU is CPRI (Common Public Radio Interface) and proprietary so far. Now with Open RAN, its being generalized by defining open interface eCPRI to use with Split 7 option for 5G.

# EN-DC - Dual Connectivity (Option 3a) diagram diagram2 diagram3
   - that 5G New Radio used by UE for the access network, and works in conjunction with the 4G LTE access network and the 4G core, also known as the EPC

# NE-DC - 5G NR assisted LTE connected with 5GC (Option 7a, uses ng-eNB with gNB) diagram

# NGEN-DC - LTE assisted 5G NR connected with 5GC (Option 3, uses en-gNB with gNB) diagram

Carrier Aggregation - multiple carrier channels can be aggregated for LTE-Adv UE to increase the bandwidth and thus rate (Each aggregated carrier of CA is referred to as a component carrier, CC. The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is 100 MHz)
In FDD the number of aggregated carriers can be different in DL and UL, number of UL CC's is always <= the number of DL CC's. The individual CC's can also be of different bandwidths
In TDD the number of CCs as well as the bandwidths of each CC will normally be the same for DL and UL. 

Carrier Aggregation explained (3gpp.org)
ARFCN -  Absolute Radio-frequency Channel Number
Frequencies - diagram diagram2
Cell
Channels
Beam
Antenna
MIMO - Multi Input/ Multi Out (Single User-SU, Multi User-MU)
TDD
FDD
PAGING - MME initiated, RRC Initiated (eNB)

etc ...

2. How is RAN evolving in 4G, 5G, to 6G

O-RAN

3GPP considered the BBU split concept (DU and CU) from the beginning for 5G. In a 5G cloud RAN architecture, the BBU functionality is split into two functional units: a distributed unit (DU), responsible for real time L1 and L2 scheduling functions, and a centralized unit (CU) responsible for non-real time, higher L2 and L3. In a 5G cloud RAN, the DU physical layer and software layer are hosted in an edge cloud datacenter or central office, and the CU physical layer and software can be co-located with the DU or hosted in a regional cloud data center. While CUs will maintain BBU-like functionalities, DUs that are software based will be more than RRH in terms of processing capacities

RU - Radio Unit (mounted on tower) - Fronthaul to DU/ BBU
 - Fronthaul latency is constrained to 100 microseconds
DU - Distributed Unit (connected with RU) - Midhaul
 - located close to RU, and runs the RLC, MAC, and parts of the PHY layer. 
 - There are three purposes of separating DU from RU:
       1. To reduce cost – less intelligent RU costs less,
       2. Ability to look at a sector of RUs at once and not just an individual RU – this will help to enable features like CoMP, and 
       3. As processing is done in the DU, resources can be pooled resulting in pooling gains
 - Midhaul connects the CU with the DU via F1 interface with latency of around 1 millisecond.
CU - Central Unit (aggregated DUs, and connected to Core) - Backhaul

 - located close to core network and runs the RRC and PDCP layers

ref - 
1#New_RAN_Architecture_options(2,3,4,7)
2# DU-CU connectivity with AMF/ UPF etc
3# DU CU functional splits
4# Mapping of eNB in external and internal network

In terms of distances supported on each interfaces
RU to DU (Fronthaul) - within 10 kms 
DU to CU (Midhaul) - 80 kms
CU to Core (Backhaul) - 200 kms

The gNB consists of a CU and one DU connected to the CU via Fs-C and Fs-U interfaces for CP and UP respectively. A CU with multiple DUs will support multiple gNBs. The split architecture enables a 5G network to utilize different distribution of protocol stacks between CU and DUs depending on midhaul network. The CU controls the operation of several DUs over the midhaul interface

3. 5G Features in brief
 - NR-Lite/ NR-Light/ RedCap (Reduced Capacity) -
 - IoT/ MTC/ M2M/ NB-IoT etc


4. 3GPP roadmap

- 5G spec work started with Release 15 (Phase1), followed by rel 16, 17 and 18 (5G-Adv)
- Release 18 is about to be frozen in coming mid year 2022, Post that the 3GPP would be named as 5G advanced and later to that would begin defining the usecase and requirements for 6G

5. Release contents

5.1 Release 15 Features (5G kicks off)

5.2 Release 16 Features
Enhancements to 
 - Slicing
 - Analytics
 - SBA
 - Location Based Services
 - CAP-IF (for IoT)
Time Sensitive Communication
Non-Public Network - (S-NPN by non-PLMN and NP-NPN - by PLMN)
V2X Communication
5G-SRVCC
Mobile Communication for Railways
Satellite Access
Optimized UE Radio capability signaling

5.3 Release 17 Features [Summary]
Beamforming and multiple-input, multiple-output (MIMO)
User equipment power savings
Positioning
Dynamic spectrum sharing
NR Coverage
Non-Public Networks (Private/ Enterprise Network)
Ultra-reliable, low-latency communication
Small data transmission
Edge Computing
Data networks analytics (NWDAF Node)
*Reduced-capability user equipment/ RedCap/ NR-Lite/ NR-Light
*Non-terrestrial networks (NTN) 

5.3 Release 18 Features (5G-Advanced) [PDF] [summary]



Important milestones
1. Dual-Connectivity - introduced in Release 15, is informed by network using SIB2 to UE
2. 


Important feature








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