Split Options for 5G Radio Access Networks

Paul Arnold, Nico Bayer, Jakob Belschner, Gerd Zimmermann

Technology InnovationDeutsche Telekom AG

IntroductionTwo splits envisioned in the 5G RAN

Control-Plane / User-Plane Split

“CU/DU”-Split (Central and Distributed Units)

Central and Distributed Units

Central and Distributed Units

› Centralization of parts of the processing in a central unit

› Motivation: Simplified implementation of interference coordination, multi-connectivity, traffic-steering, …

› Several options to split the protocol stack, each with different demand on the underlying interface (xHaul)

› Flexibility to implement centralization in different deployments

Split Options in the Protocol Stack

Examples for data rate requirements on the xHaul interface

0,0

2,0

4,0

6,0

8,0

10,0

12,0

8 7-1 7-3 6

xHau

l dat

a ra

te in

Gb

it/s

3GPP Split Option

2 antenna ports / 2 layers

4 antenna ports / 4 layers

8 antenna ports / 8 layers

0,1

1,0

10,0

100,0

1000,0

10000,0

8 7-1 7-3 6

xHau

l dat

a ra

te in

Gb

it/s

3GPP Split Option

LTE 20 MHz

5G AIV (< 6 GHz /100 MHz)5G AIV (> 6 GHz /400 MHz)

LTE Potential 5G Air Interfaces

Control-Plane / User-Plane Split

Control-Plane / User-Plane Split

› Categorize network functions into Control-Plane (CP) and User-Plane (UP) functions

› Define standardized interfaces for interaction between CP and UP

› Consistent control over network elements from different vendors

› Avoid replacement of UP in case CP is modified

› More flexible network

Pros

› Tight coupling of CP / UP Full separation might be complex

› Standardization for all interfaces is required

› Additional effort in terms of testing

Cons

Interactions between CP and UP

1: DL Buffer Status

2: Payload Selection

3: Payload Selection, DL Resource Assignment, UL Grants

4: Retransmission Control

5: Broadcast Channel Information

6: Coding Scheme

7: Antenna Mapping, Precoder, Modulation Scheme

8: Reference Symbols, Synchronization Channels

9: Antenna Weights

10: Channel State Information (from UL Sounding)

11: Channel State Information (CQI Reporting),

UL Scheduling Request

12: HARQ Status

Radio Unit (RU)

1

PD

CP

RLC

(asyn

ch.)

RLC

(syn

ch.)

MA

C H

igh (

MU

X)

MA

C L

ow

(H

AR

Q)

FE

C C

odin

g

Scra

mblin

g

Modula

tion

Laye

r M

appin

g

Dig

ital B

eam

form

ing Resource Element

Mapping & IFFT

Resource Element

Mapping & IFFT

D/A

Conversion

D/A

Conversion

Antenna

#1

Antenna

#N

2 3 4 6 7

RLC

(asyn

ch.)

RLC

(syn

ch.)

MA

C H

igh (

DeM

UX

)

MA

C L

ow

(H

AR

Q)

FE

C D

ecodin

g

Descra

mblin

g

Laye

r D

em

appin

g

Dem

odula

tion

Equaliz

ation

Resource Element

Demapping & FFT

Resource Element

Demapping & FFT

A/D

Conversion

A/D

Conversion

S1-U*

S1-U*

Downlink User

Plane

Uplink User

Plane

Control PlaneICIC

S1-C*

8

Cell

Config

5

10

Analo

gB

eam

form

ing

(RF

Pre

codin

g)

Antenna

#1

Antenna

#M

Analo

g

RF

Com

bin

ing

Short-Term Scheduler 9

1112

RRC

RU

Config

PD

CP

X2-C*

3GPP Split

Option

3 87-3654 7-12

Overall Network Architecture

Proposed Overall Network Architecture› Forwarding of data in the

transport network through based on SDN– Implements CP/UP split

› Central Access Controller (CAC) is the centralized network element in Radio Access Network– Separated into CP and UP part

› xHaul-Interface to the radio sites– Flexible adaptation depending

on the network deployment

Use Case: Multi-Connectivity

1

PD

CP

(incl. F

ast

Sw

itch o

r

Packet

Duplic

ation)

PHY

2

S1-U*

Downlink User Plane

Low

er

CP

AIV

1

S1-C*

8

Cell

Config

5

1012

RUMACRLC

PHY RUMACRLC

PHY RUMACRLC

PHY RUMACRLC

QoS

/ S

lice

Enfo

rcem

ent

AIV 2

AIV 1

AIV 1

AIV 2

Uplink User Plane

S1-U*

76

Short-Term Scheduler

11

43

QoS

/ S

lice

Contr

ol

RR

C

PD

CP

(incl. F

ast

Sw

itch o

r

Packet

Duplic

ation)

Multi-C

ell/

-AIV

Resourc

e

Mappin

g

Com

mon H

igher

CP L

ow

er

CP

AIV

2

Central Unit (CU) Distributed Unit (DU)

… … ………………………

› Multi-Connectivity is important in 5G, especially for:– mmWave Radio

– Ultra-Reliable Communication

› Figure shows potential implementation based on CU / DU split option 2

› Additional CP functions for:– Traffic Steering (Multi-Cell / Multi-

AIV Resource Mapping)

– Quality of Service

– Network Slicing

2

Summary and Conclusions

Summary and Conclusions

› Two split options under discussion for 5G– Central and distributed unit (“CU/DU split”)– Control-Plane / User-Plane split

› CU/DU split– Important for multi-connectivity, interference coordination, traffic-steering– Split at lower layers can lead to extremely high data rates on the interface

› Control-Plane / User-Plane split– Important for flexible future networks and consistent control functions– Tight coupling in the RAN make full separation complex

Thank Youhttps://metis-ii.5g-ppp.eu/