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
› 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
› 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
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
› 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
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