5G Fundamentals:
5G has been introduced within the release 15 version of the 3GPP
specifications, whereas 4G was introduced within release 8.
5G has been specified based upon the requirements of the following
use cases:
- enhanced
Mobile Broadband (eMBB)
- Ultra Reliable
and Low Latency Communications (URLLC)
- massive Machine Type Communications (mMTC)
The Radio Access Network (RAN) belonging to 4G is known as Long
Term Evolution (L TE), whereas the RAN belonging to 5G is known as New Radio
(NR).
* NR has been standardized to allow tight interworking with L TE.
Tight interworking supports the inter connection of L TE and NR Base Stations.
These Base Stations can then be used in combination to serve the population of User
Equipment (UE). 5G network architectures based upon tight interworking between
L TE and NR are known as Non-Standalone (NSA)
- Non-Standalone architectures allow a smooth and relatively
simple evolution towards a complete end-to-end 5G System (5GS).
- Non-Standalone architectures allow re-use of existing L TE Base
Stations and existing 4G Core Networks. ln general, a software upgrade is
sufficient to allow interworking with a set of NR Base Stations
* Standalone (SA) NR Base Stations provide connectivity to a 5G
Core Network. The combination of NR Base Station and 5G Core Network is known
as a 5G System (SGS). The benefits of 5G are maximized when using a 5G System
* NR Base Stations have a flexible architecture that supports a
range of deployment options:
- a 'classical' Base
Station architecture can be adopted to keep the hardware within a single
cabinet
- alternatively, the Base
Station can be split into a Centralized Unit (CU) and a Distributed Unit (DU).
The CU accommodates the higher protocol stack layers, while the DU accommodates
the lower protocol stack layers. A single CU can host a large number of DU
(typically> 100), while each DU can host multiple cells (typically> 6)
- in addition, the CU can
be split into Control Plane (CP) and User Plane (UP) functions. This allows
independent scaling of the CP and UP processing capabilities. It also allows
the two functions to be deployed at different geographic locations. UP
functions may be located in close proximity to the DU to help reduce user plane
latency, while CP functions may be centralized to pool resources.
- all deployment options
can use either passive or active antenna. Passive antenna arc connected to
radio modules using RF feeder cables whereas active antenna are connected to
baseband processing hardware using high speed fiber.
* Congestion within the lower operating bands, combined with a
requirement for wider channel bandwidths has led to the specification of both
low and high operating bands for 5G. Release I 5 has adopted the use of
Frequency Range I ( 450 MHz to 6 GHz) and Frequency Range 2 (24.25 GHz to 52.60
GHz). Frequency Range I supports channel bandwidths from 5 to 100 MHz, whereas
Frequency Range 2 supports channel bandwidths from 50 LO 400 MHz It Frequency
Range 1 includes operating bands which support Frequency Division Duplexing
(FDD), Time Division Duplexing (TDD), Supplemental Downlink (SOL) and
Supplemental Uplink (SUL), whereas Frequency Range 2 supports only TDD. 3GPP
has specified mechanisms to allow dynamic changes to the uplink and downlink
transmission pattern used by TDD
* The NR air-interface uses Cyclic Prefix OFDM (CP-OFDM) in both
the uplink and downlink directions. In addition, Discrete Fourier Transform
Spread OFDM (DFT-S-OFDM) can be used to help improve coverage in the uplink
direction. Both waveforms can use QPSK, l 6QAM, 64QAM and 256QAM. DFT-S-OFDM
can also use n/2 BPSK in areas of weak coverage
* Subcarrier spacings of 15, 30 and 60 kHz are supported within
Frequency Range 1, while sub-carrier spacings of 60, 120 and 240 KHz arc
supported within Frequency Range 2. The 240 KHz subcarrier spacing is only used
for the transmission of Synchronization Signals and the Physical Broadcast
Channel (PBCH). Smaller sub-carrier spacings have longer symbol durations which
allow support for larger cell ranges. Larger subcarrier spacings have shorter
symbol durations which allow support for lower latencies
* Beamforming and MIMO arc important for both the uplink and
downlink of the NR air-interface. These can be combined within the context of
massive MlMO (mMIMO). Beamforming is particularly important to improve the link
budget when using Frequency Range 2. Multi-User MIMO (MU-MlMO) can be used to
improve spectrum efficiency when lJE have sufficient spatial separation
* Both 4G and SG have been designed to support Packet Switched
(PS) services. 4G supports the speech service using Voice over LTE (VoLTE),
whereas 5G supports the speech service using Voice over NR (VoNR). 4G networks
support Single Radio Voice Call Continuity (SRVCC) to allow inter-system handover
towards the Circuit Switched (CS) domain belonging to either 30 or 2G. Release
15 does not support SRVCC for 5G but Packet Switched inter-system handovers
from 5G to 4G are possible. SRVCC from 5G to 3G is specified within the release
16 version of the 3GPP specifications.
Difference between 5G and 4G overall Architecture:
4G :
5G:
5G:
Non-Roaming 5G System Architecture :
Network Function:
- Termination of RAN CP interface (N2).
- Termination of NAS
signaling, NAS ciphering and integrity protection
- Mobility Management
(MM) layer NAS termination
- Session Management (SM)
layer NAS forwarding/ Provide transport for SM messages between UE and SMF / Transparent
proxy for routing SM messages.
- Manages the security
context
- Registration management
- Connection management.
- Reachability management
- Mobility Management.
- Lawful intercept (for
AMF events and interface to LI System).
- Provide transport for SMS
messages between UE and SMSF.
- Location Services
management for regulatory services.
- Provide transport for Location
Services messages between UE and LMF as well as between RAN and LMF
- EPS Bearer ID
allocation for interworking with EPS
- Apply mobility related
policies from PCF (e.g. mobility restrictions)
- Provisioning of
external parameters (Expected UE behaviour parameters or Network configuration
parameters).
- In addition the AMF may include the following functionality to support non-3GPP access networks.
SMF - Session Management Function
- In addition the AMF may include the following functionality to support non-3GPP access networks.
SMF - Session Management Function
The SMF performs the session
management functions that are handled by the 4G MME, SGW-C, and PGW-C.
- Session
Management e.g. Session Establishment, modify and release, including tunnel maintain between UPF and AN node.
- UE
IP address allocation & management (including optional Authorization).
The UE IP address may be received from a UPF or from an external data network.
- Termination
of NAS signaling for session management (SM)
- Sends
QoS and policy information to RAN via the AMF
- Downlink
data notification
- Select
and control UPF for traffic routing. The UPF selection function enables
Mobile Edge Computing (MEC) by selecting a UPF close to the edge of the
network.
- Acts
as the interface for all communication related to the offered user plane
services.
- It
determines how the policy and charging for these services are applied.
- Lawful
intercept — control plane
- Support
of header compression
- Roaming functionality:
- Handle
local enforcement to apply QoS SLAs (VPLMN).
- Charging
data collection and charging interface (VPLMN).
- Lawful
intercept (in VPLMN for SM events and interface to LI System).
- Support
for interaction with external DN for the transport of signaling for PDU Session
authentication/authorization by external DN.
- Instructs UPF and NG-RAN to perform redundant transmission on N3/N9 interfaces.
UPF - User Plane Function
- Instructs UPF and NG-RAN to perform redundant transmission on N3/N9 interfaces.
UPF - User Plane Function
The UPF is part of the data plane of the SGW and PGW. In the context of the CUPS
architecture:
EPC
SGW-U + EPC PGW-U → 5G UPF
The
UPF performs the following functions:
-
Anchor
point for Intra-/Inter-RAT mobility (when applicable).
-
Allocation
of UE IP address/prefix (if supported) in response to SMF request.
-
External
PDU Session point of interconnect to Data Network.
-
Packet
routing & forwarding (e.g. support of Uplink classifier to route traffic
flows to an instance of a data network, support of Branching point to support
multi-homed PDU Session).
-
Packet
inspection (e.g. Application detection based on service data flow template and
the optional PFDs received from the SMF in addition).
-
User
Plane part of policy rule enforcement, e.g. Gating, Redirection, Traffic
steering).
-
Lawful
intercept (UP collection).
-
Traffic
usage reporting.
-
QoS
handling for user plane, e.g. UL/DL rate enforcement, Reflective QoS marking in
DL.
-
Uplink
Traffic verification (SDF to QoS Flow mapping).
-
Transport
level packet marking in the uplink and downlink.
-
Downlink
packet buffering and downlink data notification triggering.
-
Sending
and forwarding of one or more "end marker" to the source NG-RAN node.
-
Functionality
to respond to Address Resolution Protocol (ARP) requests and / or IPv6
Neighbour Solicitation requests based on local cache information for the
Ethernet PDUs. The UPF responds to the ARP and / or the IPv6 Neighbour
Solicitation Request by providing the MAC address corresponding to the IP
address sent in the request.
-
Packet
duplication in downlink direction and elimination in uplink direction in GTP-U
layer.
-
TSN
Translator functionality to hold and forward user plane packets for
de-jittering when 5G System is integrated as a bridge with the TSN network.
AUSF - Authentication Server Function
The AUSF performs the authentication function of 4G HSS.
The Authentication
Server Function (AUSF) is in a home
network and performs authentication with a UE. It makes the decision on UE authentication.
It obtains the UE authentication information from the UDM.
The UDM performs parts of the 4G HSS function.
UDR
- Unified Data Repository
It obtains the UE authentication information from the UDM.
The UDM performs parts of the 4G HSS function.
-
Generation
of 3GPP AKA Authentication Credentials.
-
User
Identification Handling (e.g. storage and management of SUPI for each
subscriber in the 5G system).
-
Support
of de-concealment of privacy-protected subscription identifier (SUCI).
-
Access
authorization based on subscription data (e.g. roaming restrictions).
-
UE's
Serving NF Registration Management (e.g. storing serving AMF for UE, storing
serving SMF for UE's PDU Session).
-
Support
to service/session continuity e.g. by keeping SMF/DNN assignment of ongoing
sessions.
-
MT-SMS
delivery support.
-
Lawful
Intercept Functionality (especially in outbound roaming case where UDM is the
only point of contact for LI).
-
Subscription
management.
-
SMS
management.
-
5GLAN
group management handling.
-
Support
of external parameter provisioning (Expected UE Behaviour parameters or Network
Configuration parameters).
- To provide this functionality, the UDM uses subscription data (including authentication data) that may be stored in UDR, in which case a UDM implements the application logic and does not require an internal user data storage and then several different UDMs may serve the same user in different transactions.
UDR
- Unified Data Repository
The Unified Data
Repository (UDR) supports the following functionality:
- Storage and retrieval of subscription data by
the UDM.
- Storage and retrieval of policy data by the PCF.
- Storage and retrieval of structured data for
exposure.
- Application data (including Packet Flow
Descriptions (PFDs) for application detection, AF request information for
multiple UEs, 5GLAN group information for 5GLAN management).
The Unified Data
Repository is located in the same PLMN as the NF service consumers storing in
and retrieving data from it using Nudr. Nudr is an intra-PLMN interface.
AF
— Application Function
Performs the same function as the EPC AF.
-
Application
influence on traffic routing
-
Accessing
NEF
-
Interaction
with the policy framework for policy control.
PCF - Policy Control Function
The
5G PCF performs the same function as the PCRF in 4G networks.
-
Provides
policy rules for control plane functions. This includes network slicing,
roaming and mobility management.
-
Accesses
subscription information for policy decisions taken by the UDR.
-
Supports
the new 5G QoS policy and charging control functions.
NEF - Network Exposure Function
The
Network Exposure Function has appeared in the 5G standards as an intelligent,
service-aware “border gateway” that will enable the external AFs to communicate
with the 5G Network Functions in a secure manner.
NSSF
— Network Slice Selection Function
NSSF redirects traffic to a network slice.
Network slices may be defined for different classes of subscribers.
The NSSF performs the following functions:
-
Selecting
of the Network Slice instances to serve the UE
-
Determining
the allowed NSSAI
-
Determining
the AMF set to be used to serve the UE
SMSF - Short Message Service Function
In 5G networks, the SMSF supports the
transfer of SMS over NAS. In this capacity, the SMSF will conduct subscription
checking and perform a relay function between the device and the SMSC (Short
Message Service Centre), through interaction with the AMF (Core Access and
Mobility Management Function).
GMLC - Gateway Mobile Location Centre
The Gateway Mobile Location Centre (GMLC) is a control
plane system that interfaces with emergency and commercial LCS clients and the
operator's network to provide the location of a mobile device, required to
support Location Based Services (LBS).
5G Protocol Stack- User Plane
Function:
NG control plane interface (NG-C):
The NG control plane interface (NG-C) is defined between the
NG-RAN node and the AMF.
NG-C provides the following functions:
- NG interface management;
- UE context management;
- UE mobility management;
- Transport of NAS
messages;
- Paging;
- PDU Session Management;
- Configuration Transfer;
- Warning Message Transmission.
The Xn User plane (Xn-U) interface is defined between two NG-RAN
nodes.
Xn-U provides non-guaranteed delivery of user plane PDUs and
supports the following functions:
- Data forwarding
- Flow control
- Flow control
Xn Control Plane- (Xn-C protocol Stack)
The
Xn control plane interface (Xn-C) is defined between two NG-RAN nodes.
The Xn-C interface supports the following functions:
- Xn interface management;
- UE mobility management,
including context transfer and RAN paging;
- Dual connectivity
User Plane-
The figure below shows the protocol stack for the user plane,
where SDAP, PDCP, RLC and MAC sublayers (terminated in gNB on the network side)
perform the functions listed in below-
Control Plane
In LTE networks, X2 and S1 interface is defined as an interface between RAN nodes and between RAN and Core Network.
5G is expected to operate
in two modes as non -standalone and standalone mode of operation.
For non-stand operation, the specification defined the extension for S1 and X2 interfaces whereas for
standalone operation new interfaces are defined.
These new interfaces are listed below:
- Interface
between RAN Node as Xn
- Interface
between RAN and Core Network as NG
- Interface
for Function Split and Open Interface as F1/E1 within RAN Node
- The interface between PHY and
Radio as eCPRI or F2.
A gNB may consist of a gNB-Control Unit (CU) and one or more gNB-Distributed Units (DUs), and the interface between gNB-CU and gNB-DU is called F1. The NG and Xn-C interfaces for a gNB terminate in the gNB-CU. The maximum number of gNB-DUs connected to a gNB-CU is only limited by an implementation. As per 3GPP specifications, one gNB-DU connects to only one gNB-CU but implementations that allow multiple gNB-CUs to connect to a single gNB-DU. One gNB-DU may support one or more cells (sector).
The F1 interface supports signaling exchange and data transmission between the endpoints, separates Radio Network Layer and Transport Network Layer, and enables the exchange of UE-associated and non-UE-associated signaling.
Further F1 interface functions are divided into F1-Control Function (F1-C) and F1-User Function (F1-U).
F1-Control Plane (F1-C) Operations:
- F1
Interface Management Functions: It is consist of F1 setup, gNB-CU
Configuration Update, gNB-DU Configuration Update, error indication and reset function.
- System
Information Management Functions: The gNB-DU is responsible for the scheduling and broadcasting of system information. For system information broadcasting, the encoding of NR-MIB and SIB1 is performed by the gNB-DU,
while the encoding of other SI messages is performed by the gNB-CU. The F1
interface also provides signaling support for on-demand SI delivery,
enabling UE energy saving.
- F1 UE
Context Management Functions: These functions are responsible for the establishment and modification of the necessary UE context. The
establishment of the F1 UE context is initiated by the gNB-CU and the
gNB-DU can accept or reject the establishment based on admission control criteria (e.g., the gNB-DU can reject a context setup or modification request in case resources are not available). In addition, an F1 UE
context modification request can be initiated by either gNB-CU or gNB-DU.
The receiving node may accept or reject the modification. The F1 UE
context management function can be also used to establish, modify and release Data Radio Bearers (DRBs) and Signaling Radio Bearers (SRBs).
- RRC
Message Transfer Function: This function is responsible for the transferring of RRC messages from the gNB-CU to the gNB-DU, and vice versa.
F1-U (User Plane) Functions:
- Transfer of User Data: This function allows to transfer user data between gNB-CU and gNB-DU.
- Flow Control Function: This function allows to control the downlink user data transmission towards the gNB-DU. Several functionalities are introduced for improved performance on data transmission, like fast re-transmission of PDCP PDUs lost due to radio link outage, discarding redundant PDUs, the re-transmitted data indication, and the status report.
Note:
To optimize the location of different RAN functions according to different scenarios and performance requirements, the gNB-CU can be further separated into its CP and UP parts (the gNB-CU-CP and gNB-CU-UP, respectively). The interface between CU-CP and CU-UP is called E1 which purely a control plane interface. The overall RAN architecture with CU-CP and CU-UP separation is shown in the above figure.
- Transfer of User Data: This function allows to transfer user data between gNB-CU and gNB-DU.
- Flow Control Function: This function allows to control the downlink user data transmission towards the gNB-DU. Several functionalities are introduced for improved performance on data transmission, like fast re-transmission of PDCP PDUs lost due to radio link outage, discarding redundant PDUs, the re-transmitted data indication, and the status report.
Note:
E-UTRAN New Radio - Dual
Connectivity (EN-DC) is a technology that enables the introduction of 5G services
and data rates in a predominantly 4G network. UEs supporting EN-DC can connect
simultaneously to LTE Master Node eNB (MN-eNB) and 5G-NR Secondary Node gNB
(SN-gNB). This is called Non-stand alone (NSA) operation. Whereas in stand-alone (SA) operation, UE is connected to gNB only.
Mobility Support Requirements:
The following connected-mode mobility scenarios
supported is required from the CU-DU split.
- Inter-gNB-DU
Mobility: The UE moves from one gNB-DU to another within the same gNB-CU.
- Intra-gNB-DU
inter-cell mobility: TheUE
moves from one cell to another within the same gNB-DU, supported by UE
Context Modification (gNB CU initiated) procedure.
- EN-DC
Mobility with Inter-gNB-DU Mobility using MCG SRB: The UE moves from one gNB-DU to another within the same gNB-CU when only MCG SRB is available during EN-DC operation.
- EN-DC
Mobility with Inter-gNB-DU Mobility using SCG SRB: The UE moves from one gNB-DU to another when SCG SRB
is available during EN-DC operation.
Control and User Plane Separation with Higher Layer Split (HLS)
To optimize the location of different RAN functions according to different scenarios and performance requirements, the gNB-CU can be further separated into its CP and UP parts (the gNB-CU-CP and gNB-CU-UP, respectively). The interface between CU-CP and CU-UP is called E1 which purely a control plane interface. The overall RAN architecture with CU-CP and CU-UP separation is shown in the above figure.
The gNB-CU-CP hosts the RRC and the
control-plane part of the PDCP protocol; it also terminates the E1 interface
connected with the gNB-CU-UP and the F1-C interface connected with the gNB-DU.
The gNB-CU-CP hosts the user-plane part of the PDCP protocol of the gNB-CU for
an en-gNB, and the user plane part of the PDCP protocol and the SDAP protocol
of the gNB-CU for a gNB.
The gNB-CU-UP terminates the E1 interface connected with the
gNB-CU-CP and the F1-U interface connected with the gNB-DU. A gNB may consist
of a gNB-CU-CP, multiple gNB-CU-UPs, and multiple gNB-DUs. The gNB-CU-CP is
connected to the gNB-DU through the F1-C interface, and gNB-CU-UP is connected
to the gNB-DU through the F1-U interface. One gNB-CU-UP is connected to
only one gNB-CU-CP, but implementations allowing a gNB-CU-UP to connect to
multiple gNB-CU-CPs e.g. One gNB-DU
can be connected to multiple gNB-CU-UPs under the control of the same
gNB-CU-CP. One gNB-CU-UP can be connected to multiple DUs under the control of
the same gNB-CU-CP. The basic functions of the E1 interface include E1
interface management function and E1 bearer context management function.
5G NR gNB Logical Architecture and It’s Functional Split
Options-
The logical architecture of gNB is shown in the above figure with Central Unit (CU) and Distributed Unit (DU). F1-C and F1-U
provide control plane and user plane connectivity over F1 interface.
In this architecture, Central Unit
(CU) and Distribution Unit (DU) can be defined as follows.
Central Unit (CU): It is
a logical node that includes the gNB functions like Transfer of user data,
Mobility control, Radio access network sharing, Positioning, Session Management
etc., except those functions allocated exclusively to the DU. CU controls the
operation of DUs over the front-haul (Fs) interface. A central unit (CU) may also
be known as BBU/REC/RCC/C-RAN/V-RAN.
Distributed Unit (DU): This
logical node includes a subset of the gNB functions, depending on the
functional split option. Its operation is controlled by the CU. Distributed
Unit (DU) also known with other names like RRH/RRU/RE/RU.
General description of split
options :
There are 8 possible options can be
configured-
Details explanation of split option:
Option 1 (1A-like split)
Option 4 (RLC-MAC
split) :
Option 5 (Intra MAC
split) :
Option 1 (1A-like split)
- The
function split in this option is similar as 1A architecture in DC. RRC is in
the central unit. PDCP, RLC, MAC, physical layer and RF are in the distributed
unit.
Option 2 (3C-like split)
- The
function split in this option is similar as 3C architecture in DC. RRC, PDCP
are in the central unit. RLC, MAC, physical layer and RF are in the distributed
unit.
Option 3 (intra RLC split)
- Low
RLC (partial function of RLC), MAC, physical layer and RF are in distributed
unit. PDCP and high RLC (the other partial function of RLC) are in the central
unit.
Option 4 (RLC-MAC split)
- MAC,
physical layer and RF are in distributed unit. PDCP and RLC are in the central
unit.
Option 5 (intra MAC split)
- RF,
physical layer and some part the MAC layer (e.g. HARQ) are in the distributed
unit. Upper layer is in the central unit.
Option 6 (MAC-PHY split)
- Physical
layer and RF are in the distributed unit. Upper layers are in the central unit.
Option 7 (intra PHY split)
- Part
of physical layer function and RF are in the distributed unit. Upper layers are
in the central unit.
Option 8 (PHY-RF split)
- RF
functionality is in the distributed unit and upper layer are in the central
unit.
NOTE: The options represented
consist of a non-exhaustive list. The assumption on protocols and functions
definition was based on what was available during the study phase.
Details explanation of split option:
Option 1 (1A-like split)
Description: In this split option, RRC is in the
central unit. PDCP, RLC, MAC, physical layer and RF are in the distributed
unit, thus the entire user plane is in the distributed unit.
Benefits and Justification:
- This
option allows a separate U-plane while having a centralized RRC/RRM.
- It
may in some circumstances provide benefits in handling some edge computing or
low latency use cases where the user data needs to be located close to the
transmission point.
- Because
of the separation of RRC and PDCP, securing the interface in practical
deployments may or may not affect the performance of this option.
- It
needs to be clarified whether and how this option can support aggregation based
on alternative 3C.
- The
function split in this option is similar to 1A architecture in DC.
Option 2
(PDCP/RLC split)
Option 2-1 Split U-plane only (3C like split)
Option 2
(PDCP/RLC split)
Option 2-1 Split U-plane only (3C like split)
Description: In this split option, RRC, PDCP are in the
central unit. RLC, MAC, physical layer and RF are in the distributed unit.
Benefits and Justification:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can
facilitate the management of traffic load between NR and E-UTRA transmission
points.
- Fundamentals for achieving a PDCP-RLC split
have already been standardized for LTE Dual Connectivity, alternative 3C.
Therefore this split option should be the most straightforward option to
standardize and the incremental effort required to standardize it should be
relatively small.
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.
Benefits and Justification:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can facilitate the management of traffic load
between NR and E-UTRA transmission points.
- This
option enables centralization of the PDCP layer, which may be predominantly
affected by UP process and may scale with UP traffic load.
- This
option allows a separate U-plane while having a centralized RRC/RRM.
Cons:
- Coordination
of security configurations between different PDCP instances for Option 2-2
needs to be ensured
Option 3 (High
RLC/Low RLC Split)
Option 3 (High
RLC/Low RLC Split)
This option splits Low
RLC (partial function of RLC), MAC, physical layer and RF are in distributed
unit. PDCP and high RLC (the other partial function of RLC) are in the central
unit.
Two
approaches based on Real-time/Non-Real-time function split are as follows:
Option
3-1 Split based on ARQ
Description:
- Low RLC
maybe composed of segmentation functions;
- High RLC
maybe 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. Here, High RLC segments RLC
PDU based on the status reports while Low RLC segments RLC PDU into the
available MAC PDU resources.
Benefits and
Justification:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can
facilitate the management of traffic load between NR and E-UTRA transmission
points.
- This split
option may also have better flow control across the split.
- Centralization
gains: ARQ located in the CU may provide centralization or pooling gains.
- The
failure over transport network may also be recovered using the end-to-end ARQ
mechanism at CU. This may provide protection for critical data and C-plane
signaling.
- DUs
without functions of RLC may handle more connected mode UEs as there is no RLC
state information stored and hence no need for UE context.
- This
option may provide an efficient means for implementing integrated access and
backhaul to support self-backhauled NR TRPs.
NOTE: As part of the analysis with RAN2, there is
no consensus on the following benefits and drawbacks from RAN2 point of view.
Benefits and
Justification:
- This
option may have the advantage of being more robust under non-ideal transport
conditions because the ARQ and packet ordering is performed at the central
unit.
- It may
reduce processing and buffer requirements in DU due to absence of ARQ protocol
- Could be
used over multiple radio legs of different DUs for higher reliability (U-Plane
and C-Plane) [Pending to multi-connectivity]
- This
option may provide an efficient way for implementing intra-gNB RAN-based
mobility.
Cons:
- Comparatively,
the split is more latency-sensitive than the split with ARQ in DU, since
re-transmissions are susceptible to transport network latency over a split
transport network.
Option 3-2 Split based on TX RLC and RX RLC
Description:
- 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 with downlink transmission.
- High RLC
maybe composed of receiving TM RLC entity, receiving UM RLC entity and a
receiving side of AM except the routing function and reception of RLC status
report, which are related with uplink transmission.
Transmitting: Tx RLC receives RLC SDU from PDCP and
transmits these packets under the format indicator of MAC.As soon as RLC
receives the PDU request from MAC, RLC must assemble the MAC SDU under the
format indicator of MAC and submit the MAC SDU to MAC. In order to adapt the
transport network between CU and DU, it is critical that Tx RLC is placed in DU.
Receiving: Routing receives RLC PDU from MAC and judges
CONTROL PDU/DATA PDU, then submits DATA PDU to Rx RLC and CONTROL PDU to Tx
RLC. When PDCP/RLC reestablishment procedure is triggered, placing Rx RLC in CU
is critical in order to real-timely deliver data packets to PDCP.
Benefits and
Justification:
Option3-2 not only is insensitive to the transmission
network latency between CU and DU, but also uses interface format inherited
from the legacy interfaces of PDCP-RLC and MAC-RLC. Some benefits of Option3-2
are as follows:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can
facilitate the management of traffic load between NR and E-UTRA transmission
points.
- Flow
control is in the CU and for that a buffer in the CU is needed. The TX buffer
is placed in the DU, so that the flow controlled traffic from the CU can be
buffered before being transmitted. Flow control can be done depending on
fronthaul conditions
- As Rx
RLC is placed in CU, there is no additional transmission delay of PDCP/RLC
reestablishment procedure when submitting the RLC SDUs to PDCP
- This
option does not induce any transport constraint, e.g. transport network
congestion. MAC submits RLC PDUs as a whole packet to RLC rather than RLC
sending RLC SDUs to PDCP.
Cons:
- Compared
to the case where RLC is not split, STATUS PDU of AM Rx RLC may lead to
additional time delay. Because STATUS PDU must be submitted through PDCP-Tx RLC
interface from CU to DU before Tx RLC in DU transmits it over air interface,
which may lead to additional transport delay.
- Due
to performing flow control in the CU and RLC Tx in the DU two buffers are
needed for transmission, one at the CU, which allows to flow control data
submission to the RLC Tx, and one at the DU in order to perform RLC TX
Option 4 (RLC-MAC
split) :
Description: In this
split option, RRC, PDCP and RLC are in the central unit.MAC, physical layer and
RF are in the distributed unit.
Benefits and Justification: In the context of the
LTE protocol stack a benefit is not foreseen for option 4. This might be
revised with NR protocol stack knowledge.
Option 5 (Intra MAC
split) :
Description:
Option 5 assumes the following distribution:
- RF,
physical layer and lower part of the MAC layer (Low-MAC) in the Distributed
Unit
- Higher
part of the MAC layer (High-MAC), RLC and PDCP in the Central Unit
Therefore by splitting the MAC layer into 2 entities (e.g.
High-MAC and Low-MAC), the services and functions provided by the MAC layer
will be located in the Central Unit (CU), in the Distributed Unit (DU), or in
both. An example of this distribution and its justification is given below.
In High-MAC sublayer:
The centralized
scheduling in the High-MAC sublayer will be in charge of the control of
multiple Low-MAC sublayers. It takes high-level centralized scheduling
decision.
The inter-cell interference coordination in the High-MAC
sublayer will be in charge of interference coordination methods such as JP/CS
CoMP.
In Low-MAC sublayer:
Time-critical functions
in the Low-MAC sublayer include the 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.
Radio specific functions in the Low-MAC sublayer can for perform
scheduling-related information processing and reporting. It can also
measure/estimate the activities on the configured operations or the served UE’s
statistics and report periodically or as requested to the High-MAC sublayer.
Depending on the
different implementations of the intra-MAC functional split, the following pros
and cons can be defined:
Benefits and Justification:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can
facilitate the management of traffic load between NR and E-UTRA transmission
points.
- Reduce
the bandwidth needed on fronthaul, dependent on the load of RAN-CN interface;
- Reducing
latency requirement on fronthaul (if HARQ processing and cell-specific MAC
functionalities are performed in the DU);
- Efficient
interference management across multiple cells and enhanced scheduling technologies
such as CoMP, CA, etc., with multi-cell view;
Cons:
- Complexity
of the interface between CU and DU;
- Difficulty
in defining scheduling operations over CU and DU;
- Scheduling
decision between CU and DU will be subject to fronthaul delays, which can
impact performances in case of non-ideal fronthaul and short TTI;
- Limitations
for some CoMP schemes (e.g. UL JR).
Option 6 (MAC-PHY split) :
Description:
The MAC and upper layers are in the central unit (CU). PHY layer and RF are in
the DU. The interface between the CU and DUs carries data, configuration, and
scheduling-related information (e.g. MCS, Layer Mapping, Beamforming, Antenna
Configuration, resource block allocation, etc.) and measurements.
Benefits and Justification:
- This
option will allow traffic aggregation from NR and E-UTRA transmission points to
be centralized. Additionally, it can
facilitate the management of traffic load between NR and E-UTRA transmission
points.
- This
option is expected to reduce the fronthaul requirements in terms of throughput to
the baseband bitrates as the payload for Option 6 is transport block bits.
- Joint
Transmission is possible with this option as MAC is in CU.
- Centralized
scheduling is possible for Option 6 as MAC is in CU.
- It
allows resource pooling for layers including and above MAC.
Cons:
- This
split may require subframe-level timing interactions between MAC layer in CU
and PHY layers in DUs. Round trip fronthaul delay may affect HARQ timing and
scheduling.
5G BASIC
Reviewed by LTE/IMS reference
on
May 24, 2020
Rating:
Reviewed by LTE/IMS reference
on
May 24, 2020
Rating:
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