LTE/SAE Engineering Overview
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Radio Access Protocols
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Contents
Radio Access Protocol Stack...4.1 SCTP (Stream Control Transmission Protocol) ...4.2 PDCP And RRC ...4.3 MAC PDU Structure ...4.4 MAC Random Access Procedure ...4.5 MAC Scheduling Functions...4.6 Resource Multiplexing...4.7 QoS and Priority Handling...4.8 HARQ Operation ...4.9 RLC Modes ...4.10 RLC PDU Structure...4.11 PDCP Operation ...4.12 Integrity Protection ...4.13 RRC Functions...4.14 RRC States ...4.15 Radio Resource Management Functions...4.16 Network Nodes and Areas ...4.17 System Acquisition...4.18 Attach Procedures...4.19 Idle Mode Procedures ...4.20 Tracking Areas and Paging...4.21 LTE Handover Types ...4.22
LTE/SAE Engineering Overview
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Radio Access Protocols
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Objectives
At the end of this section you will be able to:
list the general set of protocols employed for E-UTRA above the physical layer and list their main functions
outline the general structure of the E-UTRA MAC layer
describe the format of a typical MAC PDU
describe the functions performed by the physical layer and the MAC in support of random access procedures
outline the MAC scheduling concept
outline the methods employed by the MAC to multiplex user traffic
list the basic QoS levels described so far for the E-UTRAN specifications
describe the methods employed by the MAC to manage retransmission and other HARQ functions
outline the basic functions of the RLC layer and describe the layout of the RLC PDU
describe the differences between the RLC connection modes – transparent, unacknowledged and acknowledged
demonstrate an understanding of the role and functions of the PDCP layer
list the functions of the RRC layer and describe the two RRC states
outline the main RRM functions of the E-UTRAN
outline the basic procedures followed in respect of cell search, network attach, idle mode, connection establishment and handover
identify the handover options for radio access through the E-UTRAN
LTE/SAE Engineering Overview
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Radio Access Protocols
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Non-Access
Stratum (NAS) Non-Access
Stratum (NAS)
RRC RRC
PDCP PDCP
RLC RLC
MAC MAC
Physical Layer Physical Layer
User Equipment eNB Evolved Packet Core
Radio Access Protocol Stack
The Long Term Evolution of UMTS has demanded a reduction in the complexity of the relationship between the UE and the RAN. This in turn allows a reduction in the complexity of the access network itself.
Formerly, UMTS air interface protocols operating from the UE interacted with spilt destinations; the physical layer operated between the UE and the Node B, while the MAC, RLC, PDCP and RRC were all forwarded to the RNC.
In the E-UTRA all air interface protocols operate between the UE and the eNB only. The protocol layers are shown in the diagram.
The MAC layer is responsible for random access management, scheduling, priority, handling, multiplexing, HARQ error correction and transport format selection.
The RLC layer is responsible for managing different transfer modes, transfer of upper-layer PDUs, error correction through ARQ, concatenation, segmentation and reassembly of RLC SDUs (Service Data Units).
Further Reading: 3GPP TS 36.300
LTE/SAE Engineering Overview
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SCTP Signalling Associations eNB
eNB eNB
EPC Nodes
SCTP (Stream Control Transmission Protocol)
In UMTS Release 99, Iu interface traffic was carried over ATM (Asynchronous Transfer Mode) with different AAL (ATM Adaptation Layer) types being used to encapsulate and transport control and user plane traffic. The Release 5 specifications provided an upgrade path to ‘IP Transport’, which allowed existing U-plane traffic formats to be encapsulated by IP in place of ATM.
The Release 8 EPS specifications describe an all-IP network environment in which the message formats themselves change, using the S1AP and X2AP protocols for control-plane traffic, which are both carried over IP. In order to provide effective transport for signalling messages, the protocol used above IP at layer 4 must provide fast and reliable data transmission.
The protocols most often seen at layer 4 in an IP protocol stack, UDP and TCP, cannot provide a good enough service for signalling traffic. UDP provides a fast but connectionless service in which there are no facilities to retransmit errored packets, remove duplicate packets, or ensure sequenced delivery. TCP provides the required reliability but suffers from timing problems caused by, for example, head-of-line blocking whilst a lost or errored packet is retransmitted.
SCTP is a general-purpose transport protocol designed by the IETF for message-oriented applications such as signalling. It overcomes the problems identified with TCP and UDP in a
conventional IP protocol stack by breaking message flows between two peer devices into a number of separate ‘streams’ – each stream manages its own retransmission process, so that retransmission of an errored packet belonging to one stream does not slow down delivery of packets belonging to other streams. SCTP also introduces network-level fault tolerance with its multihoming feature, which allows one device to be connected to IP links in multiple networks.
SCTP was originally designed as a reliable method of carrying SS7 (Signalling System No. 7) signalling messages over an IP network, but it can carry messages for a wide variety of other communications protocols too.
Further Reading: 3GPP TS 23.401; IETF RFC 2690
Radio Access Protocols
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Segm.
Scheduling / Priority Handling
PCCH
Security Security Security Security
Downlink
PDCP is responsible for:
header compression, using RoHC (Robust Header Compression)
ciphering of U- and C- plane data
integrity protection for C-plane data
transfer of U-plane and RRC data
The RRC layer is responsible for:
creation of BCH (Broadcast Channel) system information
creation and management of the PCH (Paging Channel)
RRC connection management between eNB and UEs, including generating temporary identifiers such as the C-RNTI (Cell Radio Network Temporary Identifier) mobility-related functions such as measurement reporting, inter-cell handover and inter-eNB UE context handover
QoS management
direct transfer of messages from the NAS (Non-Access Stratum) to the UE
Further Reading: 3GPP TS 36.300
LTE/SAE Engineering Overview
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Padding
MAC header MAC payload
MAC PDU
MAC PDU Structure
E-UTRA upper-layer data is formatted into transport blocks. Transport blocks pass through several stages of formatting before arriving at the MAC layer, where they are placed into MAC PDUs.
The basic structure of a MAC PDU consists of a MAC header part and MAC payload part. The payload part contains variable numbers of MAC SDUs carrying data, MAC CEs (Control Elements) and, if required, a padding field. The fields within a MAC PDU vary in size. The PDU itself, however, must be sized to fit into the physical layer resource blocks to which they will be mapped. Downlink frames must be transmitted in their entirety; there cannot be any gaps in transmission, but the
resource block structure may not fit exactly with the amount of data being carried in a PDU, hence the need for padding.
The MAC header part consists of one or more MAC sub-headers. Each MAC sub-header relates to one of the MAC SDUs, MAC control elements and, under some circumstances, the padding elements in the MAC payload part. The order of MAC sub-headers matches the order of the MAC payload elements to which they relate.
Most MAC sub-headers are made up of six elements: two reserved bits, one extension field bit, one LCID (Logical Channel Identity), one ‘F-field’ bit and one length indicator. The extension field ‘E’ bit indicates whether there are any further sub-headers before the payload part. The LCID identifies the logical channel identity for the corresponding MAC SDU or, if the sub-header relates to a MAC control element or padding, it acts as a type field. The ‘F-field’ indicates the size of the length indicator (either 7 or 15 bits). The length indicator indicates the length of the corresponding MAC SDU or control element in octets.
Transport blocks are not permitted to overlap across multiple MAC PDUs since a maximum of one MAC PDU can be transmitted per transport block per UE. A transport block will be transmitted within the TTI, which for E-UTRA is defined as a subframe period, or 1 ms.
Further Reading: 3GPP TS 36.321
Radio Access Protocols
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PBCH – RACH Preamble, Contention parameters
RACH Preamble
C-RNTI, TA, Power Adjustment
MAC Random Access Procedure
The random access procedure is handled by the MAC and the physical layer and operates using a combination of the PRACH on the uplink and the PDCCH.
UEs are informed of the range of random access preambles available in the current cell via the PBCH, as are the contention management parameters (maximum number of retries, back-off variables, etc.) currently in force.
When a random access event is required, the UE will perform the following functions:
review and randomly select a preamble
check the BCCH for the current PRACH configuration; this will indicate the location and periodicity of PRACH resources in uplink subframes
calculate open loop power control parameters – initial transmit power, maximum transmit power and power step
discover contention management parameters
As with legacy WCDMA systems, once the UE transmits an initial RACH it will wait a specified period of time (RA_WINDOW) for a response before backing off and retrying. Open loop power control ensures that each successive retry will be at a higher power level.
Upon receipt of a successful uplink RACH preamble, the eNB will calculate power adjustment and timing advance parameters for the UE based on the strength and delay of the received signal and schedule an uplink capacity grant to enable the UE to send further details of its request. If necessary, it will also assign a RNTI (Radio Network Temporary Identifier) for the UE to use for radio mobility.
Further Reading: 3GPP TS 36.321
LTE/SAE Engineering Overview
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Data Data Data Data Data
Data Data Data Data Data
MAC Downlink Assignment (PDCCH) MAC Uplink Grant(PDCCH)
RRC (DL-SCH) Semi-Persistent scheduling
MAC Scheduling Functions
The main function of the MAC is to manage the shared access to a common transmission medium by multiple devices. This is achieved through the eNB’s scheduling function. Resource allocation will be performed on the basis of a scheduling algorithm, the specifics of which are not defined by the standards. However, channel performance, data buffer fill, UE power capability and traffic priority are likely to be considered.
When a UE establishes an RRC relationship with an eNB it is assigned a C-RNTI, which will uniquely identify that UE in that cell. The C-RNTI will be used to address any control and scheduling messages to or from the UE. Each UE is capable of establishing multiple EPS bearers, which are the NAS traffic and signalling connections that travel from the UE to the core network.
Allocations take the form of one or more PRB (Physical Resource Block), which will be populated using a specified MCS (Modulation and Coding Scheme). The allocations can be made for one or more TTI periods.
LTE offers three scheduling modes. The first, known as ‘dynamic scheduling’, involves the use of MAC downlink assignment messages and uplink grant messages in the PDCCH to allocate resources as required. Dynamic scheduling is intended for typical bursty packet data traffic.
For VoIP (Voice over IP) traffic where regular and reliable allocation of resources is required to meet more demanding QoS requirements, LTE offers ‘persistent scheduling’. This is achieved through a combination of RRC signalling in the DL-SCH (Downlink Shared Channel), for the initial specification of the resource allocation interval, and MAC signalling in the PDCCH for more specific PRB and MCS information. The result is a lower overhead in the PDCCH for these regular resource allocations.
The third scheduling option, known as ‘semi-persistent scheduling’, is used specifically for the purpose of resource allocation for the establishment or reconfiguration of a persistent scheduled resource., i.e. for the transport of RRC messages relating to the persistent scheduled resource. In this case an SPS (Semi Persistent Scheduling) will be used to address the UE, which is different from the UE’s C-RNTI.
Further Reading: 3GPP TS 36.321, 36.331
Radio Access Protocols
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SAE LCID 10
LCID 12
Voice bearer
Data bearer C-RNTI – a12b
LCID 10 L E LCID
12 L E
PDCCH Scheduling Grant to C-RNTI a12b
Resource Multiplexing
Each separate traffic flow, carrying one or more upper-layer EPS bearer connections, is assigned an LCID at the physical layer.
The LCID is used to differentiate between logical data flows belonging to the same UE and is therefore be significant under the umbrella of one C-RNTI.
Scheduling of non-multicast/broadcast traffic takes place on a ‘per UE’, or more precisely, on a ‘per C-RNTI’ basis. The allocation of uplink or downlink capacity to a UE is signalled on the PDCCH by the inclusion of the UE’s C-RNTI and the parameters of the allocated capacity.
A MAC PDU will be mapped into the capacity thus signalled. The MAC PDU format makes it possible to multiplex a number of upper-layer SDUs and control messages together, the position of each within the PDU being flagged by MAC sub-headers showing the MAC SDU’s LCID.
In this scenario, each MAC PDU will carry traffic for one UE only, but will be capable of multiplexing together traffic from several parallel services connected to that UE.
Further Reading: 3GPP TS 36.321
LTE/SAE Engineering Overview
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Downlink:
Guaranteed Bit Rate (GBR)
Aggregate Maximum Bit Rate (AMBR)
Uplink:
Prioritized Bit Rate (PBR) Maximum Bit Rate (MBR)
QoS and Priority Handling
QoS definition is the responsibility of the RRC layer but also has a bearing on the functionality of the MAC scheduler.
On the downlink, SAE bearers are assigned a QoS level of either GBR (Guaranteed Bit Rate) or AMBR (Aggregate Maximum Bit Rate).
GBR is applied to individual bearers, whilst AMBR is applied to a group of aggregated bearers which have been given no individual rate guarantees but which must share bandwidth with other members of the group.
On the uplink, EPS bearers are first assigned a priority level, determined by the traffic type or some other differentiator.
Each priority group in a cell is assigned a PBR (Prioritized Bit Rate), which limits the bit rates available to members of each group. Additionally, each bearer will be assigned its own specific MBR (Maximum Bit Rate).
Scheduling for uplink allocations is handled on the basis of decreasing priority. For example, EPS bearers with the highest priority are scheduled first, then others in decreasing priority until capacity is exhausted.
Further Reading: 3GPP TS 36.300, 36.331
Radio Access Protocols
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Punctured Transmission
Pattern 1
Pattern 2
Pattern 3 Original data block
Chase Combining Incremental Redundancy
N-process stop and wait subchannels
Sequential transport blocks assigned to different HARQ
subchannels
A B C A B C A B C
HARQ Operation
The ARQ process is managed by the RLC layer, while HARQ is handled by the physical layer and the MAC. Both processes are employed to manage the transmission and retransmission of transport blocks. ARQ performs retransmission of RLC PDUs carried by RLC acknowledged mode connections that fail to arrive and are therefore not acknowledged. Based on ACK/NACK responses from the peer RLC entity, ARQ will simply retransmit failed PDUs in sequence.
HARQ is designed to speed up the retransmission cycle and to reduce the effect of retransmission problems such as head-of-line blocking. It does this in two ways. Firstly, HARQ employs complex TB (Transport Block) coding methods which initially reduce the amount of data transferred per block. This is achieved using ‘puncturing’ techniques, where specific bits in each TB are ‘knocked out’ of the block following a puncturing algorithm. The reduced size of each punctured TB therefore increases the overall capacity of the air interface. For punctured blocks that are received unerrored, the inverse of the puncturing process restores the missing data ready for the block to be processed.
HARQ has two methods of dealing with errored blocks – chase combining and incremental
redundancy. Chase combining simply retransmits the errored block and the receiving station attempts to build a ‘good’ copy of the original data from the received versions. Incremental redundancy is more complex and uses three puncturing algorithms on the original transport block. If the first copy of the block, punctured with the first algorithm, arrives with errors, the retransmitted block will be created using an alternative puncturing method. Again, the receiving station attempts to build a ‘good’ copy of the data from the blocks received.
The other technique employed by HARQ is ‘N-process Stop and Wait’. In this technique the sequential stream of TBs travelling over a bearer is divided into a number of logical HARQ
subchannels – for example, the 1st, 4th, 7th, etc. blocks could form subchannel A, the 2nd, 5th, 8th, etc. subchannel B and so on. Retransmission of an errored TB belonging to subchannel A only affects subsequent TBs belonging to that subchannel. TBs belonging to other subchannels will not be
affected by the need to stop and wait for the missing TB to be retransmitted.
Further Reading: 3GPP TS 36.321
LTE/SAE Engineering Overview
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