LTE, Long Term EvolutionCarlo Vallati
C. VallatiUniversity of Pisa, Italy
LTE, Long Term EvolutionAn introduction
LTE, Long Term EvolutionCarlo Vallati
Introduction LTE (Long Term Evolution) is a standard for wireless
communication of high speed data for mobile phones and data terminals
Developed by 3GPP (3rd Generation Partnership Project), a consortium of telecommunication associations formed for defining communication standards
LTE, Long Term EvolutionCarlo Vallati
Introduction LTE is the last major step in mobile radio
communications A long history of standards have been defined since
1990
LTE, Long Term EvolutionCarlo Vallati
Introduction Peak Data Rate: target 150 Mbps (downlink) and
50 Mbps (uplink) (20 MHz spectrum allocation, 2x1 MIMO)
Throughput: Downlink target is 3-4 times better than HSDPA. Uplink target is 2-3 times better than HSUPA
LTE, Long Term EvolutionCarlo Vallati
Introduction Latency: The one-way transit time for a packet
traveling from UE to EnB and vice versa shall be less than 5 ms
Quality of Service: End-to-end Quality of Service (QoS) shall be supported
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Introduction Mobility: The system should be optimized for low
mobile speed (0-15km/h), but higher mobile speeds shall be supported as well including high speed train environment as special case
Interworking: Interruption time for shall be less than 300 ms for real time services and less than 500 ms for non real time services
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Architecture
EPCEvolved Packet Core Network
E-UTRANUMTS Terrestrial Radio Access Network
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ArchitectureProtocol stack
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PHY Layer
BS controls and regulates the access to the shared wireless,
both uplink and downlink
Standard defines the way the shared wireless means is accessed
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PHY Layer Downlink/Uplink multiplexing:
v in time (Time Division Duplexing – TDD)v in frequency (Frequency Division Duplexing - FDD)
TDD
Frequ
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TIME
FDD
Freq
uen
cyTIME
DL
UL
LTE, Long Term EvolutionCarlo Vallati
PHY Layer The generic radio frame for TDD and FDD has a
duration of 10ms and consists of 10 sub-frames or TTI with a duration of 1ms
Each TTI can be assigned for either downlink or uplink transmission
At the beginning of each TTI the eNodeB broadcasts the allocation of the resources
1ms
10 ms
1ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms 1ms
Radio Frame
TTI
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PHY Layer LTE uses OFDMA for the downlink.
v OFDMA meets the LTE requirement for spectrum flexibility v enables cost-efficient solutions for wide carriers with high
peak rates
It is a well established technology, for example in standards such asv IEEE 802.11a/g v 802.16 (WiMax)v Digital Video Broadcast (DVB)
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PHY Layer The frequency spectrum is divided into multiple sub-
carriers, which are orthogonal to each other
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PHY Layer Orthogonal Frequency-Division Multiple Access
(OFDMA) is a multi-user version of the popular OFDM digital modulation scheme.
Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users over the time
Frequ
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om
ain
Time Domain
User 1
User 2
User 3
OFDMA
LTE, Long Term EvolutionCarlo Vallati
PHY Layer Resource Block
Freq
uen
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Dom
ain
Time Domain
180 kHz = 12 subcarriersSubcarrier spacing = 15 kHz
1 slot = 0.5 ms =7 OFDM symbols
1 subframe =1 ms= 1 TTI
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PHY Layer OFDMA time-frequency multiplexing
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PHY Layer From the perspective of the MAC layer, each slot is the
basic allocation unit, called Resource Block (RB) A subset is dedicated to control signaling:
v downlink scheduling information, transport format, resource allocation, and Hybrid-ARQ information
v uplink scheduling grant
Resource Block
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PHY Layer Link adaptation is already known from HSPA as
Adaptive Modulation and Coding Modulation and coding for the data channel is not
fixed, but it is adapted according to radio link quality Each UE provides a feedback about channel quality The amount of bits transmitted on a resource block
depends on the modulation adopted
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PHY Layer The LTE uplink transmission scheme for FDD and
TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access)v SC-FDMA signal processing has some similarities with
OFDMA signal processing, so parameterization of downlink and uplink can be harmonized
v OFDMA in uplink would consume too much transmission power for UEs
LTE, Long Term EvolutionCarlo Vallati
PHY Layer In terms of resource allocation SC-FDMA is similar to
OFDM signal, but…v in SC-FDMA, only contiguous carriers can be allocated
to the same user
Frequency
Dom
ain
Time Domain
User 1
User 2
User 3
OFDMA
User 1
Freq
uency
Dom
ain
Time Domain
User 1
User 2
SC-FDMA
User 1User 3
User 3
User 3
User 3
User 3
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MAC Layer LTE MAC defines:
v format of the messages exchanged in the LTE networkv sequence of control messages for service operations
and data transmission
MAC Header MAC PDU
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MAC Layer The connection point between PHY layer and MAC
layer is represented by the Hybrid Automatic Repeat reQuest (HARQ)
The HARQ is an stop-and-wait protocol with a maximum number of retransmission
HARQ is a combination of forwarding error coding (FEC) and error detection (ED) using ARQ
UE 1 UE 3
Downlink TTI
UPLINK TTI
UE 2
ACK NACK ACK
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MAC Layer In addition, HARQ can be used with soft combining Two transmissions cannot be independently decoded
without error The receiver combine them to increase decoding
probability Two possible mode: chase combining and
incremental redundancy
PACKET
1st tx 2nd tx
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MAC Layer MAC header can contain one or more control
elements (CE) CE carry control information directed to UE or NodeB Most important CEs are:
v Channel Quality Indicator (CQI)v Buffer Status Report (BSR)
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MAC Layer Channel Quality Indicator (CQI)
v a measurement of the communication quality of the wireless channel
v high CQI value indicates high quality channelv computed using signal-to-noise ratio (SNR)v reported from UEs to NODEBs through the control
channels or embedded into the data channel (control element into the MAC PDU header)
CQI
SNR
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MAC Layer Buffer Status Report (BSR)
v contains quantized information on the amount of data waiting for transmission in the UE buffers
v is sent into an uplink MAC PDU header control element (CE)
BSR
MAC SDUBUFFER
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MAC Layer LTE Scheduling Request (SR)
v it allows the UE to request uplink-transmission resources from the eNB
v The SR conveys a single bit of information, indicating that the UE has new data to transmit
v The SR mechanism is one of two types: dedicated SR (D-SR) : an allocated resource block is used, random access-based SR (RA-SR), no uplink resource block
are available, a random procedure shared with other Ues is executed
SR
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MAC Layer LTE Random Access Procedure
RA Preamble
RA Message (Ue Identity, BSR,etc.)
RA Response (timing advance, UL Grant, etc.)
RA contention resolution (UL grant, DL assignment)
Further uplink/downlink transmissions
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MAC Layer From the perspective of the MAC layer, each time slot is
viewed as a contiguous list of Resource Blocks (RBs) Scheduling is done in the base station (eNodeB) Scheduler assigns resources in terms of Resource Blocks and
Modulation to be used to encode them Fr
eq
uen
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om
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Time Domain
UE 16 RBs
UE 212 RBs
UE 38RBs
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MAC Layer Scheduling policy: opportunism vs fairness
Freq
uen
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Time Domain
UE 3Fr
eq
uen
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Time Domain
UE 2
UE 1
UE 3
Fairness Opportunism
Good Ch Cond
Bad Ch Cond
UE 4
UE 1 UE 2
UE 4
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RLC Layer RLC Layer is the interface between upper layers and
MAC layers.
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RLC Layer Segmentation
v The RLC layer is used to format and transport traffic between the UE and the eNB
RLC Layer
MAC Layer
IP Layer
Segmentation
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RLC Layer Reordering and duplicate detection Automatic Repeat reQuest (ARQ) using ED codes
IP
PDCP
RLC
MAC
PHY
RLC
MAC
PHY
UE
NODE BPDU 1
ACK 1
PDU 1
PDU 2
PDU 3
LTE, Long Term EvolutionCarlo Vallati
RLC Layer RLC provides three different reliability modes for
data transportv Acknowledged Mode (AM)
Concatenation, segmentation and reassembly of RLC SDUs
Reordering of RLC data PDUsDuplicate detectionAutomatic Repeat reQuest
v Unacknowledged Mode (UM)Concatenation, segmentation and reassembly of RLC
SDUsReordering of RLC data PDUsDuplicate detection
v Transparent Mode (TM)no functions
LTE, Long Term EvolutionCarlo Vallati
Thanks for the attentionComments or questions?
Carlo [email protected]
Dipartimento di Ingegneria dell'InformazioneUniversity of Pisa, Italy