LTE, Long Term Evolution An introduction - unipi.itcalcolatori.iet.unipi.it/seminari/2012-04-02/lte_carlo_2012_last.pdf · LTE, Long Term Evolution An introduction. LTE, Long Term

LTE, Long Term EvolutionCarlo Vallati

C. VallatiUniversity of Pisa, Italy

LTE, Long Term EvolutionAn introduction


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


Introduction LTE is the last major step in mobile radio

communications A long history of standards have been defined since

1990


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


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


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


Architecture

EPCEvolved Packet Core Network

E-UTRANUMTS Terrestrial Radio Access Network


ArchitectureProtocol stack


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


PHY Layer Downlink/Uplink multiplexing:

v in time (Time Division Duplexing – TDD)v in frequency (Frequency Division Duplexing - FDD)

TDD

Frequ

en

cy

TIME

FDD

Freq

uen

cyTIME

DL

UL


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


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)


PHY Layer The frequency spectrum is divided into multiple sub-

carriers, which are orthogonal to each other


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

en

cy D

om

ain

Time Domain

User 1

User 2

User 3

OFDMA


PHY Layer Resource Block

Freq

uen

cy

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


PHY Layer OFDMA time-frequency multiplexing


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


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


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


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


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


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


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


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)


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


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


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


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


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

cy D

om

ain

Time Domain

UE 16 RBs

UE 212 RBs

UE 38RBs


MAC Layer Scheduling policy: opportunism vs fairness

Freq

uen

cy D

om

ain

Time Domain

UE 3Fr

eq

uen

cy D

om

ain

Time Domain

UE 2

UE 1

UE 3

Fairness Opportunism

Good Ch Cond

Bad Ch Cond

UE 4

UE 1 UE 2

UE 4


RLC Layer RLC Layer is the interface between upper layers and

MAC layers.


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


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


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


Thanks for the attentionComments or questions?

Carlo [email protected]

Dipartimento di Ingegneria dell'InformazioneUniversity of Pisa, Italy

Documents

LTE, Long Term Evolution An introduction - unipi.itcalcolatori.iet.unipi.it/seminari/2012-04-02/lte_carlo_2012_last.pdf · LTE, Long Term Evolution An introduction. LTE, Long Term