1

Long Term Evolution

Technology training(Part 1)

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Outline

• LTE and SAE overview• LTE radio interface architecture• LTE radio access architecture• LTE multiple antenna techniques

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LTE/SAE OVERVIEWPart 1

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Mobile broadband (3GPP)Release Standardized Commercial Major features

3GPP R99 1999 2000 •Bearer services •64 kbit/s CS•384 kbit/s PS•Location services•Call services: compatible with GSM

3GPP R5 2002 2006 • IP Multimedia Subsystem (IMS)• IPv6, IP transport in UTRAN• Improvements in GERAN•HSDPA

3GPP R6 2004 2007 • Multimedia broadcast and multicast•Improvements in IMS•HSUPA•Fractional DPCH

3GPP R7 2007 2008 •Enhanced L2•64 QAM , MIMO•VoIP over HSPA•CPC - continuous packet connectivity•FRLC - Flexible RLC

3GPP R8 2008 2010 •DC-HSPA+ (Dual Cell HSPA+)•HSUPA 16QAM

3GPP R8 (LTE) 2008 2010 •New air interface (OFDM/SC-FDMA)•New core network

• 3G continues to evolve• Standardized through 3GPP• 3G gracefully evolves into 4G –

starting from R7 and R8• Date rates

– R99: 0.4Mbps UL, 0.4Mbps DL– R5: 0.4Mbps UL, 14Mbps DL– R6: 5.7Mbps UL, 14Mbps DL– R7: 11Mbps UL, 28Mbps DL– R8: 50Mbps UL on LTE, 160 Mbps

DL on LTE, 42Mbps DL on HSPA

• Two branches of the standards– HSPA : Gradual performance

improvements at lower incremental costs

– LTE: revolutionary changes with significant performance improvements (higher cost, first step towards IMT advanced)

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LTE Releases

• LTE – has an “evolution path” of its own• Evolution is towards IMT-Advanced (LTE advanced)• LTE advanced – spectral efficiency 30bps/Hz (DL), 15bps/Hz (UL)

Release Standardized Commercial Major features

3GPP R8 (LTE) 2008 2010

•Multi antenna support•Channel dependent scheduling•Bandwidth flexibility•ICIC (Intercell Interference Coordination)•Hybrid ARQ •FDD + TDD support

3GPP R9 (LTE) 2009•Dual layer beam forming •Network based UE positioning•MBSFN (Multicast/Broadcast Single Frequency Network)

3GPP R10 (LTE)LTE Advanced 2010

•Multi antenna extension•Relaying•Carrier aggregation •Heterogeneous networks (HetNet’s)

Note: This presentation focuses on R8 features

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LTE requirements • Outlined in 3GPP TR 29.913• Seven different areas

– Capabilities– System performance– Deployment related aspects– Architecture and migration– Radio resource management– Complexity, and– General aspects

• Capabilities– DL data rate > 100 Mbps in 20 MHz– UL data rate > 50 Mbps in 20MHz– Rate scales linearly with spectrum– Latency user plane: 5ms (transmission of

small packet from UE to edge of RAN)– Latency control plane: transmission time

from camped state – 100ms, transmission time from dormant state 50 ms

– Support for 200 mobiles in 5MHz, 400 mobiles in more than 5MHz

• System performance– Baseline is HSPA Rel. 6– Throughput specified at 5% and 50%– Maximum performance for low mobility

users (0-15km/h)– High performance up to 120 km/h– Maximum supported speed 500km/h– Cell range up to 100km– Spectral efficiency for broadcast 1 b/s/Hz

Performance measure

DL target relative to base line

UL target relative to baseline

Average throughput per MHz

3-4 times 2-3 times

Cell edge user throughput per MHz

2-3 times 2-3 times

Spectrum efficiency (bit/sec/Hz)

3-4 times 2-3 times

Throughput requirements relative to baseline

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LTE requirements (2)• Deployment related aspects

– LTE may be deployed as standalone or together with WCDMA/HSPA and/or GSM/GPRS

– Full mobility between different RANs– Handover interruption time targets

specified

• Spectrum flexibility– Both paired and unpaired bands – IMT 2000 bands (co-existence with

WCDMA and GSM)– Channel bandwidth from 1.4-20MHz

Non-real time services (ms)

Real time services (ms)

LTE to WCDMA 500 300

LTE to GSM 500 300

Handover interruption time

LTE duplexing options

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LTE requirements (3)

• Architecture and migration– Single RAN architecture– RAN is fully packet based with support

for real time conversational class– RAN architecture should minimize

“single points” of failure– RAN should simplify and reduce

number of interfaces– Radio Network Layer and Transport

Network Layer interaction should not be precluded in interest of performance

– QoS support should be provided for various types of traffic

• Radio resource management– Support for enhanced end to end QoS– Support for load sharing between different

radio access technologies (RATs)

• Complexity– LTE should be less complex than

WCDMA/HSPA

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SAE design targets

• SAE – Service Architecture Evolution• SAE = core network• Requirements placed into seven categories

– High level and operational aspects– Basic capabilities– Multi-access and seamless mobility– Man-machine interface aspects– Performance requirements for Evolved 3GPP system– Security and privacy– Charging aspects

• SAE requirements mainly non access related (highlighted ones have impact on RAN)

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Basic principles – Air interface• Downlink OFDM• OFDM = Orthogonal Frequency

Division Multiplexing• OFDM = Parallel transmission on

multiple carriers• Advantages of OFDM

– Avoid intra-cell interference – Robust with respect to multi-path propagation

and channel dispersion

• Disadvantage of OFDM– High PAPR and lower power amplifier

efficiency

• Uplink DFTS-OFDM (SC-FDMA)• DFTS = DFT spread OFDM• SC-FDMA = Single carrier FDMA• Advantages (all critical for UL)

– Signal has single carrier properties– Low PAPR– Similar hardware as OFDM– Reduced PA cost– Efficient power consumption

• Disadvantage – Equalizer needed (not critical from UL)

DL modulation

UL modulation

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Basic principles – Air interface• Shared channel transmission

– Only PS support– No CS services

• Fast channel dependent scheduling

– Adaptation in time – Adaptation in frequency– Adaptation in code

• Hybrid ARQ with soft combining– Chain combining– Incremental redundancy

ARQ reduces required Eb/No

Scheduler takes the advantage of time-frequency variations of the channel

One shared channel simplifies the overall signaling

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Basic principles – air interface

• MIMO support– MIMO = Multiple Input Multiple Output– Use of multiple TX / RX antennas– Three ways of utilizing MIMO

• RX diversity/TX diversity• Beam forming• Spatial multiplexing (MIMO with space time

coding)

– MIMO transmission in Rayleigh fading environment increases theoretical capacity by a factor equal to number of independent TX RX paths

– As a minimum LTE mobiles have two antennas (possibly four)

Note: Rayleigh fading de-correlates the paths and provides multiple uncorrelated channels

Outline of spatial multiplexing idea

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Basic principles – air interface• ICIC – Inter-cell interference

coordination• LTE affected by inter-cell

interference (more than HSDPA)• In LTE interference avoidance

becomes scheduling problem• By managing resources across

multiple cells inter-cell interference may be reduced

• Standard supports exchange of interference indicators between the cells

One possible implementation of ICIC. Cell edge implements N=3. Cell interior implements N=1.

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SAE-Architecture• SAE – flat architecture

– Core network,– RAN

• RAN consist of single elements: eNode B

– Single element simplifies RAN– No single point of failure

• Core network provides two planes– User plane (through SGSN)– Control plane (through MME)

• Interfaces– S1-UP (eNode B to SGSN)– S1-CP (eNode B to MME)– X2 between two eNode Bs (required for

handover)– Uu (UE to eNode B) UE – user equipment (i.e. mobile)

eNode B – base stationSGSN – Support GPRS Serving NodeGGSN – Gateway GPRS Serving NodeMME – Mobility Management EntityPCRF - Policy and Charging Rules function

LTE Network layout

SAE = System Architecture Evaluation

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LTE protocol-control plane

NAS – Non Access StratumRRC – Radio Resource ControlPDCP – Packet Data Convergence ProtocolRLC – Radio Link ControlMAC – Medium Access Control

S1-AP – S1 Application SCTP – Stream Control Transmission Prot.IP – Internet Protocol

Note: LTE control plane is almost the same as WCDMA (PDCP did not exist in WCDMA control plane)

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LTE protocol- user plane

PDCP – Packet Data Convergence ProtocolRLC – Radio Link ControlMAC – Medium Access Control

GTP-U - GPRS Tunneling Protocol

Note: LTE user plane is identical to UMTS PS side. There is no CS in LTE – user plane is simplified.

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LTE protocol – X2

• Connects all eNodeB’s that are supporting end user active mobility (handover)

• Supports both user plane and control plane

• Control plane – signaling required for handover execution

• User plane – packet forwarding during handover

Control plane

User plane

GTP-U: GPRS tunneling protocolSTCP: Stream Transmission Control Protocol

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Channel structure• Channels – defined on Uu

• Logical channels – Formed by RLC– Characterized by type of information

• Transport channels– Formed by MAC – Characterized by how the data are

organized

• Physical channels– Formed by PHY– Consist of a group of assignable radio

resource elementsUu interface

Note: LTE defines same types of channels as WCDMA/HSPA

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LTE - channel mapping

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Logical channels

• BCCH – Broadcast Control CH– System information sent to all UEs

• PCCH – Paging Control CH– Paging information when addressing UE

• CCCH – Common Control CH– Access information during call establishment

• DCCH – Dedicated Control CH– User specific signaling and control

• DTCH – Dedicated Traffic CH– User data

• MCCH – Multicast Control CH– Signaling for multi-cast

• MTCH – Multicast Traffic CH– Multicast data

LTE ChannelsRed – common, green – shared, blue - dedicated

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Transport channels• BCH – Broadcast CH

– Transport for BCCH

• PCH – Paging CH– Transport for PCH

• DL-SCH – Downlink Shared CH– Transport of user data and signaling.

Used by many logical channels

• MCH – Multicast channel– Used for multicast transmission

• UL-SCH – Uplink Shared CH– Transport for user data and signaling

• RACH – Random Access CH– Used for UE’s accessing the network

LTE ChannelsRed – common, green – shared

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PHY Channels• PDSCH – Physical DL Shared CH

– Uni-cast transmission and paging

• PBCH – Physical Broadcast CH– Broadcast information necessary for accessing the network

• PMCH – Physical Multicast Channel– Data and signaling for multicast

• PDCCH – Physical Downlink Control CH– Carries mainly scheduling information

• PHICH – Physical Hybrid ARQ Indicator – Reports status of Hybrid ARQ

• PCIFIC – Physical Control Format Indicator– Information required by UE so that PDSCH can be

demodulated (format of PDSCH)

• PUSCH – Physical Uplink Shared Channel– Uplink user data and signaling

• PUCCH – Physical Uplink Control Channel– Reports Hybrid ARQ acknowledgements

• PRACH – Physical Random Access Channel– Used for random access

LTE ChannelsRed – common, green – shared

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Time domain structure• Two time domain structures

– Type 1: used for FDD transmission (may be full duplex or half duplex)– Type 2: used for TDD transmission

• Both Type 1 and Type 2 are based on 10ms radio frame

Radio frame : Type 1

Radio frame : Type 2

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TDD frame configurations

• Different configurations allow balancing between DL and UL capacity

• Allocation is semi-static • Adjacent cells have same

allocation • Transition DL->UL

happens in the second subframe of each half-frame

Note: TDD frame structure allows co-existence between LTE TDD and TD-SCDMA

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Allocatable resources

• LTE – radio resource = “time-frequency chunk”

• Time domain 1 frame = 10 sub-frames 1 subframe = 2 slots 1 slot = 7 (or 6) OFDM

symbols • Frequency domain

1 OFDM carrier = 15KHz

Resource Block (RB) = 12 carriers in one TS (12*15KHz x 0.5ms)

Note: In LTE resource management is along three dimensions: Time, Frequency, Code

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Bandwidth flexibility• LTE supports deployment from 6RBs to 110 RBs in 1 RB increments• 6RBs = 6 x 12 x 15KHz = 1080KHz -> 1.4MHz (with guard band)• 110RBs = 110 X 12 X 15KHz = 19800KHz -> 20MHz (with guard band)• Typical deployment channel bandwidths: 1.4, 3, 5, 10, 15, 20 MHz• Straight forward to support other channel bandwidths (due to OFDM)

• UE needs to support up to the largest bandwidth (i.e. 20MHz)

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UE States• UE may be in three states

– Detached: not connected to the network– Idle: attached to the network but not active– Connected: attached and active

• UE tracking– Detached state: UE position unknown– Idle state: UE position know with the Tracking Area (TA) resolution – Connected: UE location known to the eNodeB resolution

Note: Both the UE states and UE tracking are simpler than in UMTS

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3GPP Specifications• All 3GPP specs are available at http://www.3gpp.org

– RAN 1 36.2xx series PHY layer– RAN2 36.3xx series Layers 2 and 3– RAN3 36.4xx series S1 and X2 interfaces– RAN4 36.1xx series Core performance requirements– RAN5 36. 5xx series Terminal conformance testing

Example specs organization

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Section review1. What are 3GPP broadband

cellular technologies?2. What releases of 3GPP standard

contains LTE?3. What were target DL and UL

throughputs for LTE?4. What does SAE stand for?5. What are components of the CS

part of the LTE core network?6. What is the access scheme used

on the DL?7. What is the role of fast scheduler

on LTE DL?8. What is the smallest allocateable

resource in LTE DL?

9. What is Radio Block (RB)?10.What are spectrum bandwidth

deployment options for LTE?11. How many radio blocks are in

20MHz deployment?12.Does LTE support TDD

deployment?13.What are three UE States

supported by LTE?

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LTE RADIO ACCESSPart 2

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Overview

• Overview of OFDM/OFDMA• LTE Downlink transmission• Overview of DFTS-OFDM• LTE Uplink transmission• Multi-antenna transmission

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Single carrier transmission• Data are used to modulate amplitude/phase (frequency) of a single carrier• Higher data rate results in wider bandwidth• Over larger bandwidths ( > 20KHz), wireless channel is frequency selective• As a result of frequency selectivity the received signal is severely distorted • Channel equalization needed• Complexity of equalizer increases rapidly with the signal bandwidth requirements

Transmission of single carrier in mobile terrestrial environment

Note: over small portion of the signal spectrum, fading may be seen as flat

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Multi-carrier transmission• Channel fading over smaller frequency bands – flat (no need for equalizer)• Divide high rate input data stream into many low rate parallel streams• At the receiver – aggregate low data rate streams

Signal for each stream experiences flat fading

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FDM versus OFDM

• OFDMA minimizes separation between carriers

• Carriers are selected so that they are orthogonal over symbol interval

• Carrier orthogonality leads to frequency domain spacing Df=1/T, where T is the symbol time

• In LTE carrier spacing is 15KHz and useful part of the symbol is 66.7 microsec

Note: orthogonality between carriers in time domain allows closer spacing in frequency domain.

FDM versus OFDM

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OFDM transmitter/receiver• Practically OFDM TX/RX is implemented using IFFT/FFT• Use of the IFFT/FFT at the baseband means that there is no need for

separate oscillators for each of the OFDM carriers• FFT (IFFT) hardware is readily available – TX/RX implementation is simple

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Guard time• Duration of the OFDM symbol is chosen to be much longer than the multi-path

delay spread• Long symbols imply low rate on individual OFDM carriers• In multipath environment long symbol minimizes the effect of channel delay spread• To make sure that there is no ISI between OFDM symbols – guard time is inserted

OFDM symbols without guard time OFDM symbols with guard time

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Cyclic prefix• Guard time eliminates ISI between OFDM symbols• Multipath propagation degrades orthogonality between carriers within an

OFDMS symbol• To regain the orthogonality between subcarriers – cyclic prefix is used• Cyclic prefix fills in the guard time between the OFDM symbols

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Block diagram of full OFDM TX/RX

• LTE supports numerous AMC schemes• AMC adds additional level of adaptation to the RF channel• Size of CP depends on the amount of dispersion in the channel• Two CP are used: normal (4.7 us) and extended (16.7 us)

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OFDMA time-frequency scheduling

• Minimum allocateable resource in LTE is Resource Block pair

• Resource block pair is 12 carriers wide in frequency domain and lasts for two time slots (1ms)

• Depending on the length of cyclic prefix RB pair may have 14 or 12 OFDM symbols

• PHY channels consist of certain number of allocated RB pairs

• Overhead channels are typically in a predetermined location in time frequency domain

• Within a RB different AMC scheme may be used

• Allocation of the radio block is done by scheduler at eNode B

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LTE DOWNLINK TRANSMISSIONPart 3

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LTE OFDMParameter Value

Bandwidth (MHz) 1.4 3 5 10 15 20Frame /subframe

duration 10/1 ms

Subcarrier spacing 15KHz

Useful symbol part 66.7us

FFT size 128 256 512 1024 1536 2048Resource blocks 6 15 25 50 75 100Number of used

subcarriers 72 180 300 600 900 1200

Cyclic prefix length Normal: 5.1us for first symbol in a slot and 4.7us for other symbols , Extended: 16.7us

OFDM symbols /slot 7 (normal CP), 6 (extended CP)

Error coding 1/3 convolutional (signaling); 1/3 turbo (data)

Basic timing unit: Ts = 1/(2048 x 15000) ~ 23.552 ns

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Detailed time domain structure

TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols

TCP-e: 512 Ts (16.7 us) for all symbols

Need for two different CP:1. To accommodate environments

with large channel dispersion2. To accommodate MBSFN (Multi-

Cast Broadcast Single Frequency Network) transmission

In case of MBSFN it may be beneficial to have mixture of sub-frames with normal CP and extended CP. Extended CP is used for MBSFN sub-frames

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Exercise – OFDM data rate capability at the PHY

Case 1. Normal CP (no MIMO)Resource block: 12 carriers x 14 OFDM symbols = 168 resource elementsEach resource element carries one modulation symbolFor 64 QAM: 1 symbol = 6 bitsNumber of bits per subframe = 168 x 6 = 1008 bits/subframeRaw PHY data rate = 1008/1ms = 1,008,000 bits/sec/resource block (180KHz)For 20MHz, Raw PHY data rate = 100 RB x 1,008,000 bits/sec/RB = 100.8Mbps

Case 2. Extended CP (no MIMO)Resource block: 12 carriers x 12 OFDM symbols = 144 resource elementsEach resource element carries one modulation symbolFor 64 QAM: 1 symbol = 6 bitsNumber of bits per subframe = 144 x 6 = 864 bits/subframeRaw PHY data rate = 864/1ms = 864,000 bits/sec/resource block (180KHz)For 20MHz, Raw PHY data rate = 100 RB x 864,000 bits/sec/RB = 86.4Mbps

Note: with the use of MIMO the rates are increased

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Downlink reference signals• For coherent demodulation – terminal needs channel estimate for each subcarrier• Reference signals – used for channel estimation• There are three type of reference signals

1. Cell specific DL reference signals – Every DL subframe– Across entire DL bandwidth

2. UE specific DL reference signals– Sent only on DL-SCH– Intended for individual UE’s

3. MBSFN reference signals– Support multicast/broadcast

Note: Reference signals are staggered in time and frequency. This allows UE to perform 2-D complex interpolation of channel time-frequency response

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Cell specific reference signals

• DL transmission may use up to four antennas• Each antenna port has its own pattern of reference signals• Reference signals are transmitted at higher power in multi-

antenna case• Reference signals introduce overhead

– 4.8% for 1 antenna port– 9.5% for 2 antenna ports– 14.3 % for 4 antenna ports

• Reference symbols vary from position to position and from cell to cell – cell specific 2 dimensional sequence

• Period of the sequence is one frame

Four port TX

Two port TX

One port TX

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• There are 504 different Reference Sequences (RS)• They are linked to PHY-layer cell identities • The sequence may be shifted in frequency domain – 6 possible shifts• Each shift is associated with 84 different cell identities (6 x 84 = 504)• Shifts are introduced to avoid collision between RS of adjacent cells• In case of multiple antenna ports – only three shifts are useful• For a given PHY Cell ID - sequence is the same regardless of the bandwidth used –

UE can demodulate middle RBs in the same way for all channel bandwidths

Cell specific reference signals (2)

Shifts for single port transmission

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UE Specific RS

• UE specific RS – used for beam forming• Provided in addition to cell specific RS• Sent over resource block allocated for DL-SCH (applicable only

for data transmission)

Note: additional reference signals increase overhead. One of the most beneficial use of beam forming is at the cell edge – improves SNR

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PHY channels supporting DL TX

• SCH – allows mobile to synchronize to the DL TX during acquisition

• PBCH – used to broadcast static portion of the BCCH

• PDSCH – carries user information and signaling from upper layers of protocol stack

• PDCCH – channel used by MAC scheduler to configure L1/L2 and assign resources (DL scheduling and UL grants)

• PCFICH – explains to the UE the format of the DL transmission

• PHICH – support for HARQ on the uplink

• PUCCH – support for HARQ on the downlink

Channels required for DL transmission

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Summary of PHY DL channels

Transport channel Coding scheme PHY Channel Modulation

DL-SCH Turbo 1/3 PDSCH QPSK, 16-QAM, 64-QAM

BCH Convolutional 1/3 PBCH QPSK

PCH Turbo 1/3 PDSCH QPSK

MCH Turbo 1/3 PMCH QPSK, 16-QAM, 64-QAM

L1/L2 Control Coding scheme PHY Channel Modulation

CFI (Channel format Indicator)

Block code R=1/16 PCFICH QPSK

HI (HARQ information) Repetition 1/3 PHICH BPSK

DCI (Downlink control Information)

Convolutional 1/3 with rate matching

PDCCH QPSK

Services to upper layers

L1/L2 signaling

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Downlink L1/L2 signaling• Signaling that supports DL transmission• Originates at L1/L2 (no higher layer data

or messaging)• Consists of

– Scheduling assignments and associated information required for demodulation and decoding of DL-SCH

– Uplink scheduling grants for UL-SCH– HARQ acknowledgements– Power control commands

• L1/L2 signaling is transmitting in first 1-3 symbols of a subframe – control region

• Size of control region may vary dynamically – always whole number of OFDM symbols (1,2,3)

• Signaling – beginning of the subframe– Reduces delay for scheduled mobiles– Improves power consumption for non-scheduled

mobiles

Three different PHY channel types

1. PCFIC (PHY Control Format Indicator Channel)2. PHICH (PHY – Hybrid ARQ Channel)3. PDCCH (PHY Downlink Control Channel)

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PCFICH• PCFICH – PHY Channel Format Indicator Channel• Indicates to UE the size of the control region (1,2 or 3 OFDM symbols)• PCFICH value may be 1, 2 or 3 (0 is reserved for future use)• Decoding of PCFICH is essential for UE operation

– Encoded with 1/16 repetition code– Uses QPSK modulation– Mapped to the first symbol of each subframe– 16 resource elements in 4 groups of 4 (RE Groups)– Location of the resource elements depends on cell identity

Processing of PCFICH

Note: REGs of the PCFICH are spread in frequency domain to achieve frequency diversity

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PHICH• PHICH = PHY Hybrid-ARQ Indicator Channel• HARQ acknowledgements for UL-SCH transmission• As many PHICH channels as the number of UEs in the cell• A set of PHICH channels is multiplexed on the same resource elements (8 normal

CP, 4 extended CP)• Transmitted in the first OFDM symbol of the subframe• Occupies 3 resource element groups (REGs) = 12 resource elements (RE)• PHICH response comes 4 sub-frames after PU-SCH

Processing of PHICH

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PDCCH• PDCCH = Physical Downlink Control Channel• Used for

– DL scheduling assignments– UL scheduling grants– Power control commands

• PDCCH message occupies 1,2,4 or 8 Control Channel Elements (CCEs)• CCE = 9 Resource Element groups (REGs) = 36 Resource Elements (REs)• One PDCCH carrier one message with a specific Downlink Control Information (DCI)• Multiple UE-s scheduled simultaneously -> Multiple PDCCH transmissions in a subframe

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PDCCH DCIs• PDCCH carrier Downlink Control Information (DCI)• Multiple DCI formats are defined based on type of information

Format Purpose Content # of bits (FDD)

0 UL PUSCH grant RB assignment, MCS, hopping flag, NDI, cyclic shift of DM-RS, CQI, …

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1 DL PDSCH grant for single code word

Resource allocation header, RB allocation, MCS, HARQ, HARQ PID, …

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1A Compact DL PDSCH grant of single code word

Similar to format 1, but with smaller flexibility 44

1A RACH initiated by PDCCH order Localized/distributed VRB assignment flag, preamble index, PRACH message mask index

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1B Compact DL PDSCH grant with pre-coding information

Similar to 1, but with distributed VRB flag, reduced RB allocation flexibility, transmit PMI and pre-coding

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1C Very compact DL PDSCH grant Reduced payload for improved coverage, always uses QPSK on associated PDSCH, restricted RB assignment, No HARQ, …

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1D Compact DL PDSCH grant with pre-coding information and

power offset

Same as 1, but with reduced RB allocation flexibility and addition of distributed VRB transmission flag. Transmit PMI information for

pre-coding, DL power offset

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2 MIMO DL grant Same as 1, but for MIMO transmission 76

2A Compact MIMO DL grant Same as 1A, but for MIMO transmission 68

3 2-bit UL power control TPC for 14 UEs plus 16 bit CRC 44

3A 1-bit UL power control TPC for 28 UEs plus 16 bit CRC 44

DCI formats of PDCCH

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PDSCH

• DL-SCH = DL Shared channel• Used for user data coming from upper

layers (both signaling and payload)• Optimized for low latency and high data rate• Individual steps in the processing chain

operate on data blocks – enables parallel processing

• Many different adaptation modes– Modulation– Coding– Transport block size– Antenna mapping (TX diversity, beam forming,

spatial multiplexing)

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Time/Frequency location of PBCH and SS - FDD

• PBCH = Physical Broadcast Channel

– Used for BCH transport channel

• SS = Synchronization Signal– P-SS = Primary Synchronization

Signal– S-SS = Secondary

Synchronization Signal – SS are used only on Layer 1 – for

system acquisition and Layer 1 cell identity

Note: PBCH and SS use innermost part of the spectrum. This way the system acquisition is the same regardless of deployed bandwidth

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Time/Frequency location of PBCH and SS - TDD

• PBCH = Physical Broadcast Channel

– Used for BCH transport channel

• SS = Synchronization Signal– P-SS = Primary Synchronization

Signal– S-SS = Secondary

Synchronization Signal – SS are used only on Layer 1 – for

system acquisition and Layer 1 cell identity

Note: The position of the P-SS is different in TDD and FDD. By acquiring P-SS, the UE already knows if the system is FDD or TDD.

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Synchronization Channel (SCH)• SCH – first channel acquired by UE• Based on SCH, UE determines eNode B PHY cell identity• 504 possible PHY layer cell IDs• 168 groups with 3 identities per group• SCH consist of 2 signals

– PSS (Primary Synchronization Signal)– SSS (Secondary Synchronization Signal)

• 3 possible PSS sequences: NID(2) = 0,1, 2

• 168 possible SSS sequences: NID(1) = 0,1, …, 167

• Cell ID: NIDcell = 3* NID

(1) + NID(2)

For FDD (frame type 1)• PSS is transmitted on OFDM symbol 7 in the first time slot of subframe 0 and 5• SSS is transmitted on OFDM symbol 6 in the first time slot of subframe 0 and 5For TDD (frame type 2)• PSS is transmitted on OFDM symbol 3 in the first time slot of subframe 1 and 6• SSS is transmitted on OFDM symbol 6 in the first time slot of subframe 0 and 5

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PBCH

• PBCH = PHY Broadcast Channel• PBCH provides PHY channel for static part

of Broadcast Control Channel (BCCH)• BCCH carriers RRC System Information

(SI) messages • SI messages carry System Information

Blocks (SIBs)• SI-M is a special SI that carrier Master

Information Block (MIB)

• In LTE BCCH is split into two parts– Primary broadcast: Carriers MIB and provides UE

with fast access to vital system broadcast information. Primary broadcast is mapped to PBCH

– Dynamic broadcast: Carries all SIBs that contain quasi-static information on system operating parameters. Dynamic broadcast is mapped to PDSCH

Mapping of the BCCH information

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PCH• PCH = Paging Channel• Transmitted over PDSCH (messages), PDCCH (paging indicator) • LTE support DRX (UE sleeps between paging occasions)

– LTE defines DRX cycle– UE is assigned to P-RNTI (Paging – Radio Network Temporary Identifier)– P-RNTI is set on PDCCH– UE that finds set P-RNTI reads PCH on PDSCH to determine if it is being paged

• DRX cycle compromise– Long cycle: good battery life, higher paging delay– Short cycle: faster paging response, shorter UE battery life

Mapping of PCCH

DRX and paging

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Section review1. Explain the main idea behind

OFDM?2. How is OFDMA different from

FDMA?3. What is the role of cyclic prefix

(CP) in OFDM?4. What are DL reference signals?5. How are cell specific reference

signals linked to cell’s physical identity?

6. What is the role of PCFICH?7. What is the role of PHICH?8. What is the channel used for user

data and higher layer signaling?

9. What is SCH?10.What portion of the time-frequency

resources is occupied by SCH?11. What is the duration of LTE

frame?12.How many subframe are in LTE

frame?13.What is the time duration of one

LTE time slot?

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DFTS-OFDM• DFTS-OFDM = DFT Spread OFDM• Also known as s Single Carrier FDMA (SC-FDMA)• Used on RL of LTE• Advantages:

– Lower PAPR than OFDM (4dB for QPSK and 2dB for 16-QAM)– Orthogonality between the users in the same cell– Low complexity TX/RX due to DFT/FFT

• Disadvantage:– Needs an equalizer at the Node B RX– Need for some synchronization in time domain

Outline of the DFTS-OFDM

Note: In DFTS-OFDM, M < N

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DFTS-OFDM TX/RX chain

Note: the TX/RX of DFTS-OFDM is almost the same as OFDM. The DFT pre-coding / decoding and equalization are done in software

64

Uplink user multiplexing• Two ways of mapping the output of the DFT

– Consecutive carriers: Localized DTFS-OFDM– Distributed carriers: Distributed DTFS-OFDM

• Distributed OFDM has benefit of frequency diversity

Note 1: Mapping between output of the OFDM and carriers is performed by MAC schedulerNote 2: Spectrum bandwidth may be allocated in dynamic fashion

Localized DFTS-OFDM Distributed DFTS-OFDM

65

Uplink frame format

TCP: 160Ts (5.1us) for first symbol, 144Ts (4.7us) for other six symbols

TCP-e: 512 Ts (16.7 us) for all symbols

Need for two different CP:1. To accommodate environments

with large channel dispersion2. To accommodate MBSFN (Multi-

Cast Broadcast Single Frequency Network) transmission

Note: UL and DL frame formats are identical

66

PHY channels supporting UL TX

• PRACH – initial random access and UL timing alignment

• PUSCH – channel used for transmission of user data and upper layer signaling

• PUCCH – uplink control channel used for scheduling requests for synchronized UEs

• PDCCH – uplink scheduling grants• PHICH – HARQ feedback channel

supporting UL transmission

67

Uplink reference signals (1)

• Used for uplink channel estimation• Two types of sequences

– Data demodulation Reference Signal (DM-RS)– Sounding Reference Signal (SRS)

• DM-RS– Sent on each slot transmission to help

demodulate data– Occupies center part of the slot transmission

(symbols 4) in both transmission slots– Use same bandwidth as the UL data (multiples

of 12 carrier RBs)– Properties of DM-RS sequences

• Small power variations in frequency domain• Small power variations in time domain

68

Uplink reference signals (2)

• SRS– Allow network to estimate channel quality

across entire band– Used by MAC scheduler to perform

frequency dependent scheduling– Optional implementation– UE can be configured to send SRS

sequence at time intervals from 2ms to 160ms

– Two modes of operation• Wideband SRS – UE send the sequence across

the entire spectrum• Hopping SRS – UE sends narrowband

sequence that hops across different parts of the spectrum

69

PUSCH

• PUSCH = PHY Shared channel• PUSCH carries UL-SCH (user data/higher

layer signaling)• During data transmission L1/L2 signaling also

mapped o PUSCH – preserve single carrier TX• Resources allocated to the UE on per

subframe basis• Allocation is done in PRB (12 carriers by 1 ms)• Modulation used may be QPSK, 16-QAM or

64-QAM (optional)• Allocated PRBs may be hopped from subframe

to subframe• Two modes of hopping

– Intra subframe and inter subframe– Only inter subframe

• Hopping may be on the basis of explicit grants from Node B or following predefined cell-specific mirroring patterns

Example: 2 UE’s, 10MHz (50 RB)

Note: Frequency hopping provides frequency diversity and interference

averaging for the UL transmission

70

PUCCH• PUCCH = PHY Uplink Control Channel• Used for L1/L2 signaling

– Scheduling request– ACK/NACK/DTX for DL-SCH transmission– Feedback on DL channel quality (CQI/PMI/RI)

• Used only when there is no scheduled PUSCH transmission (single carrier TX)

• Uses PRBs at the very end of the allocated channel bandwidth

– Increases frequency diversity– Allows scheduling of larger resource “chunks” for

uplink transmission

• Number of PRBs is configured by the network in a semi-static manner

• Bandwidth of a single resource block in a subframe is shared by several UE’s

– Economical use of allocated resources– Reduces signaling overhead Note: PUCCH performs frequency

hopping between two slots of a subframe

71

PUCCH formats

PUCCH format Modulation Purpose Bits/subframe

1 On/off keying Scheduling requests N/A

1a BPSK ACK/NACK for SIMO 1

1b QPSK ACK/NACK for MIMO 2

2 QPSK CQI/PMI/RI 20

2a QPSK+BPSK CQI/PMI/RI+ACK/NACK for SIMO

21

2b QPSK+QPSK CQI/PMI/RI+ACK/NACK for MIMO

22

Note 1: There are 2 formats: Format 1 (1, 1a and 1b) and Format 2 (2, 2a and 2b)Note 2: PUCCH power offset depends on the PUCCH format

72

PUCCH – Format 1• Small in size (1 or 2 bits)• Used for

– DL HARQ ACK/NACK for MIMO/SIMO– Scheduling request

• By using different cyclic shifts and different covers sequences, multiple users may be multiplexed on the same PUCCH resource

• Typically there are 6 shifts and 3 cover sequences – 18 UE’s per PUCHH resource

Note: Format 1 is repeated in two corresponding slots in the subframe

73

PUCCH – Format 2• Larger in size (20, 21 or 22 bits)

− 10 bits for CQI report− 2 bits for ACK/NACK

• Used for– DL HARQ ACK/NACK for MIMO/SIMO– Scheduling request– CQI/PMI and RI information

• By using different cyclic shifts of the CAZAC sequence multiple UE’s may be multiplexed on one PUCCH resource

• Format 1 and 2 share the same basic format

Processing of CQI report

Note: for Format 2, both CQI report and ACK/NACK information are sent

74

PRACH

• PRACH = PHY Random Access Channel• Physical channel used in support of random

access• In LTE initial access is handled only on PHY, all

the signaling is sent through UL-SCH (PUSCH)• PRACH carries one of 64 preambles • Available preambles are signaled in SIB-2• UE selects a preamble based on the amount of

data it needs to send on UL-SCH (this way Node B knows how to reserve resources)

• PRACH preamble is sent over PRACH time frequency resource

– Occupies middle 1.08MHz of spectrum– Same spectrum regardless of total LTE bandwidth– PRACH access subframe may occur every 1, 2, 5, 10 or

20 ms (20 ms – optional, only in synchronized networks)– Subframe allowed for access – signaled on SIB-2,

paremeter PRACH_Configuration index UL time frequency resources for PRACH

75

Section review1. Why is OFDM not suitable for UL

transmission?2. What is PAPR?3. What is DFTS-OFDM?4. What are two types of UL

reference signals?5. Why is there need for sounding

reference signals?6. How often can a mobile configured

to send SRS signals?7. What is PUSCH?8. What is PUCCH?9. What are PUCCH formats?

10.What information is carried on PUCCH?

11. What is PRACH?12.How does UE learn what

preamble sequences are available for PRACH?

76

MULTIPLE ANTENNA TECHNIQUESPart 3

77

Multi antenna configuration• LTE uses of multiple antennas at

both communication ends• LTE standard requires support for

– 4 antennas at the eNodeB– 2 antennas at the UE

• Multiple antennas may be used in three principle ways

– Reception/transmission diversity– Beam forming– Spatial multiplexing (MIMO antenna

processing)

• Downlink MIMO– TX diversity – Beam forming or SDMA – Spatial multiplexing

• Uplink MIMO – Multi user MIMO (SDMA)

Downlink MIMO

Uplink MIMO

Note: UL MU MIMO avoids use of multiple PAs at the UE

78

DL transmit diversity• Two implementations

– Cyclic Delay Diversity (CDD)– Space-Time Transmit Diversity (STTD)

• CDD– Multiple antenna elements are used to

introduce additional versions of the signal that are cyclically delayed

– UE perceives these signals as additional multi-paths

– Assuming low correlations between TX antennas –created “multi-paths” fade independently – source of diversity

• STTD– Uses Space-Frequency Block Codes– Special encoding (SFBC) makes the

channel matrix unitary (full rank)– Reference symbols are used to

estimate and invert channel matrix

CDD TX diversity

SFBC TX diversity

79

TX Diversity - CDD

• OFDM is robust with respect to multi-path propagation (within CP interval)

• CDD simulates multi-path propagation

• No modification in RX signal processing – UE ‘sees’ single antenna transmission in dispersive environment

Processing in case of 2 antenna CDD TX diversity

Note: Extension of CDD to more than 2 antennas is straightforward. Each antenna has its own cyclic delay.

80

TX Diversity – 2 TX SFBC• Data sent to different antenna are encoded using SFBC

– 2 symbols at the time for 2 antennas TX diversity– Open loop

12

2

1*2

2*1

22

21

*12

2*1

*2

21

12

2

1ˆn

nn

nn

n

n

n

rr

hhhh

hh

aa

hhhh

rr

s

Hs

SFBC in case of 2 TX diversity

Note 1: UE needs to have good estimate of the channel – estimate obtained using PHY reference sequences

81

TX Diversity – 4 TX SFBC• Data sent to different antenna are encoded using SFBC

– 4 symbols at the time for 4 antennas TX diversity– TX diversity operates on a resource element group (REG)– Open loop

Note 1: 4 TX SFBC diversity may be seen as two 2 TX SFBC diversity transmissions multiplexed in time

SFBC in case of 4 TX diversity

82

Spatial multiplexing• Basic idea: fading channel

provides uncorrelated parallel paths for data transmission

2

1

2

1

2221

1211

nn

ss

hhhh

r

nHrHWr 1

2

11

2

1 ˆˆˆˆ

ss

ss

Example: 2 by 2

Capacity benefit of SM MIMO

RTL

L

RL

NNNNS

NNN

BWC

,min

1log2

NT - number of TX antennasNR - number of RX antennas

0 2 4 6 8 10 12 140.00

2.00

4.00

6.00

8.00

10.00

12.00

C/W (1,1)C/W (1,2)C/W (2,2)

S/N (dB)

Spec

tral

effi

cien

cy (b

ps/H

z)

83

Spatial multiplexing in LTE• Two types

– Open loop (used high speed scenarios)• Large delay Cyclic Delay Diversity (CDD)

– Closed loop (used in low speed scenarios)• Mobile provides channel feedback to eNode B

Feedback Closed loop spatial multiplexing Open loop spatial multiplexing

PMI (Pre-coded matrix indicator)

PMI feedback from UE based on instantaneous channel state

No feedback from UE. Fixed pre-coding at eNode B implementing cyclic delay diversity

(CDD)

CQI (Channel quality indicator) Separate CQI for each code word Aggregate CQI (one value)

RI (Rank indicator) Based on the rank of estimated channel matrix(indicates number of spatial channels)

Based on the rank of estimated channel matrix when SFBCs are used

Closed loop spatial

multiplexing

84

Code word – layer mapping• LTE uses either 1 or 2 code words• Code words are mapped onto layers

– 1 layer for 1 codeword– 2, 3 or 4 layers for 2 code words

• Number of modulation symbols in each layer is the same

– Accomplished through numerous transport-block formats and sizes

• Through a pre-coding matrix the layers are mapped onto the antennas

– There is a set of pre-defined pre-coded matrices

– Through PMI, UE recommends to eNodeB which pre-coded matrix to use

– eNodeB may not follow UE’s recommendation – informs UE about pre-coding matrix through explicit signaling

Mapping between code-words and layers

Note: layers are mapped to antennas one symbol at the time

85

Antenna configurations

Transmission modes Description Comments

1 Single antenna (Port 0) Used for SISO and SIMO transmission

2 Transmit diversity Used in low SNR and high mobility

3 Open loop spatial multiplexing(large delay CDD)

Beneficial in high SNR and rich multipath environment

4 Closed loop spatial multiplexing(Rank 2, 3 or4)

Beneficial in high SNR and rich multipath environment

5 Multi-user MIMO Beneficial in high SNR environment for interference reduction

6 Closed loop Rank = 1 Beneficial in low SNR environments

7 Single antenna port (Port 5) Used for beam forming of antenna arrays

86

SIMO/MIMO mode selection

Note: Detection of the environment type and best use of MIMO/SIMO is one of the tasks for scheduler – major differentiating factor between different equipment vendors

87

Section review1. What is MIMO?2. What is receive diversity?3. What is transmit diversity?4. What is beam forming?5. What is SDMA?6. What is spatial multiplexing?7. How much is capacity of link

increased using spatial multiplexing?

8. What is CQI?9. What is RI?10.How is RI used by the scheduler?

11. What is the main idea behind SFBC?

12.What is CDD?13.Explain the main idea behind

CDD?