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Long Term Evolution. Technology training (Part 1). Outline. LTE and SAE overview LTE radio interface architecture LTE radio access architecture LTE multiple antenna techniques. Part 1. LTE/SAE overview. Mobile broadband (3GPP). 3G continues to evolve Standardized through 3GPP

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Long term evolution

Long Term Evolution

Technology training

(Part 1)


Outline
Outline

LTE and SAE overview

LTE radio interface architecture

LTE radio access architecture

LTE multiple antenna techniques


Lte sae overview

Part 1

LTE/SAE overview


Mobile broadband 3gpp
Mobile broadband (3GPP)

  • 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)


Lte releases
LTE Releases

Note: This presentation focuses on R8 features

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)


Lte requirements
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

Throughput requirements relative to baseline


Lte requirements 2
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

Handover interruption time

LTE duplexing options


Lte requirements 3
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


Sae design targets
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)


Basic principles air interface
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)

UL modulation

DL modulation


Basic principles air interface1
Basic principles – Air interface

  • Sharedchannel 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

One shared channel simplifies the overall signaling

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

ARQ reduces required Eb/No


Basic principles air interface2
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)

Outline of spatial multiplexing idea

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


Basic principles air interface3
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.


Sae architecture
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)

LTE Network layout

UE – user equipment (i.e. mobile)

eNode B – base station

SGSN – Support GPRS Serving Node

GGSN – Gateway GPRS Serving Node

MME – Mobility Management Entity

PCRF - Policy and Charging Rules function

SAE = System Architecture Evaluation


Lte protocol control plane
LTE protocol-control plane

NAS – Non Access Stratum

RRC – Radio Resource Control

PDCP – Packet Data Convergence Protocol

RLC – Radio Link Control

MAC – 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)


Lte protocol user plane
LTE protocol- user plane

PDCP – Packet Data Convergence Protocol

RLC – Radio Link Control

MAC – 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.


Lte protocol x2
LTE protocol – X2

Control plane

GTP-U: GPRS tunneling protocol

STCP: Stream Transmission Control Protocol

User plane

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


Channel structure
Channel structure

Uu interface

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

  • Channels – defined on Uu

  • Logical channels

    • Formed by RLC

    • Characterized by typeof 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 elements



Logical channels
Logical channels

Red – common, green – shared, blue - dedicated

LTE 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


Transport channels
Transport channels

Red – common, green – shared

LTE 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


Phy channels
PHY Channels

LTE Channels

Red – common, green – shared

  • 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


Time domain structure
Time domain structure

Radio frame : Type 1

Radio frame : Type 2

  • 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


Tdd frame configurations
TDD frame configurations

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

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


Allocatable resources
Allocatable resources

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

  • Time domain

    • 1 frame = 10 sub-frames

    • 1 subframe = 2 slots

    • 1 slot = 7 (or 6) OFDM symbols

  • Frequency domain

    • 1 OFDM carrier = 15KHz

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

LTE – radio resource = “time-frequency chunk”


Bandwidth flexibility
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)


Ue states
UE States

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

  • 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


3gpp specifications
3GPP Specifications

Example specs organization

  • 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


Section review
Section review

  • What are 3GPP broadband cellular technologies?

  • What releases of 3GPP standard contains LTE?

  • What were target DL and UL throughputs for LTE?

  • What does SAE stand for?

  • What are components of the CS part of the LTE core network?

  • What is the access scheme used on the DL?

  • What is the role of fast scheduler on LTE DL?

  • What is the smallest allocateable resource in LTE DL?

  • What is Radio Block (RB)?

  • What are spectrum bandwidth deployment options for LTE?

  • How many radio blocks are in 20MHz deployment?

  • Does LTE support TDD deployment?

  • What are three UE States supported by LTE?


Lte radio access

Part 2

LTE Radio Access


Overview
Overview

Overview of OFDM/OFDMA

LTE Downlink transmission

Overview of DFTS-OFDM

LTE Uplink transmission

Multi-antenna transmission


Single carrier transmission
Single carrier transmission

Transmission of single carrier in mobile terrestrial environment

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

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


Multi carrier transmission
Multi-carrier transmission

Signal for each stream experiences flat fading

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


Fdm versus ofdm
FDM versus OFDM

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

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


Ofdm transmitter receiver
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


Guard time
Guard time

OFDM symbols with guard time

OFDM symbols without 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


Cyclic prefix
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


Block diagram of full ofdm tx rx
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)


Ofdma time frequency scheduling
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


Lte downlink transmission

Part 3

LTE Downlink Transmission


Lte ofdm
LTE OFDM

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


Detailed time domain structure
Detailed time domain structure

Need for two different CP:

To accommodate environments with large channel dispersion

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

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

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


Exercise ofdm data rate capability at the phy
Exercise – OFDM data rate capability at the PHY

Case 1. Normal CP (no MIMO)

Resource block: 12 carriers x 14 OFDM symbols = 168 resource elements

Each resource element carries one modulation symbol

For 64 QAM: 1 symbol = 6 bits

Number of bits per subframe = 168 x 6 = 1008 bits/subframe

Raw 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 elements

Each resource element carries one modulation symbol

For 64 QAM: 1 symbol = 6 bits

Number of bits per subframe = 144 x 6 = 864 bits/subframe

Raw 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


Downlink reference signals
Downlink reference signals

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

  • For coherent demodulation – terminal needs channel estimate for each subcarrier

  • Reference signals – used for channel estimation

  • There are three type of reference signals

  • Cell specific DL reference signals

    • Every DL subframe

    • Across entire DL bandwidth

  • UE specific DL reference signals

    • Sent only on DL-SCH

    • Intended for individual UE’s

  • MBSFN reference signals

    • Support multicast/broadcast


Cell specific reference signals
Cell specific reference signals

Two port TX

Four port TX

One port TX

  • 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


Cell specific reference signals 2
Cell specific reference signals (2)

Shifts for single port transmission

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


Ue specific rs
UE Specific RS

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

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)


Phy channels supporting dl tx
PHY channels supporting DL TX

Channels required for DL transmission

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


Summary of phy dl channels
Summary of PHY DL channels

L1/L2 signaling

Services to upper layers


Downlink l1 l2 signaling
Downlink L1/L2 signaling

  • Three different PHY channel types

  • PCFIC (PHY Control Format Indicator Channel)

  • PHICH (PHY – Hybrid ARQ Channel)

  • PDCCH (PHY Downlink Control Channel)

  • 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


Pcfich
PCFICH

Processing of PCFICH

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

  • 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


Phich
PHICH

Processing of 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


Pdcch
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


Pdcch dcis
PDCCH DCIs

DCI formats of PDCCH

PDCCH carrier Downlink Control Information (DCI)

Multiple DCI formats are defined based on type of information


Pdsch
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)


Time frequency location of pbch and ss fdd
Time/Frequency location of PBCH and SS - FDD

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

  • 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


Time frequency location of pbch and ss tdd
Time/Frequency location of PBCH and SS - TDD

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.

  • 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


Synchronization channel sch
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 5

    For 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


PBCH

Mapping of the BCCH information

  • 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


PCH

DRX and paging

Mapping of PCCH

  • 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


Section review1
Section review

  • Explain the main idea behind OFDM?

  • How is OFDMA different from FDMA?

  • What is the role of cyclic prefix (CP) in OFDM?

  • What are DL reference signals?

  • How are cell specific reference signals linked to cell’s physical identity?

  • What is the role of PCFICH?

  • What is the role of PHICH?

  • What is the channel used for user data and higher layer signaling?

  • What is SCH?

  • What portion of the time-frequency resources is occupied by SCH?

  • What is the duration of LTE frame?

  • How many subframe are in LTE frame?

  • What is the time duration of one LTE time slot?


Dfts ofdm
DFTS-OFDM

Note: In DFTS-OFDM, M < N

Outline of the 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


Dfts ofdm tx rx chain
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


Uplink user multiplexing
Uplink user multiplexing

Note 1: Mapping between output of the OFDM and carriers is performed by MAC scheduler

Note 2: Spectrum bandwidth may be allocated in dynamic fashion

Distributed DFTS-OFDM

Localized DFTS-OFDM

  • 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


Uplink frame format
Uplink frame format

Need for two different CP:

To accommodate environments with large channel dispersion

To accommodate MBSFN (Multi-Cast Broadcast Single Frequency Network) transmission

Note: UL and DL frame formats are identical

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

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


Phy channels supporting ul tx
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


Uplink reference signals 1
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


Uplink reference signals 2
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


Pusch
PUSCH

Example: 2 UE’s, 10MHz (50 RB)

Note: Frequency hopping provides frequency diversity and interference averaging for the UL transmission

  • 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


Pucch
PUCCH

Note: PUCCH performs frequency hopping between two slots of a subframe

  • 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


Pucch formats
PUCCH formats

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


Pucch format 1
PUCCH – Format 1

  • 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

  • Small in size (1 or 2 bits)

  • Used for

    • DL HARQ ACK/NACK for MIMO/SIMO

    • Scheduling request


Pucch format 2
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

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

Processing of CQI report


Prach
PRACH

UL time frequency resources for 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, paremeterPRACH_Configuration index


Section review2
Section review

  • Why is OFDM not suitable for UL transmission?

  • What is PAPR?

  • What is DFTS-OFDM?

  • What are two types of UL reference signals?

  • Why is there need for sounding reference signals?

  • How often can a mobile configured to send SRS signals?

  • What is PUSCH?

  • What is PUCCH?

  • What are PUCCH formats?

  • What information is carried on PUCCH?

  • What is PRACH?

  • How does UE learn what preamble sequences are available for PRACH?


Multiple antenna techniques

Part 3

Multiple antenna techniques


Multi antenna configuration
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


Dl transmit diversity
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


Tx diversity cdd
TX Diversity - CDD

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

Processing in case of 2 antenna CDD TX diversity

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


Tx diversity 2 tx sfbc
TX Diversity – 2 TX SFBC

SFBC in case of 2 TX diversity

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

  • Data sent to different antenna are encoded using SFBC

    • 2 symbols at the time for 2 antennas TX diversity

    • Open loop


Tx diversity 4 tx sfbc
TX Diversity – 4 TX SFBC

SFBC in case of 4 TX diversity

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

  • 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


Spatial multiplexing
Spatial multiplexing

Capacity benefit of SM MIMO

NT - number of TX antennas

NR - number of RX antennas

Example: 2 by 2

Basic idea: fading channel provides uncorrelated parallel paths for data transmission


Spatial multiplexing in lte
Spatial multiplexing in LTE

Closed loop spatial multiplexing

  • 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


Code word layer mapping
Code word – layer mapping

Mapping between code-words and layers

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

  • 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



Simo mimo mode selection
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


Section review3
Section review

  • What is MIMO?

  • What is receive diversity?

  • What is transmit diversity?

  • What is beam forming?

  • What is SDMA?

  • What is spatial multiplexing?

  • How much is capacity of link increased using spatial multiplexing?

  • What is CQI?

  • What is RI?

  • How is RI used by the scheduler?

  • What is the main idea behind SFBC?

  • What is CDD?

  • Explain the main idea behind CDD?


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