LTE Overview_IEEE

8/7/2019 LTE Overview_IEEE

http://slidepdf.com/reader/full/lte-overviewieee 1/49

Technical Overview of 3GPP Long Term Evolution (LTE)

Feb. 8, 2007

Hyung G. Myung ([email protected])



Technical Overview of 3GPP LTE | Feb. 8, 07 | Hyung G. Myung 1

OutlineOutlineOutlineOutline

Introduction

LTE Downlink Physical Layer (OFDMA)

LTE Uplink Physical Layer (SC-FDMA)

Summary and Conclusions

LTE Physical Layer Procedures

LTE System Architecture




DisclaimerDisclaimerDisclaimerDisclaimer

• 3GPP LTE standardization process is still on-going at currentmoment.

⇒ Many of the technical details presented here may changeor evolve into different forms.



Introduction


LTE DL PHY Layer (OFDMA)

LTE UL PHY Layer (SC-FDMA)

LTE PHY Layer Procedures



http://slidepdf.com/reader/full/lte-overviewieee 5/49Technical Overview of 3GPP LTE | Feb. 8, 07 | Hyung G. Myung 4

3GPP Evolution3GPP Evolution3GPP Evolution3GPP Evolution

• Release 99 (Mar. 2000): UMTS/WCDMA

• Rel-5 (Mar. 2002): HSDPA

• Rel-6 (Mar. 2005): HSUPA

• Rel-7 (???, 2007): DL MIMO, IMS (IP Multimedia Subsystem),

optimized real-time services (VoIP, gaming, push-to-talk).

• Long Term Evolution (LTE)

– 3GPP work on the Evolution of the 3G Mobile System started in

November 2004.– Currently, standardization in progress in the form of Rel-8.

– Spec scheduled to be finalized by the end of 2007/early 2008.

– Target deployment in 2010.



Requirements of LTERequirements of LTERequirements of LTERequirements of LTE

• Peak data rate

– 100 Mbps DL/ 50 Mbps UL within 20 MHz bandwidth.

• Up to 200 active users in a cell (5 MHz)

• Less than 5 ms user-plane latency

• Mobility– Optimized for 0 ~ 15 km/h.

– 15 ~ 120 km/h supported with high performance.

– Supported up to 350 km/h or even up to 500 km/h.

• Coverage

– Performance should be met for 5 km cells with slight degradationfor 30 km cells. Up to 100 km cells not precluded.



Requirements of LTERequirements of LTERequirements of LTERequirements of LTE ---- cont.cont.cont.cont.

• Enhanced multimedia broadcast multicast service (E-MBMS)

• Spectrum flexibility– 1.25 ~ 20 MHz

• Enhanced support for end-to-end QoS



Key Features of LTEKey Features of LTEKey Features of LTEKey Features of LTE

• Multiple access scheme

– DL: OFDMA with CP.

– UL: Single Carrier FDMA (SC-FDMA) with CP.

• Adaptive modulation and coding

– DL modulations: QPSK, 16QAM, and 64QAM

– UL modulations: QPSK and 16QAM

– Rel-6 Turbo code: Coding rate of 1/3, two 8-state constituentencoders, and a contention-free internal interleaver.

• Advanced MIMO spatial multiplexing techniques

– (2 or 4)x(2 or 4) downlink and uplink supported.• Multi-layer transmission with up to four streams.

– Multi-user MIMO also supported.

• ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer.



Key Features of LTEKey Features of LTEKey Features of LTEKey Features of LTE ---- cont.cont.cont.cont.

• Power control and link adaptation

• Implicit support for interference coordination

• Support for both FDD and TDD

• Possible support for operating as single frequency network

(SFN) to support MBMS– Time-synchronized common waveform transmitted from multiple

cells.



Introduction









LTE Network ArchitectureLTE Network ArchitectureLTE Network ArchitectureLTE Network Architecture

• E-UTRAN (Evolved Universal Terrestrial Radio Access Network)

NB: NodeB (base station)

RNC: Radio Network ControllerSGSN: Serving GPRS Support NodeGGSN: Gateway GPRS Support Node

RNC RNC

SGSN

GGSN

NB NB NB NB

UMTS 3G: UTRAN

eNB

aGW (MME/UPE)

aGW (MME/UPE)

S1

X2

E-UTRAN

EPC (Evolved Packet Core)

eNB eNB

eNB

eNB: E-UTRAN NodeB

aGW: Access Gateway MME: Mobility Management Entity UPE: User Plane Entity




LTE Network ArchitectureLTE Network ArchitectureLTE Network ArchitectureLTE Network Architecture ---- cont.cont.cont.cont.

ePDG

EvolvedPacket Core

GPRS Core

Trusted non 3GPP IP A

ccess

WLAN

3GPP IPAccess

S2b

WLAN

Access NW

S5b

IASA

S5a

SAE

Anchor

3GPP

Anchor

S4

SGiEvolved RAN

S1

Op.

IP

Serv.

(IMS,

PSS,

etc…)

Rx+

GERAN

UTRAN

Gb

Iu

S3

MME

UPE

HSS

PCRF

S7

S6

SGSN

S2a

* 3GPP TR 23.882




eNB


• eNB

– All radio-related functions.

• MME

– Manage/store UE control

plane context.

– UE authentication.

– Mobility management.

• UPE– Manage/store UE context.

– Packet routing/forwarding.

aGW (MME/UPE)

aGW (MME/UPE)

S1

X2

E-UTRAN

EPC (Evolved Packet Core)

eNB eNB

eNB

eNB: E-UTRAN NodeBaGW: Access Gateway MME: Mobility Management Entity UPE: User Plane Entity





internet

eNB

RB Control

Connection

Mobility Cont.

eNB

Measurement

Configuration &

Provision

Dynamic

Resource

Allocation

(Scheduler)

RRC

PHY

MME

UPES1

MAC

PDCP

Inter Cell RRM

Radio

Admission

Control

RLC

E-UTRAN aGW

U-Plane

Protocol

Stack

C-PlaneProtocol

Stack

PDCP: Packet Data Convergence ProtocolNAS: Non-Access Stratum

* Details in 3GPP TS 36.300




PHY Layer Transport ChannelsPHY Layer Transport ChannelsPHY Layer Transport ChannelsPHY Layer Transport Channels

• DL transport channel types

– Broadcast Channel (BCH)

– Downlink Shared Channel (DL-SCH)

– Paging Channel (PCH)– Multicast Channel (MCH)

• UL transport channel types

– Uplink Shared Channel (UL-SCH)– Random Access Channel (RACH)




LTE Frame StructureLTE Frame StructureLTE Frame StructureLTE Frame Structure

• Two radio frame structures defined.

– Generic frame structure: FDD and TDD.

– Alternative frame structure: TDD only.

• Generic radio frame has duration of 10 ms. It consists of 20slots. A slot has a duration of 0.5 ms. 2 slots comprise a

subframe.

• A resource block (RB) spans 12 subcarriers over a slot duration

of 0.5 ms. One subcarrier has bandwidth of 15 kHz.

#0#0 #1#1 #2#2 #3#3 #19#19

One slot, T slot = 15360×T s = 0.5 ms

One radio frame, T f = 307200×T s=10 ms

#18#18

One subframe

* T s = 1/(15000×2048) sec

* Generic radio frame structurea.k.a. TTI (Transmission Time Interval)




LTE Layer 2LTE Layer 2LTE Layer 2LTE Layer 2

• Layer 2 has three sublayers

– MAC (Medium Access Control)

– RLC (Radio Link Control)– PDCP (Packet Data Convergence Protocol)

Multiplexing

...

HARQ

Scheduling / Priority Handling

Transport Channels

MAC

RLC

PDCP

Segm.ARQ

Segm.ARQ

Logical Channels

Radio Bearers

ROHC ROHC

SAE Bearers

Securtiy Security

DL UL

ROHC: Robust Header CompressionSAE: System Architecture Evolution




MACMACMACMAC----Layer Logical ChannelsLayer Logical ChannelsLayer Logical ChannelsLayer Logical Channels

• Control channels: Transfer of control-plane information.

– Broadcast Control Channel (BCCH), Paging Control Channel

(PCCH), Common Control Channel (CCCH), Multicast ControlChannel (MCCH; only used for MBMS), Dedicated ControlChannel (DCCH)

• Traffic channels: Transfer of user-plane information.

– Dedicated Traffic Channel (DTCH) and Multicast Traffic Channel(MTCH; only used for MBMS).




RRC LayerRRC LayerRRC LayerRRC Layer

• Terminated in eNB on the network side.

• Functions

– Broadcast

– Paging

– RRC connection management

– RB (Radio Bearer) management– Mobility functions

– UE measurement reporting and control

• RRC states– RRC_IDLE

– RRC_CONNECTED




MAC and RRC ControlMAC and RRC ControlMAC and RRC ControlMAC and RRC Control

Integrity protected

Ciphering (FFS)

No integrity protection

No ciphering

No integrity protection

No ciphering Security

HighLimitedNone or very limitedExtensibility

LongerShortVery shortControl delay

~ 10-6 (after ARQ)~ 10-3 (after HARQ)~ 10-2 (no retransmission)Signalling reliability

RRC messageMAC control PDUL1/L2 control channelSignalling

RRCMACControl entity

RRC controlMAC control




Resource Scheduling of Shared ChannelsResource Scheduling of Shared ChannelsResource Scheduling of Shared ChannelsResource Scheduling of Shared Channels

• Dynamic resource scheduler

resides in eNB on MAC layer.

• Radio resource assignment based

on radio condition, traffic volume,

and QoS requirements.

• Radio resource assignmentconsists of:

– Physical Resource Block (PRB)– Modulation and Coding Scheme

(MCS)

Fixed

Dy

namic




Radio Resource ManagementRadio Resource ManagementRadio Resource ManagementRadio Resource Management

• Radio bearer control (RBC)

• Radio admission control (RAC)

• Connection mobility control (CMC)

• Dynamic resource allocation (DRA) or packet scheduling (PS)

• Inter-cell interference coordination (ICIC)

• Load balancing (LB)




Other FeaturesOther FeaturesOther FeaturesOther Features

• ARQ (RLC) and HARQ (MAC)

• Mobility

• Rate control

• DRX (Discontinuous Reception)

• MBMS

• QoS

• Security



Overview of 3GPP LTE









DL OverviewDL OverviewDL OverviewDL Overview

• DL physical channels

– Physical Downlink Shared Channel (PDSCH)

– Physical Downlink Control Channel (PDCCH)– Common Control Physical Channel (CCPCH)

• DL physical signals

– Reference signal (RS)– Synchronization signal

• DL baseband signal generation

OFDM

Mapper

OFDM signal

generationLayer

Mapper

Layer

Mapper

Scrambling

Precoding

Modulation

Mapper

Modulation

Mapper

Modulation

Mapper

Modulation

Mapper

OFDM

Mapper

OFDM signal

generationScrambling

Related to MIMO transmission




DL Slot StructureDL Slot StructureDL Slot StructureDL Slot Structure

One downlink slot, Tslot

subcarriers

N

BW

DL

subcarriers

N

BW

DL

N

BW

DL

Resource element

OFDM symbolsDLsymbN OFDM symbolsDLsymbN

One resource block, NBW

subcar

riers

RB

One resource block, NBW

subcar

riers

RB

(16.67/512)

(16.67/384)

(16.67/256)

(16.67/128)

(16.67/64)(16.67/32)Long

(4.69/144)

× 6,(5.21/160)

×1

(4.69/108)

× 6,

(5.21/120)

× 1

(4.69/72) ×

6,

(5.21/80) ×

1

(4.69/36) ×

6,

(5.21/40) ×

1

(4.69/18) ×

6,

(5.21/20) ×

1

(4.69/9) ×

6,

(5.21/10) ×

1*

ShortCP

length(μμμμs/sam

ples)

7/6Number of OFDM symbols

per sub frame

(Short/Long CP)

120190160130115176

Number of

occupied

sub-carriers

204815361024512256128FFT size

30.72 MHz(8 × 3.84

MHz)

23.04 MHz(6 × 3.84

MHz)

15.36 MHz(4 × 3.84

MHz)

7.68 MHz(2 × 3.84

MHz)

3.84 MHz1.92 MHz(1/2 × 3.84

MHz)

Samplingfrequency

15 kHzSub-carrier

spacing

0.5 msSlot duration

20 MHz15 MHz10 MHz5 MHz2.5 MHz1.25 MHzTransmission BW

#0#0 #1#1 #2#2 #3#3 #19#19


One radio frame, T f = 307200×T

s=10 ms

#18#18

One subframe

* 3GPP TR 25.814




DL Reference SignalDL Reference SignalDL Reference SignalDL Reference Signal

R0

R0

R0

R0

R0

R0

R0

R0

0=l 6=l 0=l 6=l

R0

R0

R0

R0

R0

R0

R0

R0

0=l 6=l 0=l 6=l

R0

R0

R0

R0

R0

R0

R0

R0

0=l 6=l 0=l 6=l

R1

R1

R1

R1

R1

R1

R1

R1

0=l 6=l 0=l 6=l

R0

R0

R0

R0

R0

R0

R0

R0

0=l 6=l 0=l 6=l

R0

R0

R0

R0

R0

R0

R0

R0

0=l 6=l 0=l 6=l

R1

R1

R1

R1

R1

R1

R1

R1

0=l 6=l 0=l 6=l

R1

R1

R1

R1

R1

R1

R1

R1

0=l 6=l 0=l 6=l

even-numbered slots odd-numbered slots

R3

R3

R3

R3

0=l 6=l 0=l 6=l

R0

R0

R0

R0


R0

R0

R0

R0

0=l 6=l 0=l 6=l

R1

R1

R1

R1


R1

R1

R1

R1

0=l 6=l 0=l 6=l


R2

R2

R2

R2

0=l 6=l 0=l 6=l

One

virtual antenna

Two virtual antennas

Four virtual antennas

Antenna 0 Antenna 1 Antenna 2 Antenna 3

Not used for transmission on this antenna

Reference symbols on this antenna

( )lk ,elementResource

*For generic frame with normal CP




DL Reference SignalDL Reference SignalDL Reference SignalDL Reference Signal ---- cont.cont.cont.cont.

• 2D RS sequence is generated as the symbol-by-symbolproduct of a 2D orthogonal sequence (OS) and a 2D pseudo-random sequence (PRS).

– 3 different 2D OS and ~170 different PRS.

– Each cell (sector) ID corresponds to a unique combination of one OS and one PRS ⇒ ~510 unique cell IDs.

• CDM of RS for cells (sectors)of the same eNodeB (BS)

– Use complex orthogonal spreading codes.

• FDM of RS for each antenna in case of MIMO




DL MIMODL MIMODL MIMODL MIMO

• Support up to 4x4 configuration.

• Support for both spatial multiplexing (SM) and Tx diversity (TxD).

– SM

• Unitary precoding based scheme with codebook based feedback from user.

• Multiple codewords

– TxD: SFBC/STBC, switched TxD, CDD (Cyclic Delay Diversity)considered.

• MU-MIMO supported.




Basic Idea of UnitaryBasic Idea of UnitaryBasic Idea of UnitaryBasic Idea of Unitary PrecodingPrecodingPrecodingPrecoding

• Parallel decomposition of a MIMO channel

1x

2x

t N x

1y

2y

r N y

11h21h

1r N h

r t N N h

11 11 1 1

1

t

r r r t t r

N

N N N N N N

h hy x n

y h h x n

y H x n

= ⋅ +

⇒ = ⋅ +

⋯

⋮ ⋮ ⋱ ⋮ ⋮ ⋮

⋯

H H

H H H H

I

H H H

x ny

H UDV y UDV x n

U y U U DV x U n

U y D

y x

U

D

V n

n

x

=

= ⇒ = +

= +

= +

= +

ɶ ɶɶ

ɶ ɶ ɶ* Narrowband channel

I. E. Telatar, “Capacity of Multi-Antenna Gaussian Channels,” Europ. Trans. Telecommu., vol. 10, Nov./Dec. 1999, pp. 585-595.




Practical UnitaryPractical UnitaryPractical UnitaryPractical Unitary PrecodingPrecodingPrecodingPrecoding SystemSystemSystemSystem

• For subcarrier k (total of M subcarrriers).

Unitary Precoding

MIMO Channel

H k

Receiver

Feedback Processing:

(Averaging& quantization of V k’s)

⊕

k X

ˆk k k

X V X =ɶ

k k H X

k N

k Y

k Z

( )⋅F

ˆ ( )k k V F V =

{ } { }ˆ ˆ(Estimation of ) , ; 0, , 1 , ; 0, , 1= = = = − = = −ɶ … …H

k k k k k k k H H U D V V V k M V V k M




Single Carrier FDMASingle Carrier FDMASingle Carrier FDMASingle Carrier FDMA

• What is Single Carrier FDMA (SC-FDMA)?

– Utilizes single carrier modulation and frequency domain

equalization.– Has similar performance and essentially the same overall structur

e as those of OFDMA system. Also, referred to as DFT-spreadOFDMA.

– Has low PAPR because of its inherent single carrier transmitter structure.

– An attractive alternative to OFDMA, especially in the uplink communications where lower PAPR greatly benefits the mobileterminal in terms of transmit power efficiency.

• For more technical information, a tutorial is available athttp://hgmyung.googlepages.com/scfdma .




UL OverviewUL OverviewUL OverviewUL Overview

LB #0CP LB #1C

P LB #2CP LB #3C

P LB #4CP LB #6C

P

1 slot = 0.5 ms

LB #5CP

Used for RS

• UL physical channels

– Physical Uplink Shared Channel (PUSCH)

– Physical Uplink Control Channel (PUCCH)

• UL physical signals

– Reference signal (RS)

– Random access preamble

#0#0 #1#1 #2#2 #3#3 #19#19


One radio frame, T f = 307200×T s=10 ms

#18#18

One subframe




UL Signal GenerationUL Signal GenerationUL Signal GenerationUL Signal Generation

Serial-to-Parallel

M -pointIDFT

N -pointDFT

Zeros

{ }0 1 1, ,N

x x x −… Parallel-to-Serial

{ }0 1 1, , M x x x −ɶ ɶ ɶ…

SubcarrierMapping

subcarrier

0

M-1

Zeros

Modulation

mapper

Modulation

mapper

SC-FDMA

mapper

SC-FDMA

mapper SC-FDMAsignal gen.SC-FDMAsignal gen.Scrambling

One Block




UL Transmission ParametersUL Transmission ParametersUL Transmission ParametersUL Transmission Parameters

(3.65/7)

or (7.81/15)33.33/38/6466.67/75/1280.51.25

(3.91/15)

or (5.99/23)33.33/75/12866.67/150/2560.52.5

(4.04/31)

or (5.08/39)33.33/150/25666.67/300/5120.55

(4.1/63)

or (4.62/71)33.33/300/51266.67/600/10240.510

(4.12/95)

or (4.47/103)33.33/450/76866.67/900/15360.515

(4.13/127)

or (4.39/135)33.33/600/102466.67/1200/20480.520

CP duration

(µµµµs/# of subcarriers)

SB size (µµµµs/# of occupied subcarriers

/FFT size)

LB size (µµµµs/# of occupied subcarriers

/FFT size)

Slot duration

(ms)

Bandwidth

(MHz)

* 3GPP TR 25.814




UL Reference SignalUL Reference SignalUL Reference SignalUL Reference Signal

• Two types of UL RS

– Demodulation (DM) RS ⇒ Narrowband.

– Sounding RS: Used for UL resource scheduling ⇒ Broadband.

• RS based on Zadoff-Chu CAZAC (Constant Amplitude ZeroAuto-Correlation) polyphase sequence

– CAZAC sequence: Constant amplitude, zero circular auto-correlation, flat frequency response, and low circular cross-correlation between two different sequences.

2

2 , 0,1,2, , 1; for even

2

( 1)2 , 0,1,2, , 1; for odd

2

k

r k j qk k L L

L

r k k j qk k L L

L

ea

e

π

π

− + = −

+ − + = −

=

⋯

⋯

* r is any integer relatively prime

with L and q is any integer.

B. M. Popovic, “Generalized Chirp-like Polyphase Sequences with Optimal Correlation Properties,”IEEE Trans. Info. Theory, vol. 38, Jul. 1992, pp. 1406-1409.




UL RS MultiplexingUL RS MultiplexingUL RS MultiplexingUL RS Multiplexing

subcarriers

User 1

User 2

User 3

subcarriers

FDM Pilots CDM Pilots




UL RS MultiplexingUL RS MultiplexingUL RS MultiplexingUL RS Multiplexing ---- cont.cont.cont.cont.

• DM RS

– For SIMO: FDM between different users.

– For SU-MIMO: CDM between RS from each antenna

– For MU-MIMO: CDM between RS from each antenna

• Sounding RS

– CDM when there is only one sounding bandwidth.

– CDM/FDM when there are multiple sounding bandwidths.









ll hll hll hll h




Cell SearchCell SearchCell SearchCell Search

• Cell search: Mobile terminal or user equipment (UE) acquirestime and frequency synchronization with a cell and detectsthe cell ID of that cell.

– Based on BCH (Broadcast Channel) signal and hierarchical SCH(Synchronization Channel) signals.

• P-SCH (Primary-SCH) and S-SCH (Secondary-SCH) are transmitt

ed twice per radio frame(10 ms) for FDD.

• Cell search procedure

1. 5 ms timing identified using P-SCH.

2. Radio timing and group ID found from S-SCH.3. Full cell ID found from DL RS.

4. Decode BCH.

C ll S hC ll S hC ll S hC ll S h




Cell SearchCell SearchCell SearchCell Search ---- cont.cont.cont.cont.

Cell site with 20-MHz transmission bandwidth

SCH

Center carrier frequency

Step 1:Cell search using synchronizationchanneldetect center 1.25 spectrum ofentire 20-MHz spectrum

Example: 10-MHz UE in 20-MHz cell site, SCH bandwidth = 1.25 MHz and BCH bandwidth = 1.25 MHz

Step 2:BCH reception

BCH

BCHreception

Initiate data transmission using assigned spectrum

Step 3:UE shifts to the center carrier frequencyassigned by the system and initiatesdata transmission

R d AR d AR d AR d A




Random AccessRandom AccessRandom AccessRandom Access

• Non-synchronized random access.

• Open loop power controlled with power ramping similar to

WCDMA.

• RACH signal bandwidth: 1.08 MHz (6 RBs)

• Preamble based on CAZAC sequence.

CP Preamble

TCP TGP

RA slot = 1 ms

* TCP = 0.1 ms, TGP = 0.1 ms

UE eNB

Random Access Preamble1

Random Access Response 2

Scheduled Transmission3

Contention Resolution 4








S d C lS d C lS d C lS d C l




Summary and ConclusionsSummary and ConclusionsSummary and ConclusionsSummary and Conclusions

• On-going standardization in the form of 3GPP Release 8.

– Spec by the end of 2007/early 2008 and target deployment in2010.

• LTE air-interface.

– Downlink: OFDMA– Uplink: SC-FDMA

• Support for both FDD and TDD.

• Flexible spectrum allocation (1.25 ~ 20 MHz).

• Advanced MIMO system.

R f d RR f d RR f d RR f d R




References and ResourcesReferences and ResourcesReferences and ResourcesReferences and Resources

• http://hgmyung.googlepages.com/scfdma– LTE & SC-FDMA references

• LTE and SC-FDMA– H. Ekström et al., “Technical Solutions for the 3G Long-TermEvolution,” IEEE Commun. Mag., vol. 44, no. 3, March 2006, pp.38–45

– H. G. Myung et al., “Single Carrier FDMA Technique for Uplink

Wireless Transmission,” IEEE Vehicular Technology Magazine, Sep.2006– H. G. Myung, “Single Carrier Orthogonal Multiple Access

Technique for Broadband Wireless Communications,” PhDDissertation, Polytechnic University

• 3GPP LTE Activities– http://www.3gpp.org/tb/home.htm

• RAN WG1: Layer 1• RAN WG2: Layer 2 & 3

R f d RR f d RR f d RR f d R tttt




References and ResourcesReferences and ResourcesReferences and ResourcesReferences and Resources ---- cont.cont.cont.cont.

• 3GPP Release 8 Spec– http://www.3gpp.org/ftp/Specs/html-info/36-series.htm

• 36.201: Physical layer; General description

• 36.211: Physical Channels and Modulation• 36.212: Multiplexing and channel coding• 36.213: Physical layer procedures

• 36.214: Physical layer; Measurements• 36.300: E-UTRA and E-UTRAN; Overall description (layer 2&3 info)

• 36.401: E-UTRAN; Architecture description– http://www.3gpp.org/ftp/Specs/html-info/25814.htm (old)

• 3GPP LTE Layer 1 Contribution Papers (Tdoc)– http://www.3gpp.org/ftp/Specs/html-info/Meetings-R1.htm

LTE Overview_IEEE

Documents

Transcript of LTE Overview_IEEE