3G/4G Mobile Communications Systems · 3G/4G Mobile Communications Systems. Chapter VI: Physical Layer of LTE 2 Slide 2. Physical Layer of LTE OFDM and SC-FDMA Basics ... Physical

Dr. Stefan BrückQualcomm Corporate R&D Center Germany

3G/4G Mobile Communications Systems

Chapter VI: Physical Layer of LTE

2

Chapter VI: Physical Layer of LTE

Slide 2

Physical Layer of LTE

� OFDM and SC-FDMA Basics

� DL/UL Resource Grid

� Downlink Operation

� Downlink Physical Channels

� Uplink Operation

� Uplink Physical Channels

3

� Uplink Physical Channels

� UE Categories

Slide 3

LTE Key Radio Features (Release 8)� 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, 16QAM, and 64QAM (optional for UE)

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

4

and a contention-free internal interleaver

� ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer

� Advanced MIMO spatial multiplexing techniques� (2 or 4)x(2 or 4) downlink and 1x(2 or 4) uplink supported

� Multi-layer transmission with up to four streams in DL

� Multi-user MIMO also supported in UL and DL

� Implicit support for interference coordination

� Support for both FDD and TDD

Slide 4

LTE Frequency Bands

� LTE will support all band classes currently specified for UMTS as well as additional bands

5 Slide 5

LTE Duplexing Modes

� LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) to provide flexible operation in a variety of spectrum allocations around the world.

� Unlike UMTS TDD there is a high commonality between LTE TDD & LTE FDD

6

� Slot length (0.5 ms) and subframe length (1 ms) is the same as LTE FDD with the same numerology (OFDM symbol times, CP length, FFT sizes, sample rates, etc.)

� UL/ DL switching pointsdesigned to allow co-existence with UMTS-TDD(TD-CDMA, TD-SCDMA)

Slide 6

LTE Half-Duplex FDD

� In addition to FDD & TDD, LTE supports also Half-Duplex FDD (HD-FDD)

� HD-FDD is like FDD, only the UE cannot transmit and receive at the same time

7

� Note, that the eNodeB can still transmit and receive at the same time to different UEs; half-duplex is enforced by the eNodeB scheduler

� Reasons for HD-FDD� Handsets are cheaper, as no duplexer is required

� More commonality between TDD and HD-FDD than compared to full duplex FDD

� Certain FDD spectrum allocations have small duplex space; HD-FDD leads to duplex desense in UE

Slide 7

OFDM Basics – Overlapping Orthogonal� OFDM: Orthogonal Frequency Division Multiplexing

� OFDMA: Orthogonal Frequency Division Multiple-Access

� FDM/FDMA is nothing new: carriers are separated sufficiently in frequency so that there is minimal overlap to prevent cross-talk.

conventional FDM

8

� OFDM: still FDM but carriers can actually be orthogonal (no cross-talk) while actually overlapping, if specially designed → saved bandwidth !

frequency

OFDM

frequency

saved bandwidth

Slide 8

OFDM Basics – Waveforms

� Frequency domain: overlapping sinc functions� Referred to as subcarriers

� Typically quite narrow, e.g. 15 kHz

� Time domain: simple gated sinusoid functions

4 5 6 7 8 9

x 105

-0.2

0

0.2

0.4

0.6

0.8

1

freq

∆∆∆∆f = 1/T

T = symbol time

9

� Time domain: simple gated sinusoid functions� For orthogonality: each symbol has an

integer number of cycles over the symbol time

� fundamental frequency f0 = 1/T

� Other sinusoids with fk = k • f00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

time

T = symbol time

Slide 9

OFDM Basics – The Full OFDM Transceiver

10 Slide 10

OFDM Basics – Cyclic Prefix� ISI (between OFDM symbols) eliminated almost completely by inserting a

guard time

� Within an OFDM symbol, the data symbols modulated onto the subcarriers are only orthogonal if there is an integer number of sinusoidal cycles within the receiver window

OFDM Symbol OFDM Symbol OFDM Symbol

TG TG

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� Filling the guard time with a cyclic prefix (CP) ensures orthogonality of subcarriers even in the presence of multipath � elimination of same cell interference

CPUseful OFDM symbol time

OFDM symbol


OFDM symbol


OFDM symbol

Slide 11

Comparison with CDMA – Principle� OFDM: Particular modulation symbol is carried over a relatively long symbol

time and narrow bandwidth� LTE: 66.6 µsec symbol time and 15 kHz bandwidth

� For higher data rates send more symbols by using more sub-carriers →increases bandwidth occupancy

� CDMA: Particular modulation symbol is carried over a relatively short symbol time and a wide bandwidth� UMTS HSPA: 4.17 µsec symbol time and 3.84 Mhz bandwidth

� To get higher data rates use more spreading codes

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� To get higher data rates use more spreading codes

frequency

time

symbol 0

symbol 1

symbol 2

symbol 3

frequency

time

symbol 0

symbol 1

symbol 2

symbol 3

CDMA OFDM

Slide 12

Comparison with CDMA – Time Domain� Short symbol times in CDMA lead to ISI in the presence of multipath

� Long symbol times in OFDM together with CP prevent ISI from multipath

1 2 3 4

1 2 3 4

1 2 3 4

CDMA symbols

Multipath reflections from one symbol significantly overlap subsequent symbols → ISI

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� Long symbol times in OFDM together with CP prevent ISI from multipath

1 2CPCP

1 2CPCP

1 2CPCP

Little to no overlap in symbols from multipath

Slide 13

Comparison with CDMA – Frequency Domain

14

� In CDMA each symbol is spread over a large bandwidth, hence it will experience both good and bad parts of the channel response in frequency domain

� In OFDM each symbol is carried by a subcarrier over a narrow part of the band → can avoid send symbols where channel frequency response is poor based on frequency selective channel knowledge → frequency selective scheduling gain in OFDM systems

Slide 14

OFDM Basics – Choosing the Symbol Time for LTE� Two competing factors in determining the right OFDM symbol time:

� CP length should be longer than worst case multipath delay spread, and the OFDM symbol time should be much larger than CP length to avoid significant overhead from the CP

� On the other hand, the OFDM symbol time should be much smaller than the shortest expected coherence time of the channel to avoid channel variability within the symbol time

� LTE is designed to operate in delay spreads up to ~5µs and for speeds up to

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� LTE is designed to operate in delay spreads up to ~5µs and for speeds up to 350km/h (1.2ms coherence time @ 2.6GHz). As such, the following was decided:� CP length = 4.7 µs

� OFDM symbol time = 66.6 µs(= 1/20 the worst case coherence time)

∆f = 15 kHz

CP

~4.7 µs ~66.7 µs

Slide 15

Scalable OFDM for Different Operating Bandwidths

� With Scalable OFDM, the subcarrier spacing stays fixed at 15 kHz regardless of the operating bandwidth � 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz,

20 MHz

� Symbol time is fixed to 66.6 µs

common channels

10 MHz bandwidth

20 MHz bandwidth

5 MHz bandwidth

1.4 MHz bandwidth

3 MHz bandwidth

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� Symbol time is fixed to 66.6 µs

� The total number of subcarriers is varied in order to operate in different bandwidths� This is done by specifying different FFT

sizes (i.e. 512 point FFT for 5 MHz, 2048 point FFT for 20 MHz)

� Influence of delay spread, Doppler due to user mobility, timing accuracy, etc. remain the same as the system bandwidth is changed � robust design

centre frequency

Slide 16

LTE Downlink Frame Format

slot = 0.5ms slot = 0.5ms

subframe = 1.0ms

Radio frame = 10ms

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� Subframe length is 1ms

� Consists of two 0.5ms slots

� 7 OFDM symbols per 0.5ms slot � 14 OFDM symbols per 1ms subframe

� In UL center SC-FDMA symbol used for the data demodulation reference signal (DM-RS)

OFDM symbol

Slide 17

LTE Downlink Frame Structure

Spectrum allocation 1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Slot duration 0.5 ms

Sub-frame duration 1.0 ms ( = 2 slots)

Sub-carrier spacing15 kHz

(7.5 kHz for MBMS)

Sampling frequency

1.92 MHz(1/2 × 3.84) 3.84 MHz 7.68 MHz

(2 × 3.84)15.36 MHz(4 × 3.84)

23.04 MHz(6 × 3.84)

30.72 MHz(8 × 3.84)

Sampling rates are

multiples of UMTS chip

rate, to ease

implementation of

dual mode UMTS/LTE

terminals

Subframe length relevant to the latency requirement

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frequency (1/2 × 3.84) 3.84 MHz (2 × 3.84) (4 × 3.84) (6 × 3.84) (8 × 3.84)

FFT size 128 256 512 1024 1536 2048

Number of sub-carriers 75 150 300 600 900 1200

OFDM symbols per slot 7 (short CP), 6 (long CP)

CP length

Short4.69 µµµµs x 6

5.21 µµµµs x 1

Long 16.67 µµµµs

FFT size scales to

support larger

bandwidth �

Scalable OFDM

Slide 18

Downlink/Uplink Resource Grid

� Resource Element (RE)� Fundamental element in time/frequency

grid (1 subcarrier x 1 symbol)

� Resource Block (PRB)� Minimum resource set for DL/UL data

channel assignment (12 subcarriers x 1 slot)

� Physical Resource Block (180 kHz x 1

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� Physical Resource Block (180 kHz x 1 ms)

� Virtual Resource Block (same size as PRB)

� Resource Block Group (RBG)� Group of Resource Blocks

Slide 19

DL Logical, Transport and Physical ChannelsLTE makes heavy use of shared

channels � common control,

paging, and part of broadcast

information carried on PDSCHPCCH: paging control channel

BCCH: broadcast control channel

CCCH: common control channel

DCCH: dedicated control channel

DTCH: dedicated traffic channel

BCCHPCCH CCCH DCCH DTCH MCCH MTCHDownlinkLogical channels

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PCH: paging channel

BCH: broadcast channel

DL-SCH: DL shared channel

BCHPCH DL-SCH MCH

DownlinkTransport channels

DownlinkPhysical Channels

PDSCH PDCCHPBCH PHICHPCFICHSCHDL-RS PMCH

Slide 20

Carries DL traffic

Carries basic system broadcast information

Allows mobile to get timing and frequency sync with the cell

Physical Channels to Support LTE Downlink

21

eNode-BDL resource allocation

HARQ feedback for DL

CQI reporting

MIMO reporting

Time span of PDCCH

Slide 21

Downlink Operation

� UE reports CQI (channel quality indicator), PMI (precoding matrix indicator), and RI (rank indicator) in PUCCH or PUSCH

� Scheduler at eNB dynamically allocates DL resources to UE

� eNB sends user data in PDSCH

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� UE reads PCFICH every subframe and determines the number of OFDM symbols occupied by PDCCH

� UE reads PDCCH to determine the assigned DL resources (PRB and MCS) for a specific Tx mode

� UE attempts to decode the received packet and sends ACK/NACK using PUCCH or PUSCH

Slide 22

Downlink Physical Channels in LTE

23 Slide 23

Physical Channel Resource Allocation

� P-SCH and S-SCH Resource Allocation� Subframe 0 and 5 of every 10th frames

� Middle of bandwidth (6 PRBs)

� PBCH Resource Allocation� Subframe 0 every 10th frames

� Middle of bandwidth (6 PRBs)

� PDCCH, PCFICH, PHICH allocated to the

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� PDCCH, PCFICH, PHICH allocated to the (at most) three OFDM symbols in each subframe

� The remaining time – frequency resources can be allocated for data transmission in the PDSCH

Slide 24

Downlink Reference Signals

� Three Types of DL Reference Signals (DL-RS)

� Cell-specific Reference Signals� Associated with PDSCH multiple antenna port transmission

� Used by UE for coherent demodulation and channel estimation

� UE-specific Reference Signals� UE-specific RS are supported for single antenna port transmission (Rel. 8)

25

� UE-specific RS are supported for single antenna port transmission (Rel. 8)

� In Rel. 10 UE-specific RS are also introduced for multiple antenna port transmission

� MBSFN Reference Signals � Associated with MBSFN transmission

Slide 25

DL cell-specific RS for multiple Tx Antennas

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� RS are allocated on a per antenna port basis

� Antenna 0 and 1� Transmitted on 2 OFDM symbols every slot

� 6 subcarrier spacing and 2x staggering (45 kHz frequency sampling)

� Antenna 2 and 3� Transmitted on 1 OFDM symbols every slot

� 6 subcarrier spacing with 2x staggering across slots

� Same frequency spacing for normal and extended CP Slide 26

Spatial Multiplexing� Code Word

� Transport block format : CRC encoded data

� Transmission layer� Sub stream resulting from a mapping of modulated code word symbols

� Number of layers ≤ number of antenna ports

� Code book� Quantized set of spatial combination vectors for precoding of symbols layer for transmission on

antenna ports

� Rank of MIMO channel� Number of independents TX/RX channels offered by MIMO for spatial multiplexing

27

� Rank ≤ min(NTx, NRx)

� UE indicates channel quality (CQI), pre-coding matrix (PMI) and rank (RI)

M Tx N Rx

V

RIHVUH Λ=

UHSelect

# code

words

Modulation

+ coding

PMICQI

Modulation

+ coding

Demod +

decode

Demod +

decode

precoding

Layer

mappingH

Slide 27

PDSCH Transmission Scheme

� Codewords (maximum of 2)� One codeword for rank 1 transmission

� Two codewords for rank 2/3/4 transmission

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� Layer mapping� Number of layers depend on the number of Tx antennas and the channel rank

� Fixed mapping schemes of codewords to layers

� Tx antennas (maximum of 4)� Potentially up to 4 layers

� Precoding� Used to support spatial multiplexing

� Code book based precoding

� Seven different transmission modes are supported in Rel. 8Slide 28

PDSCH Transmission Modes (Rel. 8)

� Mode 1: Uses a single Tx antenna at eNB together with cell-specific RS

� Mode 2: Uses transmit diversity based on Alamouti scheme

� Mode 3: Open loop spatial multiplexing� No UE feedback

� Exploits cyclic delay diversity (CDD)

� Mode 4: closed loop spatial multiplexing� Up to 2 codewords and 4 layers

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� Up to 2 codewords and 4 layers

� Rank (RI) and precoding (PMI) feedback

� Mode 5: Multi-user MIMO� Single codeword and single layer per UE

� UE reports PMI but no RI

� Mode 6: closed loop Rank = 1 precoding (restricted Mode 4)� No RI reports are needed

� Mode 7: same as Mode 1 with UE-specific RS

Slide 29

Multiple Antenna Techniques Supported in LTE

� SU-MIMO� Multiple data streams sent to the same user

(max. 2 codewords)

� Significant throughput gains for UEs in high SINR conditions

� MU-MIMO or Beamforming� Spatial Division Multiple Access (SDMA) � Different data streams sent to different users

using the same time-frequency resources

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using the same time-frequency resources

� Improves throughput even in low SINR conditions (cell-edge)

� Works even for single antenna mobiles

� Transmit diversity (TxDiv)� Improves reliability on a single data stream

� Fall back scheme if channel conditions do not allow spatial multiplexing

� Useful to improve reliability on common control channels

Slide 30

Precoding in Transmission Mode 4

� For two Tx antennas 4 single layer precoding vectors and 2 dual layer precoding matrices are supported� See table below (TS 36.211)

� For four Tx antennas 16 precoding vectors/matrices are supported per layer � The construction is based on Householder transformation

� For details, see TS 36.211

31 Slide 31

PDSCH Transmission Mode Configuration

32 Slide 32

CQI/PMI/RI Reporting

� CQI/PMI/RI reporting is either on PUSCH or PUCCH, periodic or aperiodic

� Aperiodic CQI/PMI/RI reporting is defined by the following characteristics:� The report is scheduled by the eNB via the PDCCH

� Transmitted together with uplink data on PUSCH

� From the frequency span perspective these reports can be:� Frequency selective: UE Selected Subband CQI and Higher Layer Configured

Subband CQI

� Frequency non-selective: Wideband CQI reports

33

� When a CQI report is transmitted together with Uplink data on PUSCH, it is multiplexed with the transport block by L1 � The CQI report is not part of the uplink transport block

� Periodic CQI/PMI/RI reporting is defined by the following characteristics:� Periodic CQI reports are sent on PUCCH

� From the Frequency span perspective these reports can be:� Frequency selective: UE Selected Subband CQI

� Frequency non-selective: Wideband CQI reports

Slide 33

CQI Definition

34 Slide 34

Downlink Peak Rates

bandwidth

# of parallel streams supported

1 2 4

1.4 MHz 5.4 Mbps 10.4 Mbps 19.6 Mbps

3 MHz 13.5 Mbps 25.9 Mbps 50 Mbps

5 MHz 22.5 Mbps 43.2 Mbps 81.6 Mbps

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10 MHz 45 Mbps 86.4 Mbps 163.2 Mbps


20 MHz 90 Mbps 172.8 Mbps 326.4 Mbps

Assumptions: 64QAM, code rate =1, 1OFDM symbol for L1/L2, ignores subframes with P-BCH, SCH

Slide 35

LTE Uplink Transmission Scheme (1/2)

� To facilitate efficient power amplifier design in the UE, 3GPP chose single carrier frequency domain multiple access (SC-FDMA) in favor of OFDMA for uplink multiple access.� SC-FDMA results in better PAPR

� Reduced PA back-off � improved coverage

� SC-FDMA is still an orthogonal

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� SC-FDMA is still an orthogonal multiple access scheme� UEs are orthogonal in frequency

� Synchronous in the time domain through the use of timing advance(TA) signaling� Only need to be synchronous

within a fraction of the CP length

� 0.52 µs timing advance resolution

Node BUE C

UE B

UE A

UE A Transmit Timing

UE B Transmit Timing

UE C Transmit Timing

αααα

ββββ

γγγγ

Slide 36

LTE Uplink Transmission Scheme (2/2)� SC-FDMA implemented using an OFDMA front-end and a DFT pre-coder,

this is referred to as either DFT-pre-coded OFDMA or DFT-spread OFDMA (DFTS-OFDMA)� Advantage is that numerology (subcarrier spacing, symbol times, FFT

sizes, etc.) can be shared between uplink and downlink

� Can still allocate variable bandwidth in units of 12 sub-carriers

� Each modulation symbol sees a wider bandwidth

37

+1 -1 -1 +

1 -1 -1 -1 +1 +

1 +1 -1

DFT pre-coding

Slide 37

SC-FDMA Signal

� SC-FDMA uses DFT precoding of user data

38

� SC-FDMA uses DFT precoding of user data� Individual bits mapped across multiple frequencies

� DFT size (N) defines number of subcarriers allocated to user data

� Time domain signal more resembles a single carrier signal� Peak-to-average power ratio (PAPR) is reduced

Slide 38

Localized and Distributed SC-FDMA

� Localized Assignment� Uses consecutive subcarriers

� Simpler to implement

� Used in LTE

� Distributed Assignment� Distributes subcarriers across

frequency bands

� Increases frequency diversity

39

� Increases frequency diversity

� Not applied in LTE

Slide 39

Comparison of OFDMA and SC-FDMA

40 Slide 40

Figure taken from http://cp.literature.agilent.com/litweb/pdf/5989-7898EN.pdf

Downlink/Uplink Resource Grid

� Resource Element (RE)� Fundamental element in time/frequency

grid (1 subcarrier x 1 symbol)

� Resource Block (PRB)� Minimum resource set for DL/UL data

channel assignment (12 subcarriers x 1 slot)

� Physical Resource Block (180 kHz x 1

41

� Physical Resource Block (180 kHz x 1 ms)

� Virtual Resource Block (same size as PRB)

� Resource Block Group (RBG)� Group of Resource Blocks

Slide 41

UL Logical, Transport and Physical Channels

CCCH: common control channel

DCCH: dedicated control channel

DTCH: dedicated traffic channel

42

RACH: random access channel

UL-SCH: UL shared channel

PUSCH: physical UL shared channel

PUCCH: physical UL control channel

PRACH: physical random access channel

Slide 42

Physical Channels to Support LTE Uplink

Random access for initial

access and UL timing

alignment

Carries UL Traffic

UL scheduling request for

time synchronized IEs

43

UL scheduling grant

time synchronized IEs

HARQ feedback for UL

eNode-B

Slide 43

Uplink Operation

� If UE does not have UL-SCH resources, UE send SR (scheduling request) on PUCCH

� Scheduler at eNB allocates resources to UE in terms of UL grant on PDCCH� Assigned resources: PRB and MCS

� UE sends user data on PUSCH

44

� If eNB decodes the UL data successfully, it sends ACK on PHICH

Slide 44

Uplink Physical Channels in LTE

45 Slide 45

PUSCH

� PUSCH may carry� UL data

� ACK/NACK for DL data

� CQI/PMI/RI

� The allocation of PRB resources is continuous

� Frequency hopping is supported to

46

� Frequency hopping is supported to obtain frequency diversity� Intra- and inter-subframe hopping

are supported

� Demodulation-RS (DM-RS ) are embedded in the SC-FDMA symbols

Slide 46

PUCCH

� PUCCH may carry� ACK/NACK for DL data

� CQI/PMI/RI

� Scheduling request

� PUCCH and PUSCH are never transmitted simultaneously� Reason: Reduction of PAPR

47 Slide 47

� PUCCH uses one PRB in each of the two slots in a subframe

� Multiple UEs may be assigned the same PRB resource for PUCCH transmission � Assignment of different cyclic shifts

of scrambling sequence and orthogonal spreading sequences

� (DM-RS are embedded in the SC-FDMA symbols

MIMO Support is Different in Downlink and Uplink

� Downlink� Supports SU-MIMO, MU-MIMO, TxDiv

48

� Uplink� Initial release of LTE does only support MU-MIMO with a single transmit antenna

at the UE � Desire to avoid multiple power amplifiers at UE

Slide 48

Uplink Peak Rates

bandwidthHighest Modulation

16 QAM 64QAM

1.4 MHz 2.9 Mbps 4.3 Mbps

3 MHz 6.9 Mbps 10.4 Mbps


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Assumptions: code rate =1, 2PRBs reserved for PUCCH (1 for 1.4MHz), no SRS, ignores subframes with PRACH,takes into account highest prime-factor restriction

Slide 49

LTE-Release 8 User Equipment Categories

Category 1 2 3 4 5

Peak rate Mbps

DL 10 50 100 150 300

UL 5 25 50 50 75

Capability for physical functionalities

RF bandwidth 20MHz

Modulation DL QPSK, 16QAM, 64QAM

UL QPSK, 16QAM QPSK,

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UL QPSK, 16QAM QPSK,16QAM,64QAM

Multi-antenna

2 Rx diversity Assumed in performance requirements.

2x2 MIMO Not supported Mandatory

4x4 MIMO Not supported Mandatory

Slide 50

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3G/4G Mobile Communications Systems · 3G/4G Mobile Communications Systems. Chapter VI: Physical Layer of LTE 2 Slide 2. Physical Layer of LTE OFDM and SC-FDMA Basics ... Physical