LTE intro sessionLTE intro session MODULE 2: LTE Frames

Lunch ’n’ learnJUNE 3rd, 2010

QUICK REVIEWQ– Why LTE ?

› Higher data rates (targeting more than 200 Mbits/s on DL)› Flatter Architecture› Scalability (Easier than before)› Scalability (Easier than before)› Evolution for GSM/EDGE, IS-95/EV-DO, UMTS/HSPA

– And from a more technical point of view:

› OFDM / MIMO / HOM (upto 64 QAM) › Lesser Latency› All-IP backbone

› * “LTE advanced” to target 1 Gb/s on DL

Ericsson Internal

QUICK REVIEWQFlatter Architecture: Lower Latency & Deployment simplicity

– eNodeB = ~ NodeB/BTS + RNC/BSC functionality combined– X2 interface between eNB’s ; S1 from eNB’s to GateWays (evolved core); y ( )– eNodeB’s handle HO between themselves (serving) & target eNodeB’s

Ericsson Internal

OFDM REview

S S S S Suubcarrier-1

ubcarrier-2

ubcarrier-3

ubcarrier-4

bcarrier-N

Frequency

Carrier 1 has a peak where

Use many carriers that are equally spaced:1 2 3 4 5

all other carriers have a null

k 0 1 N 1frequency

1

k = 0, 1, … , N-1

Ts = Symbol Time

1

Ericsson Internalsk T

kff 10

sTf 1

OFDM REviewTime-Frequency Domain

4-QAM

16-QAM

64-QAM

Subcarrie

Subcarrie

Subcarrie

Subcarrie

Subcarrie

Frequency

er-N

er-1

er-2

er-3

er-4

OFDM Slot/FrameIFFT

IFFTS1 S2 S3 S4 S5 S6 S7 SN

SN 1SN SN SN 4OFDM Symbol

S

Time Bandwidth

Guard TimeIFFTSN+1SN+

2

SN+3

SN+4 (FTT duration) S2N

Advantages of OFDM technologies• Higher spectral efficiency in real-life time dispersive channels

• More robust – less multi-path interference

• Easy to integrate MIMO technologies

• Simpler receiver to cope with real-life time dispersive channels lower cost Ericsson Internal

OFDM-A REviewMulti-user AccessMulti user Access

lot)

User #1 payload

User #2 payload

Tim

e (o

ne s

User #3 payload

T

User #4 payload

User #5 payload

• Multi user payloads are mapped part sub-carriers

Frequency

• Allow fast dynamic bandwidth and channel allocation

• Multi-user diversity is exploited in time slot and sub-carrier

MIMO Overview

7Ericsson Internal

OFDM Transceiver

8

OFDM Generator (QAM)(Q )

› Serial source(s)› Parallelized (1:N)› IFFT -> spreads all symbols over sub carriers (N:1 in time)

9Ericsson Internal

IFFT

› An OFDM system treats the source symbols as though they are in the frequency domain

Creating the OFDM Signal from Subcarriers

frequency domain.› These symbols are input to an IFFT (inverse fast Fourier transform)

block bringing the symbols into the time domain.› The IFFT takes N symbols at a time, where N is the number of

subcarriers in the system Each of the N input symbols has a symbolsubcarriers in the system. Each of the N input symbols has a symbol period of T seconds.

› Recall that each input signal is an orthogonal sinusoid› The output of the IFFT is summation of the N sinusoids

Th bl k f N t t l f th IFFT k i l OFDM› The block of N output samples from the IFFT make up a single OFDM symbol.

› The resultant time domain signal is transmitted across the channel.› An FFT block is used at the receiver to bring the signal into the

f d i Id ll th FFT t t ill b th i i l b lfrequency domain. Ideally, the FFT output will be the original symbols that were sent to the IFFT at the transmitter.

10

OFDM TX

An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with basebanddata on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation (QAM) or phase-shift keying (PSK). This composite baseband signal is typically used to modulate a main RF carrier.

11Ericsson Internal

g y y

OFDM RX

The receiver picks up the signal which is then quadrature-mixed down to baseband using cosine and sine waves at the carrier frequency. This also creates signals centered on 2fc, so low-pass filters are used to reject these. The baseband signals are then sampled and digitised using analogue-to-digital converters (ADCs), and a forward FFT is used to convert back to the frequency domain.

12Ericsson Internal

What does it mean to be Orthogonal?g

› If you take two signals, multiply them together and then› If you take two signals, multiply them together and then integrate over some interval, and the result is zero, then they are orthogonal in that interval. The frequencies must be integer multiplesbe integer multiples.

– The same is true for CDMA / WCDMA. The difference with OFDM is the basic function is a sinusoid instead of a code.

1x(t)=Sin(t) y(t)=Sin(2t)

time

0)(*).( dttytx0

Esignal = 0

t e

13

-1

Orthogonality IN OFDMg y› With N sub-carriers, M symbols modulating each sub-carrier, the OFDM symbol would have

‘M to the power N’ combined symbols.

› OFDM signal can be expressed as:

› Xk are the data symbols, T is the OFDM symbol time.

› Due to the sub-carrier spacing, the parallel signals are orthogonal over each symbol period.› Due to the sub carrier spacing, the parallel signals are orthogonal over each symbol period. › This can be expressed as:

Ericsson Internal

Orthogonality

1

x(t)=Sin(t) y(t)=Sin(3t)

1

-1

0

Esignal = 0

Sin(t).Sin(3t) time

This only works for integer multiples (= harmonics)

signal

Si ( t) Si (1 25 t)

1x(t)=Sin(t) y(t)=Sin(1.25t)

timeSin(t).Sin(1.25t)

Esignal > 0-1

0

time

15

-1

Orthogonality

x(t)=Sin(t) y(t)=Cos(t)

1

-1

0

Sin(t).Cos(t) time

O th l

-1Esignal = 0

Orthogonal:

• Sin(t) and Sin(nt)

• Sin(t) and Cos(t)Sin(t) and Cos(t)

Integration over a complete period

16

Multipath Fadingp g

F d d P thFaded Path

Direct Path

Faded Pathaded at

17

Effects of Multipath Fadingp g

Deep nulls in data.

18

Delay Spready p

› Received signal consists of a direct signal and reflections from other objects in the receive path The reflected signalfrom other objects in the receive path. The reflected signal arrives at a different time than the direct signal, thus spreading the received signal energy.

› Delay spread is the time between the arrival of the first and last multipath signal seen by the receiver.

› The delay spread causes inter symbol interference since› The delay spread causes inter-symbol interference since the delayed signal overlaps with the following symbols.

› This can cause significant errors in high bit rate systems– As the bit rate is increased the inter symbol interference also

increases

19

Delay Spready p

20

Solution

› OFDM lowers the delay spread by splitting the high rate data stream into multiple lower rate streams transmitteddata stream into multiple lower rate streams transmitted simultaneously. The signal dispersion in time is decreased because the symbol duration increases for lower rate

b isubcarriers.› Inter symbol interference can almost be eliminated

completely by introducing a guard time in every OFDMcompletely by introducing a guard time in every OFDM symbol.

› In the guard time, the OFDM symbol is cyclically extended id i i i fto avoid intercarrier interference.

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Cyclic PrefixCyclic Prefix

› The last part of a symbol pulse is copied and attached to the beginning. This becomes the cyclic prefix.

› In the presence of multipath, the delayed component falls within the guard periods of the next symbol.Th d i d t i i f ti hi h ill b th ll i th› The guard period contains information which will be thrown away allowing the original symbol to remain intact.

› As long as the multipath components fall within the window of the cyclic prefix, inter symbol interference can be avoided.

22

Cyclic Prefix combats ISIy

› In the guard region, Tg, the resulting contributions from both signals will cause ISI Since the guard interval isboth signals will cause ISI. Since the guard interval is ignored by the receiver, there is no degradation of performance.

23

Scaling and rotation of symbol only

Draw Back to Cyclic Prefixy

› Adds overhead to the system and thus the overall throughput by transmitting information with has no usethroughput by transmitting information with has no use

› The length of the CP is critical.– If it is too small, it may not prevent ISI– If it is too large, the additional overhead becomes significant

› Typically, the CP must be 4-6 times the delay spread.Thi i ll ifi d f ti f th b l ti› This is usually specified as a fraction of the symbol time without the CP.

– For example, if the symbol time is X microseconds, the CP may be ¼, 1/8, 1/16, or 1/32 of X.

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LTE AttributesUses OFDM/FFT technology (typically 15 Khz spaced subcarriers)

SC-FDMA in UL due to PAPR issues

MIMO (upto 4x4 for now)

AMC / HOM– Upto 64 QAM

Lower Latency to enable real-time traffic– < 20msec RTD

Scalable B/W– 1.4, 3, 5, 10, 15, 20 MHz

Higher Capacity and Peak Rates– 200 Mbps DL, 100 Mbps UL (with 4x4 MIMO). Targetting 1G in future

Modulation and Channel Coding

> Downlink:Downlink:• Conventional OFDM with cyclic prefix• QPSK, 16-QAM and 64-QAM supported• Asynchronous adaptive H ARQ• Asynchronous adaptive H-ARQ

> Uplink:• SC-FDMA• QPSK, 16-QAM supported now (64-QAM in future ?)• Synchronous H-ARQ • UE-specific UL scrambling is supported but optionalp g pp p

> Turbo code is channel code for bearer traffic

> Tail-biting convolutional coding for control signaling

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LTE Frame Structure Type 1› Radio frame is 10 msec long and consists of 20 slots, numbered from 0 to 19.› Slot: Consists of 7 OFDMA symbols of 71.4usec each (average) with normal CP; 6 with

extend CP. Tslot = 0.5 msec.› Subframe: Two consecutive slots, so subframe duration is 1 msec. There’re 10

subframes in 1 radio frame. There’re 7 symbols per slot; 2 slots per subframe; 10 subframes per radio frame

› TTI = 30720*Ts = 1 msec (2 slots or 1 subframe)› TTI = 30720 Ts = 1 msec (2 slots or 1 subframe).

› Type 1 applies to both FDD and TDD– In FDD, 10 subframes are available for downlink transmission and 10 subframes are available for

uplink transmissions in each 10 ms interval. DL and UL are separated in frequency domain.uplink transmissions in each 10 ms interval. DL and UL are separated in frequency domain.– For TDD, a subframe is either allocated to downlink or uplink transmission. Subframe 0 and

subframe 5 are always allocated for downlink transmission.

One radio frame, Tf = 307200Ts=10 ms

#0 #1 #2 #3 #19

One slot, Tslot = 15360Ts = 0.5 ms

#18Frame

structure type 1

One subframe

type 1

Frame Structure - FDD

10 ms

Downlink radio frame #i F1

Uplink radio frame #i F2

NTA x TS time units

y T i i f th d li k di f

frequ

ency Transmission of the downlink radio frame

from the UE starts before the corresponding uplink radio frame at the UE

time

DL LTE System Parameters (example)

29

UL LTE System Parameters (example)

30

Resource Block (RB), Resource Element (RE)( ) A scheduler resource block or simply an RB consists of two time slots and 12 subcarriers

– has 168 physical resource elements (12 subcarriers X 7 symbols X 2 slots)– RE is 1 symbol (71.4usec) x 1 subcarrier (15 kHz)– RB represents the minimum resource allocation

O i b d idth f 12*15 kH 180 kH 1 i d– Occupies a bandwidth of 12*15 kHz = 180 kHz over 1msec period

Downlink – 168 REs total

16 REs used/reserved for RS

Uplink • 168 REs total• 24 REs Dem. Reference

0 1 2 3 4 5 6 0 1 2 3 4 5 6

Resource Block (RB)

– 16 REs used/reserved for RS– 20 REs used for PDCCH – 132 REs used for data

• 12 REs could be used for sounding• 144/132 REs used for data (with/without SR)

RE0 1 2 3 4 5 6 0 1 2 3 4 5 6

012345

RS ant 0

RS ant 1

Demodulation ReferenceSoundingR f6

7891011

PDCCH

Data

Reference

eque

ncy

(SC

)

1 ms (2 slots, or 1 sub-frame)time

Fre

DL Resource Blocks and Resource Elements One downlink slot, Tslot

Resource element

riers

Resource blockRBsc

DLsymb NN resource elements

bcar

riers

Nsc

subc

arr

RB

NR

BDL

sub

NscR

B

N

32

OFDM symbolsDLsymbN OFDM symbolsDLsymbN

Downlink Frame Structure- PDSCH (Physical Downlink Shared Channel)

0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6012345

Slot 0 Slot 1 Slot 2 Slot 3 Slot 19<----- 1 subframe = 1ms ----->

Slot 16 Slot 17 Slot 18

RS antenna 056789

101112131415

...RS antenna 1PDCCH Data

151617181920212223 . . S

C)

577578579580581582583584

.. ..

Freq

uenc

y (S

584585586587588589590591592593594

...time

594595596597598599600

Uplink Frame Structure- PUSCH (Physical Uplink Shared Channel)- PUSCH (Physical Uplink Shared Channel)

› PUSCH – Can be scheduled for one or more RB’s– For data transmission Demodulation

ReferenceSounding

0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 60

Slot 19Slot 0 Slot 1 Slot 2 Slot 3

Sou d gReference

0123456789

1011

uenc

y (S

C)

Scheduler Resource Block (RB)time

Freq

THANK YOU

Q&A & Back Up slides

Ericsson Internal

LTE GlossaryAMC Adaptive Modulation and Coding PDSCH Physical Downlink Shared Channel

CCPCH Common Control Physical Channel PRB Physical Resource Block

CCE Control Channel Element PSC Primary Synch channel

CP C clic P efi PUCCH Ph sical Uplink Cont ol ChannelCP Cyclic Prefix PUCCH Physical Uplink Control Channel

CQI Channel Quality Indicator PUSCH Physical Uplink Shared Channel

CRC Cyclic Redundancy Check QAM Quadrature Amplitude Modulation

DL Down Link RB Resource Block

eNode-B Evolved Node-B RE Resource Element

EVRC Enhanced Variable Rate Codec RLC Radio Link Control

FBSS Fast Basestation Switching RRC Radio Resource Control

FDD Frequency Division Duplex RS Reference Signal

H-ARQ Hybrid-Automatic Repeat Request SC-FDMA Single-Carrier Frequency Division Multiple Access

LTE Long Term Evolution SISO Single Input Single OutputLTE Long Term Evolution SISO Single Input Single Output

MAC Medium Access Control SR Scheduling Request

MCW Multi Code Word SRB Scheduler Resource Block

MCS Modulation and Coding Scheme SRS Sounding Reference Signal

MDHO Macro Diversity Hand-Off SSC Secondary Synch Channel

MIMO Multiple Input Multiple Output TB Transport Block

MU-MIMO Multi-User MIMO TD Transmit Diversity

OFDM Orthogonal Frequency Division Multiplexing TTI Transmit Time Interval

PBCH Physical Broadcast Channel UE User Equipment

PCFICH Physical Control Format Indicator Channel UL Up LinkPCFICH Physical Control Format Indicator Channel UL Up Link

PDCCH Physical Downlink Control Channel v-MIMO Virtual MIMO

PHICH Physical Hybrid ARQ Indicator Channel

Ericsson Internal

Coherence BandwidthTime domain view Freq. domain view

)(tx

Time domain view

)( fX

Range of freq over

which response is flat d l d

High correlation of amplitude

which response is flatBc

delay spread

between two different freq.

components

Ericsson Internal

Coherence TimeTime domain view Frequency domain viewTime domain view Frequency domain view

signal bandwidthsymbol time

fc+fdfc-fd

Coherence Time: Time interval

over which channel impulseTc

over which channel impulse

responses are highly

correlated

Ericsson Internal