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MobileCommProfessionals, Inc.
Your Partner for Wireless Engineering Solutions
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Objective
Understand LTE Duplexing
Single Transmitter FDMA Principle
Multi carrier principle
OFDMA and SC FDMA PrincipleMultipath Propagation
Cyclic Prefix
OFDMA and SC FDMA
Transmitter Receiver
OFDM and SC FDMA Key Parameters
Resource Block
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Duplexing and Multiple Access
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Legacy- Single Transmitter
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FDMA Principle
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LTE: Multi-Carrier Principle
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The Rectangular Pulse
Advantages:
Simple to implement: there is nocomplex filter system required to
detect such pulses and to generatethem.
The pulse has a clearly definedduration. This is a major advantagein case of multi-path propagationenvironments as it simplifies handling
of inter-symbol interference.
Disadvantage:
It allocates a quite huge spectrum
However the spectral power densityhas null points exactly at multiplesof the frequency fs = 1/Ts.
This will be important in OFDM.
time
amplitude
Ts fs1
Ts
Time Domain
frequency f/fs
spectralpowerdensity
Frequency Domain
fs
FourierTransform
InverseFourierTransform
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OFDMA Principle
Transmits hundreds or even thousands of separately modulatedradio signals using orthogonal subcarriers spread across a
wideband channel
Orthogonality:
The peak (centrefrequency) of onesubcarrier
intercepts thenulls of theneighbouringsubcarriers
15 kHzin LTE: fixedTotal transmission bandwidth
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OFDM Basics
Data is sent in parallelacross the set of subcarriers, each subcarrieronly
transports a part of the whole transmission
The throughputis the sum of the data rates of each individual (or used)
subcarriers while the power is distributed to all used subcarriers
FFT( Fast Fourier Transform) is used to create the orthogonalsubcarriers.
The number of subcarriers is determined by the FFT size ( by the bandwidth)
Power
Frequency
Bandwidth
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OFDM Signal
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OFDM: Nutshell
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Frequency-Time Representation
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FFT/IFFT
It can be shown that the OFDM signal may be obtained by transformingLdata symbols by the IFFT, where L is the number of subcarriers.
Therefore, OFDM transmitter and receiver are implemented using IFFTand FFT respectively.
The size of the FFT should be chosen carefully as a balance betweenprotection against multipath (i.e. ISI), temporal variations (i.e. ICI), anddesign cost/complexity.
LTE FFT period is 66.67 usec, corresponding to the 15 KHz subcarrierseparation.
IFFT FFT
d1d2
dL
d1d2
dL
Time-domain(to be transmitted)
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Motivation for OFDMA
Good performance in frequency selective fadingchannels
Low complexity of base-band receiver
Good spectral properties and handling of multiplebandwidths
Link adaptation
Frequency domain scheduling
Compatibility with advanced receiver and antenna
technologies.
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Challenges
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1) ISI
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Solution: CP
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2) Multi-Carrier Modulation
The center frequencies must be spaced so that interference between different
carriers, known as Adjacent Carrier Interference ACI, is minimized; but not toomuch spaced as the total bandwidth will be wasted.
Each carrier uses an upper and lower guard band to protect itself from its
adjacent carriers. Nevertheless, there will always be some interference
between the adjacent carriers.
frequency
fsubcarrier
f0 f1 f2 fN-1fN-2
fsub-used
ACI = Adjacent Carrier Interference
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Solution: OFDM Multi-Carrier
OFDM allows a tight packing of small carrier
called the subcarriers - into a given frequency band.
No ACI (Adjacent Carrier Interference) in OFDM
due to the orthogonal subcarriers !
PowerDen
sity
PowerDens
ity
Frequency (f/fs) Frequency (f/fs)
SavedBandwidth
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3)Inter-Carrier Interference (ICI)
The price for the optimum subcarrier spacing is the sensitivity of OFDMto frequency errors.
If the receivers frequency slips some fractions from the subcarrierscenter frequencies, then we encounter not only interference betweenadjacent carriers, but in principle between all carriers.
This is known as Inter-Carrier Interference (ICI) and sometimes alsoreferred to as LeakageEffectin the theory of discrete Fouriertransform.
One possible cause that introduces frequency errors is a fast movingTransmitter or Receiver (Doppler effect).
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f0 f1 f2 f3 f4
P
I3
I1I4I0
ICI=Inter-CarrierI
nterference
Frequency Drift
Two effects begin to work: Subcarrier has no longer its
power density maximum- soloose of signal energy.
The rest of subcarriers haveno longer a null point here.So we get some noise fromthe other subcarrier.
OFDM T itt
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LowPass
cos(2fct)
-sin(2fct)
I
Q
ModulationMapper
IFFT
s0
ModulationMapper
s1
ModulationMapper
sN-1
b10 ,b11,
Serial toParallel
Converter(Bit
Distrib.)
b20 ,b21,
bN-1 0
BinaryCodedData
.
.
.
D
Ax0, x1, , xN-1 IQSplit
LowPass
D
A
R
freq.f1f2f0 fN-1
s0
s1 sN-1s2
FrequencyDomain
timet1t2t0 tN-1
x0x1
xN-1
x2
TimeDomain
CP/Guard
Generation
I
Q
Time Domain Signal
FrequencyDomain Signal:(Collection ofSinusoids)
Each entry to the IFFT modulecorresponds to a different sub-carrier
Each sub-carrier is modulatedindependently by ModulationSchemes:
BPSK,QPSK, 16QAM, 64QAM
OFDM Transmitter
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reference(pilot)
ChannelCorrection
Demodulator
Bit Mapping
j
I
Q
A
D
ChannelEstimation
RF
LowNoise
Amp.
+Bandp
ass
A
D
AGCAutomatic
Gain Control
De-rotator
signalstrength
LNA gain
Frequency And Timing Sync
signalautocorreation
pha
secorrection
timee
adjust
.
.
.
s0
s1
sN-1
channel
response
s0
Bit Mappings1
Bit MappingsN-1
.
.
.
.
.
.
.
.
.
B10 ,B11,
B20 ,B21,
BN-1 0
BitDistrib
ution
Soft BitCodedData
freq.f1f2f0 fN-1
s0s1 sN-1
s2
Frequency Domain
Time Domain
timet1t2t0 tN-1
y0y1yN-1
x2
QPSK
Im
Re
10
11
00
01
skd11
d10
OFDM Receiver
Windowing+
FFT
FrequencyD
omain
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OFDM Key Parameters
2) Subcarrier Spacing (f = 15 KHz)The Symbol time is
Tsymbol = 1/f = 66,7s
f
TSYMBOL
TCP SYMBOL
TCP
TS
Frequency
Time
Powerdensity
Amplitude
1) Variable Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz
Frequency
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3) The number of Subcarriers Nc
If BW = 20MHz Transmission BW = 20MHz2MHz = 18 MHz
the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers
TransmissionBandwidth [RB]
Transmission Bandwidth Configuration [RB]
Channel Bandwidth [MHz]
Resourceblock
Channeledge
Channeledge
DC carrier (downlink only)Active Resource Blocks
OFDM Key Parameters
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4) FFT (Fast Fourier Transform) size Nfft
For a bandwidth BW = 20 MHzNc = 1200 subcarriers not a power of 2
The next power of 2 is 2048 the rest 2048 -1200
848 padded with zeros
5. Sampling rate fs
This parameter indicates what is the sampling frequency:
fs = Nfft x f
Example: for a bandwidth BW = 5 MHz (with 10% guard band)
The number of subcarriers Nc = 4.5 MHz/ 15 KHz = 300
300 is not a power of 2 next power of 2 is 512 Nfft = 512Fs = 512 x 15 KHz = 7,68 MHz fs = 2 x 3,84 MHz which is the chip rate inUMTS!!
The sampling rate is a multiple of the chip rate from UMTS/ HSPA. Thiswas acomplished because the subcarriers spacing is 15 KHz. Thismeans UMTS and LTE have the same clock timing!
OFDM Key Parameters
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Bandwidth
(NCf)
1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz
Subcarrier Fixed to 15 kHz Spacing (f)
Symbol Tsymbol= 1/f = 1/15kHz = 66.67s
duration
Sampling rate,
fS(MHz)
1.92 3.84 7.68 15.36 23.04 30.72
Data
Subcarriers (NC)
72 180 300 600 900 1200
NIFFT
(IFFT Length)
128 320 512 1024 1536 2048
Number ofResource Blocks 6 15 25 50 75 100
Symbols/slot Normal CP=7; extended CP=6
CP length Normal CP=4.69/5.12sec., Extended CP= 16.67sec
OFDM Recap
OFDMA Ch ll
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OFDMA Challenges
1) Tolerance to frequency offset(Inter carrier Interference-ICI)
2) High Peak-to-Average Power Ratio
(PAPR)
Frequency
ICI
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SC FDMA
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SC FDMA nd OFDMA
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SC-FDMA and OFDMA
OFDMA transmits data in parallelacross multiple subcarriers
SC-FDMA transmits data in series employing multiple subcarriers
In the example:
OFDMA: 6 modulation symbols ( 01,10,11,01,10 and 10) are
transmitted per OFDMA symbol, one on each subcarrier
SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA
symbol using all subcarriers. The duration of each modulation
symbol is 1/6thof the modulation symbol in OFDMA
OFDMA SC-FDMA
SC FDMA and OFDMA
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OFDM SC-FDMA
Difference in transmission: for SC-FDMA there is an extra block onthe transmission chain: the FFT block
which should spread the input modulation symbols over all theallocated subcarriers
SC-FDMA and OFDMA
OFDMA vs SC FDMA: QPSK
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OFDMA vs SC-FDMA: QPSK
From: TS 36.211.
SC FDMA P i i l
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SC-FDMA Principles
PAPR is the same as that used for the input modulation symbols
This could be achieved by transmitting Nmodulation symbols in series at N times therate.
One can see that the SC-FDMA symbol which ishaving 66.66s is containing N sub-symbols
N = 6 in the example shown
In Time domain only one modulation symbolis transmitted at a time.
The number of subcarriers which could be allocated for transmissionshould be multiple of 2,3 and/or 5
This limitation is imposed by the input of the FFT block which isbefore the IFFT. This enables efficient implementation of theFFT.
Note that also the number of Resource Blocks should bemultiple of 2,3 or/and 5
SC FDMA P inciples
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The FFT output size is always smaller than the IFFT input size
FFT
IFFT
.
.
.
Subcarriers
allocated for oneUE
Subcarriersallocated to
other users orset to zero
This is because a typical cells uplinkcapacity will be greaterthan 180kHz
Other UEs will be assigned other groups ofsubcarriers to use across the uplink channelbandwidth.
No two UEs will be assigned the same180KHz block to use simultaneously.
As not all sub-carriers are used by themobile station, many of them are set to zeroin the diagram
Notethat if the output size of the FFT is equalto the size of the IFFT input then the overalleffect is nullsince the two operations (FFT andIFFT are complementary)
SC-FDMA Principles
SC FDMA Principles
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Adjusting the data rate in SC-FDMA
Halved SC-FDMAsub-symbol
duration
Initialbandwidth
SC-FDMAsub-
symbolduration
Doubledbandwidth
If the data rate increases more bandwidth is needed to transmit more modulationsymbols (when data rate is doubled the resource allocation in the frequency domain
is also doubled). The individual transmission is now shorter in time but wider inthe frequency domain.
For double data rate the amount of inputs in transmitter doubles and the sub-symbol duration(Time) is halved. Note that the SC-FDMA is still 67 s
Double the datarate
SC-FDMAsymbol 67s
SC-FDMAsymbol 67s
SC-FDMA Principles
In the example 6 modulation symbols are sent initially and 12 modulations for double data rate
SC FDMA: Multiplexing
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SC-FDMA: Multiplexing
One user always continuous in frequencySmallest uplink bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in downlink)
Largest uplink bandwidth: 20 MHz (same for OFDMA in downlink)
In time domain the granularity for resource allocation is 1 msfor one user (same for OFDMA in downlink)
User 2 f
User 1 f
f
Receiver
User 1 User 2
Bandwidth Distribution
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Bandwidth Distribution
CarrierBandwidth
(MHz)
Number ofSub-Carriers
1.4 72
3 198
5 330
10 660
15 990
20 1320
Resource: Element Block Grid
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Resource: Element, Block, Grid
[source: 3GPP TR 25.814]
LTE Reference Signals (R)areInterspersed Among ResourceElements
The Usage of RE
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The Usage of RE
Resource elements
reserved for
reference symbols
Control Channel
Region (1-3 OFDM symbols)
One subframe (1ms)
12
sub
carriers
Frequency
TimeData
Region
Duplexing FDD/TDD
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DuplexingFDD/TDD
FDD
..
..
..
..
Downlink Uplink
Frequency band 1
Frequency band 2
.. ..Single frequency
band
TDD
Frame Structure: Generic
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Frame Structure: Generic
Radio Frame Type 1 FDD
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Radio Frame Type 1 - FDD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
radio frame 10 msec
subframe 1 msec Type 1
0
0 1 2 3 4 5 67 OFDM symbols (short CP)
Radio Frame Type 2 TDD
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Slot
f
time
UL/DLcarrier
radio frame 10 ms
subframe 0
DwPT
S
GP
UpPT
S
half frame
subframe 2subframe 3subframe 1 subframe 4 subframe 5
DwPT
S
GP
UpPT
S
half frame
subframe 7subframe 8subframe 6 subframe 9
Slot
f
time
UL/DLcarrier
radio frame 10 ms
subframe 0
DwPTS
GP
UpPTS
half frame
subframe 2subframe 3subframe 1 subframe 4 subframe 5
half frame
subframe 7subframe 8subframe 6 subframe 9
Downlink Slot Uplink or Downlink Special SlotUplink Slot
Radio Frame Type 2 - TDD
Special Subframe
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Special Subframe
DwPTS (Downlink Pilot Timeslot Channel) Can contain synchronization, PDSCH and PDCCH.
The DwPTS is used for downlink synchronization.
Primary synchronization signal transmitted in the first OFDM symbol of theDwPTS.
Secondary synchronization signal transmitted in the last OFDM symbol ofsubframe 0 (immediately preceding to the DwPTS).
Resources not used for synchronization signals can be used for data,reference signals and control signaling.
UpPTS (Uplink Pilot Timeslot Channel) Used by eNB to determine the received power level and the received
timing from the UE.
Resources not used for reference signals(sounding and/or demodulationreference signals) can be used for random access.
No PUCCHis transmitted in UpPTS.
GP (Guard Period) The guard period between DwPTS and UpPTS determines the maximum cell
size.
TDD Frame Configurations
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TDD Frame Configurations
Configuration1 DL:UL=2:2 (or 3:2)
Configuration2 DL:UL=3:1 (or 4:1)
D S UDownlink Special Uplink
Uplink-
downlink
configuration
Downlink-to-Uplink
Switch-point
periodicity
Subframe number
0 1 2 3 4 5 6 7 8 9
0 5 ms D S U U U D S U U U
1 5 ms D S U U D D S U U D
2 5 ms D S U D D D S U D D
3 10 ms D S U U U D D D D D
4 10 ms D S U U D D D D D D
5 10 ms D S U D D D D D D D6 5 ms D S U U U D S U U D
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Summary
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Summary
Understand LTE Duplexing
Single Transmitter FDMA Principle
Multi carrier principle
OFDMA and SC FDMA PrincipleMultipath Propagation
Cyclic Prefix
OFDMA and SC FDMA
Transmitter Receiver
OFDM and SC FDMA Key Parameters
Resource Block
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HAPPY LEARNING
MobileCommProfessionals, Inc.www.mcpsinc.com
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