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LTE Course for Technical Personnel
Summer School ATHENA 2011
LTE
Air Interface
History - Details
 1G FDMA (NMT, AMPS, TACS) 80’s
- Voice (analog traffic, digital signaling)
 2G TDMA (GSM, D-AMPS, PDC...
3G Evolution
 HSPA Evolution
– gradually improved performance at a low additional cost prior to
the introduction of LTE
...
R99
LTE
HSPA
evolved
HSPA
Enhanced UplinkHSDPA
3GPP Rel 99/4 Rel 5 Rel 6
WCDMA EvolvedWCDMA
Rel 7 Rel 8
LTE
HSPA Evolved
L...
Telephony
WWW
@
Office
TV
MobileHome
Why LTE/SAE ? - Mobile Triple Play
- Telephony, Data and Video/TV
all service deliver...
LTE – Targets
 High data rates
– Downlink: >100 Mbps
– Uplink: >50 Mbps
– Cell-edge data rates 2-3 x HSPA Rel. 6 (@ 2006)...
EPS Architecture
eNB eNB
eNB
MME/S-GW MME/S-GW
S1
X2
X2
X2
SAE
(System Architecture
Evolution)
LTE
(Long Term Evolution)
E...
source
source
PCRF
Overall Architecture
X2-UP
S1-UP (User Plane)
EPC
S1-CP (Control Plane)
E-UTRAN
eNodeBeNodeB
S11
MME
S-GW
P-GW
S5/S8
...
Typical Implementation of SAE/LTE
- combined SGSN/MME
Iub
3G
(HSPA & DCH)
S1-UP
UTRAN
Node B
Internet
Evolved
Packet
Core
...
Multiple Access Approaches
Frequency
Division
Multiple
Access
Each User has a unique
frequency
(1 voice channel per user)
...
LTE Physical Layer
 Flexible bandwidth
– Possible to deploy in 6 different bandwidths
up to 20 MHz
 Uplink: SC-FDMA with...
Time-domain
Structure FDD
Normal CP, 7 OFDM
symbols per slot
TCP Tu  66.7 s
#0 #1 #9
One OFDM symbol
One slot (0.5 ms) =...
Segmentation, ARQ
Ciphering
Header Compr.
Hybrid ARQHybrid ARQ
MAC multiplexing
Antenna and
resrouce mapping
Coding + RM
D...
MAC Layer
Segmentation, ARQ
Ciphering
Header Compr.
Hybrid ARQHybrid ARQ
MAC multiplexing
Antenna and
resrouce mapping
Cod...
Channel mapping
UL-SCHPCH DL-SCH
PCCH
Logical Channels
“type of information”
(traffic/control)
Transport Channels
“how and...
Tx & Rx physical layer processing
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC check
Radio Chan...
Tx Coding
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC check
Radio Channel
OFDM(IFFT) OFDM(FFT)...
CRC Coding – error detection
Cyclic-Redundancy Check (CRC) Coding
– Identifies any corrupted data left
after error correct...
FEC Coding
 Error Correction
Help the receiver correct bit errors caused by the air interface.
– How do you correct error...
FEC Coding Approaches
– Block Codes (Hamming Codes, BCH Codes, Reed-Solomon
Codes)
– Continuous Codes (Convolutional Codes...
FEC Coding
Original Data
00011011...
FEC
Generator
FEC Encoded data
1010011100110110...
Original Data
00011011
Viterbi/
Tu...
Tail Biting Convolutional Encoder
D D D DD D
G0 = 133 (octal)
G1 = 171 (octal)
G2 = 165 (octal)
kc
)0(
kd
)1(
kd
)2(
kd
 ...
Radio Channel problems -
Multipath Propagation
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC che...
Multipath Propagation
• Up to date cellular systems have used single carrier modulation
schemes almost exclusively.
• LTE ...
Multipath Propagation
 One user’s signal reflects off many objects
 The received signal contains many time-delayed
repli...
Multipath Propagation
- and the resulting impulse response
Multipath Propagation gives rise to:
1. InterSymbol Interferenc...
Multipath Propagation
- and the resulting impulse response
Fast fading (Rayleigh fading)
τ0 τ1 τ2 t(μs)
P0
P1
P2
Power
(dB...
Multipath Propagation
- and the resulting impulse response
InterSymbol Interference (ISI)
τ0 τ1 τ2 t(μs)
P0
P1
P2
Power
(d...
Multipath Fading
Direct Signal
Reflected Signal
Combined Signal
Path loss and Fast fading
Power
distance
Time between fades is related to
• RF frequency
• Geometry of multipath vectors
•...
Block Interleaving
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC check
Radio Channel
OFDM(IFFT) ...
Block Interleaving
Time
Amplitude
To
decoder
Original Data Samples
1 2 3 4 5 6 7 8 9
Interleaving
Matrix
1 2 3
4 5 6
7 8 9...
Scrambling in LTE
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC check
Radio Channel
OFDM(IFFT) O...
Interference in LTE
PROBLEM STATEMENT
In LTE, a frequency reuse of 1 will typically be used. This means that
all cells use...
Scrambling in LTE
 Cell specific bit-level scrambling used in LTE for all
datastreams in UL and DL
– used in order to ach...
Modulation
Coding
Scrambling
Modulation
CRC
Decoding
Descrambling
Demodulation
CRC check
Radio Channel
OFDM(IFFT) OFDM(FFT...
Modulation
Next step after channel coding and scrambling is modulation.
Modulation: A process that maps blocks of scramble...
Modulation
 The sub-carriers are modulated with a certain modulation scheme
– maps the data bits into a carrier phase and...
OFDM Principle
 Parallel transmission using a large number of narrowband “sub-carriers”
 “Multi-carrier” transmission
 ...
Background
 OFDM – Orthogonal Frequency Division Multiplex
– a modulation scheme, not a multiple-access scheme
 Basic pr...
Orthogonal Frequency Division Multiplexing
Principles
Benefits
+ Frequency diversity
+ Robust against ISI
+ Easy to implem...
Orthogonal Frequency Division Multiplexing
Principles
Coherence time - Physics
coherence time in electromagneti wave theor...
Orthogonal Frequency Division Multiplexing
Principles
Coherence time - Physics
Waves of different frequencies interfere
to...
Orthogonal Frequency Division Multiplexing
Principles
Coherence time - Telecom
In communications systems, a communication ...
Orthogonal Frequency Division Multiplexing
Principles
1 11( ) ( ) ( )t ty t x t t h t  
1
( )th t
2 22( ) ( ) ( )t ty t...
OFDM multicarrier transmission
 Single carrier transmission
– each user transmits and receives data stream with only one ...
Fourier transform
)]([)()( 2
txFdtetxfX ftj
 



  fxFdfefXtx ftj 12
)()( 


  

 
 f
f
f
ee
fj
d...
OFDM Transmission
freq
=
freq
*
freq
t
OFDM Transmission
freq
freqtime
freq
time
-10 -5 0 5 10
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6...
Discrete Fourier Transform (DFT)
 N-point DFT:
 N-point IDFT:
 Discrete sequence of sampled signal, discrete spectrum
...
Orthogonality
Time domain Frequency domain
Example of four subcarriers within one OFDM symbol Spectra of individual subcar...
Orthogonal Frequency Division Multiplexing
 Receiver integrates for symbol integral
 Orthogonality criteria:
     ...
OFDM and Multicarrier Transmission
(A)
(E)
(D)
(C)
(B)
1cf f 2cf f 3cf f 4cf f 5cf f
1
T
1
T
Orthogonal
Non-orthogona...
Guard time and Cyclic Prefix
Direct path:
Reflected path:
Integration interval
 We rely on that the subcarriers are ortho...
FFT-based OFDM System
Serial-to-
Parallel
Converter
Signal
Mapper
IFFT
Parallel-
to-Serial
Converter
Guard
Interval
Insert...
Links and references
www.3gpp.org
 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial
...
36.201 – Physical layer general description
36.211 – Physical channels and modulation
36.212 – Multiplexing and channel co...
Links and references
LTE Air Interface
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LTE Air Interface

  1. 1. LTE Course for Technical Personnel Summer School ATHENA 2011 LTE Air Interface
  2. 2. History - Details  1G FDMA (NMT, AMPS, TACS) 80’s - Voice (analog traffic, digital signaling)  2G TDMA (GSM, D-AMPS, PDC) and CDMA (IS-95) 90’s - Voice, SMS, CS data transfer ~ 9.6 kbit/s (50 kbit/s HSCSD)  2.5G TDMA (GPRS) 00’s - PS data transfer ~ 50 kbit/s 2.75G TDMA (GPRS+EDGE) 00’s - PS data ~ 150kbit/s  3-3.5G WCDMA (UMTS) and CDMA 2000 00’s - PS & CS data transfer ~ 14-42 Mbit/s (HSPA/HSPA+), Voice, SMS  3.9G OFDMA (LTE/SAE) 10’s - PS Data and Voice (VoIP) ~ 100Mbit/s  4G IMT Advanced source
  3. 3. 3G Evolution  HSPA Evolution – gradually improved performance at a low additional cost prior to the introduction of LTE  LTE – improved performance in a wide range of spectrum allocations HSUPA MBMS Rel 6 MIMO HOM CPC Rel 7Rel 4R99 HSDPA Rel 5 4G Further enhancements WCDMA/HSPAWCDMA HSPA Evolution Rel 8 LTE LTE Advanced source
  4. 4. R99 LTE HSPA evolved HSPA Enhanced UplinkHSDPA 3GPP Rel 99/4 Rel 5 Rel 6 WCDMA EvolvedWCDMA Rel 7 Rel 8 LTE HSPA Evolved LTE HSPA+ 3GPP standard evolution  Initial packet data in Rel 99/Rel 4  High Speed Downlink Packet Access in Rel 5  Enhanced Uplink in Rel 6  ”High Speed Packet Access+” in Rel 7 e.g.: – Multiple Input Multiple Output (MIMO) – Higher order modulation DL/UL  Long Term Evolution in Rel 8 384 kbps 14.4 / 5.8 Mbps 28 / 12 Mbps >100 Mbps
  5. 5. Telephony WWW @ Office TV MobileHome Why LTE/SAE ? - Mobile Triple Play - Telephony, Data and Video/TV all service delivered by one network
  6. 6. LTE – Targets  High data rates – Downlink: >100 Mbps – Uplink: >50 Mbps – Cell-edge data rates 2-3 x HSPA Rel. 6 (@ 2006)  Low delay/latency – User plane RTT: < 10 ms RAN RTT (fewer nodes, shorter TTI) – Channel set-up: < 100 ms idle-to-active (fewer nodes, shorter messages, quicker node resp.)  High spectral efficiency – Targeting 3 X HSPA Rel. 6 (@ 2006 )  Spectrum flexibility – Operation in a wide-range of spectrum allocations, new and existing – Wide range of Bandwidth: 1.4, 1.6, 3.0/3.2, 5, 10, 15 and 20 MHz, FDD and TDD  Simplicity – Less signaling, Auto Configuration e-NodeB – ”PnP”, ”Simple as an Apple”  Cost-effective migration from current/future 3G systems  State-of-the-art towards 4G  Focus on services from the packet-switched domain
  7. 7. EPS Architecture eNB eNB eNB MME/S-GW MME/S-GW S1 X2 X2 X2 SAE (System Architecture Evolution) LTE (Long Term Evolution) EPC (Evolved Packet Core) E-UTRAN EPS (Evolved Packet System) UE source
  8. 8. source
  9. 9. source
  10. 10. PCRF Overall Architecture X2-UP S1-UP (User Plane) EPC S1-CP (Control Plane) E-UTRAN eNodeBeNodeB S11 MME S-GW P-GW S5/S8 X2-CP P-CSCF S7/Gx Network & Service management OSS-RC EMA MM DNS/ENUM HSS S-CSCF I-CSCF IMS Control layer Platforms / Concepts TSP/NSP or TSP/IS DNS/ ENUM MGC MGW SUN IS A-SBG CPP / RBS6000 Juniper/ Redback WPP GERAN UTRAN Broadband Wired Access GPRS Packet Core SGSN GGSN CDMA2000 HRPD (EV-DO) WLAN N-SBG Internet S6a CS Core MSC GWMSC PSTN PDSN S1-AP, X2-AP H.248 ISUP Diameter S3 S4 GTP-C Gxa S103 S2a RNC Other SIP/UDP or SIP/TCP Rx+ User data RTP/UDP GTP/UDP S101 IMS Connectivity layer Service LayerAS AS AS Application Servers MTAS S6d Uu source
  11. 11. Typical Implementation of SAE/LTE - combined SGSN/MME Iub 3G (HSPA & DCH) S1-UP UTRAN Node B Internet Evolved Packet Core S1-CP Iu-CP LTE Gi S4/S11 SAE BTS Gb Abis 2G GERAN BSC SGSN/ MME P/S-GW RNC X2-UP E-UTRAN eNodeBeNodeB X2-CP Gn
  12. 12. Multiple Access Approaches Frequency Division Multiple Access Each User has a unique frequency (1 voice channel per user) All users transmit at the same time AMPS, NMT, TACS Each Transmitter has a unique Scrambling Code Each Data Channel has a unique Channelization code Many users share the same frequency and time IS-95, cdma2000, WCDMA Code Division Multiple Access Spread Spectrum Multiple Access Each User has a unique time slot Each Data Channel has a unique position within the time slot Several users share the same frequency IS-136, GSM, PDC Time Division Multiple Access Orthogonal Frequency Division Multiple Access Each User and each channel has a unique Time and Frequency Resource Many users are separated in frequency and/or time LTE, Wimax (WLAN 802.11a,g, DAB radio)
  13. 13. LTE Physical Layer  Flexible bandwidth – Possible to deploy in 6 different bandwidths up to 20 MHz  Uplink: SC-FDMA with dynamic bandwidth (Pre-coded OFDM) – Low PAPR  Higher power efficiency – Reduced uplink interference (enables intra-cell orthogonality )  Downlink: Adaptive OFDM – Channel-dependent scheduling and link adaptation in time and frequency domain  Multi-Antennas, both RBS and terminal – MIMO, antenna beams, TX- and RX diversity, interference rejection – High bit rates and high capacity TX RX frequency frequency  Harmonized FDD and TDD concept – Maximum commonality between FDD and TDD  Minimum UE capability: BW = 20 MHz 10 15 20 MHz3 fDL fUL FDD-only fDL fUL Half-duplex FDD fDL/UL TDD-only Δf=15kHz 180 kHz User #2 scheduledUser #1 scheduled User #3 scheduled 1.4 5 source
  14. 14. Time-domain Structure FDD Normal CP, 7 OFDM symbols per slot TCP Tu  66.7 s #0 #1 #9 One OFDM symbol One slot (0.5 ms) = 7 OFDM symbols One subframe (1 ms) = two slots One radio frame (10 ms) = 10 subframes = 20 slots #2 #3 #4 #5 #6 #7 #8 •PBCH sent in subframe #0, slot 1, symbol 0-3 over 4 consequtive radio frames (40 ms) •SCH sent in subframe #0 and #5, slot 0 and 10, symbol 5-6 (4-5 in case of extended CP) PBCH S-SCH P-SCH S-SCH P-SCH frequency Δf=15kHz 180 kHz User #2 scheduledUser #1 scheduled User #3 scheduled source
  15. 15. Segmentation, ARQ Ciphering Header Compr. Hybrid ARQHybrid ARQ MAC multiplexing Antenna and resrouce mapping Coding + RM Data modulation Antenna and resource mapping Coding Modulation Antenna and resource assignment Modulation scheme MACscheduler Retransmission control Priority handling, payload selection Payload selection RLC #i PHY PDCP #i User #i User #j MAC Concatenation, ARQ Deciphering Header Compr. Hybrid ARQHybrid ARQ MAC demultiplexing Antenna and resrouce mapping Coding + RM Data modulation Antenna and resource demapping Decoding Demodulation RLC PHY PDCP MAC eNodeB UE Redundancy version IP packet IP packet EPS bearers E-UTRA Radio Bearers Logical Channels Transport Channels Physical Channels Radio interface structure source
  16. 16. MAC Layer Segmentation, ARQ Ciphering Header Compr. Hybrid ARQHybrid ARQ MAC multiplexing Antenna and resrouce mapping Coding + RM Data modulation Antenna and resource mapping Coding Modulation Antenna and resource assignment Modulation scheme MACscheduler Retransmission control Priority handling, payload selection Payload selection RLC #i PHY PDCP #i User #i MAC IP packet MAC layer for the LTE access can be compared to the Rel-6 MAC- hs/MAC-e and covers mainly similar functionality: • HARQ, • priority handling (scheduling), • transport format selection • DRX control source
  17. 17. Channel mapping UL-SCHPCH DL-SCH PCCH Logical Channels “type of information” (traffic/control) Transport Channels “how and with what characteristics” (common/shared/mc/bc) Downlink Uplink PDSCH Physical Channels “bits, symbols, modulation, radio frames etc” MTCH MCCH BCCH DTCH DCCH DTCH DCCH CCCH PRACH RACH CCCH MCH BCH PUSCHPBCH PCFICH PUCCH -CQI -ACK/NACK -Sched req. -Sched TF DL -Sched grant UL -Pwr Ctrl cmd -HARQ info MIB SIB PMCH PHICHPDCCH ACK/NACK PDCCH info Physical Signals “only L1 info” RS SRSP-SCH S-SCH RS -meas for DL sched -meas for mobility -coherent demod -half frame sync -cell id -frame sync -cell id group -coherent demod -measurements for UL scheduling source
  18. 18. Tx & Rx physical layer processing Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel Not shown: Rate Matching HARQ MIMO mapping... OFDM(IFFT) OFDM(FFT) Tx Rx
  19. 19. Tx Coding Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel OFDM(IFFT) OFDM(FFT) Tx Rx
  20. 20. CRC Coding – error detection Cyclic-Redundancy Check (CRC) Coding – Identifies any corrupted data left after error correction function in receiver – CRC is used for checking BLER (Block Error Ratio) in the outer loop power control Checksum 24 bits 110010110011 Original Data 244 bits CRC Generator Original Data 1001011010.. CRC Generator Re-Generated Checksum 110010110001 Transmitter Receiver If Checksums do not match, there is an error Received Data 1001010010.. Received Checksum 110010110011 RF Transmission Path The longer the checksum, the greater is the accuracy of the process. Why??? Answer: Various combinations of errors in the data and the checksum would produce the same checksum. The longer the checksum the less likely it is for this to happen. Example: 24 bits of binary information represents 16 777 216 (224) different combinations
  21. 21. FEC Coding  Error Correction Help the receiver correct bit errors caused by the air interface. – How do you correct errors at the receiver? Send message many times? 010010110, 010010110, 010010110, 010010110, 010010110,    Forward Error Correction! Up to 6x data expansion... But the most powerful results Advantage: The more times the data is transmitted the better is the error protection. Disadvantage: However the bandwidth is also increased proportionally Need to find a FEC technique with minimum BW requirements!!
  22. 22. FEC Coding Approaches – Block Codes (Hamming Codes, BCH Codes, Reed-Solomon Codes) – Continuous Codes (Convolutional Codes, Turbo Codes)  Data is processed continuously through FEC generator  Resulting data stream has built-in redundancy that can be extracted to correct bit errors. – LTE uses Turbo codes with rate 1/3 for DL-SCH transmissions. – Convolutional coding used for BCH source
  23. 23. FEC Coding Original Data 00011011... FEC Generator FEC Encoded data 1010011100110110... Original Data 00011011 Viterbi/ Turbo Decoder Transmitter Receiver RF Transmission Path source
  24. 24. Tail Biting Convolutional Encoder D D D DD D G0 = 133 (octal) G1 = 171 (octal) G2 = 165 (octal) kc )0( kd )1( kd )2( kd  Constraint length 7  Coding rate 1/3 source
  25. 25. Radio Channel problems - Multipath Propagation Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel OFDM(IFFT) OFDM(FFT) Tx Rx
  26. 26. Multipath Propagation • Up to date cellular systems have used single carrier modulation schemes almost exclusively. • LTE uses OFDM rather than single carrier modulation • single carrier systems face extreme problems with multipath induced channel distortion • A measure of multipath distortion is provided by delay spread  describes the amount of time delay at the receiver from a signal traveling from the transmitter along different paths.
  27. 27. Multipath Propagation  One user’s signal reflects off many objects  The received signal contains many time-delayed replicas
  28. 28. Multipath Propagation - and the resulting impulse response Multipath Propagation gives rise to: 1. InterSymbol Interference (ISI) 2. Fast fading (Rayleigh fading) source
  29. 29. Multipath Propagation - and the resulting impulse response Fast fading (Rayleigh fading) τ0 τ1 τ2 t(μs) P0 P1 P2 Power (dB) Impulse response
  30. 30. Multipath Propagation - and the resulting impulse response InterSymbol Interference (ISI) τ0 τ1 τ2 t(μs) P0 P1 P2 Power (dB) P2,τ1 P0,τ0 P1,τ2 Impulse response Direct signal Reflected signal Path delay difference ISI
  31. 31. Multipath Fading Direct Signal Reflected Signal Combined Signal
  32. 32. Path loss and Fast fading Power distance Time between fades is related to • RF frequency • Geometry of multipath vectors • Vehicle speed: Up to 4 fades/sec per kilometer/hour path loss Rayleigh Deep fade caused by destructive summation of two or more multipath reflections
  33. 33. Block Interleaving Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel OFDM(IFFT) OFDM(FFT) Tx Rx
  34. 34. Block Interleaving Time Amplitude To decoder Original Data Samples 1 2 3 4 5 6 7 8 9 Interleaving Matrix 1 2 3 4 5 6 7 8 9 Transmitter Interleaved Data Samples 1 4 7 2 5 8 3 6 9 RF Transmission Path Interleaved Data Samples 1 4 7 2 5 8 3 6 9 Errors Clustered De- Interleaving Matrix 1 2 3 4 5 6 7 8 9 De-Interleaved Data Samples 1 2 3 4 5 6 7 8 9 Receiver Errors Distributed Solution: use a block-interleaving technique as shown: source
  35. 35. Scrambling in LTE Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel OFDM(IFFT) OFDM(FFT) Tx Rx
  36. 36. Interference in LTE PROBLEM STATEMENT In LTE, a frequency reuse of 1 will typically be used. This means that all cells use the same frequency band(s). For UEs close to the cell border, this will lead to massive interference in both UL and DL. Solutions 1. In order to reduce this inter cell interference, a cell specific bit- level scrambling is applied for all transmissions in both UL and DL. 2. Inter Cell Interference Co-ordination (ICIC). ICIC co-ordinates the radio resource allocations (scheduling) between neighboring cells that experience problems
  37. 37. Scrambling in LTE  Cell specific bit-level scrambling used in LTE for all datastreams in UL and DL – used in order to achieve interference randomization between cells  No frequency planning (freq reuse 1) – massive inter-cell interference mitigated by scrambling and interference co-ordination techniques (e.g. ICIC)  Common scrambling used for cells in broadcast/multicast service transmissions (MBMS)
  38. 38. Modulation Coding Scrambling Modulation CRC Decoding Descrambling Demodulation CRC check Radio Channel OFDM(IFFT) OFDM(FFT) Tx Rx
  39. 39. Modulation Next step after channel coding and scrambling is modulation. Modulation: A process that maps blocks of scrambled bits (bit rate) onto symbols (symbol rate or baud rate)  Over the air interface we apply digital modulation techniques; a digital signal modulates an analog carrier  different symbols correspond to a specific amplitude and/or phase shift of the carrier wave. Three different modulation schemes are supported in E-UTRAN; • QPSK (Quadrature Phase shift keying) • 16-QAM (16 Quadrature Amplitude Modulation) • 64-QAM (64 Quadrature Amplitude Modulation) QPSK is a pure phase modulation  it has constant amplitude, 16-QAM and 64-QAM both uses a combination of phase and amplitude.
  40. 40. Modulation  The sub-carriers are modulated with a certain modulation scheme – maps the data bits into a carrier phase and amplitude (symbols)  E-UTRAN user data channels supports QPSK, 16QAM and 64QAM  16QAM allows for twice the peak data rate compared to QPSK  64QAM allows for three times the data rate compared to QPSK  Higher order modulation more sensitive to interference – Useful mainly in good radio channel conditions (high C/I, Little or no dispersion, Low speed) e.g. Close to cell site & Micro/Indoor cells  BPSK is used for some signaling (PHICH) 2 bits/symbol 4 bits/symbol 6 bits/symbol 64-QAM16-QAMQPSK 1 bit/symbol BPSK 0 1 00 11 10 01 1111 111111
  41. 41. OFDM Principle  Parallel transmission using a large number of narrowband “sub-carriers”  “Multi-carrier” transmission  Implemented with IFFT (Inverse Fast Fourier Transform) at transmitter and FFT at receiver side  Uplink uses similar approach, but with precoder to achieve single carrier properties f = 15 kHz 20 MHz (example) S/P f1 f2 fM  IFFT Coded and modulated data split f1 f2 fM filter FFT Tx Rx P/S source
  42. 42. Background  OFDM – Orthogonal Frequency Division Multiplex – a modulation scheme, not a multiple-access scheme  Basic principle known since the 50’s – Kineplex system by Collins, …  Popular/feasible with the ‘discovery’ of FFT in 1965 and its efficient implementation in HW – Bell Labs, 1971, implementation through FFT – Used by LTE, DAB, DVB, WiMax, xDSL, …
  43. 43. Orthogonal Frequency Division Multiplexing Principles Benefits + Frequency diversity + Robust against ISI + Easy to implement + Flexible BW + Suitable for MIMO + Classic technology (WLAN, ADSL etc) Drawbacks - Sensitive to Doppler and freq. errors - High PAPR (not suitable for uplink) - Overhead • Orthogonal: all other subcarriers zero at sampling point • Delay spread << Symbol time < Coherence time • Tu – symbol time per subcarrier -> Subcarrier spacing f = 1/Tu f source See next slides
  44. 44. Orthogonal Frequency Division Multiplexing Principles Coherence time - Physics coherence time in electromagneti wave theory is the time over which a propagating wave may be considered coherent. it is the time interval within which its phase is, on average, predictable. Coherence time is related to the classical uncertainty principle and it relates the bandwidth (spread in frequency) of signal or wave to its temporal extent df = 1/dt Thus phenomena with long coherence times will be sharply peaking with respect to their spectrum, i.e., they will be composed of less frequencies. The limit of this is an infinite coherence time, which would mean the signal is composed of a singular frequency
  45. 45. Orthogonal Frequency Division Multiplexing Principles Coherence time - Physics Waves of different frequencies interfere to form a pulse if they are coherent Spectrally incoherent waves interferes to form continuous wave with a randomly varying phase and amplitude
  46. 46. Orthogonal Frequency Division Multiplexing Principles Coherence time - Telecom In communications systems, a communication channel may change with time  Coherence time is the time duration over which the channel impulse response is considered to be not varying. Important: Such channel variation is much more significant in wireless communications systems, due to doppler effects.
  47. 47. Orthogonal Frequency Division Multiplexing Principles 1 11( ) ( ) ( )t ty t x t t h t   1 ( )th t 2 22( ) ( ) ( )t ty t x t t h t   Coherence time - EXAMPLE simple model a signal x(t) transmitted at time t1 will be received as: Where is the channel impulse response (CIR) at time t1 A signal transmitted at time t2 will be received as Now, if is relatively small, the channel may be considered constant within the interval t1 to t2. Coherence time (Tc) will therefore be given by And from Clark’s model 2 1 ( ) ( )t th t h t 2 1cT t t  0,423 c d T f 
  48. 48. OFDM multicarrier transmission  Single carrier transmission – each user transmits and receives data stream with only one carrier at any time  Multicarrier transmission – a user can employ a number of carriers to transmit data simultaneously – FFT to replace the banks of sinusoidal generators IFFT 1cos(2 )f t 2cos(2 )f t cos(2 )Nf t ( )s t  ( )s t S/P bk      N k tfj k k ebtx 1 2       N k ftkj kebtx 1 2 x(t) x(t) x1(t) x2(t) xN(t) tfj e 12 tfj e 22 tfj N e 2 source
  49. 49. Fourier transform )]([)()( 2 txFdtetxfX ftj        fxFdfefXtx ftj 12 )()(           f f f ee fj dtetdtettF fjfj ftjftj sinc )sin( 2 1 )()()]([ 2 1 2 1 22                  Periodic and non-periodic signals, continuous spectrum  Example for rectangular pulse shape: source
  50. 50. OFDM Transmission freq = freq * freq t
  51. 51. OFDM Transmission freq freqtime freq time
  52. 52. -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 OFDM Transmission 1subcarrier f 2subcarrier f 3subcarrier f 4subcarrier f -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1.5 -1 -0.5 0 0.5 1 1.5 2 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 -2 -1 0 1 2 3 4 -10 -5 0 5 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 Time domain Frequency domain
  53. 53. Discrete Fourier Transform (DFT)  N-point DFT:  N-point IDFT:  Discrete sequence of sampled signal, discrete spectrum  Four-point DFT: multiplying with {1, -1, j, -j}     21 0 nkN j N n X k x n e          21 0 nkN j N k x n X k e     0x 1x 2x 3x 0X 1X 2X 3X j j 1 j 1 1 j 1 0X 1X 2X 3X 0x 1x 2x 3x  Fast Fourier Transform: FFT  Derived to “radix-4 algorithm”  N-point DFT – N2 multiplications or phase rotation – N2 complex additions  FFT multiplications additions  2 3 log 2 8 N N       2logN N source
  54. 54. Orthogonality Time domain Frequency domain Example of four subcarriers within one OFDM symbol Spectra of individual subcarriers * 1 2( ) ( ) 0x t x t dt    * 1 2( ) ( ) 0X f X f df    source
  55. 55. Orthogonal Frequency Division Multiplexing  Receiver integrates for symbol integral  Orthogonality criteria:           fTj T tffj T tfjtfj T e fT fT dte dteedttxtx k             2 0 2 0 *22 0 * 2112 sin21 1          1 0 1 0 2 N k k N k tfj k txebtx k if , n is a non-zero integer, i.e. , thenfT n  n f T   12 0  source
  56. 56. OFDM and Multicarrier Transmission (A) (E) (D) (C) (B) 1cf f 2cf f 3cf f 4cf f 5cf f 1 T 1 T Orthogonal Non-orthogonal Orthogonal, n=3 Orthogonal, n=2 Orthogonal, n=1 (OFDM) source
  57. 57. Guard time and Cyclic Prefix Direct path: Reflected path: Integration interval  We rely on that the subcarriers are orthogonal  Short subcarrier spacing (f=15 kHz)  long symbol duration  Intersymbol interference is eliminated almost completely by introducing a guard interval with zero padding in every OFDM symbol. Direct path: Reflected path: Integration interval
  58. 58. FFT-based OFDM System Serial-to- Parallel Converter Signal Mapper IFFT Parallel- to-Serial Converter Guard Interval Insertion Serial Data Input x bits 0d 1d 1nd 0s 1s 1ns D/A & Low pass Filter Up- Converter Down- Converter A/D Guard Interval Removal Serial-to- Parallel Converter FFT One-tap Equalizer Signal Demapper Parallel- to-Serial Converter Serial Data Output 0 ˆdx bits 1 ˆd 1 ˆ nd 0 ˆs 1 ˆs 1 ˆ ns Channel )(ts Time Frequency Subchannels Fast Fourier Transform Guard Intervals Symbols source
  59. 59. Links and references www.3gpp.org  Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2, 36.300  Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, 36.211  Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding , 36.212  Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures, 36.213  Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer; Measurements , 36.214  LTE Physical Layer – General Description, 36.201 A good book:  3G Evolution – HSPA and LTE for Mobile Broadband, Academic Press 2007 Erik Dahlman; Stefan Parkvall; Johan Sköld; Per Beming
  60. 60. 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 overall description 36.302 – Services provided by the physical layer 36.304 – UE Functions related to idle mode 36.306 – UE radio access capabilities 36.321 – Medium Access Control (MAC) Protocol Specification 36.322 – Radio Link Control (RLC) Protocol Specification 36.323 – Packet Data convergence Protocol (PDCP) Protocol Specification 36.331 – Radio Resource Control (RRC) Protocol Specification 36.101 – UE radio transmission and reception (FDD) 36.104 – BTS radio transmission and reception (FDD) 36.113 – Base station EMC 36.133 – Requirements for support of Radio Resource Management (FDD) 36.141 – Base station conformance testing (FDD) 36.401 – E-UTRA Architecture Description 36.410 – S1 interface general aspects & principle 36.411 – S1 interface Layer 1 36.412 – S1 interface signalling transport 36.413 – S1 application protocol S1AP 36.414 – S1 interface data transport 36.420 – X2 interface general aspects and principles 36.421 – X2 interface layer1 36.422 – X2 interface signalling transport 36.423 – X2 interface application part X2AP 36.442 – UTRAN Implementation Specific O&M Transport All specifications can be found on the web site www.3gpp.org LTE Specifications 23.002 – Network Architecture 23.003 – Numbering, addressing and identification 23.009 – Handover Procedures 23.048 – Security mechanisms for USIM application 23.401 – GPRS enhancements for eUTRA 23.907 – QoS Concept 24.301 – NAS Protocol for Evolved Packet System (EPS) 24.302 – Access to the EPC via non 3GPP networks 33.401 – System Architecture Evolution (SAE); Security Architecture
  61. 61. Links and references
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