SYSTEM ARCHITECTURE ADVANCED SYSTEM ARCHITECTURE LUO Chapter18.1 and 18.2 Introduction to OFDM 2013/Fall-Winter Term Monday 12:50 Room# 1-322 or 5F Meeting Room Instructor: Fire Tom Wada, Professor 12/9/2013 1
What is OFDM? OFDM =Orthogonal Frequency Division Multiplexing Many orthogonal sub-carriers are multiplexed in one symbol What is the orthogonal? How multiplexed? What is the merit of OFDM? What kinds of application? 12/9/2013 2
Outline Background, history, application Review of digital modulation FDMA vs. Multi-carrier modulation Theory of OFDM Multi-path Real OFDM systems examples 12/9/2013 3
Why OFDM is getting popular? State-of-the-art high bandwidth digital communication start using OFDM Terrestrial Video Broadcasting in Japan and Europe ISDB-T / DVB-T ADSL High Speed Modem WLAN such as IEEE 802.11a/g/n/ac WiMAX as IEEE 802.16d/e Economical OFDM implementation become possible because of advancement in the LSI technology 12/9/2013 4
Brief history of OFDM First proposal in 1950 s Theory completed in 1960 s DFT implementation proposed in 1970 s Europe adopted OFDM for digital radio broadcasting in 1987 OFDM for Terrestrial Video broadcasting in Europe and Japan ADSL, WLAN(802.11a) 12/9/2013 5
Digital modulation basics Digital modulation modulates three parameters of sinusoidal signal. A, θ k fc, s( t) A cos( 2 f t ) Three type digital modulation: ASK : Amplitude Shift Keying PSK : Phase Shift Keying FSK : Frequency Shift Keying c k OFDM uses combination of ASK and PSK such as QAM, PSK 12/9/2013 6
Symbol Waveform Digital Information 1 0 1 0 0 carrier ASK PSK FSK Symbol length 12/9/2013 7
Multi bit modulation carrier BPSK 1 0 1 0 0 1bit per symbol 10 11 01 00 01 QPSK 2bit per symbol Symbol length 12/9/2013 8
Mathematical expression of digital modulation Transmission signal can be expressed as follows s( t) cos(2 f cos cos(2 f a k s( t) k cos, k Re[( a k b ) e j2 fc t s(t) can be expressed by complex base-band signal ( a jb ) k e j2 fc t k k c jb t ) k c k t) sin sin(2 f sin k Indicates carrier sinusoidal Digital modulation 12/9/2013 9 ] k c t) ( a jb ) e Digital modulation can be expressed by the complex number k k j2 fc t
Constellation map (a k + jb k ) is plotted on I(real)-Q(imaginary) plane data a k b k 00 π/4 01 3π /4 11 5π /4 1 1 2 2 1 1 2 2 1 1 2 2 QPSK Q I 10 7π /4 1 2 1 2 12/9/2013 10
Quadrature Amplitude Modulation (QAM) 16QAM Q 64QAM Q I I 12/9/2013 11
Summary of digital modulation Type of modulation: ASK,PSK,FSK,QAM OFDM uses ASK,PSK,QAM Digital modulation is mathematically characterized by the coefficient of complex base-band signal ( a jb ) k k Q Plot of the coefficients gives the constellation map I 12/9/2013 12
Frequency Division Multiple Access (FDMA) Old conventional method (Analog TV, Radio etc.) Use separate carrier frequency for individual transmission Occupied BW Channel separation f c1 f c2 f c3 f cn Carrier frequency Guard band Radio frequency 12/9/2013 13
Multi-carrier modulation Use multiple channel (carrier frequency) for one data transmission data DEMULTIPLEX cos( 2 f 1 t) cos( 2 f 2 t) cos( 2 f 1 t) cos( 2 f 2 t) LPF LPF MULTIPLEX data cos( 2 f Nt) cos( 2 f Nt) LPF 12/9/2013 14
Spectrum comparison for same data rate transmission Multi carrier frequency Single carrier frequency OFDM frequency 12/9/2013 15
OFDM vs. Multi carrier OFDM is multi carrier modulation OFDM sub-carrier spectrum is overlapping In FDMA, band-pass filter separates each transmission In OFDM, each sub-carrier is separated by DFT because carriers are orthogonal Condition of the orthogonality will be explained later Each sub-carrier is modulated by PSK, QAM Thousands of PSK/QAM symbol can be simultaneously transmitted in one OFDM symbol 12/9/2013 16
Single Carrier vs. Multi Carrier 12/9/2013 Fire Tom Wada, Univ. of the Ryukyus 17
OFDM carriers OFDM carrier frequency is n 1/T Symbol period T f 1 0 T cos( 2 1 f 0 t 1 ) cos( 2 2 f0 t 2) cos( 2 3 f 0 t 3 ) cos( 2 4 f0 t 4) cos( 2 5 f 0 t 5 ) cos( 2 6 f 0 t 6 ) 12/9/2013 18
Sinusoidal Orthogonality m,n: integer, T=1/f 0 0 0 0 T T T T cos( 2 mf 0t) cos( 2 nf 0t) dt 2 0 T sin( 2 mf 0t) sin( 2 nf 0t) dt 2 0 cos( 2 mf t) sin( 2 nf t) dt 0 0 0 ( m n) ( m n) ( m n) ( m n) Orthogonal Orthogonal Orthogonal 12/9/2013 19
A sub-carrier of f=nf 0 a cos( 2 nf t) b sin( 2 nf t) n 0 n 0 2 2 n n 0 n n a b cos( 2 nf t ), tan 1 b a n n Amplitude and Phase will be digitally modulated n cycles Time t=0 t=t 12/9/2013 20
Base-band OFDM signal N 1 sb ( t) an cos( 2 nf 0t) bn sin( 2 nf 0t) n 0 T n=0 n=1 n=2 n=3 n=4 n=5 n=6 s B (t) 12/9/2013 21
Pass-band OFDM signal S B (t) is upcoverted to pass-band signal S(t) f c frequency shift N 1 s( t) an cos 2 ( f c nf 0) t bn sin 2 ( f c nf 0) t n 0 12/9/2013 22
Actual OFDM spectrum f c +(k-1)f 0 f c +kf 0 f c +(k+1)f 0 12/9/2013 23
OFDM power spectrum Total Power spectrum is almost square shape 12/9/2013 24
OFDM signal generation N 1 Direct method needs N digital modulators N carrier frequency generator Not practical s( t) an cos 2 ( f c nf 0) t bn sin 2 ( f c nf 0) t n 0 In 1971, method using DFT is proposed to OFDM siganal generation 12/9/2013 25
OFDM signal generation in digital domain Define complex base-band signal u(t) as follows Perform N times sampling in period T u k Nf s ( t) Re u( t) B N 1 j2 nf0t u( t) d e, d a jb n 0 n n n n 1 k 2 j N j2 nf N 1 0 Nf0 dn e dn e n 0 nk n 0 N 1 2 j dn e N k 0 1 2 N 1 (,,,, ) n 0 0 u(k) = IFFT (d n ) = IFFT(a n + jb n ) 12/9/2013 26 nk N
OFDM modulator cos( 2 f Ct) Bit stream M A P S / P I-DFT P / S Real Imag sin( ) 2 f t C generated AIR 0~d N-1 BPF 12/9/2013 27
OFDM demodulator (Too simple) Channel T u n e r cos( 2 f Ct) π/2 LPF LPF A / D S / P DFT P / S Bit Stream 12/9/2013 28 D E M A P
Summary of OFDM signal Each symbol carries information Each symbol wave is sum of many sinusoidal Each sinusoidal wave can be PSK, QAM modulated Using IDFT and DFT, OFDM implementation became practical Time Symbol period T=1/f 0 12/9/2013 29
Multi-path Delayed wave causes interference Path 2 Direct Path Building Base Station Path 3 Mobile Recept ion 12/9/2013 30
Multi-pass effect No multi-path Symbol k-1 Symbol k Symbol k+1 T=1/f 0 Sampling Period Multi-path Direct Delayed Sampling Period Inter symbol interference (ISI) happens in Multi-path condition 12/9/2013 31
Guard Interval T g T g OFDM symbol(1/f 0 ) T g Copy signal By adding the Gurard Interval Period, ISI can be avoided T g OFDM symbol (1/f 0 ) Direct Delayed Sampling Period 12/9/2013 32
Multi-path By adding GI, orthogonality can be maintained However, multi-path causes Amplitude and Phase distortion for each sub-carrier The distortion has to be compensated by Equalizer 12/9/2013 33
18.2.5 Channel Estimation and Equalizer Reception signal s R (t) CTF Frequency domain Equalization 12/9/2013 Fire Tom Wada, Univ. of the Ryukyus 34
18.2.6 OFDM communication and broadcasting systems 12/9/2013 Fire Tom Wada, Univ. of the Ryukyus 35
OFDM system Measurement Signal Analyzer OFDM Communication System 2013/1/21 Fire Tom WADA 36
Power Signal Analyzer 1 2 3 4 5 6 7 8 9 2540 Mega Hz 2550 Mega Hz Frequency 2013/1/21 Fire Tom WADA 37
Measured OFDM Spectrum 2013/1/21 Fire Tom WADA 38
Video Demo 2013/1/21 Fire Tom WADA 39
That s all for introduction Feature of OFDM 1. High Frequency utilization by the square spectrum shape 2. Multi-path problem is solved by GI 3. Multiple services in one OFDM by sharing subcarriers (3 services in ISDB-T) 4. SFN 5. Implementation was complicated but NOW possible because of LSI technology progress 12/9/2013 40
REAL OFDM SYSTEMs 1. OFDM WLAN 2. OFDMA transceiver design (LTE) 12/9/2013 Fire Tom Wada, Univ. of the Ryukyus 41
EXAMPLE 1) WLAN Wireless LAN 802.11a/g 2013/1/21 Fire Tom WADA 42
OFDM WLAN SYSTEM SPEC 802.11a: 5GHz BAND 802.11g: 2.4GHz BAND 2013/1/21 Fire Tom WADA 43
WLAN frame format and receiver tasks 2013/1/21 Fire Tom WADA 44
WLAN Block Diagram 2013/1/21 Fire Tom WADA 45
EXAMPLE 2) LTE Long Term Evolution 2013/1/21 Fire Tom WADA 46
LTE is OFDMA transceiver 2013/1/21 Fire Tom WADA 47
LTE Frame Structure 10ms 1ms Frame Subframe0 Subframe1 Subframe2 Subframe3 Subframe4 Subframe5 Subframe6 Subframe7 Subframe8 Subframe9 Slot0 Slot1 Slot2 Slot3 Slot4 Slot5 Slot6 Slot7 Slot8 Slot9 Slot10 Slot11 Slot12 Slot13 Slot14 Slot15 Slot16 Slot17 Slot18 Slot19 0,5ms 500us 5.21us 4.69us 4.69us 4.69us 4.69us 4.69us 4.69us 66.6us 66.6us 66.6us 66.6us 66.6us 66.6us 66.6us Normal CP CP S0 CP S1 CP S2 CP S3 CP S4 CP S5 CP S6 Extended CP (15KHz) 500us 16.6us 66.6us 16.6us 66.6us 16.6us 66.6us 16.6us 66.6us 16.6us 66.6us 16.6us 66.6us S0 S1 S2 S3 S4 S5 CP CP CP CP CP CP 2013/1/21 Fire Tom WADA 48
OFDMA and SC-FDMA Down Link is OFDMA Up Link us SC-FDMA 2013/1/21 Fire Tom WADA 49
Base Station TX and RX 2013/1/21 Fire Tom WADA 50
User Terminal TX and RX 2013/1/21 Fire Tom WADA 51
Summary High performance Computer and Communication support current IT Society. Main stream Technology of High Bandwidth Wireless Communication is based on OFDM method. OFDM power spectrum is rectangle then high efficiency is realized. WLAN end LTE examples are briefly covered. 2013/1/21 Fire Tom WADA 52
Matlab lab on Dec/14 th 1:00PM Dec/14 th Saturday,1pm to 5pm is Matlab lab at computer center, please prepare your login ID Building 1 Building 2 GO TO Computer Center 2F 12/9/2013 53