Introduction to OFDM Fire Tom Wada Professor Information

Introduction to OFDM Fire Tom Wada Professor, Information Engineering, Univ. of the Ryukyus Chief Scientist at Magna Design Net, Inc wada@ie. u-ryukyu. ac. jp http: //www. ie. u-ryukyu. ac. jp/~wada/ 10/31/2011 1

What is OFDM? n n OFDM =Orthogonal Frequency Division Multiplexing Many orthogonal sub-carriers are multiplexed in one symbol n n What is the orthogonal? How multiplexed? What is the merit of OFDM? What kinds of application? 10/31/2011 2

Outline n n n Background, history, application Review of digital modulation FDMA vs. Multi-carrier modulation Theory of OFDM Multi-path Summary 10/31/2011 3

Why OFDM is getting popular？ n State-of-the-art high bandwidth digital communication start using OFDM n n n Terrestrial Video Broadcasting in Japan and Europe ADSL High Speed Modem WLAN such as IEEE 802. 11 a/g/n Wi. MAX as IEEE 802. 16 d/e Economical OFDM implementation become possible because of advancement in the LSI technology 10/31/2011 4

Japan Terrestrial Video Broadcasting service n n n n ISDB-T (Integrated Services Digital Broadcasting for Terrestrial Television Broadcasting) Service starts on 2003/December at three major cities (Tokyo, Nagoya, Osaka) Full service area coverage on 2006 5. 6 MHz BW is divided into 13 segments (~430 KHz BW) HDTV: 12 segments Mobile TV : 1 segment SDTV: 4 segment Analog Service will end 2011 10/31/2011 5

Brief history of OFDM n n n 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. 11 a) 10/31/2011 6

Digital modulation basics n Digital modulation modulates three parameters of sinusoidal signal. A, θk fc, n Three type digital modulation: n n ASK : Amplitude Shift Keying PSK : Phase Shift Keying FSK : Frequency Shift Keying OFDM uses combination of ASK and PSK such as QAM, PSK 10/31/2011 7

Symbol Waveform Digital Information 1 0 0 carrier ASK PSK FSK 10/31/2011 Symbol length 8

Multi bit modulation carrier 1 0 0 10 11 01 00 01 BPSK 1 bit per symbol QPSK 2 bit per symbol Symbol length 10/31/2011 9

Mathematical expression of digital modulation n Transmission signal can be expressed as follows n s(t) can be expressed by complex base-band signal　　　　　 Indicates carrier sinusoidal Digital modulation can be expressed by the complex number 10/31/2011 10

Constellation map n (ak + jbk) is plotted on I(real)-Q(imaginary) plane ak data 00 π/4 01 3π /4 11 5π /4 10 7π /4 10/31/2011 bk QPSK Q I 11

Quadrature Amplitude Modulation (QAM) 16 QAM 64 QAM Q Q I 10/31/2011 I 12

Summary of digital modulation n 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 Q n Plot of the coefficients gives the constellation map 10/31/2011 I 13

Frequency Division Multiple Access (FDMA) n n Old conventional method (Analog TV, Radio etc. ) Use separate carrier frequency for individual transmission Occupied BW fｃ１ Carrier frequency 10/31/2011 Channel separation f ｃ2 f ｃ3 Guard band f ｃN Radio frequency 14

Japan VHF channel assignment Channel number Frequency (MHz) 1 90 -96 2 96 -102 3 102 -108 4 170 -176 5 176 -182 6 182 -188 7 188 -194 8 192 -198 9 198 -204 10 204 -210 11 210 -216 12 216 -222 10/31/2011 n Channel Separation = 6 MHz 15

Multi-carrier modulation n Use multiple channel (carrier frequency) for one data transmission LPF MULTIPLEX DEMULTIPLEX data LPF 10/31/2011 16

Spectrum comparison for same data rate transmission Multi carrier Single carrier OFDM 10/31/2011 frequency 17

OFDM vs. Multi carrier n n 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 n n 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 10/31/2011 18

OFDM carriers n OFDM carrier frequency is n・ 1/T Symbol period T 10/31/2011 19

Sinusoidal Orthogonality n m, n: integer, T=1/f 0 Orthogonal 10/31/2011 20

A sub-carrier of f=nf 0 n Amplitude and Phase will be digitally modulated n cycles Time t=0 10/31/2011 t=T 21

Base-band OFDM signal Ｔ n=0 n=1 n=2 n=3 n=4 n=5 n=6 s. B(t) 10/31/2011 22

How an, bn are caluculated from s. B(t) - Demodulation Procedure - n n n According to the sinusoidal orthogonality, an, bn can be extracted. In actual implementation, DFT(FFT) is used N is roughly 64 for WLAN, thoudand for Terrestrial Video Broadcasting 10/31/2011 23

Pass-band OFDM signal n n SB(t) is upcoverted to pass-band signal S(t) fc frequency shift 10/31/2011 24

Actual OFDM spectrum fｃ＋(ｋ-1)ｆ０ 10/31/2011 fｃ＋ｋｆ０ fｃ＋(ｋ+1)ｆ０ 25

OFDM power spectrum n Total Power spectrum is almost square shape 10/31/2011 26

OFDM signal generation n Direct method needs N digital modulators N carrier frequency generator Not practical n In 1971, method using DFT is proposed to OFDM siganal generation 10/31/2011 27

OFDM signal generation in digital domain n n Define complex base-band signal u(t) as follows Perform N times sampling in period T u(k) = IFFT (dn) = IFFT(an + jbn) 10/31/2011 28

OFDM modulator Real M Bit A stream P S / P I-DFT P / S Imag generated 0~dＮ－１ 10/31/2011 AIR ＢＰＦ 29

OFDM demodulation dn = FFT(u(k)) 10/31/2011 30

OFDM demodulator (Too simple) Channel T u n e r LPF A / D π/2 S / P DFT LPF Bit Stream 10/31/2011 P / S D E M A P 31

Summary of OFDM signal n n 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/f０ 10/31/2011 32

Multi-path n Delayed wave causes interference 10/31/2011 33

Multi-pass effect No multi-path Symbol k-1 Symbol k T=1/f０ Symbol k+1 Sampling Period Multi-path Direct Delayed Sampling Period n Inter symbol interference (ISI) happens in Multi-path condition 10/31/2011 34

Guard Interval Tg Tg OFDM symbol(1/f 0) Tg Copy signal n By adding the Gurard Interval Period, ISI can be avoided Tg OFDM symbol (1/f 0) Direct Delayed 10/31/2011 Sampling Period 35

Multi-path n n n 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 10/31/2011 36

Multiple Frequency Network f 3 f１ f 1 Area 3 Area 4 Area 1 f 2 n Area 2 10/31/2011 Frequency utilization is low 37

Single Frequency Network f１ f１ f 1 Area 3 Area 4 Area 1 f１ Area 2 10/31/2011 n If multi-path problem is solved, SFN is possible 38

That’s all for introduction n Feature of OFDM 1. 2. 3. 4. 5. 10/31/2011 High Frequency utilization by the square spectrum shape Multi-path problem is solved by GI Multiple services in one OFDM by sharing subcarriers (3 services in ISDB-T) SFN Implementation was complicated but NOW possible because of LSI technology progress 39