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OFDM

OFDM

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  • 1. Introduction to OFDM Fire Tom Wada Professor, Information Engineering, Univ. of the Ryukyus Chief Scientist at Magna Design Net, Inc [email_address] http://www.ie.u-ryukyu.ac.jp/~wada/ 06/07/09 System Arch 2007 (Fire Tom Wada)
  • 2. 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?
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 3. Outline
    • Background, history, application
    • Review of digital modulation
    • FDMA vs. Multi-carrier modulation
    • Theory of OFDM
    • Multi-path
    • Summary
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 4. Why OFDM is getting popular ?
    • State-of-the-art high bandwidth digital communication start using OFDM
      • Terrestrial Video Broadcasting in Japan and Europe
      • ADSL High Speed Modem
      • WLAN such as IEEE 802.11a/g/n
      • WiMAX as IEEE 802.16d/e
    • Economical OFDM implementation become possible because of advancement in the LSI technology
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 5. Japan Terrestrial Video Broadcasting service
    • 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
    • 5.6MHz BW is divided into 13 segments (~430KHz BW)
    • HDTV: 12 segments
    • Mobile TV : 1 segment
    • SDTV: 4 segment
    • Analog Service will end 2011
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 6. 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)
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 7. Digital modulation basics
    • Digital modulation modulates three parameters of sinusoidal signal.
    • A, θ k fc,
    • Three type digital modulation:
      • ASK : Amplitude Shift Keying
      • PSK : Phase Shift Keying
      • FSK : Frequency Shift Keying
    06/07/09 System Arch 2007 (Fire Tom Wada) OFDM uses combination of ASK and PSK such as QAM, PSK
  • 8. Symbol Waveform 06/07/09 System Arch 2007 (Fire Tom Wada) 1 0 1 0 0 Digital Information carrier ASK PSK FSK Symbol length
  • 9. Multi bit modulation 06/07/09 System Arch 2007 (Fire Tom Wada) 1 0 1 0 0 carrier BPSK 1bit per symbol QPSK 2bit per symbol 10 11 01 00 01 Symbol length
  • 10. Mathematical expression of digital modulation
    • Transmission signal can be expressed as follows
    • s(t) can be expressed by complex base-band signal          
    06/07/09 System Arch 2007 (Fire Tom Wada) Indicates carrier sinusoidal Digital modulation Digital modulation can be expressed by the complex number
  • 11. Constellation map
    • (a k + jb k ) is plotted on I(real)-Q(imaginary) plane
    06/07/09 System Arch 2007 (Fire Tom Wada) QPSK I Q data a k b k 00 π/4 01 3π /4 11 5π /4 10 7π /4
  • 12. Quadrature Amplitude Modulation (QAM) 06/07/09 System Arch 2007 (Fire Tom Wada) I Q I Q 16QAM 64QAM
  • 13. 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
    • Plot of the coefficients gives the constellation map
    06/07/09 System Arch 2007 (Fire Tom Wada) I Q
  • 14. Frequency Division Multiple Access (FDMA)
    • Old conventional method (Analog TV, Radio etc.)
    • Use separate carrier frequency for individual transmission
    06/07/09 System Arch 2007 (Fire Tom Wada) Radio frequency f c1 f c 2 f c 3 f c N Carrier frequency Occupied BW Channel separation Guard band
  • 15. Japan VHF channel assignment
    • Channel Separation = 6MHz
    06/07/09 System Arch 2007 (Fire Tom Wada) 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
  • 16. Multi-carrier modulation
    • Use multiple channel (carrier frequency) for one data transmission
    06/07/09 System Arch 2007 (Fire Tom Wada) data LPF LPF LPF data DEMULTIPLEX MULTIPLEX
  • 17. Spectrum comparison for same data rate transmission 06/07/09 System Arch 2007 (Fire Tom Wada) frequency Single carrier frequency OFDM frequency Multi carrier
  • 18. 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
    06/07/09 System Arch 2007 (Fire Tom Wada) Thousands of PSK/QAM symbol can be simultaneously transmitted in one OFDM symbol
  • 19. OFDM carriers
    • OFDM carrier frequency is n ・ 1/T
    06/07/09 System Arch 2007 (Fire Tom Wada) Symbol period T
  • 20. Sinusoidal Orthogonality
    • m,n: integer, T=1/f 0
    06/07/09 System Arch 2007 (Fire Tom Wada) Orthogonal Orthogonal Orthogonal
  • 21. A sub-carrier of f=nf 0
    • Amplitude and Phase will be digitally modulated
    06/07/09 System Arch 2007 (Fire Tom Wada) n cycles t=0 t=T Time
  • 22. Base-band OFDM signal 06/07/09 System Arch 2007 (Fire Tom Wada) T n=0 n=1 n=2 n=3 n=4 n=5 n=6 s B (t)
  • 23. How a n ,b n are calculated from s B (t) - Demodulation Procedure -
    • According to the sinusoidal orthogonality, a n ,b n can be extracted.
    • In actual implementation, DFT(FFT) is used
    • N is roughly 64 for WLAN, thoudand for Terrestrial Video Broadcasting
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 24. Pass-band OFDM signal
    • S B (t) is upcoverted to pass-band signal S(t)
    • f c frequency shift
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 25. Actual OFDM spectrum 06/07/09 System Arch 2007 (Fire Tom Wada) f c +kf 0 f c + ( k -1) f 0 f c + ( k +1) f 0
  • 26. OFDM power spectrum
    • Total Power spectrum is almost square shape
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 27. OFDM signal generation
    • Direct method needs
      • N digital modulators
      • N carrier frequency generator
      •  Not practical
    • In 1971, method using DFT is proposed to OFDM signal generation
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 28. OFDM signal generation in digital domain
    • Define complex base-band signal u(t) as follows
    • Perform N times sampling in period T
    06/07/09 System Arch 2007 (Fire Tom Wada) u(k) = IFFT (d n ) = IFFT(a n + jb n )
  • 29. OFDM modulator 06/07/09 System Arch 2007 (Fire Tom Wada) M A P S / P I-DFT P / S Real BPF generated 0 ~d N-1 AIR Bit stream Imag
  • 30. OFDM demodulation 06/07/09 System Arch 2007 (Fire Tom Wada) d n = FFT(u(k))
  • 31. OFDM demodulator (Too simple) 06/07/09 System Arch 2007 (Fire Tom Wada) T u n e r S / P DFT P / S A / D LPF Channel π/2 LPF D E M A P Bit Stream
  • 32. 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
    06/07/09 System Arch 2007 (Fire Tom Wada) Time Symbol period T=1/f 0
  • 33. Multi-path
    • Delayed wave causes interference
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 34. Multi-pass effect
    • Inter symbol interference (ISI) happens in Multi-path condition
    06/07/09 System Arch 2007 (Fire Tom Wada) T=1/f 0 Symbol k Symbol k-1 Symbol k+1 Sampling Period No multi-path Sampling Period Multi-path Direct Delayed
  • 35. Guard Interval T g
    • By adding the Gurard Interval Period, ISI can be avoided
    06/07/09 System Arch 2007 (Fire Tom Wada) OFDM symbol(1/f 0 ) Copy signal T g T g Direct Delayed OFDM symbol (1/f 0 ) T g Sampling Period
  • 36. 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
    06/07/09 System Arch 2007 (Fire Tom Wada)
  • 37. Multiple Frequency Network
    • Frequency utilization is low
    06/07/09 System Arch 2007 (Fire Tom Wada) Area 1 Area 2 Area 3 Area 4 f 1 f2 f3 f1
  • 38. Single Frequency Network
    • If multi-path problem is solved, SFN is possible
    06/07/09 System Arch 2007 (Fire Tom Wada) Area 1 Area 2 Area 3 Area 4 f 1 f 1 f 1 f1
  • 39. That ’ s all for introduction
    • Feature of OFDM
      • High Frequency utilization by the square spectrum shape
      • Multi-path problem is solved by GI
      • Multiple services in one OFDM by sharing sub-carriers (3 services in ISDB-T)
      • SFN
      • Implementation was complicated but NOW possible because of LSI technology progress
    06/07/09 System Arch 2007 (Fire Tom Wada)