• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
(Ofdm)
 

(Ofdm)

on

  • 5,743 views

OFDM

OFDM

Statistics

Views

Total Views
5,743
Views on SlideShare
5,713
Embed Views
30

Actions

Likes
0
Downloads
551
Comments
0

2 Embeds 30

http://www.slideshare.net 15
http://www.nexgwireless.com 15

Accessibility

Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    (Ofdm) (Ofdm) Presentation Transcript

    • 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)
    • 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)
    • 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)
    • 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)
    • 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)
    • 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)
    • 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
    • Symbol Waveform 06/07/09 System Arch 2007 (Fire Tom Wada) 1 0 1 0 0 Digital Information carrier ASK PSK FSK Symbol length
    • 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
    • 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
    • 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
    • Quadrature Amplitude Modulation (QAM) 06/07/09 System Arch 2007 (Fire Tom Wada) I Q I Q 16QAM 64QAM
    • 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
    • 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
    • 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
    • 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
    • Spectrum comparison for same data rate transmission 06/07/09 System Arch 2007 (Fire Tom Wada) frequency Single carrier frequency OFDM frequency Multi carrier
    • 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
    • OFDM carriers
      • OFDM carrier frequency is n ・ 1/T
      06/07/09 System Arch 2007 (Fire Tom Wada) Symbol period T
    • Sinusoidal Orthogonality
      • m,n: integer, T=1/f 0
      06/07/09 System Arch 2007 (Fire Tom Wada) Orthogonal Orthogonal Orthogonal
    • 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
    • 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)
    • 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)
    • 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)
    • 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
    • OFDM power spectrum
      • Total Power spectrum is almost square shape
      06/07/09 System Arch 2007 (Fire Tom Wada)
    • 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)
    • 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 )
    • 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
    • OFDM demodulation 06/07/09 System Arch 2007 (Fire Tom Wada) d n = FFT(u(k))
    • 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
    • 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
    • Multi-path
      • Delayed wave causes interference
      06/07/09 System Arch 2007 (Fire Tom Wada)
    • 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
    • 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
    • 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)
    • 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
    • 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
    • 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)