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Understanding of OFDM

Understanding of OFDM. Prof. Hyuckjae Lee Dept. of Electrical Engr. KAIST. OFDM ?. Need for Wideband Multimedia Service Limited Spectrum Bandwidth Not Friendly Transmission Environment Multipath fading effect Frequency selective channel Doppler effect

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Understanding of OFDM

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  1. Understanding of OFDM Prof. Hyuckjae Lee Dept. of Electrical Engr. KAIST

  2. OFDM ? • Need for Wideband Multimedia Service • Limited Spectrum Bandwidth • Not Friendly Transmission Environment • Multipath fading effect • Frequency selective channel • Doppler effect • Single Carrier vs. Multi-Carrier Modulation • CDMA system for single carrier modulation • OFDM system for multi-carrier modulation

  3. Multipath Fading (1)

  4. Multipath Fading (2) : Intersymbol interference  freq. selective fading : No ISI  flat fading

  5. Multicarrier Modulation (2) • Bandpass filter and oscillator bank • Scheme 1: high complexity due to the filters with sharp transition • Scheme 2: orthogonally overlapped spectrum for high bandwidth efficiency and lower filter complexity ( more than two times effective)

  6. Fourier series can represent any periodic time domain signal by a summation of harmonically related sinewaves.For example, the square wave (equivalent 1, 0, 1, 0, 1, 0, ...) data signal shown here can be constructed from sinewaves of descending amplitudes, spaced, in this example, at odd multiples of the fundamental frequency of the square wave.If we wished to represent the 1, 0, 1, 0, 1, 0, ... pattern perfectly, an infinite number of sinusoidal components would be required, implying that we need an infinite channel bandwidth! Review of Fourier Series

  7. Orthogonality • The trigonometric functions are orthogonal 1, cos x, sin x, cos 2x, sin 2x,…., cos nx, sin nx on the interval as  Orthogonal on the multiple intervals of any harmonic periods. If the intervals are not the multiple of any harmonic periods, they are not orthogonal.

  8. Orthogonal Frequency Division Multiplexing (1) • All subchannels are orthogonally overlapped without bandlimiting filter  efficient implementation by use of IFFT / FFT • Transmission signal is rectangular windowed in time-domain  each subchannel’s spectrum is sinc function

  9. OFDM (2) - OFDM increases the spectral efficiency by allowing subchannels to overlap

  10. OFDM (3)

  11. Cyclic Prefix zero-valued guard interval cyclic prefix no ISI but ICI no ISI and ICI

  12. Applications • Military Application (1950’s – 1960’s) • KINEPLEX and Kathryn • Broadcasting • Digital Audio Broadcasting(DAB) , Digital Terrestrial TV Broadcasting(DVB) • Wired Communications (DMT) • Asymmetric Digital Subscriber Line(ADSL) • Very-High-Bit-Rate Digital Subscriber Line(VDSL) • Power-Line Communication • Wireless ATM and Wireless LAN • Magic WAND, IEEE 802.11a & g , HIPERLAN/2 • Flash-OFDM for Wireless Internet Service • WiBro and WiMax Services • 4G LTE-Advanced

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