Transcription of Introduction to OFDM
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1E225C Lecture 16 OFDM IntroductionEE225 CEE225 CIntroduction to OFDMlBasic idea Using a large number of parallel narrow-band sub-carriers instead of a single wide-band carrier to transport informationlAdvantages Very easy and efficient in dealing with multi-path Robust again narrow-band interferencelDisadvantages Sensitive to frequency offset and phase noise Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitterlAdopted for various standards DSL, , DAB, DVB2 Multipath can be described in two domains: time and frequencytimetimeSinusoidal signal as inputtimetimeSinusoidal signal as outputfFrequency responseTime domain: Impulse responseFrequency domain: Frequency responsetimeImpulse responsetimetimeModulationtechniques: monocarrier vs. multicarrierTo improve thespectral efficiency:To use orthogonal carriers (allowing overlapping)Eliminate band guards between carriers Selective Fading Very short pulses ISI iscompartively long EQs are then very long Poor spectral efficiencybecause of band guardsDrawbacks It is easy to exploitFrequency diversity Flat Fadingpercarrier N long pulses ISI iscomparatively short N short EQsneeded Poor spectral efficiencybecause of band guardsAdvantagesFurthermore It allows to deploy2D coding techniques Dynamic signallingNcarriersBPulse length ~N/BSimilar toFDM tech
6 Cyclic Prefix T g T τ max T x Multi-path components Sampling start T 802.11a System Specification l Sampling (chip) rate: 20MHz l Chip duration: 50ns l Number of FFT points: 64 l FFT symbol period: 3.2µs l Cyclic prefix period: 16 chips or 0.8µs » Typical maximum indoor delay spread < 400ns » OFDM frame length: 80 chips or 4µs
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