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Orthogonal Frequency Division Multiplexing - OFDM

Orthogonal Frequency Division Multiplexing - OFDM. Power response [dB]. 20. 15. 10. 5. 0. -5. -10. Frequency. Frequency-Selective Radio Channel. Time. 1 Channel (serial). Channels are transmitted at different frequencies (sub-carriers). 2 Channels. 8 Channels.

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Orthogonal Frequency Division Multiplexing - OFDM

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  1. Orthogonal Frequency Division Multiplexing - OFDM

  2. Power response [dB] 20 15 10 5 0 -5 -10 Frequency Frequency-Selective Radio Channel Multipath Radio Channel

  3. Time 1 Channel (serial) Channels are transmitted at different frequencies (sub-carriers) 2 Channels 8 Channels In practice: 50 … 8000 Channels (sub-carriers) Concept of parallel transmission Channel impulse response

  4. Signal is “broadband” 2 Channels Frequency 8 Channels Frequency Channels are “narrowband” Concept of parallel transmission Channel transfer function Channel impulse response Frequency Time 1 Channel (serial) Frequency

  5. Impact of fading High bit rate S(f) H(f) f r(t) s(t) f h(t) t t S(f) Low bit rate f t r(t) s(t) t t

  6. Frequency Selective fading flat fading s(t) H(f) t r(t) f p1(t) h(t) t p2(t) t p3(t) p3(t) t p4(t) p4(t) t

  7. Concept of an OFDM signal Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Conventional multicarrier techniques frequency Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Ch.3 Ch.5 Ch.7 Ch.9 Saving of bandwidth 50% bandwidth saving Orthogonal multicarrier techniques frequency

  8. FDM  OFDM • Frequency Division Multiplexing • OFDM GAIN IN SPECTRAL EFFICIENCY

  9. OFDM = Orthogonal FDM Carrier centers are put on orthogonal frequencies ORTHOGONALITY - The peak of each signal coincides with zero crossing of other signals Subcarriers are spaced by 1/ Ts OFDM DEFINITION

  10. Orthogonal Subcarriers

  11. S-P Cos 2f1t Input bits s(t) + Cos 2f8t A simplified view Rep P-S Cos 2f1t r(t) Output bits Cos 2f8t

  12. Alternate View

  13. OFDM Theory

  14. Discrete Time Equivalent • Inverse Discrete Fourier Transform  s(n) =  dk exp( j 2 k n/N) • N-point IDFT  N2 complex multiplications • Inverse Fast Fourier Transform • Radix 2 N-point IFFT  (N/2). log2 N • Radix 4 N-point IFFT  (3/8). N. ( log 2 N - 2) N -1 k=0

  15. Complexity of OFDM versus Single Carrier • Key difference between OFDM and single carrier transmission is FFT versus equalizer • Complexity of 64 point radix-4 FFT in IEEE 802.11a OFDM=96 million multiplications per second • 16 taps OQPSK or GMSK Equalizer for same data rates above needs 768 million multiplications per second • OFDM order of magnitude less complex

  16. Cyclic Prefix

  17. Generation of ICI

  18. Cyclic Extension

  19. Cyclic Prefix

  20. Cyclic Extension

  21. OFDM ADVANTAGES • OFDM is spectrally efficient • IFFT/FFT operation ensures that sub-carriers do not interfere with each other. • OFDM has an inherent robustness against narrowband interference. • Narrowband interference will affect at most a couple of subchannels. • Information from the affected subchannels can be erased and recovered via the forward error correction (FEC) codes. • Equalization is very simple compared to Single-Carrier systems

  22. OFDM ADVANTAGES • OFDM has excellent robustness in multi-path environments. • Cyclic prefix preserves orthogonality between sub- carriers. • Cyclic prefix allows the receiver to capture multi- path energy more efficiently. • Ability to comply with world-wide regulations: • Bands and tones can be dynamically turned on/off to comply with changing regulations. • Coexistence with current and future systems: • Bands and tones can be dynamically turned on/off for enhanced coexistence with the other devices.

  23. OFDM DRAWBACKS • High sensitivity inter-channel interference, ICI • OFDM is sensitive to frequency, clock and phase offset • The OFDM time-domain signal has a relatively large peak-to-average power ratio • tends to reduce the power efficiency of the RF amplifier • non-linear amplification destroys the orthogonality of the OFDM signal and introduces out-of-band radiation

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