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OFDM

OFDM. Lecture 9. Basics of Radio Propagation. Exponential. Power. 0.1 -1 m (10-100 msecs). Short-term Fading. Long-term Fading. 10-100 m (1-10 secs). Distance. Gain (in volts). Fading. Delay Spread rms = 5 m secs. Time. 3.0 secs. 2.0 secs. 2.5 secs. Frequency Selective Fading.

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OFDM

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  1. OFDM Lecture 9

  2. Basics of Radio Propagation Exponential Power 0.1 -1 m (10-100 msecs) Short-term Fading Long-term Fading 10-100 m (1-10 secs) Distance

  3. Gain (in volts) Fading Delay Spread rms = 5msecs Time 3.0 secs 2.0 secs 2.5 secs Frequency Selective Fading Frequency Selective Fading Channels can provide -- time diversity (can be exploited in DS-CDMA) -- frequency diversity (can be exploited in OFDM)

  4. TDMA, CDMA, and OFDM Wireless Systems • Time Division Multiple Access (TDMA) is the most prevalent wireless access system to date • GSM, ANSI-136, EDGE, DECT, PHS, Tetra • Direct Sequence Code Division Multiple Access (DS-CDMA) became commercial only in the mid 90’s • IS-95 (A,B, HDR,1x,3x,...), cdma-2000 (3GPP2), W-CDMA (3GPP) • Orthogonal Frequency Division Multiplexing (OFDM) is perhaps the least well known • can be viewed as a spectrally efficient FDMA technique • IEEE 802.11A, .11G, HiperLAN, IEEE 802.16 OFDM/OFDMA options

  5. TDMA (with FDMA) Principle Carriers Power Freq. Time Time-slots

  6. Direct Sequence CDMA Principle (with FDMA) User Code Waveforms Power Freq. Time

  7. OFDM (with TDMA & FDMA) Principle Tones Carriers Power Freq. Time-slots Time

  8. What is an OFDM System ? • Data is transmitted in parallel on multiple carriers that overlap in frequency IIT Madras

  9. Generic OFDM Transmitter OFDM symbol bits Serial to Parallel Pulse shaper FEC LinearPA IFFT & DAC fc add cyclic extension view this as a time to frequency mapper Complexity (cost) is transferred back from the digital to the analog domain! IIT Madras

  10. Add Cyclic Prefix Parallel/ Serial Serial/ Parallel IFFT OFDM Transmitter -- contd. • S/P acts as Time/Frequency mapper • IFFT generates the required Time domain waveform • Cyclic Prefix acts like guard interval and makes equalization easy (FFT-cyclic convolution vs channel-linear convolution) IIT Madras

  11. Remove Cyclic Prefix Parallel/ Serial Serial/ Parallel FFT OFDM Receiver • Cyclic Prefix is discarded • FFT generates the required Frequency Domain signal • P/S acts like a Frequency/Time Mapper IIT Madras

  12. Generic OFDM Receiver Slot & Timing AGC Sync. Error P/S and Detection Sampler FFT Recovery fc gross offset VCO Freq. Offset Estimation fine offset (of all tones sent in one OFDM symbol) IIT Madras

  13. OFDM Basics • To maintain orthogonality where • = sub-carrier spacing • = symbol duration • If N-point IDFT (or FFT) is used • Total bandwidth (in Hz) = • = symbol duration after CP addition IIT Madras

  14. Time T Condition for Orthogonality Base frequency = 1/T T= symbol period IIT Madras

  15. Sync Basis Functions(of equal height for single-ray channel) Shape gets upset by (a) Fine Frequency Offset (b) Fading IIT Madras

  16. OFDM -- PHY layer tasks • Signals sent throh wireless channels encounter one or more of the following distortions: • additive white noise • frequency and phase offset • timing offset, slip • delay spread • fading (with or without LoS component) • co-channel interference • non-linear distortion, impulse noise, etc • OFDM is well suited for high-bit rate applications IIT Madras

  17. Effect of Delay Spread in OFDM • Delay spread easily compensated in OFDM using : • Cyclic Prefix (CP) which is longer than the delay spread • Thereby, converting linear convolution (with multipath channel) to effectively a circular convolution • enables simple one-tap equalisation at the tone level Example: IEEE 802.11 A (and also in HiperLAN) Data Payload CP 3.2msecs 0.8msecs However, the frequency selectiveness could lead to certain tones having very poor SNR=> poor gross error rate performance IIT Madras

  18. Comparing TDMA, CDMA, and OFDM

  19. Output (Rx signal) Input (Tx signal) channel DS-CDMA versus OFDM DS-CDMA can exploit time-diversity a0 Impulse Response h(t) a3 time Frequency Response H(f) OFDM can exploit freq. diversity freq. IIT Madras

  20. Comparing Complexity of TDMA, DS-CDMA, & OFDM Transceivers TDMA CDMA OFDM Difficult, and requires sync. channel (code) Very elegant, requiring no extra overhead Easy, but requires overhead (sync.) bits Timing Sync. Easy, but requires overhead (sync.) bits Gross Sync. Easy Fine Sync. is Difficult Freq. Sync. More difficult than TDMA Complexity is high in Asynchronous W-CDMA Usually not required within a burst/packet Timing Tracking Modest Complexity Freq. Tracking Easy, decision-directed techniques can be used Modest Complexity (using dedicated correlator) Requires CPE Tones (additional overhead) Channel Equalisation Modest to High Complexity (depending on bit-rate and extent of delay-spread) RAKE Combining in CDMA usually more complex than equalisation in TDMA Frequency Domain Equalisation is very easy Analog Front-end (AGC, PA, VCO, etc) Complexity or cost is very high Fairly Complex (power control loop) Very simple

  21. Comparing Performance of TDMA, DS-CDMA, & OFDM Transceivers TDMA CDMA OFDM Fade Margin (for mobile apps.) Modest requirement (RAKE gain vs power- control problems) Required for mobile applications Required for mobile applications Range increase by reducing allowed noise rise (capacity) Range Very easy to increase cell sizes Difficult to support large cells (PA , AGC limitations) Modest (in TDMA) and High in MC-TDMA Re-use planning is crucial here Re-use & Capacity Modest FEC Requirements FEC is usually inherent (to increase code decorrelation) FEC is vital even for fixed wireless access FEC optional for voice Variable Bit-rate Support Powerful methods to support VBR (for fixed access) Very elegant methods to support VBR Low to modest support Very High (& Higher Peak Bit-rates) Spectral Efficiency Poor to Low Modest

  22. Proprietary OFDM Flavours Wireless Access (Macro-cellular) Flash OFDM from Flarion www.flarion.com Vector OFDM (V-OFDM) of Cisco, Iospan,etc. www.iospan.com Wideband-OFDM (W-OFDM) of Wi-LAN www.wi-lan.com -- Freq. Hopping for CCI reduction, reuse -- 1.25 to 5.0MHz BW -- mobility support -- 2.4 GHz band -- 30-45Mbps in 40MHz -- large tone-width (for mobility) -- MIMO Technology -- non-LoS coverage, mainly for fixed access -- upto 20 Mbps. Wi-LAN leads the OFDM Forum -- many proposals submitted to IEEE 802.16 Wireless MAN Cisco leads the Broadand Wireless Internet Forum (BWIF)

  23. Wireless Advances Spatial Multiplexing Transmit Diversity Spectral Efficiency OFDM Turbo Coding Link Adaptation Sectorisation Space-Time Coding CCI Suppression Transmit Diversity Freq. Hopping Smart Antennas Receive Diversity Fixed Beamforming Power Control Range Multi-user Detection Re-use Efficiency

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