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Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling

Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling. Klaus Witrisal Signal Processing and Speech Communication Lab Technical University Graz, Austria VL: Mobile Radio Systems, Ch. 5: “Wideband Systems” 24-Nov-05. Outline. Introduction What is OFDM?

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Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling

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  1. Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling Klaus Witrisal Signal Processing and Speech Communication Lab Technical University Graz, Austria VL: Mobile Radio Systems, Ch. 5: “Wideband Systems” 24-Nov-05

  2. Outline • Introduction • What is OFDM? • Multipath fading radio-channel • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM in Practice • Summary

  3. PSD PSD f f * -fc fc Radio- channel e.g. Audio 0110 01101101 Receiver: Source decoding Decoding / deinter-leaving OFDM de-modulation Down-converter, I/Q-demod. Info Sink I/Q RF What is OFDM? • Modulation technique • Requires channel coding • Solves multipath problems Transmitter: I/Q RF OFDM modulation Source coding Channel coding / interleaving I/Q-mod., up- converter Info Source

  4. Reflections from walls, etc. Time dispersive channel Impulse response: Problem with high rate data transmission: inter-symbol-interference t p ( ) (PDP) t [ns] Multipath Propagation Multipath Radio Channel

  5. Transmitted signal: Received Signals: Line-of-sight: Reflected: The symbols add up on the channel  Distortion! Inter-Symbol-Interference Delays Multipath Radio Channel

  6. Outline • Introduction • What is OFDM? • Multipath fading radio-channel • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM in Practice • Summary

  7. 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 (1) Channel impulse response OFDM Technology

  8. Power response [dB] 20 15 10 5 0 -5 -10 Frequency The Frequency-Selective Radio Channel • Interference of reflected (and LOS) radio waves • Frequency-dependent fading Multipath Radio Channel

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

  10. 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 Implementation and System Model

  11. Outline • Introduction • What is OFDM? • Multipath fading radio-channel • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM in Practice • Summary

  12. Symbol (QPSK) of sub-carrier i at time k Other symbol-alphabets can be used as well (BPSK, m-QAM) Baseband signal is generated by DSP xi,k Im Re Window function Sub-carrier Generating the OFDM signal (1)

  13. Rectangular Window of duration T0 Has a sinc-spectrum with zeros at 1/ T0 Other carriers are put in these zeros  sub-carriers are orthogonal T0 N sub-carriers: resembles IDFT! Spectrum of the modulated data symbols Magnitude Frequency

  14. xi,k Im Re Generating the OFDM signal (2) x0,k s0,k serial-to-parallel x1,k IDFT (IFFT) s1,k parallel-to-serial xn sn xN,k sN,k N data symbols: (in frequency- domain) Base-band signal (time-domain)

  15. Idea of Guard Interval (GI) Insertion of guard interval (cyclic prefix): 1 OFDM symbol FFT-part time Channel impulse response (shorter than GI): t Cyclicconvolution of transmitted signal with channel impulse response  multiplication in frequency-domain Introduction

  16. Guard interval (2) - Cyclic extension

  17. OFDM Symbol Configuration (1) System Proposal

  18. OFDM System Model • Multiplication of data symbols with (complex-valued) channel transfer-function: Introduction

  19. I/Q I/Q Channel coding / interleaving Symbol mapping (modulation) OFDM modulation (IFFT) Guard interval 0110 010101001 1 OFDM symbol N symbols Receiver FFT-part Decoding / deinter-leaving symbol de-mapping (detection) OFDM demod. (FFT) Guard interval removal time I/Q I/Q Channel impulse response: Channel est. Time sync. OFDM Block Diagram Transmitter Introduction

  20. Outline • Introduction • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM System Design • Parameter selection • Implementation Issues • Summary and Applications

  21. Data rate; modulation order Guard interval length FFT symbol length Channel impulse response x(4 … 10) Nr. of carriers Channel Parameters are needed • Other constraints: • Nr. of carriers should match FFT size • and data packet length • considering coding and modulation schemes Design of an OFDM System Introduction

  22. OFDM Symbol Configuration (1) OFDM System Design

  23. Spectral Shaping by Windowing OFDM System Design

  24. OFDM Symbol Configuration (2) • Not all FFT-points can be used for data carriers • Lowpass filters for AD- and DA-conversion • oversampling required • DC offsets; carrier feedtrough; etc. Design of an OFDM System

  25. Outline • Introduction • What is OFDM? • Multipath fading radio-channel • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM in Practice • Summary

  26. Advantages of OFDM • Solves the multipath-propagation problem • Simple equalization at receiver • Computationally efficient • For broadband systems more efficient than SC • Supports several multiple access schemes • TDMA, FDMA, MC-CDMA, etc. • Supports various modulation schemes • Adaptability to SNR of sub-carriers is possible • Elegant framework for MIMO-systems • All interference among symbols is removed

  27. time domain signal (baseband) 0.2 0.1 0 -0.1 imaginary real -0.2 0 20 40 60 80 100 120 140 160 180 200 sample nr. Problems of OFDM (Research Topics) • Synchronization issues: • Time synchronization • Find start of symbols • Frequency synchr. • Find sub-carrier positions • Non-constant power envelope • Linear amplifiers needed • Channel estimation: • To retrieve data • Channel is time-variant OFDM Technology

  28. ... conj. … (M times) conj. ...  Correlation-based Frequency-sync. • Correlation of duplicated parts of OFDM signal • e.g.: Cyclic prefix (Guard interval - GI): Guard interval (M samples) FFT-part (L samples) si: • Peak at optimum position • Phase  frequency-offset • Received signal with f-offset: • Constant phase offset between samples spaced by L Introduction

  29. Outline • Introduction • What is OFDM? • Multipath fading radio-channel • Principle of OFDM • OFDM Implementation and System Model • Advantages and Disadvantages • OFDM in Practice • Summary

  30. Applications of OFDM • Wireless LAN • IEEE802.11a/g • HYPERLAN • DAB, DVB, etc. • Digital Audio/Video Broadcasting • xDSL (Digital Subscriber Line) • uses Discrete Multitone (DMT) Summary and Applications

  31. Summary – Essential “Ingredients” • IFFT & FFT • For efficient implementation • Guard interval insertion • Obtaining simple equalization • Removing all IS- & IC-interferences • Error correction coding • To restore bits that are lost on weak sub-carriers

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