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

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

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

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

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

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

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

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

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

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

Generating the OFDM signal (1) 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 Window function Re Im xi,k Sub-carrier

Spectrum of the modulated data symbols 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 Magnitude Frequency N sub-carriers: resembles IDFT!

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 Re Im xi,k N data symbols: (in frequency- domain) Base-band signal (time-domain)

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

Guard interval (2) - Cyclic extension

OFDM Symbol Configuration (1) System Proposal

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

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

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

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

OFDM Symbol Configuration (1) OFDM System Design

Spectral Shaping by Windowing OFDM System Design

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

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

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

Problems of OFDM (Research Topics) 20 40 60 80 100 120 140 160 180 200 -0.2 -0.1 0.1 0.2 time domain signal (baseband) sample nr. imaginary real 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

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: conj.  ... … (M times) Peak at optimum position Phase  frequency-offset Received signal with f-offset: Constant phase offset between samples spaced by L Introduction

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

Summary and Applications 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

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