ofdm

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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 : 

Outline Introduction 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? : 

What is OFDM? Modulation technique Requires channel coding Solves multipath problems OFDM modulation Source coding I/Q-mod., up- converter I/Q RF Channel coding / interleaving Transmitter: Info Source

Multipath Propagation : 

Multipath Radio Channel Multipath Propagation Reflections from walls, etc. Time dispersive channel Impulse response: Problem with high rate data transmission: inter-symbol-interference

Inter-Symbol-Interference : 

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

Outline : 

Outline Introduction 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) : 

OFDM Technology Concept of parallel transmission (1) Channel impulse response

The Frequency-Selective Radio Channel : 

Multipath Radio Channel The Frequency-Selective Radio Channel Interference of reflected (and LOS) radio waves Frequency-dependent fading

Concept of parallel transmission (2) : 

OFDM Technology Concept of parallel transmission (2) Channel impulse response 1 Channel (serial) Channel transfer function Frequency Frequency Time

Concept of an OFDM signal : 

Implementation and System Model 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 Saving of bandwidth Ch.3 Ch.5 Ch.7 Ch.9 Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Conventional multicarrier techniques Orthogonal multicarrier techniques 50% bandwidth saving frequency frequency

Outline : 

Outline Introduction 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) : 

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

Spectrum of the modulated data symbols : 

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 Frequency Magnitude

Generating the OFDM signal (2) : 

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

Idea of Guard Interval (GI) : 

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

Slide 16: 

Guard interval (2) - Cyclic extension

OFDM Symbol Configuration (1) : 

System Proposal OFDM Symbol Configuration (1)

OFDM System Model : 

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

OFDM Block Diagram : 

Introduction OFDM Block Diagram Transmitter

Outline : 

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 : 

Introduction Design of an OFDM System

OFDM Symbol Configuration (1) : 

OFDM System Design OFDM Symbol Configuration (1)

Spectral Shaping by Windowing : 

OFDM System Design Spectral Shaping by Windowing

OFDM Symbol Configuration (2) : 

Design of an OFDM System 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.

Outline : 

Outline Introduction 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 : 

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) : 

OFDM Technology 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

Correlation-based Frequency-sync. : 

Introduction 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

Outline : 

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

Applications of OFDM : 

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 – Essential “Ingredients” : 

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

OFDM Block Diagram : 

OFDM Technology OFDM Block Diagram Transmitter