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A Convenient Framework for Time-Frequency Processing in Wireless Communications

OFDM as a Multicarrier Modulation:. A Convenient Framework for Time-Frequency Processing in Wireless Communications. Jacek Ilow, Ph.D. Associate Professor. Department of Electrical and Computer Engineering Dalhousie University Halifax,Canada. J.ilow@dal.ca http://www.dal.ca/~jilow

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A Convenient Framework for Time-Frequency Processing in Wireless Communications

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  1. OFDM as a Multicarrier Modulation: A Convenient Framework for Time-Frequency Processing in Wireless Communications Jacek Ilow, Ph.D. Associate Professor Department of Electrical and Computer Engineering Dalhousie University Halifax,Canada J.ilow@dal.ca http://www.dal.ca/~jilow http://radio-1.ee.dal.ca/~ilow

  2. OFDM History • High-data-rate communications systems are limited not by noise, but often more significantly by the intersymbol interference (ISI) due to the memory of the dispersive communications channel. • If the symbol rate exceeds the duration of channel impulse response (CIR), mechanisms must be implemented in order to combat the effects of ISI. • Channel equalization techniques can be used to suppress the echoes caused by the channel. • Significant research efforts have been invested into the development of such channel equalizers • Another approach is to utilize an FDM system which employs a set of subcarriers in order to transmit information in parallel subchannels over the same channel. • The data throughput of each channel is only a fraction of the data rate of the single-carrier system having the same throughput.

  3. OFDM – Orthogonal Frequency Division Multiplexing 2001-05-31 Frequency Selective Channel Why OFDM? Single Carrier Multicarrier • Uses the entire bandwidth • Splits bandwidth into subchannels • Short symbol times • Sends information in parallel • This causes ISI • OFDM: orthogonal subcarriers OFDM is a considerable option when the channel introduces ISI Applications: ADSL, DAB, DVB, Hiperlan/2, ...

  4. FDM (Frequency Division Multiplexing) Vs. OFDM Power Frequency FDMA Bc Time Frequency cos(2pf0t) Bm Frequency channel BPF X Serial To Parallel (S/P) X BPF Time s(t) S cos(2pf1t) BPF X cos(2pfN-1t)

  5. OFDM History • In 1971, Weinstein suggested using a digital implementation based on the DFT. • The DFT is by its nature cyclically redundant in the frequency domain. • The associated harmonically related frequencies can be used as a set of subchannels carriers required by the OFDM system.

  6. Multipath can be described in two domains: time and frequency Time domain: Impulse response time time time Impulse response . Frequency domain: Frequency response time time time time f Sinusoidal signal as input Sinusoidal signal as output Frequency response

  7. Modulationtechniques: monocarrier vs. multicarrier N carriers Similar to FDM technique B Pulse length ~ N/B – Data are shared among several carriers and simultaneously transmitted Drawbacks Advantages Furthermore – Selective Fading – Flat Fading per carrier – It is easy to exploit – Very short pulses – N long pulses Frequency diversity – ISI is comparatively short – ISI is compartively long – It allows to deploy 2D coding techniques – N short EQs needed – EQs are then very long – Dynamic signalling – Poor spectral efficiency – Poor spectral efficiency because of band guards because of band guards To improve the spectral efficiency: Eliminate band guards between carriers To use orthogonal carriers (allowing overlapping) Channel Channelization Guard bands B Pulse length ~1/B – Data are transmited over only one carrier

  8. N carriers Symbol: 8 periods of f0 B Symbol: 4 periods of f0 B Orthogonal Frequency Division Modulation Symbol: 2 periods of f0 Transmit + f f Channel frequency response . Data coded in frequency domain Transformation to time domain: each frequency is a sine wave in time, all added up. Decode each frequency bin separately Receive time f Time-domain signal Frequency-domain signal

  9. Each subcarrier is modulated at a low enough rate that dispersion (ISI) is not a problem. Subcarriers must be spaced so that they do not interfere. S(f) f1 fN-1 f0 Bandwidth, B cos(2pf0t) P/S Detector x LPF Detector x LPF r(t) cos(2pf1t) Detector x LPF cos(2pfN-1t) Demodulator

  10. Orthogonal Frequency Division Multiplexing (OFDM) OFDM is a special case of multicarrier transmission that permits subchannels to overlap in frequency without mutual interference  increased spectral efficiency. OFDM exploits signal processing technology to obtain cost-effective means of implementation. Mulitple users can be supported by allocating each user a group of subcarriers. Bandwidth, ~ B/2

  11. Spectrum of OFDM Signal When N is large, the power spectral density (PSD) of the transmitted signal is PSD of OFDM Signal

  12. OFDM transmission system (time continuous) OFDM Basics

  13. Mathematical Description of an OFDM System 1/2 • time continuousrepresentation of anOFDM transmitter: • time discreterepresentation ofan OFDM transmitter: OFDM Basics

  14. Mathematical Description of an OFDM System 2/2 • time discreterepresentation ofan OFDM receiver: • Complete System: OFDM Basics

  15. Symbol Rate Model of an OFDM System OFDM Basics

  16. To combat the time dispersion: including ‘special’ time guards in the symbol transitions c o p y C P  T T c Including a “cyclic prefix” Furthemore it converts Linear conv. = Cyclic conv. (Method: overlap-save) Including the Cyclic Prefix Without the Cyclic Prefix Symbol: 8 periods of fi Symbol: 8 periods of fi CP Y ( t ) i Y ( t ) i Channel: Passing the channel h(n) Passing the channel h(n) – n h ( n ) = ( 1 ) / n n = 0 , … , 2 3 ¹ Y ( t ) i Final transient remains within the CP Initial transient remains within the CP The inclusion of a CP maintains the orthogonality Initial transient Decaying transient Loss of orthogonality Y ( t ) Y ( t ) j j Symbol: 4 periods of fi Symbol: 4 periods of fi CP functions: – It acomodates the decaying transient of the previous symbol – It avoids the initial transient reachs the current symbol

  17. OFDM Modulator cos(2pfct) S/P IDFT P/S Real x Bit Stream BPF s(t) S Mapping Img x sin(2pfct) OFDM Demodulator x LPF S/P DFT P/S r(t) cos(2pfct) A/D Received Bit Stream Demapping p/2 x LPF

  18. IEEE 802.11 Wireless LAN • IEEE 802.11 standard: • unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz and 802.11b 802.11a

  19. Frequency Band

  20. The 802.11 Protocol Stack www.ieee802.org/11/

  21. t4 t5 t6 GI2 T1 T2 t1 t2 t3 t9 t10 t7 t8 GI GI OFDM Symbol OFDM Symbol Short training sequence: AGC and frequency offset Long training sequence: Channel estimation 802.11a System Specification • Sampling (chip) rate: 20MHz • Chip duration: 50ns • Number of FFT points: 64 • FFT symbol period: 3.2ms • Cyclic prefix period: 16 chips or 0.8ms • Typical maximum indoor delay spread < 400ns • OFDM frame length: 80 chips or 4ms • FFT symbol length / OFDM frame length = 4/5 • Modulation scheme • QPSK: 2bits/sample • 16QAM: 4bits/sample • 64QAM: 6bits/sample • Coding: rate ½ convolutional code with constraint length 7

  22. OFDM Transmitter(HIPERLAN/2 / IEEE802.11a) • channel coding (convolutional codes with Viterbi decoding) • IDFT: discrete realized filter bank (very efficient FFT) • cyclic prefix / guard interval (GI) prevents intersymbol interference (ISI) OFDM Basics

  23. OFDM Receiver (HIPERLAN/2 / IEEE802.11a) • Synchronization • FFT window position (time domain) • sample and modulation frequency correction • Pre equalizer (PE) for impulse compression • OFDM: Orthogonal Frequency Division Multiplexing • separate multiplicative channel correction on each subcarrier • equalizer coefficient design: en = 1 / Cn  circular convolution OFDM Basics

  24. Channel Estimation (CE): Training Symbols • burst structure of HIPERLAN/2 and IEEE802.11a • short symbols for AGCand raw synchronization • training sequence (TS): 2 identical symbols per subcarrier (52) • data OFDM symbols with 48 user data and 4 pilot symbolseach • pilot symbols for fine synchronization (insufficient for channel estimation) Channel Estimation

  25. Spectrum Mask Power Spectral Density -20 dB -28 dB -40 dB -11 -9 -30 -20 9 11 20 30 f carrier Frequency (MHz) • Requires extremely linear power amplifier design.

  26. Carrier orthogonality by Discrete Multi-Tone (DMT) modulation enables their partial overlap • DMT used in Digital Subscribe Line (xDSL) • Usable frequency band is separated into 256 small frequency bands (or subchannels) of 4.3125 kHz each (ADSL) • Within each subchannel, modulation uses quadrature amplitude modulation (QAM) • By varying the number of bits per symbol within a subchannel, the modem can be rate-adaptive • DMT uses the fast Fourier transform (FFT) algorithm for modulation and demodulation

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