Salient features

1. Salient features • BASIC IDEA : Channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading. • Multicarrier transmission :Subcarriers are orthogonal each other in frequency domain. • Time-domain spreading: • Spreading is achieved in the time-domain by repeating the same information in an OFDM symbol on two different sub-bands => Frequency Diversity. • Frequency-domain spreading: • Spreading is achieved by choosing conjugate symmetric inputs for the input to the IFFT (real output) • Exploits frequency diversity and helps reduce the transmitter complexity/power consumption.

2. OFDM Transceiver RF Tx DAC Binary Input Data Add Cyclic extension & Windowing QAM mapping Pilot Insertion S - P P - S Coding Interleaving IFFT FFT Remove Cyclic extension QAM demapping Channel Correction P - S S - P Decoding De-Interleaving Binary Output Data Timing & Freq. Sync. RF Rx ADC

3. Input Vector IFFT Mapped to Output Time Series, Up-Sampled, Converted Via DAC to Waveform, and I-Q Up-Converted

4. The IFFT as Signal Generator and Interpolator

5. Adjacent Symbol Interference (ASI) Symbol Smearing Due to Channel

6. Guard Interval Inserted Between Adjacent Symbols to Suppress ASI

7. Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference (ACI) and retain orthogonality

8. Data Length Defines Sinc Width:Spectral Spacing Matches Width

9. Extended Data Length Reduces Sinc Width: Spectral Spacing Preserved

11. Selection of OFDM parameters • Bandwidth, bit rate, delay spread • Guard time Tg • 2 to 4 times delay spread  2 to 4 • depends on the order of modulation employed • Symbol duration > Guard time to maximize SNR • More subcarriers, smaller spacing, implementation complexity, more sensitivity to phase noise & frequency offset, high PAPR • Symbol duration  5 x Guard time ( 1-dB SNR loss ) • Ts = 5 x Tg  Tofdm = Ts + Tg • Subcarrier spacing f = 1 / Ts • Number of subcarriers = 3-dB BW / f

12. Example : • Bit rate = 20 Mbps • Tolerable delay spread = 200 ns • Bandwidth < 15 MHz • Tg = 800 ns • Tofdm = 5 x Tg + Tg = 4.8 sec • f = 1 / 4 sec = 250 KHz • Number of bits in one OFDM symbol = 20 Mbps x 4.8 sec = 96 • 16-QAM with rate ½ Conv. Coding  2 bits / symbol / subcarrier  48 subcarriers  48 x 250 KHz = 12 MHz < 15 MHz • QPSK with rate ¾ coding  1.5 bits / symbol / subcarrier  64 subcarriers  64 x 250 KHz  16 MHz > 15 MHz • 64 point IFFT / FFT  16 zero subcarriers  oversampling Given requirements

13. OFDM ADVANTAGES • OFDM is spectrally efficient • IFFT/FFT operation ensures that sub-carriers do not interfere with each other. • OFDM has an inherent robustness against narrowband interference. • Narrowband interference will affect at most a couple of subchannels. • Information from the affected subchannels can be erased and recovered via the forward error correction (FEC) codes. • Equalization is very simple compared to Single-Carrier systems

14. OFDM ADVANTAGES • OFDM has excellent robustness in multi-path environments. • Cyclic prefix preserves orthogonality between sub- carriers. • Cyclic prefix allows the receiver to capture multi- path energy more efficiently. • Ability to comply with world-wide regulations: • Bands and tones can be dynamically turned on/off to comply with changing regulations. • Coexistence with current and future systems: • Bands and tones can be dynamically turned on/off for enhanced coexistence with the other devices.

15. OFDM DRAWBACKS • High sensitivity inter-channel/carrier interference, ICI • OFDM is sensitive to frequency, clock and phase offset • The OFDM time-domain signal has a relatively large peak-to-average power ratio • tends to reduce the power efficiency of the RF amplifier • non-linear amplification destroys the orthogonality of the OFDM signal and introduces out-of-band radiation

16. OFDM Symbol: Time and Spectra Channel Input and Output

17. Test Bench: Demonstration of Receiver I-Q Imbalances, Carrier Offset, and Timing Offset

18. Time and Spectra of Sparse OFDM Symbol

19. Carrier Offset: 4% of FFT Bin Width

20. Time and Spectra With Frequency Offset = 0.1 Bin

21. Timing Offset: 10% of Sampling Time Period

22. Timing Clock Offset: 5% of Sampling Time Period per Frame

23. Time and Spectra With Sample Clock Offset = 1.02 fs

24. Time and Spectra With Sample Clock Offset = 0.98 fs

25. Gain Imbalance: 10% Error

26. Phase Imbalance: 0.1 Radian Error

27. I-Q Mixer Imbalance; 20% Gain, 0.2 Radians

28. Differential Delay to I/Q Mixers, 10% of Sample Interval

29. Power Amplifier Non-Linearity

30. OFDM based Applications • Wireless LAN standards using OFDM are • HiperLAN-2 in Europe • IEEE 802.11a, .11g • OFDM based Broadband Access Standards are getting defined for MAN and WAN applications • 802.16 Working Group of IEEE • 802.16 -- single carrier, 10-66GHz band • 802.16a, b -- 2-11GHz, MAN standard

31. IEEE 802.11a Overview • Carrier frequency= 5 GHz • Total allotted bandwidth= 20 MHz x 10 = 200MHz • Size of the FFT= 64 • Number of data subcarriers= 48 • Number of Pilot subcarriers= 4 • FFT period= 3.2 µs • Channel bandwidth used= 64/3.2 µs => 20 MHz

32. Typical Configuration • 52 subcarriers, 64 point FT/IFFT • Symbol time 4 µs • Guard time 800 ns • BPSK, QPSK, 16-QAM, 64-QAM • Coding rates 1/2,3/4,2/3 • Bit rates 6,12,18,24,36,48,54 Mbps • Channel spacing 20 MHz • Tolerable delay spread about 250 ns at 24 Mbps

33. DFT (FFT) as Signal Generatorfor Complex Sinusoids

34. DFT (FFT) As Signal Analyzer for Complex Sinusoids

35. Radix-2 FFT Flow Diagrams

36. OFDM Modulation With IFFTand Interpolator

37. OFDM Demodulation With FFT

38. OFDM Transceiver

39. Linear Versus Circular Convolution

40. Fast Circular Convolution with the FFT