Week8_ECE9308_2020_Modulation_Part_II_OFDM.pptx

OFDM

1. Introduction to Orthogonal Frequency Division Multiplexing (OFDM) Prof. Xianbin (Warren) Wang March 20 23

2. Outline What’s OFDM? OFDM stands for Orthogonal Frequency Division Multiplexing Multicarrier modulation technique Principles of OFDM Transmitter Receiver Advantages / Disadvantages Peak-to-Average Power Ratio Sensitivity to carrier frequency offset OFDM System Design Channel Estimation Time and Frequency Domain Synchronization

3. f OFDM is a multicarrier modulation technique that divides high-speed serial information signal into multiple lower-speed sub-signals. Transmission is simultaneously at different frequencies in parallel. Different subcarriers are overlapped in the frequency domain to improve the spectrum efficiency. What is OFDM? (I) y(n) f f 1 f n-1 + S/P Data X X 1 X N-1

4. Single Carrier System Sequential Transmission of Waveforms Waveforms are Short Duration T Waveforms Occupy Full System Bandwidth 1/T What is OFDM? (II)

5. Multi-Carrier System Parallel Transmission of Waveforms Waveforms are Long Duration MT Waveforms Occupy 1/M th Of System Bandwidth 1/T What is OFDM? (III)

6. OFDM: Dense Multichannel System Conventional multichannel system Uses Non-Overlapping Adjacent Channels. Channels separated by some guard band in frequency domain OFDM Multichannel System 50% Overlap of Adjacent Channels Channels separated by Half Their Two Sided bandwidth What is OFDM? (IV)

7. Why do we need OFDM? (I) h c (t) = å k a k d (t - t k ) where k = 0, … , K -1 a k : path gain (complex) t = 0 normalize relative delay of first path D k = t k - t difference in time-of-flight | a | | a 1 | | a 2 | D 1 D 2 a a 1 a 2

8. Why do we need OFDM? (II) 2T s 4T s T s Single carrier modulated communication system requires complex adaptive equalization to remove resulting ISI. Multipath can cause destructive interference and time dispersion (inter symbol interference) With OFDM, the ISI can be effectively mitigated as the symbol duration in OFDM system is significantly longer.

9. r(t) = a s(t- t ) + a 1 s(t- t 1 ) + a 2 s(t- t 2 ) + a 3 s(t- t 3 ) channel Input (Tx signal) Output (Rx signal) Impulse Response h(t) t 3 - t time a 3 a freq. Frequency Response H(f) Why do we need OFDM? (III)

10. subchannel frequency magnitude carrier channel 2T s 4T s T s Why do we need OFDM? (IV)

11. Outline What’s OFDM? OFDM stands for Orthogonal Frequency Division Multiplexing It’s multicarrier modulation technique Principles of OFDM Transmitter Receiver Advantages / Disadvantages Peak-to-Average Power Ratio Sensitivity to carrier frequency offset OFDM System Design Channel Estimation Time and Frequency Domain Synchronization

12. P/S QAM decoder FEQ frequency domain equalizer S/P QAM encoder N -IFFT cyclic prefix P/S D/A + transmit filter N -FFT S/P remove cyclic prefix TRANSMITTER RECEIVER N subchannels N complex samples N complex samples N subchannels Receive filter + A/D multipath channel OFDM Transceiver Bits 00110

13. Continuous Time: Orthogonal Time Signal Set OFDM Principle (I)

14. Discrete Time: Orthogonal Time Signal Set OFDM Principle (II)

15. Easy Modulation of Orthogonal Carriers Using IDFT OFDM Transmitter

16. x(t) h(t) y(t) t t t t t Adjacent Symbols Symbol Channel Distorted Symbol OFDM ISI Removal: Cyclic Prefix (I)

17. Cyclic Prefix Add the last part of the packet to the beginning of the signal Duration of the CP larger than multipath delay spread Orthogonality of the subcarriers not affected. Simple frequency domain equalizer can be used. OFDM ISI Removal: Cyclic Prefix (II)

18. OFDM is a block transform method. A “block” consists of a single OFDM symbol and its cyclic prefix. A new block follows each previous block, and so on. OFDM ISI Removal: Cyclic Prefix (III)

19. OFDM ISI Removal: Cyclic Prefix (IV)

20. Cyclic Prefix and OFDM Equalization (I) Received Analog Signal: Received Digital Signal: Question : At the output of the FFT, does for OFDM signal without cyclic prefix?

21. Cyclic Prefix and OFDM Equalization (II) Answer: NO !!! FFT-domain multiplication results in time-domain circular convolution. Solution : Force cyclic convolution by making x(n) appear periodic to the channel. The Result : adding a cyclic prefix of equal or greater length than the channel impulse response .

22. Cyclic Prefix and OFDM Equalization (III) DFT properties Prefix and postfix extension convert linear convolution to cyclic convolution Equalization : multiply FFT output vectors Y with . Downside : data rate reduced by factor . Transmitted time-domain data block Cyclic prefix N -point data block

23. OFDM In-band Pilots In-band pilots are subcarriers which always available for reference. Receiver needs to reliably estimate the channel and detect signal presence, synchronization.

24. Easy Demodulation of Orthogonal Carriers Using DFT OFDM Receiver

25. Channel Estimation & Interpolation Frequency Domain Channel Estimation Interpolation between in-band Pilots Tones Boosted Pilots for Better Estimate Interpolation Method

26. OFDM Transceiver

27. OFDM Modulator

28. Transmitting Procedure Summary Encode the incoming data Insert In-band pilots Multiplex data and pilots IFFT modulation Cyclic prefix insertion

29. Receiver Procedure Timing Synchronization Removal of Cyclic prefix FFT demodulation Channel estimation Equalization Data recovery

30. OFDM Demodulator Y(n)  

31. Symbol Duration and Subcarrier Spacing To maintain orthogonality , let where = sub-carrier spacing = symbol duration If N-point IDFT (or FFT) is used Total bandwidth (in Hz) = , symbol duration after CP addition

32. Outline What’s OFDM? OFDM stands for Orthogonal Frequency Division Multiplexing It’s multicarrier modulation technique Principles of OFDM Transmitter Receiver Advantages / Disadvantages Peak-to-Average Power Ratio Sensitivity to carrier frequency offset OFDM System Design Channel Estimation Time and Frequency Domain Synchronization

33. OFDM Advantages & Disadvantages Negligible ISI High Spectral Efficiency Robust to Multipath Simple receiver design High peak-to-average power ratio Sensitive to frequency offsets and phase noise Advantages Disadvantages

34. OFDM Advantages Lower equalization complexity compared to single-carrier modulation due to efficiency of FFT algorithm Immune to intersymbol interference caused by multipath channel with cyclic prefix ( or guard time) Higher bandwidth efficiency compared to conventional FDM and single carrier modulation system Spectrum is very flat ( hard for a single-carrier system which requires very sharp pulse shaping filters)

35. OFDM Challenges Subject to frequency offset and random phase shift among each subcarrier Very high peak to average power ratio (PAPR) Subject to narrow band interference ( OFDM with FEC can efficiently mitigate this problem) Sensitive to nonlinear distortion because the spectrum of each carrier overlapped tightly

36. PAPR (1/4) Large peak-to-average ratio (PAPR) problem

37. PAPR(2/4) OFDM signal at any time instant is the summation of N subcarrier signals. Each carrier is multiplied by a independent modulated constellation generated from the input data. f S(f) f 1 f 2 f n F output + S/P Data d 1.m d 2.m d N.m

38. PAPR (3/4) High peak-to-average power ratio It increased complexity of the analog-to-digital and digital-to-analog converters It reduced efficiency of the RF power amplifier The PAPR puts a stringent requirement on the power amplifier and reduces the efficiency in the sense that a higher input backoff factor is needed before the peaks in the signal experience significant distortion due to power amplifier nonlinearity.

39. PAPR (4/4) [7]

40. Example of PAPR

41. Power Amplifier Nonlinearity

42. Power Amplifier Non-Linearity

43. 16-QAM ( a =0.2) Envelope Statistics

44. PAPR in Clipped OFDM

45. Clipping Effect on OFDM Spectra

46. Timing Offset Impact The cyclic prefix and the channel estimator provide some immunity to small time offsets Larage time offset leads to intersymbol interference time no offset time offset

47. Frequency Offset Impact Due to base carrier frequency difference between the transmitter and the receiver Leading to serious inter-channel-interference (ICI) Must be estimated and compensated by using specific preambles or pilots No CFO CFO in presence

48. Constellation with Frequency Offset

49. STA STA STA STA AP AP ESS BSS BSS Existing Wired LAN Infrastructure Network Overview, 802.11 Architecture

50. Wireless Networking (Wi-Fi) Data Link Layer over radio frequencies Many standards Notably IEEE 802.11

51. OFDM 802.11a

52. OFDM 802.11a

53. OFDM Systems System Transform Size Number Carriers Channel Spacing kHz Bandwidth MHz Sample Rate MHz Symbol Duration  sec Data Rate Mbits/s HyperLAN/2 64 52 4 312.5 16.25 20 3.2 0.8 6-54 802.11a 64 52 4 312.5 16.56 20 3.2 0.8 6-54 DAB 2048 1024 1712 842 4.464 7.643 9.174 224 0.68-14.92 DVB-T 2048 8192 1536 1.00 1.536 2.048 24/48/96 msec 3.072 ADSL 256 (down) 64 (up) 36-127 7-28 4.3125 1.104 1.104 231.9 0.64-8.192

54. OFDM 802.11a

55. OFDM Pilot in DVB-T DVB-T Data Continuous Pilot Scattered Pilot TPS Carriers Frequency Spectrum Continual Pilots Scattered Pilots. Transmission Protocol Signals TPS.