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Ofdm for wireless

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Ofdm for wireless

  1. 1. OFDM FOR WIRELESS COMMUNICATION Y SUNIL RAJ KUMAR M.Tech 2nd year Reg. NO: 14304029 Dept. of electronics engineering Pondicherry university
  2. 2. CONTENT LAYOUT 1. INTRODUCTION TO MULTI-CARRIER SYSTEM 2. OFDM SYSTEM MODEL & GENERATION OF SUB CARRIERS 3. FADING 4. GUARD TIME & CYCLIC EXTENSIONS 5. WINDOWING 6. CHOICE OF OFDM PARAMETERS 9/9/2015 Course Code :EENG 564 Directed Study 2
  3. 3. INTRODUCTION TO MULTI-CARRIER SYSTEM Frequency Division Multiplexing Multicarrier Modulation Multicarrier Modulation OFDM Discrete Multi-tone (DMT) MC-CDMA MC-DS-CDMA MT-CDMA 9/9/2015 Course Code :EENG 564 Directed Study 3
  4. 4. APPLICATIONS • Wideband communication over mobile radio: Mobile radio FM, Dig. Cellular telephony, WLAN, WMAN, UWB, . . . . . • Digital subscriber lines: ADSL, HDSL, VHDSL, . . . . . . • Digital audio broadcasting • Digital video broadcasting • HDTV broadcasting • Optical communication – HFC • Underwater communications 9/9/2015 Course Code :EENG 564 Directed Study 4
  5. 5. Single Carrier QAM Modulator HT(f) HT(f) M-ary symbol to quadrature Signal encoding × × + cos(w0t) - sin(w0t) {d0, d1, d2, d3, . . . . } {a0 , a1 , a2 , a3 , . . . } {b0 , b1 , b2 , b3 , . . . } b0 b1 b2 O/P a0 a1 a2 9/9/2015 Course Code :EENG 564 Directed Study 5
  6. 6. Multi-Carrier Comm. System Modulator Modulator Modulator De- MUX + . . ,d1 i, d1 i+1, d2 i+2, . . r bps . ,d3 i, d3 i+1, . . . ,dM i, dM i+1, . . f0 f0+B f0+2B f0+(M-!)B Signal spectrum p(t) p(t) p(t) f0 f0+B f0+(M-1)B Subcarrier MC signal {. . ,d1 i, d2 i, . . dM i, d1 i+1, d2 i+1, . . dM i+1, . . } Mr bps9/9/2015 Course Code :EENG 564 Directed Study 6
  7. 7. OFDM SYSTEM MODEL & GENERATION OF SUB CARRIERS 9/9/2015 Course Code :EENG 564 Directed Study 7
  8. 8. Frequency Division Multiplexing (FDM) • Frequency Division Multiplexing (FDM) has been used for a long time to carry more than one signal over a telephone line. • FDM divides the channel bandwidth into sub channels and transmits multiple relatively low rate signals by carrying each signal on a separate carrier frequency. • To ensure that the signal of one sub channel did not overlap with the signal from an adjacent one, some guard- band was left between the different sub channels. 9/9/2015 Course Code :EENG 564 Directed Study 8
  9. 9. Orthogonal Frequency Division Multiplexing (OFDM) • In order to solve the bandwidth efficiency problem, orthogonal frequency division multiplexing was proposed, where the different carriers are orthogonal to each other. With OFDM, it is possible to have overlapping sub channels in the frequency domain, thus increasing the transmission rate. • This carrier spacing provides optimal spectral efficiency. Today, OFDM has grown to be the most popular communication system in high-speed communications. • OFDM is becoming the chosen modulation technique for wireless communications. OFDM can provide large data rates with sufficient robustness to radio channel impairments. 9/9/2015 Course Code :EENG 564 Directed Study 9
  10. 10. OFDM is a special case of FDM 9/9/2015 Course Code :EENG 564 Directed Study 10
  11. 11. An example of OFDM using 4 sub-carriers • few bits are 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1,… 9/9/2015 Course Code :EENG 564 Directed Study 11
  12. 12. An example of OFDM using 4 sub-carriers 9/9/2015 Course Code :EENG 564 Directed Study 12
  13. 13. OFDM signal in time and frequency domain 9/9/2015 Course Code :EENG 564 Directed Study 13
  14. 14. GENERATED OFDM SIGNAL 9/9/2015 Course Code :EENG 564 Directed Study 14
  15. 15. FADING • The attraction of OFDM is mainly because of its way of handling the multipath interference at the receiver. • Multipath phenomenon generates two effects (a) Frequency selective fading and (b) Inter symbol interference (ISI). • The "flatness" perceived by a narrow-band channel overcomes the frequency selective fading. On the other hand, modulating symbols at a very low rate makes the symbols much longer than channel impulse response and hence reduces the ISI. • Use of suitable error correcting codes provides more robustness against frequency selective fading. • The insertion of an extra guard interval between consecutive OFDM symbols can reduce the effects of ISI even more 9/9/2015 Course Code :EENG 564 Directed Study 15
  16. 16. FADING: • DEFINING FADING: the path from the transmitter to the receiver either has reflections or obstructions, we can get fading effects. In this case, the signal reaches the receiver from many different routes, each a copy of the original. Each of these rays has a slightly different delay and slightly different gain. The time delays result in phase shifts which added to main signal component (assuming there is one.) causes the signal to be degraded 9/9/2015 Course Code :EENG 564 Directed Study 16
  17. 17. Fading…. • a)the signal we want to send and the channel frequency response are well matched. • (b) A fading channel has frequencies that do not allow anything to pass. Data is lost sporadically. • (c) With OFDM, where we have many little sub-carriers, only a small sub-set of the data is lost due to fading a b c 9/9/2015 Course Code :EENG 564 Directed Study 17
  18. 18. Fading….. • OFDM signal offers an advantage in a channel that has a frequency selective fading response . • Instead Of the whole symbol being knocked out, we lose just a small subset of the (l/N) bits. With proper coding, this can be recovered. • The BER performance of an OFDM signal in a fading channel is much better than the performance of QPSK/FDM which is a single carrier wideband signal . 9/9/2015 Course Code :EENG 564 Directed Study 18
  19. 19. GUARD TIME & CYCLIC EXTENSIONS 9/9/2015 Course Code :EENG 564 Directed Study 19
  20. 20. GUARD TIME & CYCLIC EXTENSIONS….. • The PSK symbol and its delayed version • Move the symbol back so the arriving delayed signal peters out in the gray region. No interference to the next symbol • we just extend the symbol, then the front of the symbol which is important to us since it allows figuring out what the phase of this symbol is, is now corrupted by the “splash”. • we move the symbol back and just put in convenient filler in this area, then not only we have a continuous signal but one that can get corrupted and we don’t care since we will just cut it out anyway before demodulating 9/9/2015 Course Code :EENG 564 Directed Study 20
  21. 21. • Cyclic prefix is copy of back of the symbol we add to the front of the symbol. • Slide the symbol to start at the edge of the delay spread time and then fill the guard space with a copy of what turns out to be tail end of the symbol . • We will be extending the symbol so it is 1.25 times as long, to do this, copy the back of the symbol and glue it in the front . • We can add cyclic prefix just once to the composite OFDM signal. The prefix is any where from 10% to 25% of the symbol time. • the addition of cyclic prefix which mitigates the effects of link fading and inter symbol interference, increases the bandwidth. 9/9/2015 Course Code :EENG 564 Directed Study 21
  22. 22. 9/9/2015 Course Code :EENG 564 Directed Study 22
  23. 23. WINDOWING • OFDM signals consists of a number of unfiltered QAM sub carriers, as a result the out of band spectrum decreases rather slowly, according to sinc function. • Spectra of 16,64,and 256 subcarriers are plotted for larger number of subcarriers, the spectrum goes down more rapidly in the beginning which is caused by the fact that the side lobes are closer together. • To make the spectrum go down rapidly, windowing can be applied to the individual OFDM symbols. • Windowing an OFDM symbol makes the amplitude go smoothly to zero at the symbol boundaries. 9/9/2015 Course Code :EENG 564 Directed Study 23
  24. 24. WINDOWING………. • A commonly used window type is the raised cosine window • 𝒘 𝒕 = {𝟎. 𝟓 + 𝟎. 𝟓 𝐜𝐨𝐬(𝝅 + 𝒕𝝅/(𝜷𝑻𝒔)) 0 ≤ 𝒕 ≤ 𝜷𝑻𝒔} {1.0 𝜷𝑻𝒔 < 𝒕 ≤ 𝑻𝒔} {𝟎. 𝟓 + 𝟎. 𝟓𝐜𝐨𝐬( 𝒕−𝑻𝒔 𝝅 𝜷𝑻𝒔 )} 𝑻𝒔 ≤ 𝒕 ≤ 𝟏 + 𝜷 𝑻𝒔 • Here Ts is the symbol interval, which is shorter than the total symbol duration because we allow adjacent symbol to partially overlap in roll-off region. 9/9/2015 Course Code :EENG 564 Directed Study 24
  25. 25. WINDOWING………. • The practical OFDM signal is generated as fallows first Nc input QAM values are padded with zero to get N input samples that are used to calculate an IFFT. • The last Tprefix samples of the IFFT output are inserted at the start of the OFDM symbol, and the first Tpostfix samples are appended at the end. • The OFDM symbol is then added to the output of the previous OFDM symbol with delay Ts, such that there is no overlap region 0f 𝜷𝑻𝒔 • Where 𝜷 is the roll-off factor of the raised cosine window. 9/9/2015 Course Code :EENG 564 Directed Study 25
  26. 26. WINDOWING……… • Instead of windowing it is also possible to use conventional filters techniques to reduce the out of band spectrum • Windowing and filtering are duel techniques, multiplying an OFDM signal by an window means the spectrum is going to be a convolution of the spectrum of the window function with asset impulse functions at the subcarriers frequencies. • When filtering is applied ,a convolution is done in the time domain and the OFDM spectrum is multiplied by the frequency response of the filter • Digital filtering techniques are more complex to implement than windowing 9/9/2015 Course Code :EENG 564 Directed Study 26
  27. 27. CHOICE OF OFDM PARAMETERS • OFDM system design, a number of parameters are up for consideration • Such as the 1. Number of subcarriers 2. Guard time 3. Symbol duration 4. Sub carrier spacing 5. Modulation type per sub carrier 9/9/2015 Course Code :EENG 564 Directed Study 27
  28. 28. CHOICE OF OFDM PARAMETERS……. • The choice of parameters is influenced by system requirements • Usually there are three main requirements to start with 1. Available bandwidth 2. Required bit rate 3. Tolerable delay spread 9/9/2015 Course Code :EENG 564 Directed Study 28
  29. 29. CHOICE OF OFDM PARAMETERS…… • The delay spread is directly dictates the guard time.as a rule the guard should be about two to four times the root-mean-squared delay spread • This value depends on the type coding and QAM modulation. • Higher order QAM is more sensitive to ICI and ISI than QPSK, while heavier coding obviously reduces the sensitivity to such interference. • Now that the guard time has been set, the symbol duration can be fixed. • To minimize the signal-to-noise ratio (SNR) loss caused by guard time, it is desirable to have the symbol duration much larger than the guard time 9/9/2015 Course Code :EENG 564 Directed Study 29
  30. 30. • Large symbol duration means more subcarriers with smaller subcarrier spacing, a larger implementation complexity, and more sensitivity to phase noise • Practical design choice is to make the symbol duration at least five times the guard time. • The number of subcarriers follows directly as the required -3-dB bandwidth divided by the subcarrier spacing, which is the inverse of the symbol duration less the guard time • Another way to determine the subcarrier is Required bit rate divided by the bit rate per subcarrier • The bit rate per subcarrier is defined by the modulation type ,coding rate, and symbol rate 9/9/2015 Course Code :EENG 564 Directed Study 30

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