Concepts of Multimedia Processing and Transmission

1. Concepts of Multimedia Processing and Transmission IT 481, Lecture #12 Dennis McCaughey, Ph.D. 27 November, 2006

2. Broadcast Network Schematic • SFN: All transmitters operate on a single Radio Frequency (RF) • MFN: Each transmitter operates on a different frequency IT 481, Fall 2006

3. Frequency Usage Schematic IT 481, Fall 2006

4. Terrestrial Drivers • Terrestrial broadcasts are omnidirectional • Multiple copies of the same signal may arrive at the receiver with slightly different delays and thus interfere with each other • Multipath = (direct path signal + reflected signal + refracted signal) • Intersymbol Interference (ISI) • Limits the bit rate that may be achieved IT 481, Fall 2006

5. Multipath IT 481, Fall 2006

6. COFDM Channel Interface Components IT 481, Fall 2006

7. Overhead Bits IT 481, Fall 2006

8. COFDM Modulator IT 481, Fall 2006

9. COFDM Transmitter Showing Constellation Mapping IT 481, Fall 2006

10. Operation • The OFDM modulator consists of the block in the diagram that is labeled 'IDFT', which stands for inverse discrete Fourier transform. • In reality, the actual process carried out is the inverse fast Fourier transform (IFFT), because the IFFT is, as the name suggest, a fast way to calculate the IDFT. • The IDFT calculates the following equation: IT 481, Fall 2006

11. COFDM Receiver IT 481, Fall 2006

12. COFDM Receiver Showing Constellation Mapping IT 481, Fall 2006

13. Operation • The signals are received at the antenna, • The signals are I/Q down-converted from RF to generate the real (I) and imaginary (Q) streams, • Low-pass filtered (LPF) and digitized in the analogue to digital converters (ADC, one ADC for each stream). • Following the ADC, the cyclic prefix is stripped off and the remaining sampled values are serial to parallel converted and once there is a full block of samples • The DFT is calculated (in reality the FFT is calculated as the FFT requires far fewer multiplications to be carried out than the DFT). IT 481, Fall 2006

14. Symbol Generation IT 481, Fall 2006

15. Example • N = 5 bits are mapped into 1symbol of duration Ts • Each symbol bit is modulated onto a different carrier frequency using on-off keying (OOK) • Carriers are DC, fs, 2fs, 3fs, 4fs • fs = 1/Ts • Bits Per Carrier BPSK = 1 QPSK = 2 (4QAM) 16QAM = 4 64QAM = 8 • Bits Per Symbol • BPSK = N • QPSK = 2N • 16QAM = 4N • 64QAM = 8N IT 481, Fall 2006

16. QPSK and QAM Constellations IT 481, Fall 2006

17. QPSK Bit Rates IT 481, Fall 2006

18. Receiver Operation • Receiver waits a short period of time called the guard interval, Tg, before starting to process the received symbol • Ensure receipt of all delayed versions of the direct path signal • Processing entails • Determining which of the N= 5 carriers are received • Demodulating the subcarrier modulated symbols to recover the bits (QPSK, 16QAM etc) IT 481, Fall 2006

19. Why Orthogonality? • If the subcarriers are orthogonal they do not interfere with each other • Simplifies the construction and recovery of the bit-symbol-bit stream sequence • If the subcarriers are cleverly spaced this orthogonality is preserved in the presence of multipath • Orthogonality is implemented by spacing the subcarriers at multiples of 1/Ts IT 481, Fall 2006

20. What is Orthogonality? • Two signals x and y are orthogonal if: • If the two signals do not overlap in either time or frequency they are orthogonal • However the important case here is when they do. IT 481, Fall 2006

21. Need to define two functions Useful Properties Achieving Orthogonality • Fourier Transform Pairs IT 481, Fall 2006

22. Fourier Transform Pairs IT 481, Fall 2006

23. Important Fourier Transform Properties IT 481, Fall 2006

24. Combined Shifting and Scaling IT 481, Fall 2006

25. Frequency Domain Symbol IT 481, Fall 2006

26. Guard Interval avoids intersymbol interference It lowers the maximum encoded bit rate Typically Tg = Ts/4 For MFM networks For SFN networks Guard Interval IT 481, Fall 2006

27. Example • Since a FFT and IFFT are used the nearest powers of 2 are: • 2048 (1705) • 8192 (6817) IT 481, Fall 2006

28. DVB-T/H Transmitter NOKIA IT 481, Fall 2006

29. DVB-T Frame Format IT 481, Fall 2006

30. 4-Carrier example • Frame = [00,01,10,11] • Modulation = QPSK (4QAM) IT 481, Fall 2006

31. I Signal Plots [0,Ts] IT 481, Fall 2006

32. Q Signal Plots [0,Ts] IT 481, Fall 2006

33. Composite Signal [0,Ts] IT 481, Fall 2006

34. N = 4; M = 4; msg_b = [0 0 0 1 1 0 1 1] msg_b = reshape(msg_b,2,[ ]) msg_d = [1,2]*msg_b(:,:) msg_a = qammod(msg_d,M); A = diag(msg_a); x = zeros(N,128); for k = 1:N x(k,:) = linspace(0,i*(k-1)*2*pi(),128); end y = ones(1,N)*A*exp(x); plot(real(y),'-r','LineWidth',2) hold on plot(imag(y),'-b','LineWidth',2) legend('I','Q','Fontweight','bold') plot(zeros(1,128),'--k','LineWidth',2) hold off grid on title('Cumulative I and Q Components','Fontsize',14) grid on z = (fft(y)); s = qamdemod(z(1:N),M) s = dec2bin(s) s = reshape(s',1,[ ]) Scale = eye(N)/sqrt(2); Scale(1,1) = 1; figure; hold on plot(real(Scale*A*exp(x))','LineWidth',2) ylim([-1.1 1.1]) title('In-Phase Carriers','Fontsize',14) legend('00','01','10','11') plot(zeros(1,128),'--k','LineWidth',2) grid on hold off figure; hold on plot(imag(Scale*A*exp(x))','LineWidth',2) ylim([-1.1 1.1]) title('Quadrature Carriers','Fontsize',14) legend('00','01','10','11') plot(zeros(1,128),'--k','LineWidth',2) grid on hold off Matlab Code IT 481, Fall 2006

35. References • “MPEG-4 Natural Video Coding - An overview” Touradj Ebrahimi* and Caspar Horne** • J. Henriksson, “DVB-H, Standards Principles and Services”, Nokia HUT Seminar T-111.590 Helsinki Finland 2.24.2005 • F. Halsall, “Multimedia Communications”, Addison-Wesley, New York, 2001 IT 481, Fall 2006