Concepts of Multimedia Processing and Transmission

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Concepts of Multimedia Processing and Transmission

• IT 481, Lecture 12
• Dennis McCaughey, Ph.D.
• 23 April, 2007

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Broadcast Network Schematic

• SFN All transmitters operate on a single Radio
  Frequency (RF)
• MFN Each transmitter operates on a different
  frequency

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Frequency Usage Schematic
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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

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Multipath
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COFDM Channel Interface Components
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Overhead Bits
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COFDM Modulator
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COFDM Transmitter Showing Constellation Mapping
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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

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COFDM Receiver
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COFDM Receiver Showing Constellation Mapping
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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).

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Symbol Generation
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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

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QPSK and QAM Constellations
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QPSK Bit Rates
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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)

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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

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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.

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Achieving Orthogonality

• Need to define two functions

• Useful Properties

• Fourier Transform Pairs

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Fourier Transform Pairs
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Important Fourier Transform Properties
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Combined Shifting and Scaling
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Frequency Domain Symbol
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Guard Interval

• Guard Interval avoids intersymbol interference
• It lowers the maximum encoded bit rate
• Typically Tg Ts/4
• For MFM networks

• For SFN networks

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Example

• Since a FFT and IFFT are used the nearest powers
  of 2 are
• 2048 (1705)
• 8192 (6817)

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DVB-T/H Transmitter
NOKIA
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DVB-T Frame Format
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4-Carrier example

• Frame 00,01,10,11
• Modulation QPSK (4QAM)

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I Signal Plots 0,Ts
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Q Signal Plots 0,Ts
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Composite Signal 0,Ts
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Matlab Code

• N 4
• M 4
• msg_b 0 0 0 1 1 0 1 1
• msg_b reshape(msg_b,2, )
• msg_d 1,2msg_b(,)
• msg_a qammod(msg_d,M)
• A diag(msg_a)
• x zeros(N,128)
• for k 1N
• x(k,) linspace(0,i(k-1)2pi(),128)
• end
• y ones(1,N)Aexp(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

• Scale eye(N)/sqrt(2)
• Scale(1,1) 1
• figure
• hold on
• plot(real(ScaleAexp(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(ScaleAexp(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)

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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