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Concepts of Multimedia Processing and TransmissionPowerPoint Presentation

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

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

Frequency Usage Schematic

IT 481, Fall 2006

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

Multipath

IT 481, Fall 2006

COFDM Channel Interface Components

IT 481, Fall 2006

Overhead Bits

IT 481, Fall 2006

COFDM Modulator

IT 481, Fall 2006

COFDM Transmitter Showing Constellation Mapping

IT 481, Fall 2006

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

COFDM Receiver

IT 481, Fall 2006

COFDM Receiver Showing Constellation Mapping

IT 481, Fall 2006

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

Symbol Generation

IT 481, Fall 2006

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

QPSK and QAM Constellations

IT 481, Fall 2006

QPSK Bit Rates

IT 481, Fall 2006

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

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

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

Useful Properties

Achieving Orthogonality- Fourier Transform Pairs

IT 481, Fall 2006

Fourier Transform Pairs

IT 481, Fall 2006

Important Fourier Transform Properties

IT 481, Fall 2006

Combined Shifting and Scaling

IT 481, Fall 2006

Frequency Domain Symbol

IT 481, Fall 2006

Guard Interval avoids intersymbol interference

It lowers the maximum encoded bit rate

Typically Tg = Ts/4

For MFM networks

For SFN networks

Guard IntervalIT 481, Fall 2006

Example

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

IT 481, Fall 2006

DVB-T Frame Format

IT 481, Fall 2006

I Signal Plots [0,Ts]

IT 481, Fall 2006

Q Signal Plots [0,Ts]

IT 481, Fall 2006

Composite Signal [0,Ts]

IT 481, Fall 2006

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 CodeIT 481, Fall 2006

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

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