L64780 LSI Logic Corporation, L64780 Datasheet - Page 33

• Manufacturer Part Number: L64780
• Description: Dvb-t Cofdm Demodulator Technical Manual 2/00
• Manufacturer: LSI Logic Corporation

2.10 Channel Estimation and Equalization
The output from the FFT block is a sequence of complex numbers, each
describing the signal received on one of the COFDM carriers. The
numbers correspond to the values, chosen from the points of the current
constellation, which were used to modulate each carrier at the modulator.
However, each carrier is received with unknown amplitude and phase
due to the combined effects of:
The purpose of the channel estimation and equalization block is to
correct these effects so that the complex numbers at its output would, if
plotted on an Argand diagram, correspond to points of the transmitted
constellation (for example, QPSK, 16 QAM, or 64 QAM) except for any
superimposed noise or interference.
The transmitted DVB-T signal contains “scattered pilots,” which are
distributed among the data cells in a regular pattern (see
These are transmitted with known values: the imaginary part is always
zero, while the real part has a fixed amplitude. The sign of the real part,
however, is determined by the carrier number (according to a pseudo-
random function that also is used in a similar way to determine the real
part of continual pilots).
Figure 2.6
The Channel Estimation and Equalization Block compares each received
scattered-pilot cell with the known transmitted value (derived from a
Channel Estimation and Equalization
K
min
= 0
The channel through which the RF signal has passed (which, in
general, is frequency selective)
Any minor error in the FFT timing window
Scattered Pilot Structure
K
K
max
max
Figure
= 1704 if 2K
= 6816 if 8K
Symbol 67
Symbol 0
Symbol 1
Symbol 2
Symbol 3
2.6).
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