Terrestrial Transmission of Digital Television Signals (DVB-T) - Digital Video and Audio Broadcasting Technology - page 359

(NCO). This mixer and the NCO are then used for correcting the frequency
of the DVB-T signal but because the oscillators lack accuracy, the receiver
must also be locked to the transmitted frequency by means of automatic
frequency control (AFC). This is done by the AFC evaluating the continual
pilots after the Fast Fourier Transform (FFT). If the receiver frequency dif-
fers from the transmitted frequency, all the constellation diagrams will ro-
tate more or less quickly clockwise or anticlockwise. The direction of rota-
tion simply depends on whether the deviation is positive or negative and
the speed depends on the magnitude of the error. It is then only necessary
to measure the position of the continual pilots in the constellation diagram.
The only factor of interest with respect to the frequency correction is the
phase difference of the continual pilots from symbol to symbol, the aim
being to reduce this phase difference to zero. The phase difference is a di-
rect controlled variable for the AFC, i.e. the NCO frequency is changed
until the phase difference becomes zero. The rotation of the constellation
diagrams is then stopped and the receiver is locked to the transmitted fre-
quency.
The FFT signal processing block, the sampling window of which is con-
trolled by the time synchronization, transforms the COFDM symbols back
into the frequency domain, providing again 2048 or 8192 real and imagi-
nary parts. However, these do not as yet correspond directly to the carrier
constellations. Since the FFT sampling window is not placed precisely
over the actual symbol, there exists a phase shift in all COFDM subcarri-
ers, i.e. all constellation diagrams are twisted. This means that the contin-
ual and scattered pilots are no longer located on the real axis, either, but
somewhere on a circle, the radius of which corresponds to the amplitude of
these pilots. Furthermore, channel distortions must be expected due to
echoes or amplitude response or group delay. This, in turn, means that the
constellation diagrams can also be distorted in their amplitude and can be
additionally twisted to a greater or lesser extent. However, the DVB-T sig-
nal carries a large quantity of pilot signals which can be used as measuring
signal for channel estimation and channel correction in the receiver. Over
the period of twelve symbols, scattered pilots will have come to rest at
every third carrier position, i.e. information about the distortion in the
channel is available at every third carrier position. Measuring the ampli-
tudes and phase distortion of the continual and scattered pilots enables the
correction function for the channel to be calculated, rotating the constella-
tion diagrams back to their nominal position. In addition, the amplitude
distortion is removed and the constellation diagrams are compressed or
expanded in such a way that the pilots come to rest at the correct position
at their nominal position on the real axis.