PS20251 Datasheet, PDF (17/25 Pages) List of Unclassifed Manufacturers – Dual COFDM demodulator with PID filters and MRC diversity

PS20251
3.23Diversity operation
Figure 7 Diversity operation
Diversity reception is a receiving process that combines more than one received signal to obtain a better
result than that obtained from using just one signal. The diversity technique used by the PS20251 is
called spatial diversity, and relies on two antennas receiving the same broadcast signal. The two
antennas must be spaced apart by a distance that is significant when compared to the wavelength of the
RF signal that is being received. This is so that the signals from each antenna suffer different
impairments, but the combination can yield acceptable results.
The diversity combining technique used in the PS20251 is called Maximum Ratio Combining (MRC) and is
the optimum technique that can be employed. This combination of the signals occurs immediately after the
FFT block in the demodulator, before the Viterbi and Reed-Solomon forward error correction (FEC) blocks
(see Figure 2). The secondary demodulator supplies the output information from its FFT block to a circuit
in the primary demodulator which combines this information with its own FFT data and passes the resulting
information to the Viterbi block within the primary demodulator.
The operations of the Intel DVB-T demodulators are internally controlled by a state machine. This
significantly reduces the software overhead required by the device driver. During acquisition, first the
symbol synchronisation has to be achieved. Then the FFT block is aligned in terms of frequency. Once this
is achieved the Viterbi block can start synchronisation. In a diversity application, the state machine in the
primary demodulator will start to acquire Viterbi lock when the FFT block in either the primary or secondary
channel has achieved symbol lock and frequency lock.
The quality of the signal present at the input to the primary channel necessary to achieve FFT lock does
not have to be high. Indeed, a signal with a carrier to noise ratio of 0dB is sufficient, and will also provide
TPS information from either the primary or secondary demodulator.
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