PS20251 Datasheet, PDF (10/25 Pages) List of Unclassifed Manufacturers – Dual COFDM demodulator with PID filters and MRC diversity

PS20251
3 Demodulator Functional Description
A functional block diagram of the PS20251 OFDM demodulator is shown in Figure 4. This accepts an IF
analogue signal and delivers a stream of demodulated soft decision data to the on-chip Viterbi decoder.
Clock, timing and frequency synchronization operations are all digital and there are no analogue control
loops except the AGC. The frequency capture range is large enough for all practical applications. This
demodulator has novel algorithms to combat impulse noise as well as co-channel and adjacent channel
interference. If the modulation is hierarchical, the OFDM outputs both high and low priority data streams.
Only one of these streams is FEC-decoded, but the FEC can be switched from one stream to another with
minimal interruption to the transport stream.
Figure 4 OFDM demodulator diagram
The FEC module shown in Figure 5 consists of a concatenated convolutional (Viterbi) and Reed-
Solomon decoder separated by a depth-12 convolutional de-interleaver. The Viterbi decoder operates on
5-bit soft decisions to provide the best performance over a wide range of channel conditions. The trace-
back depth of 128 ensures minimum loss of performance due to inevitable survivor truncation, especially
at high code rates. Both the Viterbi and Reed -Solomon decoders are equipped with bit-error monitors.
The former provides the bit error rate (BER) at the OFDM output. The latter is the more useful measure
as it gives the Viterbi output BER. The error collecting intervals of these are programmable over a very
wide range.
PLESSEY SEMICONDUCTORS LTD
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