Complete Communications Engineering

At the input of a DVB-T receiver is an OFDM-modulated RF signal from a terrestrial TV channel.

DVB-T Receiver
fig 1 DVB-T Receiver

The desired TV channel OFDM signal is amplified (level adjusted), pre-filtered from adjacent channels and shifted from the RF band to the baseband where it’s represented as a complex baseband signal. This signal is an input to the OFDM demodulator. The OFDM demodulator outputs a sequence of complex QAM samples organized into symbols. Symbols are organized in frames where each frame has 1512 symbols (2K mode) or 6048 symbols (8K mode). These symbols are demapped to bit substreams in the demapper, and deinterleaved in the symbol deinterleaver and the bit-deinterleaver.

In the bit-deinterleaver, v substreams are multiplexed into two bit-streams. These bit-streams are sent through the de-puncturing block and then are sent to the convolutional error correction decoder which is based on the Viterbi algorithm. After error correction, the signal is processed in the outer deinterleaver. The output of the outer deinterleaver is sent to a Reed-Solomon error correction decoder, then the signal is descrambled and sync is modified to form the output MPEG transport stream.

OFDM Demodulator
Fig 2 OFDM Demodulator

The OFDM demodulator block is shown in Fig.2. A pre-filter is first, followed by a frequency rotator that corrects the input frequency offset error. Next, the polyphase filter adjusts the sampling rate. After frequency offset and timing correction, the signal goes through the main filter where adjacent channels are attenuated. The co-channel filter attenuates carrier frequencies from co-channel analog TV signals. This filter is optional.

Next the signal goes through the Fast Fourier Transform (FFT) block where the time-to-spectral domain transform occurs. To work properly, the FFT block must get information about the starting and ending of each symbol. This information is recovered by symbol synchronization block. After the FFT, the slope correction filter is applied to correct the overall FFT sample slope. From the slope corrected symbol samples, the set of inserted pilots is extracted. Based on these pilots, information for carrier recovery, timing correction and the signal equalizer is extracted.

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