Technical Description of The Receiver
The receiving STB adopts techniques which are dual to those ones used in the transmission.
- Front-end and ADC: the analogue RF signal is converted to base-band and transformed into a digital signal, using an analogue-to-digital converter (ADC).
- Time and frequency synchronization: the digital base band signal is searched to identify the beginning of frames and blocks. Any problems with the frequency of the components of the signal are corrected, too. The property that the guard interval at the end of the symbol is placed also at the beginning is exploited to find the beginning of a new OFDM symbol. On the other hand, continual pilots (whose value and position is determined in the standard and thus known by the receiver) determine the frequency offset suffered by the signal. This frequency offset might have been caused by Doppler effect, inaccuracies in either the transmitter or receiver clock, and so on. Generally, synchronization is done in two steps, either before or after the FFT, in such way to resolve both coarse and fine frequency/timing errors. Pre-FFT steps involve the use of sliding correlation on the received time signal, whereas Post-FFT steps use correlation between the frequency signal and the pilot carriers sequence.
- Guard interval disposal: the cyclic prefix is removed.
- OFDM demodulation: this is achieved with an FFT.
- Frequency equalization: the pilot signals are used to estimate the Channel Transfer Function (CTF) every three subcarriers. The CTF is derived in the remaining subcarriers via interpolation. The CTF is then used to equalize the received data in each subcarrier, generally using a Zero-Forcing method (multiplication by CTF inverse). The CTF is also used to weigh the reliability of the demapped data when they are provided to the Viterbi decoder.
- Demapping: since there are Gray-encoded QAM constellations, demapping is done in a "soft" way using nonlinear laws that demap each bit in the received symbol to a more or less reliable fuzzy value between -1 and +1.
- Internal deinterleaving
- Internal decoding: uses the Viterbi algorithm, with a traceback length larger than that generally used for the basic 1/2 rate code, due to the presence of punctured ("erased") bits.
- External deinterleaving
- External decoding
- MUX adaptation
- MPEG-2 demultiplexing and source decoding
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