Complete Communications Engineering

This system transmits an MPEG-2 or MPEG-4 family digital audio/video stream using an orthogonal frequency-division multiplexing (OFDM) modulation with channel coding through terrestrial channels. In DVB-T OFDM, modulation can work in two possible modes: 2K and 4K based on the size of the OFDM transform.

DVB-T Transmitter

Fig 1 DVB-T Transmitter

The input signal is a sequence of standard MPEG transport stream packets. Each packet consist of 288 bytes. First that sequence is scrambled for energy dispersion and packet synchronisation is modified. After that it goes through a Reed-Solomon encoder and 16 bytes for the error protection are added to the each packet. The individual packet length becomes 304 bytes. Then the modified packets go through a convolutional interleaver with depth 12 and a convolutional error correction coder.

The system allows for a range of punctured convolutional codes based on a mother convolutional code of rate 1/2 with 64 states. This allows selection of the most appropriate level of error correction for a given service or data rate. The generator polynomials of the mother code are G1 = 171OCT and G2 = 133OCT. The system uses puncturing of the mother coded signal. The puncturing code rates are 2/3, 3/4, 5/6 and 7/8. Next, inner interleaving is applied. The inner interleaving consists of bit-wise interleaving followed by symbol interleaving.

The input of the bit-wise interleaver, which consists of up to two bit streams, is demultiplexed into v sub-streams, where v = 2 for QPSK, v = 4 for 16-QAM, and v = 6 for 64-QAM. The demultiplexing is defined by the mapping algorithm. Each sub-stream from the demultiplexer is processed by a separate bit interleaver. There are up to six interleavers depending on v.

Bit interleaving is performed only on the useful data. The block size is the same for each interleaver, but the interleaving sequence is different in each case. The bit interleaving block size is 126 bits, so the block interleaving process is repeated exactly twelve times per OFDM symbol of useful data in the 2K mode, and forty-eight times per symbol in the 8K mode.

Symbol interleaver maps v bit words onto the 1512 (2K mode) or 6048 (8K mode) active carriers per OFDM symbol.

The symbol interleaver output is mapped into QAM I and Q symbol constellations.

The transmitted signal is organized in frames. Each frame has a duration of TF, and consists of 68 OFDM symbols that are numbered from 0 to 67. Each symbol contains 1705 carriers in the 2K mode and 6817 carriers in the 8K mode. The symbol duration (TS) consists of two parts: a useful part (TU) and a guard interval (Δ). The guard interval contains a cyclic continuation of the useful part and is inserted before it. Four values for the guard interval may be utilized:

Δ = TU/4; TU/8; TU/16; TU/32.

One super-frame consists of 4 frames.

All symbols contain data and reference information. Since the OFDM signal comprises many separately-modulated carriers, each symbol is divided into cells, each corresponding to the modulation of one carrier during one symbol.

Additional information is inserted into data frames as scattered pilot cells, continual pilot carriers, or Transmission Parameter Signalling (TPS) carriers. The insertion pattern depends on the particular frame number.

The pilots can be used for frame synchronization, frequency synchronization, time synchronization, channel estimation, transmission mode identification and can also be used to follow the phase noise.

After additional information insertion, the OFDM transform occurs. OFDM transform is a discrete inverse Fourier transform. Then guard interval is inserted for each symbol.

The sequence of OFDM symbols are shifted in frequency to the desired channel band and transmitted.

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