DVB-T2 - Technical Details

Technical Details

The processing workflow is as follows:

• Input pre-processing
  • Physical Layer Pipe (PLP) creation: adaptation of Transport Stream (TS), Generic Stream Encapsulation (GSE), Generic Continuous Stream (GCS), or Generic Fixed-length Packetized Stream (GFPS)
• Input processing
  • Mode adaptation
    • Single PLP (mode 'A'): data are assembled in groups called BaseBand Frames (BBFRAMEs), with lengths of bits, defined by modulation and coding (MODCOD) parameters, in a 'normal' length or 'short' length version
      • Input interface
      • CRC-8 encoding
      • BaseBand (BB) header insertion
    • Multiple PLPs (mode 'B')
      • Input interface
      • Input stream synchronization
      • Delay compensation
      • Null packets deletion
      • CRC-8 encoding
      • BB header insertion
  • Stream adaptation
    • Single PLP (mode 'A')
      • Padding insertion
      • BB scrambling: a Pseudo Random Binary Sequence (PRBS) with generator is used to scramble completely every BBFRAME
    • Multiple PLPs (mode 'B')
      • PLP scheduling
      • Frame delay
      • In-band signaling or padding insertion
      • BB scrambling
• Bit Interleaved Coding and Modulation (BICM)
  • Forward Error Correction (FEC) encoding: each BBFRAME is converted into a FECFRAME of bits, by adding parity data. Normal FECFRAMEs are 64,800 bits long, whereas short FECFRAMEs are 16,200 bits long. The effective code rates are 32,208/64,800 (1/2), 38,688/64,800 (3/5), 43,040/64,800 (2/3), 48,408/64,800 (3/4), 51,648/64,800 (4/5), 53,840/64,800 (5/6)
    • Outer encoding: a BCH code, capable to correct 10 or 12 errors per FECFRAME, is used to compute parity data for the information data field. The BCH generator polynomial is of the 160th, 168th, or 192nd grade
    • Inner encoding: a Low Density Parity Check (LDPC) code is cascaded to the BCH
  • Bit interleaving
    • Parity bits block interleaving
    • Twist column interleaving
  • Bit demultiplexing to cell words
  • Gray mapping of cell words to constellations: either QPSK (4-QAM), 16-QAM, 64-QAM, or 256-QAM maps are used
  • Constellation rotation and cyclic quadrature (Q) delay: optionally, the constellations may be tilted counterclockwise by an amount of up to 30 degrees. Furthermore, the quadrature (imaginary) part of the cells is cyclically shifted by one cell
  • Cell interleaving
  • Time interleaving
• Frame building: the transmitted stream is organized in super frames, which are composed by T2 frames and FEF (Future Extension Frame) parts
  • Cell mapping: cells are mapped to OFDM symbols. A T2 frame is composed by a P1 symbol, one or more P2 symbols, regular data symbols, and a Frame Closing symbol (for certain configuration parameters). The P1 symbol is used for synchronization purposes, the P2 symbols convey L1 parameter configuration signaling, whereas the data symbols carry PLP data (there are three types: common PLPs, type 1 PLPs, and type 2 PLPs), auxiliary streams, and dummy symbols used as space filler
  • Frequency interleaving: random interleaving is done on every OFDM symbol (except P1)
• OFDM generation
  • Multiple-Input Single-Output (MISO) processing: Alamouti pre-processing is optionally applied to pairs of OFDM symbol cells. Given the input cells, and transmitter group 1 and 2 cells, the mapping is done as and for group 1, and as and for group 2
  • Pilot insertion and dummy tone reservation: three classes of pilot tones are added. They are either continual (fixed position), scattered (cyclically moving position), or edge (boundary positions). There are 8 different configuration for scattered pilots (PP1 ... PP8). Moreover, a number of dummy carriers are not modulated and reserved to reduce the dynamic range of the DVB-T2 output signal (it helps to combat nonlinear phenomena in power amplifiers during broadcast).
  • Inverse Discrete Fourier Transform (IDFT): classic IDFT is used to switch from the frequency domain into the time domain, after having adjusted carrier position relevant to the central transmit frequency. 1k (1024) to 32k (32768) carriers are available. There is also an extended mode, which allows to fill more data in the available bandwidth, using more active carriers and reducing the number of guard band (null) carriers.
  • Peak-to-Average-Power-Ratio (PAPR) reduction
  • Guard interval insertion: a cyclic prefix is inserted before the IDFT symbol, to recover from transmit channel echoes (multipath). Lengths from 1/128 to 1/4 of the IDFT length are allowed.
  • P1 symbol insertion: the P1 symbol is a particularly crafted 1k OFDM symbol, always inserted at the head of a T2 frame. It conveys few bits of information (spread, scrambled and DBPSK modulated), as it is mainly dedicated to fast synchronization (both in time and in frequency) at the receiver side. It is prepended and postpended by frequency shifted repetitions of itself, to ease receiver lock even if the nominal center frequency of the T2 signal is up to 500 kHz off.
  • Digital-to-Analog Conversion (DAC): the T2 samples are converted into an analog BB complex (I&Q) signal at a sample rate that depends on the channelization bandwidth. For instance, in 8 MHz wide channels, the complex sample time is 7/64 μs.