News 155 - Rohde _ Schwarz by langkunxg


									DVB-T, the new terrestrial TV standard
In February 1997 the voting procedure       first, and is expected to be taken up in    all other DVB specifications, the terres-
for the new terrestrial European Telecom-   other parts of the world too.               trial broadcasting standard is based
munications Standard ETS 300 744 (in                                                    on the requirements of potential users.
short DVB-T) ended with a unanimous         The basic technical specifications
vote*. Thus, within about two years, a      were prepared by the European DVB
                                                                                        * ETS 300 744: Digital broadcasting systems for
basis has been created for the terres-      Project and in particular by its Tech-
                                                                                        television, sound and data services; Framing
trial television of the future. This new    nical Module, in which Rohde &              structure, channel coding and modulation for
standard will be applied in Europe          Schwarz is taking an active part. Like      digital terrestrial television. March 1997

                                                                            News from Rohde & Schwarz   Number 155 (1997/III)      31

The DVB-T standard is envisaged to                        • Multiplex adaptation and energy dis-          • Pilot and TPS signals (transmission
fulfill the following requirements:                         persal: multiplex adaptation through            parameter signalling): insertion of
• Trade-off between transmission ca-                        appropriately standardized inter--              pilot carriers, some of which carry
    pacity and coverage range should                        faces, inversion of every eighth                the transmission parameters in cod-
    be possible.                                            sync byte, energy dispersal for                 ed form as modulation signals.
• The implementation of large-area                          even power distribution in the trans-         • OFDM (orthogonal frequency-divi-
    and local single-frequency networks                     mission channel.                                sion multiplex): calculation of IFFT
    should be possible.                                   • Outer coder: coder with shortened               with 2048 or 8192 carriers, with
• The system should allow transmis-                         Reed-Solomon code (204,188, t = 8),             1705 or 6817 of which containing
    sion of data containers irrespective                    capable of correcting up to eight               data or transmission parameters,
    of their actual contents (eg com-                       errored bytes in an MPEG2 trans-                1512 or 6048 being exclusively re-
    pressed video or audio data).                           port-stream packet.                             served for payload data.
• The system should be robust and                         • Outer interleaver: interleaver with           • Guard interval insertion: insertion
    allow adjacent-channel operation of                     a depth of I = 12 for dispersal                 of a guard interval for the OFDM
    analog and digital TV signals in a                      of burst errors to different packets            signal, permissible intervals are
    densely occupied frequency band.                        so that higher probability of suc-              1/4, 1/8, 1/16 or 1/32 of symbol
• The transmission standard should                          cessful error correction is achieved.           duration.
    be available well in time to enable                   • Inner coder: punctured convolu-               • Digital /analog conversion: conver-
    network operators wishing to adopt                      tional code with code rates of                  sion of digital into an analog signal.
    the standard to start operation still                   1/ 2, 2/3, 3/4, 5/6 and 7/ 8.                 • Front end: conversion to output
    in 1997.                                                                                                channel, usually in UHF range.

                                                                                                                                          To antenna

       Program   Transport
         MUX       MUX
  Video                                    MUX
                                                               Outer              Inner               Frame               Guard
                                        adaptation,   Outer              Inner
  coder                                                         inter-            inter-   Mapper    adapta-    OFDM     interval   D/A   Front end
                                          energy      coder              coder
                                         dispersal             leaver            leaver                tion             insertion

  Audio          1
                 2                                             Outer                                 Pilots &
                                        adaptation,   Outer              Inner
  Data                                    energy
                                                                inter-                                 TPS
                 n                                    coder              coder
  coder                                  dispersal             leaver                                signals

Functional block diagram of system (terrestrial channel adapter)

The DVB-T standard contains elements                      • Inner interleaver: combination of             For the socalled hierarchical modula-
of the standards for satellite and cable                    bit interleaving and symbol inter-            tion, two sets of the forward error-
transmission. The input signal, for in-                     leaving.                                      correction blocks are required and
stance, is defined in all cases as an                     • Mapper: allocation of information to          the MPEG2 transport stream is split up
MPEG2 transport stream. For error                           the individual carriers and constella-        accordingly.
protection too, well-proven techniques                      tion points; permissible modes are
have been resorted to. In contrast to                       QPSK (quadrature phase-shift key-             Rohde & Schwarz pre-series DVB-T
other systems, a multicarrier transmis-                     ing), 16QAM and 64QAM (quad-                  modulators successfully passed their
sion method was chosen for adaptation                       rature amplitude modulation), Gray            first interoperability tests and are
to the terrestrial transmission channel                     mapping.                                      at present being used in laboratory
with its multipath propagation. Chan-                     • Frame adaptation: providing syn-              and field tests in cooperation with
nel coder and modulator contain the                         chronization to frame structure of the        customers.
following functional blocks (FIG):                          signal.                                                         Dr Jürgen Lauterjung

32        News from Rohde & Schwarz         Number 155 (1997/III)

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