Digital Video Broadcasting (DVB); User guidelines for Digital ...

producing an output bit-stream at 8.16 kbit/s, 2×8.16 kbit/s, 4×8.16 kbit/s, respectively. The multiplex
provides a signalling byte which indicates the multiplex configuration to the receiver
Modulation and channel coding
In order to achieve high ruggedness against noise and interference Direct-Sequence Spread-
Spectrum (DS-SS) processing is applied before Quaternary Phase Shift Keying (QPSK) modulation,
generating a modulated signal whose bandwidth occupation is expanded and whose power spectral
density level is reduced accordingly. DS-SS technique permits the superposition of a number of coordination
signals in the frequency domain (Code Division Multiple Access, CDMA), using the same
centre frequency. For example the scenarios in Figure 1 may be efficiently implemented by using this
technique.
For system simplicity, the spreading processes are asynchronous at each terminal, therefore the
number of channels which may be superimposed is limited by mutual interference. Compared to
conventional modulations, DS-SS techniques offer significant performance improvements in the
presence of interferences (e.g. from and to co-channel narrow-band signals) and also produce less
intermodulation noise density over a non linear transponder . DS-SS signals also require less
frequency precision in the transmission/reception equipment. In figure the basic principle of Direct-
Sequence Spread-Spectrum coding is illustrated.
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