Grundig Digital Radio manual Channel coding and modulation

DIGITAL RADIO GUIDE TERRESTRIAL TRANSMISSION SYSTEMS - DRM

FAC provides information on the signal bandwidth and other such parameters, and is also used to allow service selection information for fast scanning. The SDC gives information to a receiver on how to decode the MSC, how to find alternative sources of the same data, and gives attributes to the services within the multiplex.

The MSC multiplex may contain up to 4 services, any one of which can be audio or data. The gross bit rate of the MSC is dependent on the channel bandwidth and transmission mode being used. In all cases, it is divided into 400 millisecond frames.

The FAC’s structure is also built within a 400 millisecond frame, and is designed without interleaving, for example, to ensure rapid delivery of the information it contains. The design without interleaving is also to ensure fastest decoding of basic data by the Rx before it can do the audio decoding. The channel parameters are included in every FAC frame segment. The service parameters are carried in successive frames, one service per frame. The names of the FAC channel parameters are: base/enhancement flag, identity, spectrum occupancy, interleaver depth flag, modulation mode, number of services, reconfiguration index, and reserved for future use. These use a total of 20 bits. The service parameters within the FAC are: service identifier, short identifier, conditional access, language, audio/data flag, and reserved for future use. These use a total of 44 bits.

The SDC’s frame periodicity is 1200 milliseconds. The fields of information are: multiplex description, label, conditional access, frequency information, frequency schedule information, application information, announcement support and switching, coverage region identification, time and date information, audio information, FAC copy information, and linkage data. As well as conveying these data, the fact that the SDC is inserted periodically into the waveform is exploited to enable seamless switching between alternative frequencies.

(5)Channel coding and modulation

The coding/modulation scheme used is a variety of coded orthogonal frequency division multiplexing (COFDM), which combines the OFDM with the Multi-Level Coding (MLC) based upon convolutional coding. The convolutional coding provides a level of error protection. These two main components are supplemented by time interleaving (“scrambling” of the bit stream) and the provision of pilot (predetermined value) cells for instantaneous channel estimation. All of this mitigates the effects of short-term signal fading, whether selective or flat.

Taken together, this combination provides excellent transmission and signal protection possibilities in the narrow 9 or 10 kHz channels in the LF, MF and HF broadcasting frequency bands. It can also be used for “multi-channel” DRM use; that is 18 or 20 kHz channels, using 2 contiguous ITU-R channels. This level of bandwidth will permit good stereo broadcasting.

For OFDM, the transmitted signal is composed of a succession of symbols, each including a “guard interval,” which is a cyclic time prefix that provides a “dead time” to counter intersymbol interference due to multipath delay spread. Orthogonality refers to the fact that, in the case of the design of the DRM system, each symbol contains around between 100 and 200 subcarriers spaced evenly across the 9 or 10 kHz channel in such a way that their signals do not interfere with each other (are orthogonal). The precise number of subcarriers, and other parameter considerations, are a function of the actual letter modes used: ground wave, sky wave, and highly robust transmissions.

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