Keywords

5.1 Concept of Multiplexes

The concept of the multiplex includes:

  1. 1.

    The cumulative network of programs and services that use a common frequency block.

  2. 2.

    The common frequency block modulated as the OFDM signal, in the Specifications referred to as the ensemble signal. This signal contains multiplexed programs and services organized in subchannels – so the term used simultaneously is the multiplex signal.

  3. 3.

    The common coverage area of multiplex signal of transmitters operating in the Single Frequency Network (SFN).

A sketch of a centralized organization of multiplex with its distinguished elements is shown in Fig. 5.1.

Fig. 5.1
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Scheme of transport of multiplex programs and services from the supplier to the transmitters

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Signal of multiplex is created by programs and services that use a common block of frequencies and the joint broadcasting network. The decisive element in the organization of multiplex signal is the multiplexer (Sect. 2.8). A multiplexer is a device linking signals of programs and services of different operators, with different bit rates, in the main signal in the transmission channel, i.e., the Main Service Channel. The aggregated output signal of multiplexer is fed to transmitters of the single-frequency network, one or several SFNs. In each transmitter, the logical signal of multiplex undergoes the OFDM modulation and, after synchronization with an entire network, is jointly broadcast [1].

The individual subnets of multiplex can be organized in a flexible manner.

  • Ad. 1. The bandwidth of the DAB+ frequency block (1.536 MHz) requires inclusion of several parallel programs at the same time. Delivery of individual programs and services to the DAB server may be realized through different telecom links. Description of the service transmission to DAB server is presented in Chap. 8. Ensemble operator collects programs of individual operators along with the parameters of transmission closer discussed in Chaps. 6 and 7.

  • Ad. 2. The multiplex organization, aside from the centralized form shown in Fig. 5.1, may be configured in the dispersed form, with the intermediate multiplexes connecting portion of services or programs [2].

  • Ad. 3. To bring the multiplex signal to the transmitters of each single-frequency network (SFN), different telecommunication links can be used. Transport organization of multiplex signal is described in Sect. 9. Signal processing in the convolutional encoder and the channel modulator OFDM takes place in individual transmitters. The aim of such solution is to avoid transmission of the incremented signal rate of the redundant code of convolutional encoder.

Planning of multiplex is connected with:

  • Internationally agreed blocks of the DAB frequencies

  • The authorized coverage areas of SFN networks following the cross-border arrangements

  • Compatibility with existing broadcasting services

  • Adjacent channel issues

  • Inter-block interference

  • Re-use distance for T-DAB allotment plan [3]

5.2 Planning the Coverage Area for Multiplexes

Unlike the channels in the analog broadcasting, the fundamental frequency units in system DAB are the frequency blocks of 1.5 MHz. Designing the digital radio in Europe started with the adaptation of a grid of 4 frequency blocks as 4 quarters of a single television frequency channel [4].

In its initial proposal, any multiplex can be assigned a coverage area from the proposed hypothetical grid. The four frequency blocks of multiplexes can be arranged in a checkerboard pattern of non-overlapping areas named A, B, C, and D, as in Fig. 5.2. Such division can gradually cover any area.

Fig. 5.2
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The initial proposal of multiplexes A, B, C, and D assignment to the geographical regions

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Fig. 5.3
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An example of an international plan of allocation blocks in L-band (DMB “Eureka,” in February 2013, according to working Report ECC 188)

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Allocation of additional TV channels for digital radio offers the possibility of further developing DAB multiplex networks. Based on this assumption, the initial allocation of frequency blocks for each country is adapted to the real conditions considering:

  • Limitations resulting from the international agreements

  • Limitations resulting in the bilateral cross-border arrangements between neighboring countries

  • Compatibility with other telecommunication systems within a country

The arrangements of the ITU Conference in Wiesbaden (1995) revised in Maastricht (2002) and Constanta (2007) granted to system DAB the frequency bands in VHF: 174–230 MHz and 230–240 MHz [5].

The frequency range of transmitters for terrestrial broadcasts is located in a technically reasonable limit

$$ 174\hbox{--} 240\ \mathrm{MHz}\ \left(\mathrm{VHF}\ \mathrm{range}\right) $$

tunable every 16 kHz.

Unlike in the analog radio, where the basic concept is the frequency channel corresponding to one program, in the DAB digital system, the basic units are the blocks of frequency size 1.536 MHz. The gross bit throughput of each block is 2304 kbit/s.

In one frequency block simultaneously can be emitted about ten programs in the DAB, twice more programs in the DAB+ system, and additionally some added information.

The number of programs transmitted in the block may be different. It depends on the compression ratio of sound channels and capacity of the value-added services. Certain types of digital information at the receiver can be presented as a correlated object. Such correlated components together form a multimedia object. The individual components are called monomedia .

The main carrier of programs and information in the stream of DAB logical frames is the logical main service channel (MSC). The information on division and organization of MSC is included in the logical Fast Information Channel (FIC), decoded at the receiver about 384 milliseconds earlier than the main service. This is due to omission of the time interleaver of logical frames in the FIC channel, which delays the transmission process. Adoption of constant efficiency 1/3 for convolutional encoder in the FIC channel secures robust transmission. A faster transmission in the FIC channel is required in order to prepare the demultiplexer to the deconvolution of frames of individual programs and the frames of digital information.

Basic parameters of the output DAB+ frames are presented in Table 5.1.

Table 5.1 The DAB+ signal mode I parameters [1]
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A single transmitter covers with the program the local area. If the program is to cover a larger area, the coverage requires a network of transmitters. The most economical solution, from point of view of the optimal application of the spectrum, is a single-frequency network (SFN) [6]. Such network requires the transmission of formatted DAB multiplex signal (DAB ensemble) to each of the transmitters in a coordinated, synchronized way. Since transmission costs increase with throughput, it is appropriate to transfer signal without redundancy generated by the convolutional encoder. The transmission network protocols guarantee its own signal security. For these reasons, the convolutional encoder with redundant bits has been moved from the DAB server to the DAB transmitters. In each of the DAB transmitters of the SFN, the multiplex signal is encoded in accordance with the given set of parameters and then modulated, amplified, filtered, and emitted by the antenna. New data on the SFN networks with examples of implementation are included in the papers [7].