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Single Frequency Networks (SFN) in Digital Terrestrial Broadcasting

Date:2019/10/18 9:56:15 Hits:

The transmission channel (path) for the terrestrial TV broadcasting is commonly and rightfully considered as the worst quality channel. Terrestrial transmission channel is asubject to many influences - additive noise and other disturbing signals (namely in the municipal and industrial agglomerations), signal echoes – so-called multipath reception. There are mainly the many echoes which mostly influence the quality of received signals. This effect leads to location- and frequency -selective fading. 

After a reflectionfrom natural and other objects, one or more variously delayed signals (echoes) come to the receiving antenna. These time delayed signals cause severe degradation of a received television signal and corresponding image particularly in the analogue television, where additional images appear shifted in the scan direction – so-called“ghosts”. In digital terrestrial television broadcasting, the effects of multipath reception were largely suppressed by choosing of sophisticated modulation methods. One of many advantages of the emerging terrestrial digital video broadcast standards DVB-T (Digital Video Broadcasting – Terrestrial defined in ETS 300744), DVB-H (Handheld), but also next services - e.g. digital audio broadcasting DAB (Digital Audio Broadcasting) or DRM (Digital Radio Mondial) is, apart from mobile reception support, the suppression of multipath reception impacts. In the all mentioned standards, digital modulation method (C)OFDM(Coded Orthogonal Frequency Division Multiplex) is used. The first symbol C in the abbreviation means, that the data stream is protected by the error correcting encoding FEC (Forward Error Correction) to detect and correct errors that occur during the transmission.

 For symbol protection,a block Reed-Solomon code is used, and for bit protection, a convolutional code with different code rates is employed. Modulation (C)OFDM is characterized by high robustness against inter-symbol interference (ISI), which would threaten the received signal and increase their error rates due to the multipath reception. Digital broadcasting in the above mentioned standards can be performed in so-called single frequency network SFN (Single Frequency Network). Reception of more delayed signals from several transmitters working in the single frequency network can be utilized even for improvement of the power efficiency of transmitters.

The application of (C)OFDM modulation is an effective, most frequently used, but only possible tool to eliminate the impacts of multipath reception. A different approach is for instance multi-sensor reception with the angularly oriented system of reception antennas and consequent complex signal processing including filtering, sampling, base-band conversion followed by location and separation of different signal sources (so-called Array Processing). A more detailed overview is beyond the scope of this paper and can be found in [4], for instance.

Principle of the SFN
The signal coverage of a certain area can be provided by a number of transmitters, transmitting the multiplex of digital television or radio signals in the identical frequency channel. Their partial signal contributions in the reception point not only do not interfere, under certain circumstances then even improve the reception. It is thus obvious that single frequency networks of digital transmitters may considerably improve the utilization of frequency bands and channels as well as energy balance of digital transmitters. Digital transmitters in SFN might be considerably lesser power for the signal coverage of the given area sufficient for quality reception. Methods of SFN cannot be used with the terrestrial analogue\ television broadcasting, where in fact all the present-day world’s television standards use amplitude vestigial sideband modulation and operate in
multi-frequency networks MFN (Multi Frequency Network).

Single frequency networks can be built only in a limited area, not over a whole country – even as small as the Czech Republic. Wherewith the SFN size is affected actually? Let us assume that in the analyzed SFN area:
• a number of transmitters DVB-T operate,
• all transmitters operate at the same frequency,
• these transmitters operate with the same and exact time synchronous digital data multiplex,
• the level of received signals anywhere in the SFN area reaches leastwise the threshold limit value (the level, which DVB-T receiver needs to able to demodulate and decode the signal properly). 

Time Synchronization of Transmitters in SFN
In a single frequency network all the individual transmitters must be exactly time synchronized. Every transmitter must broadcast absolutely identical OFDM symbol at the same time. The DVB-T modulation is structured in frames, one frame being composed of 68 OFDM symbols. Four frames make up one so-called super-frame and two super-frames make up so-called mega-frame (in the mode 2k four super-frames). With regard to different time duration of the symbol OFDM, which depends on the parameters of used modulation and encoding (mode i.e. number of carriers, code rate, guard interval etc.), the time duration of single frame can be different, too. 

Time synchronization of the all transmitted packets in the transport stream of the final data multiplex is ensured by the time signal 1 pps (pulse per second), which is acquired from the GPS system. This signal controls time synchronous insertion of the special packet MIP (Megaframe Initialization Packet) into the transport stream at the beginning of every mega-frame. Transport stream MPEG2, generated for example in the playout center (TV studio), can be carried to the individual transmitters by the diverse distribution networks (via satellite, microwave line, optical fibre, ATM networks) with different time delays. Therefore the time synchronization by the GPS signal is performed again in each of transmitters. A result of this operation is the state, whereat every DVB-T transmitter broadcasts identical OFDM symbols at exactly the same time. 

SFN Gain
Power contributions from the individual transmitters working in the single frequency network add. Therefore the single frequency network shows so-called SFN gain. This gain can be simply formulated as follows – two DVBT transmitters with the broadcast power Pv ensure in the same conditions (the same directivity and antenna gain) better signal coverage (greater values of the field intensity) in the receiver position than a single transmitter with the double broadcast power 2Pv. Quantitative expression of the SFN gain, which depends on the receiver position and on many other factors, is beyond the scope of this paper. 

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