Digital DX

marluck

- Un articol explicativ deosebit de plăcut din numarul 1103 al revistei Tele-satellite International care explica precis toate calitatile si neajunsurilor transmisiilor terestre digitale SFN si MFN . Extraordinar de exhaustiv si bine venit in aceste clipe cand se efectueaza tranzitia de la analog la digital terestru atat in tara noastra cat si in tarile invecinate .

Single - and Multi Frequency Networks in Digital Terrestrial Television

Jacek Pawlowski

- Before the digital era, the analog TV transmitters located in neighboring areas had to use different frequencies to avoid interference. It is intuitively understandable that using the same frequency to transmit channel A from transmitter 1 and channel B from transmitter 2 is not a good solution. There would always be a location where the reception antenna would not only receive the desired signal (say, channel A) but also the undesired signal. The undesired signal (channel , even if transmitted from a completely different direction, could interfere because of electromagnetic wave reflections or scattering on various obstacles (like buildings, masts, etc.). So the reception antenna properly directed to receive channel A from transmitter 1 has always a chance to pick up some signal from transmitter 2. But what if the two transmitter broadcast the same channel ? If they use the same frequency to transmit the same analog channel, the reception antenna could again receive signals from both transmitters simultaneously. In practice, one of the signals would always be delayed in relation to the other one. That’s because the distance between the reception antenna and any of the transmitters would be different. Time delays cause phase shifts what in turn leads either to constructive or destructive interference.

Fig. 1. Different signal paths in terrestrial Tv :
[url=http://www.99poze.ro/p/54ae7a486150bbac2aa95055aef149b9.jpg] Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT

Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT

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This would lead to signal fading. Additionally, due to time delays the “ghost” effect would be visible on the TV screen. The multipath effect resulting from rejections and scattering also would take place. Since analog TV used simple amplitude modulation for video, there was no other way to avoid interference but to ensure that the transmitters which cover overlapping areas had to use different frequencies. The same frequency channel is re-used only in regions separated by a large distance, to avoid harmful co-channel interference. Such arrangement is called Multi Frequency Network or MFN. In COFDM modulation digital data is not transmitted continuously but with pauses among the symbols. These pauses are called the guard intervals. The guard interval should be chosen long enough to allow all symbol echoes to reach the receiver. So that when the next symbol comes there are no longer echoes from the previous one. If all delayed signals (symbol echoes) come to the receiver within the guard intervals, it is possible to process the signals so smartly to avoid fading. DVB-T permits to choose one of the following values for guard interval: 1/4, 1/8, 1/16, 1/32. The longer the interval the more echoes we can cancel but less useful data we can send in the same unit of time. To compensate for that, the number of carriers must be increased. If such digital signal is transmitted from two transmitters on the same frequency, the signal originating from the more distant transmitter is seen as a long delay echo. And this is the important factor for designing single frequency network (SFN): if we used powerful transmitters located far away from one another, we would have long echoes – at least in some areas. Therefore, it is usually assumed that SFN requires a dense grid of small power transmitters. For a medium sized European country like Poland, it is assumed that some 350 transmitters with 40 m high antenna masts would be required. Creating such dense grid of transmitters is costly and takes time.

Fig. 2. In a Multi-Frequency Network the neighboring transmitters must use different frequencies :
[url=http://www.99poze.ro/p/9d6ddbe49ac6ebbd8a869601148a3121.jpg] Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT

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- On the other hand, MFN can use the existing infrastructure of analog transmitters and antennae. In such configuration, the channels are transmitted on different frequencies and we do not have to care about the echoes from neighboring transmitters. We still have echoes resulting from the same transmitter (self interference) because there are still signal rejections from different objects. But those echoes are shorter and do not pose serious problems for COFDM modulation. Another advantage of MFN is the fact that it is convenient to use it in the interim period when analog and digital TV are broadcast concurrently. That’s because the viewers do not have to rotate their antennae to catch new digital signal nor do they have to install new antennas. Analog and digital signals come from the same location. In SFN arrangements one multiplex is transmitted on the same frequency all over the country. So SFN has one tremendous advantage over MFN – it spares a lot of precious bandwidth released after the analog transmission is completely switched off. This bandwidth can be either used to transmit many more digital channels or used for other purposes. Except for better spectral eficiency described above, SFN has better power eficiency. In other words, the electric energy consumed by all SFN transmitters is lower than that in MFN system with the same reception area. SFN uses more transmitters, but these are the low power units. SFN is also much more desired for DVB-H signals, especially if SFN is built with a dense grid of small power transmitters. Such grid creates much more uniform signal level what is important for reception on the move. Moreover, if we use the same frequency, the receiver can switch from cell to cell smoothly (handover). For MFN networks a second tuner is required in the receiver to check signals from the neighboring cells. SFN is also the better choice if we need to ensure the reception inside the buildings or in those places were it is difficult or impossible to install a roof-top antenna. It is not difficult to figure out that SFN is more demanding from technological point of view. It requires very good synchronization of all transmitters both in time domain and in frequency domain. Usually, GPS is used to synchronize all transmitters. Technology experts when comparing both solutions sometimes overlook non-technical aspects. For example, in some countries there are many regions that transmit local content to local viewers. Such local broadcasts are of little interest to the people living at the other side of the country. One can not split networks into regional or local area networks with different content using SFN. This is particularly a challenge for geographically large countries. Practically, SFN network requires everything be transmitted throughout the whole country.

Fig. 3. SFN uses a grid of low power transmitters for more uniform Field strength :
[url=http://www.99poze.ro/p/51fc47d6440068ee727ae0f471017cb3.jpg] Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT

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- As usually, it is not so easy to say which arrangement is better. It depends on the circumstances in a given country. One of the reasonable approaches could be to use MFN in the intermediate period and once the digital switch over is over and analog channels no longer exist, start building SFN. SFN will not only release bandwidth but also enable DVB-H reception almost everywhere. To become really popular, portable DVB-H devices should be operational everywhere where a mobile phone is used today. There are even some proposals to use the existing GSM infrastructure for DVB-T/H SFN transmitters. Another approach, justified especially in large countries, could be a mixed solution: MFN and SFN together. In this way, one could have different local multiplexes delivered to different regions, and nationwide digital multiplexes delivered all over the country. Before we conclude this article, a word about North America. COFDM modulation has been chosen for the terrestrial transmissions by DVB and ISDB, but the American standardization body ATSC has opted for 8VSB. Although 8VSB has not been designed with SFN in mind, this modulation is also good in ghost cancellation. So, at least theoretically, it is also possible to build SFN networks in North America.

Table 1. A comparison of pro’s and con’s of SFN and MFN networks :
[url=http://www.99poze.ro/p/bd6f4dfc7c232af67e5a0a05b484839e.jpg] Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT Retele digitale terestre Tv SFN si MFN - Detalii diverse DTT

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- Spor la studiu ... :-)

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