Advantages of Digital Broadcasting
Digital broadcasting is more than superb picture and sound quality, high definition picture, wide screen format, multichannel sound and more choice. There is one key factor that has significantly contributed to the success of digital broadcasting. This key property is the possibility that all transmitters within some area can transmit on the same frequency. This kind of operation is called a Single Frequency Network (SFN) and it significantly contributes to the efficient use of radiofrequency spectrum.
Most digital broadcasting transmission standards use COFDM - Coded Orthogonal Frequency Division Multiplex, a multi-carrier modulation scheme. The number of carries varies from 2k or 8k carriers in DVB-T system to many ten thousand carriers in the latest standards. The orthogonal frequency spacing of sub-carriers avoids inter-carrier interference, which permits each of the many thousand sub-carriers to carry from two bits of data in QPSK to eight bits in 256QAM. The “Coded” means that the transmitted data contains actual data and additional information for protection (FEC - Forward Error Correction).
Because of finite speed of radio waves at each reception point the propagation delay from various transmitters is different. Therefore, without some mechanism there would be interference receiving digital data from more than one transmitter operating on the same frequency.
The first part of the solution is that each subcarrier transmits its payload (symbol) during an elementary period of several hundred microseconds which is much longer than the multi-path propagation delay. The second part of the solution is that symbol time is prolonged with a “cyclic prefix”. This is called a guard interval.
During this guard interval time the same symbol with different reception times from different transmitters can be received without any interference. This is true until the difference in propagation delays is greater than the guard interval. This also means that short time echoes have no destructive effect on the reception. The length of the guard interval defines the size of the area where we can use SFN network. It is usually measured in tens of kilometers.
In order to operate within a Single Frequency Network, transmitters must transmit the same data and must be synchronized to transmit the same symbol at any time. The later is achieved by inserting synchronization packets into the transport stream. This allows each transmitter to wait until the indicated time to start broadcasting the particular packet. This time-synchronization method allows equalization of the distribution time through the network used to deliver the transport stream to each transmitting site.
The frequency of transmitters operating in SFN network must also be synchronized. Usually it is done with a GPS frequency and time reference. This allows the network to reach the accuracy and stability needed for SFN synchronization - better than 1 Hz in the frequency domain and 1 microsecond in the time domain.
Using single frequency networks means using only
one frequency in particular area. This significantly reduces the number of
frequencies needed to cover arbitrary territory. How important is this factor is evident, for example, near Italian border where a special agreement for dividing UHF channels has been developed.
Of course, the cost of the network is higher and usage of SFN repeaters (gap fillers) is very limited, but the overall gain of SFN networks is huge. The price we have to pay is lower than the price of the most valuable part of the system – the frequency.