Terrestrial Digital television - DVB-T (Modulación COFDM)
The majority of the times, our antenna “looks” at the repeater and “only” we have a way between emitter and receiver. Other times, mainly in cities, the antenna is oriented correctly but the signal can arrive to him from two sites (one direct and another one, for example, by bounce in a building). In this one case one “double image” took place, since the receiver dealed with the two signals with a slight difference time (the bounced signal arrived something later).
Interval of Guard.
|If in transmission COFDM we concentrated in two neighboring symbols, for example symbol n and symbol n+1 and within the time domain, first we transmitted symbol n and when we have finished, we began to transmit the symbol n+1. By the circumstance previously set out and reflected in the image, the receiver receives the symbols so many times as trajectories arrive. First in arriving, it will be evidently direct between emitter and the receiver. While the receiver takes the symbol and it demodulates it, the same symbol arrives (that came by another way) giving rise at interferences. With the purpose of to avoid this type of interferences, in the emitter a time interval is inserted after the transmission of each symbol, denominated guard interval. During this In that time, the receiver ignores the signals that receives, because it considers them echoes. The technical characteristics of DVB-T come defined by the system that it uses: COFDM are:||Carriers|
|6817, of which 6048 are of useful information||System of Modulation|
|64 QAM (modulation in quadrature and amplitude of 64 symbols||Value of FEC|
|2/3: It means that of three bits, 2 bits are useful and 1 bit is of redundancy to correct the errors in the reception system. Those errors take place during the transmission.||Interval of guard|
Each group of carriers is separated one of another one in 1116 cycles (Hertz). Soon we last of 896 useful symbol of a microsecond and interval of guard of 224 microseconds, giving rise to 1120 microseconds.
Corrector RS is an algorithm that is used to codify a signal that later in the receiver can correct the errors in digital transmissions DVB.
In DVB-T, the way to work it is to add 16 Bytes to each “elementary package of the transport flow”.
We have, therefore PES = 188Bytes + 16Bytes =204 Bytes, that allow to correct in the Decoder a maximum of 8 Bytes of error by PES, using the algorithm Reed-Solomon corrector.
Calculation of the capacity of the channel
This value can be obtained from approximated way of the following form.
We have 6048 carriers with useful information (the one that interests to us). We have the time to send that information is 1120 microseconds. Of it, we have:
1120/6048 = 0,1851 μS
As in that time, we must pass the 6 bits (we have a 64 QAM), we obtain a value of 0,1851 μS/6 = 0,0308641 μS
This gives a speed us of 1/0,0308641 μS = 32,4 Mbps As we have a FEC of 2/3, means that 1 of each three loses 32,4 Mz * 2/3 = 21,6 Mbps The capacity of the useful Channel comes conditional by
Reed-Solomon corrector, so that
21,6 FU = Mbps *188/204 = 19.91 Mbps
In the image superior we have the variations in diverse countries of the surroundings, where the rate of transference varies according to we changed the parameters of the system.
To calculate the effectivo bandwidth in Mbps of the DVB-T in Italy and Scandinavia.
We do not confuse the terms.COFDM (Coded Orthogonal Frequency Division Multiplexing) is the very complex technique of broadband modulation that transmits digital information, with systems of correction of errors in the receiver, modulation of the information in multiple orthogonal carrier frequencies, where each of them is modulated in Amplitude and Phase (Modulation QAM), next to a system to fight the effects multitrajectory (guard interval). QAM (Quadrature amplitude modulation is
used technique to modulate in the carrier, as much in amplitude as in phase.
The incorporation of 4G supposed to eat part of the band of the UHF one, as we see in the following image
The channels between 61 and 69 passed to use of the telephony 4G
The complete system of transmission and reception of the DVB-T, of simplified form, stays as in the following image
In the image inferior, one is how the carriers are sent, within his bandwidth with the necessary intervals of guard. We must consider that, the system allows to send several multiplexadas signals, therefore, at every moment of time, the carriers correspond to a specific program of TV
We know that we have two types of multiplexers. In this case, we concentrated in the time multiplexer to explain how to pass several channels through a channel. In image GIF we see how once in a while, only one signal (in this case conversation) happens through a single cable. And how we interpreted that in the DVB-T? The set of the 6817 carriers with their bandwidth of 8 MHZ, changes in the time, according to the needs of each program, being generated every so often time, 6817 carriers for each program.
In 5º, one becomes to send the 6817 of Channel 1