Improved digital video broadcasting
Jan 1, 2007 12:00 PM, BY LARS-INGEMAR LUNDSTRñM
DVB-S2 is the new toolbox for satellite communications engineers.
HDTVs are now affordable consumer products, with transmission standards adopted worldwide. However, there are still details to sort out, such as how to choose the appropriate balance between quality and distribution capacity.
The 720p and 1080i and the even more demanding 1080p formats are hungry for more information capacity. There are two ways to deal with this problem: DVB-S2 and MPEG-4. This article will discuss the new opportunities that DVB-S2 makes possible for the satellite communications engineer.
Increased efficiency
Figure 1. Constellation diagram for QPSK
Click image to enlarge.
DVB-S2 can increase the capacity of a satellite transponder by about 30 percent. The capacity of a standard 33MHz satellite transponder, operating with 27.5 mega symbols per second and FEC of 3/4, may increase from 38Mb/s to about 50Mb/s using DVB-S2. Together with the use of MPEG-4, it theoretically would be possible to carry up to six 8Mb/s HDTV channels on one transponder. Using DVB-S and MPEG-2, the same transponder could carry just two 16Mb/s HDTV channels.
The magic is achieved by replacing the DVB-S inner convolutional code (Viterbi) with the more efficient inner low-density parity-check (LDPC) code and replacing the outer Reed-Solomon (204,188, T=8) code with a concatenated Bose, Ray-Chaudhuri, Hocquenghem (BCH) code. The concatenated LDPC/BCH FEC makes it possible to get much closer to the Shannon limit, about 1.2dB, compared with about 3dB for DVB-S.
A powerful toolbox
DVB-S2 is a toolbox of several modulation schemes and includes improved filter roll-off factors and FEC. Using higher modulation schemes, such as 8PSK, 16APSK and 32APSK, it is possible to increase the capacity from the satellite transponders.
Figure 2. Constellation diagram for 8PSK
Click image to enlarge.
Modulation modes higher than QPSK are not new. 8PSK and 16QAM were a part of the DVB-DSNG specification that allowed more spectrum-efficient modulation modes to be used for DSNG.
In DVB-S2, both broadcasting and DSNG applications are covered by a single specification. The number of FEC rates has been increased considerably; the DVB-S specification included 1/2, 2/3, 3/4, 5/6 and 7/8. In the new DVB-S2 specification, 1/4, 1/3, 2/5, 3/5, 4/5, 8/9 and 9/10 have been added to the list. And there is a reason for the increase in flexibility. DVB-S2 is designed to be used in a much more flexible way than its predecessors DVB-S and DVB-DSNG.
The choice of modulation mode
Before choosing a modulation mode, it is important to note that constant envelope modulation modes, such as QPSK and 8PSK, can operate at saturation. Saturated transponders are important to broadcast services because there is an automatic gain control keeping the transponders at full output power independent of uplink attenuation. This assures a constant output equivalent isotropically radiated power level from the satellite.
Figure 3. The modulation mode and code rate may change from frame to frame in the physical layer of the DVB-S2 signal. The frames may be assigned to different transport streams. BPSK is used for the highly protected header of each frame of the physical RF layer. APSK can only be used in transponders operating in linear mode.
Click image to enlarge.
QPSK provides two bits per symbol, while 8PSK provides three bits per symbol. (See Figure 1 above and Figure 2 on page 12.) As a result, 8PSK may carry 50 percent more information within the same bandwidth than QPSK. But this requires 50 percent more transmission power or a 50 percent improvement in the antennas used for reception. High-power satellites and LNBs with extremely low noise figures make 8PSK popular for broadcasting applications.
Modulation modes with alternating envelopes, such as 16QAM (DVB-DSNG), 16APSK and 32APSK, need to be operated in transponders in linear mode. These transponders operate at considerable back-off and will not provide maximum output power.
On the other hand, several carriers originating from different uplinks may be operated simultaneously in the same transponder. SNG applications using a single channel per carrier may benefit from the 16QAM and 32QAM modes. 16QAM and 16APSK signals contain four bits in each symbol and carry twice the bit rate as QPSK within the same bandwidth. But it costs twice the amount of power in the link budget. 32APSK carries five bits per symbol and provide a bit rate of two-and-a-half times the bit rate of the QPSK signal. But the link budget deteriorates proportionately.
In my own experience, 16QAM is harder to use than a constant envelope modulation mode, such as 8PSK, because of its sensitivity to amplitude variations. Power spikes in HPAs, supply power stability and even amplitude variations because of other carriers being switched on and off in the same (linear) transponder may cause pixelation and black frames.
On modulation, DVB-S2 includes roll-off factors of 20 percent, 25 percent and 35 percent, which improve the possibilities for smarter frequency planning. A 20 percent roll-off factor allows for considerably steeper filtering than the 35 percent used for DVB-S. (See Figure 3.)
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