Improved digital video broadcasting

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

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.

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.

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.)

The choice of code rate

There are three coding rates that include more redundant bits than information bits: 1/4, 1/3 and 2/5. At QPSK 1/4, it is possible to decode the signal at the subnoise level -2.35dB. These extreme code rates allow communication during poor conditions often found in DSNG applications.

The physical layer frames

The RF layer of the DVB-S2 signal is divided into physical layer frames that do not need to use the same coding and modulation. Each physical frame starts off with a 90-bit (symbol) BPSK sequence, which is a highly protected 7/64 block code header. The header includes synchronization and information related to signalling. This is followed by either 16,200 bits (180 × 90) or 64,800 bits (720 × 90), protected by the powerful concatenated LDPC/BCH FEC.

The longer 64,800-bit FEC frame provides better protection but introduces more latency than the shorter 16,200-bit FEC frame. Therefore, the short FEC frame should be chosen in applications where latency is critical, and the longer frame should be used to optimize signal protection.

In conventional broadcasting applications, latency is of no importance, and the longer frame may be chosen. Internet traffic applications are latency critical, so the shorter frame is the best choice. By using different coding and modulation for different frames, several new possibilities are created. It should also be mentioned that pilots can be added to the signal to facilitate carrier recovery.

Constant coding and modulation

The simplest way to operate DVB-S2 is with constant coding and modulation (CCM), which is similar to how DVB-S signals are used. In CCM, the same modulation mode and FEC is used for all physical layer frames. The point of using DVB-S2 in CCM mode, compared with using DVB-S, is the improved protection achieved by the new inner and outer codes, providing 30 percent improved capacity. This improvement is of great value for HDTV broadcasting systems. However, in CCM mode, the full potential of the DVB-S2 physical layer frame structure is not used.

Variable coding and modulation

In DVB-S broadcast applications, QPSK and a fixed FEC rate may be used for years, whereas DVB-S2 may be quite the opposite. Several transport streams can be combined into one transponder operating at saturation, provided the envelope of the signal is kept at a constant level (QPSK and 8PSK). However, the multiple transport streams may be assigned to different physical layer frames, making it possible to use different modulation and code rates for different streams.

Depending on the application, it might be possible to make different trade-offs between capacity and robustness of the transmission. For example, a transponder that carries both SD and HD signals, with less protection for the HD signals, benefits with an increased bit rate. This may put a higher demand on receiver dish size for HDTV, but it might still be acceptable in some systems.

Professional services that allow for larger dishes may therefore use worse code rates to maximize bit rate and can be integrated into broadcasting transponders operating at saturation. This may include different kinds of data traffic, such as Internet backbone, that previously were forced to use separate carriers for full modulation and coding rate selection flexibility.

Another application using the DVB-S2 multiple transport stream is the distribution of DVB-T multiplexes to terrestrial transmission sites. The bit rate of a DVB-T terrestrial multiplex may be 23Mb/s. Until now, it has not been possible to carry more than one of these in a saturated transponder. Two multiplexes would have required separate carriers and a transponder operating in linear mode.

Using DVB-S2, the two multiplexes may be assigned to different physical layer frames and would be kept separated until received at the terrestrial transmission site. DVB-S2 would also provide the extra bit rate required to fit both multiplexes in one transponder.

Adaptive coding and modulation

Perhaps the most sophisticated use of DVB-S2 is the adaptive coding and modulation (ACM) mode, which can be used to optimize point-to-point applications. In the ACM mode, there is a return path from the receiver to the transmitting uplink. This return path provides the instantaneous update of the Eb/No figure at the receive site available at the uplink station. This may be used to change coding and modulation to optimize the bit rate throughput. This means the rain margins can provide increased bit rate during clear sky conditions, thus increasing the average throughput of the system. This may be a significant improvement, especially for Internet backbone connections and other kinds of data traffic.

In DSNG applications, a narrow band return path to the uplink van may provide optimum throughput under tough conditions by changing the code and modulation mode accordingly. If forced to a mode with a lower bit rate, this may have to be compensated by decreasing the bit rate on the encoder side and so on.

Backwards compatibility

It has not been many years since DVB-S was introduced, so it is not easy to immediately change to DVB-S2. HDTV means that there will soon be a large population of DVB-S2/MPEG-4 receivers. However, the DVB-S receivers will still be in use for many years to come.

There are several ways to obtain backwards compatibility. (See Figure 4.) However, this means making a compromise between the performance of the DVB-S and DVB-S2 components of the signal. One way to compromise is to use hierarchical modulation.

Using 8PSK with the two symbols of each quadrant placed closer together than in the original 8PSK constellation diagram causes the DVB-S receivers to believe that they are receiving a QPSK signal while the DVB-S2 receivers detect all eight symbols. (See Figure 5.) Using clever hierarchical mapping, it is possible to combine SD signals for DVB-S receivers with HD signals for DVB-S2 receivers in the same transponder.

Because this backwards compatibility comes at the price of compromise, it may be better to fully exploit the capabilities of DVB-S2 in separate transponders and to keep the DVB-S transponders as they are for some years to come.

The alternative transmission modes and the concept of putting both broadcasting and professional contribution techniques into the same standard will probably make professional DVB-S2 equipment cheaper because the same chipsets may be used for all applications. The new DVB-S2 standard, with all its possibilities, calls for innovative use and will keep satellite communications engineers occupied for years to come.

Lars-Ingemar Lundström has managed several large broadcasting projects and is currently in charge of development of business media systems for Teracom, Sweden. His book “Understanding Digital Television” is available from several booksellers and directly from the publisher