DVB-S2 and spectral efficiency
Industry experts and vendors alike recognize that DVB-S2 satellite modulation offers significant technical advantages. A properly configured DVB-S2 system based on modern transmission equipment can easily justify the outlay required by dramatically reducing operational expenses. This article will examine the benefits of DVB-S2, and how broadcast and cable operators can successfully implement and maximize their investment by deploying a DVB-S2 system.
In digital communications, spectral efficiency is a measure of what bit rate a given RF bandwidth can sustain. While not universally defined, it is often expressed as a ratio of megabits per second to megahertz used in the active channel band. For example, in the ATSC broadcast system, 19.39Mb/s of data can be transmitted in a 6MHz channel. The spectral efficiency of the system could be calculated by dividing 19.39 by six — yielding an approximate efficiency of 3.23 bits per hertz.
The DVB-S2 system is highly flexible; depending on certain modulation parameters, it could provide spectral efficiencies anywhere between 0.5 and 4.5 bits per hertz. Higher spectral efficiencies allow an operator to inject more data into a leased transponder, reducing operational expenses significantly for many operators. However, the highest spectral efficiencies can only be achieved under certain conditions. To understand the trade-offs involved in configuring a DVB-S2 system, it is important to know the factors that contribute to spectral efficiency. The effort is worthwhile, and as demonstrated later in this article, optimal configuration can result in significant savings.
Calculating DVB-S2 spectral efficiency
Transmitted data is divided into frames before forward error correction (FEC) is applied, and certain headers describing the transmission are added. This FEC frame size is the first factor that drives spectral efficiency. The standard allows for two different frame sizes: 64,800 bits (“normal” frames) or 16,200 bits (“short” frames). Normal frames are more efficient, while short frames can reduce end-to-end system latency. Most broadcast video systems use normal framing.
The frames described above include a certain number of FEC bits. Two layers of FEC are applied to each frame to provide protection against RF interference. The DVB-S2 system first applies a fixed FEC algorithm known as BCH encoding. Next, a user-configurable proportion of additional FEC bits are added via a process known as LDPC encoding. The amount of LDPC encoding is represented by a fraction known as the FEC code rate.
The FEC code rate contributes directly to the transmission's spectral efficiency. A lower ratio of transport stream bits to total frame bits indicates that more FEC bits are being included in the frame. This lowers spectral efficiency while increasing link margin (resilience to noise). As an example, in a system configured for 64,800 bit frames, with a FEC code rate of 1/4, only 15,928 bits of transport data can be encoded in a frame. In contrast, 64,800 bit frames with 9/10 FEC coding each contain 58,112 bits of transport stream data. A quick calculation shows that, in the latter configuration, approximately 9/10 of the frame data is used for carriage of video. Table 1 shows transport bits per frame for various configurations, reproduced from the DVB-S2 standard itself. The standard uses the designation Kbch - 80 to represent transport bits per frame. The subtraction of 80 bits accounts for the space occupied by the baseband header, which contains information indicating, in this case, that the transmission carries a transport stream.
DVB-S2 also allows for the use of several different symbol constellations. These constellations are simply mappings that define how data bits are mapped to symbols in the RF domain. The constellations supported by DVB-S2 are illustrated in Figure 1, as reproduced from the DVB-S2 standard.
The fact that symbols are closer together for “higher order” constellations in the diagrams is significant. This closer spacing correlates to more difficulty for receivers trying to distinguish between symbols; thus, higher-order constellations provide less resilience to RF interference. However, the higher order constellations allow each symbol to represent more data bits. For example, the 16APSK constellation provides 16 symbols, each of which can represent four data bits (24 = 16). The number of bits per symbol is another important factor in determining spectral efficiency. The DVB-S2 standard uses ηMOD to represent bits per symbol.
We have now arrived at a sufficient understanding to begin approximating DVB-S2 spectral efficiency for many configurations. We will begin by calculating the number of transport stream data bits per symbol for a single frame. To determine this value, choose the appropriate constant for transport bits per frame (Kbch-80) as defined in the previously reproduced table, and divide by the number of symbols per frame. The number of symbols per frame can be calculated by taking the number of bits per frame and dividing it by the number of bits per symbol. (See Equation 1.)
The equation approximates spectral efficiency based on frame size, constellation and FEC. This value “approximates” spectral efficiency, but there are a few more factors that need be considered in order to paint the picture in its entirety. The first of these is a feature of DVB-S2 known as pilots. Pilots are not RF carriers in the traditional modulation sense; rather, in DVB-S2, they are simply repeating patterns of symbols that are injected at fixed intervals between symbols containing data. Their purpose, however, is the same as an RF carrier-based pilot — to allow the demodulating device to more easily lock on to the RF transmission. On average, pilots reduce efficiency by about 2.4 percent, although, as will be demonstrated below, this number can vary depending on modulation parameters.
Modulated symbols in DVB-S2 are logically grouped into slots. A slot is simply a grouping of 90 successive symbols, with a pilot string of 36 symbols introduced after every 16 slots — that is, after every 1440 symbols of data. One final factor affects spectral efficiency: Before transmission, a physical layer frame header (PL header) is prepended to each frame. This header allows the receiver to automatically detect modulation parameters, simplifying user configuration of a receiving device. The physical layer frame header adds one slot (90 symbols' worth of transmission time) to each frame, which imparts an additional reduction in efficiency.
An illustration of pilot insertion and physical layer framing is shown in Figure 2.
Thus, to calculate final spectral efficiency, one must account both for pilots and for the PL header. Unfortunately, because frames are comprised of an integer number of symbols, pilots do not add a fixed percentage of overhead; the number of pilots for each frame length and constellation will be different. The number of pilots per frame can be calculated by Equation 2, where the integer function rounds down to the nearest integer.
Subtract one from the number of slots per frame to ensure that a pilot is not placed as the last slot of a transmitted PL frame. Thus, the final impact of framing on spectral efficiency is given by Equation 3.
At this point, we have described all factors affecting spectral efficiency in a single-stream DVB-S2 transmission. Equation 4 shows the end-to-end efficiency calculation considering all of the relevant framing factors.
Impact on broadcast operations
Why does any of this matter to a system operator? Understanding the DVB-S2 system can help a broadcast engineer properly specify parameters for a DVB-S2 transmission. The industry has recently begun to demonstrate that higher-order constellations and DVB-S2 modulation are practical using recently deployed equipment.
Let us compare a DVB-S2 transmission to a DVB-S transmission with the following example. A satellite operator has leased a 36MHz transponder, which is currently being used for DVB-S, 3/4, QPSK transmission. This DVB-S system provides a spectral efficiency of 1.5 bits per hertz. The scenario in Figure 3 will replace this system with a DVB-S2 system using 16APSK modulation with 3/4 FEC and pilots off. This transmission boasts a spectral efficiency of 2.97 bits per hertz.
Additionally, transmissions must be spaced apart to account for the roll-off factor of the root-raised cosine filter used to shape the bandwidth of a satellite transmission. This filter is applied to reduce intersymbol interference and is required by the DVB-S and S2 standards. DVB-S requires a roll-off factor of 0.35. RF roll-off is another parameter that DVB-S2 has improved upon, allowing the user to select a narrower filter with 0.20 roll-off.
Figure 4 illustrates transponder usage for the DVB-S system, showing four separate transmissions being uplinked on this transponder, with bit rates of 8Mb/s, 10Mb/s, 12Mb/s and 8.75Mb/s. As is apparent, the transponder bandwidth is completely saturated.
In contrast, the DVB-S2 system can carry those same transport streams twice in the same bandwidth, plus an additional transport stream at 7.6Mb/s.
This represents a bandwidth increase of 220 percent. If an operator is paying $2.5 million per year to lease each 36MHz transponder, this improvement could save an equivalent 65 percent in operating expenses, reducing costs by $1.1 million annually for this one transponder alone!
This article has addressed the benefits that accompany an optimally implemented DVB-S2 system and described the principals upon which operators can achieve significant cost savings. When selecting a modulation system, there are other factors to consider as well. Most notably, an operator must carefully consider the link margin required, based on the equipment available at both the transmit and receive site, as well as the satellite RF band being used and relevant geographic conditions. However, a carefully planned DVB-S2 system is an excellent investment, and understanding why can help any operator plan a successful deployment.
Russ Van Der Werff is a systems engineer at Sencore.
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