Digital Compression

Is There a Better Way to Maximize the Throughput of my Satellite Capacity?

The first use of digital circuits for communications was for telephony applications. One uncompressed telephone circuit made up a 64 KBps circuit. Soon digital standards were developed to combine telephony circuits:

  • Twenty-four 64 KBps circuits became a 1.544 KBps circuit (or the Unites States Standard T-1)
  • Thirty-two 64 KBps became a 2,048 KBps (or a European E-1), and
  • Digital Service Level 3 (DS-3) equals 44.436 MBps,

These circuit standards are still in use today. As digital compression technology developed, a 64 KBps circuit handled more telephone quality circuits. With today’s advanced compression technology, up to eight-plus telephone circuits can be placed within a 64 KBps circuit.

The same is true with video carriers. When digital video first arrived on the scene, the default digital single channel per carrier (SCPC) video carrier was at 8.448 MBps at QPSK, ¾ FEC (Forward Error Correction) using Reed Solomon (RS) 188/204 coding. Those carrier parameters conveniently fit into a 9 MHz slot (when transponders were typically multiples of 36 MHz wide). Up to four 9 MHz SCPC carrier slots could be created on a 36 MHz transponder. Today, an acceptable standard definition video can be transmitted using 4 MBps in as little as 4.5 MHz, or even slightly less.

Our broadcast customers also utilize multiple channels per carrier (MCPC), where multiple distribution channels can fit into one carrier in a 36 MHz slot. The idea of placing multiple channels in a single carrier is advantageous as it allows for the maximum amount of throughput (MBps) in the transponder. As an added benefit, all the power of the transponder is maximized into one carrier. This means the MCPC carrier can reach smaller antennas than if each channel was individually uplinked on its own carrier.

The introduction of the Digital Video Broadcast-Satellite (DVB-S) standard made digital transmission anywhere around the world very easy to do and established the QPSK, ¾ FEC and 188/204 RS coding as the most efficient digital method. Using this coding method, a digital video signal can be transmitted to the same legacy 3.8m receive antennas that received analog video. In fact, the DVB-S standard has been the dominant digital transmission standard for 15 years—a very long time in technology terms.

With the introduction of High Definition (HD) television, more MBps are needed for each channel. The challenge is to fit multiple HD channels into a single transponder. The more HD channels you can place into a transponder, the more cost-efficient it is for the broadcaster as the cost of the transponder is spread out over multiple channels—or ‘revenue streams’ from their perspective. Using DVB-S standard for HD transmission means fewer channels broadcast per carrier as the HD channels use more MBps per channel than standard definition channels.

The introduction of DVB-S2 has made great strides again using today’s newest compression technologies. This relatively new standard also allows for more throughput in the same transponder space than DVB-S. The question comes up, “Is there a better way to maximize the throughput of my satellite capacity?” The answer is yes, and maybe no. Using the advanced compression techniques in DVB-S2, a significant savings in transponder Occupied Bandwidth can be achieved. Or, more MBps can be squeezed into the same transponder space. However, you must allow for sufficient transponder power margin, needed to transmit the carrier.

Looking back at the Tech Talk link analysis article, we know that every combination of modulation scheme, FEC, etc., requires a minimum Eb/No to achieve a threshold of performance—in this case a Bit Error Rate (BER) of 1 x 106. Using the standard DVB-S (QPSK, ¾ FEC with 188/204 RS), the Eb/No threshold to achieve a BER of 1 x 106 is 5.9 dB.

When you compare Eb/No thresholds to some of the higher DVB-S2 coding schemes (Table), you will see that the standard DVB-S schemes still leave a lot of the transponder power available. This is important to know as the extra power could be used to receive the carrier into antennas smaller than the 3.8m used in this example.

The DVB-S2 schemes have higher throughput, however, use more power of the transponder to do so. Using the DVB-S2 higher throughput schemes may prohibit the use of smaller antennas within a network.

Carrier Parameters Maximum Throughput within 36 MHz Eb/No Threshold (1×106) Extra Transponder Power Margin (dB)*
with 188/204 RS
41.470 MBps 5.9 7.8
DVB-S QPSK, 7/8 FEC with 188/204 RS 48.383 MBps 6.8 6.1
67.500 MBps 5.6 5.8
8/9 FEC
79.990 MBps 8.3 2

(Assumptions: Galaxy C-band, 3.8m downlink CONUS, Circuit Availability=99.96%)
*Extra transponder power is after the 99.96% availability is met.

As you can see, the DVB-S2 can add more than 30 percent more MBps per transponder than DVB-S. The information is ‘packed in’ tighter than DVB-S, but the new digital compression techniques used in DVB-S2 mean the Eb/No threshold are relatively the same. However, the amount of transponder power needed increases with the use of higher modulation schemes and data rates. This factor of more MBps per transponder versus transponder power needed must be balanced to ensure the proper bandwidth use and power margins for the desired circuit availability on an annual basis.

As Intelsat customers demand more capacity and greater throughput, we will undoubtedly adopt future compression standards, and we will continue to strive to be at the forefront of the implementation of those new standards. In the meantime, join me for the next edition of Tech Talk when I will delve into the inner workings of ‘the transponder.’

One of the many valuable tools on MyIntelsat, the company’s customer extranet, includes an link budget tool. To gain access to MyIntelsat, Intelsat customers should connect with their Intelsat Sales Director or simply send an email request to [email protected].