How Much Bandwidth Does a Satellite Have?

The answer, of course depends on several factors, modulation, symbol rate, number of active transponders, etc... For this example though we're going to look at the absolute maximum for everything. Let's say that we have a satellite that has 24 C-band and 24 Ku band transponders, each being 40 MHz wide - a common configuration for a newly launched satellite. This gives us a total of 1.92 GHz of raw RF bandwidth.

The symbol or baud rate is the number of times per second that the electrical signal changes. Depending on the modulation, a single symbol can encode one or more bits. The most common modulation for digital satellite transmissions is phase shift keying -PSK. Binary PSK has two symbols and encodes one bit per symbol, quaternary PSK has four symbols and encodes two bits per symbol, and so on. Each increase in the number of symbols allows another bit to be encoded. While modulations used for terrestrial communications can have as many as 256 symbols encoding 8 bits per symbol, the highest rate commonly used on satellites is octal PSK, often shortened to 8PSK, having eight symbols encoding 3 bits each.

For television signals the highest symbol rate in common use is 30 million symbols per second (MS/s) per transponder. To get the bit rate, you multiply the symbol rate by the number of bits that can be encoded by each symbol. In this case we multiply 30,000,000 by 3 to get 90,000,000 bits per second or 85.83 Mb/s per transponder. Since we have 24 transponders on each band that gives us 2,060 Mb/s for each band for a total per-satellite bandwidth of 4,120 Mb/s or 4 Gb/s. The forward error correction will take up some of this bandwidth, and the higher the error correction code rate is, the higher its overhead will be but it's generally under 10%.

Comparison With Other Devices

Some of these are academic, since there's no way anyone would ever upload uncompressed audio or video to a satellite - it's just too expensive. It's the same reason you almost never see analog channels anymore. More often than not you'll see MPEG 4/H.264 streams, with some MPEG 2 streams mixed in for compatibility with older equipment.

 It does give you some idea of how much everything costs, though. The last time I checked (a long time ago) it cost about $90,000 a month to lease a C-band transponder and twice that for a Ku-band transponder. At those rates, each SD channel would cost $3,100 per month on C-band and $6,200 per month on Ku-band, while each HD channel would cost $9,000 per month on C-band and $18,000 per month on Ku band.