Symbol Rate

This page refers to the outlink multi-Mbit/s carrier from the hub which is shared amongst all vsat users to download internet web pages etc. The carrier is similar to a DVB-S carrier which carries several MPEG TV programmes.

The carrier on the satellite is made up of a sequence of joined together pulses to make a continuous signal.  Each pulse is a symbol.  According to the modulation method each symbol represents 1, 2 or 3 etc bits of transmission rate data.  

In phase shift keying (PSK) modulation each pulse is a burst of carrier signal with its sinewave zero crossing point timing adjusted forwards or backwards in time to constitute a phase shift.  Phase shifts of 180 deg apply in BPSK, 90 deg in QPSK etc. A phase shift of 90 deg represents a time shift of 1/4 of a full cycle of the sinewave.   The closer the spacing phase shifts, the more difficult it is to distinguish between them at the receive end, so for for each higher order PSK schemes more carrier to noise ratio is required.    

As a general rule if you have bandwidth to spare, then use a lower order modulation or a higher rate FEC (like 1/2 or 2/3) to spread the signal out.  If you have power to spare then use a higher order modulation and/or lower rate FEC (like 3/4 or 7/8). Ideally you want to use all of both the available bandwidth and power simultaneously to obtain the highest user information rate. 

If you use larger receive dishes you will always be able to increase the system capacity.  If you are doing a point to point link it is worth using larger dishes - spend more on the antennas and used advanced modulation technique modems, like Vipersat CDM-570L, to save on the space segment costs.  If you have thousands of receive dishes then the aggregate cost of these is significant and you will want to allow smaller receive dish sizes even though this reduces system capacity and increases space segment costs.

Forward error correction

Forward error correction is applied to the customer's information data at the transmit end.

so transmission data rate = customer information rate x 1/ (FEC rate).  

FEC rate is typically in the range 1/2 to 7/8 so the transmission data rate is always significantly more than the customer information rate.  

This page provides a key formula:  

SR = Symbol Rate
DR = Data Rate = the information rate. This is the same as the customer information rate if there is no framing, supervisory, conditional access or encryption overhead added to the data stream in the modem.  DVB modems add significant overheads.
CRv = Viterbi  forward error correction (FEC) Code Rate.  Eg. 1/2, 2/3, 3/4, 5/6, 7/8
CRrs = Reed Solomon forward error correction (FEC) Code Rate. Eg. 188/204

If some other type of FEC coding method is chosen, such as Turbo coding, just use whatever FEC rate is selected (e.g. 5/16, 21/44, 3/4, 7/8, 0.95 )

m = modulation factor (transmission rate bits per symbol).   BPSK=1, QPSK=2, 8PSK=3, 8QAM =3, 16QAM=4 etc  

Formula:   SR = DR / (m x CRv x CRrs)

DVB-S Carrier bandwidth

The bandwidth of the carrier at the -3.8 dB points is approx the same as the symbol rate.
The bandwidth of the carrier at the -12 dB points is approx 1.28 times the symbol rate.

The expression "occupied bandwidth" is used to refer to a bandwidth 1.19 times the symbol rate, approx -10 dB points..

The allocated bandwidth, i.e. spacing between carriers needs to be approx 1.35 to 1.4 times the symbol rate.  If you put the carriers too close together you will start to see more adjacent carrier interference. If you put them too far apart you will waste expensive bandwidth, so choose some compromise that includes some, but not too much interference.  A suggested adjacent carrier interference allowance in link budgets is 28 dB on each side.  You choose.  If you can avoid high spectral density carriers adjacent to low spectral density carriers it will help.

For example: Symbol rate=27.5 Msym/s. Bandwidth = -1 dB 20.9 MHz, -2 dB 24.2 MHz, -3 dB 26.25 MHz, -3.8 dB 27.5 MHz, -4 dB 27.7 MHz, -6 dB 30.3 MHz, -12 dB 35 MHz.  For a spectrum analyser view of the spectrum see this page

Some examples:

A really good explanation of digital modulation is given in http://www.educatorscorner.com/media/AN_5965-7160E.pdf (warning large file 650 k bytes)

Last amended 4 Oct 2007
6 April 2004: link to dvb-s carrier spectrum analyser plot added.
20 Sept 2004:  link to Agilent Application note 1298 - Digital Modulation in Communications systems added (warning large pdf file 650 k bytes).
4 October 2006.  Turbo code, adjacent carrier interference, allocated bandwidth and occupied bandwidth mentioned.
4 Jan 2007, 4 Oct 2007:  Table of example modulation methods, FEC coding and FEC rates added, amended