Satellite Communications

Technical Support

Megahertz Vs. Megabits Conversion

There is often confusion about Mhertz (Mhz) versus Mega-bits-per-second (Mbps). This is similar to the same confusion between mega bits and mega bytes.

Keep in mind that communications data rates (bandwidth) is variable as related to the type compression and modulation with this efficiency impacting the occupied analog bandwidth. Some compression can give you a 1:1 relationship, but others definitely do not. It's a lot like fitting a square meal in a round can.

It all depends on the modulation, error correction schemes and the quality of signals (EbNo - bit error rate BER as it relates to signal to noise ratio S/N).

If you use QPSK modulation and FEC 7/8 you can expect 1.2bits/Hz.

If you use 16QAM FEC 5/6, expect 2.4 bits/Hz.

The higher the modulation efficiency, more power will be needed. To determine the amount of power needed, a link budget needs to be performed. However, let's not put the cart in front of the horse. In order to calculate the occupied bandwidth, you first need to determine the "symbol rate".

The symbol rate = Data Rate x Inverse of code rate x Modulation index.
The Modulation index is 0.5 for QPSK, 0.333 for 8PSK and 0.25 for 16QAM. Let's take 34.368Mbps with 3/4 rate FEC and QPSK as an example:

34.368 x 4/3 x 0.5 = 22.912 Msps

In the ideal world, the occupied bandwidth would be the same as the symbol rate, but "brickwall" filter shapes are near impossible to achieve. Distortion is also present, so we use a rolled off filter with a spreading factor.

Having determined the symbol rate, we now multiply by the spreading factor of the filter. A typical Intelsat filter has a spreading factor of 1.2, so continuing with our example, the occupied bandwidth = 22.912 x 1.2 = 27.494 MHz

Assuming we are not alone on the transponder, the carrier will normally be adjacent to another one so there is a guard band inserted making typical channel spacing of 1.35 x symbol rate.

If the service is DVB or TCM then Reed Solomon FEC may be used in addition to the Viterbi so you need to multiply the date rate by the inverse of RS code rate so increasing the sysmbol rate a further 8-12%.

If the service carries an engineering service circuit (ESC), this will
increase still further the data rate (x16/15 for IBS and +96kbps for IDR).

If you are looking to minimize bandwidth, then Turbo coding is the way to go.

For implementation:

Going from QPSK to 8PSK is quite easy (so long as your system phase noise is good).

Going to 16QAM with high bit rates (and obtaining theoretical Bit Error Rate perfomance) is a quantum leap and not for the in-experienced, since you need to ensure your entire link (uplink earth station, satellite and downlink earth station) are all group delay equalised, amplitude response is flat and all amplifiers are linearised for minimum AM/PM distortion. 16QAM works best if you are a large teleport with large antenna.