Link Margin Measurement

September 1, 2009.

Since the introduction of the DVB-S standard, the digital satellite transmission technology has evolved with more efficient modulation schemes allowing more “bits by Hertz” and with more robust error protection techniques. This has meant the evolution towards a 2nd generation standard called DVB-S2 that optimises the available resources.

The main advantage of DVB-S2 is that it can transmit up to 30% more information compared to DVB-S, considering a signal with the same C/N and bandwidth conditions. This bit-rate increase combined with the use of MPEG-4 AVC compression provides a better solution for the broadcast of High Definition contents.

The inconvenient of DVB-S2 is that the BCH and LDPC error correction mechanisms, whilst providing an outstanding protection to the system generates a more sudden drop of the called “cliff effect”. Hence, a 0.1 dB variation implies to move from the good reception to a situation where the information can not be recovered and therefore services can not be viewed.

This minimum error margin for the DVB-S2 signals is extremely critical so knowing how far we are from the point of failure is very important. The LINK MARGIN in the TV EXPLORER II, TV EXPLORER II+ and TV EXPLORER HD field strength meters informs about the dBs signal quality can decrease before  to reach the digital cliff, this is to say, the point where the error correction system (FEC) can not recover the information. In other words, its value indicates the margin of quality of the signal .

The minimum C/N required for a “Quasi Error Free” reception is expressed in terms of Bit Energy to Noise Density (Eb/No), being both directly related. The Eb/No is a very important parameter to determine the error rate.

Link Margin measurement

The higher the LINK MARGIN is, the higher the quality of the received signal and the farther we are from the cliff. The closer to 0 dB, the worst the quality of the received signal and therefore the closer to the point where the system stops to operate.

Link Margin measurement
A small disruption causes the drop to the digital cliff.

When the LINK MARGIN is 0 dB, we are in the limit of the correction. The services can be displayed, although occasionally can be seen some artifact, but only a slight signal degradation can cause the drop through the cliff. The system will stop to work immediately.