MER (Modulated Error Ratio) What is it ?
Unfortunately for us, simply measuring a signal or looking at it on a spectrum analyser is not enough to troubleshoot or check the health of a digital system.
In order to measure BER accurately in a laboratory, a known bit pattern is continuously transmitted. The receiver compares what it knows has been sent with what should be received. If the error rate is low, that measurement can take several minutes. Even so, in the real world the receiver cannot know what bit pattern is being sent, so the actual measurement is an estimation.
Relying of BER as the sole indicator of the system performance is risky, because of the “cliff edge” characteristic of digital systems. If performance is right on the cliff edge (and hence appears satisfactory) there may be very little margin to prevent the entire system from failing.
What is really required is an “internal” examination of the signal stream to determine what is wrong. If the received signal strength is so low that errors are being received, then pre-correction BER can be used as a gauge of the system performance, but if no errors are being received MER is a much better measurement of performance.
Modulation error ratio is a measurement of the received SNR in a digital system. MER concerns itself with errors caused by phase noise, distortion, frequency response errors, group delay, echo, level compression and imperfections in the signal path. The measurement takes into account the performance of dish, LNBF and cable.
In effect it is a measurement of the end-to- end performance of the system.
Whether mathematically calculated or visually displayed, MER relates to the received signal constellation. In simple terms, it is the average symbol or bit power, divided by the average error power, calculated thus:
MER (dB)=10log (average symbol power/average error power)
The Constellation Display, is a visual representation of errors encountered in a digital system. Normally the display is divided into 4 quadrants. This is what a display with no errors looks like.
MER measures the “fuzziness” of the display resulting for the effects of various factors. The two examples show the stages of MER degradation in a 16 QAM system. Notice the degradation difference between the two displays.
Ideally, a prefect signal would have all bits land at exactly the same positions over time. Impairments to the signal mean that the bits land at slightly different positions making the display appear fuzzy. In the real world, the higher the MER the better. Typical readings on Optus C1 Pay TV services using a 65cm dish and good LNBF will give MER readings in the vicinity of 14-16dB.
MER is a complex subject and we have only touched on the surface in this simple discussion. For more detailed information (from which some of the information presented here was sourced), there are two good articles written by Ron Hranac (Cisco Systems) and Bruce Currivan (Broadcom) describing digital modulation systems and measurements.
The link below will lead you there.
