Modulation Error Ratio and Signal-to-Noise Ratio Demystified Presented by Sunrise Telecom Broadband …a step ahead

Introduction Viewing the Constellations Demystified seminar prior to viewing this seminar is recommend. Modulation Error Ratio (MER) in digital systems is analogous to Signal-to-Noise or Carrier-to-Noise used in analog systems. Determining the MER of a digital signal is a critical part of determining how much margin the system has before failure. Unlike analog systems where you can see degradations in Carrier to Noise performance, a poor MER is not noticeable on the picture right up to the point of system failure.

Noise Effect Comparison Between Analog and Digital Systems Effect of Noise on Analog Systems (Gradually Poorer C/N) 35 dB C/N 25 dB C/N 20 dB C/N 45 dB C/N Effect of Noise on Digital Systems (Gradually Poorer MER) 34 dB MER 23 dB MER 22.5 dB MER 22 dB MER Noise has very little effect on digital systems until the system fails completely

Analog Carrier-to-Noise Carrier-to-Noise ratio in analog systems is simply a measure of the ratio of peak video carrier power over the noise in the channel, over the system bandwidth expressed in dB. This type of measurement can be done on a digital channel as well, but unfortunately does not provide the complete picture. Analog C/N Measurement

Quadrature Modulation Unlike analog video, QAM modulation not only uses Amplitude Modulation, but also phase modulation. Simply measuring the carrier level relative to the noise level does not take into account any phase noise that may also be present on the signal. Carrier Amplitude Modulation Carrier Amplitude Modulation Carrier Phase Shift Analog Video AM Modulation QAM Modulation

Other Noise and Interference In addition to amplitude and phase noise, other impairments can also effect the quality of the signal in similar ways to noise. Examples of other disturbances that will effect the performance: Implementation Loss due to limitations of A/D and D/A converters and sampling. Reflections not cancelled out by the adaptive equalizer. Non-Linear distortions such as laser clipping and amplifier compression.

Decision Boundaries Each location on the constellation is framed by decision boundaries. If the signal falls within these boundaries, the correct data will be received. If because of noise or other interference it falls in an adjacent area the data will be in error.

Noise Effects on the Constellation Because the position on the constellation depends on both carrier amplitude and carrier phase, both amplitude and phase noise will affect the location on the constellation. Amplitude noise will change the distance from the origin and phase noise will change the rotational position. Other types of noise and interference can effect the symbol in all directions. Effect of Amplitude Noise Effect of Amplitude Noise Effect of Phase Noise Phase of Carrier Other Types of Noise and Interference (Effects Symbol in all Directions) Amplitude of Carrier

Modulation Error Ratio and Error Vector Magnitude. In order to take into account both the amplitude, phase noise and other impairments on the carrier two different measurements have been developed, Modulation Error Ratio, (MER) and Error Vector Magnitude, (EVM). MER and EVM can be directly correlated with each other since they are essentially the same measurement, only performed and specified in different ways. MER and EVM effectively assign a value to the “fuzziness” of the constellation clusters. The further the dots are from their ideal location, the poorer the MER or EVM. MER and EVM can be considered a figure of merit for the QAM signal that includes all types of impairments, not just noise as in Carrier to Noise in analog systems.

Constellation Display The constellation display gives a visual indication of how far the noise is moving the signal from its ideal locations on the constellation. The further from the ideal locations, the poorer the MER or EVM. The shape of the cluster gives information on the type of impairment. Constellation With “Good” MER Constellation With “Poor” MER

Modulation Error Ratio RMS error magnitude average symbol magnitude MER is defined as follows: MER is expressed in dB. 10 log RMS Error Magnitude Ideal Symbol Average Symbol Magnitude

MER and EVM Since the MER and EVM measurements are very similar, they give essentially the same information about the signal. MER was chosen as the preferred measurement for cable TV by the SCTE because of it’s similarity to the Analog Carrier-to-Noise measurement expressed in dB that most people the cable industry are familiar with. MER and EVM are sometimes referred to as Signal-to-Noise by some manufacturers, but this is not technically correct.

What is a Good MER? 64 QAM set top converters require better than 23 dB MER to operate. To allow for system degradation a margin of at least 3 or 4 dB is preferred. 256 QAM set top converters require better than 28 dB MER to operate. To allow for degradation a margin of at least 3 dB is preferred. Typically the maximum MER displayed on portable analyzers is about 34 to 35 dB.

CM1000 Cable Modem System Analyzer Conclusions MER is the digital equivalent of Carrier-to-Noise in analog systems. Poor MER does not effect picture quality right up until the system begins to fail. MER takes into account not only amplitude noise, but also phase noise. Determining the MER of a signal is a key part of determining the margin from failure of the channel. CM1000 Cable Modem System Analyzer AT2000 Spectrum Analyzer