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OptiSystem applications:
Signal quality ratios

Introduction


Signal quality ratios

When designing or analyzing the performance of an analog or digital link, the following signal metrics
are commonly used:

Carrier to noise ratio (C/N)
Noise power density (No)Energy per bit to noise density (EbNo)
Energy per symbol to noise density (EsNo)

C/N is calculated from the ratio of the carrier power to the noise level over the specified bandwidth of
the transmission system. It is used to assess the quality of the analog communication channel (thesignal that is used to carry the digitally modulated information envelope).

EbNo and EsNo are, respectively, the ratios of the energy per information bit and symbol divided by
the noise power density. They are frequently used to measure and compare the performance of digitalmodulation links.

The following application note briefly describes how to capture and measure C/N, No, EbNo and EsNofor a typical QAM modulation system

REFS[1] “Digital Transmission: Carrier-to-Noise Ratio, Signal-to-Noise Ratio, and Modulation Error Ratio”, White Paper (CiscoSystems & Broadcom Corp), 2006
[2] “Signal-to-Noise, Carrier-to-Noise, EbNo”, Wolfgang Damm (Dir. of Product Marketing), Noisecom.
http://www.noisecom.com/resource-library/webinars/sn-cn-ebno-webinar ; accessed 23 June 2017[3] “Intuitive Guide to Principles of Communications: Link Budgets”, 1998/2002 Charan Langton,
http://complextoreal.com/wp-content/uploads/2013/01/linkbud.pdf ; accessed 20 June 20172Design setupThe example design below represents a 16 QAM digital modulation scheme transmitted over a 20 GHz electrical carrier
signal. The signal metrics for C/N, No (power spectral density), EbNo and EsNo are calculated using the Component scriptof the electrical power meter (EPM)

An electrical filter is used to account formeasurement system/channel bandwidth

The received carrier power and noise iscaptured with an electrical power meter. The
signal metrics are then calculated and reported
as custom results3

Signal quality metric calculations
This part of the component script is used toaccess the global parameter settings

The VBScript below is associated with
the EPM Visualizer and is used tocalculate the signal quality metrics basedon the measured carrier power andmeasured noise power

The receiver bandwidth is accessed from theparameter “Bandwidth” of the Rect Filt component
The local results for Signal Power and Noise Power areaccessed here. As there is a pre-filter before the EPM,
the power is integrated over the filter bandwidth(represents the measurement system bandwidth)

C/N Ratio = Carrier power/Noise power

PSD (No/2) = Noise Power/Receiver Bandwidth. The PSD model in OptiSystem uses a complex signal
representation (+/- frequencies). To account for the negative frequencies (1/2 of noise spectrum), thecalculated value for the noise density is reduced by 3 dB.

EsNo = C/N x Receiver Bandwidth/Symbol Rate
EbNo = C/N x Receiver Bandwidth/Bit RateAs the receiver bandwidth is set to the symbol rate (5 GHz) EsNo
will equal C/N4

RFA (1): Signal and noise power analysis
Carrier power

Carrier toNoise ratio

Noise power

C/N ~ 44 dBThe C/N calculation here is for the total integrated signal power
and noise power over 5 GHz BW. As the Signal and noise
spectrum are relatively flat there is a close match between OSA
measurements and the component calculation (43.8 dB)

EsNo and C/N are identical as the system bandwidth and symbol
rate are both at 5 GHz.

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RFA (2): PSD analysis
To verify the PSD, select the check box for Power spectral density under the Graphs tab of the RFA PostFilt
visualizer (the Y-axis units will change to dBm/Hz). The PSD in OptiSystem assumes the double-sided noiseconvention and thus a factor of -3 dB is applied to the PSD calculation for the EPM component script.
Note: The resolution bandwidth of the spectrum analyzer will change the position of the noise floor. For thecase below it was un-selected to match the sampling rate of the simulation.

PSD ~ -204 dBm/Hz

PSD calculation is adjusted by 3dB to account for double sideband noise representation

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