How to measure spectral efficiency

[chee]

i would like to compare the spectral efficiency of my free space optic system. Does Optisystem 12 have the feature to measure spectral efficiency?

[Ravil]

Hi chee,

Didn’t quite understand your goal. Are you trying to measure it directly? In general, the spectral efficiency of a digital communication system is measured in bit/s/Hz. It is the net bitrate (useful information rate excluding error-correcting codes). Knowing the parameters of your system, it can be calculated as a maximum throughput bit-rate divided by the bandwidth in hertz of a communication channel or a data link. Let me know if you have more questions about that part…

[chee]

for my case, how would i measure my maximum throughput? is the bandwidth of the communication link is equal to the bandwidth i used in the transmitter laser?

[Damian Marek]

You could use an OSA to estimate the amount of bandwidth your link uses and then divide the bit rate by that amount.

[chee]

May i know what is an OSA?

[chee]

nvm. got it. optical spectrum analyzer

[Ravil]

Hi chee,

Unfortunately, I am not in my lab and don’t have an access to OptiSystem to look at the scheme you are attached. However, in general, the maximum bit-rate throughput is defined by your PRBS settings and it is limited by maximum bir-error ratio or signal-to-noise ratio allowed for your communication system. The bandwidth of the communication link is defined by modulator, not the transmitter laser, and can be measured by Spec. Analyzer. Do you have a baseband system, by the way?

[chee]

i am not sure what what u meant by baseband system. i am using LiNb Mach-Zehnder modulator. according to the spectrum analyzer, this is the result i got. i get the value of bandwidth by subtracting the minimum value from maximum value. the value i got is 2e-10m (1.5e18 hz). is this correct?

[Ravil]

Hi chee,

By its definition, the baseband telecommunication system doesn’t use modulation scheme (i.e., it transmits a base information signal) and its spectrum is different (in most cases its twice shorter) then the spectrum of modulated signal. Not sure what min and max values you exactly mean… In general, the full width of pulse spectrum can be determined at the level of -20dB (or 99% of its maximum power). This width can be used for spectral efficiency calculations.

[Ravil]

By the way, I am not exactly sure how you obtained a frequency bandwidth range of 1.5e18 Hz for you wavelength bandwidth of 2e-10m (0.2 nm). According to my quick calculations, a frequency bandwidth range of 7e13 Hz corresponds 0.2 nm of wavelength bandwidth. It looks like you used the relation between a single frequency and a wavelegnth: f=c/lambda. In your case you are dealing with the bandwidth (multiple frequencies) you should consider that equqtion through differential.

[chee]

at the right side of the diagram of the optical analyzer, there is a set of wavelength values, center, start and stop. can i use the stop value to minus the start value to get my bandwidth?

[Damian Marek]

Hi Chee,

The spectral efficiency is related to the spectrum that contains the information you are transmitting. In this case the spectrum analyzer is showing a range larger than you would actually need to transmit that signal. In a WDM system the spectral range used is the channel spacing.

Regards

[Ravil]

Hi Chee,

As I mentioned before (and it was indicated by Damian in the previous message), to calculate the spectral efficiency of a certain channel (for telecommunication system which uses modulation, i.e. not baseband) you need to consider the channel spectrum of your system. If in your case you are using WDM system with the determined frequency grid, you can refer to this grid to find your bandwidth (see Damian message). You can measure this spectrum (for your task) in a channel after your performing modulation.

[Author]

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