[sfink]

What kind of wacky unit is the spectral efficiency of bits/sec/Hz? Isn't that the same as 'bits'?

I mean, I'm guessing it's just saying the bit rate depends on the signal frequency, but it's still funny to see canceling units.

Related: a "spacing" measured in GHz.

Again, it makes sense, but it struck me that someone could write a total spoof paper with nonsense units and I wouldn't be able to tell the difference!

[namibj]

It's the normal unit/measurement of spectral efficiency. You have X Hz bandwidth, and you multiply it by the spectral efficiency to get the data rate.

The fact that the time-unit cancels out just shows that spectral efficiency is independent of any sense of time one might have.

Measuring "spacing" in GHz makes sense if you consider the way heterodyne mixing shifts the signal around, without affecting the spacing of sub-carriers.

[mstresh]

Spectral efficiency is also a nifty metric because, given some bandwidth and signal-to-noise ratio, there is an upper bound to which you can compare a result [0].

Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.

[0] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

[mmmBacon]

The metric is a measure of how much of the frequency spectrum a particular modulation format takes up. For example traditional OOK (on-off-keying) has a spectral efficiency of 1bit/s/Hz. In coherent optical communications we take ideas from radio to send more bits in the same amount of BW by encoding information in the phase and amplitude. For QPSK, we send 2 bits/s/Hz. In optics we use 2 orthogonal polarizations at the same time so we call this DP-QPSK. This doubles the spectral efficiency to 4bits/s/Hz.

One can keep going to higher order modulations to improve the spectral efficiency but the SNR required increases exponentially with constellation order.