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The link spectral efficiency of a digital communication system is measured in bit/s/Hz.

We have that 1 Hz = 1 s$^{-1}$.

So, $\left((1 \,\text{bit}) /1 \text{s} \right)\, / \left({\frac{1}{1 \text{s}}} \right) = 1 \, \text{bit}$.

Thus, why don't we just say then that spectral efficiency if measured in bits, but wherever I read it's bit/s/Hz? Is there some special meaning to this?

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The units of spectral efficiency should be thought of as (bits per second) per Hertz. It measures the ratio of the data rate to the bandwidth. Yes, dimensional analysis shows that the dimension is equivalent to just bits but using that simplified unit for spectral efficiency does not show what the meaning of the term is. So, it is better to use (bits per second) per Hertz as the unit, or, for those whose (US) keyboards have broken shift keys and cannot type parentheses (the ( and ) come out as 9 and 0 respectively), as bits per second per Hertz or bits/second/Hertz. (Typing / does not need the shift key).

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  • $\begingroup$ And that twice as much, as bit, being the unit of Shannon Information, actually has no unit in itself: 1b = 1 $\endgroup$ – Marcus Müller Feb 18 '18 at 21:15

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