I’m surprised you don’t know the theory behind 8PSK 🙂 Actually each symbol = 3 bits.
But what is one symbol ? In GMSK, one symbol is simply a certain phase.
In 8PSK, a symbol is a certain phase + a certain amplitude (high amplitude or low amplitude).
Because of that, when you measure the signal, you’ll actually measure an average of high and low amplitudes. Therefore the rxlevel is lower.
FYI, the GMSK emmits always in “high” amplitude.
Your second question is about the “physical” capacity of a radio timeslot ?
One timeslot lasts 0.577 ms. During this amount of time, it’s possible to pack “x” symbols.
In GMSK, x symbols = x bits
In 8PSK, x symbols = 3*x bits.
Therefore the bitrate is increased by 3.
Don’t forget that a timeslot rate in GMSK is 22 kb/s, containing useful + protection + signalling
16kb/s is the timeslot rate on an E1 interface (Abis or Ater, for instance), which contains only the useful signal + some signalling.
If your quesiton is about E1 timleslots, then… it’s simple : each radio timeslot in 8PSK will be carried by “n” terrestrial timeslots.
For instance, 1 RTS in MCS9 will be carried by 4.5 TTS.