Bandwidth efficiency in m - ary psk

[erece]

In M-ary psk
Rb=Rs*log M log with base 2
Rb- bit rate
Rs - symbol rate
then bandwidth required for passband signal is B=Rs
and bandwidth effeciency = Rb/B=log M log with base 2.
Is the above formula for bandwidth efficiency correct ?
But in simon haykin it is given that bandwidth efficiency is (log M )/2 log with base 2

 

[Kholdstare]

The channel bandwidth required to pass M-ary PSK is B=2/T
Now, T = Tb (log2 M) and Rb = 1/Tb
So rho = Rb/B = (log2 M) /2