Concept:
The bit rate of base baseband binary pulse amplitude modulation (PAM) with a raised-cosine spectrum is given as:
\({\rm{Bit\ rate}} = \frac{{2\left( {{\rm{BW}}} \right)}}{{\left( {1 + {\rm{α }}} \right)}}\)
Where,
BW= Transmission Bandwidth of PAM signal.
α = Roll-off factor. (α < 1)
Calculation:
Given: Bit rate = 56 kbps, α =.25
we know that,
\({\rm{Bit\ rate}} = \frac{{2\left( {{\rm{BW}}} \right)}}{{\left( {1 + {\rm{α }}} \right)}}\)
So,
\(56 = \frac{{2\left( {{\rm{BW}}} \right)}}{{1 + 0.25}} \)
B.W. = 28 × 1.25
B.W. = 35 kHz