Communication Systems

The number of telephone conversations by a fibre of band width 40MHz, with much less intensity loss, are:
In sky wave communication which of the frequency will be reflected back by the ionospheric layer having electron density $$4 \times 10^6/m^3$$ ?
The power of a  AM transmitter is 100 W. If the modulation index is 0.5 and the transmission is  having single side band, the percentage of useful power is:
In a communication system, noise is most likely to affect the signal
Cellular Mobile works in the frequency range of :