Analysis of QPSK Satellite link and BER

A satellite communication link transmits a QPSK signal at a rate of 10 Msps. The transmitter and receiver have ideal RRC filters with roll-off factor r = 0.25. At the input to the QPSK demodulator in the receiver, the CNR is 18.5 dB under clear air conditions (no rain in the link).

a. What is the bandwidth occupied by the QPSK signal?

b. What is the BER at the output of the receiver in clear air conditions, and the average time between errors? Give your answer in years, days, or hours as appropriate. Assume an implementation margin of 0.5 dB.

c. Rain in the satellite to earth path causes the overall CNR in the earth station receiver to fall to 15.5 dB. The link has an implementation margin of 0.5 dB.
What is the BER at the output of the receiver, and the average time between errors? Give your answer in hours, minutes, or seconds, as appropriate.

d. Why is it necessary to include an implementation margin when estimating the BER expected in a real digital communications link?