$2$ Design a FDMA system for multiplexing $4$ signals. Three of these signals are bandlimited to $3$ kHz$,$ while the fourth one is bandlimited to $12$ kHz$.$ The first carrier available is $200$ kHz and a guardband of $2$ kHz is used.$$

a$-$ Draw the spectrum of the composite signal and determine the required bandwidth if DSB and SSB are used.$$

b$-$ Sketch the block diagram of the transmitter.$$

c$-$ Sketch the block diagram of the receiver.