thank you everyone. i have implemented the system with 4 TX/RX. no doubt the BER and the Q factor are getting improved with this approach till 5000km. but if i am increasing the distance beyond that, with aperture diameter value 18 cm, for both transmitter and receiver, then the BER is not in acceptable limits.
so if i increase the aperture diameter say to about 25cm, then the system is improved. but this is going to increase the cost of the overall system and make it bulky too as abhishek and sam sung said above.
so i am not sure to implement this.
please suggest me whether this approach is good or some other option for improving the system at increased distance.