Consider the above multi-user communication system: a) Explain what type of MAC is being used by this system? b) Show that this set of codes are orthogonal {{1,1,1,1},{1,+1,1,+1} and { 1,1,+1,+1}}? c) Given the transmitted data bits in the first interval only: 1,+1, and +1 from User 1, User 2, and User 3 respectively, and using the different codes, give the sum signal at the receiver. d) If the receiver wants to detect the 1st bit (+1) for User 3 , explain what type of operation is needed, and show how does it work