where Sk is the modulated M-ary QAM symbol for the kth user. It should be note that, in cognitive-CDMA systems, one common spectrum availability vector, i.e., ATx =ATx 0 ,ATx 1 ,...,ATx N−1, is shared among all users at transmitter sides,and each user adopt a distinct PR poly phase vector for phase coding to accomplish multiple access capability.
The downlink transmitted signal in (7) is expressed in matrix as