DS/CDMA with two sets of orthogonal spreading sequences and iterative detection

In this letter we introduce a direct-sequence code-division multiple access (DS/CDMA) concept which accommodates a higher number of users than the spr eading factor N, Each of the available orthogonal spreading sequences of le ngth N is assigned to one of the first N users which employ a common pseudo noise (PN) scrambling sequence. When the number of users K exceeds N, say K = N + M with M < N, the M additional users reuse M of those orthogonal spr eading sequences but in combination with another PN scrambling sequence. An iterative multistage detection technique is used to cancel interference be tween the two sets of users when K > N, The proposed technique thus accommo dates N users without any mutual interference and a number of additional us ers at the expense of a small signal to-noise ratio penalty.