Double orthogonal codes for increasing capacity in MC-CDMA systems

In this paper, for the first time, the notion of double orthogonal multi-carrier code division multiple access (DO/MC-CDMA) is introduced. Complex spreading codes of length N, capable of supporting 2N-1 users with exact orthogonality are used in the proposed system which is why it is named as double orthogonal. DO/MC-CDMA can be considered as a kind of carrier interferometry MC-CDMA (CI/MC-CDMA) (B. Natarajan et al., 2001) and therefore has all its privileges over conventional codes, capable of going beyond N users, such as the pseudo-orthogonal Gold codes. In addition, there is no need for pseudo-orthogonal codes for adding up N-1 extra users to the MC-CDMA N user limit; thus offering enhanced performance and lower BER. A frequency selective Rayleigh fading channel is analyzed, and it is shown that the performance of proposed system matches that of orthogonal Hadamard-Walsh codes up to N user limit; whereas the DO/MC-CDMA shows a much better performance compared with CI/MC-CDMA for extra users up to 2N-1.