Oversaturating synchronous CDMA systems using collaborative coding

Oversaturated synchronous CDMA system is proposed based on collaborative coding, where data bits of group of L + s users are jointly one-to-one mapped onto 2/sup L+s/ L-dimensional signal vectors. Instead of the unique signature per each user, the L-dimensional signature subspace is used to transmit bits of L + s users (s > 0). All signature subspaces are orthogonal to each other which simplifies the optimal receiver structure. With signal space dimension N, the number of users is K = N(1 + s/L). Preferable collaborative codes are found using the sphere packing theory for L = 2...5, s = 1, 2. Trade-off between oversaturation efficiency and energy loss/gain is evaluated against the conventional non-oversaturated orthogonal CDMA.