Rotated spreading sequences for broadband multicarrier-CDMA

We present an extension of the pure Walsh-Hadamard spreading sequences for MC-CDMA. By rotating the phase of the modulated signal before spreading, the signal space is exploited more efficiently. After spreading, the Euclidean distance of each superposed constellation is distributed more favorably in the signal space than without rotating. Our novel approach combines the more favorable distributed Euclidean distance, caused by rotation, and a multiuser detector (MUD). A MUD exploits the more favorably distributed Euclidean distance, which a single-user detector, like, e.g., MMSE, is not able to do. The novel combination of these two techniques offers up to 3 dB gain for 4-QAM in comparison to the non rotated Walsh-Hadamard sequences in an MC-CDMA system. The performance gain gets lost in a coded system applying a convolutional code with a Hamming distance of ten. A gain of 1 dB can be retrieved by applying convolutional codes with higher code rates, that offer a lower Hamming distance of three for a fully loaded system.