Multi User Detection and Channel Estimation with Rotated Transforms for MC-CDMA

In this paper 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 the spreading the Euclidean distance of each superposed constellation is distributed more favorable in the signal space than without rotating. The novel approach of this paper combines the more favorable distributed Euclidean distance, caused by rotation, and a multiuser detector (MUD). A MUD does exploit the more favorable distributed Euclidean distance, which a single-user detector, like e.g. the 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.