Rotated spreading matrices in a coded MC-CDMA system

We investigate rotated Walsh-Hadamard spreading matrices for a coded MC-CDMA system in the synchronous downlink with robust channel estimation. Rotated Walsh-Hadamard spreading sequences increase the signal space diversity. The signal space diversity improves the performance in frequency and time selective fading channels. The Hamming distance of the convolutional code and the detector at the receiver define the improvements at common bit error rates of 10^-5 for the rotated spreading matrices. By applying rotated in comparison to standard Walsh-Hadamard spreading codes a higher throughput is achieved at channel code rates greater than 2/3. The improvements based on the rotated spreading sequences are similar in the case of a robust pilot aided channel estimator. Low complexity detectors, like the minimum mean square error detector do not benefit for the 4-QAM modulation scheme from the rotated scheme. However, through the rotation of the data symbols any loss is impossible because the average Euclidean distance between all spread chips is the same with or without rotating the data symbols. In a system with rotated spreading and an MLSSE detector at high channel code rates even the soft parallel interference canceller is outperformed.