A fast OFDM-CDMA user demultiplexing architecture

A fast algorithm based on a butterfly structure is presented that demultiplexes the symbols of a particular type of MC-MA (Multi-Carrier Multiple-Access) modulation previously proposed for indoor radio communications [1]. A special transform is used to packetize the different symbols on the down-link such that the code sequences associated with the different symbols are transmitted synchronously at the base station. The input to the FFT bins is defined as a symboldependent combination of Walsh-Hadamard codes [2][3][4]. We derive a fast arquitecture that can combine the common redundancy found in the Walsh-Hadamard and the Inverse Fourier Transform. In the direct implementation, the receiver would compute the IFFT (as in OFDM) before the Walsh Hadamard transform. The proposed algorithm evaluates both transforms in a single step. A general expression for the butterfly weights is derived and the savings in computational complexity in terms of the frame length is evaluated.