Despreading of WCDMA HSDPA signals using FFT based on the equivalent representation of MC-MC CDMA and WCDMA

We propose an method of implementing simultaneously despreading operation for high speed downlink packet access (HSDPA) wideband code division multiple access (W-CDMA) signals using fast Fourier transform (FFT). HSDPSA is a part of the W-CDMA standard and it provides an access method for high speed data communications. The basic idea of the HSDPA is similar to that of the orthogonal frequency division multiplex (OFDM), namely, instead of using multiple frequency to transmit symbols, HSPDA uses multiple spreading codes to transmit symbols. Although the idea is same, they uses different transformation for multiplexing: OFDM uses the fast Fourier transformation (FFT) and, on the other hand, HSDPA (WCDMA) uses Walsh-Hadamard transformation (WHT). Therefore, it becomes hard to implement these two technologies into a single configuration. The proposed method implement the despreading process as a combination of an FFT and a post-processing. By computer simulation, it is shown that the proposed method provides same BER performance with that of the direct despreading calculation. Moreover, we show that, by generalizing the configuration of the proposed method, WCDMA can be expressed as a special case of the multicoded multicarrier (MC-MC) CDMA. MC-MC CDMA is a newly proposed multiplexing method and is a mixture of the OFDM and CDMA technologies. We show that all of OFDM, MC-MC CDMA and WCDMA can be implemented by a combination of an FFT and a multiplication of orthogonal matrix. Hence, using the proposed method, we can implement those three technologies into a single configuration.