A study on MMSE combining for MC-CDMA

In MC-CDMA system, the combining method based on MMSE criterion is well known as one of the methods that reduce multiple access interference. There are two cases of code spreading; in frequency domain and in time domain. In this paper, we analyse the receiving characteristics using MMSE combining in time-domain code spreading. MMSE combining in time domain cannot work effectively in slow fading channels, because time diversity affected by code spreading cannot be obtained. So, we propose a code spreading method to maximally bring out the effect of MMSE; in this method chips of the spreading code are separated spatially in the time domain so as to reduce time correlation of received level fluctuation and obtain time diversity effect as much as possible. We clarified the proposed method dramatically improved receiving characteristics in slow-to-fast fading environments as time interval between chips is wider.