Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

2MC-CDMA and MC-DS-CDMAIn this chapter, the different concepts of the combination of multi-carrier transmission withspread spectrum, namely MC-CDMA and MC-DS-CDMA, are detailed and analyzed.Several single-user and multi-user detection strategies and their performance in terms ofBER and spectral efficiency in a mobile communications system are examined.2.1 MC-CDMA2.1.1 Signal StructureThe basic MC-CDMA signal is generated by a serial concatenation of classical DS-CDMA and OFDM. Each chip of the direct sequence spread data symbol is mappedon to a different sub-carrier. Thus, with MC-CDMA the chips of a spread data symbolare transmitted in parallel on different sub-carriers, in contrast to a serial transmissionwith DS-CDMA. Let’s assume K be the number of simultaneously active users 1 in anMC-CDMA mobile radio system.Figure 2-1 shows multi-carrier spectrum spreading of one complex-valued data symbold (k) assigned to user k. The rate of the serial data symbols is 1/T d . For brevity, butwithout loss of generality, the MC-CDMA signal generation is described for a single datasymbol per user as far as possible, such that the data symbol index can be omitted. Inthe transmitter, the complex-valued data symbol d (k) is multiplied with the user specificspreading codec (k) = (c (k)0 ,c(k) 1 ,...,c(k) L−1 )T (2.1)of length L = P G ,whereP G is the processing gain. The chip rate of the serial spreadingcode c (k) before serial-to-parallel conversion is1T c= L T d(2.2)and is L times higher than the data symbol rate 1/T d . The complex-valued sequenceobtained after spreading is given in vector notations bys (k) = d (k) c (k) = (S (k)0 ,S(k) 1 ,...,S(k) L−1 )T . (2.3)1 Values and functions related to user k are marked by the index (k) ,wherek may take on the values 0, ..., K − 1.Multi-Carrier and Spread Spectrum Systems Second Edition K. Fazel and S. Kaiser© 2008 John Wiley & Sons, Ltd