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

MC-CDMA 83where Q different user groups transmit simultaneously in one OFDM symbol. Each usergroup has a specific set of sub-carriers for transmission, which avoids interference betweendifferent user groups. Assuming that each user group applies spreading codes of lengthL, the total number of sub-carriers isN c = QL, (2.93)where each user exploits a subset of L sub-carriers for data transmission. Depending onthe coherence bandwidth (f ) c of the channel, it can be sufficient to apply spreadingcodes with L ≪ N c to obtain the full diversity gain [20, 26].To exploit the frequency diversity of the channel optimally, the components of thespread sequences s q ,q = 0,...,Q− 1, transmitted in the same OFDM symbol are interleavedover the frequency. The interleaving is carried out prior to OFDM. The OFDMsymbol duration (including the guard interval) isT ′s = T g + QLT c . (2.94)Only one set of L spreading codes of length L is required within the whole MC-CDMAsystem. This set of spreading codes can be used in each subsystem. An adaptive sub-carrierallocation can also increase the capacity of the system [2, 13].2.1.9.3 M&Q-ModificationThe M&Q-Modification combines the flexibility of the M- andtheQ-Modification. Thetransmission of M data symbols per user and, additionally, the splitting of the users in Qindependent user groups according to the M&Q-Modification is illustrated in Figure 2-13.The total number of sub-carriers used isN c = MQL, (2.95)where each user only exploits a subset of ML sub-carriers for data transmission due to theOFDMA component introduced by Q-Modification. The total OFDM symbol duration(including the guard interval) results inT ′s = T g + MQLT c . (2.96)A frequency interleaver scrambles the information of all sub-systems prior to OFDM toguarantee an optimum exploitation of the frequency diversity offered by the mobile radiochannel.The M-, Q-, and M&Q-Modifications are also suitable for the uplink of an MC-CDMAmobile radio system. For Q- andM&Q-Modifications in the uplink only the inputs ofthe frequency interleaver of the user group of interest are connected in the transmitter;all other inputs are set to zero.Finally, it should be noted that an MC-CDMA system with its basic implementation orwith any of the three modifications presented in this section could support an additionalTDMA component in the up- and downlink, since the transmission is synchronized onOFDM symbols.