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

114 Hybrid Multiple Access SchemesThe SS-MC-MA and MC-CDMA systems have the following similarities:– SS-MC-MA and MC-CDMA systems exploit frequency diversity by spreading eachdata symbol over L sub-carriers.– Per subsystem, the same data detection techniques can be applied with SS-MC-MAand MC-CDMA systems.– ISI and ICI can be avoided in SS-MC-MA and MC-CDMA systems, resulting insimple data detection techniques.Their main differences are:– In SS-MC-MA systems, CDM is used for the simultaneous transmission of the dataof one user on the same sub-carriers, whereas in MC-CDMA systems, CDM is usedfor the transmission of the data of different users on the same sub-carriers. Therefore,SS-MC-MA is an OFDMA scheme on the sub-carrier level whereas MC-CDMA isa CDMA scheme.– MC-CDMA systems have to cope with multiple access interference, which is notpresent in SS-MC-MA systems. Instead of multiple access interference, SS-MC-MAsystems have to cope with self-interference caused by the superposition of signalsfrom the same user.– In SS-MC-MA systems, each sub-carrier is exclusively used by one user, enablinglow complex channel estimation, especially for the uplink. In MC-CDMA systems,the channel estimation in the uplink has to cope with the superposition of signals fromdifferent users, which are faded independently on the same sub-carriers, increasingthe complexity of the uplink channel estimation.After this comparative introduction of SS-MC-MA, the uplink transmitter and the assignedreceiver are described in detail in this section.Figure 3-5 shows an SS-MC-MA uplink transmitter with channel coding for the dataof user k. The vectord (k) = (d (k)0 ,d(k) 1 ,...,d(k) L−1 )T (3.10)pilot symbolgeneratordata sourceof user kchannelencodersymbolmapperinterleaverserial-to-parallelconverterspreaderc (0)OFDMd (k) s+withuser specificxfrequencyL−1 mapper. . .spreaderc (L−1)serial-to-parallelconverter0. . .Figure 3-5SS-MC-MA transmitter of user k