Multiple users adaptive modulation schemes for MC-CDMA

Multi-carrier CDMA (MC-CDMA) unites CDMA and OFDM together and potentially offers the advantages of both technologies. Under a frequency-selective fading channel, the subchannel frequency functions have different amplitude and phase. While the frequency domain spreading process gains some advantage from the frequency diversity available, it does not make the most of the different sub-channel conditions. Instead, the subcarriers should be adaptively loaded according to their states. This would improve the performance in a way similar to what adaptive bit loading offers in OFDM. The paper investigates this topic and proposes a configuration that not only permits multiple users to employ adaptive modulation, but also leads to an equivalent sub-carrier concept that enables a group of sub-carriers to be represented by an equivalent sub-carrier of a conventional OFDM modem, thereby allowing various powerful bit/power loading schemes, originally developed for OFDM, to be directly deployed to MC-CDMA.