The combination of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) is generally termed Multi-carrier CDMA (MC-CDMA). For future high-data-rate wireless applications, MC-CDMA has emerged as the most promising multiple access scheme due to its numerous advantages in both uplink and downlink communications. Carrier Interferometry (CI) code used as a spreading sequence for its inherently strong orthogonality nature; of length N can supports 2N users. In present work, a communication system which can support up to $3N$ number of users with variable data rate by allocating excess users with odd-even CI codes, has been proposed. In present work high power and spectral efficient CI/MC-CDMA system capable of variable data rate has been developed and its different quality parameters (i.e., peak to average power ratio (PAPR), bit error rate (BER), SNR, average data rate (ADR) etc.) have been examined and finally system is optimized with respect to number of subcarriers, number of users and SNR to achieve acceptable values of BER, PAPR and ADR. So PAPR reduction and as well as efficient receiver design are the main works done in this book.