This paper introduces the principles of interferometry to multicarrier code division multiple access (MC-CDMA). Specifically, we propose the use of MC-CDMA with novel carrier interferometry (CI) complex spreading codes. The CI/MC-CDMA method, applied to mobile wireless communication systems, offers enhanced performance and flexibility relative to MC-CDMA with conventional spreading codes. Specifically, assuming a frequency selective Rayleigh-fading channel, CI/MC-CDMAs performance matches that of orthogonal MC-CDMA using Hadamard-Walsh codes up to the MC-CDMA user limit; and, CI/MC-CDMA provides the added flexibility of going beyond users, adding up to 1 additional users with pseudo orthogonal positioning. When compared to MC-CDMA schemes capable of supporting greater than users, CI/MC-CDMAs performance exceeds that of MC-CDMA. Additionally, this new system is analyzed in the presence of phase jitters and frequency offsets and is shown to be robust to both cases. Index Terms-Carrier interferometry, complex sequences, frequency diversity, multicarrier code division multiple access (MC-CDMA). I. INTRODUCTION M ULTICARRIER code division multiple access (MC-CDMA) [1] has emerged as a powerful alternative to conventional direct sequence CDMA (DS-CDMA) [2] in mobile wireless communications. In MC-CDMA, each user's data symbol is transmitted simultaneously over narrow-band subcarriers, with each subcarrier encoded with a 1 or 1 (as determined by an assigned spreading code). Multiple users are assigned unique, orthogonal (or pseudo-orthogonal) codes. That is, while DS-CDMA spreads in the time domain, MC-CDMA applies the same spreading sequences in the frequency domain. When perfectly orthogonal code sequences are transmitted over slow, flat fading channels with perfect synchronization, the performance of DS-CDMA and MC-CDMA is equivalent, as the orthogonal multiuser interference vanishes completely. However, in reality, wide-band CDMA signals sent over multipath channels experience more severe channel distortions and the resulting channel dispersion (i.e., frequency selectivity) erodes the orthogonality of CDMA signals. In such cases, it turns out to be far more beneficial to harness the signal energy Manuscript

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