Contents I. Introduction
Multi-carrier code division multiple access (MC-CDMA) is a combination of code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) techniques. Hence, it combines the benefits of CDMA with the natural robustness to frequency selectivity offered by OFDM. It gained recognition as a candidate to support multimedia services in wireless communication because of its convenience for high-bit-rate applications. However, MC-CDMA system is not suitable for mobile communication systems due to the frequency offset between the transmitter and receiver or introduced by Doppler shift in high mobility environment. With this frequency offset, the orthogonality among all the subcarriers is lost and intercarrier interference (ICI) is generated. Many methods have been proposed in the literature to mitigate the frequency-offset problem to cancel the ICI for OFDM system. In [1], authors developed low-complexity minimum mean-square error and decision-feedback equalizer receivers to suppress ICI based on the fact that the ICI power mainly comes from a few neighbouring subcarriers. Many data aided estimation [2][3] and blind estimation [4][5][6] schemes were proposed to estimate the frequency offset. Later an effective method known as the ICI self-cancellation scheme [7] has been proposed in which copies of the same data symbol are modulated on adjacent subcarriers using optimized weights. In [9], data-conjugate method of ICI self cancellation is proposed. There are only a few ICI cancellation technologies for MC-CDMA system [10] gives a detailed analysis of the MAI and ICI for MC-CDMA system In [11], the OFDM ICI self-cancellation is applied for MC-CDMA system with Hadamard-Walsh codes to reduce ICI and improve BER performance. To compensate for the network capacity (in terms of the number of users) loss in the proposed ICI self-cancellation method, a second set of orthogonal Hadamard-Walsh codes on the quadrature subcarriers are introduced. [12] gives total ICI cancellation by using orthogonality of ICI coefficient matrix, significantly by quantizing normalized frequency offset, but the complexity increases linearly with the number of quantizing levels. However, all existing ICI cancellation methods are not without their drawbacks. Even though all the existing ICI cancellation methods reduce ICI and improve BER performance for the system, the performance improvement is very limited. The more important, most of the existing ICI cancellation methods achieve the ICI reduction and BER performance improvement at the cost of lowering the transmission rate and reducing the bandwidth efficiency. There do exist some methods that do not reduce the date rate, however, such methods produce even less reduction in ICI. Nevertheless, only a few of these ICI cancellation methods are applied for MC-CDMA system with carrier frequency offset to improve the performance of mobile MC-CDMA system.
