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Carrier frequency offset (CFO) is a major contributor to the inter-carrier interference in orthogonal frequency division multiplexing (OFDM) systems. In order to overcome CFO at the receiver, a new OFDM symbol structure based on extending the transmitted symbol by inserting extra bits at the front of it before transmission is applied in this paper. The proposed method is capable of correcting frequency offset in the range of multiples of subcarrier spacing depending on the number of added extra bits. This is performed at the expense of transmitted power and data rate due to the insertion of extra bits. Simulation results indicate the performance improvement over both additive white Gaussian noise (AWGN) and multipath fading channels.

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Multi-carrier code division multiple access (MC-CDMA) systems are based on the combination of orthogonal frequency division multiplexing (OFDM) with CDMA. Various MC-CDMA schemes have been proposed in the literature. In this paper a system model for MC-CDMA is described in its basic form. The performance measures of MC-CDMA system is considered for Rayleigh fading environments. The bit error rate performance of MC-CDMA systems with both maximal ratio combining (MRC) detection and minimum mean square error (MMSE) detection are carried out by computer simulation. The results show that MMSE detection is more roubst than the MRC over Rayleigh fading environments. In addition, the behavior of the MC-CDMA system is compared with that one of DS-CDMA system. The results show that the MC-CDMA with MMSE detection outperforms the DS-CDMA in the practical case of Rayleigh fading environments.

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Third generation (3G) cellular communication standards are based on Wideband Code Division Multiple Access (WCDMA). The wideband signals experience frequency-selective fading due to multi-path propagation, hence increasing the Multiple Access Interference (MAI). To combat this effect, an efficient channel estimation algorithm, based on the Maximum Likelihood (ML) approach is derived. The algorithm handles the situation of multiuser, and multi-path by estimating a composite Channel Impulse Response (CIR). This estimation is performed in the reverse link, for each user, without needs of estimating each individual channel parameters. This has the achievement of reducing the computational complexity. The efficiency of the proposed channel estimator is measured in terms of Mean Squared Error (MSE), handling capacity, loss due to estimation, as well as resistance to MAI.

Orthogonal frequency division multiplexing (OFDM) is a technique which gives high quality of service (QOS) to the users by mitigating the fading signals as well as high data rates in multimedia services. However, the peak-to-average power ratio (PAPR) is a technical challenge that reduces the efficiency of RF power amplifiers. In this paper, we propose the combined Hadamard transform and modified meu-law companding transform method in order to lessen the effects of the peak-to-average power ratio of the OFDM signal. Simulation results show that the proposed scheme reduces PAPR compared to other companding techniques as well as the Hadamard transform technique when used on its own.