This paper deals with carrier frequency offset (CFO) estimation based on the minimum variance distortionless response (MVDR) criterion without using specific training sequences for interleaved orthogonal frequency division multiple access (OFDMA) uplink systems. In the presence of large CFOs, the estimator is proposed to find a new CFO vector based on the first-order Taylor series expansion of the one initially given. The problem of finding the new CFO vector is formulated as the closed form of a generalized eigenvalue problem, which allows one to readily solve it. Since raising the accuracy of residual CFO estimation can provide more accurate residual CFO compensation, this paper also present a decision-directed MVDR approach to improve the CFO estimation performance. However, the proposed estimator can estimate CFOs with less computation load. Several computer simulation results are provided for illustrating the effectiveness of the proposed blind estimate approach.

In this paper, a scheme for constructing the flat frequency spectrum of interleaved frequency division multiple access (IFDMA) is proposed. Since IFDMA is one of the single carrier modulation schemes, the frequency spectrum components are fluctuated and depend on the information data sequence. Even if IFDMA modulation scheme makes frequency spectrum dispersive for obtaining frequency diversity gain, frequency diversity gain is reduced by the fluctuation of frequency spectrum. In addition, in decision directed channel estimation (DDCE), which achieves good channel estimation accuracy in fast fading environment, the accuracy of channel transfer function estimated at the significant attenuated frequency component is much degraded. In the proposed technique, a random phase sequence is multiplied to the information data sequence for constructing the flat frequency spectrum. As a result, the frequency diversity gain is enlarged and the accuracy of channel estimation by DDCE is improved. Furthermore, we consider the blind estimation technique for the random phase sequence selected by transmitter. We show the effects of the proposed scheme by computer simulation.

This paper proposes a scattered-pilot-OFDM reception scheme employing turbo inter-carrier interference (ICI) cancellation in the fast varying fading environments of mobile communications. In the OFDM transmission, the orthogonality among the subcarriers cannot hold due to large Doppler shift, and the OFDM signal suffers from severe degradation due to ICI. The proposed receiver carries out two modes: (i) a coherent detection (CD) mode, and (ii) a turbo ICI cancellation (TC) mode. Initially, the receiver performs the CD mode. When any decision errors are detected, it shifts from the CD mode to the TC one that carries out both the ICI cancellation and the channel estimation by using the decoder output (the log likelihood ratio). In addition, the iteration of the TC mode can improve the accuracy of the channel estimation and ICI cancellation ability. Computer simulations following specifications for the mobile reception mode in the digital terrestrial television broadcasting demonstrate that the receiver can effectively cancel ICI due to the fast varying fading, and that its average BER performance is much better than that of CD.

When the Alamouti code is applied (as a space-time block code) to an OFDM system with transmit diversity, the simple Alamouti decoding requires that each subchannel is flat-fading and constant over two symbol periods (Alamouti codeword period). The second requirement makes the Alamouti decoding scheme not suitable for time varying channels. In this Letter, we propose a new decision directed receiver to better accommodate time varying channels.