In this paper, we propose a simple frequency-domain interpolation technique for the discrete Fourier transform (DFT). This interpolation technique can significantly improve the frequency and phase resolution capabilities of the DFT without increasing the size of the DFT (the number of points used for the DFT). This new technique employs a dividing point in the amplitude and phase spectra. Suitable areas of application include joint estimation of fine frequency and phase offsets in burst-mode digital transmission.

We consider interference free decision feedback channel estimation (DFCE) for M-ary orthogonal modulation in direct sequence/code division multiple access (DS/CDMA) systems. The proposed DFCE uses the maximum combiner output in a RAKE receiver as the decision feedback information, enabling M-ary orthogonal signals to be demodulated coherently and a RAKE receiver to use a maximal ratio combining (MRC) scheme. Under multiuser environments, a multistage parallel interference cancellation (PIC) scheme is combined with the DFCE in order to cancel the interference. Accurate knowledge of the channel coefficient estimated by the proposed DFCE is used to regenerate the signal of each user for the multistage PIC scheme. According to the results of our simulations, the performance of coherent demodulation using the proposed system is significantly improved in comparison with conventional noncoherent demodulation.

Various studies have shown that adaptive antenna array techniques are good one to cancel multipath and multiuser interference. However, many of them need training signals to estimate the desired signal or have heavy complexity to compute the algorithmic processing. An antenna array receiver in a direct sequence/code division multiple access (DS/CDMA) system with M-ary orthogonal modulation is proposed. At every antenna element a channel estimation technique is used, which refers to the maximum correlator output of a RAKE receiver. The estimated channel coefficients enable the signal to be detected coherently with no need of training signals. Moreover, the antenna weights, which are determined by the estimated channel coefficients, make it possible to focus the receiver on the desired signal and to suppress the interference signal. Consequently, the proposed receiver has a 2D (dimensions) RAKE structure which can exploit both spatial diversity and temporal diversity. Simulation results show that the performance of the proposed scheme improves 1.5 dB in E/sub b//N/sub 0/ at a BER=10/sup -3/ over the receiver of noncoherent detection. The results also show that at a BER=10/sup -2/ the proposed receiver improves about 2.3 times and 3 times the system capacity with 3 and 5 antenna elements, respectively.