Narrowband interference (NBI) is one of the mostimportant problems in an OFDM system, which not only degrades the signal performance on the overlapped subcarriers, but also affects the signal performance on the nearby subchannels due to the spectral leakage effect of DFT demodulation at the receiver. In this paper, we consider the NBI problem caused by narrow band digital communication systems, and propose a novel NBI cancellation method. We consider the NBI reconstruction problem including NBI central frequency and its bandwidth estimation. Simulation results show that this method can achieve about 6dB signal to interference plus noise ratio (SINR) gain if the interference bandwidth is about twice of that OFDM bins and the system SINR is about 0 dB. As the bandwidth of NBI decreases, more SINR gain will be obtained. Meanwhile, We also consider the system implementation problem, it has shown that its implementation complexity is not high based on the current digital signal processing techniques. If zoom FFT is employed in our system, the calculation complexity can be reduced considerably1.

In recent years, combining multiuser detection (MUD) and channel decoding has received considerable attention. The maximum a posteriori (MAP) criterion-based iterative multiuser detector greatly improves the system performance and can approach the performance of single-user coded systems. However, its complexity increases exponentially with the number of users and can become prohibitive for systems with a medium-to-large number of users. In this paper, a reduced complexity MAP-based iterative MUD based on the use of a soft sensitive bits algorithm is proposed for coded multicarrier code-division multiple-access systems. It is shown that it can greatly reduce the computational complexity with a minimal penalty in performance compared to the conventional optimal scheme.

In this paper, we consider the subchannel detection problem in forward links for the multicarrier DS-CDMA system when some different subchannel allocation policies are used. An optimal subchannel decision algorithm is proposed based on the maximum-likelihood (ML) criterion. Theoretical analysis and simulation results are presented. Furthermore, we discuss the parameter selection problem on the length of the training sequences in the subchannel allocation scheme in [8], [12] by using the proposed ML detection algorithm. The results show that the subchannel allocation scheme in [8], [12] is feasible since only a few symbols overhead is required.

In this paper, we present a new transmit diversity scheme with power control by using two transmit antennas in which the power control unit is added to adaptively suit the channel fading variation. Compared to the transmit diversity scheme (STD, one space time coding scheme) proposed by Alamouti and the traditional maximal ratio combining (MRC) diversity scheme employed at the receiver, simulation results indicate that the new scheme has considerable performance gain. We also discuss the effects of the imperfect channel parameter estimation on the performance of the system. Simulation results show that the new system is more robust to the estimation error of channel fading parameters than the STD. As the signal to noise ratio is relatively high, the diversity scheme with power control is more sensitive to the channel estimation error compared to the MRC. But when the channel estimation accuracy is relatively high, the diversity scheme with power control still has better performance than the ideal MRC as the BER is about 1×10−3.

Space-time coding is a powerful scheme that combines channel coding, modulation, and multiple transmit antennas to achieve higher data rates and combat fading in wireless systems. In this letter, we propose a multiple-input multiple-output minimum mean-square error spatial-filtering-based daptive antenna arrays method to suppress cochannel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time tructure, thereby significantly improving the system's interference suppression ability without significant bit error rate performance degradation.