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.

equency selective fading channels. However, OFDM systems can be extremely sensitive and vulnerable to synchronization errors. In this paper, we present a scheme for performing timing recovery that includes symbol synchronization and sampling clock synchronization in OFDM systems. The scheme is based on pilot subcarriers. In the scheme, we use a件path time delay estimation method to improve the accuracy of the correlation-based symbol synchronization methods, and use a delay-locked loop (DLL) to do the sampling clock synchronization. It is shown that by using this scheme, the mean square values of the symbol timing estimation error can be decreased by several orders of magnitude compared to the common correlation methods in both the AWGN and multipath fading channels. In addition, the scheme can track the symbol timing drift caused by the sampling clock frequency offsets.

An innovative and effective algorithm is proposed to estimate the spread-spectrum sequence of a direct spread-spectrum sequence (DSSS) signal in non-cooperative communication systems. In particular, a measurement function, which is used for sequence synchronisation, and a detection statistic, which is for sequence estimation chip by chip, are developed based on the idea of correlation. Only linear complex addition and multiplication are required for the implementation of the algorithm. Theoretical analysis and computer simulations verify the effectiveness of the algorithm.

The system capacity and performance of multicarrier code-division multiple-access (MC-CDMA) communication systems can be significantly enhanced by jointly employing MAP-based multiuser detection (MUD) and channel decoding techniques. In this paper, a group-oriented soft iterative MUD based on the combination of smart antennas and iterative MAP-based MUD is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. In this method, all the users are first grouped into several groups according to their impinging direction of arrivals (DOAs). All users with similar DOAs are classified into the same group and then low-complexity MAP-based iterative MUD is employed in each group. Because spatial filtering cannot suppress all the interference between the groups, interference cancellation among the groups is used prior to MUD within each group. It is shown that the proposed group-oriented soft iterative MUD algorithm can significantly reduce the computational complexity compared with the conventional optimal MAP-based MUD schemes. It is also demonstrated that the performance of the proposed algorithm can approach that of a single-user coded MC-CDMA system with an antenna array in additive white Gaussian noise and frequency selective fading channels.

This paper proposes a SNR estimation scheme based on Gibbs sampler [1]-[6]. This scheme can estimate SNR using a very short received sequence, and does not require any prior information of the transmitted symbol. Compared with the existing estimators, the performance of this method is better when real SNR is larger than 5 dB in both single path channel and multi-path channel.