In this letter, we propose two low complexity algorithms for least square (LS) and minimum mean square error (MMSE) based multi-cell joint channel estimation (MJCE). The algorithm for LS-MJCE achieves the same complexity and mean square error (MSE) performance as the previously proposed most efficient algorithm, while the algorithm for MMSE-MJCE is superior to the conventional ones, in terms of either complexity or MSE performance.

In this letter, a low complexity multi-cell joint channel estimation (MJCE) scheme is proposed. With proper arrangement of the multi-cell midamble matrix and channel impulse response (CIR) vector, the MJCE operation is formulated to solve a block-Toeplitz linear system. The block-Levinson algorithm is adopted to solve this problem instead of the Cholesky algorithm. Our results show that the proposed MJCE scheme can be a practical choice with significantly lower complexity, compared with the previous schemes with the Cholesky algorithm.

In recent years, Pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. The technique is based on precoding of the transmitted signal relying on knowledge of the channel estimation response before transmission. This a priori channel state information is available in Time Division Duplexing (TDD) systems, since the same channel is used both in uplink and downlink. In practice, the error in channel estimation in Pre-Rake system occurs due to time variability in mobile radio channel. Most previous works on Pre-Rake in TDD CDMA have not taken into consideration the effect of imperfect channel estimation. In this paper, we present Pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler Frequency and compare it with the ideal Pre-Rake system. Numerical analysis and computer simulations were carried out to obtain the system error probability.

Conventional multiuser interference canceling is always done in the uplink. In this paper, a novel multiuser interference canceling scheme is introduced. It can be used in the downlink of the TDD-CDMA systems. The multiuser interference canceling is done through the joint optimization of both the spreading and despreading waveform for different users, instead of the despreading code only. The idea is to take the advantages of exploitation, rather than mitigation, of the multipath propagation effects. In TDD-CDMA systems, the channel parameter of the downlink can be estimated from the uplink. We can calculate the spreading and despreading code by using the instantaneous channel parameters, so they can accommodate to the characteristic of the multipath channel. The maximum receive signal to interference plus noise ration (SINR) criterion is used to optimize the spreading and despreading code. An iteration algorithm is given to create the optimal spreading and despreading code. Numerical results show that our scheme can greatly decrease the multiple access interference (MAI), hence greatly increase the SINR, compared with the conventional spreading code such as Gold code. Numerical results of the proposed scheme assuming different number of active users, Doppler frequency and coefficient estimation error are also given.

A novel multipath diversity scheme used in TDD-CDMA systems is presented. A FIR filter is added in the transmitter, while a RAKE combiner is used in the receiver. The optimum FIR filter problem may be viewed as an eigenvalue problem. The multiple antennas system is also analyzed. Results show that this new scheme can greatly improve the output Signal to Noise Ratio (SNR) compared with conventional RAKE receiver or Pre-RAKE diversity system.