| The multiple-input multiple-output (MIMO) system has been considered as one of the most important techniques owing to the significantly high capacity improvement compared with the traditional single-input single-output (SISO) wireless channel. With the development of the telecommunication system, the Channel State Information (CSI) at the transmitter is possible to be obtained with the cost of all system. In some cases, if the communication environment is slowly time varying, the CSI at the transmitter is possible to be obtained by feedback in frequency-division duplex (FDD) systems or the reciprocal principle in time-division duplex (TDD) systems. When CSI is achieved at the transmitter, the downlink precoding algorithms can be designed with these advantages: From one hand, the receiver error propagation can be avoided. From the other hand, the complexity of user mobile stations can be greatly reduced. As a result, how to realize the precoding algorithms in MIMO systems are hot-spot in communication researching area.In this paper, we bring a Transmit Power Allocation (TPA) matrix into the conventional MIMO-THP system for point-to-point transmission. In tradition MIMO-THP system, the MIMO channel is divided into multiple parallel sub-channels via matrix factorization. The performances of sub-channels are different. The average system performance is dominated by the worst sub-channel. As a result, we first introduce the TPA matrix to the conventional MIMO-THP system and an effective TPA method that minimizes the bit-error rate (BER) performance under sum-power constraint is proposed. Second, this method is capable of achieving further performance improvement because the precoding loss factor is considered in the derivations. When employed to an uncoded QAM, a tightly approximation expression of the complementary cumulative distribution function (Q-function) is introduced. Finally, a closed-form expression of TPA scheme is derived.After considering the outdoor multi-user transmission scenario, a THP structure for the downlink multi-user MIMO-THP system is proposed. During the derivation, unified expressions of the user signal to interference-plus-noise ratio (SINR) under ZF and MMSE criterions are achieved from the THP structure. According to the unified expressions, a closed-form solution of TPA matrix is determined with the minimum bit error rate (BER) criterion under sum-power constraint. It is referred to as the minimum BER-THP (MBER-THP) algorithm. Furthermore, an improved MBER-THP (IMBER-THP) algorithm is proposed under the novel structure, which invokes the precise precoding loss factor for performance improvement. Finally, the IMBER-THP algorithm is applicable to the ordered MIMO-THP system for maximizing the SNR of all users and achieves better performance than conventional ordered MIMO-THP systems. |
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