Researches On Unitary Space-time Modulation In High-Speed Railway Communication Systems

When the wireless communications occurs between the relay on a high-speed train and the base station on the ground, because the train is always in plain area with a very fast velocity, the fading channel always exhibits significantly spatially correlated and con-tinuously fast time-varying (SCCFTV) characteristics. For a multiple antenna system, an efficient non-coherent technology called Unitary Space-Time Modulation (USTM) is very suited for high-speed mobile environment as it does not require the receiver to know the real-time channel state information (CSI). The thesis focuses on the transmitter/receiver design and performance analysis of the USTM system in high-speed railway scenarios involving the point-to-point, point-to-multipoint and multipoint-to-multipoint link. The main contents are summarized as follows.Firstly, for the point-to-point link, the optimal non-coherent receiver of the USTM system over SCCFTV fading channels is investigated. It is shown that, even though the system shows error floor effect at very high signal-to-noise ratio(SNR) region, the optimal receiver has a significant performance improvement in the low and medium SNR regime when compared to the non-coherent receiver without any channel information. Besides, the high spatial correlation of the channel causes the loss of the coding gain in the medium SNR regime, and the system performance can also not be improved by the awareness of the channel spatial correlation information at the receiver. In view of this, a simplified non-coherent receiver for high-speed railway scenario is proposed, which only utilizes the velocity information of high-speed train and omits the CSI estimation to reduce the computing complexity. The theoretical expressions for pair-wise error probability (PEP) of the proposed receiver, the optimal receiver and the suboptimal receiver are derived, respectively. Analysis and simulation results show that the proposed receiver can significantly outperform the non-coherent receiver without any channel knowledge and is very close to the optimal receiver in terms of bit-error rate performance.Secondly, a joint precoder and receiver design for USTM system over SCCFTV fad-ing channels is proposed. The design of precoders uses the transmit spatial correlation matrix of the channel, aiming at minimizing the mean-square error of channel estimation, where the receivers utilizes all of channel correlation information. Simulation results show that the proposed scheme can significantly outperform the optimal non-coherent receiver without precoding. At high enough spatial correlation and normalized maximum Doppler shift, our proposed scheme can even outperform the optimal non-coherent receiver on the spatially independent fading channel. Moreover, the PEP of proposed algorithm is derived, which are shown to corroborate with our simulation results.Thirdly, an optimal coherent receiver for USTM over continuously fast-varying fad-ing channels is proposed, which utilizes incomplete channel information, i.e., imperfect channel state information and the time correlation of channel response. Analysis and simulation results show that the error performance of the proposed receiver is better than that of the mismatched coherent receiver and optimal non-coherent receiver under the continuously time-varying Rayleigh fading channel. As the velocity of high-speed train varies over a wide range in high-speed railway scenarios, there is a wide range of Doppler frequency shift accordingly. In this situation, USTM provides more robust performance than the space-time block code(STBC), which shows that it is generally more suitable for high-speed railway communications.For a point-to-multipoint link, the transmitter/receiver design and error performance evaluation of a novel Multiplicative Superposition Signaling Scheme (MSSS) on the two-user multiple-input-multiple-output (MIMO) broadcast channel are studied, respectively. Different users may have different moving speed and therefore have different channel estimation capability. In this scenario, the exact CSI is available at the low-speed user (coherent user) while the high-speed user (non-coherent user) has no idea of the CSI. USTM and Multiary phase shift keying modulation with full spatial multiplexing are adopted by the non-coherent and coherent users, respectively. While we apply the non-coherent maximum likelihood detection to non-coherent user, for the coherent user, we propose two hard detection schemes based on maximum likelihood (ML) and minimum mean square error(MMSE), respectively, and a low-complexity soft detection scheme based on conditional expectation. Simulation results of error performance are presented for comparisons of different schemes. Furthermore, we derive the union bound of the symbol error rate for the ML-based hard detection scheme and the upper bound of that for the MMSE-based hard detection scheme.Finally, for a multipoint-to-multipoint link, the combination transmission scheme of unitary space-time codes(USTC) and blind interference alignment(BIA) over two-user heterogeneous X Channel is presented, where non-coherent user undergoes time-selective flat fading channel and coherent user undergoes frequency-selective and static fading chan-nel. Even if neither the transmitters nor the receivers know the channel state information, the receivers can effectively implement the interference alignment and decoding. As the non-coherent user cannot known the CSI, the non-coherent signal transmission is neces-sary, that is, desired signals of both user are modulated by USTM. Our simulation result shows that, compared with orthogonal transmission scheme without BIA, the proposed scheme possesses higher diversity gain and lower error rate in high SNR region.