Research On Key Technologies Of Short-Range Multi-Antenna Wireless Communications

To meet the demand of low power consumption,ubiquitous access requirement and extremely high spectral efficiency for the next generation mobile communication systems, the multi-antenna techniques in the short-range scenarios have attracted significant attention. The most striking feature is that the size of the array aperture is comparable with the transceiver distance and the spherical-wave characteristic of electromagnetic signal propagation is more obvious. Hence the plane-wave assumption in long-range communications is not appropriate in short-range communications. However, the research on the communication capability and the methods of realizing the communication capability in short-range multi-antenna channels are still insufficient, and this thesis investigates these two aspects.For the communication capability, we investigated the capacity and spatial degrees of freedom of line-of-sight(LOS) channels in short-range multi-antenna channels. For the methods of realizing the communication capability, we investigated the increased capacity by the antenna placement optimization and the precoding transmission for short-range multi-antenna channels. The details of the investigation are as follows:Firstly, the analytical expression for the capacity of multiple-input two-output short-range LOS channels with the spherical-wave model is derived without channel state information(CSI) at the transmitter. Based on this obtained capacity expression, the exact analytical expressions for the probability density function(PDF), cumulative distribution function(CDF), and expectation of the capacity are derived in two-input two-output short-range LOS channels. Meanwhile, the ergodic capacity of multiple-input two-output short-range Ricean channels is then investigate in rich scattering environments. It is shown by the analytical and simulation results that:The capacity of short-range multi-antenna LOS channels is determined by the wavelength, the number of transmit antennas, the inter-element spacings, the transceiver distance and the orientations of the arrays. And the capacity fluctuates significantly with these parameters.The capacity statistics by the spherical-wave model is determined by the array-aperture to transceiver-distance ratio. Meanwhile, the probability of higher capacity can be obtained for the bigger array-aperture to transceiver-distance ratio.The ergodic capacity of multiple-input two-output short-range Ricean channels with rich scattering environments is determined by the Ricean K-factor and the antenna placement. And the maximum ergodic capacity can be obtained by designing the antenna placement. In particular, when the signal-to-noise ratio(SNR) is 20 d B and the Ricean K-factor is 5d B, the maximum ergodic capacity is 12.85bps/Hz for the three-input two-output array by designing designing the antenna placement.Secondly, by using the spherical-wave model, the analytical expressions for the spatial degrees of freedom of short-range LOS channels and long-range LOS channels are respectively derived. Both the analytical and simulation results show that the spatial degrees of freedom of long-range LOS channels for uni-polarized linear arrays is equal to 1, whereas multiple degrees of freedom can be offered by appropriately designing array orientations and sizes in short-range communications with the spherical-wave model.Thirdly, the increased capacity of short-range multi-antenna LOS channels is obtaied by designing the antenna placement. For the two-input two-output antenn array with arbitrary orientations in fixed wireless access, considering the joint effects of path loss and phase differences, we derive the explicit analytical expressions for the optimal antenna placement which maximize the capacity of short-range LOS channels. For the mobile station moved in indoor, based on an ant colony algorithm, we propose the scheme of the transmit antenna placement optimization which increases the area capcity.Finally, by using the spherical-wave model, we derive the analytical expressions for the transmit directions and the power allocation of the precoding transmission in short-range LOS channels and Ricean channels with rich scattering environments, respectively. Meanwhile, a necessary and sufficient condition for optimality of beamforming and spatial multiplexing is derived. Both the analytical and simulation results show that the capacity by the precoding transmission is greater than or equal to that by the equal power in all directions, and the difference becomes more significant with the absolute value of the cosine of the angle between the two spatial signatures. In particular, when the SNR is 5d B and the Ricean K-factor is 30 d B, the difference is 0.7bps/Hz for the two-input three-output antenna array.The research results in this dissertation for short-range multi-antenna wireless communications can be applied to the future wireless communication systems such as next generation wireless network, wireless broadband access network, and the Internet of things, which are of great practical guiding significance.