Spatial Multidimensional Cooperative Transmission Theories And Key Technologies. Lin Bai
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of MS satellites, as shown in Fig. 1.3. Among them, each satellite is equipped with ML transmitting antenna arrays.
The frequency-selective MIMO satellite communication channel can be described by its channel matrix H(f). Due to the characteristics of the satellite communication system, the link is actually a non-fading and shadowless LOS channel. In groundwireless communication systems, we have demonstrated that orthogonal channels in the LOS channel can provide optimal channel capacity,34 which requires that the channel response between the transmitting and receiving antennas meets special requirements and is quasi-static. Since groundwireless system terminals are almost mobile, the assumption of quasi-static channels is not true in groundcellular mobile systems.
Fortunately, in the satellite communication system, the ground station has a very low movement speed relative to the satellite in most cases. The geometric arrangement of the receiving and transmitting antenna arrays is almost constant in a short time, and thus, the LOS channel can be approximated as static. Therefore, the satellite channel has unique advantages in realizing channel capacity optimization. Through the cooperative multi-beam transmission technology of constellation, we can achieve theoretically optimal antenna configuration, thereby increasing the capacity gain of the satellite communication system.
Fig. 1.3. Downlink of space-based cooperative transmission system.
Regardless of the noise generated during signal propagation, the propagation process of a frequency stationary signal from constellation in MIMO channel can be expressed as
where the groundreceiving signal vector is y = [y1, . . . , ymE]T, the transmitting signal vector of constellation is x = [x1, . . . , xms]T, and the channel matrix is
For a MIMO system, the highest spectral efficiency of the channel can be calculated by Telatar’s famous formula.35
where (·)H is the transpose of matrix and ρ is the linear signal-to-noise ratio of the channel. The signal-to-noise ratio of the channel is defined as SNR = 10 lg(ρ) = EIRP + (G − T)−κ − β [dB], where EIRP, (G−T), κ, and β are the effective isotropic radiated power, the quality factor, the Boltzmann constant, and the logarithm of downlink bandwidth, respectively. Since the distance between the satellite and the ground is much larger than the distance between array antennas, each element in the transfer matrix H can be considered to be of the same magnitude. Therefore, the transfer matrix H of the MIMO channel satisfying the maximum multiplexing gain is an orthogonal matrix. The theoretically optimal channel capacity can be achieved by adjusting the distance between the antennas and the distance between the constellations. The accessibility and conditions of the optimal channel capacity will be discussed in detail in Chapter 8.
1.4Summary
Traditional wireless communication mainly uses ground-based wireless communication systems. However, with the rapid development of wireless communication systems, it is increasingly requiring higher spectrum utilization, greater system capacity, more flexible network coverage, and lower construction costs. With the continuous improvement of aerospace technology and the rapid growth of the types and quantities of space-based and air-based platforms, the space–air–ground integrated information network consisting of satellites, stratospheric balloons, and various aerospace vehicles is developed. This chapter mainly summarizes the characteristics and development process of the ground-based, air-based, and space-based wireless communication systems. In the following chapters, we will elaborate on the space–air–ground integrated cooperative transmission theories and key technologies.
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