Capacity of HAP-MIMO channels for high-speed train communications

Providing high data-rates to high-speed train (HST) in Ka-band is one of the interesting applications of high-altitutde platform (HAP). Theoretically in the high signal-to-noise ratio (SNR) regime, we can increase the channel capacity, hence the data-rate, proportional to the minimum number of antennas at the transmitter or receiver by implementing the multiple-input multiple-output (MIMO) systems. However, strong line-of-sight (LOS) exists in the HST wireless channel, which yields the MIMO channel matrix to be ill-conditioned and thus, the MIMO system fails to reach the multiplexing gain. Assuming uniform linear array (ULA) configurations at the transceiver, previous works have shown that in channels with strong LOS, capacity is maximized by obeying the antenna separation product (ASP) at the transceiver. Nevertheless, this product depends on the elevation angle between the incoming wave and the HST moving direction. Therefore, different train antenna separation distance values are required while the train is travelling in the HAP coverage area, which makes it unrealistic in practice. It is shown here that in Ka-band, the distance values change slowly for the elevation angles of interest. Assuming a 2×2 MIMO systems, an antenna separation distance of 83 cm at the train is a good compromise value which still achieve the same multiplexing gain.