HST (High-Speed Train) operation in FR2:
Unlike FR1, the FR2 case assumes that dedicated UE are mounted on the rooftop of the train to avoid penetration loss, and that this UE provides a link to serve users inside the train as a kind of mobile router. Operation in FR2 usually requires beam sweeping, where the UE/BS switches the transmitter (TX) and receiver (RX) beams to transmit/receive the signal, but this need is significantly reduced in the HST scenario. To reduce the amount of time it takes to search for the best TX/RX beam, it is assumed that the roof-mounted UE consists of two antenna panels; one RX beam pointing forward and the other RX beam pointing backward.
Two deployment scenarios have been considered for BS deployment in FR2. In scenario A, the BS is trackside, and in scenario B, the BS is non-trackside. By covering these two scenarios, the specification enables FR2 deployments in any mixture of deployment scenarios.
For scenario A, our investigation shows that due to the line-of-sight (LoS) path to the UE, excellent SNR can be obtained with a single TX and single RX beam. There is no need for beam management (that is, the procedure to keep the best TX/RX beam).
For scenario B, our investigation shows similar results to scenario A: the LoS path to the UE results in excellent SNR. It is possible to operate with single TX and RX beams, but the beam sweeping around 2 TX and 3 RX beams work best.
BS antenna deployment in FR2 can be either bi-directional or uni-directional. In uni-directional deployments, the BS antennas (TRxP) are placed along the track pointing in the same direction, whereas in bi-directional deployments the TRxP point both ways. Bi-directional deployment reduces the distance between train and antenna, but the Doppler shift changes rapidly when the UE panel is switched.
