  Millimeter wave (mmWave) communications are vulnerable to blockages and node
mobility due to the highly directional signal beams. The emerging
Reconfigurable Intelligent Surfaces (RISs) technique can effectively mitigate
the blockage problem by exploring the non-line-of-sight (NLOS) path, where the
beam switching is realized by digitally configuring the phases of RIS elements.
To date, most efforts have been made in the stationary scenario. However, when
considering node mobility, beam tracking algorithms designed specifically for
RIS are needed in order to maintain the NLOS link. In this paper, a fast
RIS-based beam tracking algorithm is developed by partly transforming the large
amount of signaling time into the calculation happens at base station in a
mmWave system with mobile users. Specifically, the differential form of optimal
RIS configuration is exploited as the updating beam tracking parameter to avoid
complex channel estimation procedure. The RIS-based beam tracking problem is
then transformed into an optimization problem whose solution is found by a
calculation-based search. Finally, by training on a small set candidate,
RIS-based beam tracking is realized. The effectiveness and efficiency of the
proposed RIS-based beam tracking algorithm is evaluated by simulations. It
shows that the proposed algorithm has near-optimal performance with dramatic
savings in terms of signaling time.
