A shaft structure of a liftable screen cover includes a screen cover, a cover reinforcement plate and a shaft set. In one embodiment, an interface circuit exchanges signals with another electronic device via a signal transmission line. Rotor outlet passageways in the peripheral wall are arranged at an angle to cause the head to spin in a turbine-like fashion when air flow is directed from the chamber and out through the passageways. Upon movement of an object point, focusing is carried out with the fifth lens group G5. A first array is repeated in the frequency domain on a first OFDM symbol of the sub-frame, a second array is repeated in the frequency domain on a second OFDM symbol of the sub-frame and a third array is repeated in the frequency domain on a third OFDM symbol of the sub-frame, and each of the first, second and third arrays consists of six contiguous symbols in the frequency domain. The video conference is implemented between the remote user and the user in response to the user accepting the invitation.