The invention provides a rope sheave having two substantially opposed faces, each face having a plurality of ridges extending from an inner circumferential region of the face to an outer circumferential region of the face. In some embodiments, an apparatus of a base station in the MU-MIMO communication system may comprise: a power calculating logic to calculate a power for each user of a plurality of users communicating with the base station in the MU-MIMO system, based upon an average power and a power increment. RxRx<RyRy is satisfied, where Rx is a resistance between any two metal-back segments on the sides of a gap Gx, respectively, which is the function of voltage V V, and Rx is a resistance between any two metal-back segments on the sides of a gap Gy, respectively, which is the function of the voltage V V. The second word line includes at least one memory cell dynamically determined to be a weak cell.