Mechanical interaction in a multi-actuator is suppressed by utilizing a trapezoidal driving current. Fundamental performance of this method is examined experimentally and analytically.

Millimeter-wave (mmw) frequency bands, especially 60GHz unlicensed band, are considered as a promising solution for gigabit short range wireless communication systems. IEEE standard 802.11ad, also known as WiGig, is standardized for the usage of the 60GHz unlicensed band for wireless local area networks (WLANs). By using this mmw WLAN, multi-Gbps rate can be achieved to support bandwidth-intensive multimedia applications. Exhaustive search along with beamforming (BF) is usually used to overcome 60GHz channel propagation loss and accomplish data transmissions in such mmw WLANs. Because of its short range transmission with a high susceptibility to path blocking, multiple number of mmw access points (APs) should be used to fully cover a typical target environment for future high capacity multi-Gbps WLANs. Therefore, coordination among mmw APs is highly needed to overcome packet collisions resulting from un-coordinated exhaustive search BF and to increase total capacity of mmw WLANs. In this paper, we firstly give the current status of mmw WLANs with our developed WiGig AP prototype. Then, we highlight the great need for coordinated transmissions among mmw APs as a key enabler for future high capacity mmw WLANs. Two different types of coordinated mmw WLAN architecture are introduced. One is distributed antenna type architecture to realize centralized coordination, while the other is autonomous coordination with the assistance of legacy Wi-Fi signaling. Moreover, two heterogeneous network (HetNet) architectures are also introduced to efficiently extend the coordinated mmw WLANs to be used for future 5th Generation (5G) cellular networks.

The authors have studied the demagnetization phenomenon which is observed in a conventional CoCrTa/CoZrNb double-layered (DL) perpendicular recording medium. The authors have also investigated the effects of an in-plane hard magnetic layer in a triple-layered (TL) perpendicular recording medium. The in-plane hard magnetic underlayer is made of CoSm or CoCrTa/Cr and is laid under the CoZrNb soft magnetic layer. In the DL medium, a demagnetization phenomenon i.e. decrease of the readback signal, is observed when the CoCrTa layer has a strong perpendicular orientation and the CoZrNb underlayer has a low coercivity. The amount of the signal decrease depends strongly on the accumulated disk revolutions. This demagnetization is considered to be caused by fact that the recorded magnetization in the CoCrTa layer is reduced by the magnetic field generated from the domain walls in the CoZrNb layer, since the CoZrNb layer is very sensitive to a magnetic environment such as geo-magnetism and domain walls move as the disk rotates. On the other hand in the TL medium, the hard magnetic layer has an effect of pinning the magnetic domain in the CoZrNb layer, by which the demagnetization problem is successfully prevented. The hard magnetic layer remarkably reduces the domain walls in the CoZrNb layer and contributes to medium noise reduction. Thus the TL medium presents a higher SN ratio than DL medium.