Abstract: An apparatus and a method for estimating a Doppler frequency in a mobile communication system are provided. The apparatus includes an estimator for temporarily estimating a maximum Doppler frequency based on channel values sampled according to a first period in a frame, a detector for calculating at least one correlation value between at least one channel value additionally sampled in the frame and at least one channel value sampled over the first period, and for determining whether aliasing occurs based on the correlation value, and a determiner for determining a final maximum Doppler frequency according to the occurrence or non-occurrence of the aliasing.

Abstract: A hard disk drive apparatus to control a flying height of a magnetic head and a method thereof are provided. The hard disk drive apparatus includes a disk, a magnetic head to read data from the disk or to record data on the disk, a flying height maintaining unit to control a flying height of the magnetic head and to uniformly maintain the flying height above the disk, and a controller to control the flying height maintaining unit to maintain a first height as the flying height of the magnetic head in a seek mode, wherein the first height is greater than a second height at which a write operation or a read operation is performed by the magnetic head.

Abstract: A DC bias feedback circuit stabilizes the I.sub.ds of an active MESFET with respect to process-dependent I.sub.dss variations to improve yield and reduce the effect of process variations.