A new and distinct cultivar of Verbena plant named Usbenal21, characterized by its semi-upright and outwardly spreading plant habit; vigorous growth habit; large violet-colored flowers; and resistance to Powdery Mildew. In the broadband wireless communication system, a scanning method of a serving base station includes: receiving information on a frequency use status of a neighboring base station from an upper network entity; and after determining a recommended scan frequency by using the received information, transmitting a scanning response message including the recommended scan frequency. A rail assembly on the base has a length.