Figure 1From: Joint Power Control, Base Station Assignment, and Channel Assignment in Cognitive Femtocell NetworksCognitive Radio Operation. The arrows show the movement of secondary clients from one spectrum hole to another. A spectrum hole is used to describe unused spectrum resource. The vertical axis represents the unit of spectrum resource. For WiMAX and LTE, spectrum resource is classified by frequency while for W-CDMA(UMTS), the spectrum is classified by code.Back to article page