In order to ease the control message exchange, a constantly available control channel is demanded in cognitive radio ad hoc networks. Due to the characteristics of the cognitive radio network, available channel sets vary with both time and space. A globally fixed in-band control channel is not always ready to use, so a local and dynamic control channel is more preferred. In this paper, a spatially varied control channel assignment scheme, called SVC, is developed. SVC takes advantage of the spatial similarity among the available channel sets which neighboring secondary users hold. The secondary users believe that their neighbors would have the same control channel choices as theirs if they value the channels in the same way. Each user can observe the environment and make the decision independently. As a result, SVC is scalable and has the ability to recover immediately from the sudden appearance of primary users. Besides, a simple neighbor discovery mechanism is also introduced to solve the problem that most of the control channel assignment schemes require neighbor information in advance.