White space: Definitional perspectives and their role in exploiting spectrum opportunities

The U.S. Federal Communications Commission (FCC) defines white space as the channels that are unused at a specific location or time. For futuristic cognitive radio (CR) based applications and communication networks, white space detection plays an important role. In fact, the proper white space understanding is a prerequisite for effective communication in support of a wide range of information technology systems. Moreover, by clearly defining the white space, the business and technical scenarios for white space usage can be clearly defined and their implementation will be simplified. Also, the decisions of regulatory bodies and telecommunications policy makers for auctions of particular spectrum bands can be facilitated by a thorough white space understanding. White space detection is a critical aspect of Dynamic Spectrum Access (DSA) which ultimately can help in overcoming bandwidth shortages. A major portion of the DSA research to date has been limited to the dimensions of time, frequency, and geographical location while neglecting other perspectives for the detection of white spaces. Generally, what exactly is a white space and how do white spaces differ in various modern contexts of wireless networks? This paper strives to answer these questions by reviewing the conventional white space definitions and exploring advanced perspectives on white spaces that can be used for CR communications. We propose a novel classification of white spaces based on the combination of three perspectives, namely signal dimension, licence, and transmission strategy, and outline open areas for future research on exploiting white spaces for CR communication.