Automated spectrum trading mechanisms: understanding the big picture

Public regulatory agencies have traditionally assigned radio-electric spectrum in a static way. This has led to an almost fully assigned but sparsely and unevenly used spectrum, in which it is becoming more difficult to accommodate the increasing demand of wireless communication. This work presents a general view of automated spectrum trading, one of the mechanisms proposed to improve spectrum efficiency. Licensed operators would be able to lease their unused bandwidth to unlicensed ones in secondary markets, satisfying real time demands from users. This results in a higher and more dynamic use of spectrum while providing incentives to spectrum owners for allowing secondary users to access their unused spectrum. Several approaches can be found in this research area combining game theory, economic models and auction design, among others. We describe and organize the main objectives and challenges of spectrum trading, and present a comprehensive classification and explanation of the existing research lines, showing how different works addressed each relevant issue, discussing the benefits and drawbacks of each approach. Finally, we highlight future research trends in this topic and identify critical but possibly overlooked problems.