Secondary Use of Radio Spectrum: A Feasibility Analysis Arnon Tonmukayakul and Martin B.H. Weiss School of Information Sciences University of Pittsburgh Pittsburgh, PA 15260 25 August 2004 Abstract Enhancing spectrum efficiency and use is a significant task of regulatory authorities worldwide. A number of measurement studies of spectrum utilization have indicated that spectrum is sporadically used in many geographical areas and times. Low utilization and increased demand for the radio spectrum resource suggests the notion of secondary use, which allows unused parts of spectrum to become available temporarily for commercial purposes. The secondary use of spectrum is one of the promising ideas that can mitigate unsatisfied spectrum demand, potentially without major changes to incumbents. In this paper, we intend to outline the issues and discuss further study with the goal to determine what the necessary conditions are for spectrum sharing to be feasible. We consider the basic elements of wireless communications that utilize radio spectrum space (signals and the channels) in our analysis. The signals and channels of potential interest are expected to exist in at least three dimensions: geographical space, time, and frequency. Different signals/channels occupy different subspaces, therefore allowing us to locate and distinguish one from another. The model provides useful graphical information to clarify the concerned topics at hands. This feasibility study of the secondary use takes several factors into consideration, including the availability of spectrum, interference, mobility, practicality of communications, and service applications. We perform preliminary studies of three basic scenarios relating to three basic methods of separating channels – Frequency Division (FDM), Time Division (TDM), and Code Division Multiplexing (CDM). Exploring the problems of providing secondary use of spectrum gives us the ability to consider basic obstacles to secondary use, including why primary users would allow secondary use and, equally important, under what circumstances secondary users might emerge. The reality of identifying the pre-conditions for secondary use is a crucial step towards higher level of efficient spectrum utilization.