Spectrum Coexistence Issues: Challenges and Research Directions

Spectrum congestion is a problem to both commercial and military systems. A recent Presidential Council of Advisors on Science and Technology (PCAST) study suggests that this problem is further amplified for military systems because not only demand for more bandwidth is increasing but portion of the spectrum primarily allocated to the military is recommended for commercial use. PCAST report also suggests that spectrum congestion is mainly due to inefficient use of spectrum rather than spectrum scarcity. This paper deals with challenges evolving due to coexistence of various commercial and military communications systems in wideband non-contiguous spectrum. A non-contiguous spectrum environment is characterized by spectrum holes fragmented across frequency range of interest. %This paper, in particular, focuses on different types of interference which will affect the quality of a spectrum hole and its utility for spectrum sharing application. In particular, we discuss the impact of spectrum sharing from the system perspective including hardware design, physical (PHY) layer intricacies, and medium access control (MAC) layer spectrum management. Since the number of spectrum holes required is application dependent, non-contiguous waveform design and pertinent design modifications and MAC layer improvements are also an intrinsic part of this paper.