1. IEEE Communications Surveys & Tutorials: In a comprehensive survey on wireless coexistence, the authors define the core problem: "The problem of coexistence arises when two or more otherwise independent wireless systems operate in the same frequency band and in close physical proximity." This establishes that managing frequency use is the central issue.
Source: Vanlin Sathya, et al. "A Survey of Wireless Coexistence: Challenges, Solutions, and Opportunities." IEEE Communications Surveys & Tutorials, vol. 18, no. 1, Firstquarter 2016, pp. 22-46, Section I (Introduction). DOI: 10.1109/COMST.2015.2471153.
2. University Courseware (Carnegie Mellon University): Course materials on wireless networks emphasize that the physical layer's primary challenge is sharing the wireless medium (spectrum). Techniques like Frequency Division Multiple Access (FDMA), which is the basis for channel planning, are presented as fundamental methods to allow multiple systems to operate simultaneously without interference.
Source: Ganti, R. K. (2012). 18-759: Wireless Networks, Lecture 3: Wireless Physical Layer. Carnegie Mellon University, Department of Electrical and Computer Engineering. Slide 12, "Multiple Access".
3. University Courseware (Stanford University): In lectures on wireless communication, the concept of interference as the limiting factor in wireless network capacity is a core topic. The necessity of orthogonal resource allocation (such as using different frequency channels) is highlighted as the primary method to mitigate this interference and enable coexistence.
Source: Goldsmith, A. (2009). EE359: Wireless Communications, Lecture 1: Introduction to Wireless Communications. Stanford University. Section on "Challenges in Wireless Communications".