You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Advances in cognitive radio networks: A survey 
Chapter Title:  Measurements-Based Sensing and Modeling: 
1. Abstract of Cited Paper (BIB001):  This paper describes the development, deployment and early results of a long-term, wide-band (30 MHz - 6 GHz) spectrum observatory system focused on downtown Chicago and the immediately surrounding areas. Previous short term studies have indicated that more than 80% of the spectral capacity may be unused, even in United States urban centers. The goals of this research effort include the validation of these early results, the understanding of trends in spectral usage over long periods (years), the observation of usage patterns and anomalies, and the detection of the positions of spectral "holes" in time and space. The current status in the attainment of these goals will be shared in this paper. This information, once obtained, vetted and appropriately analyzed will be extremely useful in the development of cognitive radios and other dynamic access network systems, and in the proper establishment of policies and regulations to govern the deployment of such systems.

Literature Review:
By collecting data over a long period of time at many base stations, BIB001 provides a unique analysis of cellular primary usage. The collected data is dissected along different dimensions to characterize the primary usage. With the aid of spectrum observatory, BIB002 extends short-term spectrum usage measurements to study the spectrum usage trend over long periods, observes spectrum usage patterns, and detects the positions of spectrum white space in time and spatial domains. Such information can be greatly helpful in developing good dynamic access protocols and governing secondary systems.