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: Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review 
Chapter Title:  B. Hidden Primary User Problem 
1. Abstract of Cited Paper (BIB001):  In this paper we present an experimental study that comprehensively evaluates the performance of three different detection methods proposed for sensing of primary user signals in cognitive radios. For pilot and energy detection, our measurement results confirmed the theoretical expectations on sensing time performance. However, a physical implementation of these detectors in the presence of real noise uncertainties, analog impairments and interference allowed us to establish practical bounds on the detectable signal levels. In the case of collaborative detection, our analysis of experimental data collected in indoor environments identified the design parameters that can significantly improve the sensing gain: adaptive threshold, spatial separation and multiple antennas. 
2. Abstract of Cited Paper (BIB002):  Dual site spectrum measurements have been made in the public safety band in Howard County, Maryland, USA. The public safety band is of interest because of its obvious importance and the increasing need to determine whether improved spectral utilization would accommodate increased usage for public safety. Two receiver suites were synchronized and used to measure concurrent spectra at separations between a few meters up to a few kilometers. These measurements prove useful in examining issues of spectrum sensing for dynamic spectrum access, including receiver sensitivity, primary user signal detection, adjacent channel interference, and policy performance with local and cooperative sensing. We analyze the collected data to characterize the usage of this public safety network and provide insights into how white space can be identified and utilized. We propose a class of opportunistic access strategies and demonstrate how collected data can be used to evaluate the performance of such schemes.

Literature Review:
The hidden primary user problem is caused by several factors such as: -severe multipath fading or shadowing observed by secondary users while scanning for primary users. In this condition, the cognitive radio devices causes, unwanted interference to the primary user (receiver) as the primary transmitter, signal cannot be detected due to the location of devices. Cooperative sensing has been proposed as a means of handling hidden primary user problem BIB001 BIB002 BIB003 . Cooperative sensing is discussed in detailed in the later part of this paper.