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:  F. Security 
1. Abstract of Cited Paper (BIB001):  Cognitive Radio (CR) is a promising technology that can alleviate the spectrum shortage problem by enabling unlicensed users equipped with CRs to coexist with incumbent users in licensed spectrum bands without inducing interference to incumbent communications. Spectrum sensing is one of the essential mechanisms of CRs that has attracted great attention from researhers recently. Although the operational aspects of spectrum sensing are being investigated actively, its security aspects have garnered little attention. In this paper, we describe an attack that poses a great threat to spectrum sensing. In this attack, which is called the primary user emulation (PUE) attack, an adversary's CR transmits signals whose characteristics emulate those of incumbent signals. The highly flexible, software-based air interface of CRs makes such an attack possible. Our investigation shows that a PUE attack can severely interfere with the spectrum sensing process and significantly reduce the channel resources available to legitimate unlicensed users. As a way of countering this threat, we propose a transmitter verification procedure that can be integrated into the spectrum sensing mechanism. The transmitter verification procedure employs a location verification scheme to distinguish incumbent signals from unlicensed signals masquerading as incumbent signals. Two alternative techniques are proposed to realize location verification: Distance Ratio Test and Distance Difference Test. We provide simulation results of the two techniques as well as analyses of their security in the paper. 
2. Abstract of Cited Paper (BIB002):  Over the past few years there has been a growing demand for radio resources and at the same time these resources are under utilized due to static spectrum allocation techniques. Dynamic spectrum access (DSA) has been thought of as a solution that would satisfy both the growing demand for radio resources and to efficiently utilize the spectrum. The radio devices that have the capability to dynamically sense the spectrum and access the under utilized bands are called cognitive radios (CR). There are two broad classes of users in CR, the primary user is a licensed user of a particular radio frequency band and the secondary users are unlicensed users who cognitively operate without causing harmful interference to the primary user. In this paper we consider a denial attack on centralized DSA networks where a malicious secondary user masquerades as a primary user and effectively shuts off access to all other secondary users. Note that this problem is unique to CR due to the distinction between primary and secondary users. We propose a public key cryptography based primary user identification mechanism that prevents malicious secondary users from masquerading as primary users. We show that the proposed identification mechanism and the associated key management are computationally light weight. We also discuss some advantages and limitations of the proposed identification mechanism.

Literature Review:
In Cognitive radio, an unauthorized user can change its air interface to look like a primary user. However, this phenomenon misleads the spectrum sensing performed by legitimate primary users. This type of attack is investigated in BIB002 . The possibilities of primary user emulation (PUE) attacks are realistic due to the facts that CR is highly reconfigurable due to the fact that they are software based air interface BIB001  . In order to stop such attacks, a robust transmitter verification scheme that can distinguish between legitimate incumbent primary signal transmitters and secondary signal transmitters needs to be designed BIB001 . The task of differentiating an incumbent primary signals user from secondary users becomes challenging when the requirement described in FCC's NPRM 03-322, which states that " no modification to the incumbent system should be required to accommodate opportunistic use of the spectrum by secondary user signal". The major technical challenge in spectrum sensing is distinguishing primary signals from secondary user signal. A public key encryption is proposed in BIB003 . The primary user encrypts its identification with its private key and appends the encrypted value (signature) to its transmission. All secondary users scan for the signature, during the sensing period, the signature from various base stations. The base station then verifies these signatures. Since the primary user knows its signature, a malicious secondary user cannot produce a valid signature.