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:  E. Power Control in a CR Network 
1. Abstract of Cited Paper (BIB001):  In this paper, we consider a cognitive radio system in fading wireless channels and propose an opportunistic power control strategy for the cognitive users, which serves as an alternative way to protect the primary user's transmission and to realize spectrum sharing between the primary user and the cognitive users. The key feature of the proposed strategy is that, via opportunistically adapting its transmit power, the cognitive user can maximize its achievable transmission rate without degrading the outage probability of the primary user. If compared with the existing cognitive protocols, which usually try to keep the instantaneous rate of the primary user unchanged, our strategy relieves the cognitive users from the burden of detecting and relaying the message of the primary user and relaxes the system synchronization requirements. The achievable rate of a cognitive user under the proposed power control strategy is analyzed and simulated, taking into account the impact of imperfect channel estimation. A modified power control strategy is also proposed to reduce the sensitivity of our strategy to the estimation errors. Its effectiveness is verified by the simulations. 
2. Abstract of Cited Paper (BIB002):  We present a collaborative algorithm to enable opportunistic spectrum access for cognitive radios in the presence of multiple co-channel transmitters. A spectrum hole detection and estimation technique based on received signal strength observations is developed, which allows the coexistence of both licensed and unlicensed transmitters. We address the issue of how to perform collaborative spectrum sensing in the presence of multiple co-channel transmitters and how to determine the maximum transmit power that can be used for a given frequency channel by a cognitive radio while avoiding harmful interference to the licensed network. We provide some simulation results to validate the feasibility of our approach. 
3. Abstract of Cited Paper (BIB003):  Dynamic spectrum management can drastically improve the performance of wireless networks struggling under increasing user demands. However, performing efficient spectrum allocation is a complex and difficult process. Current proposals make the problem tractable by simplifying interference constraints as conflict graphs, but they face potential performance degradation from inaccurate interference estimation. In this paper, we show that conflict graphs, if optimized properly, can produce spectrum allocations that closely match those derived from the physical interference model. Thus we propose PLAN, a systematic framework to produce conflict graphs based on physical interference characteristics. PLAN first applies an analytical framework to derive the criterion for identifying conflicting neighbors, capturing the cumulative effect of interference. PLAN then applies a local conflict adjustment algorithm to address heterogeneous interference conditions and improve spectrum allocation efficiency. Through detailed analysis and experimental evaluations, we show that PLAN builds a conflict graph to effectively represent the complex interference conditions and allow the reuse of efficient graph-based spectrum allocation solutions. PLAN also significantly outperforms the conventional graph model based solutions. 
4. Abstract of Cited Paper (BIB004):  We consider power control in cognitive radio networks where secondary users identify and exploit instantaneous and local spectrum opportunities without causing unacceptable interference to primary users. We qualitatively characterize the impact of the transmission power of secondary users on the occurrence of spectrum opportunities and the reliability of opportunity detection. Based on a Poisson model of the primary network, we quantify these impacts by showing that (i) the probability of spectrum opportunity decreases exponentially with respect to the transmission power of secondary users, where the exponential decay constant is given by the traffic load of primary users; (ii) reliable opportunity detection is achieved in the two extreme regimes in terms of the ratio between the transmission power of secondary users and that of primary users. Such analytical characterizations allow us to study power control for optimal transport throughput under constraints on the interference to primary users. Furthermore, we reveal the difference between detecting primary signals and detecting spectrum opportunities, and demonstrate the complex relationship between physical layer spectrum sensing and MAC layer throughput. The dependency of this PHY-MAC interaction on the application type and the use of handshake signaling such as RTS/CTS is illustrated.

Literature Review:
In order to manage the interference among secondary users, or avoid harmful interference to primary users due to secondary spectrum usage, various power control schemes are considered in CR networks to coordinate spectrum sharing. Power control in opportunistic spectrum access (OSA) is studied in BIB005 , which models the packet transmission from source to destination in OSA as crossing a multi-lane highway. If a secondary user tries to use high transmission power to reach the destination in one hop, it has to wait until the primary user is inactive; on the other hand, it can take more advantage of the spectrum opportunities with lower transmission while relying on the intermediate users on the path to destination. The impact of transmission power on the occurrence of spectrum opportunities is investigated in BIB005 , and it is shown that the optimal transmission power of secondary users decreases monotonically with the traffic load of the primary network. Dynamic programming has been used in designing optimal power and rate control strategy, in order to maximize the long-term average rate for a secondary user BIB001 . An opportunistic power control strategy is proposed in BIB002 , which enables the cognitive user to maximize its transmission rate while guaranteeing that the outage probability of the primary user is not degraded. A collaborative spectrum sensing scheme that considers signal strength, localization and collaboration in the presence of multiple co-channel primary and secondary transmitters is proposed in BIB003 . The allowed maximum transmitter power of a secondary user in a given channel is determined, using a distributed database containing co-channel transmitter information including location, error estimates, power, etc. Conflict graph is commonly adopted to describe the interference constraints among users, where the node in a graph represents a user, and an edge between a pair of nodes represents the existence of interference. A systematic framework to produce conflict graphs based on physical interference model is presented in BIB004 , which characterizes the cumulative effect of interference while making it possible to use graph theory to solve spectrum allocation problems under physical interference constraints.