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:  D. Spectrum Sensing and Access 
1. Abstract of Cited Paper (BIB001):  Opportunistic spectrum access (OSA) that allows secondary users to independently search for and exploit instantaneous spectrum availability is considered. The design objective is to maximize the throughput of a secondary user while limiting the probability of colliding with primary users. Integrated in the joint design are three basic components: a spectrum sensor that identifies spectrum opportunities, a sensing strategy that determines which channels in the spectrum to sense, and an access strategy that decides whether to access based on potentially erroneous sensing outcomes. This joint design is formulated as a constrained partially observable Markov decision process (POMDP), and a separation principle is established. The separation principle reveals the optimality of myopic policies for the design of the spectrum sensor and the access strategy, leading to closed-form optimal solutions. Furthermore, it decouples the design of the sensing strategy from that of the spectrum sensor and the access strategy, and reduces the constrained POMDP to an unconstrained one. Numerical examples are provided to study the tradeoff between sensing time and transmission time, the interaction between the physical layer spectrum sensor and the MAC layer sensing and access strategies, and the robustness of the ensuing design to model mismatch. 
2. Abstract of Cited Paper (BIB002):  We design distributed spectrum sensing and access strategies for opportunistic spectrum access (OSA) under an energy constraint on secondary users. Both the continuous and the bursty traffic models are considered for different applications of the secondary network. In each slot, a secondary user sequentially decides whether to sense, where in the spectrum to sense, and whether to access. By casting this sequential decision-making problem in the framework of partially observable Markov decision processes, we obtain stationary optimal spectrum sensing and access policies that maximize the throughput of the secondary user during its battery lifetime. We also establish threshold structures of the optimal policies and study the fundamental tradeoffs involved in the energy-constrained OSA design. Numerical results are provided to investigate the impact of the secondary user's residual energy on the optimal spectrum sensing and access decisions. 
3. Abstract of Cited Paper (BIB003):  This paper considers the design of efficient strategies that allow cognitive users to choose frequency bands to sense and access among multiple bands with unknown parameters. First, the scenario in which a single cognitive user wishes to opportunistically exploit the availability of frequency bands is considered. By adopting tools from the classical bandit problem, optimal as well as low complexity asymptotically optimal solutions are developed. Next, the multiple cognitive user scenario is considered. The situation in which the availability probability of each channel is known is first considered. An optimal symmetric strategy that maximizes the total throughput of the cognitive users is developed. To avoid the possible selfish behavior of the cognitive users, a game-theoretic model is then developed. The performance of both models is characterized analytically. Then, the situation in which the availability probability of each channel is unknown a priori is considered. Low-complexity medium access protocols, which strike an optimal balance between exploration and exploitation in such competitive environments, are developed. The operating points of these low-complexity protocols are shown to converge to those of the scenario in which the availability probabilities are known. Finally, numerical results are provided to illustrate the impact of sensing errors and other practical considerations.

Literature Review:
Due to energy and hardware constraints, a secondary user may not be able to sense the entire spectrum space and can only access a limited number of channels from those it has sensed. To optimize spectrum access while considering physical layer spectrum sensing and primary user's traffic statistics, a decision-theoretic approach based on partially observable Markov decision process (POMDP) is proposed in BIB001 , which can optimize secondary users' performance, accommodate spectrum sensing error, and protect primary users from harmful interference. A separation principle BIB002 reveals the optimality of myopic policies for the spectrum sensor design and access strategy, and reduces the complexity of the POMDP formulation by decoupling the design of sensing strategy from the design of the access strategy. An extension of BIB001 is presented in BIB003 which incorporates the secondary user's residual energy and buffer state in the POMDP formulation for spectrum sensing and access. Cognitive medium access has also been modeled as a multiarmed bandit problem in BIB004 , and an efficient access strategy is developed that achieves a good balance between exploring the availability of other free bands and exploiting the opportunities that have been identified. Multi-cognitive user scenario is also considered, which is modeled as a game.