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: Survey on coexistence of heterogeneous wireless networks in 2.4 GHz and TV white spaces 
Chapter Title:  Coexistence under presence of PUs 
1. Abstract of Cited Paper (BIB001):  Cognitive radio is viewed as a disruptive technology innovation to improve spectrum efficiency. The deployment of coexisting cognitive radio networks, however, raises a great challenge to the medium access control (MAC) protocol design. While there have been many MAC protocols developed for cognitive radio networks, most of them have not considered the coexistence of cognitive radio networks, and thus do not provide a mechanism to ensure fair and efficient coexistence of cognitive radio networks. In this paper, we introduce a novel MAC protocol, termed fairness-oriented media access control (FMAC), to address the dynamic availability of channels and achieve fair and efficient coexistence of cognitive radio networks. Different from the existing MACs, FMAC utilizes a three-state spectrum sensing model to distinguish whether a busy channel is being used by a primary user or a secondary user from an adjacent cognitive radio network. As a result, secondary users from coexisting cognitive radio networks are able to share the channel together, and hence to achieve fair and efficient coexistence. We develop an analytical model using two-level Markov chain to analyze the performance of FMAC including throughput and fairness. Numerical results verify that FMAC is able to significantly improve the fairness of coexisting cognitive radio networks while maintaining a high throughput.

Literature Review:
As mentioned in the previous section, in TVWSs, a particularity that makes the coexistence of CR networks challenging is the presence of PUs, whose transmission may not necessarily follow a predictable pattern. Hence, coordination among heterogeneous secondary networks under the presence of PUs consists of an importance design. We point out that although the PUs issue is mentioned in a number of CR coexistence standards (e.g. TVWSs database in IEEE 802.19.1 19 ), the problem of how to react when PUs arrive has not been specified. For example, in IEEE 802.19.1, it is mentioned that a PU list in TVWSs database can provide PU information to CMs to manage spectrum hopping of the coexisting networks, but the problem of coexistence under the presence of PUs is not specified. Zhao and colleagues BIB002 BIB001 develop a solution to this problem. However, their solution requires that the locations of PUs are prior known to the coexisting networks and are fixed. Such assumptions may be too strong although impractical in many cases. Even in the cases where the assumptions hold, their solution may not be effective or even fail to work if PUs are geographically close to cognitive nodes, which makes it difficult to distinguish the primary signal from secondary signal. Given the above argument, a primary building block enabling coexistence of CR networks with the presence of PUs is the detection of primary signals. Once the primary signal is detected, distributed mechanisms should be devised to let cognitive nodes react and coordinate the spectrum access among them. We illustrate the challenge in this phase via the following example. Two CR networks (A and B) operate on a band of 30 MHz. A occupies first 20 MHz and B occupies last 20 MHz with an overlapped 10-MHz band in the middle part that they share. The event that a PU arrives at first 10 MHz will not only trigger A to relocate its spectrum but also probably impact B who might need to adapt its operation band.