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:  Classification by spectrum bands 
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. 
2. Abstract of Cited Paper (BIB002):  A number of wireless standards (e.g., IEEE 802.11af and IEEE 802.22) have been developed or are currently being developed for enabling opportunistic access in TV white space (TVWS) using cognitive radio (CR) technology. When heterogeneous CR networks that are based on different wireless standards operate in the same TVWS, coexistence issues can potentially cause major problems. Enabling collaborative coexistence via direct coordination between heterogeneous CR networks is very challenging due to incompatible MAC/PHY designs of coexisting networks. Moreover, the direct coordination would require competing networks or service providers to exchange sensitive control information that may raise conflict of interest issues and customer privacy concerns. In this paper, we present an architecture for enabling collaborative coexistence of heterogeneous CR networks over TVWS, called Symbiotic Heterogeneous coexistence ARchitecturE (SHARE). By mimicking the symbiotic relationships (i.e., the interspecific competition process) between heterogeneous organisms in a stable ecosystem, SHARE establishes an indirect coordination mechanism for spectrum sharing between heterogeneous CR networks via a mediator system, which avoids the drawbacks of direct coordination. Analytical and simulation results show that SHARE allocates spectrum among coexisting networks in a weighted-fair manner without any inter-network direct coordination. 
3. Abstract of Cited Paper (BIB003):  Cognitive radio (CR) technologies have led to several wireless standards (e.g., IEEE 802.11af and IEEE 802.22) that enable secondary networks to access the TV white-space (TVWS) spectrum. Different unlicensed wireless technologies with different PHY/MAC designs are expected to coexist in the same TVWS spectrum-we refer to such a situation as heterogeneous coexistence. The heterogeneity of the PHY/MAC designs of coexisting CR networks can potentially exacerbate the hidden terminal problem. This problem cannot be addressed by the conventional handshaking/coordination mechanism between two homogeneous networks employing the same radio access technology. In this paper, we present a coexistence protocol, called Spectrum Sharing for Heterogeneous Coexistence (SHARE), that mitigates the hidden terminal problem for the coexistence between two types of networks: one that employs a time-division-multiplexing-based MAC protocol and one that employs a carrier-sense-multiple-access-based MAC protocol. Specifically, SHARE utilizes beacon transmissions and dynamic quiet periods to avoid packet collisions caused by the hidden terminals. Our analytical and simulation results show that SHARE reduces the number of packet collisions and guarantees weighted fairness in partitioning the spectrum among the coexisting secondary networks. 
4. Abstract of Cited Paper (BIB004):  Compared to the homogeneous coexistence problems studied in previous chapters, the coexistence of heterogeneous wireless systems in TV “white space” (TVWS) is a more challenging issue to address. 
5. Abstract of Cited Paper (BIB005):  Recently, tremendous utilization of wireless networks has led to sever scarcity of radio spectrum resources. TV White Spaces (TVWSs) as novel bands enabling Cognitive Radio (CR) technology to improve spectrum resources utilization, have attracted significant standardisation efforts such as IEEE 802.11af, IEEE 802.16h and 802.22. As these heterogeneous networks may operate on the same channels of TVWSs, network coexistence problem cannot be avoided and is particularly challenging given the heterogeneous MAC/PHY layer protocols and operation parameters (e.g., tx power) employed in coexisting networks. In this paper, we develop a coexistence mechanism called ecological Species Competition based HEterogeneous networks coexistence MEchanism (SCHEME). Inspired by ecology based species competition model, SCHEME uses an ecological spectrum allocation method to assign available spectrums. Through both theoretical and simulation analysis, we demonstrate that SCHEME can achieve stable and fair spectrum allocation among coexisting networks. 
6. Abstract of Cited Paper (BIB006):  To meet the diverse traffic demands of different applications, emerging WLAN standards have been incorporating a variety of channel widths ranging from 5 to 160 MHz. The coexistence of variable-width channels imposes a new challenge to the 802.11 protocols, since the 802.11 MAC is agnostic of, and thus incapable of, adapting to the PHY-layer spectrum heterogeneity. To address this challenge, we uncover the cause and effect of variable-width channel coexistence, and develop a MAC-layer scheme, called Fine-grained Spectrum Sharing (FSS), that solves the general problem of fair and efficient spectrum sharing among users with heterogeneous channel-widths. Instead of deeming its spectrum band as an atomic block, an FSS user divides the spectrum into chunks, and adapts its chunk usage on a per-packet basis. FSS's spectrum adaptation is driven by a decentralized optimization framework. It preserves the 802.11 CSMA/CA primitives while allowing users to contend for each spectrum chunk, and can opportunistically split a wide-band channel or bond multiple (discontiguous) chunks to ensure fair and efficient access to available spectrum. In making such adaptation decisions, it balances the benefit from discontiguous chunks and the cost of guardband – a unique tradeoff in WLANs with heterogeneous channel-widths. Our in-depth evaluation demonstrates that FSS can improve throughput by multiple folds, while maintaining fairness of spectrum sharing for heterogeneous WLANs.

Literature Review:
Perhaps the most natural classification criterion of networks coexistence is their operating spectrum bands. Using this criterion, the representative solutions could mainly be divided into two spectrum bands, the 2.4 GHz ISM band, mainly from 2400 to 2484.5 MHz, and the TVWSs, mainly on very high frequency/ultra high frequency (VHF/UHF) bands ranging from 54 to 806 MHz in the case of USA. The reasons that these two bands have attracted both academic and industrial research attentions are as follows: The ISM spectrum bands are radio bands reserved internationally for the use of radio frequency (RF) energy for industrial, scientific and medical purposes other than telecommunications. Due to its license-free nature, many communication equipments are designed to operate on this spectrum band, among which two representative ones are WiFi  and ZigBee.  Given their ubiquitous presence in daily life and personal applications, designing coexistence mechanisms between them becomes a pressing topic. The FCC has recently authorized license exemption for operations of CR devices in TV bands left available by the transition from analogue to digital TV systems.  Given the overcrowding of the ISM band and a number of desirable signal propagation properties of TV bands, a large number of emerging standards based on CR have targeted them as their operation spectrums.   [16] 19 Heterogeneity by nature, these networks are expected to coexist in the same spectrum band, thus calling for efficient coexistence protocols among them. In what follows, a summary of coexistence solutions in these two spectrum bands would be stated. WiFi-WiFi coexistence: Since there are only three independent channels which can be used by different WiFi networks simultaneously, when there are more WiFi networks, they are required to coordinate among them to share the spectrum. In this regard, Zhang and Shin 20 study the partial spectrum overlapping between WiFi networks and propose a distributed mechanism called adaptive subcarrier nulling (ASN) to enable coexistence among partially overlapped WiFi networks by sharing overlapped spectrum. And in the work by Han et al., BIB007 Zhang's group introduces a coexistence 14,16 a two-degree coexistence mechanism is designed in IEEE 802.22 (networks coexistence and cells coexistence) to guarantee that distinct channels are chosen between neighbour base stations (BSs) and no conflicts exist in terminals' communication; coexistence beacons are employed in ECMA 392 to allow adjacent devices channel sharing. In IEEE 802.16h,  two types of coexistence mechanisms, uncoordinated mechanism and coordinated mechanism, are proposed in heterogeneous and homogeneous scenarios, respectively. Besides the solutions in existing standards, a number of novel coexistence mechanisms have been proposed for specific scenarios and applications. Zhao and colleagues BIB002 BIB001 proposed a fair MAC protocol to solve coexistence problem between CR networks and between CR networks and PUs using a three-state model. Bian et al. BIB004 introduced Spectrum Sharing for Heterogeneous Coexistence (SHARE) mechanism to mitigate hidden terminal problem and Symbiotic Heterogeneous Coexistence Architecture (SHARE) to establish coordination between heterogeneous CR networks. BIB003 BIB005 But a calculating fault in the work by Bian and colleagues BIB003 BIB005 has been found, so Zhang et al. BIB006 provided a better ecology-based spectrum sharing mechanism.