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This paper provides a comprehensive survey on the coexistence of cellular and IEEE 802.11 standards from a holistic viewpoint that takes into account the coexistence of all existing and future cellular and IEEE 802.11 standards in all the available unlicensed spectrum bands. Unlike existing survey works focusing mostly on any unlicensed band and/or standard, we start by giving an overview of unlicensed spectrum bands, including 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. We then review the operation of cellular technologies, namely Long-Term Evolution Unlicensed (LTE-U), Licensed Assisted Access (LAA), and New Radio Unlicensed (NR-U), worldwide in the unlicensed spectrum bands. Further, we summarize scenarios and categories of coexistence mechanisms, conditions for a fair coexistence, and coexistence-related features. An extensive study on the coexistence mechanisms, deployment scenarios, as well as standardization efforts for the coexistence between cellular and IEEE 802.11 standards, is carried out. Finally, we highlight the coexistence challenges and open problems, the convergence of the Third Generation Partnership Project (3GPP) and IEEE standards, as well as future research directions. Moreover, to provide insights on the relative measures, we also carry out comparative studies of several key concerns with regard to the coexistence, namely unlicensed spectrum band, regulatory requirement, coexistence mechanism, and cellular standardization effort. Each study presents a comparison among potential features of one of these concerns in tabular forms. Finally, we summarize key lessons that are learned and discussed throughout the paper.

In this paper, we present a framework to give a comprehensive review of how to improve the spectrum utilization of millimeter-wave (mmWave) systems using indoor small cells in multi-operator network scenarios. More specifically, the framework concerns with the improvement of the utilization of the 28 GHz mmWave spectrum allocated to an arbitrary number of mobile network operators (MNOs) in a country using numerous spectrum allocation techniques, namely Static and Equal Spectrum Allocation (SESA), Flexible and Unequal Spectrum Allocation (FUSA), and Countrywide Full Spectrum Allocation (CFSA). A number of spectrum utilization improvement mechanisms such as spectrum trading, spectrum sharing, and spectrum reusing are then exploited into SESA, FUSA, and CFSA techniques in four major-interconnected domains, including time, frequency, power, and space. Using the Equal Likelihood Criterion and the properties of left-justified Pascal’s triangle, the average capacity, spectral efficiency (SE), and energy efficiency (EE) performance metrics for each spectrum allocation technique in each domain are derived. Extensive system-level numerical and simulation results and analyses are carried out to evaluate the performance of each technique in each domain for example scenario of a country with four MNOs. Overall, it is shown that, in the power-domain, CFSA outperforms SESA and FUSA (when operating either at the interweave or at the underlay spectrum access technique), whereas, in the time-domain and frequency-domain, SESA and FUSA outperform CFSA, in terms of the average capacity, SE, and EE. Finally, we show that CFSA in the power-domain outperforms SESA and FUSA operating in any domain to achieve the prospective SE and EE requirements for the sixth-generation (6G) mobile networks.

The traditional static allocation of a portion of the licensed spectrum specified for a country to a mobile network operator (MNO) is no longer sufficient to address the required spectrum demand, as well as efficient to utilize the allocated spectrum. To overcome these constraints, i.e. to increase the spectrum availability and utilization, in this paper, we present a new paradigm for the spectrum allocation called licensed countrywide full-spectrum allocation (LCFSA) to allocate the licensed countrywide full 28 GHz millimeter-wave (mmWave) spectrum to each MNO of a country to operate its small cells per building subject to avoiding co-channel interference (CCI). Since most data is generated indoors, the countrywide full mmWave spectrum allocated to an MNO is considered reusing further to its in-building small cells. CCI avoidance in both the time-domain and frequency-domain is presented along with deducing the conditions for optimality. We derive average capacity, spectral efficiency (SE), energy efficiency (EE), and cost efficiency (CE) for LCFSA, and perform extensive numerical and simulation results and analyses for a country consisting of four MNOs. It is shown that LCFSA with both the time-domain and frequency-domain CCI avoidance schemes provide similar performance improvement in average capacity, SE, EE, and CE by about 296%, 164%, 75%, and 60% respectively for no interfering user equipments (UEs), whereas by about 59.2%, 6.1%, 37.2%, and 0.45% respectively for all interfering UEs with the frequency-domain CCI scheme. Moreover, it is found that the improvement in the above metrics does not change with the spectrum reuse factor (RF) and the number of buildings of small cells per macrocell L, and the impact of interfering UEs can be compensated by adjusting RF. Finally, we show that LCFSA can satisfy the SE and EE requirements for sixth-generation (6G) systems by adjusting either RF or L, or both.

This paper presents a tactic to realize numerous in-building small cell base station (SBS) architectures for dynamic spectrum sharing by varying the number of physical transceivers as well as the number, amount, and characteristics of operating spectrums per SBS. Each SBS architecture is defined as a Type and the spectrum sharing mechanism of each Type of architecture is detailed and mathematically analyzed to define existing dynamic spectrum sharing techniques suggested in the literature. The high external wall penetration loss of a building and the eICIC technique are exploited to manage co-channel interference generated due to sharing the spectrum of one system to another. System-level capacity, spectral efficiency, and energy efficiency performance metrics are derived for each SBS architecture to show the relative outperformance of one to another. It is found that, unlike energy efficiency (EE), the spectral efficiency (SE) response of an SBS architecture is directly affected by the channel characteristics as well as the number and amount of operating spectrum bands. However, the number of transceivers of an SBS does not have a noticeable impact on both SE and EE so long as the feature of an operating spectrum is not altered. Finally, we show that each Type of the proposed SBS architectures can achieve the prospective SE and EE requirements for fifth-generation (5G) mobile networks.

In this paper, we propose two spectrum utilization improvement approaches (SUIAs), namely SUIA 1 and SUIA 2, based on whether or not the licensed 28 GHz millimeter-wave (mmWave) spectrum is allocated equally to each mobile network operator (MNO) in a country. Principally, SUIA 1 concerns with improving spectrum utilization if the 28 GHz spectrum is allocated equally to each MNO statically, also termed as static licensed spectrum allocation (SLSA). Whereas, SUIA 2 concerns with improving spectrum utilization if the spectrum is allocated unequally to each MNO in a flexible manner, also termed as flexible licensed spectrum allocation (FLSA). Since sharing and reusing of the licensed spectrum allocated to each MNO using either SLSA or FLSA can be exploited, and the secondary spectrum trading is required only in case of SLSA to redistribute the assigned spectra among MNOs, SUIA 1 employs SLSA, spectrum trading, spectrum sharing, and spectrum reusing techniques, whereas SUIA 2 employs FLSA, spectrum sharing, and spectrum reusing techniques. We present mathematical models for each technique and derive average capacity, spectral efficiency (SE), and energy efficiency (EE) metrics for SUIA 1 and SUIA 2. Extensive numerical and simulation results and analyses for SUIA 1 and SUIA 2 are carried out. It is shown that the spectrum reusing technique influences mostly SE and EE performances. Moreover, SE and EE performances of an MNO in SUIA 1 do not differ considerably from that in SUIA 2 such that depending on the spectrum allocation policies and practices in a country, either SUIA 1 or SUIA 2 can be employed. Finally, it is shown that by applying both SUIA 1 and SUIA 2, the prospective average SE and average EE requirements for sixth-generation (6G) mobile networks can be satisfied.

The future 5th generation (5G) mobile communication system will be required to support diverse services. Because of the difficulty in addressing the requirements of all these services using the same physical network, “network slicing,” which provides a customized logical network corresponding to service requirements, has recently attracted a great deal of attention. In the radio access network (RAN), the appropriate locations of base station functionalities to meet service requirements and achieve efficient network operation differ according to service type. Therefore, we have proposed a novel RAN slicing architecture, which has logical networks (slices) that choose an appropriate functional split option and its placement, to adjust to providing any service. This architecture achieves high flexibility and large scalability by employing a software-defined network and network function virtualization. A prototype is then developed based on the proposed architecture employing a unified controller of open-source software. It can construct slices for three service types: enhanced mobile broadband, ultra-reliable and low-latency communication, and massive machine-type communication, with flexibly located functionalities. In this paper, an experimental evaluation, considering the points of bandwidth consumption in midhaul, the impact of midhaul delay, communication latency, and isolation among slices, shows that the slices have specific performance benefits for each service type and also to achieve efficient network operation. This suggests that the proposed architecture is able to construct slices in a single infrastructure with different features according to the services required and to accommodate various services efficiently.

In this paper, we present a novel technique for sharing satellite spectrum with terrestrial-mobile 3-dimensional (3D) in-building small cells, namely femtocells. Both the space-satellite and the terrestrial-mobile systems (TMSs) operate at their respective licensed spectrums. However, due to the scarcity of the available spectrum, a huge amount of data generated indoors, and a high external wall penetration loss of a 3D building, we propose to reuse the satellite spectrum to small cells deployed only in 3D buildings of the TMS. The proposed spectrum sharing technique is detailed by identifying and addressing the major related issues. The co-channel interference generated because of the co-existence of the satellite user equipments (UEs) in the small-cell coverage is analyzed, and the handover procedures for a satellite UE toward and away from any small cell are discussed. The almost blank subframe (ABS)-based enhanced intercell interference coordination (eICIC) technique is employed to small cells in order to assure the quality-of-service (QoS) between small-cell UEs and satellite UEs. We derive the system-level capacity, spectral efficiency, and energy efficiency performance metrics for the proposed technique by varying the number of buildings over the coverage of a macrocell and deduce the condition for optimality for both the energy efficiency and the spectral efficiency. With an extensive system-level simulation and numerical analysis, we justify the impact of using the ABS-based eICIC technique toward provisioning QoS in terms of radio resource allocation. Furthermore, we show the outperformance of the proposed technique in terms of spectral efficiency and energy efficiency over the ones expected for the fifth generation (5G) network requirements as well as achieved by numerous existing techniques. Finally, we discuss the radio resource scheduler implementation as well as the significance, challenge, and future research perspective of the proposed spectrum sharing technique.

In this paper, we propose multiple communication systems incorporating an integrated satellite-mobile system (iSMS) and an autonomous terrestrial-mobile system (aTMS), and a technique to share multiple reused spectrum bands of iSMS and aTMS with in-building small cell base stations (SBSs) of aTMS by exploiting high external wall penetration loss of a building. The aTMS consists of a set of small cells enabled with either a single-band or multiband and deployed in a number of buildings. The iSMS consists of a satellite station integrated with the complementary ground components (CGC) stations which are deployed alongside small cells one-to-one basis. An interference management scheme to coordinate interference, a global resource scheduler to allocate resources, and an algorithm for the technique is proposed. We derive expressions for capacity, spectral efficiency, and energy efficiency under various SBS configurations for L buildings where L ≥ 1. An optimal value of L satisfying both energy and spectral efficiencies and an optimal number of almost blank subframes (ABSs) are derived. With extensive analysis, we show that the spectral efficiency improves linearly with the number of spectrum bands per SBS as well as L. However, the energy efficiency improves largely for low values o L and is hardly affected by the number of spectrum bands per SBS. An optimal value of L is obtained by choosing a slope on the energy efficiency curve for 0 ≤ L ≤ 100. The multiband enabled SBSs to provide with the best capacity, spectral efficiency, and energy efficiency performances of all SBS configurations. Furthermore, we show that the proposed technique outperforms several existing techniques in terms of energy efficiency, spectral efficiency, and average capacity per user equipment (UE) and can surpass the requirements of spectral and energy efficiencies for the fifth generation (5G) systems. Finally, we point out the significance, challenges, and future research perspectives of the proposed technique.

In this paper, we propose a technique for cloud coordinated clustering and scheduling of small cells, i.e., femtocells, in CRAN to improve capacity and spectral efficiency in 3D in-building environments. Control-and user-plane of each cell is decoupled. The clustering technique consists of two levels of clustering such that level 1 clustering defines an initial 3D femtocell cluster size. Level 2 clustering defines a cooperating set of femtocells per floor within each level 1 cluster to explore joint transmission coordinated multipoint (CoMP) during off-states of their transmit power. Femtocells are split at the MAC layer, and a mechanism is presented to model the bandwidth and latency fronthaul constraints. We derive the system level aggregate capacity, spectral efficiency, and energy efficiency. We then develop a heuristic algorithm and discuss its scheduler implementation. With numerical and simulation results, we show with an example scenario that the proposed technique can achieve the maximum CoMP capacity gain of 44% per building and 32.15% in overall system level. Finally, we demonstrate that the proposed technique can achieve the performance requirements of 5G cellular systems and point out the significance and further research issues.

In this paper, we propose a hybrid interweave–underlay spectrum access and reuse technique for the dynamic spectrum access and reuse of the countrywide 28 GHz millimeter-wave (mmWave) spectrum to in-building small cells of each mobile network operator (MNO) in a country. For the spectrum access, the proposed technique explores both interweave and underlay spectrum access techniques, whereas, for the spectrum reuse, it considers reusing the countrywide spectrum to each three-dimensional (3D) cluster of small cells in a building. To access the countrywide spectrum, each MNO is considered by paying a licensing fee following its number of subscribers. We present the 3D clustering of in-building of small cells and derive average capacity, spectral efficiency (SE), and energy efficiency (EE). We then perform extensive numerical and simulation results and analyses for an MNO of a country consisting of four MNOs. It is shown that, for no spectrum reuse to in-building small cells, the proposed technique improves average capacity and SE by 3.63 and 2.42 times, respectively, whereas EE improves by 72.79%. However, for vertical spatial reuse of six times (as an example) to small cells in a building, average capacity, SE, and EE improve further by 21.77 times, 14.51 times, and 95.66%, respectively. Moreover, the proposed technique can satisfy SE and EE requirements for sixth-generation (6G) mobile systems by horizontal spatial reuse of the countrywide spectrum to small cells of about 40.62%, 9.37%, and 6.25% less buildings than that required by the traditional static licensed spectrum access (SLSA) technique.

In this paper, we propose a dynamic exclusive-use spectrum access (DESA) method to improve the overall licensed millimeter-wave (mmWave) spectrum utilization of all mobile network operators (MNOs) in a country. By exploiting secondary spectrum trading, the proposed DESA method shares partly and exclusively the licensed mmWave spectrum of one MNO to another in a dynamic and on-demand basis for a certain agreement term. We formulate the proposed DESA method for an arbitrary number of MNOs in a country. We then present an iterative algorithm to find the optimal amount of shared spectrum for each MNO, which is updated at each agreement term. We derive average capacity, spectral efficiency, energy efficiency, and cost efficiency performance metrics for all MNOs countrywide and present extensive numerical and simulation results and analyses for an example scenario of a country with four MNOs each assigned statically with an equal amount of 28-GHz mmWave spectrum. By applying DESA, we show that MNOs with a lack of minimum licensed spectra to serve their data traffic can lease at the cost of payment of the required additional spectra from other MNOs having unused or under-utilized licensed spectra. Moreover, it is shown that the overall countrywide average capacity, spectral efficiency, energy efficiency, and cost efficiency can be improved, respectively, by 25%, 25%, 17.5%, and 20%. Furthermore, we show that, by applying DESA to all MNOs countrywide, the expected spectral efficiency and energy efficiency requirements for sixth-generation (6G) mobile systems can be achieved by reusing the same mmWave spectrum to 20% fewer buildings of small cells. Finally, using the statistics of subscribers of all MNOs, we present a case study for fifth-generation (5G) networks to demonstrate the application of the proposed DESA method to an arbitrary country of four MNOs.

The sixth-generation (6G) mobile networks are expected to operate at a higher frequency to achieve a wider bandwidth and to enhance the frequency reuse efficiency for improved spectrum utilization. In this regard, three-dimensional (3D) spatial reuse of millimeter-wave (mmWave) spectra by in-building small cells is considered an effective technique. In contrast to previous works exploiting microwave spectra, in this paper, we present a technique for the 3D spatial reuse of 28 and 60 GHz mmWave spectra by in-building small cells, each enabled with dual transceivers operating at 28 and 60 GHz bands, to enhance frequency reuse efficiency and achieve the expected spectral efficiency (SE) and energy efficiency (EE) requirements for 6G mobile networks. In doing so, we first present an analytical model for the 28 GHz mmWave spectrum to characterize co-channel interference (CCI) and deduce a minimum distance between co-channel small cells at both intra- and inter-floor levels in a multistory building. Using minimum distances at both intra- and inter-floor levels, we find the optimal 3D cluster size for small cells and define the corresponding 3D spatial reuse factor, such that the entire 28 and 60 GHz spectra can be reused by each 3D cluster in each building. Considering a system architecture where outdoor macrocells and picocells operate in the 2 GHz microwave spectrum, we derive system-level average capacity, SE, and EE values, as well as develop an algorithm for the proposed technique. With extensive numerical and simulation results, we show the impacts of 3D spatial reuse of multi-mmWave spectra by small cells in each building and the number of buildings per macrocell on the average SE and EE performances. Finally, it is shown that the proposed technique can satisfy the expected average SE and EE requirements for 6G mobile networks.

Addressing high capacity at low power as a key design goal envisages achieving high spectral efficiency (SE) and energy efficiency (EE) for the next-generation mobile networks. Because most data are generated in indoor environments, an ultra-dense deployment of small cells (SCs), particularly within multistory buildings in urban areas, is revealed as an effective technique to improve SE and EE by numerous studies. In this paper, we present a framework exploiting the four most interconnected-domain, including, power, time, frequency, and space, in the perspectives of SE and EE. Unlike existing literature, the framework takes advantage of higher degrees of freedom to maximize SE and EE using in-building SCs for 5G and beyond mobile networks. We derive average capacity, SE, and EE metrics, along with defining the condition for optimality of SE and EE and developing an algorithm for the framework. An extensive system-level evaluation is performed to show the impact of each domain on SE and EE. It is shown that employing multiband enabled SC base stations (SBSs) to increase operating spectrum in frequency-domain, reusing spectrum to SBSs more than once per building in spatial-domain, switching on and off each in-building SBS based on traffic availability to reduce SBS power consumption in power-domain, and using eICIC to avoid co-channel interference due to sharing spectrum with SBSs in time-domain can achieve massive SE and EE. Finally, we show that the proposed framework can satisfy SE, EE, as well as user experience data rate requirements for 5G and beyond mobile networks.

In this paper, we present two spectrum sharing techniques for a multisystem, incorporating an integrated satellite-mobile system and an autonomous terrestrial-mobile system (iSMS/aTMS), namely orthogonal spectrum sharing (OSS) and non-orthogonal spectrum sharing (nOSS) techniques. aTMS consists of numerous small cells deployed in several buildings, and iSMS consists of a satellite station integrated with complementary ground component (CGC) stations deployed within buildings. By exploiting the high external wall penetration loss of a building, the iSMS spectrum is shared with small cells per building in OSS, and small cells per 3-dimensional (3D) cluster per building in nOSS. An interference management scheme, to avoid interference in apartments with collocated CGC stations and small cells, was developed and an optimal number of almost blank subframes (ABSs) per ABS pattern period (APP) was defined. System-level capacity, spectral efficiency, and energy efficiency performance metrics were derived. Furthermore, we present an algorithm for both OSS and nOSS techniques. With extensive simulation and numerical analysis, it is shown that the proposed nOSS significantly outperforms OSS in terms of spectral efficiency and energy efficiency, and both techniques can meet the expected spectral efficiency and energy efficiency requirements for the fifth-generation (5G) mobile networks.

In this paper, we propose a technique to share the licensed spectrums of all mobile network operators (MNOs) of a country with in-building small cells per MNO by exploiting the external wall penetration loss of a building and introducing the time-domain eICIC technique. The proposed technique considers allocating the dedicated spectrum Bop per MNO only its to outdoor macro UEs, whereas the total spectrum of all MNOs of the country Bco to its small cells indoor per building such that technically any small indoor cell of an MNO can have access to Bco instead of merely Bop assigned only to the MNO itself. We develop an interference management strategy as well as an algorithm for the proposed technique. System-level capacity, spectral efficiency, and energy efficiency performance metrics are derived, and a generic model for energy efficiency is presented. An optimal amount of small indoor cell density in terms of the number of buildings L carrying these small cells per MNO to trade-off the spectral efficiency and the energy efficiency is derived. With the system-level numerical and simulation results, we define an optimal value of L for a dense deployment of small indoor cells of an MNO and show that the proposed spectrum sharing technique can achieve massive indoor capacity, spectral efficiency, and energy efficiency for the MNO. Finally, we demonstrate that the proposed spectrum sharing technique could meet both the spectral efficiency and the energy efficiency requirements for 5G mobile networks for numerous traffic arrival rates to small indoor cells per building of an MNO.

In this paper, we show how to realize numerous spectrum licensing policies by means of time-domain enhanced inter-cell interference coordination (eICIC) technique to share both the licensed and unlicensed spectrums with small cells in order to address the increasing demand of capacity, spectral efficiency, and energy efficiency of future mobile networks. Small cells are deployed only in 3-dimensional (3D) buildings within a macrocell coverage of a mobile network operator (MNO). We exploit the external wall penetration loss of each building to realize traditional dedicated access, co-primary shared access (CoPSA), and licensed shared access (LSA) techniques for the licensed spectrum access, whereas, for the unlicensed spectrum access, the licensed assisted access (LAA) technique operating in the 60 GHz unlicensed band is realized. We consider that small cells are facilitated with dual-band, and derive the average capacity, spectral efficiency, and energy efficiency metrics for each technique. We perform extensive evaluation of various performance metrics and show that LAA outperforms considerably all other techniques concerning particularly spectral and energy efficiencies. Finally, we define an optimal density of small cells satisfying both the spectral efficiency and energy efficiency requirements for the fifth-generation (5G) mobile networks.