Types Of Base Stations Research Articles

Exploring power consumption reduction in centralized radio access for energy‐efficient centralized‐Internet of Things implementation

AbstractThe advent of new wireless and Internet of Things (IoT) technology has increased the dependency on the cellular base stations (BS). The rapid growth in IoT and wireless devices impinges many challenges on cellular services providers and manufacturers. The Centralized Radio Access Network is an energy and cost‐efficient solution for the next‐generation BS architecture. In this article, we have proposed a centralized‐IoT (C‐IoT) architecture based on centralized BS (C‐BS). We evaluated different base station technologies and compared the C‐BS with the traditional distributed BS (D‐BS) architecture for power consumption. A mathematical model for power consumption for both types of BS is proposed. We have analyzed the impact of variation in traffic load on power consumption for both types of BS architecture. The simulation result shows that C‐BS architecture has the potential to save around 40% to 50% energy compared with D‐BS architecture.

Dynamic User Association for Resilient Backhauling in Satellite–Terrestrial Integrated Networks

The satellite-terrestrial integrated networks (STINs) have gradually become a new class of effective ways to satisfy the requirements of a higher capacity and stronger connection in the future communications. In contrast with terrestrial networks, the fast periodic motion of satellites results in the dynamic time-varying features of STIN, which further leads to frequent changes in the connectivity of satellite-terrestrial links and the backhaul capacities of satellite networks. To balance the accessible capacity of STIN under the intermittent connectivity and dynamic backhaul capacity, an effective user association mechanism is needed. In this article, a dynamic user association (DUA) mechanism with task classification is proposed to meet the requirements of load balancing and the user task processing. First, a STIN model is constructed with low earth orbit satellites and the three types of base station, which are a macro base station, small cell base station, and low earth orbit based base station. After that, the optimization problem is formulated via jointly considering the task classification, the load condition of base stations, and the backhaul capacity of low earth orbit based base stations. Then, the DUA mechanism is proposed to find the most suitable base station serving each user. In DUA, a dynamic cell range extension algorithm is developed to adjust the load of STIN in terms of the resilient backhaul capacity, and a greedy-based user-centric user association with task classification algorithm is proposed to find the base station, which has the maximum rate and minimum load for each user and to meet the requirements of user task processing. The simulation results show that the proposed DUA can enhance the load balance and guarantee the task processing demand of STIN compared with the reference signal receiving power association and the max-sum rate association algorithms.

Performance Analysis of MISINR User Association in 3-D Heterogeneous Cellular Networks

The continuously growing mobile traffic demand is pushing wireless networks to dense heterogeneous architectures with different types of base stations (BSs). In such dense heterogeneous networks (HetNets), especially in the high-rise building 3-D environment, the users need to determine which BS to be associated with before communication. Different user association schemes lead to different system performance. In this paper, a maximum instantaneous signal-to-interference-plus-noise ratio (MISINR) user association scheme is studied in the dense urban environment. A general 3-D model is utilized to model the dense urban HetNets deployed in the high-rise buildings. The coverage probability and average user rate are theoretically derived by using stochastic geometry in a general case and an interference-limited case, respectively. Numerical and simulation results demonstrate that the MISINR user association scheme has the best performance compared with other two association schemes in 3-D HetNets. From the results, it is interesting to observe that the expressions of coverage probability and average user rate can be simplified to closed-forms in the interference-limited case.

A Comprehensive Survey on Mobility Management in 5G Heterogeneous Networks: Architectures, Challenges and Solutions

With the rapid increase in the number of mobile users, wireless access technologies are evolving to provide mobile users with high data rates and support new applications that include both human and machine-type communications. Heterogeneous networks (HetNets), created by the joint installation of macro cells and a large number of densely deployed small cells, are considered an important solution to deal with the increasing network capacity demands and provide high coverage to wireless users in future fifth generation (5G) wireless networks. Due to the increasing complexity of network topology in 5G HetNets with the integration of many different base station types, in 5G architecture mobility management has many challenges. Intense deployment of small cells, along with many advantages it provides, brings important mobility management problems such as frequent handover (HO), HO failure, HO delays, ping-pong HO and high energy consumption which will result in lower user experience and heavy signal loads. In this paper, we provide a comprehensive study on the mobility management in 5G HetNet in terms of radio resource control, the initial access and registration procedure of the user equipment (UE) to the network, the paging procedure that provides the location of the UE within the network, connected mode mobility management schemes, beam level mobility and beam management. Besides, this paper addresses the challenges and suggest possible solutions for the 5G mobility management.

Key Challenges, Drivers and Solutions for Mobility Management in 5G Networks: A Survey

Ensuring a seamless connection during the mobility of various User Equipments (UEs) will be one of the major challenges facing the practical implementation of the Fifth Generation (5G) networks and beyond. Several key determinants will significantly contribute to numerous mobility challenges. One of the most important determinants is the use of millimeter waves (mm-waves) as it is characterized by high path loss. The inclusion of various types of small coverage Base Stations (BSs), such as Picocell, Femtocell and drone-based BSs is another challenge. Other issues include the use of Dual Connectivity (DC), Carrier Aggregation (CA), the massive growth of mobiles connections, network diversity, the emergence of connected drones (as BS or UE), ultra-dense network, inefficient optimization processes, central optimization operations, partial optimization, complex relation in optimization operations, and the use of inefficient handover decision algorithms. The relationship between these processes and diverse wireless technologies can cause growing concerns in relation to handover associated with mobility. The risk becomes critical with high mobility speed scenarios. Therefore, mobility issues and their determinants must be efficiently addressed. This paper aims to provide an overview of mobility management in 5G networks. The work examines key factors that will significantly contribute to the increase of mobility issues. Furthermore, the innovative, advanced, efficient, and smart handover techniques that have been introduced in 5G networks are discussed. The study also highlights the main challenges facing UEs' mobility as well as future research directions on mobility management in 5G networks and beyond.

Using Clustering Techniques to Plan Indoor Femtocell Base Stations Layout in Multi-floors

Long-term evolution ‘LTE’ advanced-based heterogeneous networks are used to improve spectral efficiency per unit area. Nowadays, the network specialists try to increase the capacity of indoor by adding indoor base stations, but other problems appear to them because they are adding them without considering critical constraints such as the number of users, type of base stations, and type of wall, indoor and outdoor interference. This paper proposed a system using cluster techniques to facilitate the process of cell planning that involves locating and configuring infrastructure for mobile networks. The density-based spatial clustering of applications with noise (DBSCAN) algorithm was modified to solve the problem of coverage indoor area, with the existence of obstacles to choose suitable femtocell stations which satisfy minimum path loss and maximize coverage area and minimize cost with guaranteed path loss threshold. Subsequently, Hierarchical Agglomerative Clustering (HAC) was modified to maximize Signal to Interference plus Noise Ratio SINR. Suitable penetrations of 4 and 5 G are used and different types of obstacles taking into consideration their penetration values. The results of this algorithm indicate that the proposed algorithm conducts to decrease the number of Femtocells so minimize the cost and maximize SINR with a grantee pass loss threshold.

PHY-Layer Cover-Free Coding for Wireless Pilot Authentication in IoV Communications: Protocol Design and Ultra-Security Proof

Wireless channel state information (CSI) from intelligent vehicles to the roadside unit (RSU) is a must for vehicleto-infrastructure (V2I) communications in Internet of Vehicles, but easily suffers the risks of being attacked due to the publicly known and deterministic characteristic of PHY-layer pilots that are employed for CSI acquisition. This incurs the issue of wireless pilot authentication (WPA), that is, verifying the authenticity of pilots and claimed CSIs. In this paper, we, for multiantenna V2I orthogonal frequency division multiplexed communications, develop a PHY-layer cover-free (PHY-CF) coding theory to build up a secure WPA (SWPA) protocol. Here, we encode and convey vehicle pilot signals into diversified subcarrier activation patterns (SAPs) on the time-frequency domain by employing cover-free coding. We redesign the decoding procedure using the signal independence characteristic such that those encoded SAPs, though camouflaged by malicious signals and superimposed onto each other in wireless environment, could be separated, identified and decoded into the original pilots securely. For this protocol, we prove that perfect pilot conveying and separation could be both guaranteed. We formulate the pilot identification error probability (IEP) and show how PHY-CF coding could help position the location of attacker and reduce IEP to further achieve ultrasecurity. Considering 20 MHz long-term evolution bandwidth, we prove that the number of co-time co-frequency vehicles that are securely authenticated achieves up to 19 × X for X serving sectors of base station type RSU and the latency time of uplink data access is up to 1.5 ms, thus furthering the autonomous driving. Computer simulations comprehensively verify those benefits of proposed SWPA protocol.

Probabilistic caching placement in UAV-assisted heterogeneous wireless networks

In this paper, we investigate the probabilistic caching placement in a heterogeneous wireless network, where there are several types of Unmanned Aerial Vehicle Base Station (UAV-BS) placed following the independent homogeneous Poisson Point Process (PPP), and each type of UAV-BS has a different cache capacity. We measure the network transmission performance by introducing the average service success probability, which is the probability that each user’s request can be successfully serviced by the nearest UAV-BS among several types. Taking into account stochastic geometry and Zipf distribution as well as the Signal-to-Noise Ratio (SNR) coverage model, we provide the cache hit probability and the successful transmission probability to obtain the average service success probability. The system performance is optimized by maximizing the average service success probability, through optimizing the probability caching placement. As this problem is non-convex in general, and we turn to use Genetic Algorithm (GA) to solve the placement strategy. Numerical and simulation results are provided to demonstrate that the proposed caching placement strategy is superior to the conventional Most Popular Content (MPC) strategy, especially in the high SNR regime. Moreover, we note that when the small-capacity UAV-BS uses the MPC strategy and the large-capacity UAV-BS uses the proposed caching strategy, the sub-optimal result can be obtained with reduced implementation complexity.

CPPHA: Capability-based Privacy-Protection Handover Authentication Mechanism for SDN-based 5G HetNets

Ultra-dense Heterogeneous network (HetNet) technique can significantly improve wireless link quality, spectrum efficiency and system capacity, and satisfy different requirements for coverage in hotspots, which has been viewed as one of the key technologies in fifth Generation (5G) network. Due to the existence of many different types of base stations (BSs) and the complexity of the network topology in the 5G HetNets, there are a lot of new challenges in security and mobility management aspects for this multi-tier 5G architecture including insecure access points and potential frequent handovers among several different types of base stations. In this paper, we integrate user capability and Software Defined Network (SDN) technique, and propose a capability-based privacy protection handover authentication mechanism in SDN-based 5G HetNets. Our proposed scheme can achieve the mutual authentication and key agreement between User Equipments (UEs) and BSs in 5G HetNets at the same time largely reduce the authentication handover cost. We demonstrate that our proposed scheme indeed can provide robust security protection by employing several security analysis methods including the BAN logic and the formal verification tool Scyther. In addition, the performance evaluation results show that our scheme outperforms other existing schemes.

Evolutionary 4G/5G Network Architecture Assisted Efficient Handover Signaling

Future wireless networks are expected to be ultra-dense and heterogeneous not just in terms of the number and type of base stations but also in terms of the number of users and the application types they access. Such a network architecture will require mobility management mechanisms that adapt rapidly to these highly dynamic network characteristics. In particular, the optimality of the handover signaling within these future network architectures will be extremely critical given their density and heterogeneity. In this paper, the optimality is relevant for both the total amount of signaling created and the total delay per handover process. In this paper, we first present a novel and optimized message mapping and signaling mechanism for the handover preparation and failure phases. We also develop a novel handover failure aware preparation signaling methodology, which accounts for the possibility of a handover failure and grants additional enhancements to the handover preparation and failure signaling phases. Through the analytical framework provided in this paper, we conduct studies to quantify the performance gains promised by the proposed mechanisms. These studies cover myriad handover scenarios as identified by 3GPP and use the statistics from cellular network operators and vendors. We then develop the idea and analytical framework for network wide analysis, in which the network wide processing cost and network occupation time for various handover failure rates are computed. Finally, we propose an evolutionary network architecture that facilitates the proposed signaling mechanism as well as assists operators in maintaining a manageable capital expenditure. It combines the current day and 3GPP proposed 5G network architecture with the software-defined networking approach. As a result, we argue that the proposed mechanisms are viable and outperform the legacy handover signaling mechanisms in terms of latency incurred, total network occupation time, number of messages generated, and total bytes transferred.

Fundamentals of Simultaneous Wireless Information and Power Transmission in Heterogeneous Networks: A Cell-Load Perspective

In a heterogeneous cellular network (HetNet) consisting of multiple different types (tiers) of base stations (BSs), the void cell event in which a BS does not have any users has been shown to exist due to user-centric BS association and its probability is dominated by the cell load of each tier. Such a void cell phenomenon has not been well characterized in the modeling and analytical framework of simultaneous wireless information and power transmission (SWIPT) in a HetNet. This paper aims to accurately exploit the fundamental performance limits of the SWIPT between a BS and its user by modeling the cell-load impact on the downlink and uplink transmissions of each BS. We first characterize the power-splitting receiver architecture at a user and analyze the statistical properties and limits of its harvested power and energy, which reveals how much of the average energy can be harvested by users and how likely the self-powered sustainability of users can be achieved. We then derive the downlink and uplink rates that characterize the cell-load and user association effects and use them to define the energy efficiency of a user. The optimality of the energy efficiency is investigated, which maximizes the SWIPT performance of the receiver architecture for different user association and network deployment scenarios.

Analysis of energy efficiency in cloud based heterogeneous RAN with large-scale antenna systems

To address the capacity requirements resulting from huge growth in mobile data traffic, the mobile network operators (MNOs) are densifying their networks with more base stations, and with more spectrum layers. The addition of base stations and spectrum layers increases the energy consumption of the access network. Hence the network needs to be optimized to lower the power consumption (e.g.) through cloud, or deployment of spectral and power efficient radio units like Large-Scale Antenna Systems (LSAS) or massive Multiple Input Multiple Output (MIMO) radio units.In this paper, we analyse the network evolution towards Cloud based Radio Access Network (C-RAN) for a mix of base stations with Macro and LSAS Remote Radio Units (RRU). We derive the computational complexity of the base station components using a flexible power model. We evaluate the need for base station architecture split, when C-RAN is used with LSAS radio units and large antenna configurations. We use a combination of simulators viz.NS3 (for LTE radio network simulation) and CloudSIM Plus (for Cloud based base station simulation). For each base station type, we compare the computational complexity of different sub-components. Further, based on the selected power model, we study the effect of base station's scheduling algorithm (viz.) Proportionate-Fair or Round-Robin, on the computational complexity of the various base station sub-components. We consider Time and Wavelength Division Multiplexed Passive Optical Network (TWDM-PON) for the high capacity low latency fronthaul network. Using the overall radio cluster throughput and power consumption information from Cloud base station, Radio site, and fronthaul network, we assess the energy efficiency metrics for the Cloud RAN and LSAS (with different antenna configurations) with Macro RRUs as reference.Through the simulations, we observe that, the computation complexity was relatively higher for RR scheduling (compared to PF scheduling). But the variations of energy efficiency metric were similar for both the scheduling schemes for different base station configurations.We observe that compared to Macro2TR, LSAS variants are highly energy efficient. This gain is mainly associated with higher throughput due to MU-MIMO (where multiple UEs are accommodated in the same set of radio resources) and lower power consuming components in LSAS. Between LSAS32TR, LSAS64TR and LSAS128TR, we find that LSAS32TR had lower energy efficiency. Further, we observe that the energy efficiency of LSAS64TR is almost same as LSAS128TR, though LSAS64TR supports just half the number of MU-MIMO layers. This implies that the additional number of layers that LSAS128TR system supports, is compensated by power consumed to support the higher antenna chains and spatially multiplexed layers.Thus, MNOs interested in deploying high spectral efficiency solution along with high energy efficiency, may prefer to deploy LSAS64 systems for tower-top deployments of Radio unit deployment, since it offers higher energy efficiency with lower wind-load effect due to smaller antenna size.

Ultra-Dense 5G Small Cell Deployment for Fiber and Wireless Backhaul-Aware Infrastructures

In this paper, we study the cell planning problem for a two-tier cellular network containing two types of base stations (BSs), i.e., with fiber backhaul, referred to as wired BSs (W-BSs), and BSs with wireless backhaul, referred to as unwired-BSs (U-BSs). In-band full-duplex wireless communications is used to connect U-BSs and W-BSs. We propose an algorithm to determine the minimum number of W-BSs and U-BSs to satisfy given cell and capacity coverage constraints. Furthermore, we apply our proposed non-dominated sorting genetic algorithm II (NSGA-II) to solve both cell planning and joint cell and backhaul planning problem to minimize the cost of planning, while maximizing the coverage simultaneously. Additionally, the considered cell planning program is developed into an optimization by including the problem of minimizing the cost of fiber backhaul deployment. In order to analyze the performance of the proposed algorithm, we study three different deployment scenarios based on different spatial distributions of users and coverage areas. The results show the superiority of our proposed NSGA-II algorithm for both cell planning and joint cell and backhaul planning to other well-known optimization algorithms. The results also reveal that there is a tradeoff between cell deployment costs and SINR/rate coverage, and W-BSs are placed in congested areas to consume less resources for wireless backhauls. Similarly, a tradeoff between cell and fiber deployment costs and SINR/rate coverage is observed in planning. We show that for realistic scenarios desirable solutions can be selected from the Pareto front of the introduced multi-objective problem based on given cellular operator policies.

Energy-constrained transmission on hybrid communication networks

The real communication systems usually consist of different types of agents, e.g. computers, routers, base stations and mobile phones, forming various hybrid communication networks. Furthermore, in many cases, those agents are energy-constrained resulting in a limited lifetime of the communication networks. We proposed a hybrid communication network model composed of energy-constrained base stations and mobile users, and further gave a novel energy-aware gateway selection strategy to balance the energy consumption of base stations. We developed a new metric of nodes, called node forwarding strength, which measures the intrinsic forwarding capability of nodes and is basically dependent on the network structure and routing protocol. Based on this metric, we further derived the critical packet generation rate of traffic congestion, average number of transmission hops, and network lifetime. By investigating the influence of factors on the network lifetime, we obtained the optimal user moving speed and gateway selection parameter corresponding to the maximum network lifetime. Our work may provide some clues for the designing and optimization of real hybrid communication systems.

Millimeter-Wave NOMA Transmission in Cellular M2M Communications for Internet of Things

Massive connectivity and low latency are two important challenges for the Internet of Things (IoT) to achieve the quality of service provisions required by the numerous devices it is designed to service. Motivated by these challenges, in this paper we introduce a new millimeter-wave nonorthogonal multiple access (mmWave-NOMA) transmission scheme designed for cellular machine-to-machine (M2M) communication systems for IoT applications. It consists of one base station (BS) and numerous multiple machine type communication (MTC) devices operating in a cellular communication environment. We consider its down-link performance and assume that multiple MTC devices share the same communication resources offered by the proposed mmWave-NOMA transmission scheme, which can support massive connectivity. For this system, a novel MTC pairing scheme is introduced the design of which is based upon the distance between the BS and the MTC devices aiming at reducing the system overall overhead for massive connectivity and latency. In particular, we consider three different MTC device pairing schemes, namely: 1) random near and the random far MTC devices; 2) nearest near and the nearest far MTC devices (NNNF); and 3) nearest near and the farthest far MTC device. For all three pairing schemes, their performance is analyzed by deriving closed-form expressions of the outage probability and the sum rate. Furthermore, performance comparison studies of the three MTC device pairing schemes have been carried out. The validity of the analytical approach has been verified by means of extensive computer simulations. The obtained performance evaluation results have demonstrated that the proposed cellular M2M communication system employing the mmWave-NOMA transmission scheme improves outage probability as compared to equivalent systems using mmWave with orthogonal multiple access schemes.

Security Requirement and Standards for 4G and 5G Wireless Systems

In this article, we give an overview on the security requirements and standards for the fourth generation (4G) long term evolution (LTE) and the fifth generation (5G) wireless systems. Compared with the third generation (3G) universal mobile telecommunication system (UMTS), the LTE system not only improves the network capacity and coverage but also provides better performance of data rate, access latency, energy efficiency (EE), and spectrum efficiency (SE) [1]. With the increasing data usage and the new multimedia applications, LTE technologies have been proposed by the 3rd generation partnership project (3GPP) as the broadband mobile wireless networks with packet switched, IP-based technology solely, fully inter-working with different access technologies, different types of base stations and relay nodes. To support more use cases with unique service requirements, 5G wireless systems are developed. There are eight advanced features for 5G wireless systems, 1-10 Gbps connections to end points in the field, 1 millisecond latency, 1000x bandwidth per unit area, 10-100x number of connected devices, 99.999% availability, 100% coverage, 90% reduction of network energy usage and up to ten years battery life for low power devices [2]. These advanced features make 5G wireless systems capable of supporting many new use cases, new industry applications, a multitude of devices and applications to connect society at large. Due to the introduction of new characteristics, there are many new security requirements in the design of the security architectures of the 4G LTE and 5G wireless systems.

Analysis of Dual Connectivity for Downlink Access in 5G Heterogeneous Network

Dual-Connectivity architecture allows each user’s equipment (UE) to simultaneously communicate with two kinds of serving base stations (BSs) via two different radio-interfaces. Dual-Connectivity paradigm emerges as a promising solution to boost system capacity in fifth-generation heterogeneous network (HetNet), deploying multiple kinds of BSs, such as Femto BSs (FBSs), Pico BSs (PBSs) and Macro BSs (MBSs). In this paper, a comprehensive study of Dual-Connectivity in the downlink HetNet is analyzed via the framework of stochastic geometry, where the deployments of the BSs are modeled as mutually independent homogeneous Poisson point processes. In the conventional Dual-Connectivity, only the combination of MBS and SBS is available to access. In this paper, we consider that each UE can be simultaneously associated with any two types of BSs. The analysis is divided into two parts. We first derive the association probabilities of different combinations for Dual-Connectivity and then evaluate the average downlink throughput with the help of the association probabilities and the distance distributions to the serving BS. Moreover, we analyze the impact of the relative density of FBSs (or PBSs) on the association probabilities of different cases. Also, we compare the average downlink throughput of our considered Dual-Connectivity with those of conventional Dual-Connectivity and single association. Numerical results prove the validity of the theoretical analyses and demonstrate that our considered Dual-Connectivity owns superior performance on the throughput in HetNet, especially in the dense deployed SBS HetNet.

Spectrum mobility management in cognitive two‐tier networks

SummaryRecently, new type of small Base Station known as “femtocell” has emerged. Femtocells are deployed by customers with minimal or no radio frequency planning in arbitrary locations. Thus, operators must use new approaches rather than the classic network planning and optimization. Self‐organization techniques will allow femtocells to integrate themselves into the network, learn about their environment, and adjust their parameters (power, frequency) accordingly. In this paper, a new spectrum mobility management algorithm is proposed. It allows intelligent use of the spectrum by secondary user (femtocell guest user) accessing spectrum holes unoccupied by primary users (subscriber user) and handing over to new channels or femtocells when primary user appears. Our solution aims to have more adaptive and aware communication system, which can make better use of available natural resources.

Load-Aware Performance Analysis of Cell Center/Edge Users in Random HetNets

For real-time traffic, the link quality and call blocking probability (both derived from coverage probability) are realized to be poor for cell edge users (CEUs) compared to cell center users (CCUs) as the signal reception in the cell center region is better compared to the cell edge region. In heterogeneous networks (HetNets), the uncoordinated channel access by different types of base stations determine the interference statistics that further arbitrates the coverage probability. Thus, the spectrum allocation techniques have major impact on the performance of CCU and CEU. In this paper, the performance of CCUs and CEUs in a random two-tier network is studied for two spectrum allocation techniques namely: 1) co-channel (CSA), and 2) shared (SSA). For performance analysis, the widely accepted conception of modeling the tiers of HetNet using independent homogeneous Poisson point process (PPP) is considered to accommodate the spatial randomness in location of BSs. To incorporate the spatial randomness in the arrival of service and to aid the load-aware analysis, the cellular traffic is modeled using spatio-temporal PPP. Under this scenario, we have developed an analytical framework to evaluate the load-aware performance, including coverage and blocking probabilities, of CCUs and CEUs under both spectrum allocation techniques. Further, we provide insight into achievable area energy efficiency for SSA and CSA. The developed analytical framework is validated through extensive simulations. Next, we demonstrate the impact of traffic load and femto access points density on the performance of CCUs/CEUs under CSA and SSA.

Modeling and Analysis of Downlink Heterogeneous Cellular Networks

Owing to the irregular deployment of different types of base stations (BSs), cellular networks are now becoming more and more complex. In this study, a tractable and accurate framework based on hybrid model is proposed for heterogeneous cellular networks (HCNs). According to the distribution characteristics of different types of BSs in HCNs, the plane distributed macro BSs are termed as tier-1 cells, whereas the small cells, which are spatial distributed, are modeled as tier-2. First, we derive the probability of a target user communicate with macro BS or small cell. Second, we calculate the coverage probability that the target user communicate with macro or small cell respectively. Finally, we obtain the general closed expressions of whole coverage probability. In some special cases, the expressions present simple closed forms and can be calculated easily. Compared with the presented model, the simulation results show that our model is accurate within 1-2dB.