Low-power Base Stations Research Articles

RF-EMF Exposure near 5G NR Small Cells.

Of particular interest within fifth generation (5G) cellular networks are the typical levels of radiofrequency (RF) electromagnetic fields (EMFs) emitted by 'small cells', low-power base stations, which are installed such that both workers and members of the general public can come in close proximity with them. In this study, RF-EMF measurements were performed near two 5G New Radio (NR) base stations, one with an Advanced Antenna System (AAS) capable of beamforming and the other a traditional microcell. At various positions near the base stations, with distances ranging between 0.5 m and 100 m, both the worst-case and time-averaged field levels under maximized downlink traffic load were assessed. Moreover, from these measurements, estimates were made of the typical exposures for various cases involving users and non-users. Comparison to the maximum permissible exposure limits issued by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) resulted in maximum exposure ratios of 0.15 (occupational, at 0.5 m) and 0.68 (general public, at 1.3 m). The exposure of non-users was potentially much lower, depending on the activity of other users serviced by the base station and its beamforming capabilities: 5 to 30 times lower in the case of an AAS base station compared to barely lower to 30 times lower for a traditional antenna.

Enhanced Handover Mechanism in Long Term Evolution (LTE) Networks

Femtocell is a low power base station, wireless access point designed especially for homes and small organizations. It is promising technology for operators to improve their capacity and for users to give indoor coverage. As mobile users are increasing day by day so the legacy system is unable to provide such a high data rates to all these users. In this case femtocells play a key role to offload the data traffic from macro base station. The implementation of femtocell has posed so many challenges like interference, localization, access control and mobility management. The aim of this paper is to present an enhanced algorithm for handover in Hand-In scenario. In already existing algorithms handover is decided on the basis of a single parameter but here we have simulated an algorithm that considers multiple parameters instead of a single parameter for handover. Through this algorithm, the most suitable femtocell will be selected for handover, hence number of handovers will be decreased. Simulation results show that the system performance has been improved.

Secure resource management in beyond 5G heterogeneous networks with decoupled access

Coverage, capacity and throughput can be enhanced significantly by offering a hybrid solution consisting of microwave (μW) high power base station (HPB) underlaid millimetre wave (mW) low power base station (LPB) augmented by downlink uplink decoupled (DU-De) user association strategy in N-tier heterogeneous networks (HetNets). Presence of diversity in the HetNets pave the way for marlacious attacks by eavesdropper to wiretap the channel of legal users. However, secure user association, power and μW & mW sub-carriers allocation employing DU-De strategy has not been investigated in the past. We formulates mathematical models for DU-De strategy and downlink uplink coupled (DU-Co) strategy to investigate secure user association, power and sub-carrier in μW and mW band allocation for secrecy rate maximization in N-tier HetNets. The nature of formulated problems is complex, challenging and NP-hard. We have used ɛ-optimal algorithm to solve the formulated problems and achieve ɛ-optimal solution. Extensive simulation results in terms of secure user association and average secrecy rate show the effectiveness of the DU-De strategy over the DU-Co strategy in HetNets.

Improving the Performance of Heterogeneous Network Systems in Machine Learning-based 5G Mobile Communication System

Mobile traffic, which has increased significantly with the emergence of Fourth generation longterm evolution (4G-LTE) communications and advances in video streaming services, is still currently increasing at an incredible pace. Fifth-generation (5G) mobile communication systems, which were developed to deal with such a drastic increase in mobile traffic, aim to achieve ultra-high-speed data transmission, low latency, and the accommodation of many more connected devices compared to 4G-LTE systems. 5G communication uses high-frequency bandwidth to implement these features, which leads to an inevitable drawback of a high path loss. In order to overcome this disadvantage, small cell technology was developed, and is defined as small, low-power base stations that can extend the network coverage and solve the shadow area problem. Although small cell technology has these advantages, different problems, such as the effects of interference due to the deployment of a large number of small cells and the differences in devices accessing the network, need to be solved. To do so, it is necessary to develop an algorithm for a service method. However, general algorithms have difficulties in responding to the diverse environment of mobile communication systems, such as sudden increase in traffic in certain areas or sudden changes in the mobile population, and machine learning technology has been applied to solve this problem. This study employs a machine learning algorithm to determine small cell connections. In addition, a 5G macro system, the application of small cells, and the application of machine learning algorithms are compared to determine the performance improvement in the machine learning algorithm. Moreover, Support Vector Machine (SVM), Logistic Regression and Decision Tree algorithm are employed to show a training method that uses basic training data and a small cell on-off method, and the performance enhancement is verified based on this method.

RESEARCH OF THE PROCEDURE OF INITIAL ACCESS TO THE FIFTH GENERATION COMMUNICATION NETWORK

Traditional mobile networks have bandwidth and infrastructure limitations. Each base station has a limited number of users and data capacity. 5G technology uses advanced technologies, such as massive MIMO and beamforming, to increase data rates and spectrum efficiency. These features, and the large number of small cells, allow many 5G users to receive low latency and high traffic without overloading the base station. Small cells work the same way as regular cells, but use new technology and millimetre waves (5G New Radio). Low-power base stations can be easily deployed. Cell search is a procedure by which the user equipment (UE) obtains time and frequency synchronization with the base station and decodes the cell ID of the mobile device. System information is broadcast information that is periodically transmitted by the base station on the downlink. This is very important information for the mobile device (UE) needed to connect to the base station (gNB). The UE reads system information to determine the location of mobile devices when turned on, to select and re-select a cell. System information provides the following information: system frame number, system bandwidth, PLMN, mobile subscriber selection and redial thresholds, etc., required to access the network. Physical Cell ID (PCI) is used to distinguish mobile devices in the network radio communication. PLMN (Public land Mobile Network) – mobile network service area. Is a set of all mobile devices of one cellular network operator. The UE scans the bandwidth in the sync raster. The synchronization raster indicates the frequency positions of the synchronization unit that can be used by the UE to detect the system when there is no explicit indication of the position of the synchronization unit. In the next step, the UE decodes the primary and secondary synchronization signals to obtain the Physical Cell ID (PCI) and the DMRS PBCH. The base station transmits SSB packets that contain one SSB per beam. The beam direction changes for each SSB transmission. The following steps are performed to complete the synchronization procedure. The UE selects the preferred beam and decodes the PBCH MIB information, such as SFN, SSB index, raster offset, default DL numerology, RMSI configuration, DM-RS location, and cell prohibition information. If the cell is not locked, the UE attempts to decode the physical downlink control channels (PDCCH) to obtain the frequency location of the PDSCH resources. A study of the model of restoring the synchronization of symbols and frames in the QPSK system, caused by a variable synchronization error. A study of the formation of the radio channel at the physical level.

A Study of Scheduling Algorithms in LTE -Advanced HetNet

Long Term Evolution (LTE), known as 4G technology, is considered the choice for Third Generation Partnership Project (3GPP) and 3GPP2 mobile operators. LTE is expected to bring an improved user experience with full mobility support.   However, to cope with the increasing number of mobile data subscribers that compete for the limited radio resources, effective network Quality of Service (QoS) mechanisms are necessary. A solution is to introduce small cells by adding low-power base stations to the existing macro- eNBs (Evolved Node B). The result is a Heterogeneous Network (HetNet) with large macro-cells combined with small cells (micro-, pico- and femto-cells) to cover areas not covered by the Macro-cells. Practically, HetNet is introduced in LTE-Advanced which is an extended version of LTE with more advanced features. Introducing HetNet in LTE-A brings challenges especially in terms of physical integration, QoS support and interference management. In this research, an architecture of HetNet integration is proposed. After that, some QoS scheduling algorithms are deployed for different types of on-going applications. Then, through the use of Riverbed Modeler, their performance parameters are studied with extensive simulations. Finally, an intensive comparison is done among the algorithms followed by a detailed discussion.

Joint DL/UL Decouple User Association in Microwave and mmWave Enabled Beyond 5G Heterogeneous Networks

Beyond fifth-generation (5G) heterogeneous networks (HetNets) are facing the challenge of accommodating enormous mobile users and data traffic with scarce spectrum resources in the microWave ( $\mu $ Wave) band. In this work, we consider both challenges in HetNets comprising large size, high power base station (LHB) and relay operating in $\mu $ Wave band and small size, low power base station (SLB) and device-to-device (D2D) operating in millimeter wave (mmWave) band. We formulate user association optimization problems to pitch user association schemes based on downlink uplink decoupled (DU-DPL) access against traditional downlink uplink coupled (DU-CPL) access in HetNets to gauge the performance of these access schemes in terms of accommodating users and spectrum efficiency in $\mu $ Wave and mmWave bands. Formulated problems are non-deterministic polynomial-time hard (NP-hard) and solved using $\epsilon $ -optimal algorithm. Simulation results show the edge of DU-DPL access over DU-CPL access in terms of users association and spectrum efficiency.

Parameter Adaptation and Situation Awareness of LTE-R Handover for High-Speed Railway Communication

In the evolution of railway mobile communications from Long Term Evolution for Railway ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LTE-R</i> ) to the future 5th Generation Wireless System ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5G</i> ), the rapid increase in the number of low-power base station nodes along the railway has brought more frequent handovers. The current handover parameter selection mechanism often relies on the on-site measured results in a limited number of discrete scenarios. It cannot deal with the continuous changing characteristics of the high-speed railway mobile communication environment, which leads to a serious lack of accuracy, adaptability and intelligence. This article hopes to construct a parameter-adaptive handover mechanism suitable for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5G</i> in the high-speed railway dedicated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LTE-R</i> communication system. The mechanism first uses the interaction of Temporal-Difference( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TD</i> )-learning-based reinforced agents to obtain high-speed railway handover performance and network performance in different combinations of speeds and handover parameters, and continuously updates the accumulated rewards used to target optimization, obtaining a Discrete TD value cube with closely related handover performance. Further, based on the Discrete TD value cube, we use the approximation function method for the completion of “continuous” situation of handover parameter selection, and construct a continuous TD value cube and the corresponding performance cubes. Our experimental results prove that TD learning agents with function approximation can accurately estimate and predict the handover performance and network performance of state combinations with different speeds and handover parameters, and further show that the handover parameter adaptation mechanism based on the Inference ability can find the optimal handover parameters to improve the handover performance and network performance.

Analysis of twin-layer cellular networks and frequency-swapping aided femtocell relying on fractional frequency reuse

In the 5th generation wireless systems, one of the effective technologies to enhance the attainable performance is to use heterogeneous networks, consist of macro base stations and low power base stations such as femto base stations. In order to achieve high spectral efficiency, as one of the most promising trends for heterogeneous networks, the low power base stations have to reuse the available spectrum of macro base stations. However, by reusing the available spectrum, the resulting co-channel interference increases the outage probability of the network. In this paper, first, a new spectrum access strategy has been proposed for a two-tier heterogeneous network consisting of distributed antenna system-aided macrocells and femtocells. Then, the outage probability of the outdoor macro users, as the main users of the network, has been investigated. The simulation results indicate that by applying the proposed spectrum access strategy, the outage probability of the outdoor macro users in the considered network will be equal to the outage probability of the outdoor macro users in the absence of femto base stations. In fact, the effect of interference from femto base stations with the outdoor macro users will be eliminated and the outage probability of the outdoor macro users will be significantly reduced comparing to the state of the art strategy.

A Holistic Experimental Static Bias Point Identification Method for Low Power Wireless RF Amplifier

A holistic and effective experimental method of optimum static gate bias identification is presented here for a modern wireless Radio Frequency (RF) power amplifier operating in 1900 MHz band. The amplifier is targeted for low complexity, high efficiency and low power Base Station (BS) having RF conducted power of less than 1 watt at its output load. Through the measured results, it is shown that out of various other static gate bias points obtained, the optimum gate bias condition is achieved at 2.4 volts. At this bias point setting, the PA delivers a modulated output power of +24 dBm in a standalone mode and with external RF linearization circuits added, the channel power enhanced upto +26 dBm and the Adjacent Channel Power Ratio (ACPR) is improved by 5.3 dB. At linear back-off power level, the achieved efficiency is 13.2%.

Delay-Oriented Cross-Tier Handover Optimization in Ultra-Dense Heterogeneous Networks

To meet the ever-growing demand and service requirement, especially for delay-sensitive services like vehicular communications, small cells consisting of low-power base stations (BSs) are deployed to undertake the traffic pressure of BSs in macro cells. The vehicle may handover from the macro cell tier to the small cell tier when moving toward a small cell. In this paper, we mainly analyze and optimize the cross-tier handover performance in heterogeneous networks from the perspective of delay characteristics. Based on the stochastic geometry theory, we deduce the expression of the effective capacity which reflects the delay to the rate before and after the handover; moreover, a resource reservation scheme is proposed. Based on this scheme, a closed-form expression of the blocking probability for delay-sensitive users is derived. Finally, based on the convex optimization theory, a joint optimization of effective capacity and blocking probability is carried out. The simulations show that the proposed optimization algorithm will greatly reduce the blocking probability while keeping the effective capacity relatively constant, especially beneficial for the delay-sensitive users. The theoretical and simulation results can be applied to the handover strategy and resource reservation strategy selection in heterogeneous networks.

Statistical model of the human RF exposure in small cell environment

Small cells are one of the solutions to face the imperative demand on increasing mobile data traffic. They are low-powered base stations installed close to the users to offer better network services and to deal with increased data traffic. In this paper, the global exposure induced in such networks as a whole from user equipment and base stations has been investigated. As the small cell is close to the user, the propagation channel becomes highly variable and strongly susceptible by environmental factors such as the road traffic. An innovative statistical path loss model is constructed based on measurements on two French commercial LTE small cells, operating at LTE 1800 MHz and 2600 MHz. This statistical path loss model is then used to assess global exposure of the adult proportion of a population in a scenario composed of a street lined with buildings and indoor and outdoor data users.

Throughput and Energy Efficiency of Wireless Powered Multi-tier MIMO HetNets

Wireless powered communications have been recently proposed as a variable power solution for the heterogeneous networks (HetNets) with small and flexible deployments of low-power base stations (BSs). In this paper, we consider the wireless powered multi-tier multi-input multi-output (MIMO) HetNets, where the multi-antenna BSs perform downlink transmission. In particular, we consider two cases of interests, i.e., 1) energy harvesting (EH) without energy beamforming (EB) tier and 2) EH with EB tier. Using tools of stochastic geometry, we perform analysis on the throughput and energy efficiency (EE) of the considered network. Closed-form and tractable results are obtained to reveal interesting insights that the proposed wireless-powered MIMO HetNets can achieve higher EE as compared to the conventional HetNets without EH, and the EE performance can be further improved by introducing well-designed EB across tiers.

Complexity of coordinated beamforming and scheduling for OFDMA based heterogeneous networks

Coordination is foreseen to be an important component of future mobile radio networks. It is especially relevant in heterogeneous networks, where high power base stations produce strong interference to an underlying layer of low power base stations. This work investigates in detail the achievable performance gains for one coordination technique—coordinated beamforming. It reveals the main factors that influence the throughput of the mobile stations. These findings are combined with an analysis of the computational complexity. As a result, a heuristic algorithm is presented that achieves results close to an exhaustive search with significantly less calculations. Detailed simulation analysis is presented on a realistic network layout.

Heterogeneous Cloud Radio Access Networks: Enhanced Time Allocation for Interference Mitigation

Responding to the unprecedented challenges imposed by the 5G technologies, mobile operators have given significant attention to Heterogeneous Cloud Radio Access Networks (H‐CRAN) due to their beneficial features of performing optimization, cost effectiveness, and improving spectral and energy efficiency performance. H‐CRAN inherits the attractive benefits of Heterogeneous Networks (HetNet) and the cloud computing by facilitating interference mitigation, scalability, and radio resource control. Consequently, H‐CRAN is proposed in this article as a cost‐effective potential solution to alleviate intertier interference and improve cooperative processing gains in HetNets by employing cloud computing. H‐CRAN can provide efficient resource sharing at the spectrum, network, and infrastructure levels. Therefore, this article proposes H‐CRAN cooperative interference mitigation method that enhances the time sharing among Radio Remote Heads (RRH) users. The study proposes an enhanced Almost Blank Subframe (ABSF) technique to increase the SINR and throughput of the small‐cell (low power base station) and macrocell users. Simulation results show that the proposed Dynamic Programming‐Diverse Almost Blank Subframe (ABSF) Pattern (DP-DAP) scheme improved the macro‐ and small‐cell users up to 56% and 35%, respectively, as compared to other state‐of‐the‐art ABSF schemes.

Virtual Small Cell Selection Schemes Based on Sum Rate Analysis in Ultra-Dense Network

Ultra-Dense Network (UDN) is regarded as a major development trend in the evolution of future networks, due to its ability to provide larger sum rate to the whole system and meet higher users’ Quality of Service (QoS). Different from the existing heterogeneous network, UDN has a smaller cell radius and a new network structure. The core concept of UDN is to deploy the low power Base Stations (BSs), i.e. Virtual Small Cells (VSCs). First, we derive an ergodic sum rate expression. To acquire the maximum ergodic sum rate of all the users, then we adopt the selection mode based on minimum distance. Due to the consideration of the computation complexity of the above VSC selection scheme, we finally propose a novel VSC selection scheme based on pattern search. The simulation results demonstrate the correctness of the ergodic sum rate expression and show the lower computation complexity of the proposed VSC selection scheme comparing with the above reference scheme.

Joint Load Balancing of Downlink and Uplink for eICIC in Heterogeneous Network

The 3GPP has proposed enhanced intercell interference coordination (eICIC) by making the macrocell silent in the almost blank subframes (ABSs) to mitigate interference for low-power base stations (BSs) in the downlink direction. However, two important challenges are introduced: 1) the underutilization for macrocell in ABS and 2) the optimal downlink association may not be optimal for uplink transmission, i.e., downlink/uplink decoupling (DUDe). In this paper, we first propose a novel method based on configuring uplink transmission in the macrocell in ABSs (UM-ABS) for eICIC to improve the efficiency of the ABS. Then, the UM-ABS and DUDe are formulated as an optimization problem of investigating the joint load balancing of downlink and uplink for eICIC in heterogeneous networks (HetNets) while considering different service classes. The problem is actually a mixed binary integer programming problem, which is difficult to solve. We provide a relaxed-rounding solution to the model. Numerical results illustrate that the proposed algorithm achieves superior performance in comparison to state-of-the-art methods in terms of system throughput and the rate gain for users.

Joint Beamforming and Power Control Technique for Femtocell Networks

The need for high data rate increases with the growing demand for wireless communication. In order to meet this demand, one of the most effective ways is deployment of small cells which are considered as a promising technique for future wireless networks. However, the deployment of these low-power base stations brings many challenges. Interference management will be one of the major drawbacks for the deployment of small cells in coverage of the macro base stations. In order to cope with interference problem, we propose a joint beamforming and power allocation technique for a two-tier network system involving femtocell and macrocell layers to design power efficient networks. The beamforming technique is applied by using partial zero-forcing method to remove the highest cross-tier interference while satisfying the macro and femtocell users’ data rate requirements. Then, we perform power allocation to further reduce interference and design power efficient femtocell networks. The performance results of the proposed technique are illustrated in terms of power consumption in femtocell networks.

Enriched $K$ -Tier HetNet Model to Enable the Analysis of User-Centric Small Cell Deployments

One of the principal underlying assumptions of current approaches to the analysis of heterogeneous cellular networks (HetNets) with random spatial models is the uniform distribution of users independent of the base station (BS) locations. This assumption is not quite accurate, especially for user-centric capacity-driven small cell deployments where low-power BSs are deployed in the areas of high user density, thus inducing a natural correlation in the BS and user locations. In order to capture this correlation, we enrich the existing $K$ -tier Poisson point process (PPP) HetNet model by considering user locations as Poisson Cluster Process with the BSs at the cluster centers. In particular, we provide the formal analysis of the downlink coverage probability in terms of a general density function describing the locations of users around the BSs. The derived results are specialized for two cases of interest: 1) Thomas cluster process , where the locations of the users around BSs are Gaussian distributed and 2) Matern cluster process , where the users are uniformly distributed inside a disc of a given radius. Tight closed-form bounds for the coverage probability in these two cases are also derived. Our results demonstrate that the coverage probability decreases as the size of user clusters around BSs increases, ultimately collapsing to the result obtained under the assumption of PPP distribution of users independent of the BS locations when the cluster size goes to infinity. Using these results, we also handle mixed user distributions consisting of two types of users: 1) uniformly distributed and 2) clustered around certain tiers.

On the Spectral and Energy Efficiency of Full-Duplex Small-Cell Wireless Systems With Massive MIMO

The achievable rate of full-duplex (FD) small-cell systems with massive multiple-input multiple-output (MIMO), in which a low-power base station (BS) equipped with large antenna arrays sends/receives data to and from multiple half-duplex (HD) users at the same time on the same frequency, is investigated. The BS uses imperfect channel state information (CSI) obtained from received pilots, nonideal hardware, and a linear transmitter and receiver, i.e., zero-forcing (ZF) or maximum-ratio transmission/maximum-ratio combining (MRT/MRC), to process the signals. The approximate closed-form expressions of the achievable rate for both the ZF and MRT/MRC processing are derived and used to analyze the effect of the number of antennas and the hardware imperfection on the self-interference (SI), which is a bottleneck of the FD systems. To maximize the spectral efficiency (SE) and the energy efficiency (EE) of this system, two nonconvex power-allocation optimization problems are formulated and solved by utilizing the sequential convex approximation technique and the fractional programming technique. Two iterative algorithms are proposed with proved local convergence. Numerical results illustrate that the analytical approximation of achievable rate matches well with the Monte Carlo simulation. It is also shown that the ZF processing has greater ability to suppress SI, compared with MRT/MRC processing. The proposed power-allocation algorithms are shown to increase the SE and EE significantly, compared with the uniform power-allocation scheme when the BS is equipped with moderately large antenna arrays.