Macro Cell Research Articles

Energy Management Framework for 5G Ultra-Dense Networks Using Graph Theory

The next-generation 5G networks are being developed with high promised capabilities. Beyond just multitudes faster data speed, 5G is expected to serve billions of connected devices and the Internet of Things (IoT), with the right trade-offs between speed, latency, and energy at an affordable cost. 5G radio networks will strongly depend on using ultra-dense integrated Small Cells (SCs) beside the Macro Cells (MCs). This kind of Ultra-Dense Networks (UDN) consisting of a large number of MCs and SCs will significantly increase network energy demands. A practical method to control energy consumption is by dynamically controlling power-saving modes in radio networks. In this paper, we propose a novel cooperative energy management framework for 5G UDN using graph theory. The 5G network is first modeled as a graph, then graph theory methods are exploited to determine the order of nodes at which power-off/on procedure is applied. We also show that significant power savings are achievable by considering only a subset of network nodes and thus reduce traffic migration and control plane signaling. We evaluated the proposed algorithm at different network densification levels and several load factors including two real-life networks. We also present the convergence of the proposed algorithm and the robustness of networks optimized using it. We also show that power savings up to 25% at full load and 65% during off-peak can be achieved using the proposed algorithm. These power savings increase further if no constraints are imposed on traffic migration and control signaling.

Enhanced interference cancellation techniques for downlink of LTE-A heterogeneous networks

The formation of Heterogeneous Network (HetNet) into the cellular system improves the overall spectral efficiency and network capacity of the system. Although the forming of the heterogeneous cellular network increases the complexity and overall interference of the system, the interference between the macro cell and smaller cells like femto/pico reduces the overall performance of the system in the downlink. In downlink, the Common Reference Signal (CRS) is used at the User Element (UE) to estimate the channel characteristics and is available throughout the entire frequency band. The presence of CRS of the neighbouring cells creates interference to the users. Linear Minimum Mean Square Error (LMMSE) based equaliser is proposed to reduce the effect of the non-colliding CRS at the UE. Enhanced space alternating generalised expectation maximisation-maximum a posteriori probability (ESAGE-MAP) estimator is used at the UE to mitigate colliding CRS interference. The simulation results have proved that the proposed LMMSE and ESAGE-MAP interference cancellation techniques perform better than the existing methods.

Ultra-Dense LEO: Integrating Terrestrial-Satellite Networks Into 5G and Beyond for Data Offloading

In this paper, we propose a terrestrial-satellite network (TSN) architecture to integrate the ultra-dense low earth orbit (LEO) networks and the terrestrial networks to achieve efficient data offloading. In TSN, each ground user can access the network over C-band via a macro cell, a traditional small cell, or a LEO-backhauled small cell (LSC). Each LSC is then scheduled to upload the received data via multiple satellites over Ka-band. We aim to maximize the sum data rate and the number of accessed users while satisfying the varying backhaul capacity constraints jointly determined by the LEO satellite based backhaul links. The optimization problem is then decomposed into two closely connected subproblems and solved by our proposed matching algorithms. Simulation results show that the integrated network significantly outperforms the non-integrated ones in terms of the sum data rate. The influence of the traffic load and LEO constellation on the system performance is also discussed.

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.

Traffic Modelling of Low Dense Femtocellular Network for Long Term Evolution

One of the features of Long Term Evolution (LTE) is the deployment of femtocells as the underlain cell of a macrocell without any intervention of frequency planning to offload the traffic. In this paper we used Markov chain to derive the expression of blocking probability for both macro and femtocell in terms of traffic parameters of the network. We developed an analytical model to find the expression of probability of forced termination (FT) using combination of mobility model and probability tree considering low dense femtocellular network. Two different trees were designed: a newly originating call which starts its session in a femtocell and that of in a macro cell. The link parameters of small scale fading of wireless network under Multiple Input Multiple Output (MIMO) are combined with the proposed traffic model to get the probability of FT of a real-life network. A new state transition chain was also developed including its solution for LTE traffic of variable bandwidth (BW) and a comparison was made with Erlang’s traffic model.

Enhanced interference cancellation techniques for downlink of LTE-A heterogeneous networks

The formation of Heterogeneous Network (HetNet) into the cellular system improves the overall spectral efficiency and network capacity of the system. Although the forming of the heterogeneous cellular network increases the complexity and overall interference of the system, the interference between the macro cell and smaller cells like femto/pico reduces the overall performance of the system in the downlink. In downlink, the Common Reference Signal (CRS) is used at the User Element (UE) to estimate the channel characteristics and is available throughout the entire frequency band. The presence of CRS of the neighbouring cells creates interference to the users. Linear Minimum Mean Square Error (LMMSE) based equaliser is proposed to reduce the effect of the non-colliding CRS at the UE. Enhanced space alternating generalised expectation maximisation-maximum a posteriori probability (ESAGE-MAP) estimator is used at the UE to mitigate colliding CRS interference. The simulation results have proved that the proposed LMMSE and ESAGE-MAP interference cancellation techniques perform better than the existing methods.

Periodic Fixed-Frequency Staggered Line Leaky Wave Antenna With Wide-Range Beam Scanning Capacity

A periodic fixed-frequency staggered line leaky wave antenna (LWA) with wide-range beam scanning capacity is proposed in this paper. The reconfigurable antenna is based on a periodic staggered line LWA. The dispersion and Bloch characteristics of the periodic antenna is analyzed by the Macro Cell Method (MCM). The open-stop band (OSB) of the antenna can be suppressed effectively using the relationship between widths of oblique feeding lines and stubs. The reconfigurable characteristic is achieved by switches and microstrip lines between the edge gaps of each unit cell. A “supercell” is established by combining several reconfigurable unit cells, and the state is controlled by connecting or disconnecting the edge gaps of each unit cell using switches. Fixed-frequency beam-scanning capacity is implemented due to different propagation constants in different supercell states. The prototyped reconfigurable antenna can scan the beam between 130° and 54° at 4.5 GHz.

Outage Analysis of Relay-Assisted mmWave Cellular Systems Employing JSDM

In this paper, the outage probabilities of relay-assisted millimeter wave (mmWave) cellular systems employing joint spatial division and multiplexing (JSDM) are investigated. It is assumed that the macro base station (BS) equipped with a large number of antennas serves the single antenna relay and users simultaneously and that the relay is located at the edge of the macro cell. Theoretical analysis of the signal-to-interference-plus-noise ratio (SINR) outage probabilities of each user with and without the second stage precoding is first obtained, respectively. The cell SINR outage probabilities are then derived. Under the noise-limited assumption, simplified closed-form expressions of the outage probabilities are given as well. The simulation results demonstrating the performance improvement due to the relay are provided. Overall, the impact of deploying a relay in the mmWave cellular systems on the outage probabilities is characterized.

TARCO: Two-Stage Auction for D2D Relay Aided Computation Resource Allocation in HetNet

In heterogeneous cellular network, task scheduling for computation offloading is one of the biggest challenges. Most works focus on alleviating heavy burden of macro base stations by moving the computation tasks on macro-cell user equipment (MUE) to remote cloud or small-cell base stations. But the selfishness of network users is seldom considered. Motivated by the cloud edge computing, this paper provides incentive for task transfer from macro cell users to small cell base stations. The proposed incentive scheme utilizes small cell user equipment to provide relay service. The problem of computation offloading is modelled as a two-stage auction, in which the remote MUEs with common social character can form a group and then buy the computation resource of small-cell base stations with the relay of small cell user equipment. A two-stage auction scheme named TARCO is contributed to maximize utilities for both sellers and buyers in the network. The truthful, individual rationality and budget balance of the TARCO are also proved in this paper. In addition, two algorithms are proposed to further refine TARCO on the social welfare of the network. Extensive simulation results demonstrate that, TARCO is better than random algorithm by about 104.90% in terms of average utility of MUEs, while the performance of TARCO is further improved up to 28.75% and 17.06% by the proposed two algorithms, respectively.

Optimal Dual-Connectivity Traffic Offloading in Energy-Harvesting Small-Cell Networks

Traffic offloading through heterogenous small-cell networks (HSCNs) has been envisioned as a cost-efficient approach to accommodate the tremendous traffic growth in cellular networks. In this paper, we investigate an energy-efficient dual-connectivity (DC) enabled traffic offloading through HSCNs, in which small cells are powered in a hybrid manner including both the conventional on-grid power-supply and renewable energy harvested from environment. To achieve a flexible traffic offloading, the emerging DC-enabled traffic offloading in 3GPP specification allows each mobile user (MU) to simultaneously communicate with a macro cell and offload data through a small cell. In spite of saving the on-grid power consumption, powering traffic offloading by energy harvesting (EH) might lead to quality of service degradation, e.g., when the EH power-supply fails to support the required offloading rate. Thus, to reap the benefits of the DC-capability and the EH power-supply, we propose a joint optimization of traffic scheduling and power allocation that aims at minimizing the total on-grid power consumption of macro and small cells, while guaranteeing each served MU’s traffic requirement. We start by studying a representative case of one small cell serving a group of MUs. In spite of the non-convexity of the formulated joint optimization problem, we exploit its layered structure and propose an algorithm that efficiently computes the optimal offloading solution. We further study the scenario of multiple small cells, and investigate how the small cells select different MUs for maximizing the system-wise reward that accounts for the revenue for offloading the MUs’ traffic and the cost of total on-grid power consumption of all cells. We also propose an efficient algorithm to find the optimal MU-selection solution. Numerical results are provided to validate our proposed algorithms and show the advantage of our proposed DC-enabled traffic offloading through the EH-powered small cells.

Security-Reliability Tradeoff for Distributed Antenna Systems in Heterogeneous Cellular Networks

In this paper, we investigate physical-layer security for a spectrum-sharing heterogeneous cellular network comprised of a macro cell and a small cell, where a passive eavesdropper is assumed to tap the transmissions of both the macro cell and small cell. In the macro cell, a macro base station (MBS) equipped with multiple distributed antennas sends its confidential information to a macro user (MU) through an opportunistic transmit antenna. Meanwhile, in the small cell, a small base station (SBS) transmits to a small user (SU) over the same spectrum used by MBS. We propose an interference-canceled opportunistic antenna selection (IC-OAS) scheme to enhance physical-layer security for the heterogeneous network. To be specific, when MBS sends its confidential message to MU through an opportunistic distributed antenna, a special signal is artificially designed and emitted at MBS to ensure that the received interference at MU from SBS is canceled out. For comparison, the conventional interference-limited opportunistic antenna selection (IL-OAS) is considered as a benchmark. We characterize the security-reliability tradeoff (SRT) for the proposed IC-OAS and conventional IL-OAS schemes in terms of deriving their closed-form expressions of intercept probability and outage probability. Numerical results show that compared with the conventional IL-OAS, the proposed IC-OAS scheme not only brings SRT benefits to the macro cell, but also has the potential of improving the SRT of small cell by increasing the number of distributed antennas. Additionally, by jointly taking into account the macro cell and small cell, an overall SRT of the proposed IC-OAS scheme is shown to be significantly better than that of the conventional IL-OAS approach in terms of a sum intercept probability versus sum outage probability.

Energy Efficiency Optimization for CoMP-SWIPT Heterogeneous Networks

In this paper, a fundamental study of energy efficiency (EE) optimization for coordinated multi-point (CoMP) simultaneous wireless information and power transfer (SWIPT) heterogeneous networks (HetNets) is provided. We aim to optimize the EE while satisfying certain quality-of-service requirements in regard to transmission rate and energy harvesting at both the macro cell and small cells. The corresponding joint beamforming and power allocation in the presence of intra- and inter-cell interference constitutes an EE maximization problem that is non-convex, and hence, very challenging to solve. In order to solve this problem, we propose to separate the beamforming design and power allocation processes. First, we adopt linear zero-forcing (ZF) beamforming to suppress the multi-user interference from both the energy harvesting users (EH-UEs) as well as the information decoding UEs (ID-UEs), thus transforming the HetNet under consideration to a virtual point-to-point system. An efficient power allocation algorithm is then developed to maximize the corresponding EE. On the other hand, the ZF strategy does not utilize the notion that interference benefits the EH-UEs. As a result, we propose a partial ZF approach by differentiating the EH-UEs and ID-UEs in order to further improve the EE. Our findings show that the EE can be significantly improved through the integration of CoMP-SWIPT in HetNets.

Physical Layer Security in Heterogeneous Networks With Pilot Attack: A Stochastic Geometry Approach

In this paper, we investigate physical layer security in a two-tier heterogeneous network with sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot attacks from the eavesdroppers, we analyze the coverage and secrecy performance using stochastic geometry. For the sub-6 GHz tier, we show that increasing the number of BS antennas is more effective than increasing BS density in improving the coverage performance, whereas densifying BS is more effective for security enhancement. For the mmWave tier, we first derive the success probability of beam alignment based on a beam sweeping-based channel training model. It is shown that the mmWave tier may outperform the sub-6 GHz counterpart in terms of both coverage and secrecy through densifying the base stations. Our results also reveal that the mmWave small cell can provide better coverage performance in the high transmission rate region, and can achieve higher security in the low redundant rate region, which reveals the advantage of using mmWave for secure communication. Numerical results verify the analysis.

A Reusable Code-Based SAR ADC Design With CDAC Compiler and Synthesizable Analog Building Blocks

This brief proposes a code-reusable design methodology for synthesizable successive approximation register (SAR) ADCs based on the digital design flow to significantly reduce design effort. The SAR ADCs are composed of a capacitor-DAC (CDAC) macro cell generated by a CDAC compiler and analog functional blocks implemented utilizing digital standard cells. Two prototypes of SAR ADCs (12-bit 100 kS/s and 11-bit 50 MS/s) are fabricated in different CMOS processes (180 nm and 28 nm). The prototype ADCs prove the effectiveness of the proposed design methodology with comparable performances with full-custom designed SAR ADCs.

RF-EMF exposure induced by mobile phones operating in LTE small cells in two different urban cities

With the huge growth in data traffic, the densification of the macro cell (MC) layer with low-powered small cell (SC) base stations (resulting in a heterogeneous network) will improve network performances in terms of radio coverage and capacity. However, this may influence the human exposure to radiofrequency electromagnetic fields (RF-EMFs). Through measurement campaigns in two different urban cities (in France and the Netherlands), the authors characterized the RF-EMF exposure induced by LTE (Long-Term Evolution) MC and SC networks, while considering radio emissions from both base stations (downlink or DL) and user equipment (uplink or UL). For an internet data usage and with respect to an MC connection, results showed that an SC connection may increase the DL exposure while decreasing the UL exposure (with a factor of 5 to 17), mainly due to the lower mobile phone emitted power and depending on whether the throughput is limited or not. Furthermore, the city with a dense network is characterized by low UL exposure and high DL exposure.

A Novel Cooperative NOMA for Designing UAV-Assisted Wireless Backhaul Networks

In this paper, we investigate the downlink transmissions in wireless backhaul (WB) networks when unmanned aerial vehicles (UAVs) are used as flying small cell base stations. We propose to employ the non-orthogonal multiple access (NOMA) on the WB transmissions and introduce a novel cooperative transmission scheme for the wireless access links. Then, we formulate an optimization problem which jointly determines the radio resource allocation at the macro cell base station (MBS) and UAVs along with the optimization of the decoding order of the NOMA process and the positions of the UAVs in space to maximize the sum achievable rate of all users. The formulated problem is a general mixed integer non-convex program, which is very difficult to solve optimally within a polynomial time. Therefore, we propose a framework based on the method of difference of convex program characterized by the Lipschitz continuity to transform and approximate the original problem into a series of convex approximate ones and develop a low-complexity algorithm to sequentially solve for each approximate problem until convergence. Numerical evaluation and analysis show that our achieved solution, under the proposed framework and developed algorithm, can outperform the other schemes which aim at either optimizing without using cooperative NOMA or do not optimize the UAVs' positions.

Interference management for D2D communications in heterogeneous cellular networks

Device-to-device (D2D) communications in cellular networks enable user equipments (UEs) to directly communicate with each other without the relay of base stations (BSs). They can improve network performance, while they also complicate interference management (IM). In this paper, the IM issue is studied for the D2D communications underlaying heterogeneous networks consisting of macro and small cells. Investigation is first made into D2D performance for UEs in different communication modes (D2D mode and device-to-cell modes). Then, a D2D feasible set is defined and derived which gives a region to allow D2D links to share network resource; and a feasible set scheme is proposed to manage the interference among communication links in order for the cellular links and D2D links to satisfy quality of service (QoS) requirements. After this, the optimal deployment of small cells is studied and an optimal deployment scheme is found that maximizes the transmission density while the QoS of links in different communication modes can be guaranteed. The analytical results are obtained and the calculations show that the proposed interference management schemes can effectively mitigate interference among cellular links and D2D links; and all the links (macro cell link, small cell link and D2D link) can satisfy the QoS requirement when they are sharing the resources. Compared to the neighbor based scheme, the proposed scheme can reduce the average outage probability up to 92%. In addition, the proposed deployment scheme can maximize the transmission capacity with guaranteed link QoSs.

Quality-of-Service Aware Game Theory-Based Uplink Power Control for 5G Heterogeneous Networks

The fifth-generation (5G) mobile communication system is expecting to support users with diverse data rate requirements by densely deploying small cells. The users attached with small cells make use of the same frequency band as the existing macro cell users, that causes severe co-channel interference and degrades the performance. To overcome this challenge, we propose a game theoretical framework for the optimal uplink power allocation for small cells, i.e., femtocell deployed underlaid macrocell. In this paper, femtocell users play a non-cooperative game to choose the optimal power to maximize the sum-rate of the system. Furthermore, an iterative quality-of-service (QoS)-aware game theory based power control (QoS-GTPC) scheme is proposed to optimize the femtocell user power taking into account macrocell user QoS requirements. Simulation results verify that the proposed QoS-GTPC scheme significantly improves the sum-rate and reduces outage and interference, as compared with conventional power control scheme.

An Efficient CoMP-based Handover Scheme for Evolving Wireless Networks

Inter-Cell Interference (ICI) will be one of main problems for degrading the performance of future wireless networks at the cell edge. This adverse situation will become worst in the presence of dense deployment of micro and macro cells. In this context, the Coordinated Muli-Point (CoMP) technique was introduced to mitigate ICI in evolving wireless networks and increase their network performance at the cell edge. Even though the CoMP technique provides satisfactory solutions of various problems at the cell edge, nevertheless existing CoMP handover schemes do not prevent unnecessary handover initialisation decisions. In this paper, a new CoMP-based handover scheme is proposed in order to minimise unnecessary handover decisions at the cell edge in conjunction with signal measurements such as Reference Signal Received Power (RSRP) and Received Signal Received Quality (RSRQ). A combination of calculations of RSRP and RSRQ facilitate a credible decision making process of CoMP mode and handover mode at the cell edge. Typical numerical experiments indicate that by triggering the CoMP mode, the overall network performance is constantly increase as the number of unnecessary handovers is progressively reduced.

Interference Coordination for 3-D Beamforming-Based HetNet Exploiting Statistical Channel-State Information

In this paper, we investigate the inter-tier interference coordination for a heterogeneous network (HetNet), where both macro and small cells work in the frequency-division duplexing mode and share the same spectrum. A 2-D large-scale antenna array is deployed at a macro-base station (BS), while conventional multiple antenna array is deployed at each small cell. First, we derive an approximation of macro-users’ signal-to-interference-plus-noise ratio (SINR), under the assumption of only statistical channel-state information (CSI) at macro-BS, and a 3-D beamforming transmission scheme is applied for macro-users. A lower-bound of small-cell users’ expected SINR is also derived using the property of complex Wishart matrix. Based on these, simple metrics are derived to measure the inter-tier interference. Then, new interference coordination algorithms are proposed to achieve a good tradeoff for the performance and traffic between macro-cells and small cells. The proposed algorithms require only a few additional statistical CSI. Moreover, we derive tractable ergodic rate approximation of both the macro-cell and small-cell users which are shown to match well with the Monte Carlo results.