Two-tier Heterogeneous Network Research Articles

Joint Uplink/Downlink Coverage and Spectral Efficiency in Heterogeneous Cellular Network

In this paper, the joint uplink and downlink signal to interference plus noise ratio distribution have been analyzed in a two-tier heterogeneous cellular network. We assume that macro and pico base stations and user equipments are uniformly distributed in the network. User could be paired to its nearest base station or to the BS from which the average received power in the downlink is maximum. Uplink in both cases is coupled to downlink. For symmetric services both up and down links should be able to carry the information flow. Correlation between uplink and downlink coverage could be guessed even under independent Rayleigh fading. The symmetric spectral efficiency gathers both uplink and downlink capacities while forcing them to happen simultaneously. We used realistic spectral efficiencies as signaled by channel quality indicators in LTE. Fractional power control has also been included in the analysis. The results of coverage and spectral efficiency for marginal uplink, marginal downlink, and joint uplink/downlink has been obtained. The assumption of independence between both links is analyzed for the complex trade off of the parameters for network design to obtain the best symmetric efficiency.

On Downlink Resource Allocation for SWIPT in Small Cells in a Two-Tier HetNet

We study the downlink resource allocation problem for simultaneous wireless information and power transfer (SWIPT) in small cells underlaying a macrocell in a two-tier heterogeneous network. For SWIPT, we consider both time-switching and power-splitting approaches. We determine downlink transmit power of small cell base stations along with time-switching/power-splitting variables for SWIPT to jointly optimize energy harvesting rate and achievable throughput of small cell users while ensuring minimum throughput of macrocell user. In the time-switching approach, the resource allocation problem is formulated as a mixed-integer non-linear programming (MINLP) problem. The formulated MINLP problem is solved by relaxing the binary integer constraint and then identifying the condition at which the obtained solution satisfies that constraint. In both the time-switching and power-splitting approaches, in the presence of non-negligible co-tier interference, the formulated problem is solved sub-optimally by iteratively maximizing the minorant of the non-convex objective function. A special case with negligible co-tier interference is considered in the time-switching approach and the optimal solution is obtained by using convex optimization techniques. Numerical results demonstrate significant gain in energy harvesting rate when macrocell users have flexible interference tolerance levels in time-switching approach, highlight the improvement in energy harvesting rate in the presence of co-tier interference signal, and reveal interesting trade-off in achievable throughput and energy harvesting rate.

Distributed Power Allocation for Femtocell Networks Subject to Macrocell SINR Balancing

We consider the power allocation for the two-tier heterogeneous network where a macrocell (MC) coexists with multiple femtocells (FCs). Each FC aims to maximize the weighted sum-rate of its users subject to MC signal-to-inter-ference-plus-noise ratio (SINR) balancing, which means that a prescribed SINR is guaranteed to MC users. We formulate a noncooperative game which captures bidirectional cross-tier interference. We develop a fully decentralized algorithm, and derive a sufficient condition for its convergence to a unique equilibrium.

Promoting femtocell cooperation through incentive for improving data rate of indoor users in underlay heterogeneous network

This study proposes an incentive-based femtocell cooperation scheme for indoor macrocell users (MUs) in an underlay two-tier heterogeneous network which helps the indoor MUs to achieve higher data rate through time sharing access from the femtocell. Due to underlay spectrum sharing nature of the femtocell, the network suffers from severe cross-tier interference. To manage this cross-tier interference, a price-based power control scheme is also proposed for the femtocell access point (FAP) whereby the macrocell base station (MBS) controls the transmit power of the FAP by pricing their resulted cross-tier interference power level subject to a maximum tolerable interference limit. A Stackelberg game has been formulated and analysed for joint pricing-based power control and incentive-based cooperation game. Numerical results show the performance of the proposed scheme in terms of achievable rate of indoor users and utilities of both the MBS and the FAP for different locations of the femtocells.

Analysis of Urban Two-Tier Heterogeneous Mobile Networks With Small Cell Partitioning

In this paper, we investigate the performance of typical indoor users in urban two-tier heterogeneous mobile networks with indoor-deployed small cell base stations (BSs) and outdoor BSs. The urban topology is modeled by a Boolean scheme of blockages, which is tied together with the deployment of outdoor and indoor BSs as well as the signal propagation characteristics. Each building is assumed to be served by an indoor small cell BS with a certain probability, which is denoted as occupation probability. Indoor and outdoor environments are partitioned by walls with a certain penetration loss. We separately account for outdoor BSs in line of sight (LOS) and non-line of sight (NLOS), as well as small cell BSs in neighboring and non-neighboring buildings. We derive expressions for the signal to interference ratio (SIR)-coverage probability and the average spectral efficiency of a typical indoor user. Numerical evaluations are performed with 3GPP compliant macro- and pico-BS scenarios. Our results exhibit that the indoor area coverage, which is a function of the building density and size, significantly affects the impact of the target building’s wall penetration loss. The analysis is simplified by various assumptions, which are verified by extensive Monte Carlo simulations.

Resource Allocation in Two-Tier Wireless Backhaul Heterogeneous Networks

This paper studies two-tier heterogeneous cellular networks, with wireless backhaul communication, consisting of macrocell and small cell tiers. A joint design of transmit beamforming, power allocation, and bandwidth partitioning for both uplink and downlink transmissions is considered. By assuming the reverse time division duplexing system, we propose a strategy to partition the bandwidth for two consecutive time slots by two separate partitioning factors. Under the proposed strategy, we formulate a constrained optimization problem with the objective of maximizing the sum rate of small cell uplink and downlink. For this non-convex problem, we leverage the sequential parametric convex approximation method to find the stationary point of the problem. In this method, a convex approximation of the problem is solved at each iteration. Furthermore, with appropriate transformations, we approximate the problem as second-order cone programming (SOCP) and propose a fast converging algorithm to attain the solution. We also evaluate the impact of imperfect channel state information by reformulating the optimization problem and applying the proposed algorithm to solve it. We conduct numerical simulations to show that the joint design of transmit beamforming, power allocation, and bandwidth partitioning leads to a better resource utilization and high spectral efficiency. Moreover, our results show that the proposed SOCP-based algorithm converges fast to a solution, which is shown to be closer to the global optimal solution achieved by the branch-and-bound algorithm compared with other works.

Dormancy Mechanism Based Power Allocation in Heterogeneous Networks: A Stackelberg Game Approach

In two-tier heterogeneous networks (HetNets) that consist of a central macrocell underlaid with spectrum-sharing dense smallcells, one primary concern is the cross-tier interference limits the network capacity. On the other hand, dormancy mechanism of smallcells is an emerging technique that can improve energy efficiency and mitigate interference. Therefore in this paper, we propose a power optimization oriented Stackelberg game considering dormancy mechanism for the downlink of two-tier HetNets. First, the smallcell dormancy mechanism is introduced to control the smallcells’ operation. Then the Stackelberg game framework is developed. A sub game based on a potential theory is further formulated to optimize the transmit power of the active smallcells, which is demonstrated to converge to a pure and unique Nash equilibrium. In this sub game, the interference pricing mechanism is employed to price the cross-tier interference caused by smallcells and hence restricts the utility function of smallcells. Simulation results show that the proposed Stackelberg game based on dormancy mechanism with interference pricing can improve the smallcell network throughput and meanwhile reduce the average transmit power of smallcells.

Energy Efficient Inter-Frequency Small Cell Discovery in Heterogeneous Networks

In this paper, using stochastic geometry, we investigate the average energy efficiency (AEE) of the user terminal (UT) in the uplink of a two-tier heterogeneous network, where the two tiers are operated on separate carrier frequencies. In such a deployment, a typical UT must periodically perform the interfrequency small-cell discovery (ISCD) process to discover small cells in its neighborhood and benefit from the high data rate and traffic offloading opportunity that small cells present. We assume that the base stations of each tier and UTs are randomly located, and we derive the average ergodic rate and UT power consumption, which are later used for our AEE evaluation. The AEE incorporates the percentage of time that a typical UT missed small-cell offloading opportunity as a result of the periodicity of the ISCD process. The additional power consumed by the UT due to the ISCD measurement is also included. Moreover, we derive the optimal ISCD periodicity based on the UT's average energy consumption (AEC) and AEE. Our results reveal that ISCD periodicity must be selected with the objective of either minimizing the UT's AEC or maximizing the UT's AEE.

Load-aware offloading strategy in two-tier heterogeneous network

Although small cell offloading technology can alleviate the congestion in macrocell, aggressively offloading data traffic from macrocell to small cell can also degrade the performance of small cell due to the heavy load. Because of collision and backoff, the degradation is significant especially in network with contention-based channel access, and finally decreases throughput of the whole network. To find an optimal fraction of traffic to be offloaded in heterogeneous network, we combine Markov chain with the Poisson point process model to analyze contention-based throughput in irregularly deployment networks. Then we derive the close-form solution of the throughput and find that it is a function of the transmit power and density of base stations. Based on this, we propose the load-aware offloading strategies via power control and base station density adjustment. The numerical results verify our analysis and show a great performance gain compared with non-load-aware offloading.

On the Superiority of Improper Gaussian Signaling in Wireless Interference MIMO Scenarios

Recent results have elucidated the benefits of using improper Gaussian signaling (IGS) as compared with conventional proper Gaussian signaling (PGS) in terms of achievable rate for interference-limited conditions. This paper exploits majorization theory tools to formally quantify the gains of IGS along with widely linear transceivers for multiple-input multiple-output (MIMO) systems in interference-limited scenarios. The MIMO point-to-point channel with interference is analyzed, assuming that received interference can be either proper or improper, and we demonstrate that the use of the optimal IGS when received interference is improper strictly outperforms (in terms of achievable rate and mean square error) the use of the optimal PGS when interference is proper. Then, these results are extended to two practical situations. First, the MIMO Z-interference channel (Z-IC) is investigated, where a trade-off arises: with IGS, we could increase the achievable rate of the interfered user while gracefully degrading the rate of the non-interfered user. Second, these concepts are applied to a two-tier heterogeneous cellular network where macrocells and small cells coexist and multiple MIMO Z-ICs appear.

Analysis of Two-Tier LTE Network with Randomized Resource Allocation and Proactive Offloading

The heterogeneity in cellular networks that comprise multiple base stations’ types imposes new challenges in network planning and deployment. The Radio Resource Management (RRM) techniques, such as dynamic sharing of the available resources and advanced user association strategies determine the overall network capacity and efficiency. This paper evaluates the downlink performance of a two-tier heterogeneous LTE network (consisting of macro and femto tiers) in terms of rate distribution, i.e. the percentage of users that achieve certain rate in the system. The paper specifically addresses the femto tier RRM by randomization of the allocated resources (1) and the user association process by introducing novel proactive offloading scheme (2). System level simulation results show that the proposed proactive offloading scheme in the association phase improves the performance of congested networks by efficiently utilizing the available femto tier resources.

Pricing‐based Stackelberg game for spectrum trading in self‐organised heterogeneous networks

In this study, the spectrum trading problem in a self-organised and two-tier heterogeneous cellular network is studied and the bandwidth sharing problem incorporated with cell range expansion technique is formulated as a Stackelberg game. Maximising the revenue of macro eNodeB (MeNB), affording minimum required bandwidth for each home eNodeB (HeNB), enhancing per femto-user throughput, and providing better quality of service for macro-users nearby each femto-cell are the main objectives of this investigation. The authors propose an iterative scheme in which MeNB and HeNBs make decisions autonomously. The MeNB as the spectrum provider decides on the unit-bandwidth price with the objective to maximise its revenue. Each HeNB, on the other side, decides on the amount of requested bandwidth as well as the number of possible nearby macro-users which will support to take advantage of MeNB discounting. The designed discounting strategy is applied for the extra bandwidth request of HeNBs to encourage them in supporting nearby macro-users. It is shown analytically that the proposed scheme converges to a unique Nash equilibrium which is Pareto optimal. Evaluating the authors’ game through different simulations demonstrates that it is all beneficial for both MeNB and HeNBs and the social welfare is maximised in the network.

An Analytical Framework for Evaluating Spectrum/Energy Efficiency of Heterogeneous Cellular Networks

Achieving high spectrum efficiency (SE) and energy efficiency (EE) is of primary importance for the sustainability of future cellular networks, but a key challenge is on balancing the tradeoff that arises when maximizing both of these performance metrics simultaneously. This paper develops a framework for analyzing the SE and EE of a two-tier heterogeneous cellular network consisting of macrocell and femtocell base stations (BSs) operating under a shared-spectrum scenario. It is shown that both the SE and the EE can be significantly enhanced with the overlaid deployment of the femto tier. However, the performance gain achievable is found to be strongly dependent on the load level and the BS power consumption attributes. A multiobjective optimization problem that maximizes the SE and the EE subject to quality-of-service (QoS) constraints is formulated and solved to give the Pareto-optimal operational regime. The novelty of this work is the quantification of the SE–EE tradeoff as a Lebesgue measure, which is defined by the Pareto-optimal regime. The developed framework is useful for studying the impact of the load on the SE–EE tradeoff, based on a strategy that exploits the varying load conditions to achieve good balance in the SE–EE tradeoff is formulated. Numerical results show that, while the improvement achieved in minimizing the SE–EE performance gap is marginal under high-load conditions, it is feasible to significantly increase the SE and the EE during low-load conditions and satisfy the users' QoS requirements by optimally adapting the density of the femto-tier BSs accessing the shared spectrum.

Optimisation of FeICIC for energy efficiency and spectrum efficiency in LTE‐advanced HetNets

Until recently, spectral efficiency (SE) has been considered as the major key performance indicator of cellular networks. Nowadays, green evolution is becoming a major trend and the telecom industry is concerned about the energy costs and carbon footprint of their network infrastructure. These concerns will get more critical in the future due to the increasing base station densities. In such a scenario, minimising the network energy consumption while maximising the data sent to the users becomes crucial for the sustained operation of the network. In this letter, stochastic geometry is used to analyse the energy efficiency (EE) and SE in a two-tier long-term evolution-advanced heterogeneous network (HetNet). Range expansion and further-enhanced inter-cell interference coordination which have been standardised in third generation partnership project Release-11 are considered. System parameters of the HetNet are optimised and the trade-offs associated between EE and SE of the network are studied.

Multichannel Analysis of Cell Range Expansion and Resource Partitioning in Two-Tier Heterogeneous Cellular Networks

Cellular heterogeneous networks (HetNets) can improve capacity by offloading users from congested macro cells to lightly loaded small cells through biased association known as cell range expansion (CRE). However, the offloaded (range-expanded) users must be protected from macro interference through time/frequency resource partitioning. In this paper, we develop an analytical framework to evaluate the performance gain due to CRE further supported by resource partitioning in two-tier (macro-pico) networks with multichannel downlinks, e.g., those based on orthogonal frequency division multiple access (OFDMA). By exploiting the flexibility in subchannel allocation offered by OFDMA, frequency-domain resource partitioning is proposed in which the macro tier is muted on a fraction of total subchannels, which are allocated exclusively to range-expanded pico users. The load perceived by a base-station is a key factor in determining its interference contribution over the network and is directly affected by user offloading and resource partitioning. Thus, the analysis of such systems must incorporate cell load. While previous studies mostly rely on full-load assumption, in this paper, we properly characterize cell load as the function of user density, association bias and resource partitioning fraction. We then, evaluate the performance in terms of average user data rate over the entire network, and also investigate the optimal choice of association bias and resource partitioning fraction.

Design and Development of Hybrid Time Synchronization Scheme to Mitigate the Phase and Frequency error for Asynchronous Link in Heterogeneous Network

The arbitrary deployment of HeNodeBs high-lights the importance of interference minimization in order to capture the best. Timing as well as synchronization are equally essential for control of HeNodeBs in 4G LTE devices. So as to sustain HeNodeBs for management, IEEE1588 master slave strategies are being used in order to maintain HeNodeBs for the control purpose. Nonetheless, as a result of poor internet connectivity for a long period HeNodeBs undergoes synchronization problems that leads to clock offset problem. Asymmetric link delays, due to phase and frequency misalignment causes the effects on interference and system QoS. Moreover, to mitigate interference and for QoS synchronization is prerequisite for heterogeneous network. This paper inspects the pros and cons of the IEEE 1588 algorithm and IEEE Precision Timing Protocol (PTP). Therefore, this paper proposed a hybrid phase and frequency synchronization scheme. Comparing to IEEE1588 and PTP, hybrid scheme can improve the clock Phase (offset), and frequency error in two-tier Heterogeneous Network. The proposed scheme is evaluated by Monte Carlo system level simulation, and the results are benchmarked with the existing techniques. The simulation outcome ensures that the hybrid time synchronization strategy is much better in Heterogeneous network with regards to phase (offset) and frequency errors

Analytical framework for power saving evaluation in two-tier heterogeneous mobile networks

Reducing the power consumption of base stations in mobile networks is an important issue. We investigate the power saving evaluation in two-tier heterogeneous mobile networks which consist of femtocells overlaid by macrocells. In the heterogeneous mobile networks, base stations without traffic load are allowed to enter the sleep mode to save power. The power saving probability that a base station enters the sleep mode and the average total power consumption of this network are complex joint-effects of various factors. Successful modelling of these complex joint-effects is critical to mobile network operators when they pursue the design of green mobile networks. In this paper we propose an analytical framework to facilitate systematic analysis. Based on the proposed analytical framework, we investigate the power saving probabilities and the average total power consumption in terms of several parameters, including the new traffic arrival rate per user, the maximum transmission power of a femtocell, the number of femtocells within a macrocell, and the number of users in the network. Numerical results show that the proposed analytical framework provides a useful and efficient method to facilitate systematic analysis and design of green mobile networks. Simulation results validate the accuracy of the proposed analytical framework.

Interference Alignment for the Uplink Heterogeneous Cellular Networks

Heterogeneous cellular networks, due to its multi-tier topological structure, provide significant improvements in terms of increased data rates and cell coverage. However, there are important intra-tier and inter-tier interference management problem to be solved. In this paper, we model a two-tier uplink heterogeneous network, which include a macrocell and K overlaid femtocells. Meanwhile we constitute a heterogeneous configuration that each femto BS is equipped with two antennas and the macro BS is equipped with A (2 ≤ A ≤ 2 K ) antennas. We proved that the sum-DoF outer bound of the network is 4 K 2 /4 K ‒ A , and the DoF outer bound for the femtocell and macrocell is (2 K ‒ A )/(4 K ‒ A ) and A /(4 K ‒ A ) respectively. Moreover, we present the achievable scheme, which is based on interference alignment. The simulation results show that the sum rate of the whole HetNet increases as the number of macro BS’s antennas increases.

Energy-Efficient Design of MIMO Heterogeneous Networks with Wireless Backhaul

As future networks aim to meet the ever-increasing requirements of high-data rate applications, dense, and heterogeneous networks (HetNets) will be deployed to provide better coverage and throughput. Besides the important implications for energy consumption, the trend toward densification calls for more and more wireless links to forward a massive backhaul traffic into the core network. It is critically important to take into account the presence of a wireless backhaul for the energy-efficient design of HetNets. In this paper, we provide a general framework to analyze the energy efficiency of a two-tier MIMO heterogeneous network with wireless backhaul in the presence of both uplink and downlink transmissions. We find that under spatial multiplexing the energy efficiency of a HetNet is sensitive to the network load, and it should be taken into account when controlling the number of users served by each base station. We show that a two-tier HetNet with wireless backhaul can be significantly more energy efficient than a one-tier cellular network. However, this requires the bandwidth division between radio access links and wireless backhaul to be optimally designed according to the load conditions.

Analysis and Optimization of FFR-Aided OFDMA-Based Heterogeneous Cellular Networks

Two-tier networks combining an operator-managed infrastructure of macrocell base stations combined with a user-deployed network of femtocells have recently emerged in the context of modern wireless standards as a solution to meet the ambitious performance requirements envisaged in 4G/5G architectures. Most often, these systems require interference coordination schemes that allow near universal frequency reuse while maintaining a considerably high signal-to-interference-plus-noise ratio levels across the coverage area. In particular, fractional frequency reuse (FFR) and its variants are deemed to play a fundamental role in the next generation of cellular systems. This paper develops an analytical framework targeting the downlink performance evaluation of FFR-aided orthogonal frequency division multiple access-based two-tier heterogeneous networks. In the considered scenario, macrocell and femtocell tiers are assumed to be uncoordinated and co-channel deployed, thus representing a worst-case scenario in terms of inter-tier interference. The proposed framework allows the evaluation of the impact produced by both inter- and co-tier interferences on the performance of either the macro-users (MUs) or the femto-users. Analytical results are used to optimize the FFR parameters as a function of, for example, the density of MUs per cell, the resource block scheduling policy, the density of femto base stations per area unit, or the degree of isolation provided by wall penetration losses. Moreover, different optimization designs of the FFR component are proposed that allow a tradeoff between throughput performance and fairness by suitably dimensioning the FFR inner and outer areas and the corresponding frequency allocation.