Macrocell Tier Research Articles

Secure Communications in Tiered 5G Wireless Networks With Cooperative Jamming

Cooperative jamming is deemed as a promising physical layer-based approach to secure wireless transmissions in the presence of eavesdroppers. In this paper, we investigate cooperative jamming in a two-tier 5G heterogeneous network (HetNet), where the macrobase stations (MBSs) at the macrocell tier are equipped with large-scale antenna arrays to provide space diversity and the local base stations (LBSs) at the local cell tier adopt non-orthogonal multiple access (NOMA) to accommodate dense local users (LUs). In the presence of imperfect channel state information, we propose three robust secrecy transmission algorithms that can be applied to various scenarios with different security requirements. The first algorithm employs robust beamforming (RBA) that aims to optimize the secrecy rate of a marcouser (MU) in a macrocell. The second algorithm provides robust power allocation (RPA) that can optimize the secrecy rate of an LU in a local cell. The third algorithm tackles a robust joint optimization (RJO) problem across tiers that seek the maximum secrecy sum rate of a target MU and a target LU robustly. We employ convex optimization techniques to find feasible solutions to these highly non-convex problems. The numerical results demonstrate that the proposed algorithms are highly effective in improving the secrecy performance of a two-tier HetNet.

Subchannel Allocation and Power Control for Uplink Femtocell Radio Networks with Imperfect Channel State Information

Femtocell technology is emerging as a key solution for mobile operators for its advantage in coverage and capacity enhancement along with its cost effectiveness. However, densely and randomly deployed femtocells while sharing the frequency spectrum of the macrocell arises a severe interference environment. In femtocells deployment, interference coming from a femtocell user affect other femtocell users and the macrocell users, where maintaining the communication of the users in both tiers is a mandatory. In this paper, a novel power control algorithm is proposed for optimizing the uplink transmission powers of femtocell users in a TDD-OFDM communication model in the presence of a channel estimation error and intra-tier interference. We consider signal to interference and noise ratio as the objective function where the proposed constraints deal with: (1) the aggregated interference coming from femtocell tier and received at the active subchannels by the macrocell tier, and (2) the maximum uplink power a femtocell user equipment is allowed to occupy per admissible subchannel. Based on Lagrangian multipliers, the proposed power control approach grants the priority in subchannel usage for macrocell user, then it allows or prohibits frequency reuse of a subchannel with the femtocell tier. A comparison is then made with a pure isolation method that does not allow femtocell user equipments to occupy the active subchannels at the macrocell tier. The numerical results of the proposed approach show a high total rate of femtocell user equipments and the average uplink power is below the maximum allowable transmission power.

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.

Techniques for Interference Mitigation Using Cooperative Resource Partitioning in Multitier LTE HetNets

Almost blank subframes (ABSs) have been proposed in the literature as a means to provide interference-free resources to mobile users. The use of ABS becomes inevitable in LTE heterogeneous cellular networks because of severe interference to small cells. Picocell edge users are typically the victims because of high interference exposure from macrocell tier. In case of hotspots with high-user density, picocells cannot provide coverage to the entire hotspot, thus forming a dense ring of macrocell users around picocells. These macrocell users are highly affected by picocell tier and hence should also be considered as victim users (VUEs). We propose that to protect these macrocell VUEs, macrocells and picocells should operate in a cooperative manner, such that not only macrocells should use ABS, but picocells should also use ABS to provide interference-free resources. We also introduce the concept of universal blanking pattern to coordinate among various base stations. We propose two methods for joint ABS density calculation for macrocells and picocells—bitrate utility-based optimization and physical resource block (PRB) allocation ratio-based formulation. Users with different bitrate demands are considered generating more realistic simulation scenarios. Exhaustive simulations illustrate that the proposed schemes offer significant improvement in VUE throughput as well as overall system throughput.

Stochastic Geometry Analysis of Downlink Spectral and Energy Efficiency in Ultradense Heterogeneous Cellular Networks

The energy efficiency (EE) is a key metric of ultradense heterogeneous cellular networks (HCNs). Earlier works on the EE analysis of ultradense HCNs by using the stochastic geometry tool only focused on the impact of the base station density ratio and ignored the function of different tiers. In this paper, a two-tier ultradense HCN with small-cell base stations (SBSs) and user equipments (UEs) densely deployed in a traditional macrocell network is considered. Firstly, the performance of the ultradense HCN in terms of the association probability, average link spectral efficiency (SE), average downlink throughput, and average EE is theoretically analyzed by using the stochastic geometry tool. Then, the problem of maximizing the average EE while meeting minimum requirements of the average link SE and average downlink throughput experienced by UEs in macrocell and small-cell tiers is formulated. As it is difficult to obtain the explicit expression of average EE, impacts of the SBS density ratio and signal-to-interference-plus-noise ratio (SINR) threshold on the network performance are investigated through numerical simulations. Simulation results validate the accuracy of theoretical results and demonstrate that the maximum value of average EE can be achieved by optimizing the SBS density ratio and the SINR threshold.

Modeling and Analyzing the Cross-Tier Handover in Heterogeneous Networks

Denser deployments of heterogeneous networks (HetNets) lead to more frequent handovers, which results in a decline of user experience as well as heavy signaling overheads to the network. Therefore, the study on handover is of great importance especially in dense HetNets. In this paper, we focus on the analysis of cross-tier handover from the macro cell tier to the small cell tier, which shows the highest handover failure rate in current standards and industry studies. Using the stochastic geometry, we propose an analytical model for the cross-tier handover processes in the HetNet. Based on the derived analytical model, the closed-form expressions for the key handover performance metrics, including the handover rate, handover failure rate, and ping-pong rate, are deduced as functions of the base station density, time to trigger and user mobility. Simulation results verify the accuracy of the proposed analytical model and closed-form theoretical analyses, which provide guidance for the deployments of HetNets and corresponding handover strategies.

Large System Analysis of Resource Allocation in Heterogeneous Networks With Wireless Backhaul

Small-cell networks and massive multiple-input multiple-output (MIMO) systems are regarded as important candidate techniques for 5G communication systems. This paper considers a heterogeneous network composed of a macrocell tier overlaid with an extremely dense tier of small-cells. In the network, the macrocell base station (BS), which applies massive MIMO, does not only serve macro user equipment units but also provides wireless backhaul for small-cell access points (APs). The wireless backhaul shares the same spectrum resource with radio access networks without creating extra spectrum resources. However, due to the densification of small-cells, the inter- and intra-tier interferences become severe. To mitigate the interferences, we use the regularized zero-forcing precoding combined with a projection technique is used at the BS in downlink (DL) to avoid interference to the APs in uplink (UL). Meanwhile, the joint linear minimum mean square error detection is applied in UL to mitigate the inter-tier interference. We derive deterministic expressions for ergodic UL and DL sum rates (SRs) by leveraging the large-dimensional random matrix theory. The expressions only depend on statistical channel information and can be used to optimize the bandwidth division between radio access links and wireless backhaul, as well as the time allocation between DL and UL operation intervals. Numerical results show that the deterministic SR equivalents are accurate and that the proposed resource allocation method can significantly improve system performance.

Online Optimization of Interference Coordination Parameters in Small Cell Networks

This paper focuses on interference coordination between the small cell and macro cell tiers of a wireless access network. We present a self-optimization mechanism for LTE-A eICIC parameters (CRE bias and ABS ratio) following a novel approach based on a model-free learning strategy, not requiring any previous knowledge about the network (e.g., topology, interference graph, and scheduling algorithms). Our proposal is built upon a stochastic optimization algorithm known as response surface methodology (RSM), that we use to find efficient eICIC configurations during network operation (online learning), adapting to changing network conditions, such as traffic or user distribution. The objective consists of optimizing a performance metric for which, in general, mathematical expression is unavailable. In particular, we consider the fifth percentile throughput defined by the 3GPP. By means of RSM, our mechanism obtains local approximations of the objective function to perform steepest ascent iterations with an adjustable level of statistical accuracy. The algorithm can be extended to account for stochastic constraints, allowing the network to optimize one performance metric while maintaining other metrics above a desired level.

Approximation Algorithms for Online User Association in Multi-Tier Multi-Cell Mobile Networks

The constantly growing wireless bandwidth demand is pushing wireless networks to multi-tier architectures consisting of a macrocell tier and a number of dense small cell deployment tiers. In such a multi-tier multi-cell environment, the classic problem of associating users to base stations becomes both more challenging and more critical to the overall network performance. Most previous analytical work is focused on designing static user-cell association algorithms, which, to achieve optimality, are periodically applied whenever there are new user arrivals, thus potentially inducing a large number of re-associations for previously arrived users. On the other hand, practical online algorithms that do not allow any such user re-association are often based on heuristics and may not have any performance guarantees. In this paper, we propose online algorithms for the multi-tier multi-cell user association problem that have provable performance guarantees, which improve previously known bounds by a sizable amount. The proposed algorithms are motivated by online combinatorial auctions, while capturing and leveraging the relative sparsity of choices in wireless networks as compared with auction setups. Our champion algorithm is a $\frac {1}{2-a^{-1}}$ approximation algorithm, where $a$ is the maximum number of feasible associations for a user and is, in general, small due to path loss. Our analysis considers the state-of-the-art wireless technologies, such as massive and multiuser MIMO, and practical aspects of the system such as the fact that highly mobile users have a preference to connect to larger cell tiers to keep the signaling overhead low. In addition to establishing formal performance bounds, we also conduct simulations under realistic assumptions, which establish the superiority of the proposed algorithm over existing approaches under real-world scenarios.

An improved cooperative spectrum sensing of femtocells with hybrid TDD access scheme in massive MIMO system

SummaryTo improve the sensing performance and achieve higher throughput of the femtocells (FCs) while alleviating the influence to the macrocell in the cognitive massive multiple‐input and multiple‐output system (MIMO), we propose in this paper a hybrid time‐division duplex (TDD) access scheme of the FCs with confidence factor–based weighted cooperative spectrum sensing (CSS). A first, we present an efficient hybrid TDD access scheme for better spectrum reuse of the FCs based on the typical TDD and reversed TDD schemes. Furthermore, confidence factor–based weighted CSS has been used for better spectrum sensing and interference alleviation. In the cognitive massive MIMO system, FC base stations will form dynamical clustering based on the channel conditions, sense the spectrum through cooperative scheme, and decide whether to access the spectrum or not. After information exchange within each FC cluster, the FC base stations can obtain and update their confidence factors and weight factors based on the sensing results and reliability. Numerical results and theoretical analysis show that the proposed scheme can get more accurate sensing results, increase the throughput and the spectrum access opportunity of the FCs, and efficiently alleviate the interference to the macrocell tier.

Inter-Cell Interference Coordination for Control Channels in LTE Heterogeneous Networks

In heterogeneous cellular networks for mobile communications, small cells are deployed within the coverage range of primary macro cells to provide for a localized capacity boost in traffic hotspots. Sharing of communication channels among the small-cell and macro-cell tiers is spectrally efficient, but causes failures of control signaling and data channels due to unmitigated co-channel interference. Consequently, the small-cell coverage range and capacity deteriorate. In Long Term Evolution (LTE) networks, the performance of control channels such as the physical downlink control channels (PDCCH) is of particular concern because they are protected only by simple interference averaging based on pseudo-random subcarrier allocation. Observing that the randomization algorithms are primarily seeded by the physical cell identifiers (PCIs) and cell radio network temporary identifier (C-RNTIs), we show that efficient interference-aware scheduling of control transmissions can be enabled by optimized allocation of PCIs, C-RNTIs and PDCCH resources. Simulations of a 3 GPP-compliant heterogeneous network show that the small-cell size can be doubled for a better macro-cell traffic offload by trading the number of active PDCCHs for a higher small-cell expansion bias. Alternatively, the small-cell PDCCH capacity can be at least tripled for high-load applications such as Voice over LTE by using selective macro-cell PDCCH muting.

Decoupled Downlink-Uplink Coverage Analysis with Interference Management for Enriched Heterogeneous Cellular Networks

Heterogeneous cellular networks (HetCNets) offer a promising solution to cope with the current cellular coverage crunch. Due to the large transmit power disparity, while following maximum power received (MPR) association scheme, a larger number of users are associated with macro-cell BS (MBS) than small-cell BSs (SBSs). Therefore, an imbalance load arrangement takes place across the HetCNets. Hence, using cell range expansion-based cell association, we can balance the load across the congested MBS. However, using MPR association scheme, users' offloading leads to two challenges: 1) macro-cell interference, in which the MBS interferes with the offloaded users, and 2) coupled downlink-uplink cell association, in which a random user associates with a single tier's base station (BS) both in uplink (UL) and downlink (DL) directions. This paper aims to address these problems while considering a two-tier scenario consisting of small-cell and macro-cell tiers. For the MBS interference mitigation, we employ a reverse frequency allocation (RFA) scheme. Besides coupled DL-UL association (Co-DUA), this paper also highlights the notion of decoupled DL-UL association (De-DUA). In De-DUA, a random user associates with two different tiers' BSs, i.e., with one tier's BS in the DL direction and with the other tier's BS in the UL direction. Our results illustrate that, in comparison with the Co-DUA, De-DUA with RFA employment achieves a better coverage performance.

Optimized Resource Management in Heterogeneous Wireless Networks

The dense and pervasive deployment of wireless small cells can boost the performance of existing macrocellular networks; however, it poses significant challenges pertaining to the cross-tier interference management. In this letter, the downlink resource allocation problem for an underlay small cell network is studied. In this network, the protection of the macrocell tier is achieved by imposing cross-tier interference constraints in the resource allocation problem. To solve the underlying mixed-integer resource allocation problem, we propose two different algorithms. The first algorithm is developed by applying the duality-based optimization approach for the relaxed problem, which enables distributed implementation. The second distributed algorithm, which enables coordination is devised based on matching theory. Simulation results show that the proposed duality-based algorithm outperforms the greedy approach by 4% in terms of sum-rate whereas the matching-based algorithm with tier-coordination yields performance gains up to 17% compared with the duality-based approach.

Ameliorated Resource Allocation in Two-Tier Femtocell–Macrocell Networks with Six Directional Antennas for Macrocells

Femtocells are deployed in the coverage area of traditional macrocell networks and can provide high data rates and good quality of service with low transmission power. In order to achieve the high spectral efficiency, allocated resources to the femtocells should be from the same licensed spectrum band of the macrocell network. Therefore, interference management is an important challenge in two-tier femtocell---macrocell networks. Previously, it was shown that cross-tier interference from macrocell to femtocell can be reduced by using directional antennas in macrocell tier. In this paper, we propose a new spectrum partitioning approach by dividing the spectrum to four sub-bands and then allocating them to different sectors of the macrocell with six directional antennas in a proper manner to reduce interferences. Therefore, first, we calculate the interference from the adjacent macrocells in different sectors. Then, based on the results of interference analysis, we propose a sub-band exchanging allocation strategy for improving the macrocells' and femtocells' performance. Moreover, we analyze the downlink outage probability of the two-tier femtocell---macrocell network based on the orthogonal-frequency-division-multiple-access. Finally, simulation and theoretical results demonstrate that the proposed cellular network achieves lower downlink outage probability of femtocell and macrocell in comparison with the previous cellular model and consequently, system throughput increases substantially.

Cross-tier interference management with a distributed antenna system for multi-tier cellular networks

In a multi-tier cellular communication system, the interference from one tier to another, denoted as cross-tier interference, is a limiting factor for the system performance. In spectrum-sharing usage, we consider the uplink cross-tier interference management of heterogeneous networks using femtocells overlaid onto the macrocells. We propose a variation of the cellular architecture and introduce a novel femtocell clustering based on interference cancellation to enhance the sum rate capacity. Our proposal is to use a distributed antenna system (DAS) as an interface to mitigate the cross-tier interference between the macrocell and femtocell tiers. By placing a DAS remote antenna unit (RAU) near a set of femtocells that experience interference from a macrocell user, the DAS can retrieve the interference symbols and feed them back to the femtocells, where each cell can perform interference cancellation when necessary. In addition, the DAS can forward the recovered data to the macrocell base station (MBS); thus, the macrocell user can reduce its transmit power to reach a RAU located closer than the MBS. By distributing the sensor nodes within the macrocell coverage, the proposed scheme can mitigate the cross-tier interference at different locations for several femtocell clusters. Our simulation results show substantial improvement in the network sum rate capacity.

Device-to-device communication in 5G cellular networks: challenges, solutions, and future directions

In a conventional cellular system, devices are not allowed to directly communicate with each other in the licensed cellular bandwidth and all communications take place through the base stations. In this article, we envision a two-tier cellular network that involves a macrocell tier (i.e., BS-to-device communications) and a device tier (i.e., device-to-device communications). Device terminal relaying makes it possible for devices in a network to function as transmission relays for each other and realize a massive ad hoc mesh network. This is obviously a dramatic departure from the conventional cellular architecture and brings unique technical challenges. In such a two-tier cellular system, since the user data is routed through other users? devices, security must be maintained for privacy. To ensure minimal impact on the performance of existing macrocell BSs, the two-tier network needs to be designed with smart interference management strategies and appropriate resource allocation schemes. Furthermore, novel pricing models should be designed to tempt devices to participate in this type of communication. Our article provides an overview of these major challenges in two-tier networks and proposes some pricing schemes for different types of device relaying.

A Hybrid Resource Management Technique for Energy and QoS Optimization in Fractional Frequency Reuse Based Cellular Networks

This paper presents a new technique for jointly optimizing energy consumption and quality of service (QoS) in heterogeneous cellular networks employing fractional frequency reuse (FFR) in the macrocell tier. The proposed technique combines radio resource management (RRM) and network routing principles to produce an effective way of utilizing the spectrum. Based on a novel method of interference mapping, the RRM is invoked, in distributed or centralized manner, to allocate the resource. In the case of centralized RRM, network routing is applied to maximize spectrum utilization. It will be shown through matching mathematical analysis and computer simulations that, relative to existing well known techniques, the proposed technique offers significant QoS improvements, in terms of guaranteed data rate per user, as well as, enhancements in the overall energy efficiency of the network.

Cognitive Hybrid Division Duplex for Two-Tier Femtocell Networks

With the exponential increase in high rate traffic given by a new generation of wireless devices, data is expected to overwhelm cellular network capacity in the near future. Femtocell networks have been recently proposed as an efficient and cost-effective approach to provide unprecedented levels of network capacity and coverage. However, the dense and random deployment of femtocells and their uncoordinated operation raise important questions concerning interference pollution and spectrum allocation. Motivated by the flexible subchannel allocation capabilities of cognitive radio, we propose a cognitive hybrid division duplex (CHDD) that is suitable for heterogeneous networks in future mobile communication systems. Specifically, our CHDD scheme has a pair of frequency bands to perform frequency division duplex (FDD) on the macrocell, while time division duplex (TDD) is simultaneously operated in these bands by underlaid cognitive femtocells. By doing so, the proposed CHDD scheme exploits the advantages of both FDD and TDD schemes: operating in FDD at the macrocell tier controls inter-tier interference, whereas operating in TDD at the femtocell tier provides to femtocells the flexibility of adjusting uplink and downlink rates together with opportunistic access benefits. Using tools from stochastic geometry, we provide a methodology on how to design efficient switching mechanisms for cognitive TDD operation of femtocells. In particular, we derive closed-form expressions for the success probability and the area spectral efficiency of the proposed CHDD scheme when the macro tier is in downlink and uplink mode. Furthermore, we propose an open access policy as a means to improve the performance of macrocell transmissions. Our analysis and numerical results show the effectiveness of introducing cognition in femtocells so as to improve the system performance of two-tier femtocell networks.

Clustering strategy based on graph method and power control for frequency resource management in femtocell and macrocell overlaid system

In order to control interference and improve spectrum efficiency in the femtocell and macrocell overlaid system (FMOS), we propose a joint frequency bandwidth dynamic division, clustering and power control algorithm (JFCPA) for orthogonal-frequency-division-multiple access-based downlink FMOS. The overall system bandwidth is divided into three bands, and the macro-cellular coverage is divided into two areas according to the intensity of the interference from the macro base station to the femtocells, which are dynamically determined by using the JFCPA. A cluster is taken as the unit for frequency reuse among femtocells. We map the problem of clustering to the MAX k-CUT problem with the aim of eliminating the inter-femtocell collision interference, which is solved by a graph-based heuristic algorithm. Frequency bandwidth sharing or splitting between the femtocell tier and the macrocell tier is determined by a step-migration-algorithm-based power control. Simulations conducted to demonstrate the effectiveness of our proposed algorithm showed the frequency-reuse probability of the FMOS reuse band above 97.6% and at least 70% of the frequency bandwidth available for the macrocell tier, which means that the co-tier and the cross-tier interference were effectively controlled. Thus, high spectrum efficiency was achieved. The simulation results also clarified that the planning of frequency resource allocation in FMOS should take into account both the spatial density of femtocells and the interference suffered by them. Statistical results from our simulations also provide guidelines for actual FMOS planning.

Performance analysis of a handoff scheme for two-tier cellular CDMA networks

A two-tier model is used in cellular networks to improve the Quality of Service (QoS), namely to reduce the blocking probability of new calls and the forced termination probability of ongoing calls. One tier, the microcells, is used for slow or stationary users, and the other, the macrocell, is used for high speed users. In Code-Division Multiple-Access (CDMA) cellular systems, soft handoffs are supported, which provides ways for further QoS improvement. In this paper, we introduce such a way; namely, a channel borrowing scheme used in conjunction with a First-In-First-Out (FIFO) queue in the macrocell tier. A multidimensional Markov chain to model the resulting system is established, and an iterative technique to find the steady-state probability distribution is utilized. This distribution is then used to find the performance measures of interest: new call blocking probability, and forced termination probability.