Analysis of Interference Mitigation in Heterogeneous Cellular Networks using Soft Frequency Reuse and Load Balancing

This paper presents efficient interference-mitigation techniques, such as fractional frequency reuse (FFR) and soft frequency reuse (SFR) schemes, in multicell orthogonal frequency division multiple access networks under the composite multipath fading environment, where Rayleigh fading is superimposed over log-normal shadowing. We begin by outlining the obtained explicit expressions of area spectral efficiency in down-link transmission for both FFR and SFR schemes. The theoretical result are shown to be very accurate with their simulated counterparts using Monte Carlo simulations.

In this paper, we propose an interference coordination scheme by an innovative combination of soft frequency reuse (SFR) and cooperative multipoint transmission (CoMP) to manage both co- tier and cross-tier interference in a randomly deployed macro-pico network modeled by Poisson Point Process (PPP). The proposed scheme is performed in three stages. First, the frequency reuse pattern of SFR for macrocells is determined based on the interference graph that describes interference relationship between macrocells. Second, the frequency reuse pattern of SFR for each picocell mainly depends on the measurement exchanged with its surrounding cells. Third, the CoMP scheme is performed on the secondary bands of macrocells to enhance the performance of Picocell-edge users and ensure fairness between various users. A scaling factor is considered to protect the data rates of macro cell-center users. In addition, the cross- component carrier (Cross-CC) proportional fair (PF) scheduling is adopted for the detailed resource block (RB) allocation in the carrier aggregation (CA) supported system to achieve the maximum of system fairness. Numerical results show that the proposed scheme can effectively improve cell-edge user data rate and ensure the fairness between users when compared with the SFR scheme and the full frequency reuse (Reuse 1) scheme.

Femtocell base station (FBS) is one solution to improve performance of indoor and outdoor user equipments (UEs) with low cost and low complexity. To gain the benefit of deployment of FBS, we consider a frequency assignment method to mitigate cross-tier interference between macrocell and femtocell. Previous studies proposed the channel assignment schemes where macrocell base stations (MBSs) and FBSs orthogonally use subbands. However, they have low efficiency in terms of frequency reuse and high complexity due to exchanging information of FBSs via network. Therefore, we consider the FBS with soft frequency reuse (SFR) scheme to improve the efficiency of frequency reuse. For performance improvement of macrocell UEs (MUEs), we propose a method to cluster FBSs with same SFR pattern. Since the proposed clustering scheme utilizes self-organization network (SON) technique without any information exchange of FBSs, it can improve the performance of MUEs with low complexity. By simulations, we confirm that FBSs with proposed clustering method mitigate cross-tier interference in downlink and increase the throughputs of MUEs.

In a Poisson Point Process (PPP) network model, in which the locations of Base Stations (BSs) are randomly distributed according to a Spatial Poisson Process, has been recently used as a tractable stochastic model to analyse the performance of downlink Heterogeneous Cellular Networks (HCNs). The HCN is modelled as a multi-tier cellular network where each tier is characterised by the transmission power level, propagation path loss exponent and density of BSs. The current works on HCN enabling Intercell Interference Coordination (ICIC) technique usually deal with Strict Frequency Reuse (FR) or Soft FR with a reuse factor of $\Delta=1$ in a Rayleigh fading channel. It has been assumed that all Base Stations (BSs) transmit continuously which leads to a reduction on the impact of number of users and RBs on network performance. In this paper, the performance of Soft FR with a reuse factor of $\Delta>1$ in Rayleigh-Lognormal fading channel is evaluated. The impact of the number of users and Resource Blocks (RBs) on Intercell Interference (ICI) are presented for Round Robin scheduling and indicator functions. The results show that there are opposite trends between coverage probability of Cell-Center User (CCU) and Cell-Edge User (CEU).

The rate distribution in heterogeneous networks (HetNets) greatly benefits from load balancing, by which mobile users are pushed onto lightly-loaded small cells despite the resulting loss in SINR. This offloading can be made more aggressive and robust if the macrocells leave a fraction of time/frequency resource blank, which reduces the interference to the offloaded users. We investigate the joint optimization of this technique - referred to in 3GPP as enhanced intercell interference coordination (eICIC) via almost blank subframes (ABSs) - with offloading in this paper. Although the joint cell association and blank resource (BR) problem is nominally combinatorial, by allowing users to associate with multiple base stations (BSs), the problem becomes convex, and upper bounds the performance versus a binary association. We show both theoretically and through simulation that the optimal solution of the relaxed problem still results in an association that is mostly binary. The optimal association differs significantly when the macrocell is on or off; in particular the offloading can be much more aggressive when the resource is left blank by macro BSs. Further, we observe that jointly optimizing the offloading with BR is important. The rate gain for cell edge users (the worst 3-10%) is very large - on the order of 5-10x - versus a naive association strategy without macrocell blanking.

We propose a coordinated inter-cell resource allocation (CICRA) approach that can reuse efficiently the radio spectrum through inter-cell interference (ICI) management over a cluster of adjoining cells in some coordinated manner. In this, the entire radio spectrum is assigned to all the cells within the cell cluster. We use a set of coordinated cell-specific resource allocation sequences in order to manage ICI. The set of sequences should be designed and assigned in a coordinated manner over all cells within the cell cluster, so that ICI from nearer interfering cells could be preferentially avoided and the average ICI level according to the loading factor (LF) could be monotonic and minimally incremented. By the virtue of such ICI-managing mechanism, the CICRA approach can govern efficiently the average ICI level only through individual resource allocation without intervention among cells. In this paper, we would apply the CICRA approach to orthogonal frequency division multiple access (OFDMA) cellular systems. A system-level simulator for an OFDMA cellular system covering surrounding cells up to 3rdtier has been implemented. Simulation results show that the proposed CICRA approach can improve noticeably the system performance in terms of both the outage probability and spectral efficiency as compared with the classical UFR scheme and the soft frequency reuse (SFR) scheme over the entire LF range.

In heterogeneous cellular networks (HCNets) smallcells are overlaid with macro-cells to handle heavy cellular data traffic in an efficient way. To achieve fast and reliable access to data with a better coverage it is necessary to offload users to the underutilized small-cells from the congested macro-cells. Sharing of the same licensed frequency spectrum by smallcells and macro-cells results in heavy cross-tier interference which significantly affects the downlink SINR. In this paper, we investigate and analyze a joint frequency-division duplex based cross-tier complementary spectrum sharing technique which is also regarded as reverse frequency allocation (RFA) scheme with load balancing. This scheme mitigates inter-cell interference (ICI) and better balances the load across the network. It also guarantees enhanced spectral efficiency and coverage probability, particularly for cell edge users (CEUs). Numerical results indicate that load balancing together with RFA is necessary to enhance the coverage of CEU in co-channel HCNets.

For small cell technology to significantly increase the capacity of tower-based cellular networks, mobile users will need to be actively pushed onto the more lightly loaded tiers (corresponding to, e.g., pico and femtocells), even if they offer a lower instantaneous SINR than the macrocell base station (BS). Optimizing a function of the long-term rate for each user requires (in general) a massive utility maximization problem over all the SINRs and BS loads. On the other hand, an actual implementation will likely resort to a simple biasing approach where a BS in tier j is treated as having its SINR multiplied by a factor A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> ≥ 1, which makes it appear more attractive than the heavily-loaded macrocell. This paper bridges the gap between these approaches through several physical relaxations of the network-wide association problem, whose solution is NP hard. We provide a low-complexity distributed algorithm that converges to a near-optimal solution with a theoretical performance guarantee, and we observe that simple per-tier biasing loses surprisingly little, if the bias values A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</sub> are chosen carefully. Numerical results show a large (3.5x) throughput gain for cell-edge users and a 2x rate gain for median users relative to a maximizing received power association.

Green and energy-saving heterogeneous cellular networks are the trend of future mobile communication networks, and there are also a series of problems such as resource allocation, load balancing, and energy-saving design. As an important component of energy consumption in cellular networks, base stations have the characteristics of regularity and periodicity of services, so the introduction of base station dormancy technology can effectively save energy while ensuring the quality of service for users. Aiming at the sleep problem of base stations in heterogeneous cellular networks in urban areas, this paper analyzes the sleep probability and power consumption model of distance-aware micro base stations in sleep mode. Aiming at minimizing system power consumption, a sleep scheme in which macro base station and micro base station cooperate horizontally is designed. For the multi-constraint problem of user transfer in horizontal cooperation, a heuristic algorithm is proposed. The results show that the proposed cooperative scheme can save a lot of energy when the user SINR loss is small .

The growth of energy consumption driven by a rise in traffic rate demand and services, requires cellular operators to extend their infrastructure to meet these demands. To address this issue, deployment of low power base stations (BSs) in traditional macrocells have recently being considered to offer high data rates and simultaneously reduce the load of the macrocellular networks. However, the massive and unplanned deployment of small cells and their uncoordinated operations raise important questions about the energy efficiency in the heterogeneous cellular networks. In this paper, the impact of deploying a number of pico BSs on the system performance of macrocellular network is investigated. We analyze the energy efficiency of the downlink macro-picocells heterogeneous network, measured by the traffic capacity divided by the power consumption of the BSs. An adaptive power control based on the iterative-waterfilling scheme has been implemented to control the power among the macro and the pico BSs. The numerical results show that the total macrocellular capacity improves as the number of pico stations increases, but the energy efficiency is improved only for a reasonable number of pico base stations. Moreover, the iterative-waterfilling power allocation scheme improves the energy efficiency even with introducing more pico base stations as compared with other power allocation schemes. This work provides essential understanding for successful deployment of power efficient heterogeneous networks.

Heterogeneous cellular networks (HCNs) constitute a necessary step in the evolution of cellular networks. In this paper, we apply the signal-to-interference ratio (SIR) meta distribution framework for a refined SIR performance analysis of HCNs, focusing on K-tier heterogeneous cellular networks based on the homogeneous independent Poisson point process (HIP) model, with range expansion bias (offloading bias) in each tier. Expressions for the b-th moment of the conditional success probability for both the entire network and each tier are derived, based on which the exact meta distributions and the beta approximations are evaluated and compared. Key performance metrics including the mean success probability, the variance of the conditional success probability, the mean local delay and the asymptotic SIR gains of each tier are obtained. The results show that the biases are detrimental to the overall mean success probability of the whole network and that the b-th moment curve (versus the SIR threshold) of the conditional success probability of each tier can be excellently approximated by the horizontal shifted versions of the first moment curve of the single-tier PPP network. We also provide lower bounds for the region of the active probabilities of the base stations to keep the mean local delay of each tier finite.

Evolution of cellular networks into dynamic, dense, and heterogeneous networks have introduced new challenges for cell resource optimization, especially in the imbalanced traffic load regions. Numerous load balancing schemes have been proposed to tackle this issue; however, they operate in a reactive manner that confines their ability to meet the top‐notch quality of experience demands. To address this challenge, we propose a novel proactive load balancing scheme. Our framework learns users' mobility and demands statistics jointly to proactively cache future contents during their stay at lightly loaded cells, which results in quality of experience maximization and load minimization. System level simulations are performed and compared with the state‐of‐the‐art reactive schemes.

Wireless cellular networks are large-scale networks in which asymptotic capacity investigation is no longer a cliche. A substantial body of work has been carried out to improve the capacity of cellular networks by introducing ad hoc communications, resulting in the so-called multihop cellular networks. Most of the previous research allows ad hoc transmissions between certain source and destination pairs to alleviate base stations' relay burden. However, since reports show that Internet data traffic is becoming more and more dominant in cellular networks, we explore in this paper the capacity of multihop cellular networks with all traffic going through base stations and ad hoc transmissions only acting as relay. We first investigate the capacity of regular multihop cellular networks where both nodes and base stations are regularly placed. By fully exploiting the link rate variability, we find that multihop cellular networks can have higher per-node throughput than traditional cellular networks by a scaling factor of log 2 n. Then, for the first time we extend our study to the capacity of heterogeneous multihop cellular networks where nodes are distributed according to a general Inhomogeneous Poisson Process and base stations are randomly placed. We show that under certain conditions multihop cellular networks can also outperform traditional cellular networks by a scaling factor of log 2 n. Moreover, both throughput-fairness and bandwidth-fairness are considered as fairness constraints for both kinds of networks.

Deploying low-power nodes (LPNs) overlaid with Macro cells is a promising strategy to improve the coverage and capacity of cellular systems. Meanwhile, cell range expansion (CRE) scheme is proposed to select a proper serving base station (BS) for users between Macro eNodeBs and LPNs, and then offloading of traffic from Macro cell is made. In this paper, first, we introduce the LPNs deployment strategy and its activation criteria in the heterogeneous system. Then, a quality of experience (QoE)-based CRE scheme to adjust the coverage area of LPNs adaptively is proposed. Different from the existing CRE schemes, the QoE values for different users with different services are considered when the users try to select an appropriate serving BS. Finally, simulation results demonstrate that the proposed scheme can improve cell edge user experience for various services as well as overall system performance.

Heterogeneous Networks (HetNets) have gained the attraction of the communication industry recently, due to their promising ability to enhance the performance of future broadband Fifth Generation (5G) networks and are integral parts of 5G systems. They can be viewed in multi-dimensional space where, each slice represents a unique tier that has its own Base Station (BS)s and User Equipment (UE)s. Different tiers cooperate with each other for their mutual benefit. Data can be interactively exchanged among the tiers, and UEs have the flexibility to switch between the tiers. The cells in such a heterogeneous cellular networks have variable sizes, shapes, and coverage regions. However, in HetNets with ultra dense BSs, the distance between them gets very small and, they suffer from very high levels of mutual interference. To improve the performance of HetNets, we have done multiple contributions in this dissertation. First, we have developed analytical derivations for optimizing pilot sequence length which is a very crucial factor in acquiring the Channel State Information (CSI) and the channel estimation process in general. Poisson Point Process (PPP) has been widely used to allocate BSs among various tiers so far. However, BS locations obtained using PPP approach may not be optimum to reduce interference. Therefore, in this dissertation, BSs locations are optimized to reduce the interference and improve the coverage and received signal power. Also, we have derived expressions for static UEs coverage probability and network energy efficiency in HetNets. A proper UE association algorithm for HetNets is a great challenge. The classic max-Signal to Interference and Noise Ratio (SINR) or max-received signal strength (RSS) user association algorithms are inappropriate solutions for HetNets as UEs in this context will tend to connect to the Macro BS, which is the one with the highest signal power. A severe load imbalance and significant inefficiency arises and impacts the performance. The aforementioned algorithms tend to associate UEs to BSs with the best received signal power or signal quality. In HetNets, usually Macro BSs are the ones transmitting the strongest signals; hence most UEs tend to associate with the Macro BS leaving Micro BSs with less load. Also, the conventional max-SINR and max-RSS algorithms do not provide adequate results in multi-tier systems. We suggest two centralized algorithms, LSTD and RTLB, for an even UE association to provide fair load distribution. However RTLB outperforms LSTD in real time scenarios as it easily and quickly adapts to rapid network changes. Furthermore, we consider the mobility of nodes. We derive coverage probability for moving UEs considering both handover and no handover scenarios. Proposed algorithms are fast enough to associate the moving users to different Micro and Macro BSs appropriately in real time. Our algorithms are proved to be feasible and provide a path towards attainable future communication systems.