Inter-cell Interference Mitigation Research Articles

Inter-cell Interference Mitigation in Multi-cell Cooperating System Based on SLNR Method

Coordinated Multi-Point transmission/reception (CoMP) can increase the performance of the cell edge users, the spectrum efficiency and mitigate the inter-cell interference among cells. In the CoMP scenario, several base stations which usually need satisfy the cooperating condition serve a single or several cell-edge users cooperatively. As we know, the CoMP techniques can be characterized into many classes. For the joint transmission system which is a classical CoMP system, a large number of data sharing and channel information exchange will be needed, where the system inevitably become comparatively complex. However, in this paper, the coordinated beam forming system will be considered, where several base stations cope with the inter-cell interference among cells, and all the base stations will only exchange the channel information. Further, the downlink multi-cell coordinated beam forming system is considered in this paper. In this downlink multi-cell coordinated beam forming system, it is crucial to design appropriate pre-coding schemes that are able to mitigate the inter-cell interference. In obtainable pre-coding schemes, the Signal-to-leakage-and-noise ratio (SLNR) pre-coding scheme is a promising pre-coding scheme in downlink multi-cell coordinated beam forming system, which makes closed-form solution accessible. Moreover, the problem in designing the pre-coding vectors is not a coupled problem since the cooperative base stations need not optimize the pre-coding vectors jointly, which are responsible for their own design of pre-coding vectors, that is, they will just carry out independent optimization. In this paper, we proposed a novel pre-coding scheme based on the SLNR scheme, which will determine the pre-coding vectors via iteration. The simulation results show that, the proposed method outperforms the conventional SLNR pre-coding scheme in bit error rate performance.

Multi-Cell Coordinated Scheduling and MIMO in LTE

This paper provides an overview of multi-cell coordinated scheduling and multiple-input multiple-output (MIMO) techniques in LTE. We first discuss single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) for LTE and identify potential techniques for further improving the performance of LTE, such as enhanced channel state information feedback and adaptive switching between SU-MIMO and MU-MIMO. Then, we address inter-cell interference mitigation techniques for downlink and uplink, respectively. Finally, potential research challenges and topics in the area are discussed.

Analysis of Different Call Admission Control Strategies and Its Impact on the Performance of LTE-Advanced Networks

The call admission control (CAC) optimizes the use of allocated channels against offered traffic maintaining the required quality of service (QoS). Provisioning QoS to user at cell-edge is a challenge where there is limitation in cell resources due to inter-cell interference (ICI). Soft Frequency Reuse is ICI mitigation scheme that controls the distribution of resources between users. In this paper, the Impact of four CAC schemes (Cutoff Priority scheme (CP), Uniform Fractional Guard Channel (UFGC), Limited Fractional Guard Channel (LFGC), New Call Bounding (NCB) scheme) at cell-edge have investigated using queuing analysis in a comparative manner. The comparison is based on two criteria. The first criterion guarantees a particular level of service to already admitted users while trying to optimize the revenue obtained. The second criterion determines the minimum of number of radio resources that provides hard constraints in both of blocking and dropping probabilities. The four schemes are compared at different scenarios of new and handover call arrival rates.

Virtual MIMO in Multi-Cell Distributed Antenna Systems: Coordinated Transmissions with Large-Scale CSIT

The virtual multiple input multiple output (MIMO) technique can dramatically improve the performance of a multi-cell distributed antenna system (DAS), thanks to its great potentials for inter-cell interference mitigation. One of the most challenging issues for virtual MIMO is the acquisition of channel state information at the transmitter (CSIT), which usually leads to an overwhelming amount of system overhead. In this work, we focus on the case that only the slowly-varying large-scale channel state is required at the transmitter, and explore the performance gain that can be achieved by coordinated transmissions for virtual MIMO with large-scale CSIT. Aiming at maximizing the achievable ergodic sum rate, the input covariances for all the mobile terminals (MTs) are jointly optimized, which turns out to be a complicated non-convex problem with a non-closed-form objective function. Further analysis reveals that the coordinated transmission problem can be recast as a Max-Min problem with a closed-form objective function and linear constraints. Then, by appealing to the successive approximation method and the saddle-point theory of concave-convex functions, we propose an iterative algorithm for coordinated transmissions with large-scale CSIT and establish its convergence. Simulation results corroborate that the proposed scheme converges quickly, and it yields significant performance gains compared to the existing schemes. Moreover, it is observed that the proposed scheme can achieve a nearly globally-optimal point under the diagonal input covariance constraint. Since the acquisition of large-scale CSIT is far less demanding than that of full CSIT, we believe that the proposed coordinated transmissions with large-scale CSIT in DASs shed some light on virtual MIMO in the making.

Joint Network Optimization and Downlink Beamforming for CoMP Transmissions Using Mixed Integer Conic Programming

Coordinated multipoint (CoMP) transmission is a promising technique to mitigate intercell interference and to increase system throughput in single-frequency reuse networks. Despite the remarkable benefits, the associated operational costs for exchanging user data and control information between multiple cooperating base stations (BSs) limit practical applications of CoMP processing. To facilitate wide usage of CoMP transmission, we consider in this paper the problem of joint network optimization and downlink beamforming (JNOB), with the objective to minimize the overall BS power consumption (including the operational costs of CoMP transmission) while guaranteeing the quality-of-service (QoS) requirements of the mobile stations (MSs). We address this problem using a mixed integer second-order cone program (MI-SOCP) framework and develop an extended MI-SOCP formulation that admits tighter continuous relaxations, which is essential for reducing the computational complexity of the branch-and-cut (BnC) method. Analytic studies of the MI-SOCP formulations are carried out. Based on the analyses, we introduce efficient customizing strategies to further speed up the BnC algorithm through generating tight lower bounds of the minimum total BS power consumptions. For practical applications, we develop polynomial-time inflation and deflation procedures to compute high-quality solutions of the JNOB problem. Numerical results show that the inflation and deflation procedures yield total BS power consumptions that are close to the lower bounds, e.g., exceeding the lower bounds by about 12.9% and 9.0%, respectively, for a network with 13 BSs and 25 MSs. Simulation results also show that minimizing the total BS power consumption results in sparse network topologies and reduced operational overhead in CoMP transmission and that some of the BSs are switched off when possible.

Multicell Coordination via Joint Scheduling, Beamforming, and Power Spectrum Adaptation

The mitigation of intercell interference is an importance issue for current and next-generation wireless cellular networks where frequencies are aggressively reused and hierarchical cellular structures may heavily overlap. The paper examines the benefit of coordinating transmission strategies and resource allocation schemes across multiple base-stations for interference mitigation. Two different wireless cellular architectures are studied: a multicell network where base-stations coordinate in their transmission strategies, and a mixed macrocell and femtocell/picocell deployment with coordination among macro and femto/pico base-stations. For both scenarios, this paper proposes a heuristic joint proportionally fair scheduling, spatial multiplexing, and power spectrum adaptation algorithm that coordinates multiple base-stations with an objective of optimizing the overall network utility. The proposed scheme optimizes the user schedule, transmit and receive beamforming vectors, and transmit power spectra jointly, while taking into consideration both the intercell and intracell interference and the fairness among the users. System-level simulation results show that coordination at the transmission strategy and resource allocation level can already significantly improve the overall network throughput as compared to a conventional network design with fixed transmit power and per-cell zero-forcing beamforming.

Robust and Efficient Multi-Cell Cooperation under Imperfect CSI and Limited Backhaul

Future cellular networks need to harvest existing spectral resources more efficiently. Hence, networks will be deployed at a higher density in order to increase the spatial reuse. This will require advanced interference mitigation techniques that allow to cope with the increased interference level. In this paper, the two-way interference channel is analyzed as a model for a typical inter-cell interference scenario. Based on this model, a new inter-cell interference mitigation approach is derived. This new approach reshapes interference by asymmetrically assigning uplink and downlink to communication pairs, i.e., one communication pair operates in uplink while an adjacent communication pair is in downlink. In addition, backhaul resources are taken into account, which are used to exchange support information between radio access points and to support the interference mitigation process. The introduced approach is compared to cooperative multi-point techniques which employ joint transmission and reception algorithms. The evaluation is done under consideration of limited backhaul resources and imperfect channel state information. It shows that assigning uplink and downlink asymmetrically is able to outperform cooperative multi-point techniques for terminals close to the cell border with gains of up to about 20 % compared to noncooperative transmission and 10 % compared to CoMP.

An Analytical Framework for Multicell Cooperation via Stochastic Geometry and Large Deviations

Multicell cooperation (MCC) is an approach for mitigating intercell interference in dense cellular networks. Existing studies on MCC performance typically rely on either oversimplified Wyner-type models or complex system-level simulations. The promising theoretical results (typically using Wyner models) seem to materialize neither in complex simulations nor in practice. To more accurately investigate the theoretical performance of MCC, this paper models an entire plane of interfering cells as a Poisson random tessellation. The base stations (BSs) are then clustered using a regular lattice, whereby BSs in the same cluster mitigate mutual interference by beamforming with perfect channel state information. Techniques from stochastic geometry and large-deviation theory are applied to analyze the outage probability as a function of the mobile locations, scattering environment, and the average number of cooperating BSs per cluster <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> . For mobiles near the centers of BS clusters, it is shown that outage probability diminishes as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ν</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ) with 0 ≤ ν <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ≤ 1 if scattering is sparse, and as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-ν</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) with ν <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> proportional to the signal diversity order if scattering is rich. For randomly located mobiles, regardless of scattering, outage probability is shown to scale as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-ν</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) with 0 ≤ ν <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ≤ 0.5. These results confirm analytically that cluster-edge mobiles are the bottleneck for network coverage and provide a plausible analytic framework for more realistic analysis of other multicell techniques.

Performance of indoor optical femtocell by visible light communication

Femotocell has been proposed and deployed to improve the indoor coverage and capacity of a cellular network. One big challenge in its deployment is the interference between the macrocell and femtocell cellular networks. In this paper we propose a new physical layer for the implementation of indoor femtocells – optical femtocells by LED-based visible light communication. A general system structure of the indoor optical femtocell network is first introduced. A combined wavelength division and code division multiple access scheme is proposed to differentiate cells and multiple users within a cell. This scheme coupled with directional beaming characteristics of the LEDs helps to mitigate intercell interference and intracell inference. The communication performance adopting the dimming compatible variable-PPM modulation suggested by the IEEE standard is analyzed. Monte-Carlo simulation is then carried out to reveal the system performance numerically under typical system settings and effects of various parameters.

A novel distributed power allocation scheme for coordinated multicell systems

Coordination between base stations (BSs) is a promising solution for cellular wireless systems to mitigate intercell interference, improving system fairness, and increasing capacity in the years to come. The aim of this manuscript is to propose a new distributed power allocation scheme for the downlink of distributed precoded multicell MISO-OFDM systems. By treating the multicell system as a superposition of single cell systems we define the average virtual bit error rate (BER) of one single-cell system, allowing us to compute the power allocation in a distributed manner at each BS. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation scheme minimizes the average virtual BER over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed scheme is compared against other power allocation schemes that have recently been proposed for precoded multicell systems based on LTE specifications. The results also show that although our power allocation scheme is based on the minimization of the virtual uncoded BER, it also has significant gains in coded systems.

Selecting the Optimal Fractional Frequency Reuse Scheme in Long Term Evolution Networks

Long Term Evolution (LTE) networks offer high capacity and are specified and designed to accommodate small, high performance, power-efficient end-user devices. One limiting factor that influences LTE performance is the interference from neighbouring cells, the so called Inter-Cell Interference (ICI). The investigation of ICI mitigation techniques has become a key focus area in achieving dense spectrum reuse in next generation cellular systems. Fractional Frequency Reuse (FFR) has been proposed as a technique to overcome this problem, since it can efficiently utilize the available frequency spectrum. This manuscript proposes a dynamic mechanism that selects the optimal FFR scheme based on a custom metric, which is called user satisfaction. In detail, the proposed mechanism divides the cell into two regions, the inner and outer region, and selects the optimal size as well as the optimal frequency allocation between these regions with main target to maximize the user satisfaction metric. The proposed mechanism is evaluated through several simulation scenarios that incorporate users' mobility and its selected FFR scheme is compared with other frequency reuse schemes in order to highlight its performance.

Multicell cooperation [Guest Editorial

It is well recognized that the conventional cellular networks can no longer meet the user performance requirements in terms of throughput and coverage, which is due to the significant increase in the number of mobile users along with their traffic demands and the limited spectrum resource. This has stimulated the search for new approaches to alleviate this problem. Recently, cooperative communications at both the base station (BS) and mobile station (MS) levels has been proposed as a key technology to address this problem. Multicell cooperation is a new and emerging communication paradigm promising significant improvement in system capacity by mitigating intercell interference. It has been shown that, with the aid of multicell cooperation, a significant improvement of cellular system performance can be achieved, and the increase in throughput can be as large as an order of magnitude. Although the advantages of multicell cooperation have been demonstrated, many problems related to the research and development of practical and effective schemes for multicell cooperation exist. They have attracted considerable research attention in both academia and industry. This special issue of IEEE Wireless Communications puts together some recent results in the area of multicell cooperation for the next generation cellular wireless networks. It includes 11 articles, a brief account for each of which is provided below.

Convergence of Price-Based Resource Allocation Algorithms in Multicellular Multicarrier Systems

Inter-cell interference mitigation is a key challenge in the heterogeneous wireless networks which are expected to use an aggressive frequency reuse factor and a high-density access point deployment to improve coverage and spectral efficiency. In this paper, the problem of resources allocation in multicell Orthogonal Frequency Division Multiple Access wireless system is considered with universal frequency reuse and target of Weighted Sum-Rate Maximization. We address multi cell modified iterative water filling as an iterative power allocation algorithm. Also, a new extension of fixed point implementation of Successive Convex Approximation for Low complExity (SCALE) algorithm to multicellular system [referred to as Multi Cell Fixed point SCALE (MCF-SCALE)] is presented and it has been shown both of them resulted to the same convergence point. It is also demonstrated that using Lagrangian multiplier instead of noise variance in Standard Yates framework (as has been used in some previous papers) is not a suitable method for proving convergence and all the previous results based on this pattern need to be revised. Finally, a new framework is presented for proving the convergence of MCF-SCALE algorithm based on Jacobi iterative algorithm. Moreover, some previous convergence criteria are shown to be interpreted as a special case of this condition.

On the Evolution of Multi-Cell Scheduling in 3GPP LTE / LTE-A

This paper provides a holistic overview of multi-cell scheduling strategies in emerging wireless systems. Towards this objective, the evolution of interference management techniques is thoroughly investigated from simple inter-cell interference coordination (ICIC) techniques towards more advanced coordinated multipoint transmissions (CoMP), while comparing and contrasting their common features and differences. Finally CoMP is explored in detail as an advanced and challenging mechanism to fully cooperate between adjacent cells in order to have an efficient resource allocation and inter-cell interference mitigation in multi-cell environments.

Joint User Scheduling and Adaptive Intercell Interference Cancelation for MISO Downlink Cellular Systems

Recently, an adaptive intercell interference (ICI) mitigation technique has been proposed for downlink cellular systems as a means to increase throughput with low system complexity. However, in the prior work, the issue of intercell user scheduling has not been considered. In this paper, we study multiple-input-single-output (MISO) downlink cellular systems that jointly design adaptive ICI cancelation (ICIC) and intercell user scheduling assuming that partial channel state information (CSI) is shared among base stations (BSs). Since the optimal solution would require high complexity, we investigate a new low-complexity algorithm that selects the best users and their beamforming strategies in terms of maximizing the weighted sum rate (WSR). To this end, we first develop a simple threshold criterion for each user to decide the preferred beamforming strategy based on the derivation of the expected signal-to-interference-plus-noise ratio (SINR). Then, according to users' feedback about their decisions, a successive user and beamforming selection algorithm is performed at the BSs. From simulation results, we show that, combined with proportional fair scheduling, the proposed scheme provides excellent throughput performance with very low computational complexity and the amount of inter-BS CSI exchange. Furthermore, we discuss an extension of the proposed scheme to limited feedback systems and observe that our algorithm also provides similar advantages over conventional schemes with quantized feedback.

3차원 주파수 재사용 패턴의 성능 및 반송파 결합을 사용하는 시스템으로의 적용 방안

Assigning different frequency resources among adjacent cells, namely the frequency reuse technique can be utilized to mitigate intercell interference, which is a major cause of performance degradation in cellular systems. Since most of conventional frequency reuse patterns are limited to the two-dimensional environment, the research for the three-dimensional frequency reuse would be beneficial especially for the implementation of femto cells in downtown office buildings. We propose frequency reuse patterns in three-dimensional space and evaluate their performance of each pattern in terms of channel capacity. In particular, we show that the proposed three-dimensional frequency reuse patterns can be applied for carrier-aggregated transmission of LTE-Advanced systems. The performance of the proposed patterns is evaluated using computer simulation.

제한된 피드백을 사용하는 결합 전송 공간 분할 다중 접속 기술을 위한 코드북 설계와 집중 스케줄링

In this paper, joint transmission space division multiple access(JT-SDMA) scheme is proposed to mitigate inter-cell interference(ICI) in cooperative wireless communications system with limited feedback. We propose a systematic design method for a codebook consisting of a finite number of unitary matrices suitable for network multiple-input multiple-output( MIMO) channel characteristics. A centralized cluster scheduling scheme is proposed to both mitigate ICI and maximizes multiuser diversity gain with limited feedback. It is shown that the proposed JT-SDMA scheme outperforms a existing coordinated SDMA scheme even in wireless network environments where sufficient multiuser diversity order can not be provided through efficient ICI mitigation.

Short-Term Multi-Cell Dynamic Resource Allocation in Future Cellular Networks

In future cellular system, it is becoming more challenging to optimize the radio resource management and maximize the system capacity whilst meeting the required quality of service from user's point of view. Traditional schemes have approached this problem mainly focusing on resources within a cell and to large extent ignoring effects of multi-cell architecture. For multi-cell systems employing intra-cell orthogonal communication channels, inter-cell interference mitigation techniques are expected to be one of the key radio resource management functions. In this paper, we propose a multi-cell coordinated and un-coordinated dynamic resource sharing algorithms and reuse techniques among base station/ relay station cells. The sub carrier allocation and power allocation are performed on the basic of sum-rate maximization, by considering the load over the cell (3-sector cell). The simulation results show the performance comparison between both coordinated and un-coordinated resource allocation schemes with different sum-rate maximization algorithms.

Fair Scheduling and Resource Allocation for Wireless Cellular Network with Shared Relays

This paper examines the shared relay architecture for the wireless cellular network, where instead of deploying multiple separate relays within each cell sector, a single relay with multiple antennas is placed at the cell edge and is shared by multiple sectors. The advantage of shared relaying is that the joint processing of signals at the relay enables the mitigation of intercell interference. To maximize the benefit of shared relaying, the resource allocation and the scheduling of users among adjacent cell sectors need to be optimized jointly. Based on this motivation, this paper formulates a network utility maximization problem for the shared relay system that considers the practical wireless backhaul constraint of matching the relay-to-user rate demand with the base-station-to-relay rate supply using a set of pricing variables. In addition, zero-forcing beamforming is used at the shared relay to separate users spatially; multiple users are scheduled in the frequency domain to maximize frequency reuse. A heuristic but efficient scheduling and resource allocation algorithm is proposed accordingly. System-level simulations quantify the effectiveness of the proposed approach, and show that the incorporation of the shared relay can improve the overall network performance and in particular significantly increase the throughput of cell edge users as compared to separate relaying.

Combating Interference: MU-MIMO, CoMP, and HetNet (Invited Paper)

Combating interference is one of the most outstanding challenges in wireless communication networks, especially for cellular networks with frequency reuse one. This issue becomes even more significant in heterogeneous networks where lower power nodes (such as pico-cell and/or femto-cell) are deployed within a macro-cell’s coverage. In this paper, we investigate interference coordination techniques in different scenarios. For homogeneous networks, we study single-cell multiuser MIMO for the purpose of intra-cell interference coordination and coordinated multi-point (CoMP) transmission for the purpose of inter-cell interference mitigation. For multi-userMIMO systems, we will discuss multi-userMIMO proportional-fair scheduling as well as multi-user precoding schemes. For CoMP transmission, we will focus on CoMP joint transmission and discuss the situations where CoMP could provide performance gains over single-cell operations. For heterogeneous networks, we investigate two specific inter-cell interference coordination techniques: time-domain solutions and power setting schemes. Furthermore, the potential extension of various CoMP schemes to heterogeneous networks will also be discussed.