Among the goals of 3GPP LTE networks are higher user bit rates, lower delays, increased spectrum efficiency, support for diverse QoS requirements, reduced cost, and operational simplicity. Resource scheduling and interference mitigation are two functions which are key to achieving these goals. This paper provides a survey of related techniques which have been proposed and shown to be promising. A brief discussion of the challenges for LTE-Advanced, the next step in the evolution, is also provided.

https://downloads.hindawi.com/journals/jece/2010/273486.pdf

A survey of scheduling and interference mitigation in LTE

The widely accepted OFDMA air interface technology has recently been adopted in most mobile standards by the wireless industry. However, similar to other frequency-time multiplexed systems, their performance is limited by inter-cell interference. To address this performance degradation, interference mitigation can be employed to maximize the potential capacity of such interference-limited systems. This paper surveys key issues in mitigating interference and gives an overview of the recent developments of a promising mitigation technique, namely, interference avoidance through inter-cell interference coordination (ICIC). By using optimization theory, an ICIC problem is formulated in a multi-cell OFDMA-based system and some research directions in simplifying the problem and associated challenges are given. Furthermore, we present the main trends of interference avoidance techniques that can be incorporated in the main ICIC formulation. Although this paper focuses on 3GPP LTE/LTE-A mobile networks in the downlink, a similar framework can be applied for any typical multi-cellular environment based on OFDMA technology. Some promising future directions are identified and, finally, the state-of-the-art interference avoidance techniques are compared under LTE-system parameters.

On Interference Avoidance Through Inter-Cell Interference Coordination (ICIC) Based on OFDMA Mobile Systems

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.

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

We address interference avoidance by resource partitioning in densely deployed femtocell networks. The main objective is to protect user equipments (UEs) that are located near the cell boundary of two or more femtocells from detrimental downlink interference. The available frequency bands are divided into subbands that are distributed among femtocells in a way that directly adjacent cells do not occupy the same subbands. For this purpose, a novel centrally controlled resource partitioning method is developed based on graph coloring that assigns subbands in terms of resource efficiency. The proposed algorithm strikes a balance between interference protection and spatial frequency reuse of subbands, and is well suited for randomly deployed wireless networks. System-level simulations reveal that cell edge capacities are significantly boosted without causing a degradation in average system throughput.

Graph-Based Dynamic Frequency Reuse in Femtocell Networks

The fifth generation mobile networks will be developed to improve area spectral and energy efficiency, and provide uniform user experience. Hyper-dense small cell deployment can move devices closer to the wireless network and satisfy 5G system requirements. The main challenge of this network deployment results from the random deployment, dynamic on-off, flexible connection to cellular core networks, and flat system architecture of 5G systems. Therefore, conventional network planning and radio resource management, which depend on a central control node, cannot be applied to small cell networks. In this article some cooperative distributed radio resource management algorithms for time synchronization, carrier selection, and power control are discussed for hyper-dense small cell deployment.

Cooperative distributed optimization for the hyper-dense small cell deployment

Future wireless packet switched cellular networks will require dense frequency reuse to achieve high capacity. At the same time, measures are required which limit the interference on the frequency carriers. It is assumed that central entities performing the task of interference coordination with global knowledge should be avoided. Rather, distributed algorithms are sought for. To this end, we propose decentralized resource allocation algorithms that enable base stations to select a pool of favorable resources with low interference based on local knowledge only. The actual user-level resource allocation from that pool will then be performed by fast schedulers operating on the preselected resources within each cell. We analyze and evaluate the proposed resource selection algorithms by introducing a simplified wireless network model and applying methods from game theory. Proving the existence of Nash equilibria shows that stable resource allocations can be reached by selfish agents. In addition to that, we perform simulations to determine the speed of convergence and the resulting equilibrium interference levels. By comparing these to an optimal global solution, which is derived by solving an integer linear program, we are able to quantify the efficiency loss of the distributed game approach. It turns out that even though the distributed game results are sub-optimal, the low degree of system complexity and the inherent adaptability make the decentralized approach promising especially for dynamic scenarios.

Decentralized inter-cell interference coordination by autonomous spectral reuse decisions

System-level simulations of packet-based orthogonal frequency-division multiple access (OFDMA) cellular networks require a proper consideration of the channel-dependent fast fading/scheduling in order to produce meaningful results. How- ever, generating the time and frequency selective fast fading channels and performing detailed channel-adaptive scheduling for each user in a multi-cellular system is computationally too expensive in most system-level simulations. To this end, we propose an efficient mapping technique that allows to accurately characterize the performance of the fast-scheduling system using two parameters: the average effective post-scheduling SINR gain and the ratio of allocated resources, both conditioned on the SINR and the velocity of a user in a scenario defined by the number of co-scheduled users. To derive these parameters for different numbers of users at different velocities and SINR levels, we perform detailed link level simulations including fast scheduling for the downlink of a Long Term Evolution (LTE) system.

A Concept for Efficient System-Level Simulations of OFDMA Systems with Proportional Fair Fast Scheduling

Interference management techniques like inter-cell interference coordination (ICIC) will play a key role in enabling high spectral efficiency in future wireless OFDMA-based cellular systems. The aim of ICIC is to lower inter-cell interference by coordinating the usage of spectrum resources among neighboring cells. Especially for the cell-edge users, avoiding the reuse of the same resources in neighboring cells yields a significant increase in SINR and thus capacity. In this paper, we consider decentralized ICIC schemes for the uplink of an LTE system in which base stations perform selfish resource allocation decisions. System level simulations in a multi-cell scenario show the convergence of the distributed schemes towards Nash equilibria. The mean cell throughput as well as the 5% CDF user throughput are compared to those achieved by frequency reuse 1 and 3 deployments. The simulation results show that the proposed schemes adapt well to varying uniform and especially non-uniform traffic loads. I. INTRODUCTION

Performance of Decentralized Interference Coordination in the LTE Uplink

Cellular systems like Long Term Evolution Advanced (LTE-Advanced) aim at reusing the whole spectrum in each cell which leads to high inter-cell interference and thus low signal to interference and noise ratio (SINR) conditions for users at the cell edge. One way to utilize multiple antennas at the base station is to perform downlink transmit beamforming which besides increasing the received power at the mobile also has the potential to reduce interference when collisions of beams from different base stations are avoided. To this end we propose an autonomous coordination mechanism that allows cells to coordinate their beams in the frequency domain. In addition to increasing the SINR, it reduces interference fluctuations (the “flash light” effect) due to uncoordinated scheduling in neighbor cells thus making the interference situation more predictable. This further boosts the throughput potential. The proposed scheme is based on feedback from mobiles to their serving base station but does not require inter-cell signaling. By means of system level simulations we show that our autonomous approach converges after few iterations and yields significant cell mean and cell-edge throughput improvements.

Jan Ellenbeck

Papers

Decentralized inter-cell interference coordination by autonomous spectral reuse decisions

A Concept for Efficient System-Level Simulations of OFDMA Systems with Proportional Fair Fast Scheduling

Performance of Decentralized Interference Coordination in the LTE Uplink

Autonomous beam coordination for the downlink of an IMT-Advanced cellular system

System‐Level Simulations with the IMT‐Advanced Channel Model