Intercell Interference Coordination in OFDMA Networks and in the 3GPP Long Term Evolution System G´ abor Fodor, Chrysostomos Koutsimanis, Andr´ as R´ acz, Norbert Reider, Arne Simonsson and Walter M¨ uller Ericsson Research Abstract— Intercell interference coordination (ICIC) in or- thogonal frequency division multiple access (OFDMA) net- works in general and in the 3GPP Long Term Evolution system in particular has received much attention both from the academia and the standardization communities. Under- standing the trade-offs associated with ICIC mechanisms is important, because it helps identify the architecture and protocol support that allows practical systems to realize potential performance gains. In this paper we review some of the recent advances in ICIC research and discuss the assumptions, advantages and limitations of some of the proposed mechanisms. We then proceed to describe the architecture and protocol support for ICIC in the 3GPP LTE system. We make the point that the 3GPP standard is formed in a flexible way such that network operators can employ the most suitable ICIC mechanism tailored to their actual deployment scenario, traffic situation and preferred performance target. I. I NTRODUCTION Intercell interference coordination (ICIC) techniques for multi-cell wireless systems including the Global Sys- tem for Mobile Communications (GSM), Enhanced Gen- eral Packet Radio Service (EGPRS), Enhanced Data Rates for GSM Evolution (EDGE), and the Universal Terrestrial Radio Access (UTRA) have been the topic of research ever since these systems started to gain popularity. Indeed, a great number of theoretical results as well as many years of practical experience exist; for a comprehensive survey see the classical paper by Katzela and Naghshineh [1]. Recently, the 3 rd Generation Partnership Project (3GPP) has been completing most of the technical specifications for the Long Term Evolution (LTE) of third generation cellular systems. The technical targets of LTE include peak data rates in excess of 300 Mbps, delay and latencies of less than 10 ms and manifold gains in spectrum efficiency. Unlike the previous generations, LTE uses orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) as the baseline for modulation and multiple access scheme respectively [2]. In addition to having a new radio inter- face, LTE is built around a flat architecture in which radio base stations operate in a distributed fashion rather than being controlled by a central entity such as a base station controller or a radio network controller (RNC). The aggressive performance targets, the new physi- cal layer and the novel flat (”no RNC”) architecture of LTE has triggered a new wave of studies - both within the academia and the industry - for radio resource management in general and interference coordination in particular. An important line of works formulate the ICIC problem as an optimization task whose objective is to maximize the multi-cell throughput subject to power con- straints, intercell signaling limitations, fairness objectives or minimum bit rate requirements [3], [4], [5]. While optimization models give an insight into the upper bounds of achievable ICIC gains, actually implementing these near optimal mechanisms are typically not feasible or economical in real systems. Indeed, the ICIC mechanisms currently studied by the 3GPP build on markedly lower complexity heuristics. From a system design perspective, ICIC mechanisms without (or with slow) intercell com- munication - building on some pre-configured (simple) OFDM resource block allocation rule - are particularly attractive. Along another line, several authors have developed so called collision models that analyze the bit/packet error rate and throughput performance of multi-cell systems typically assuming uncoordinated (random or channel dependent) allocation of subcarriers in the different cells of the multi-cell system [6]. The methodology and the numerical results of these papers are useful because they provide insight into the probability and the impact of intercell collisions, but they do not evaluate the usefulness of practical ICIC mechanisms. Therefore, during the stan- dardization process, the 3GPP has extensively studied a range of intuitively appealing and feasible interference co- ordination algorithms using advanced system simulations [12]. The outcome of these academic and industry efforts are a deep understanding of the tradeoffs coupled with interference coordination techniques, a broad consensus regarding the time scale at which practical ICIC schemes should operate [10] and a flexible support in the LTE standards suite that allows network operators to configure ICIC mechanisms that best suite their specific deployment scenario, traffic load situation and performance targets. We organize this article as follows. In the next section, we review the fundamentals of intercell interference co- ordination in the light of recent advances reported in the literature. These basic insights help us to identify the key tradeoffs associated with ICIC mechanisms. In Section III we describe a hybrid ICIC scheme that addresses _________________________________________ Manuscript received August 1, 2008; revised November 15, 2008; accepted May 25, 2009. JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 7, AUGUST 2009 445 © 2009 ACADEMY PUBLISHER