Chrysostomos Koutsimanis

Intercell Interference Coordination in OFDMA Networks and in the 3GPP Long Term Evolution System

Intercell interference coordination (ICIC) in orthogonal frequency division multiple access (OFDMA) networks in general and in the 3GPP Long Term Evolution system in particular has received much attention both from the academia and the standardization communities. Understanding 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.

Relaying operation in 3GPP LTE: challenges and solutions

With the ever growing demand of data applications, traditional cellular networks face the challenges of providing enhanced system capacity, extended cell coverage, and improved minimum throughput in a cost-effective manner. Wireless relay stations, especially when operating in a halfduplex operation, make it possible without incurring high site acquisition and backhaul costs. Design of wireless relay stations faces the challenges of providing backward compatibility, minimizing complexity, and maximizing efficiency. This article provides an overview of the challenges and solutions in the design of relay stations as one of the salient features for 3GPP LTE advanced.

A Dynamic Resource Allocation Scheme for Guaranteed Bit Rate Services in OFDMA Networks

While several previous works have considered the problem of resource (including subcarrier and power) allocation in multicell orthogonal frequency division multiple access networks, only a few contributions have explicitly taken into account the elastic nature of data applications. In this work, each user is associated with a minimum and maximum resource block requirement and the resource allocation problem consists of maximizing the overall throughput such that these requirements are met. We propose a hybrid method that partitions this problem into a centralized and a distributed algorithm that balances between maximizing the overall throughput and being feasible in real systems. By means of simulations we compare four resource allocation strategies that represent various degrees of multi- cell coordination and taking advantage of multi-user diversity. We find that a feasible dynamic coordination combined with intra-cell multi-user diversity provides large throughput gains compared with non-coordinated schemes or schemes that would limit the degree of freedom of multi-user diversity.

A Low Intercell Interference Variation Scheduler for OFDMA Networks

In orthogonal frequency division multiple access networks, link adaptation (LA) selects the appropriate modulation and coding scheme for the upcoming transmission time interval. Since LA relies on measurements of the interference level and assumes that it remains similar during the next scheduling instance, the performance of LA depends on the variation (rather than simply the level) of intercell interference (ICI). In this paper we propose a minimum variance scheduler that allows for opportunistic scheduling - and thereby to take advantage of multi-user frequency diversity - but keeps the variation of intercell interference at a low level. We build on previously proposed collision models (detailed in own work and related other papers) and study the interplay between this scheduler and two intercell interference coordination (ICIC) methods that we call random and coordinated subcarrier allocation. We find that the proposed low variance scheduling scheme together with coordinated ICIC reduces ICI and its variance and is superior to other scheduling techniques in terms of the overall system throughput.

Performance analysis of scheduling and interference coordination policies for OFDMA networks

In orthogonal frequency division multiple access systems there is an intimate relationship between the packet scheduler and the inter-cell interference coordination (ICIC) functionalities: they determine the set of frequency channels (sub-carriers) that are used to carry the packets of in-progress sessions. In this paper we build on previous work - in which we compared the so called random and coordinated ICIC policies - and analyze three packet scheduling methods. The performance measures of interest are the session blocking probabilities and the overall throughput. We find that the performance of the so-called Fifty-Fifty and What-It-Wants scheduling policies is somewhat improved by coordinated sub-carrier allocation, especially in poor signal-to-noise-and-interference situations and at medium traffic load values. The performance of the All-Or-Nothing scheduler is practically insensitive to the choice of the sub-carrier allocation policy.

Impact of CSI Optimization and CRS Selection on Performance of LTE Release 8 Networks

A key reference signal in 3GPP Long Term Evolution (LTE) Release 8 is the cell specific reference signal (CRS). For example, the LTE transmission mode 4 (TM4) uses the CRSs to derive the channel and the interference estimates, utilized in the demodulation process and for acquisition of channel state information (CSI). LTE networks may further operate using different CRS configurations. In a non-shifted CRS configuration the same time and frequency resources are used for CRS transmissions in all cells. In a shifted CRS configuration different cells transmit CRSs on resources that are shifted in frequency. The non-shifted configuration avoids that the CRSs interfere with data transmissions, but is also associated with a systematic CSI estimation error; especially noticeable at low traffic. Using the shifted configuration the CRSs interfere with data transmissions but the CSI estimation error is smaller. In this paper, the LTE downlink performance is evaluated in a heterogeneous network deployment using shifted and non-shifted CRS configurations. The network operation is optimized by means of adjusting the transmission rank selection and channel quality estimation for each CRS configuration. The results demonstrate that under such operational conditions, a non-shifted CRS configuration delivers the highest performance.