András Rácz

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.

Performance aspects of Bluetooth scatternet formation

The emergence of Bluetooth as a default radio interface allows handheld electronic devices to be rapidly interconnected into ad hoc networks. Bluetooth allows large numbers of piconets to form a scatternet using designated nodes that participate in multiple piconets. We study the performance implications of forming scatternets from piconets. The contribution of our work is twofold. First, we establish a network model and define performance metrics for Bluetooth scatternets. Our model is derived from constraints specific to the Bluetooth technology, but is sufficiently abstract to relate to the more general field of ad hoc networking. Second, using a number of simulation studies, we relate scatternet parameters to performance metrics and discover correlations between scatternet formation rules and performance. These results reveal some important performance implications of scatternet design decisions and can serve as guidelines for future scatternet formation algorithms.

A pseudo random coordinated scheduling algorithm for Bluetooth scatternets

The emergence of Bluetooth as a default radio interface allows handheld devices to be rapidly interconnected into ad hoc networks. Bluetooth allows large numbers of piconets to form a scatternet using designated nodes that participate in multiple piconets. A unit that participates in multiple piconets can serve as a bridge and forwards traffic between neighbouring piconets. Since a Bluetooth unit can transmit or receive in only one piconet at a time, a bridging unit has to share its time among the different piconets. To schedule communication with bridging nodes one must take into account their availability in the different piconets, which represents a difficult, scatternet wide coordination problem and can be an important performance bottleneck in buillding scatternets. In this paper we propose the Pseudo-Random Coordinated Scatternet Scheduling (PCSS) algorithm to perform the scheduling of both intra- and inter-piconet communication. In this algorithm Bluetooth nodes assign meeting points with their peers such that the sequence of meeting points follows a pseudo random process that is different for each pair of nodes. The uniqueness of the pseudo random sequence guarantees that the meeting points with different peers the node will collide only occasionally. This removes the need for explicit information exchange between peer devices, which is a major advantage of the algorithm. The lack of explicit signaling between Bluetooth nodes makes it easy to deploy the PCSS algorithm in Bluetooth devices, while conformance to the current Bluetooth specification is also maintained. To assess the performance of the algorithm we define two reference case schedulers and perform simulations in a number of scenarios where we compare the performance of PCSS to the performance of the reference schedulers

Applying ATM/AAL2 as a switching technology in third-generation mobile access networks

In this article we focus on the transport and switching part of third-generation mobile access networks and outline some important aspects of applying ATM in these networks. In particular, we argue that in order for the mobile access network to support low-bit-rate delay-sensitive traffic consisting of short packets, the standardization of a new ATM adaptation layer, AAL2, and associated signaling protocol has been necessary. The AAL2 protocol has been designed to support low-bit-rate delay-sensitive services (typically compressed voice) where other adaptation layers fail to deliver the required QoS and maintain efficient resource utilization at the same time. Furthermore, in order to provide mobility and soft handover support in W-CDMA-based mobile networks such as UMTS or IMT-2000, there is also a strong demand for fast connection establishment and release. Therefore, when designing ATM-based cellular access networks some specific architectural and traffic management issues need to be addressed.

Handover Performance in 3GPP Long Term Evolution (LTE) Systems

The specification of the long term evolution (LTE) of 3G systems is currently ongoing in 3GPP with a target date of ready specification at the end of 2007. The evolved radio access network (RAN) involves a new radio interface based on OFDM technology and a radically different RAN architecture, where radio functionality is distributed into the base stations. The distributed nature of the RAN architecture calls for new radio control algorithms and procedures that operate in a distributed manner, including a distributed handover scheme as well. The most important aspects of the handover procedure in LTE has been already settled in 3GPP except a few details. In this paper we give an overview of the LTE intra-access handover procedure and evaluate its performance focusing on the user perceived performance aspects of it. We investigate the necessity of packet forwarding from a TCP throughput point of view, we analyse the problem of out of order packet delivery during handover and propose a simple solution for it. Finally, we investigate the impact of HARQ/ARQ state discard at handover on the radio efficiency. The results show that neither the user perceived performance nor the radio efficiency are compromised by the relocation based handover procedure of LTE.

Voice QoS in third-generation mobile systems

We analyze voice quality and system performance in third-generation mobile communication systems. We argue that in these networks that are expected to integrate voice, data, and multimedia services, the transport network can no longer be considered as a lossless transparent traffic channel. Rather, proper dimensioning is needed even for plain voice services. Furthermore, the efficient utilization of the network resources shared by voice and data traffic requires sophisticated traffic management in the statistical multiplexing environment. The contribution of the paper is twofold: first, voice quality in the cellular transport network is analyzed and network dimensioning criteria are derived. Second, building on advances reported in the literature and taking into consideration current standardization activities, a joint performance model is established for the air interface (AI) and the transmission network (TN) allowing for the analysis of end-to-end service performance within a uniform framework. To demonstrate the applicability of this model, we point to some performance problems in the multiservice environment and suggest and evaluate traffic management actions to overcome these.

Comparison of call admission control algorithms in ATM/AAL2 based 3/sup rd/ generation mobile access networks

While several papers and standards promote ATM in combination with the ATM Adaptation Layer Type 2 (AAL2) as the basic switching and multiplexing technology for 3/sup rd/ generation mobile access networks, very little work addresses the issue of AAL2 call admission control (CAC). The hardship of AAL2 CAC comes from the fact that AAL2, unlike ATM, supports variable packet size traffic. In addition, when applied in the cellular environment, the AAL2 CAC needs to co-operate with the radio interface (RI) resource management. In this paper we develop and compare AAL2 CAC algorithms that operate on the AAL2 traffic parameters currently considered by the ITU-T. Based on this comparison we find that one of these algorithms, employing the well-known Hoeffding inequality provides a reasonable trade-off between complexity and precision.

On Scheduling and Power Control in Multi-Cell Coordinated Clusters

Recently, tight network coordination in cellular systems has been demonstrated to improve the spectrum efficiency by means of signal processing methods. However, the performance of signal processing based multi-cell coordination is sensitive to backhaul delays, channel estimation errors and imperfections in fast link control. In this paper we consider tight network coordination for fast radio resource management (RRM) including packet scheduling, power control and modulation and coding scheme selection. We use a system level simulator to analyze the uplink performance of a multi-cell coordinated system that is built around a fast backhaul transport infrastructure for the purpose of enabling coordinated RRM rather than coordinated signal processing. We find that coordinated RRM alone can provide significant performance gains, up to 50% for cell edge and cell capacity as compared to traditional single-cell configurations and that multi-cell fast power control and modulation and coding scheme selection can significantly improve the accuracy of link adaptation in terms of signal-to-interference-and-noise (SINR) distribution, while imposing lower demands on the capacities of backhaul links compared to coordinated signal processing. Therefore RRM coordination can be an efficient complement to coordinated signal processing in multi-cell coordinated clusters.

Opportunistic target SINR setting for the MIMO broadcast channel

The sum capacity of the multiple input multiple output (MIMO) broadcast channel can be approached by a linear precoding technique called block diagonalization (BD) and by its recent extension that accounts for both in-cell and other-cell interference. In this paper we consider the problem of minimizing the transmitted power subject to a capacity constraint (the power control problem) and maximizing the sum information rate subject to a power constraint (the capacity problem) for the MIMO broadcast channel using BD precoding. Unlike previous works, we do not assume that a minimum signal-to-interference-plus-noise ratio (SINR) value is prescribed for each user but impose a fairness constraint on the user SINRs. Our approach preserves the degree of freedom of optimizing the SINR targets (in both problems) and thereby it allows to improve the system performance. We use the Augmented Lagrangian Penalty Function (ALPF) method to solve the respective constrained non-convex optimization problems and study the impact of different fairness constraints on the achievable maximum sum rate or minimum transmitted power. We find that BD with target SINR selection is a promising technique for the power control as well as the capacity problem.

Effective bandwidth and associated CAC procedure for traffic streams multiplexed over a VBR link

In this paper we introduce the definition of the effective bandwidth for traffic streams that are multiplexed over a VBR link. We assume an arbitrary traffic mix multiplexed into a FIFO buffer, which has a service rate, regulated by VBR traffic parameters as opposed to the traditional CBR server capacity. In order to analytically obtain the generalized effective bandwidth we define an equivalent queuing model, which reduces the problem of the variable service rate multiplexer to a constant rate server. Following the Large Deviation Analysis of our equivalent queuing model we derive the effective bandwidth in terms of VBR link parameters. One of the appealing features of our method is that any dimensioning, resource allocation schemes that are based on the effective bandwidth concept but originally have been devised for CBR links can be easily extended to be applicable for VBR links as well. For example, one can easily build a Connection Admission Control (CAC) algorithm for a VBR link by reusing the existing methods based on the effective bandwidth concept. In this paper we also give an example for a CAC algorithm and for other practical application scenarios.