Cross-Layer Error Control Optimization in 3G LTE Dzmitry Kliazovich 1 and Fabrizio Granelli DIT - University of Trento Via Sommarive 14, I-38050 Trento, Italy E-mail: [klezovic,granelli]@dit.unitn.it Simone Redana and Nicola Riato Nokia Siemens Networks S.p.A COO RTP PT RST Via Monfalcone, 1, I-20092 Milan, Italy [Simone.Redana,Nicola.Riato]@nsn.com Abstract—3G Long-Term Evolution (LTE) is a recent effort taken by cellular industries to step into wireless broadband market. The key enhancements target an introduction of new all- IP architecture, enhanced link layer and radio access with OFDM modulation and multiple antenna techniques. In this study, we focus on the overhead deriving from the multilayer ARQ employed at the link and transport layers. To the aim of reducing unnecessary burden on the wireless link, we propose a cross-layer ARQ approach, called ARQ Proxy, which substitutes the transmission of TCP ACK packet with a short MAC layer request on the radio link. Packet identification is achieved through association of a hash function to the raw packet data. Performance of the ARQ Proxy is evaluated using EURAE extensions for ns2 simulator. Results demonstrate significant improvements in terms of system capacity, TCP throughput performance, and higher tolerance to transmission errors. 2 Keywords-3G LTE, cross-layer, ARQ proxy. I. INTRODUCTION The 3G Long-Term Evolution (3G LTE) is an attempt to step into wireless broadband taken by cellular providers and equipment vendors [1]. 3G LTE introduces evolved radio interface with major enhancement coming from the use of Orthogonal Frequency Division Multiplexing (OFDM) and multiple antenna techniques. These technologies are already available on the market and employed in WiMAX as specified in IEEE 802.16 standard [2]. Along with the evolved radio interface, 3G LTE specifies the evolution of network architecture. It is designed to be packet-based and contain less network nodes, thus reducing protocol processing overhead and leading to reduced latency and network deployment costs. Moreover, the evolved architecture is focused on packet data communications using TCP/IP protocol suite. However, TCP/IP (designed in early days of ARPANET specifically for wired networks) shows poor performance over wireless channels, mainly due to high error rates [3]. In order to compensate wireless errors, 3G LTE employs error recovery techniques at the link layer, which partially overlap with error recovery performed at the transport layer of TCP/IP. In this paper, we present a novel cross-layer ARQ optimization technique, called ARQ proxy, which jointly optimizes error recovery mechanisms operating at different layers. ARQ proxy reduces the demand for network resources consumed by the employed error recovery techniques leading to the increase of network capacity as well as throughput performance. II. 3G LTE OVERVIEW In the effort to challenge advanced communication technologies in the long run, 3rd Generation Partnership Group (3GPP) initiated the study item Evolved UTRA and UTRAN [3], aiming to complete the set of specifications for the evolved radio access in 2007 with initial product deployment in 2009. The requirements for the long-term evolution (LTE) can be summarized as follows: - Peak data rates of 100 Mb/s in the downlink and 50 Mb/s in the uplink leading to spectrum efficiency of up to 5 bit/s/Hz. - Reduced user- and control-plane latency to less than 10 ms and less than 100 ms, respectively. - Mobility is supported for up to 350 km/h. - Spectrum flexibility and reduced network costs. To achieve the above mentioned targets, new updates to the radio interface as well as modifications to the network architecture are considered. 3 A. Evolved Architecture Reduced latency and costs are two main driving requirements for architecture evolution which are accomplished with reduced number of network nodes. Fewer nodes reduce protocol-related processing along the data path as well as call setup time and control-plane related processing. Fig. 1 illustrates high level architecture for the evolved system specified in [5]. In the Evolved UTRAN, the functions of Radio Network Controller (RNC) are distributed between evolved Node B (eNB) and network core. The evolved core provides local mobility management, internetworking with previously released 3GPP systems as well as non-3GPP wireless technologies. 3G LTE relies on all-IP communications as opposed to the widely used circuit-switched approach. BTS SGSN NodeB eNodeB HSS BSC RNC GERAN UTRAN Evolved RAN GPRS Core MME UPE non 3GPP IP Access WLAN 3GPP IP Access Evolved Packet Core IMS, PSS, etc. PCRF SAE Anchor 3GPP Anchor Figure 1. 3G LTE System Architecture. 1 This work was done while Dzmitry Kliazovich was pursuing an internship focused on 3G LTE research at Siemens (now Nokia Siemens Networks), Italy. 2 The ARQ Proxy approach is currently patent-pending under EP 07425087.9 “Cross-Layer Error Recovery Optimization for 3G LTE Systems.” 3 At the moment of writing this paper 3GPP group has just initiated 3G LTE working phase. For that reason, the final details of the evolved system may differ from those presented in current paper. However, it is indented to provide the reader with the key concepts which are already agreed by the standardization committee and will most likely become a part of the standard. 1930-529X/07/$25.00 © 2007 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE GLOBECOM 2007 proceedings. 2525