LTE-SAE architecture and performance Per Beming, Lars Frid, Göran Hall, Peter Malm, Thomas Noren, Magnus Olsson and Göran Rune LTE-SAE (Long-term evolution system architecture evolution) systems promise unprecedented performance in new and existing frequency bands for 3GPP and 3GPP2 operators. The simplified and optimized architecture uses a minimum number of nodes in the user plane. In addition, new features have been introduced to simplify operation and maintenance. Ericsson s portfolio of base stations and core network products can be upgraded to LTE-SAE, and the company is developing a range of LTE base stations for new deployments. Furthermore, Ericsson s LTE mobile platforms are well positioned for different types of terminals and devices: broadband modules, fixed wireless terminals, and mobile terminals. This combination enables mobile broadband services to everyone, everywhere. Background and targets Mobile broadband is rapidly becoming a reality. By 2011, Ericsson anticipates that 1.5 billion people will have broadband (Figure 1). In addition, more than half of these people will have mobile broadband, and the majority of them will be served by HSPA/LTE networks. At present, people can surf or send e-mail with HSPA-enabled handsets and notebooks; replace their DSL modems with HSPA modems; and quickly upload and download videos or music with 3G phones. LTE, which is to be introduced in 3GPP Release 8, is the next major step in mobile radio communications. It will give a superior user experience and support even more demanding applications, such as interactive TV, user-generated videos, advanced games, and professional services. LTE uses OFDM (orthogonal frequency-division multiplexing) radio access technology together with advanced antenna technologies. In addition to LTE, 3GPP has specified a flat, IP-based network architecture as part of the system architecture evolution (SAE) effort. The aim and design of the LTE-SAE architecture and concepts are to efficiently support mass-market usage of any IP-based service. The architecture is based on, and evolved from, existing GSM/WCDMA core networks to facilitate simplified operations and smooth, cost-effective deployment. The LTE-SAE architecture reduces operating expenses (OPEX) and capital expenditures (CAPEX). The new, flat architecture, for example, means that only two node types (base stations and gateways) must scale in capacity in order to accommodate large increases in data volumes. One other area of focus has been network operation functionality, which now targets a high degree of automatic configuration. In addition, 3GPP and 3GPP2 have agreed to optimize interworking between CDMA and LTE-SAE. CDMA operators will thus also be able to evolve their networks to LTE- SAE and benefit from huge economies of scale and global chipset volumes. LTE is a versatile technology that fulfills or exceeds 3GPP requirements (Box A). Some of the most notable requirements follow below: Downlink peak rates of more than 100Mbps and roundtrip time in the radio access network (RAN) of less than 10ms. Support for flexible carrier bandwidths from less than 5MHz up to 20MHz in many new and existing spectrum bands. Support for FDD and TDD deployments. Support for handover and roaming to existing mobile networks, thereby providing ubiquitous coverage to all mobile subscribers from the very outset. Figure 1 Forecasted number of broadband subscriptions 98
Operators may introduce LTE flexibly, to match current network, spectrum, and business objectives for mobile broadband and multimedia services. Technical overview Overall architecture The main principles of the LTE-SAE architecture include a common anchor point and gateway (GW) node for all access technologies; an optimized architecture for the user plane this is a departure from four to only two node types (base stations and gateways); IP-based protocols on all interfaces; a RAN-CN functional split similar to that of WCDMA/HSPA; a split in the control/user plane between the mobility management entity (MME) and the gateway; and integration of non-3gpp access technologies using client- as well as network-based mobile IP. Figure 2 shows a simplified view of the overall LTE-SAE architecture. The gateway, which includes both packet data network (PDN) and serving gateway functionality, can be configured to serve in either or both of these roles. The PDN gateway serves as a common anchor point for all access technologies, providing a stable IP point-of-presence for all users regardless of mobility within or between access technologies. The serving gateway is the anchor point for intra-3gpp mobility. The MME functionality is kept separate from the gateways to facilitate network deployment, independent technology evolution, and fully flexible scaling of capacity. GSM and WCDMA/HSPA systems are integrated into the evolved system through standardized interfaces between the SGSN (serving GPRS support node) and the evolved core network. This includes interfaces to the MME for transferring context and establishing bearers when moving between accesses, and to the gateway for establishing IP connectivity with user equipment (UE). The gateway node thus functions as a GGSN (gateway GPRS support node) for GSM and WCDMA/HSPA terminals. The architecture also allows for a common packet core network for GSM, WCDMA/ HSPA and LTE by combining the SGSN and the MME in the same node. BOX B, OVERVIEW OF LTE-SAE TRIAL INITIATIVE In May 2007, Ericsson and other leading vendors and operators launched the LTE-SAE Trial Initiative. Objectives Drive industrialization of 3GPP LTE-SAE Demonstrate 3GPP LTE-SAE capabilities Promote 3GPP LTE-SAE to operators, vendors, analysts and regulators Founders Alcatel-Lucent Ericsson France Telecom Nokia Nokia Siemens Networks Nortel T-Mobile Vodafone Member commitments Proof-of-concept demonstrations in 2007 Interoperability tests in 2008 End-user trials in 2009 The LTE-SAE Trial initiative is open to any organization that is committed to actively contributing to the objectives. The home subscriber server (HSS) connects to the packet core through an interface that has been proposed and will likely be based on Diameter, not SS7. This will give a harmonized and simplified solution for control plane IP networking, since network signaling for policy control and charging is already based on Diameter. BOX A, 3GPP REQUIREMENTS FOR LTE-SAE The LTE base stations connect to the core network via the RAN-CN interface. The MME handles control signaling for instance, for mobility. User data is forwarded between base stations and gateway nodes over an IP-based transport infrastructure. To support high-speed handover of terminals in active mode, each LTE base station is logi- The most pronounced 3GPP requirements for LTE-SAE: Peak bit rate of more than 100Mbps in the downlink and greater than 50Mbps in the uplink Compared to HSPA Release 6 baseline: Greater spectrum efficiency: 3-4 times the baseline in the downlink, and 2-3 times the baseline in the uplink Greater average cell bit rate: 3-4 times the baseline in the downlink, and 2-3 times the baseline in the uplink Greater bit rates at cell edges: 2-3 times the baseline in both downlink and uplink 10ms RTT and 100ms transition time from idle mode (LTE access setup time) Scalable bandwidths for example, 20MHz, 15MHz, 10MHz, 5MHz and 1.25MHz Support for FDD and TDD with as many commonalities as possible Cost-effective migration and reduced CAPEX and OPEX For more details regarding the requirements, see 3GPP TR 25.913. 2 99
100 Figure 2 Simplified view of the LTE-SAE architecture. Note how cdma2000 can be integrated into the architecture. Figure 3 Architecture of radio interface protocols control and user planes. cally connected to all its neighboring base stations. The efforts to integrate cdma2000 access will yield a solution that supports seamless mobility between cdma2000 and LTE. The integration will support single- and dualradio handover, allowing for flexible migration from CDMA to LTE. Figure 2 shows how cdma2000 can be integrated into the LTE-SAE architecture. Because the existing QoS concept for GSM and WCDMA systems is somewhat complex, the LTE-SAE targets a QoS concept that combines simplicity and flexible access with backward compatibility. LTE-SAE has adopted a class-based quality-of-service (QoS) concept that gives operators a simple, yet effective solution to differentiating between packet services. LTE access After having studied a variety of multiple access technologies, 3GPP selected OFDM (orthogonal frequency-division multiplexing) for the downlink and SC-FDMA (singlecarrier frequency-division multiple access) for the uplink. These choices yield spectrum flexibility while also meeting stringent targets for throughput and spectrum efficiency. Essentially, the LTE physical layer solely provides shared channels to the higher layers using a 1ms transmission time interval (TTI). LTE relies on rapid adaptation to channel variations, employing rate adaptation and hybrid automatic repeat request (HARQ) with soft-combining in much the same way as is done in HSPA. The use of OFDM and SC-FDMA makes it possible to exploit variations in both the frequency and time domains. The subcarrier spacing in the LTE physical layer is 15kHz. The architecture of the radio interface protocol is based on that for HSPA. The names of the protocols are the same, in fact, and the functions are similar. Some distinctions stem from differences in the multiple access techniques of LTE and HSPA. Others relate to the fact that LTE is a packet-only system (that is, there are no requirements to support the legacy circuit-switched domain). Figure 3 shows the architecture of the LTE radio interface protocol. Note: Apart from the non-access stratum (NAS) protocols, all radio interface protocols terminate in the enodeb on the network side. PDCP (packet data convergence protocol) handles the header compression and security functions of the radio interface; the RLC (ra-
dio link control) protocol focuses on lossless transmission of data; and the MAC (media access control) protocol handles uplink and downlink scheduling and HARQ signaling. Similarly, the RRC (radio resource control) protocol handles radio bearer setup, active mode mobility management, and broadcasts of system information, while the NAS protocols deal with idle mode mobility management and service setup. Performance The 3GPP has thoroughly evaluated LTE access performance, showing that LTE access fulfills the stipulated requirements 2 and aptly provides the desired spectrum flexibility. Likewise, Ericsson has carried out substantial simulations of system and link-level performance. Figure 4 shows the simulated spectrum efficiency and user throughput. Spectrum efficiency is 1.7-2.7bps/Hz/cell in the downlink and 0.7bps/Hz/cell in the uplink when inter-site distance (ISD) is 500m. User throughput at the cell edge is 0.18-0.28bps/Hz/cell in the downlink and 0.022-0.05bps/Hz/cell in the uplink when simulated with 10 users with full buffers per cell. Figure 4 also shows the UTRA (UMTS terrestrial radio access) baseline, which is based on 3GPP Release 6 with basic receivers. From the peak bit rate in Figure 5 it can be seen that LTE meets and exceeds the 100Mbps downlink and 50Mbps uplink targets. In fact, with 20MHz of spectrum allocation, LTE surpasses 325Mbps in the downlink and 80Mbps in the uplink. The round-trip time (RTT) has been estimated to be 7ms; one-way delay is 3.5ms and HARQ RTT is 5ms (Figure 6). Operation and maintenance One of the most important requirements put on LTE-SAE is reduced OPEX. Selfmanaging features play a key role in achieving this goal. Self-provisioning functions, for example, reduce costs and speed up initial deployment of LTE-SAE, by enabling fast and easy installation, integration and deployment of base stations (including initial radio network setup) with a minimum of preparation and operator interaction. Likewise, selfoptimization, which includes features for automatic optimization of neighboring cells and automated tuning of parameters that control handover and other radio-resourcemanagement algorithms, targets the reduction of effort required to tune and maintain LTE-SAE networks. Figure 4 Spectrum efficiency and user throughput at cell edge. 101
Figure 5 LTE peak bit rates for different spectrum allocations. Figure 6 User plane and HARQ delay. 102 Terminals and devices From the outset, LTE has been specified and designed to accommodate small, highperformance, power-efficient, end-user devices. Besides mobile phones and laptop computers, LTE will support devices (Figure 7) that put stringent requirements on downlink and uplink bit rates for instance, TV sets and video cameras; and latency for instance, online gaming consoles. Products Ericsson is developing a complete product portfolio for LTE-SAE. Radio Access Ericsson will reuse its HSPA product packaging for LTE base stations. Moreover, the LTE and HSPA implementations are based on the same hardware and software architecture. The first release will support multiple spectrum bands as well as FDD and TDD. Using plug-in units, operators can add LTE to an existing Ericsson base station to make it dual-mode and dual-band. In this scenario, Ericsson s operations support system (OSS) will support migration and seamless management. Simplicity-of-use features include the plug-and-play base station, automated tuning, and intelligent reporting of key performance indicators (KPI). Ericsson will also develop stand-alone, high-performance macro and micro base stations. The main-remote concept will facilitate deployment at sites with limited room for new equipment or at new sites where it is difficult to obtain adequate space. Core network SAE functionality will be supported in all applicable core network products from Ericsson. The gateway functionality, which is based on an evolved gateway product offering, will support LTE traffic, provide GGSN functionality for GSM and WCDMA/HSPA, and give Mobile IP support. The MME functionality will be supported both as a standalone LTE mobility server node and as combined with SGSN functionality for GSM and WCDMA/HSPA. Ericsson s HSS and Policy Controller will also support SAE functionality. The result is a common core network for all access technologies. Mobile platforms, broadband modules and fixed-wireless terminals Ericsson s mobile platform for LTE reuses the architecture of the successful WCDMA mo-
bile platform, ensuring stability and reducing time to market and cost. Furthermore, device vendors will be certain to note Ericsson s LTE platform uses the same software interface as is found in the WCDMA mobile platform. Ericsson s first commercial LTE chipset has been optimized for data traffic and designed for use in compact mobile electronics, such as mobile phones and broadband modules for notebook computers, TV sets, video cameras and fixed-wireless terminals. Key features of the LTE chipset include downlink and uplink peak rates of 100Mbps and 50Mbps, and multi-bandwidth, quadband support. Integration of LTE with GSM and WCDMA/HSPA in the LTE mobile platform will ensure that it benefits from GSM/ WCDMA economies of scale. Consequently, it enables nationwide coverage and global roaming from day one. Topology and deployment LTE has been defined to support flexible carrier bandwidths of less than 5MHz up to 20MHz in a variety of spectrum bands for FDD and TDD deployments. Operators may thus introduce LTE in new as well as existing bands. Initially, this might be where it is easiest to deploy 10 or 20MHz carriers (for example, IMT 2000 Extension or Advanced Wireless Services bands). But eventually LTE will be deployed in every cellular band. From the outset, LTE terminals and infrastructure will support up to four frequency bands, which means worldwide deployment will take place using multiple bands. At present, the vast majority of commercial cellular systems use FDD technology. FDD is generally more efficient and represents greater device and infrastructure volumes than TDD, which is a good complement, for example, in centrum gaps between FDD uplink and downlink. Because Ericsson s LTE hardware is the same (apart from filters) for FDD and TDD, TDD operators can finally enjoy the economies of scale associated with FDD products. When defining the LTE-SAE architecture, 3GPP paid special attention to supporting flexible network configurations and to providing high service availability. In the core network, for example, the separation of the control functionality (MME) from user plane handling increases flexibility when deploying the network. Figure 7 Sample of devices that might include an LTE chipset. Figure 8 Example of network topology. 103
TERMS AND ABBREVIATIONS Standardization terminology is evolving, and as a consequence, the terms LTE and SAE are being replaced in 3GPP specifications. However, because the new terms are still not widely used, this article retains the older, more familiar terminology. Terms LTE SAE Long-term evolution, the 3GPP Release 8 of OFDM-based radio and radio access network System architecture evolution, the overall 3GPP Release 8 of the packet system connected to LTE radio New terms in 3GPP (not used in this article) EPS Evolved packet system, new term for the complete packet system, including UE, RAN and core network EPC Evolved packet core, new term for the core network part of EPS eutran Evolved UTRAN, new term for the evolved radio access network Abbreviations AWS Advanced wireless services CAPEX Capital expenditures CDMA Code-division multiple access DL Downlink DSL Digital subscriber line enb Evolved node-b FDD Frequency-division duplex FDMA Frequency-division multiple access GGSN Gateway GPRS support node HARQ Hybrid automatic repeat request HSPA High-speed packet access HSS Home subscriber server IMS IP Multimedia Subsystem ISD Inter-site distance MAC Media access control MME Mobility management entity NAS Non-access stratum OFDM Orthogonal frequency-division multiplexing OPEX Operating expenses PDCP Packet data convergence protocol PDN Packet data network QAM Quadrature amplitude modulation QoS Quality of service QPSK Quadrature phase shift keying RLC Radio link control RTT Round-trip time SC-FDMA Single-carrier FDMA SGSN Serving GPRS support node SS7 Signaling system #7 TDD Time-division duplex TTI Transmission time interval UE User equipment UL Uplink UTRA Universal terrestrial radio access WCDMA Wideband code-division multiple access To optimize the handling of packet-data traffic, operators can distribute the gateway nodes over several sites in the network, thereby reducing the use of transport resources (Figure 8). This approach also minimizes delay, which is an important prerequisite for real-time services, such as IMS multimedia telephony and high-peak-rate mobile broadband data access. The architecture also allows for centralization of MME functionality, which provides REFERENCES 1. Dahlman, Parkvall, Skold and Beming, 3G Evolution: HSPA and LTE for Mobile Broadband, Academic Press, Oxford, UK, 2007 2. 3GPP TR 25.913 Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E- UTRAN) 104 the core network control for LTE. To reduce operating costs, this functionality can be deployed in central sites that are co-located in server farms with other control nodes. The LTE-SAE architecture addresses high availability by pooling core network nodes. In practice, this means that if one core network node should fail, the base stations can connect to any other core network node inside the pool. In short, LTE-SAE provides excellent service availability. An efficient way for operators to deploy LTE-SAE architecture and functionality in an WCDMA/HSPA network is to upgrade existing network nodes. This approach is especially suitable for early deployment, when spare capacity in the WCDMA/HSPA network can be used to support LTE. One might, for example, operate a common core network for WCDMA/HSPA and LTE while sharing available control and payload processing capacity to optimize the overall use of capacity and reduce OPEX as well as CAPEX for early LTE deployments. Similarly, cdma2000 operators may introduce SAE support by evolving the core network infrastructure. Conclusion Ericsson s implementation of LTE-SAE will offer peak rates of more than 100Mbps in the downlink, round-trip time of less than 10ms, and greatly simplify operation and maintenance. The architecture has been optimized for mobile broadband services. Ericsson s implementation offers efficient integration with GSM, WCDMA/HSPA and cdma2000. Ericsson anticipates that its mobile platform for LTE will appear in a broad range of devices. LTE-SAE will benefit from the strong industry momentum and ecosystem of GSM and WCD- MA/HSPA, bringing economies of scale to terminals, devices, and infrastructure equipment.