Long Term Evolution-Advanced (LTE-A): An Introduction

Transcription

1 Long Term Evolution-Advanced (LTE-A): An Introduction Ankush Chaudhary 1 Ashish Kumar Sharma 2 Jyoti Dalal 3 Tejender Singh Rawat 4 Amity University, Haryana ABSTRACT- This paper furnishes an idea, how Evolved Universal Terrestrial Radio Access (EUTRA), known as the Long Term Evolution (LTE) technology, brings cellular communication to the fourth generation (4G). In this paper, we described the system architecture and performance objectives of the next generation accessnetwork technology being developed by 3GPP. This paper also provides an overview of Long Term Evaluation Advanced (LTE-Advanced) technology, together with possible improvements, their associated challenges, and some approaches that have been considered to tackle those challenges. LTE-Advanced is standardized in the 3GPP specification Release 10 (LTE-A) and designed to meet the 4G requirements as defined by ITU. With the foresee throughput and latency targets, spectrum flexibility, extra capacity and lower cost per bit, We have described its main technologies Carrier Aggregation, Advanced MIMO Scheme, Coordinated Multipoint Transmission, Relay Nodes and Heterogeneous Self Organizing Network. The development and integration of these elements will not end with 3GPP Release 10, but will provide the starting point for their implementation. INTRODUCTION The 3rd Generation Partnership Project (3GPP) was established in 1998, with the goal of creating a collaboration entity among various telecommunications associations, and continuing working on the radio, core network, and service architecture of a worldwide applicable 3G technology requirement [1]. As consumer demand grows for ever-richer services and connected lifestyles, LTE was initiated as a study item and its technical requirements were agreed in June The mobile industry is already hard at work defining the technical solution that will allow mobile networks to meet the growing demand for wireless broadband services [2]. LTE-Advanced does not involve the rollout of a new network, as was the case with the arrival of the third 21 generation, Instead it will be based on the deployment of a series of technological enhancements to LTE that will, in various permutations and according to various timescales, be used to boost the performance of the base LTE standard. It is for this reason that some vendors liken the relationship between LTE-A and LTE to that between HSPA+ and HSPA [3].LTE- Advanced is based on orthogonal frequency division multiplexing (OFDM) which transmits data on orthogonal sub-carriers. So the system can eliminate intra-cell interference but still suffers from inter-cell interference (ICI), the multi-cell multiple-input multiple-output (MIMO) strategy termed CoMP transmission/reception was proposed. CoMP is one of the candidate techniques for LTE-Advanced system to increase the cell average and cell edge throughput. However, multi-cell coordination remains an open research problem that will strongly influence the performance of CoMP[4].The evolution to LTE may be compelling for many operators because of the reduced capital and operating expenditures it requires over previous 3G networks. A key aspect of LTE is its simplified, flat network architecture, derived from it being an all-ip, packet-based network, and the use of new techniques to get high volumes of data through a mobile network. This allows many of the network elements involved in the data transport between an operators base stations and its core network in current cellular systems to be removed. The objective of LTE included reduced latency, higher user data rates, improved system capacity and coverage and reduced cost of operation. LTE has emerged as the technology of choice for the future for global operators and is expected to illustrate

2 more sophisticated ecosystem than WiMAX. In a Greenfield scenario for a level of data traffic, capital investment for LTE is potentially lower than for WiMAX m. The overall working cost for LTE and WiMAX base stations are probable parallel, the primary driver for lower capital investment in LTE is the potential for reduced equipment cost due to the economies of scale and full 3GPP integration. In addition to being the next mobile broadband technology for most operators worldwide, LTE offers the opportunity to deliver voice and compatibility with 2G/3G networks, which allows seamless roaming. However, voice-enabled handsets and multimodal devices are still a few years away for LTE. WiMAX can potentially offer voice, but WiMAX devices are currently lagging and may not proliferate at the same rate as LTE devices [5]. In a few years, we may find embedded LTE chips in a variety of consumer electronics, offering possible new business models for mobile operators. ARCHITECTURE OF LTE The fourth generation of wireless cellular systems has been a subject of interest for fairly a long time, since the formal definition of third generation systems was officially completed by the International Telecommunications Union Radio communication Sector (ITU-R) in By the growing demand for mobile broadband services with higher data rates and Quality of Service (QoS), 3GPP started working on two parallel projects, Long Term Evolution (LTE) and System Architecture Evolution (SAE). LTE/SAE, also known as the Evolved Packet System (EPS), represents a major step forward for the wireless industry that intend to grant a highly efficient, low-latency, packetoptimized, and more secure service. The chief design parameters of LTE-Advanced consist of OFDM (Orthogonal Frequency Division Multiplexing) and MIMO (Multiple-Input Multiple-Output) technique later research and dimensions confirmed that the system did not fully comply with ITU 4G requirements [6]. A. LTE-Advanced E-UTRAN Overview 3GPP consistent a new Evolved Packet System (EPS) architecture as part of the system architecture evolution work based on UMTS and GSM architecture evolution. LTE System Architecture includes LTE radio base stations called as E-UTRAN Node Bs, i.e. enodebs. 22 LTE consists of APs connected to one or multiple control plane Mobility Management Entities (MME) and user plane gateways. Mobility Management Entities (MME) is the Key Distributor in LTE. Both Mobility Management Entities and System Architecture Evolution gateways reside in the EPS network and connect to the APs through many to many S1 interface [7]-[8]. LTE is completely optimized for packet data access and it also supports quality of service (QoS) for exceptional data transfer needs (like browsing, download and VoIP etc). Evolved Universal Terrestrial Radio Access (E-UTRAN) is expected to support different types of services including web browsing, FTP, video streaming, VoIP, online gaming, real time video, push-to-talk and push-to-view. In Figure 1, the architecture of E-UTRAN for LTE- Advanced is shown. The core part in the E-UTRAN architecture is the enhanced Node Bs (enodebs), which offers the air interface with consumer plane and control plane protocol terminations towards the User Equipment (UE). Each of the enodebs is a logical element that serves one or several E-UTRAN cells, and the interface interconnecting the enodebs is called the X2 interface. In addition, Home enodebs (HeNodeBs, also called femtocells), which are enodebs of lower cost for indoor coverage enhancement, can be connected to the EPC directly or via a gateway that provides extra support for a large number of HeNodeBs. Further, 3GPP is considering relay nodes and sophisticated relaying strategies for network performance enhancement. The security perspective is enhanced by LTE and EPS compared to UMTS and GSM. FIG.1. LTE-ADVANCED E-UTRAN ARCHITECTURE

3 The aim of this new technology is increased coverage, better QoS performance, higher data rates and equality for different consumer [9][10]. B. LTE Interfaces LTE provides a simplified architecture compared to UTRAN, because macro diversity gains are not relevant in EUTRAN, and hence a centralized radio controller is not needed. Thus all decisions related to communication over the air interface are taken at a transmitting or receiving network node, making ultimate adaptation both to traffic and channel conditions possible. Control plane communication is executed as the application protocol over the S1- interface between the serving enodeb and the MME. User plane communication is executed as the transport protocol over the S1 interface between the enodeb and the serving gateway. In LTE, fast handovers are necessary because of the lack of macro diversity, which may cause the Signal to Interference plus noise ratio (SINR) suddenly decrease due to User Equipment moving at high velocity. Therefore, an interface called X2 is defined between the enodebs. An application protocol may be run over the X2 for handover preparation and execution, and to control transfer of the user plane packet buffers between the enodebs at handover [11]. Also Inter-cell Interference Coordination may be performed over X2. The signaling solution between enodebs appears much lighter compared to the control and reconfiguration of the transport by a centralized node. C. Evolved Packet System The architecture of Evolved Packet System (EPS) is drawn in the figure 2. The architecture the Evolved Packet System (EPS) consists of the following functional elements: FIG.2. EVOLVED PACKET SYSTEM 23 Mobility Management Entity (MME): It is the key control-node for the LTE access-network. The MME is accountable for idle mode User Equipment tracking and paging procedure including retransmissions. The MME is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a User Equipment at the initial attach and at time of intra-lte handover involving Core Network node relocation. It is also reliable for authenticating the user (by interacting with the HSS). The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the Home Subscriber Server (HSS) for roaming User Equipments. The Non Access Stratum (NAS) signaling terminates at the MME and it is also liable for generation and allocation of temporary identities to User Equipments. MME authenticate the authorization of the User Equipment to camp on the service provider s Public Land Mobile Network (PLMN) and enforces User Equipment roaming restrictions. It is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key organization. Packet Data Network Gateway (PGW): The Packet Data Network Gateway grants connectivity from the User Equipment to external packet data networks by being the point of exit and entry of traffic for the User Equipment. A User Equipment may have simultaneous connectivity with more than one PGW for accessing multiple Packet Data Networks. The PGW performs policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening. Another key role of the PGW is to act as the anchor for mobility between 3GPP and non-3gpp technologies such as WiMAX and 3GPP2. Serving Gateway (SGW): The Serving Gateway forwards user data packets and acting as the mobility anchor for the user plane during inter-enodeb handovers and as the anchor for mobility between LTE and other 3GPP technologies. For idle state User Equipments, the SGW terminates the downlink data

4 path and triggers paging when downlink data arrives for the User Equipment. It manages and stores User Equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception Home Subscriber Server (HSS): It is a fundamental information database that contains subscription-related and user-related data. The mobility management, call and session establishment support, user authentication and access authorization are the functions of the Home Subscriber Server. The Home Subscriber Server is based on pre-rel-4 Home Location Register (HLR) and Authentication centre (AuC). Access Network Discovery and Selection Function (ANDSF): The function of the ANDSF is to support the User Equipment to discover the access networks in their vicinity and to provide regulations or strategy to prioritize and manage associations to these networks. The ANDSF provides information to the User Equipment about connectivity to 3GPP and non-3gpp access networks (such as Wi-Fi). Evolved Packet Data Gateway (epdg): The most important function of the epdg is to secure the data transmission with a User Equipment connected to the EPC over an unprotected non-3gpp access. For this purpose, the epdg acts as a termination node of IPsec tunnels established with the User Equipment. THE MAIN TECHNOLOGIES OF LTE-A A. Carrier Aggregation Carrier aggregation will allow operators to combine as many as five Release-8 compatible carriers, thereby enhancing the data rates delivered to the end user without the need for contiguous frequency band allocations. So far this solution is limited to providing bandwidths with a maximum range of 100MHz, although this limit has only been imposed by foreseeable demand. Higher bandwidths could be easily supported, according to vendors. As with many network technologies, the benefits deliverable by carrier aggregation is restrained by the number of users in a cell. With high numbers of users the benefits are likely to be marginal, but if the number of users is low, then a large amount of bandwidth can be allocated to each one. There are also limits to carrier aggregation in the uplink, as devices will lack the power necessary to enable multi-carrier transmission. Carrier aggregation is likely to be one of the earliest deployed technical elements of LTE-A. B. Advanced MIMO Scheme MIMO is supported by 3GPP Releases 8 and 9 for LTE, for up to four transmit and reciever antennas in the downlink but only single antenna transmission in the uplink. In Release 10 this has been expanded to eight in the downlink, with uplink MIMO introduced with support for four transmit and eight receiver antennas. As it stands there are question marks over how practical it will be to have four antennas in the handset, however. Furthermore, according to NSN, it needs to be borne in mind that the Release 10 network will also be supporting Release 8 and 9 devices. The capacity gain from Release 10 downlink MIMO enhancements could be negative since new reference symbols create overhead for all devices. These overheads could be decreased by reducing the Release 8 and 9 specific reference symbols, the firm says, but this would prevent non LTE-A devices from operating in MIMO mode, thereby lowering the data rates they could access. C. Coordinated Multipoint Transmission (CoMP) Coordinated multipoint technology, based on Coordinated Scheduling/ Coordinated Beam forming, Joint Processing/Dynamic Cell Selection and Joint Processing/Joint Transmission, and was deemed insufficiently mature for inclusion in Release 10. Down the line it is seen as having the potential to improve cell edge performance and network capacity. D. Relay Nodes Making use of the LTE-A air interface for selfbackhaul, relay nodes allow for the deployment of small cells in locations where conventional backhaul technologies such as fixed line or microwave are unavailable or not viable. They are targeted at coverage limited environments, such as large macrocells, within the reach of which they would be used to boost coverage indoors or in urban canyons, for example. One particular benefit of relay nodes is that they are transparent to the user equipment, meaning there is no need for specialist technology within the device. E. Heterogeneous Self Organizing Network 24

5 The network architecture rather than a specific technology, the hetnet came to prominence at MWC The hetnet involves the widespread deployment of small cells among the wide area network and, in some anticipated scenarios, will function across a range of radio access technologies, with WIFI taking the strain from the cellular network in certain places, for example. NSN says that autonomous or automated interference coordination and handover optimization in such hierarchical network architectures are key aspects of heterogeneous networks. Self organizing networks will also play a role in the LTE-A environment. Some SON elements are already in existence, having first appeared in the femtocell space. But NSN says new developments to the concept are underway that will reduce cost of deployment and operation. ISSUE FOR DEPLOYMENT OF LTE With LTE already deployed in the US and Europe, the likelihood of availability is higher for the LTE chip than the WiMAX chip. In the last few years, there has been considerable progress in LTE technology and ecosystem development. They may even consider deployment of LTE in the 3G or BWA spectrum bands they plan to acquire in the auctions. Operators would need to balance their decisions with their specific needs. Mobile companies are leveraging its extensive expertise in mobile broadband innovation, including OFDM technologies (wi4 WiMAX), cellular networking (EVDOrA, HSxPA), IMS ecosystem, collapsed IP architecture, standards development and implementation, comprehensive services to deliver best-in-class LTE solutions [12]. LTE offers many advantages over competing technologies. However, in the Indian context there are several questions that need to be answered before LTE can become a credible alternative to 3G and WiMAX. Spectrum accessibility: Harmonization of the LTE spectrum in India stills lack clarity. No spectrum has yet been marked specifically for LTE. While the Indian government is expected to publish a discussion paper on 4G technologies soon, LTE auction is not slated until at least In the absence of dedicated LTE spectrum, operators may consider deployment in BWA (20 MHz of unpaired spectrum in 2.3 GHz) and 3G (paired spectrum of 2x5 MHz in 2.1 GHz) spectrum bands. Given the technology neutrality of these bands and the ability of LTE to work on different channel bandwidths in both FDD 25 and TDD4 modes, this should be feasible. In addition, approximately 120 MHz of spectrum in the 700 MHz band: an effective and cost efficient frequency band for LTE deployment could be used for LTE in the future. Gadget or appliance availability: Availability of affordable appliance is always a key driver of technology uptake. LTE enabled dongles and USB cards are already commercially available. However, it may be some time before the devices become affordable enough to gain critical mass in India. Compatibility of LTE devices with 3G and 2G will also be of utmost importance as it will ensure seamless roaming, and LTE data devices which are backward-compatible with HSPA and EDGE should start becoming available this year. High volumes in India may lead to economies of scale, and we could see a reduction in prices as well as common availability of 2G/3Gcompatible devices sooner than expected. However, LTE voice handsets are perhaps still two to three years away. Current voice overcrowding:though LTE has numerous advantages as a mobile broadband technology, any voice solution for it will take a few years or more to materialize. LTE will not serve the purpose of operators looking at 3G spectrum options to ease congestion on their current voice networks. These operators would have to incur incremental capital expenditures in 2G base stations to use 3G spectrum for LTE deployment. Scientific expansion:many operators worldwide have already committed to LTE and are actively preparing for deployments in the near future. There is an expectation that most Western operators on 3G will eventually move to LTE. However, there has been only limited commercial deployment of LTE to date. Hence, Indian operators need to be careful when considering their LTE deployment time line; given that LTE is still a relatively new technology. CONCLUSION In summary, LTE could be an attractive technology to launch on 3G spectrum for operators that can manage their voice needs through their current 2G spectrum. Also in regions, where 3G is not widely spread, LTE technology is expected to offer the next major upgrade. LTE-A helps in integrating the existing networks, new networks, services and terminals to suit the rising user demands. LTE will

6 clearly be the future technology for the wide area mobile data coverage in dense traffic areas globally. In this article, we discuss the most important characteristics of LTE; its simplified network architecture; evolved packet system in LTE; LTE interfaces; spectrum and bandwidth management. LTE is designed to provide significantly improved user experience, and will remain a strong competitor to other wireless technologies in the next decade for both developed and emerging markets. LTE reaches the targets set for IMT-Advancedin wide area evaluation scenarios. New releases of LTE-A, including technology components such as uplink MIMO and carrier aggregation, will improve LTE performance in local area scenarios, and factually realize a Gbit/s peak rate, often seen as a rate characteristic of 4G systems. REFFERENCES [1] Ian F. Akyildiz, David M. Gutierrez-Estevez, Elias Chavarria Reyes, The evolution to 4G cellular systems: LTE-Advanced Physical Communication 3 (2010) [2] E. Seidel, Initial thoughts on LTE-Advanced for 3GPP release 10, Nomor Research, May 2009 [Online] Available: or.pdf [3] 3GPP, Overview of 3GPP release 8 v.0.1.1, Tech. Rep., June [4] 3GPP, TR Relay architectures for E- UTRA (LTE-Advanced), Tech. Rep., April 2010 [Online] Available: [5] S. Sesia, I. Toufik, and M. Baker, LTE - The UMTS Long Term Evolution: From Theory to Practice, Wiley, 2009 [6] P. Lescuyer, T. Lucidarme, Evolved Packet System (EPS): The LTE and SAE Evolution of 3G UMTS, John Wiley, pages [7] 3GPP, TS , Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio access capabilities (Release 8) [8] 3GPP, TR Relay architectures for E- UTRA (LTE-Advanced), Tech. Rep., April [Online] Available: [9] Ian F. Akyildiz, David M. Gutierrez-Estevez, Elias Chavarria Reyes, The evolution to 4G cellular systems: LTE-Advanced Physical Communication 3 (2010) [10] Nokia Siemens Networks, Nokia, R single-stream proceeding for LTE-Advanced UL, Tech. Rep., February [11] Forsberg D, Leping Huang, Tsuyoshi K, Alanara S. Enhancing security and privacy in 3GPP E- UTRAN radio interface. In: IEEE 18th international symposium on Personal, Indoor and Mobile Radio Communications (PIMRC); p. 1 5 [12] Nokia, R study of UE arch. for LTE-A deployment scenarios, Tech. Rep.,March 2009 [Online] Available: GR4_50bis/Documents/R zip