LT3600 LTE-SAE Engineering Overview - V3.3

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(1)LTE/SAE Engineering Overview Course Code: LT3600. Duration: 2 days. Technical Level: 2. ... delivering knowledge, maximizing performance.... LTE courses include: . LTE/SAE Engineering Overview. . LTE Air Interface. . LTE Radio Access Network. . Cell Planning for LTE Networks. . LTE Evolved Packet Core Network. . 4G Air Interface Awareness. . Understanding Next Generation LTE. Wray Castle – leading the way in LTE training. www.wraycastle.com.

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(3) LTE/SAE ENGINEERING OVERVIEW. First published 2009 Last updated February 2012 WRAY CASTLE LIMITED BRIDGE MILLS STRAMONGATE KENDAL LA9 4UB UK. Yours to have and to hold but not to copy The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and your employer to court and claim heavy legal damages. Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs and Patents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior permission in writing of Wray Castle Limited. All of our paper is sourced from FSC (Forest Stewardship Council) approved suppliers. © Wray Castle Limited.

(4) LTE/SAE Engineering Overview. II. © Wray Castle Limited.

(5) LTE/SAE ENGINEERING OVERVIEW. CONTENTS Section 1. Introduction to LTE. Section 2. LTE OFDM Physical Layer. Section 3. LTE Higher-Layer Protocols. Section 4. Major Protocols. Section 5. Evolved Packet Core. Section 6. LTE Operation. Appendix. LTE Advanced. Glossary. © Wray Castle Limited. III.

(6) LTE/SAE Engineering Overview. IV. © Wray Castle Limited.

(7) LTE/SAE Engineering Overview. SECTION 1. INTRODUCTION TO LTE. © Wray Castle Limited. I.


(9) Introduction to LTE. CONTENTS LTE Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1 Broadband Access with LTE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2. Architecture Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.3 LTE Development and Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4 LTE Standards Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.5 LTE Key Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.6 Access Networks and the eNB (E-UTRAN Node B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7 X2 Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.8. X2 Interface Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9 X2 Deployment and Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.10 The EPC (Evolved Packet Core) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11 S1 Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.12. Evolved Packet Core ‘S’ Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.13. PDN Connectivity Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.14 NAS Identities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.15 EPS Area Identities Node Identifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.17. E-UTRA Protocols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.18. © Wray Castle Limited. III.


(11) Introduction to LTE. OBJECTIVES At the end of this section you will be able to:. . outline the evolutionary process prescribed for GSM and UMTS networks and show where LTE/SAE fits in. . explain the significance of LTE in the continued progression towards converging telecommunications and entertainment markets. . outline the overall performance aims for LTE. . identify the key air interface, radio access and core network technologies chosen for E-UTRA. . outline the basic architecture of the E-UTRAN and EPC including the eNB, the E-UTRAN interfaces and the EPC elements. . explain the role of the X2 interface in the E-UTRAN. . explain the role of the S1 interface and other possible S interfaces within the EPC. . outline the peak and average data rates that E-UTRA promises to supply and the range of services that could be carried. . describe the E-UTRA protocol stack and assign layer functions to the correct network entities. . describe the functional split between X2-U and X2-C interface variants. © Wray Castle Limited. V.

(12) LTE/SAE Engineering Overview. VI. © Wray Castle Limited.

(13) Introduction to LTE 1+ Gbit/s LTEAdvanced 100+ Mbit/s. LTE(4G) 40 Mbit/s UMTS/HSPA+ 10 Mbit/s UMTS/HSPA 1 Mbit/s EDGE Evolution. 400 kbit/s UMTS 150 kbit/s GSM/EGPRS 50 kbit/s GSM/GPRS. LTE Overview LTE (Long Term Evolution) represents the next developmental step for the 3GPP (3rd Generation Partnership Project) standards group. It provides for a continued evolutionary path from 2G GSM/GPRS (General Packet Radio Services), beyond 3G UMTS/HSPA and ultimately towards a 4G solution. UMTS (Universal Mobile Telecommunications System) has continued to build on the success of GSM (Global System for Mobile Communications) and momentum is gathering behind its significantly increased capability with the introduction of HSPA (High Speed Packet Access). The classic fixed and mobile telecommunications business models are undergoing enormous change with the move towards all-IP (Internet Protocol) switching and a total-communications service profile. Meanwhile, the last decade has seen the Internet develop into a serious business tool and fixed broadband access is fast becoming a basic commodity. This market landscape is ready for a technology that combines broadband capabilities with an efficient, scalable switching infrastructure and a flexible service delivery mechanism. LTE provides just such a solution and is designed to address growing global demand for anywhere, anytime broadband access while maintaining efficient provision of more traditional telecommunications services and maximizing compatibility and synergies with other communications systems. Although LTE most obviously represents an evolutionary path for UMTS networks it has also been designed to allow cost-effective upgrade paths from other technology starting points. For example, GSM operators now have the possibility to access 3G-like performance through EDGE (Enhanced Data rates for Global Evolution), and this in turn can be used as a direct pathway to LTE. Similarly, the interworking capabilities of the EPC (Evolved Packet Core) make it possible for CDMA (Code Division Multiple Access) to migrate radio access from 1x or 1xEV-DO (1x Evolution – Data Only) to LTE. Evolution beyond LTE has been mapped out by 3GPP with the specification of LTE-Advanced, which offers the possibility of downlink data rates (to stationary or low mobility users) of 1GBit/s or more.. Further Reading: 3GPP TS36.300 (LTE Radio Access), 23.401 (LTE Core Network) LT3600/v3.2. © Wray Castle Limited. 1.1.

(14) LTE/SAE Engineering Overview Broadcast content provider. Former ‘Fixed’ Operator. Former ‘Mobile’ Operator. New market opportunities. LTE Radio Access. LTE Radio Access. New market opportunities. Broadband Access with LTE Wide-area LTE radio access combined with the EPC represents a complete adoption of an all-IP architecture, offering broadband delivery capability with the potential for bit rates of several hundred megabits per second and QoS (Quality of Service) management suitable for real-time operation of highquality voice and video telephony. LTE has a very important role in the overall telecommunications service convergence concept. LTE could provide a key to unlocking a truly converged fixed/mobile network for the delivery of quadruple play services. Its potential bandwidth capabilities are sufficient for the support of services ranging from managed QoS real-time voice or video telephony to high-quality streamed TV. Its flat all-IP architecture means that it can act as a universal access network for a wide range of core network types.. 1.2. © Wray Castle Limited. LT3600/v3.2.

(15) Introduction to LTE. LTE. SAE. EPS UE E-UTRA. E-UTRAN. EPC. Architecture Terminology LTE is the term used to describe collectively the evolution of the RAN (Radio Access Network) into the E-UTRAN (Evolved Universal Terrestrial Radio Access Network) and the RAT (Radio Access Technology) into E-UTRA (Evolved Universal Terrestrial Radio Access). SAE (System Architecture Evolution) is the term used to describe the evolution of the core network into the EPC (Evolved Packet Core). There is also a collective term, EPS (Evolved Packet System), which refers to the combined E-UTRAN and EPC.. LT3600/v3.2. © Wray Castle Limited. 1.3.

(16) LTE/SAE Engineering Overview. LTE/SAE Design Aims 100 Mbit/s (downlink) and 50 Mbit/s (uplink) Increased cell edge bit rate 2-4 times better spectral efficiency Reduced radio access network latency Scalable bandwidth up to 20 MHz Interworking with 3G systems. LTE Development and Design Goals The debate about the structure and composition of LTE has been ongoing since at least 2004, with many different organizations promoting their preferred technological solutions for the systems. 3GPP brought some focus to the debate in June 2005 by publishing Technical Report TR 25.913 – Requirements for Evolved UTRA and UTRAN. TR 25.913 stated several objectives for the evolution of the radio interface and radio access network architecture. Targets included a significantly increased peak data rate, e.g. 100 Mbit/s (downlink) and 50 Mbit/s (uplink), and an increased ‘cell edge bit rate’ while maintaining the same site locations as are deployed for R99 (Release 99) and HSPA. Other objectives include significantly improved spectrum efficiency (e.g. two to four times that provided by Release 6 HSPA), the possibility for a significantly reduced radio access network latency for both C-plane and U-plane traffic, scaleable bandwidth with support for channel bandwidths up to 20 MHz, and support for interworking with existing 3G systems and non-3GPP-specified systems.. 1.4. © Wray Castle Limited. LT3600/v3.2.

(17) Introduction to LTE. Phase 1. Phase 2. Phase 2+. GSM900. GSM1800 GSM1900. Rel 96-98 GPRS EGPRS. Rel 99. Rel 4. Rel 5. Rel 6. Rel 7. GSM GPRS Rel 99 UMTS. Rel 8. Rel 9. EDGE. Rel 4. Rel 5. Rel 6. HSDPA IMS. HSUPA. Rel 7. Rel 8. Rel 10. Rel 11. Rel 12. GERAN enhancements. Rel 9. HSPA+. Rel 10. Rel 11. Rel 12. UTRAN enhancements. Rel 8. Rel 9. LTE/SAE. Rel 10. Rel 11. Rel 12. LTE-Advanced. LTE Standards Development Since the publication of the first GSM specifications in the late 1980s, the technologies and techniques employed by GSM networks have continually evolved and developed. GSM itself underwent a series of changes, from Phase 1 to Phase 2 and eventually to Phase 2+. Phase 2+ progressed with a series of yearly releases, starting with Release 96. The UMTS was introduced as part of Release 99 and from then onwards the 3GPP 3G network technology has also been undergoing a process of evolution. The evolutions that particularly affect the air interface are mainly contained in Releases 5, 6, 7 and 8. Releases 5 and 6 introduced HSPA – HSDPA (High Speed Downlink Packet Access) in R5 and HSUPA (High Speed Uplink Packet Access), or Enhanced Uplink, in R6. Release 7 outlines the changes necessary to deliver HSPA+ and Release 8 specifications begin to describe LTE – the Long Term Evolution of UMTS. Specification of LTE, generally described as 3.9G, was completed in Release 9. Specification of LTEAdvanced, a full 4G solution, is detailed in Release 10.. Further Reading: 3GPP Release Descriptions – www.3gpp.org/ftp/Information/WORK_PLAN/Description_Releases/ LT3600/v3.2. © Wray Castle Limited. 1.5.

(18) LTE/SAE Engineering Overview. LTE Signalling. LTE Traffic. SCTP. E-UTRA E-UTRAN. All-IP. EPC. LTE Key Technologies Tests and evaluations carried out during 2007 led to the publication of the Release 8 36-series of specifications, which began to detail the technological basis for LTE. Of the original four candidate air interface technologies, two were chosen for the final version: OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier FDMA). OFDMA is employed on the LTE downlink and is expected eventually to provide peak data rates approaching 360 Mbit/s in a 20 MHz channel. SC-FDMA is employed on the LTE uplink and may deliver up to 86 Mbit/s. SC-FDMA is also sometimes known as DFT-FDMA. In addition to the air interface technologies, LTE simplifies the range of technologies employed in other parts of the network. LTE is an ‘all-IP’ environment, meaning that all air interface, backhaul and core network interfaces will carry only IP-based traffic. The need to support different protocols for different traffic types, as was the case with R99, is therefore avoided. In this all-IP environment, layer 4 transport layer functions for signalling connections are performed using an alternative to the traditional choices, TCP (Transmission Control Protocol) or UDP (User Datagram Protocol). SCTP (Stream Control Transmission Protocol) was developed with the needs of IP-based signalling in mind and is used to manage and protect all LTE signalling services.. 1.6. © Wray Castle Limited. LT3600/v3.2.

(19) Introduction to LTE. Inter-cell RRM Radio bearer control Connection mobility control Radio admission control Measurement control Cell configuration Dynamic resource allocation. eNB. eNB (Evolved Node B). S1. Uu X2 RRC PDCP. S1. RLC MAC LTE UE. Evolved Packet Core. E-UTRAN. PHY. Access Networks and the eNB (E-UTRAN Node B) The basic building blocks of the E-UTRA access network are the eNB (Evolved Node B) plus backhaul – and nothing else. All layers of the air interface protocol stack, including the elements that previously resided in the RNC (Radio Network Controller) – RRC (Radio Resource Control), RLC (Radio Link Control) and MAC (Medium Access Control) – have been moved out to the base station. As the eNB now anchors the main backhaul link to the core network, it has also assumed responsibility for managing the PDCP (Packet Data Convergence Protocol) service, which provides header compression and ciphering facilities over the air interface. HSDPA began the process of moving RRM (Radio Resource Management) functions, such as packet scheduling, from the RNC to the Node B. In LTE, all remaining RRC functions are devolved to the eNB, meaning that there is no longer a role for a device such as the RNC. Among the RRM functions now devolved to the eNB are radio bearer control, radio admission control, connection mobility control and the dynamic allocation (via scheduling) of resources to UEs (User Equipments) in both uplink and downlink directions. Following on from innovations in R4 and R5 networks, LTE also supports the concept of flexible associations between access and core network elements, meaning that each eNB has a choice of MME (Mobility Management Entity) nodes to which to pass control of each UE. Dynamic selection of an MME for each UE as it attaches is therefore also an eNB responsibility. An eNB may be associated with MMEs belonging to different PLMNs (Public Land Mobile Networks), allowing for the easy creation of multioperator networks. The eNB also receives, schedules and transmits control channel information in its cells, including paging messages and broadcast system information, both of which are received from the MMEs. It retains many of the traditional roles associated with base stations, such as bearer management. It is responsible for routing U-plane traffic between each UE and its S-GW (Serving Gateway). The complexity of the eNB and of the decisions it is required to make are therefore much greater than for an R99 Node B The broadening of the range of services offered by the LTE EPS over time has led to the development of several specialised sub-types of eNB. Femtocell services, for example, are provided via HeNBs (Home eNBs), whilst LTE Relay facilities are offered by Relay Nodes and controlled by DeNBs (Donor eNBs). Further Reading: 3GPP TS36.300 LT3600/v3.2. © Wray Castle Limited. 1.7.

(20) LTE/SAE Engineering Overview X2-C X2–AP. SCTP IP Data link layer Physical layer. X2. X2-U User plane PDUs. GTP-U UDP IP Data link layer Physical layer. X2 Interface With the removal of the RNC from the access network architecture, inter-eNB handover is negotiated and managed directly between eNBs using the X2-C interface. In LTE implementations that need to support macro diversity, the X2-U interface will carry handover traffic PDUs (Protocol Data Units) between eNBs. X2-C (control plane) signalling is carried by the X2AP (X2 Application Protocol), which travels over an SCTP association established between neighbouring eNBs. X2AP performs duties similar to those performed by RNSAP (Radio Network Subsystem Application Protocol), which operates between neighbouring RNCs over the Iur interface in UMTS R99 networks. X2-U (user plane) traffic is carried by the existing GTP-U (GPRS Tunnelling Protocol – User plane), as employed in UMTS R99 networks. The facilities provided by the X2-U interface are only expected to be required if macro-diversity handover is supported. Both sub-types of the X2 interface travel over IP: SCTP/IP for the X2-C and UDP/IP for the X2-U.. 1.8. © Wray Castle Limited. LT3600/v3.2.

(21) Introduction to LTE X2 Interface eNode B. eNode B eNode B. eNode B. E-UTRAN IP Transport Network. eNode B. X2 Interface Architecture The X2 interface is designed to provide a logical signalling and traffic path between neighbouring eNBs. The term ‘neighbouring’ in this sense refers to eNBs that generate adjacent cells between which UEs would be expected to request handovers. The X2 interface is the functional successor to the UMTS Iur interface, which interconnects neighbouring RNCs. An eNB is only expected to support X2 interfaces to neighbouring sites with which there is a realistic possibility of handover events occurring; an individual eNB would not be required to support X2 interfaces to all eNBs in the network. Indeed, the X2 is an optional interface and all of its functions can be performed indirectly via the S1 and the MME/S-GW if direct connections are not supported.. Further Reading: 3GPP TS36.423, 36.300 LT3600/v3.2. © Wray Castle Limited. 1.9.

(22) LTE/SAE Engineering Overview. eNB eNB X2 connected directly. eNB. eNB. X2 routed via EPC. EPC Access Router. EPC IP Backbone. Logical S1 Logical X2 Physical eNB Transmission. MME. X2 Deployment and Routing If supported, logical X2 interfaces can be physically transported along either direct or indirect connections. A direct connection would require a point-to-point broadband connection to exist between the two related eNB sites. This option offers advantages in terms of resilience, in the sense that if multiple physical connections are supported the loss of one transmission link would not be catastrophic, but has disadvantages in terms of cost. If each eNB was expected to host connections to five or six neighbouring sites, for example, the costs associated with the additional transmission requirements could be unsustainably high. Another disadvantage of using direct connections to support X2 interfaces is lack of flexibility. The LTE E-UTRAN is designed to take advantage of a concept known as the SON (Self-Organizing Network). The optional SON functionality supported by the eNB allows it to attempt to establish an X2 interface connection automatically to any previously unknown local cells reported in UE measurements. Such automatic discovery and connection is only possible when all local eNBs are connected to the same common routing environment. Most X2 connections can be expected to share the eNBs core transmission link with the S1 interface. X2 traffic would then simply be routed back out towards the target eNB after arriving at a suitable E-UTRAN or EPC IP router. The benefit of this approach, which was the preferred method of carrying Iu-CS and Iu-PS connections between remote RNCs and the core network site in 3G networks, is that only one transmission link per eNB is required. The disadvantage is that only one transmission link per eNB is available, which introduces the potential for a lack of access network resilience. Operators may decide to deploy a combination of direct and indirect X2 routing, with some heavily used links between eNBs being provided with their own direct connections whilst other, less heavily used, connections are routed via the core.. Further Reading: 3GPP TS36.423, 36.300. 1.10. © Wray Castle Limited. LT3600/v3.2.

(23) Introduction to LTE HSS. NAS Security Idle State Mobility Handling. MME. PCRF. EPS bearer control. IP network. Internet Mobility Anchoring. S-GW. PDN-GW. EPC. The EPC (Evolved Packet Core) The reduced complexity in the RAN is mirrored by a similar reduction in the core network, where the EPC (Evolved Packet Core) structure consists of five main nodes, although others may be required for backwards-compatibility purposes. The MME handles control plane functions related to mobility management (authentication and security) and idle mode handling (location updates and paging), in which sense it is broadly analogous to the VLR (Visitor Location Register) or GMM (GPRS Mobility Management) functions found in legacy networks. The MME is also responsible for EPC bearer control, and so handles connection control signalling. The S-GW and PDN-GW (Packet Data Network Gateway) are broadly analogous to the SGSN (Serving GPRS Support Node) and GGSN (Gateway GPRS Support Node) found in R99 networks and perform user plane handling, switching/routing and interfacing functions. Unlike legacy systems, however, bearer control has been removed from these devices and resides with the MME. The PCRF (Policy and Charging Rules Function) is an optional network element. If deployed, it handles QoS (Quality of Service) and bearer policy enforcement and also provides charging and rating facilities. If a PCRF is not deployed then some of its functions can instead be performed by the PDN-GW. Subscriber management and security functions are handled by the HSS (Home Subscriber Server), which incorporates the functions of the legacy HLR (Home Location Register) and which is already familiar from R5 elements such as the IMS (IP Multimedia Subsystem). For backwards-compatibility purposes, SGSNs deployed to legacy parts of an operator’s network can be interfaced to both the MME (for mobility management) and the S-GW (for user plane flows). The MME then provides legacy systems with an interface to the HSS, and the S-GW and PDN-GW assume the role previously performed by the GGSN. The packet data services of legacy (GSM/GPRS, R99 and HSPA) networks and LTE/SAE systems can therefore interwork via a unified set of core network elements if required.. LT3600/v3.2. © Wray Castle Limited. 1.11.

(24) LTE/SAE Engineering Overview S1–AP. MME SCTP IP Data link layer. S1-MME. Physical layer. User plane PDUs. S1-U GTP–U UDP. S-GW. IP Data link layer Physical layer. S1 Interface Backhaul links to the core network are carried by the S1 interface. Following the general structure of the Iub interface which it replaces, traffic over the S1 is logically split into two types. S1-U flows carry user plane traffic and S1-MME flows carry mobility management, bearer control and direct transfer control plane traffic. Message structures for the S1-MME interface that operate between the eNB and the MME are defined by S1AP (S1 Application Protocol). The S1AP performs duties that can be seen as a combination of those performed by R99 RANAP (Radio Access Network Application Part) and GTP-C (GPRS Tunnelling Protocol – Control plane). To provide additional redundancy, traffic differentiation and load balancing, the S1- flex concept allows each eNB to maintain logical connections to multiple S-GWs and MMEs – there may therefore be multiple instances of the S1 interface per node. The S1-U interface employs GTP-U to create and manage user-plane data contexts between the eNB and the S-GW.. 1.12. © Wray Castle Limited. LT3600/v3.2.

(25) Introduction to LTE 2G/3G SGSN. HSS. UMTS/ GPRS. S6a. S3. UTRAN/ GERAN. S4. EIR PCRF. MME. S12. S13 S7/Gx. E-UTRA. Rx+. S11. S1-MME. IMS E-UTRAN. S5. S1-U. Interworking to MME. S–GW. SGi. IP Services. PDN–GW S2. WLAN or WiMAX. Evolved Packet Core ‘S’ Interfaces In addition to the S1 interface connecting the E-UTRAN to the EPC, a broader range of ‘S’ interfaces have been defined to identify interconnections between EPC nodes and external nodes. The gateways and the MME are the main new nodes in the EPC. They are interconnected via the S5 and S11 interfaces. The SGi interface provides a connection to the operator’s IP-based services. It is likely that this will include services managed through the IMS. In this respect the S6a interface connects the MME to the HSS, and the S7/Gx interface provides access from the PCRF to the PDN-GW. The S3 and S4 interfaces provide connectivity into the EPC from legacy 2G/3G SGSNs. However, the UTRAN may be connected directly to the EPC via the S12 interface. WLANs (Wireless Local Area Networks) or WiMAX (Worldwide Interoperability for Microwave Access) can be supported through the EPC via the S2 interface. This would require connectivity to the MME, which is provided by interfaces and interworking functions not shown in this diagram.. Further Reading: 3GPP TS23.401:4.2 LT3600/v3.2. © Wray Castle Limited. 1.13.

(26) LTE/SAE Engineering Overview. PDN Connectivity Service. Evolved Packet System EPS Bearer. PDN-GW Packet Data Network. PDN Connectivity Services The EPS is designed to provide IP connectivity between a UE and a PDN (Packet Data Network). The connection provided to a UE is referred to as a PCS (PDN Connectivity Service), sometimes known as just a PDN Connection. This consists of one or more EPS bearer that connect(s) the UE to an Access Point in a PDN-GW and traverses both the E-UTRAN and the EPC. The PDN-GW routes traffic between the EPS bearer and the external PDN. The EPS bearers, in turn, carry a Traffic Flow Aggregate that consists of one or more SDF (Service Data Flow) connections between the UE and external data services. If a UE requires additional connectivity that is only available via a different PDN-GW Access Point, then additional PDN Connectivity Services may be established in parallel.. Further Reading: 3GPP TS 23.401:4.7.1. 1.14. © Wray Castle Limited. LT3600/v3.2.

(27) Introduction to LTE. MCC. MNC. MMEI. M-TMSI. 24 bits. 32 bits. M-TMSI. GUTI. M-TMSI. 32 bits. MMEC. M-TMSI. 8 bits. 32 bits. S-TMSI. NAS Identities The main means of identifying EPS subscribers remains the IMSI (International Mobile Subscriber Identity), which is permanently assigned to a subscriber account. Temporary and anonymous identification of subscribers is provided by the GUTI (Globally Unique Temporary Identity), which is assigned by the serving MME when a UE has successfully attached and is reassigned if the UE moves to the control of a new MME. The GUTI is analogous to the legacy TMSI, but with the additional feature that its structure uniquely identifies not only the subscriber within the MME but also the MME that assigned it. The GUTI is constructed from the GUMMEI (Globally Unique MME Identifier), which identifies the MME, and the M-TMSI (MME Temporary Mobile Subscriber Identity). The M-TMSI is used to provide anonymous identification of a subscriber within an MME once that subscriber has been authenticated and attached. As with legacy TMSI use, the MME may elect to reissue the M-TMSI at periodic intervals and it will be reissued in any case if the UE passes to the control of a different MME. The GUMMEI is constructed from the MCC (Mobile Country Code), MNC (Mobile Network Code) and MME ID. The MME ID is subdivided into an MME Group ID and MMEC (MME Code). The MMEC is the MME’s index within its pool. The M-TMSI allows a subscriber to be uniquely identified within an individual MME, whereas the S-TMSI (SAE TMSI) allows subscribers to be identified within an MME group or pool. To achieve this, the S-TMSI simply adds the one-octet MMEC to the M-TMSI.. Further Reading: 3GPP TS 36.300 (E-UTRAN) and 24.301 (NAS) LT3600/v3.2. © Wray Castle Limited. 1.15.

(28) LTE/SAE Engineering Overview PLMN – MCC+MNC. MME Group ID (MMEGI). Tracking Area ID (TAI). Evolved Cell ID (ECGI) =. eNB ID + Cell ID. EPS Area Identities The EPS continues to use the PLMN identifier employed by legacy 3GPP systems, which consists of the MCC and the MNC. The MMEGI is a 16-bit identifier assigned to an individual MME Pool. The MMEGI only has to be unique within a PLMN. The TAI (Tracking Area Identifier) is analogous to the LA (Location Area) or RA (Routing Area) identifiers employed by the GERAN/UTRAN in that it is used to identify a group of cells in the access network. In the E-UTRAN the TA is the granularity with which each UE’s location is tracked. It is also the area within which a UE will be paged. The TAI consists of the network’s MCC and MNC followed by a TAC (Tracking Area Code). As in legacy systems it is necessary to be able to identify uniquely each cell in the network for call establishment, paging, handover and billing purposes. 3GPP has devised an updated Cell ID known as an ECGI (E-UTRAN Cell Global Identifier). The ECGI incorporates a unique eNB Identifier, which allows the S1 and X2 interface protocols to discover and identify the target nodes for functions such as EPS Bearer handover.. Further Reading: 3GPP TS 29.803, 23.401:5.2, 36.300:8.2. 1.16. © Wray Castle Limited. LT3600/v3.2.

(29) Introduction to LTE Gateway names/IP addresses Access Point Name (APN). MCC. MNC. MMEI. GUMMEI. 24 bits. MMEGI. 16 bits. MCC. MNC. eNB ID 20 bits. MMEC. MMEI. 8 bits. Cell ID. eCGI. 8 bits. Node Identifiers The MME is primarily a signalling node and each MME has to be accessible to and exchange control data with MMEs and other devices within its own network and in other networks elsewhere in the world. For this reason, each MME is assigned a unique and globally significant identifier known as a GUMMEI. The GUMMEI consists of the network’s MCC and MNC followed by a MMEI (MME Identifier), which in turn consists of the MMEGI and the MMEC. The MMEGI identifies the pool to which the MME belongs and the MMEC is its index within that pool. The addressing of S-GW and PDN-GW nodes follows the model for addressing legacy PS (Packet Switched) core network nodes – ultimately, each node will be identified by an IP address, which may or may not be backed up with a DNS-resolvable device name. The termination and anchor point for an EPS Bearer is an access point in a PDN-GW, which is analogous to a PDP Context terminating on GGSN APN in 2G/3G networks. Each PDN-GW AP is assigned an IP address associated with a DNS-resolvable name – the APN. The EPS ECGI is globally unique and allows individual cells to be separately identified. The ECGI is a 28-bit identifier which consists of the PLMN ID (MCC + MNC), a 20-bit eNB ID (which will be unique within a PLMN) and an 8-bit Cell ID (which will be unique within one eNB). This gives each PLMN scope to identify up to 1 million eNBs and for each eNB to control up to 256 cells.. Further Reading: 3GPP TS 23.401:5.2, 36.300 LT3600/v3.2. © Wray Castle Limited. 1.17.

(30) LTE/SAE Engineering Overview. User Equipment. eNB. Evolved Packet Core. Non-Access Stratum (NAS). Non-Access Stratum (NAS) RRC. RRC. PDCP. PDCP. RLC. RLC. MAC. MAC. Physical Layer. Physical Layer. E-UTRA Protocols In line with other aspects of E-UTRA, the air interface protocol stack has been designed to reduce complexity. Whereas an R99/HSPA-enabled Node B employs a protocol stack with a variety of RLC and MAC instances, an E-UTRA eNB employs a protocol stack with just one instance of each layer. The extent of the air interface protocol stack has also been reduced. In previous incarnations of UMTS some layers operated between the UE and the Node B, while most extended all the way to the RNC. With the elimination of the RNC, all air interface protocols in E-UTRA operate between the UE and the eNB.. 1.18. © Wray Castle Limited. LT3600/v3.2.

(31) LTE/SAE Engineering Overview. SECTION 2. LTE OFDM PHYSICAL LAYER. © Wray Castle Limited. I.


(33) LTE OFDM Physical Layer. CONTENTS Radio Carrier Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1 Spectral Efficiency in OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2 Resilience to Time Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3 Resilience to Multipath Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4 The OFDM Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5 The OFDM Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.6 Subcarrier Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7 OFDMA Resource Allocation Strategies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8. Channel Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.9 MIMO Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10 The Benefits of MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.11 Multi-User MIMO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12. Physical Layer Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.13 Channel Bandwidths and Subcarriers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.15. Frequency Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.16 Radio Channel Organization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.17. Modulation and Error Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.18 Physical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.19 The Physical Layer Timing Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.20 Type 1 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.21 Type 2 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.22 Resource Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.23 Summary of the Downlink Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.24 Summary of the Uplink Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.25. © Wray Castle Limited. III.


(35) LTE OFDM Physical Layer. OBJECTIVES At the end of this section you will be able to:. . describe an OFDM transmission system as a set of closely spaced orthogonal radio subcarriers. . illustrate the potential performance benefit for the use of OFDM as opposed to single carrier schemes. . identify typical performance characteristics of OFDM signals in multipath fading channels. . describe how channel adaptation can be used to enhance the performance of OFDM systems. . describe how scalability is achieved in OFDM systems. . describe the basic principles of MIMO operation. . identify the key benefits that can be gained from MIMO implementation. . outline the general structure of the E-UTRA physical layer. . define the term ‘bandwidth agnostic’ in the context of E-UTRA. . define the term ‘basic timing unit’ and its relevance in E-UTRA. . describe the configuration of downlink and uplink frames and list the range of frame types employed. . describe the resource allocation models employed by E-UTRA including the role of the resource block, resource grid and resource element. . list the modulation and error coding options made available in E-UTRA. . outline the function of the reference signal. . describe the functions of the E-UTRA physical channels on both uplink and downlink. . outline how control and traffic channels are mapped into the physical layer structure. © Wray Castle Limited. V.

(36) LTE/SAE Engineering Overview. VI. © Wray Castle Limited.

(37) LTE OFDM Physical Layer Spacing to next allocated carrier needs to be large. f1. f1. 5 kHz. f2. Radio Carrier Orthogonality Consider a radio carrier being modulated by a 10 kbit/s bit steam using QPSK (Quadrature Phase Shift Keying). It could be expected to see a spectral envelope following a (sin x)/ x function, as shown in the diagram, with the first null located 5 kHz from the centre frequency. In a classic FDM (Frequency Division Multiplexing) system, other radio carriers would be allocated and spaced far enough away from the first to ensure minimal adjacent channel interference. The size of the guard band required would depend on the transmitter and receiver characteristics as well as the relative powers. However, in such a system it is assumed that there is no synchronization between the potential interferers. It is this that leads to the need for large frequency spacing between adjacent carriers. In fact, if there was synchronization between adjacent channels, a much smaller frequency spacing could be used. The key is to be able to make use of the complex nature of the instantaneously transmitted spectrum. The modulation envelope is only an artificial way of indicating all possibilities over time; a snapshot at an instant in time would look different. Consider a second radio carrier allocated such that its centre frequency coincides exactly with the null in the first carrier’s envelope. It is using the same modulation scheme and carrying the same data rate. The result is as shown. Note that the carrier spacing of 5 kHz is the same magnitude as the symbol rate of 5 ksps. The spectra of the two carriers now overlaps, but as long as the carrier frequencies and the baseband data remain accurately synchronized, both can be demodulated successfully. The reason is that this relationship between centre frequency offset and symbol rate maintains a high level of orthogonality between the two radio carriers.. LT3600/v3.2. © Wray Castle Limited. 2.1.

(38) LTE/SAE Engineering Overview Centre frequency 2 QPSK subcarriers 10 kbit/s per subcarrier 15 kHz total bandwidth. f1. f2. 15 kHz Centre frequency 1 QPSK carrier 20 kbit/s 20 kHz total bandwidth. f1 20 kHz. Spectral Efficiency in OFDM Considering again the two overlapping QPSK radio carriers, it can be seen that there is a relatively large spectral efficiency gain. If the effective bandwidth of the transmitted signal is considered to be the frequency separation of the first nulls then a single QPSK carrier modulated with 10 kbit/s would have a null-to-null bandwidth of 10 kHz. However, here there are two subcarriers, each of which is carrying 10 kbit/s using QPSK. Their respective null-to-null spectra overlap by 5 kHz. This gives a collective null-to-null bandwidth for the pair of subcarriers of 15 kHz. Thus QPSK is being used to carry 20 kbit/s in a radio bandwidth of 15 kHz. Note that a single QPSK modulated carrier carrying 20 kbit/s would result in a null-to-null bandwidth of 20 kHz. The principle of independent reception of orthogonal radio carriers with overlapping spectrum can be extended by using a large number of narrowband radio carriers within one wideband channel allocation. This results in a very spectrally efficient channel that can carry high bit rates. For example, if 1000 orthogonal radio carriers were modulated using QPSK, each carrying 10 kbit/s, the net throughput for the channel would be 10 Mbit/s. This would require a total channel bandwidth of slightly more than 5 MHz. Carrying the same bit rate with QPSK modulation onto a single radio carrier would require a null-to-null bandwidth of 10 MHz. Thus OFDM (Orthogonal Frequency Division Multiplexing) almost doubles the spectral efficiency. Moreover, the resulting OFDM transmission is more resilient to multipath effects in the channel.. 2.2. © Wray Castle Limited. LT3600/v3.2.

(39) LTE OFDM Physical Layer. Low bit rate parallel streams. High-bit-rate serial stream S to P. Guard period. Useful symbol period Multipath 1 Multipath 1. Multipath 1. Resilience to Time Dispersion Spectral efficiency is not the only benefit associated with using OFDM. It also exhibits good tolerance to the effects of multipath propagation in the channel; both fading and time dispersion. Because the data rate on individual subcarriers with the channel is very low, the symbol period is correspondingly long. The resulting symbol period is typically significantly longer than the time dispersion that occurs in the channel. This means that relatively simple equalization can be used to counteract multipath even though the net rate in the whole channel is very high. Furthermore, a guard period can be inserted in every symbol period that covers the expected time dispersion for the channel. This removes most of the time dispersion distortion from the useful symbol period. This guard period is usually created by repeating a copy of the last part of the symbol at the start. In this case it is referred to as the cyclic prefix.. LT3600/v3.2. © Wray Castle Limited. 2.3.

(40) LTE/SAE Engineering Overview. Resilience to Multipath Fading Tolerance to multipath fading effects comes from the overall wideband characteristic in the channel. A narrowband channel tends to exhibit flat fading characteristics; that is to say, the fading characteristics are coherent across the whole channel bandwidth. The effects of this can be seen in the diagram. OFDM channels, on the other hand, are usually used to carry very high data rates and therefore require many subcarriers occupying a relatively large bandwidth. In most cases the bandwidth will exceed the coherence bandwidth by a large factor, so differing fading characteristics will be seen in different parts of the channel. In effect, the wide channel provides a degree of frequency diversity with a resulting improvement in performance. However, it would be wrong to assume that this benefit for OFDM results solely because the channel bandwidth is wide. A single carrier system with the same bit rate would also result in a wide radio channel. Therefore, a single carrier system also benefits from this form of frequency diversity to some extent. In the single channel system, energy from each symbol will be spread across the whole radio channel and each symbol will therefore suffer some distortion from any fading that may occur in any one part of the channel. In an OFDM system only those symbols transmitted on subcarriers in the part of the channel affected by a fade will be distorted. Symbols transmitted on other subcarriers will remain unaffected. It is then possible to adapt the subcarriers in use according to the varying fading characteristics. This means that only non-fading carriers will be used.. 2.4. © Wray Castle Limited. LT3600/v3.2.

(41) LTE OFDM Physical Layer. OFDM signal with N subcarriers {b 0 , b1 , b2 …bn }. Serial data. S. P. M-ary M-ary symbol symbol N bit parallel mapping grouping streams. I (real) N complex N N-point samples in one sine cosine complex IFFT symbol period symbols Q (imaginary). Up-conversion. D/A. fc. The OFDM Transmitter The diagram shows a block representation of the transmitter that brings together the elements of symbol mapping for QAM (Quaderature Amplitude Modulation) and the application of the IFFT (Inverse Fast Fourier Transform) in order to produce an OFDM signal. The serial data to be carried on the radio link is first passed through a serial-to-parallel conversion process. The number of parallel streams will be equivalent to the number of data-carrying subcarriers in the system. This number will usually be a power of two since this makes best use of the efficiencies offered by the IFFT. Bits on the parallel data streams will also be grouped as appropriate for the symbol constellation of M-ary QAM scheme in use. For example, for QPSK bits are grouped in pairs; for 16QAM they are grouped in fours and for 64QAM they are grouped in sixes. The next process is symbol point mapping for the bit groups on each parallel data stream. The resulting complex number symbols then form the input to an N-point IFFT where N will be a power of two equivalent to the number of subcarriers in use. The output of the IFFT will be a series of complex number digital samples representing the OFDM signal during each symbol period. At this point the cyclic prefix is added by copying the last samples onto the beginning of the symbol period. These complex real and imaginary sample values are used to form the I and Q symbol streams. Next, the I and Q branches are subsequently multiplied onto sine and cosine representations of the radio carrier. This generates a digital representation of the required multicarrier M-ary QAM modulated transmit signal. After digital-to-analogue conversion the resulting signal can be up-converted to the required channel centre frequency before amplification and transmission.. LT3600/v3.2. © Wray Castle Limited. 2.5.

(42) LTE/SAE Engineering Overview. OFDM signal with N subcarriers {b 0 , b1 , b2 …bn }. Down-conversion. A/D. fc. I (real) N complex sine samples in one N-point cosine symbol period FFT Q (imaginary). Integration and N symbol complex symbols decisions. N parallel streams. Serial data P. S. The OFDM Receiver The filtered OFDM signal is down-converted and then sampled for analogue to digital conversion. The sampling rate at this point will be factored to allow for the inclusion of the cyclic prefix. The cyclic prefix is removed and the sampled signal is separated into I and Q components. The result is a series of complex samples that are used as the input to the FFT. The FFT deconstructs the complex waveform in the symbol period to N complex values, each representing a modulation symbol on one of the subcarriers. M-ary demodulation by integration and reverse symbol mapping is performed to recover groups of bits represented by each of the received M-ary modulation symbols. Finally, parallel-to-serial conversion reconstructs that original serial bit stream.. 2.6. © Wray Castle Limited. LT3600/v3.2.

(43) LTE OFDM Physical Layer. Data subcarriers. Reference/pilot subcarriers. Upper unused/guard Subcarriers. Lower unused/guard Subcarriers. DC subcarrier. Subcarrier Assignment Different subcarriers from across the population of subcarriers created by an OFDM channel are assigned to different functions. Most subcarriers will be assigned to carry modulated user data signals. Each data subcarrier will be modulated to carry one part of the entire parallel signal being transmitted across the multi-tone channel. The data rate of each data subcarrier is determined by a combination of the symbol rate and the modulation scheme employed. In some variants of OFDM (such as that employed by WiMAX), entire subcarriers are given over to carrying ‘pilot signals’. Pilot subcarriers allow channel quality and signal strength estimates to be made and allow other control functions, such as frequency calibration, to operate. Pilots are generally transmitted at a higher power level than data subcarriers – typically 2.5 dB higher – which serves to make them more easily acquired by receiving stations. In LTE and other systems, including DVB (Digital Video Broadcasting), the same function is performed by ‘reference signals’. A reference signal, like a pilot, allows a receiving station to recalibrate its receiver and make channel estimates, but instead of occupying an entire subcarrier it is periodically embedded in the stream of data being carried on a ‘normal’ subcarrier. There are also two types of ‘null’ subcarrier – guards and the DC carrier. Nothing is transmitted on null subcarriers. Guard subcarriers separate the top and bottom data subcarriers from any adjacent channel interference that may be leaking in from neighbouring channels and, in turn, serves to limit the amount of interference caused by the OFDM channel. The more guard subcarriers that are assigned, the lower the amount of adjacent channel interference that will be caused or detected, but this also has an impact on the data throughput of the channel. The centre subcarrier of each OFDM channel – the one that has a 0 Hz offset from the channel’s centre frequency – is known as the ‘DC carrier’ and is also null.. LT3600/v3.2. © Wray Castle Limited. 2.7.

(44) LTE/SAE Engineering Overview Symbol periods (time) 0. OFDM with time multiplexing. 1. 2. User 1. 3. 4. 5. 6. 7. 8. User 2. 9. User 4. Symbol periods (time) 0. 1. 2. 3. 4. 5. 6. 7. 8. 9. User 7. User 1 User 3. OFDM with time and frequency multiplexing (OFDMA). User 8 User 2. User 4 User 9. OFDMA Resource Allocation Strategies The simplest option for multiple access in an OFDM system is to use a form of time multiplexing on the OFDM radio bearer. This is illustrated in the top part of the diagram. Each user is allocated the full channel bandwidth and all data subcarriers exclusively for a defined number of symbol periods. The greatest efficiency can be achieved if dynamic time allocation is applied so that users with higher bit rate requirements are allocated a greater proportion of time. However, in such a system the minimum resource allocation is one OFDM symbol. Even with dynamic time allocation, such an arrangement can still become very inefficient when there is strong demand for multiple lower bit rate connections, for example when multiple voice circuits are active. Consider an OFDM system operating in a 10 MHz bandwidth, with a 512-point FFT and using 16QAM. Allowing for null and reference subcarriers, such a system could transfer in the order of 1,600 bits in a single OFDM symbol period. This may seem a modest resource unit, but delay requirements must also be accounted for. For a real-time service such as voice it is essential to avoid excessive round-trip delay. To meet the delay requirement for a voice service, resources may need to be allocated, for example once every 20 ms. This would mean in a minimum bandwidth allocation to one user of 80 kbit/s (or 120 kbit/s if 64QAM is in use). Even allowing for the error protection overhead this minimum resource will significantly reduce system efficiency and its ability to benefit from optimal techniques such as discontinuous transmission and channel adaptation. Greater efficiency in resource allocation can be gained from the use of subchannelization. This involves division of resource by time and by frequency. Thus a user may be allocated a subset of the subcarriers available in the system, as illustrated in the lower part of the diagram. This approach allows much finer granulation in resource allocation and therefore greater efficiency. OFDM systems that support this are usually described as OFDMA systems.. 2.8. © Wray Castle Limited. LT3600/v3.2.

(45) LTE OFDM Physical Layer. Poor Radio Path. Node B. Interference. Modulation (QPSK/16QAM/64QAM) Error protection coding rate Adaptive fast scheduling. Channel Adaptation The quality of the radio link is affected by many factors including fading, interference and time dispersion. Terrestrial mobile radio channels, which are usually assumed to be non-line of site, can be very poor. Therefore most terrestrial cellular radio systems are designed with robust modulation schemes and large error protection overheads. However, close examination of real channel conditions shows them to be very variable in short time frames, and much of the time any given channel will show good performance. Thus the standard approach engineers the channel to deal with the worst case, which only occurs for a small amount of time. It is clear that if the channel could be adapted at a rate fast enough to track changing channel conditions then the average performance of a channel could be significantly improved. This is the principle of channel adaptation. Channel adaptation is a common approach in many broadband radio systems and in most cases involves the adaptation of the modulation scheme and the error protection overhead applied. Adaptive scheduling can also be very effective, enabling the cell to make the best use of the pool of channels allocated to different mobiles, each of which will be varying independently.. LT3600/v3.2. © Wray Castle Limited. 2.9.

(46) LTE/SAE Engineering Overview. Stream 1. Data stream Stream 2 mapping. Layer 1. Precoding matrix. Signal generation. MIMO decoding and channel estimation. Layer 2 Feedback. Power weightings and beamforming. 2x2 MIMO or Rank 2. 4x4 MIMO or Rank 4. MIMO Concept MIMO (Multiple Input Multiple Output) antenna arrays offer significant performance improvements over conventional single antenna configurations. The technique involves placing several uncorrelated antennas at both the receiving and transmitting ends of the communication link. If there are four uncorrelated antennas at the transmitter and a further four uncorrelated antennas at the receiver, then there will be 16 possible direct radio paths between the transmitter and the receiver. Each of these is open to multipath effects, creating even more radio paths between the transmitter and the receiver. These radio paths can then be constructively combined, thus producing micro diversity gain at the receiver. Since the receiver can distinguish between the various uncorrelated antennas, it is possible to transmit different data streams in different paths. The stream applied to each antenna can be referred to as a ‘layer’ and the number of antennas available at the transmitter and receiver can be referred to as ‘rank’. For example, a system operating with a 4x4 MIMO antenna array can be described as having four layers and being of rank four. The way in which data streams are mapped to layers will change the specific benefits offered by a particular MIMO implementation, and the specification of this is an important part of system design. Pre-coding may also be used to improve the MIMO system performance. Pre-coding may be adaptive and as such would be based on some source of channel estimation. This could be derived at the transmission or the reception end of the link. It is relatively easy to mount antennas on the base station in an uncorrelated manner. For a 2x2 MIMO array a single cross-polar panel could be used. A 4x4 MIMO array would require two cross-polar polar panels with suitable space separation. This is harder to achieve in a mobile. However, as for the base station, 2x2 MIMO could be achieved with cross polarization, but this could result in some undesirable directivity in the antenna.. 2.10. © Wray Castle Limited. LT3600/v3.2.

(47) LTE OFDM Physical Layer MIMO brings. Diversity gain. Array gain. Spatial multiplexing gain. Decorrelates fading through different transmission paths. Provides a beamforming effect that focuses radiated energy in the direction of the receiver. Enables multiple data streams to be transmitted on the same frequency/time resource. The Benefits of MIMO MIMO is potentially a complex technology but it can provide very significant benefits in system capability. There are three key ways in which MIMO improves system performance. Any given MIMO implementation may make use of all these benefits or may be configured to take particular advantage of one of them. Ideally, a system should be designed with sufficient flexibility in MIMO implementation to allow a system operator to choose the most suitable implementation for different environments or system goals. Diversity gain arises out of the provision of multiple antennas at the transmitting and/or receiving end of the radio link. This creates multiple transmission paths with decorrelated fading characteristics. The result is an overall improvement in channel signal-to-noise ratio leading to increased channel throughput and reliability. Array gain refers to the beamforming capability of a multiple antenna array. With suitable signalling of feedback from the receiver, or with measurements made on a return link, it is possible to direct radiated energy toward the receiver in a steered beam. The result is improved channel performance and increased throughput. Spatial multiplexing gain arises out of the orthogonality between the multiple transmission paths created by the multiple antenna array. Since the receiver can resolve independent transmission paths it is possible to map different information streams into the transmission paths, identifiable by their spatial signature. This results in a direct increase in the channel throughput in proportion to the number of separate transmission streams used.. LT3600/v3.2. © Wray Castle Limited. 2.11.

(48) LTE/SAE Engineering Overview SU-MIMO. MU-MIMO. Multi-Cell MU-MIMO. Multi-User MIMO The basic implementation of MIMO is generally referred to as SU-MIMO (Single-User MIMO). The SU-MIMO concept can be developed into MU-MIMO (Multi-User MIMO). In this case the spatial multiplexing capability of MIMO is used to multiplex a link to more than one mobile using the same time/frequency resource. The order of multiplexing available depends on the number of antennas (or rank) available at the transmitter and receiver ends of the link. For example, the diagram shows a 2x2 MIMO arrangement being used for MU-MIMO with two mobiles. In this case, the rate available to each mobile would be lower than that potentially available to a single mobile with an SU-MIMO configuration, but both mobiles are allocated the same time/frequency resource and still have the potential for diversity and array gain. Thus cell capacity is increased, but the resource can be shared between a larger number of users. The use of more than one transmitting or receiving station in this way is sometimes called virtual MIMO. It is also possible to implement MU-MIMO in one direction only with just single antennas on each of the mobiles. In this case, array and diversity gain would be reduced, but time/frequency resources can still be reused in the cell. MU-MIMO can be further developed into multi-cell MU-MIMO. In this case the data streams are mapped to the combined antenna resources of two or more base stations that provide a combined connection to multiple mobiles in multiple cells. The scenario in the diagram is in effect 4x4 MIMO but shared between two connections. Note that spatial diversity will be significant in such a scenario because of the geographical separation of the base station and of the mobiles.. 2.12. © Wray Castle Limited. LT3600/v3.2.

(49) LTE OFDM Physical Layer OFDM and SC-FDMA Bandwidth agnostic TDD and FDD SFN for MBMS MIMO operation Physical channel structure Reference signals Modulation and coding Synchronization and timing Error coding and HARQ. RRC Logical channels. MAC. Random access Power control Reporting and feedback Measurements Handover. Transport channels. Physical Layer Physical channels. Physical Layer Functions To support asymmetric services and to promote longer battery life for mobile terminals, the E-UTRA physical layer employs different technologies on the uplink and downlink: OFDMA and SC-FDMA respectively. The functions of the E-UTRA physical layer can be summarized as follows:          .  . creation and management of the uplink and downlink physical channels modulation (BPSK (Binary Phase Shift Keying), QPSK, QAM) and error coding creation of reference signals in both uplink and downlink management of the RACH (Random Access Channel) OFDMA signal generation in the downlink and SC-FDMA signal generation in the uplink modulation and up conversion synchronization procedures, including cell search procedure and timing synchronization power control procedures management of CQI (Channel Quality Indication) reporting and MIMO feedback physical uplink shared channel-related procedures, including UE sounding and HARQ (Hybrid Automatic Repeat Request) ACK/NACK detection reporting of measurement results to higher layers and the network handover measurements, idle-mode measurements, etc.. LT3600/v3.2. © Wray Castle Limited. 2.13.

(50) LTE/SAE Engineering Overview OFDM and SC-FDMA Bandwidth agnostic TDD and FDD SFN for MBMS MIMO operation Physical channel structure Reference signals Modulation and coding Synchronization and timing Error coding and HARQ. RRC Logical channels. MAC. Random access Power control Reporting and feedback Measurements Handover. Transport channels. Physical Layer Physical channels. Physical Layer Functions (continued) E-UTRA supports services in a variety of channel bandwidths. In fact, the specification explicitly labels E-UTRA as ‘bandwidth agnostic’, meaning that it has no rigidly defined or preferred channel bandwidth and can be scaled to channels of almost any size. Both FDD and TDD modes are supported, as is a ‘half duplex’ mode. E-UTRA has also been designed to work as the bearer for Multicast and Broadcast Multimedia Services (MBMS) and as such includes support for SFN (Single Frequency Network) operation. Support for advanced antenna configurations has also been designed into the specification with MIMO and beam-forming adaptive antennas both being referenced.. Further Reading: 3GPP TS 36.211. 2.14. © Wray Castle Limited. LT3600/v3.2.

(51) LTE OFDM Physical Layer. 20 MHz/1200 15 MHz/900 10 MHz/600 5 MHz/300 3 MHz/180 1.4 MHz/72. Channel bandwidths (bandwidth/subcarriers). Channel Bandwidths and Subcarriers E-UTRA/LTE is designed to work in a variety of bandwidths ranging initially from 1.4 MHz to 20 MHz. As E-UTRA is described as being ‘bandwidth agnostic’, other bandwidths, ones that allow E-UTRA to be backwards compatible with channel allocations from legacy network types, for example, could be incorporated in the future. The version of OFDMA employed by E-UTRA is similar to the versions employed by WiMAX or DVB, but with a few key differences. In systems such as WiMAX, OFDMA schemes occupying different channel bandwidths employ different subcarrier spacing, meaning that there is a different set of physical layer parameters for each version of the system. The E-UTRA scheme allows for two fixed subcarrier spacing options, 15 kHz in most cases, with an optional 7.5 kHz spacing scheme, only applicable for TDD (Time Division Duplex) operation and intended for in very large cells in an SFN. Fixing the subcarrier spacing reduces the complexity of a system that can support multiple channel bandwidths.. Further Reading: 3GPP TS 36.211, 36.101:5.5, 36.104:5.5 LT3600/v3.2. © Wray Castle Limited. 2.15.

(52) LTE/SAE Engineering Overview. FDD. TDD. Band. UL Range (MHz). DL Range (MHz). Band. UL/DL Range (MHz). 1 2 3 ... 7 8 ... 13 ... 20 ... 24. 1920 – 1980 1850 – 1910 1710 – 1785 ... 2500 – 2570 880 – 915 ... 777 – 787 ... 832 – 862 ... 1626.5 – 1660.5. 2110 – 2170 1930 – 1990 1805 – 1880 ... 2620 – 2690 925 – 960 ... 746 – 756 ... 791 – 821 ... 1525 – 1559. 33 34 35 36 37 38 39 40. 1900 – 1920 2010 – 2025 1850 – 1910 1930 – 1990 1910 – 1930 2570 – 2620 1880 – 1920 2300 – 2400. Frequency Bands There is considerable regional variation in the availability of spectrum for LTE operation and this is reflected in the standards. Along with flexibility in bandwidth there is considerable flexibility for spectrum allocation. There are no requirements for minimum band support nor for band combinations. It is assumed that this is determined by regional requirements. The standards identify a range of bands for FDD operation, ranging from frequencies of approximately 700 MHz through to frequencies in the range 2.7 GHz+. There also eight bands identified for TDD operation ranging from approximately 1900 MHz to 2.6 GHz. Considerable scope has been left in the standards to add more frequency bands as global requirements evolve.. Further Reading: 3GPP TS 36.101; 5.5, TS 36.104; 5.5. 2.16. © Wray Castle Limited. LT3600/v3.2.

(53) LTE OFDM Physical Layer 12 subcarriers. Channel bandwidth (MHz) Transmission bandwidth configuration (n x RB) Transmission bandwidth (n x RB). EARFCN (100 kHz raster). Radio Channel Organization For both uplink and downlink operation subcarriers are bundled together into groups of 12. This grouping is referred to as an RB (Resource Block). The RB also has a dimension in time and when this is combined with the frequency definition it forms the basic unit of resource allocation. The number of resource blocks available in the system is dependent on channel bandwidth, varying between 100 for 20 MHz bandwidth to just six for 1.4 MHz channel bandwidth. The nominal spectral bandwidth of an RB is 180 kHz for the standard 15 kHz subcarrier spacing. Note that this means there is a difference between the stated channel bandwidth and the transmission bandwidth configuration, which is expressed as n x RB. For example, in a 5 MHz channel bandwidth the transmission bandwidth would be approximately 4.5 MHz. This difference acts as a guard band. OFDMA channels are allocated within an operator’s licensed spectrum allocation. The centre frequency is identified by an EARFCN (E-UTRA Absolute Radio Frequency Channel Number). The precise location of the EARFCN is an operator decision, but it must be placed on a 100 kHz raster and the transmission bandwidth must not exceed the operator’s licensed spectrum.. Further Reading: 3GPP TS 36.101:5.6, 5.7; 36.104:5.6, 5.7 LT3600/v3.2. © Wray Castle Limited. 2.17.

(54) LTE/SAE Engineering Overview. Modulation Schemes. BPSK. Signalling functions only. QPSK 16QAM 64QAM. Error Coding Schemes. 1/3 Turbo Coding 1/3 CC. CRC. Optional on uplink. Traffic and most control channels BCH only. Transport Block. 24 bit CRC. Modulation and Error Protection The range of modulation schemes used in E-UTRA comprises BPSK, QPSK, 16QAM (16-state Quadrature Amplitude Modulation) and 64QAM (64-state Quadrature Amplitude Modulation). BPSK is only employed for a limited set of signalling and reference functions, while 64QAM is optional on the uplink. The range of error coding options used in E-UTRA devices is far more limited than those available to, for example, a UMTS device. For most channels the only option is one-third rate turbo coding based on convolutional coding. Broadcast traffic channels are only permitted to use 1/3 Tail Biting convolutional coding. Various control channels have been assigned either convolutional coding, block coding or simple repetition as their error coding options. In addition to error coding, transport blocks containing user and control traffic may also optionally have a CRC (Cyclic Redundancy Check) block attached. Transport blocks on connections that have CRC selected have a 24-bit CRC block appended to the end of the data container. The familiar UMTS error monitoring levels of Bit Error Rate (BER), derived from the error coding service, and BLER (Block Error Rate), derived from CRC, continue to be available in E-UTRA.. Further Reading: 3GPP TS 36.211, 36.212, 36.300. 2.18. © Wray Castle Limited. LT3600/v3.2.

(55) LTE OFDM Physical Layer. MAC. BCCH. PCCH. CCCH. DCCH. DTCH. MAC Control. Physical layer. BCH. PBCH. PDCCH. PCH. RACH. PUCCH. PHICH. PCFICH. DL-SCH. PRACH. UL-SCH. PDSCH. PUSCH. Physical signals PSS/SSS Reference signals. Physical Channels The physical layer involves the transmission and reception of a series of physical channels and physical signals. The physical signals relate to the transmission of reference signals, the PSS (Primary Synchronization Signal) and the SSS (Secondary Synchronization Signal). The PBCH (Physical Broadcast Channel) carries the periodic downlink broadcast of the RRC MasterInformationBlock message. Note that system information from BCCH (Broadcast Control Channel) is scheduled for transmission in the PDSCH (Physical Downlink Shared Channel). The PDCCH (Physical Downlink Control Channel) carries no higher layer information and is used for scheduling uplink and downlink resources. Scheduling decisions, however, are the responsibility of the MAC layer, therefore the scheduling information carried in the PDCCH is provided by MAC. Similarly the PUCCH (Physical Uplink Control Channel) is used to carry resource requests from UEs that will need to be processed by MAC. The PHICH (Physical Hybrid ARQ Indicator Channel) is used for downlink ACK/NACK of uplink transmissions from UEs in the PUSCH (Physical Uplink Shared Channel). It is a shared channel and uses a form of code multiplexing to provide multiple ACK/NACK responses. The PCFICH (Physical Control Format Indicator Channel) is used to indicate how much resource in a subframe is reserved for the downlink control channels. It may be either one, two or three of the first symbols in the first slot in the subframe. The PRACH (Physical Random Access Channel) is used for the uplink transmission of preambles as part of the random access procedure. The PDSCH and the PUSCH are the main scheduled resource on the cell. They are used for the transport of all higher-layer information including RRC signalling, service-related signalling and user traffic. The only exception is the system information in PBCH.. Further Reading: 3GPP TS 36.213, 36.211, 36.300 LT3600/v3.2. © Wray Castle Limited. 2.19.

(56) LTE/SAE Engineering Overview. Ts (Time unit) =. Ts =. 1 15,000 x 2048 1 30,720,000. Ts = c. 32.5 ns. The Physical Layer Timing Unit Almost all numbers, durations and calculations related to E-UTRA are derived from a fundamental parameter known as Ts or the basic ‘time unit’. Ts represents the ‘sampling time’ of the overall channel and is itself derived from basic channel parameters. The definition of Ts is based on a 20 MHz channel bandwidth with 15 kHz subcarrier spacing and an FFT size of 2048. Ts is calculated to be the reciprocal of the subcarrier spacing multiplied by the total number of subcarriers in the FFT, or: Ts = 1/(15,000 x 2048) seconds = 32.5 nsec Frame, subframe and slot lengths, cyclic prefix durations and many other key parameters are defined as multiples of Ts. Crucially, the value of Ts does not vary between E-UTRA physical layer configurations. As Ts stays constant, all of the key parameters used to define the E-UTRA structure also stay constant. This consistency reduces the overall complexity of E-UTRA and enables system manufacturers to scale their devices more easily to a variety of channel bandwidths and frequency bands.. Further Reading: 3GPP TS 36.211:4. 2.20. © Wray Castle Limited. LT3600/v3.2.

(57) LTE OFDM Physical Layer Frame – 10 ms (307200T s). 0. 1. 2. 3. 4. 5. 6. 7. 8. 9 10 11 12 13 14 15 16 17 18 19 Slot – 0.5 ms (15360T s). Subframe – 1 ms (30720T s). 0 1 2 3 4 5 6 0 1 2 3 4 5 6. CP. OFDM Symbol. Normal cyclic prefix (total per subframe 2048T s). CP (160/144T s) 2048Ts. 0 01 2 3 4 5 0 1 2 3 4 5. CP. OFDM Symbol. Extended cyclic prefix (total per subframe 6144T s). CP (512Ts) 2048Ts. Type 1 Frame Structure There are two basic frame types employed in E-UTRA, which are common to both uplink and downlink. Type 1 frames are employed for FDD full- and half-duplex systems, while Type 2 frames are reserved for TDD operation only. The Type 1 frame duration is 10 ms and it is divided into 20 slots, each of 0.5 ms duration. More significantly, however, for most information transmission, two slots are combined to form a subframe. Thus subframe duration is 1 ms, which corresponds to the TTI (Transmission Time Interval) for E-UTRA. Type 1 slots contain either 7 or 6 symbols, depending upon which CP (cyclic prefix) type is in use. Additionally, the length of the CP prefixed applied in a particular symbol within a slot varies, also dependent on which CP length is in use. With the normal CP, symbol 0 in each slot has a CP equal to 160 x Ts or 5.2 µsec, while the remaining symbols in the slot have slightly shorter CPs of just 144 x Ts or 4.7 µsec. When using the extended CP, all symbols are prefixed with a CP of 512 x Ts or 16.7 µsec. Scheduling occurs across a subframe period. Up to the first three symbols in the first slot of each subframe can be defined as a ‘control region’ carrying control and scheduling messages. The remaining symbols of the first and all symbols in the second slot within the subframe are then available for user traffic.. Further Reading: 3GPP TS 36.211 LT3600/v3.2. © Wray Castle Limited. 2.21.

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