LTE Course

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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. LTE/SAE ENGINEERING OVERVIEW. First published May 2009 Last amended June 2009 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. © Wray Castle Limited.

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(5) Introduction to LTE. LTE ENGINEERING OVERVIEW. Contents Section 1. Introduction to LTE. Section 2. OFDMA Principles. Section 3. The LTE Radio Interface. Section 4. Radio Access Protocols. Section 5. Evolved Packet Core. Section 6. Major Protocols. Section 7. EPC Operation. Glossary. LT3600/v1.1. © Wray Castle Limited. iii.

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(7) Introduction to LTE. Section 1. Introduction to LTE. LT3600/v1.1. © Wray Castle Limited. v.

(8) LTE/SAE Engineering Overview. vi. © Wray Castle Limited. LT3600/v1.1.

(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 (Evolved Node B) ...............................................................................1.7 X2 Interface..........................................................................................................................................1.8 The EPC (Evolved Packet Core) .........................................................................................................1.9 S1 Interface........................................................................................................................................1.10 Evolved Packet Core ‘S’ Interfaces....................................................................................................1.11 Data Rates and Services ...................................................................................................................1.12 E-UTRA Protocols..............................................................................................................................1.13. LT3600/v1.1. © Wray Castle Limited. vii.

(10) LTE/SAE Engineering Overview. viii. © Wray Castle Limited. LT3600/v1.1.

(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. LT3600/v1.1. © Wray Castle Limited. ix.

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(13) Introduction to LTE. 100+ Mbit/s. 40 Mbit/s 10 Mbit/s 400 kbit/s. LTE (4G). UMTS/HSPA+. UMTS/HSPA. UMTS EDGE Evolution 1 Mbit/s GSM/EGPRS GSM/GPRS. 150 kbit/s. 50 kbit/s. LTE Overview. 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, 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 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) Evolution, and this in turn can be used as a direct pathway to LTE. Similarly, the interworking capabilities of the Evolved Packet Core (EPC) make it possible for CDMA (Code Division Multiple Access) to migrate radio access from 1x or 1xEV-DO (1x Evolution – Data Only) to LTE.. LT3600/v1.1. © 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 (Evolved Packet Core) represents a complete adoption of an all-IP (Internet Protocol) 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 high-quality 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/v1.1.

(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/v1.1. © 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. 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/v1.1.

(17) Introduction to LTE. Phase 1. Phase 2. GSM900. GSM1800 GSM1900. Phase 2+. Rel 96-98. Rel 99. Rel 4. Rel 5. Rel 6. Rel 7. Rel 8. Rel 9. EDGE Evolution. GPRS EGPRS Rel 99 UMTS. Rel 4. Rel 5. Rel 6. Rel 7. HSDPA IMS. HSUPA. HSPA+. Rel 8. Rel 9. LTE/SAE. LTE Standards Development Since the publication of the first GSM specifications in the late 1980s, the technologies and techniques employed by GSM networks have continued to evolve and develop. 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. 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. Release 5 and 6 introduced HSPA – HSDPA (High Speed Downlink Packet Access) in Release 5 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. Some have termed LTE ‘3.9G’, while others have queried whether it should be considered to be a 4G technology.. LT3600/v1.1. © Wray Castle Limited. 1.5.

(18) LTE/SAE Engineering Overview. LTE Signalling. LTE Traffic. SCTP E-UTRA M OFD. A. E-UTRAN. All-IP. EPC. A FDM C S. LTE Key Technologies Tests and evaluations carried out during 2007 eventually led to the publication of the Release 8 36series of specifications, which begin 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 in excess of 300 Mbit/s in a 20 MHz channel. SC-FDMA is employed on the LTE uplink and may deliver in excess of 80 Mbit/s. 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/v1.1.

(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). Uu S1. X2. RRC PDCP. S1. RLC MAC PHY. LTE UE. Evolved Packet Core. E-UTRAN. Access Networks and the eNB (Evolved Node B). Access Networks and the eNB (Evolved 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. The eNB also receives, schedules and transmits control channel information in its cell, including paging messages and broadcast system information, both of which are received from the MMEs. The eNB also retains many of the more traditional roles associated with base stations, such as bearer management. The eNB 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 a more traditional R99 Node B. LT3600/v1.1. © Wray Castle Limited. 1.7.

(20) LTE/SAE Engineering Overview. X2-C X2-AP. SCTP IP Data Link Layer Physical Lyer. 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/v1.1.

(21) Introduction to LTE. HSS. • NAS Security • Idle state mobility handling • EPS bearer control. MME PCRF. 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 structure consists of five main nodes, although others may be required for backwardscompatibility 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 (Public 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) handles QoS and bearer policy enforcement and also provides charging and rating facilities. 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 EPC 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. The gateway elements form the EPC. LT3600/v1.1. © Wray Castle Limited. 1.9.

(22) LTE/SAE Engineering Overview. S1-AP. MME SCTP IP Data Link Layer Physical Layer. S1-MME. 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 S1-MME protocol (A1AP) 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.. 1.10. © Wray Castle Limited. LT3600/v1.1.

(23) Introduction to LTE. 2G/3G SGSN. HSS. UMTS/ GPRS. S6a. S3. UTRAN/ GERAN. S4. PCRF. MME. S12. LTE. S7. S11. S1-MME. Rx+. IMS E-UTRAN. S5. S1-U. Interworking to MME. SGi. IP Services. PDN-GW. S-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 has 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 interface provides access from the PCRF to the PDN-GW (Public Data Network Gateway). 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. WLAN (Wireless Local Area Network) 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.. LT3600/v1.1. © Wray Castle Limited. 1.11.

(24) LTE/SAE Engineering Overview. Channel bandwidth Modulation and 1.4 MHz 3 MHz 5 MHz 10 MHz 20 MHz MIMO coding rate 1x1 0.9 2.2 3.6 7.2 14.4 QPSK 1/2 16QAM 1/2. 1x1. 1.7. 4.3. 7.2. 14.4. 28.8. 16QAM 3/4. 1x1. 2.6. 6.5. 10.8. 21.6. 43.2. 64QAM 3/4. 1x1. 3.9. 9.7. 16.2. 32.4. 64.8. 64QAM 4/4. 1x1. 5.2. 13.0. 21.6. 43.2. 86.4. 64QAM 3/4. 2x2. 7.8. 19.4. 32.4. 64.8. 129.6. 64QAM 4/4. 2x2. 10.4. 25.9. 43.2. 86.4. 172.8. QPSK 1/2. 1x1. 0.9. 2.2. 3.6. 7.2. 14.4. 16QAM 1/2. 1x1. 1.7. 4.3. 7.2. 14.4. 28.8. 16QAM 3/4. 1x1. 2.6. 6.5. 10.8. 21.6. 43.2. 16QAM 4/4. 1x1. 3.5. 8.6. 14.4. 28.8. 57.6. 64QAM 3/4. 1x1. 3.9. 9.7. 16.2. 32.4. 64.8. 64QAM 4/4. 1x1. 5.2. 13.0. 21.6. 43.2. 86.4. Source: WCDMA for UMTS: 4th Edition – Holma & Toskala (Ed). Data Rates and Services. Data Rates and Services The potential peak data rates of 100 Mbit/s and more are only available when employing channel bandwidths of 20 MHz, which are difficult to find in most countries’ crowded radio environments. Even then, the fastest data rates will only be achievable on links that use advanced antenna techniques such as MIMO (Multiple Input Multiple Output). 2x2 MIMO, where both transmitter and receiver use two separate antennas to carry parallel streams of data over the same channel, is required for data rates of up to 170 Mbit/s, while future versions of E-UTRA that promise data rates of up to 360 Mbit/s would require 4x4 MIMO. The data rates for E-UTRA variants up to 2x2 MIMO in a 20 MHz channel are shown in the diagram. These data rates assume error coding rates of 1/2, 3/4 and 4/4, which are not currently defined in the specifications and so should only be considered to be an example of what is generically achievable with the technology. Data rates of 100 Mbit/s or more will provide users with access to almost any Internet or communications service currently available, from movie downloads and database access down to simpler communications activities such as making a telephone call or sending a text message. The capacity allocation method employed by E-UTRA has more in common with that used in HSPA than the methods used in R99 UMTS. There is no dedicated channel in LTE, meaning that bandwidth is shared between users in a flexible, on-demand way. This flexibility, coupled with the high data rates, makes E-UTRA very attractive. Although E-UTRA’s theoretical ability to provide one user in a cell with a 100 Mbit/s connection has been much discussed, network operators are more excited about the possibility of providing 1 Mbit/s connections to 100 simultaneous users in one cell.. 1.12. © Wray Castle Limited. LT3600/v1.1.

(25) Introduction to LTE. User Equipment Non-Access Stratum (NAS). eNB. RRC. RRC. PDCP. PDCP. RLC. RLC. MAC. MAC. Physical Layer. Physical Layer. Evolved Packet Core Non-Access Stratum (NAS). 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.. LT3600/v1.1. © Wray Castle Limited. 1.13.

(26) LTE/SAE Engineering Overview. 1.14. © Wray Castle Limited. LT3600/v1.1.

(27) LTE/SAE Engineering Overview. Section 2. OFDMA Principles. © Wray Castle Limited.


(29) OFDMA Principles. Contents Orthogonal Radio Carriers ...................................................................................................................2.1 Variable Channel Bandwidth................................................................................................................2.2 Factors Affecting Data Rate .................................................................................................................2.3 FFT (Fast Fourier Transform) ..............................................................................................................2.4 New Air Interface Technologies ...........................................................................................................2.5 OFDM Concepts ..................................................................................................................................2.6 The Cyclic Prefix ..................................................................................................................................2.7 Subcarrier Assignment.........................................................................................................................2.8 The ‘Brickwall’ Effect ............................................................................................................................2.9 OFDM and OFDMA............................................................................................................................2.10 OFDMA Operation .............................................................................................................................2.11 SC-FDMA Concept ............................................................................................................................2.12 SC-FDMA and OFDMA......................................................................................................................2.13 SC-FDMA Operation ..........................................................................................................................2.14 SC-FDMA Multiple Access.................................................................................................................2.15. LT3600/v1.1. © Wray Castle Limited. iii.


(31) OFDMA Principles. Objectives. At the end of this section you will be able to: . list the new set of technologies employed by E-UTRAN on the air interface, including OFDMA and SC-FDMA. . describe the basic concepts that underlie OFDM such as subcarriers, the cyclic prefix and symbols. . explain the meaning of the term ‘orthogonality’ with reference to OFDM systems. . demonstrate an understanding of the concept of orthogonal carriers and the relationship between carrier spacing and bandwidth. . outline the functionality and use of the FFT (Fast Fourier Transform) in OFDM. . outline the differences between OFDM and OFDMA and the benefits associated with each system. . explain the basic concepts that underlie SC-FDMA as used by the E-UTRA uplink. . outline the basic concepts that underlie the SCTP and the reasons for its use in EPS. LT3600/v1.1. © Wray Castle Limited. v.


(33) OFDMA Principles. FDM. Carrier Spacing. Adjacent Carrier Interference. C–1 C C+1. OFDM A Centre point of subcarrier C intersects with subcarriers C–1 and C+1. Orthogonal Radio Carriers In a standard FDM (Frequency Division Multiplexing) system, the radio carriers are spaced sufficiently far apart to minimize the effects of adjacent channel interference. High-capacity FDM systems therefore require large amounts of bandwidth. For an OFDM (Orthogonal Frequency Division Multiplexing) system to work efficiently there should be as little interference as possible between adjacent subcarriers that make up the transmitted channel. To ensure minimum interference, orthogonal radio subcarriers are selected. Orthogonality among a set of adjacent radio subcarriers occurs when the peak of one subcarrier intersects with the point where the signals and harmonics produced by neighbouring subcarriers are passing through zero. This is shown at point A. The peak power frequency of subcarrier C is also the frequency at which the signals from subcarriers C+1 and C–1 pass through zero and the point at which the secondary harmonics from subcarriers C+2 and C–2 do the same. The net effect of this is that the signals radiated by adjacent subcarriers contribute no interference to signals radiating on any particular neighbouring subcarrier. Guard bands, in the form of a number of unused subcarriers, are employed at the top and bottom ends of the channel width to ensure that minimal interference is received from services occupying adjacent channels. Channel width and subcarrier spacing are vitally important in OFDM systems – if the subcarrier spacing is incorrect then adjacent subcarriers will not be orthogonal.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.1.

(34) LTE/SAE Engineering Overview a) Initial Separation. Orthogonal Carriers. Initial Bandwidth. b) Separation Constant Non-orthogonal Carriers. Bandwidth Increased. c). Separation Increased. Orthogonal Carriers. Bandwidth Increased. Variable Channel Bandwidth If the number of subcarriers remains fixed but the bandwidth of the overall channel is increased, the separation between the subcarriers can increase. Increased separation means that each subcarrier must increase its radiated bandwidth by a comparable amount; otherwise, the orthogonality between adjacent subcarriers will be lost. Section (a) shows a set of subcarriers operating orthogonally. Section (b) shows an example of increasing the bandwidth of the centre subcarrier without increasing the separation between it and its neighbouring subcarriers. The intersection between subcarriers no longer occurs at the ‘zero’ point. Section (c) demonstrates an increase in separation and a comparable increase in subcarrier bandwidth, which shows that the orthogonality between the subcarriers can be maintained. One way of increasing the radiated bandwidth of each subcarrier is to increase the number of modulation symbols transmitted across them, as a channel’s occupied bandwidth is proportional to its modulation rate. If more modulation symbols are transmitted it means that more data is transferred across each subcarrier, which in turn increases system throughput. The symbol rate of the system must also fit in with the requirements of the receiving equipment. To ensure that all signals are received correctly, the receiver sampling rate should be slightly higher than the bandwidth of the signal used to carry it – for example, an OFDM channel with a bandwidth of 1.75 MHz may be sampled at a rate of 2 MHz. This allows for the inclusion of a symbol guard period. The sampling frequency for a given channel bandwidth is the fundamental parameter associated with OFDM capacity planning. Once this figure is known, subcarrier spacing and symbol rates can be derived.. Further Reading: 3GPP TS 36.211, 36.300 2.2. © Wray Castle Limited. LT3600/v1.1.

(35) OFDMA Principles. Carrier Spacing Number of Carriers. Modulation Schemes. Symbol Rate Number of Null Carriers. Channel Width. Factors Affecting Data Rate The data rate achievable by an OFDM system is dependent upon several interrelated factors: channel bandwidth channel frequency band number of orthogonal subcarriers separation between subcarriers number of unused or ‘null’ subcarriers modulation scheme employed on subcarriers the optimum sample rate employed by OFDM receivers The E-UTRA OFDM-based air interface avoids the complexities associated with the interrelated dependencies of OFDM parameters by employing a fixed subchannel spacing of 15 kHz. A fixed spacing eliminates the variability between subcarrier width, symbol rate, data rate and sample rate, making it a far simpler implementation of OFDM than that used by, for example, WiMAX.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.3.

(36) LTE/SAE Engineering Overview. Data In. Serial/Parallel Conversion. Data Out. Discrete Channels. Combined Waveform. Discrete Channels. Data In. Virtual Serial/Parallel Conversion. Parallel/Serial Conversion. Data Out. Inverse Fast Fourier Transform. Transmitted Waveform. Fast Fourier Transform. Virtual Parallel/Serial Conversion. FFT (Fast Fourier Transform) An OFDM transmitter can be thought of as comprising hundreds of separate radio modulators, all operating in parallel on different radio frequencies. Data is converted from serial to parallel and then fed to each separate modulator to be transmitted across a discrete radio channel. An OFDM receiver can be conceptualized as hundreds of separate radio receivers all demodulating the data from different radio channels. The data is then combined back into the original, serial data stream before being processed. This conceptual view of an OFDM system is useful for explaining the principles of OFDM, but would be expensive and unwieldy to produce and operate in real life. Real OFDM and DMT (Discrete MultiTone modulation) systems make use of a mathematical function known as FFT (Fast Fourier Transforms) to create the ‘parallel’ data signals required by a multicarrier system. The FFT is a more efficient form of the DFT (Discrete Fourier Transform), which itself provides a way of analysing the frequencies that make up a complex signal. Data is first converted from a serial stream to n parallel virtual branches, n being equal to the number of subcarriers being employed on the RF (Radio Frequency) channel. The modulation scheme to be employed on each subcarrier is selected and the inverse FFT system computes the characteristics of the wideband radio signal that would result if all of the signals were transmitted on a set of parallel radio carriers. This ‘combined’ signal is then transmitted across the radio channel employed by the system. The OFDM receiver takes the wideband signal it receives and passes it to an FFT. The FFT unit processes the received signal and determines the mix of parallel modulation symbols that would have had to have been combined together to produce a signal with those precise characteristics. The FFT process can output parallel virtual streams for each carrier, which can then be converted back into a serial stream of received data.. Further Reading: Anders E. Zonst, Understanding FFT Applications: A Tutorial. (Citrus Press, 2000) 2.4. © Wray Castle Limited. LT3600/v1.1.

(37) OFDMA Principles. Orthogonal Frequency Division Multiple Access (OFDMA) Single Carrier - Frequency Division Multiple Access (SC-FDMA). New Air Interface Technologies The Long Term Evolution of UMTS requires a number of new technologies to be deployed. Chief amongst these are the technologies used to provide air interface connections. Although other technologies were considered during the consultation and development phases, 3GPP finally settled on the use of OFDMA on the downlink and SC-FDMA on the uplink. OFDMA is a well understood and widely used technology that has formed the basis of air interface services for systems such as Wi-Fi (802.11a,g and n), WiMAX (802.16), DAB (Digital Audio Broadcasting) radio and DVB-T/H/S (Digital Video Broadcasting for Terrestrial Handheld and Satellite) television systems. SC-FDMA is a more recent variation of OFDMA, which provides a similar service but in a less powerhungry way.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.5.

(38) LTE/SAE Engineering Overview. Time Symbols 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 f15 f16. Symbol Period CP. 1 OFDM Symbol. Usable Symbol. OFDM Concepts OFDM, also known as discrete multi-tone modulation and multi-carrier modulation, is an advanced form of FDM Instead of transmitting a single modulated radio signal, as would happen in a single carrier system, OFDM transmits hundreds or even thousands of separately modulated radio signals using carriers spread across a wideband channel. Each radio carrier is known as a subcarrier. Data is sent in parallel across the set of subcarriers, each subcarrier only transporting a part of the whole transmission. The modulation rate of all subcarriers in the channel is synchronized to a central source, so all modulation symbols should be transmitted at the same points in time on all subcarriers. The time period occupied by the modulation symbols on all subcarriers is known as an OFDM symbol and represents all the data being transferred in parallel at that point in time. OFDM symbols can make use of adjustable ‘guard periods’ before the ‘useable’ part of the symbol to provide protection against multipath effects. This guard period is known as a CP (Cyclic Prefix) and is created by repeating part of the modulated RF signal for a specified period of time. The achievable symbol rate is determined by the bandwidth of the channel and the spacing between the subcarriers.. 2.6. © Wray Castle Limited. LT3600/v1.1.

(39) OFDMA Principles. Symbol Period CP. Usable Symbol. Last x samples of symbol. The Cyclic Prefix The OFDM cyclic prefix is designed to combat the ISI (Inter Symbol Interference) effects caused by multipaths and other channel impulse response effects. Multipaths cause ‘echoes’ of a previous part of the signal that, having travelled via a longer path than the primary component of the signal, arrive later in time. The cyclic prefix eliminates or masks the effects of ISI, as long as the cyclic prefix period is longer than the maximum delay spread suffered by the signal. The cyclic prefix is formed by taking a portion of the ‘useable’ part of each OFDM symbol and copying it onto the beginning of the symbol period. This is necessary, rather than just using a blank guard period, in order to maintain orthogonality between adjacent subcarriers at all points through the nominal symbol period. The cyclic prefix ratio has potentially significant consequences for the bandwidth efficiency of a channel, but these tend to be outweighed by the benefits in terms of minimized ISI. The use of the cyclic prefix to manage the effects of ISI is only permissible due to the comparatively long symbol duration enjoyed by OFDM-based systems. LTE’s typical total symbol duration (including the cyclic prefix) of 71.42 μsec compares with 3.69 μsec for GSM and just 0.26 μsec for WCDMA (Wideband Code Division Multiple Access). A cyclic prefix with a duration of around 5 μsec, which would be prohibitively expensive if employed by these other systems, is easily accommodated by an OFDMbased system, meaning that the more complex ISI management techniques employed by other systems are not required.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.7.

(40) LTE/SAE Engineering Overview. Reference Signals. Lower Guard Subcarriers. DC Carrier. Data Subcarriers. Higher Frequency Guard Subcarriers. Subcarrier Assignment Different subcarriers from across the whole population of subcarriers created by an OFDM channel are assigned to perform 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 multitone channel. The data rate of each data subcarrier is determined by a combination of the symbol rate and the modulation scheme employed. In some OFDM variants (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 E-UTRA and other systems, including DVB, 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 instead periodically embedded in the stream of data being carried on a ‘normal’ subcarrier. There are 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, serve to limit the 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 DC carrier is the centre subcarrier of each OFDM channel – the one that has a 0 Hz offset from the channel’s centre frequency – and is also null.. 2.8. © Wray Castle Limited. LT3600/v1.1.

(41) OFDMA Principles. Nominal Channel Width. Reference. Data. Null. Frequency Steep Drop Off. The ‘Brickwall’ Effect The use of null or guard subcarriers at the top and bottom ends of each OFDM channel produces a very sudden drop-off in radiated power. Power reduces far more rapidly than at the edge of a traditional single carrier channel. This affects the level of interference caused to adjacent channels. When viewed on a spectrum analyser, the steep drop-off in power seen at the edge of an OFDM channel has been dubbed the ‘brickwall effect’. A representation of this can be seen in the diagram. The difference between the radiated power level of data subcarriers and pilots can also be seen.. LT3600/v1.1. © Wray Castle Limited. 2.9.

(42) LTE/SAE Engineering Overview Symbol Periods (time) -1. 0. 1. 2. 3. 4. 1 2 3. User 1. 4. User 2. User 3. 5 6 7 OFDM. Symbol Periods (time) -1. 0. 1. 2. 3. 4. 1 2. User 1. 3. User 3. 4 5 6. User 2. 7 OFDMA. OFDM and OFDMA The 3GPP E-UTRA specifications refer to the technology employed on the downlink as OFDM, whereas a more commonly accepted term for the technology employed would be OFDMA (Orthogonal Frequency Division Multiple Access). These two technologies are almost identical; the only substantial difference is in the way capacity is allocated. Allocation of capacity in OFDM systems is generally based on time division principles. Each user is allocated the full channel and all subcarriers exclusively for a certain number of symbol periods. This allocation method can be inflexible, especially when user connections do not have enough data to send to fill up the space allocated. In these situations network capacity is wasted by carrying padding. OFDMA systems allocate capacity based on a combination of time and frequency – a certain number of subcarriers for a certain number of symbol periods. This allocation method has two main benefits. Firstly, OFDMA-based systems assign capacity to connections on the basis of a number of subcarriers for a number of symbol periods, rather than assigning the entire channel to one user at a time. This allows the amount of capacity assigned to each connection to match more closely the amount of data it has queued ready to transmit. Secondly, the ability to subdivide the subcarrier population allows the link to serve more than one user at a time.. 2.10. © Wray Castle Limited. LT3600/v1.1.

(43) OFDMA Principles. Transmitter. Modulation mapping e.g. QPSK symbols. 01 10. 10/11/01/10/10/01. I F F T. 10. S/P. 01 11 10. TX. 01 10 01/10/10/01/11/10. 10. P/S. 01 11. F F T. TX. 10. Receiver. OFDMA Operation The layout of a typical OFDMA transmitter/receiver pair is shown in the diagram. In most FDM variants, data is introduced in serial and then converted into n parallel streams, n being equal to the number of data subcarriers. The parallel data streams are then separately passed through error coding, interleaving and modulation stages before being transmitted. In reality, the process outlined above would take place in a DSP (Digital Signal Processor) rather than in discrete physical processing stages, and the eventual transmitted signal would be created using IFFT (Inverse FFT) techniques.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.11.

(44) LTE/SAE Engineering Overview. Symbols 1 2 3 4 5 6 7 8 9 10 11 12. 1. 2. 3. 4. 5. 6. 7. 8. 9. 180 kHz channel 12 x15 kHz spacing. Symbol Period CP. Usable Symbol. SC-FDMA Concept SC-FDMA, as employed on the E-UTRA uplink, can be viewed as a power-efficient adaptation of OFDM. SC-FDMA employs subcarriers, FFTs and other OFDM concepts but is designed to provide a better PAPR (Peak to Average Power Ratio) using a narrower channel. PAPR relates to the ratio between the peak power output of a signal and its average transmit strength – the higher the peaks, the greater the range of power levels over which the transmitter is required to work. Systems such as OFDM, with a high PAPR, are not best suited for use with mobile and other battery-powered devices. The lower PAPR achieved by a system like SC-FDMA makes devices that employ it much less powerhungry and therefore more suitable for mobile operation. For SC-FDMA a form of subchannel is used for resource allocation. Each subchannel occupies 180 KHz of spectrum containing 12 subcarriers. Resource allocation can be flexible, with all subcarriers in a channel being assigned to one user or separate groups of subcarriers being assigned to multiple users.. Further Reading: 3GPP TS 36.211, 36.300 2.12. © Wray Castle Limited. LT3600/v1.1.

(45) OFDMA Principles. 10 10. OFDMA. 011011011010. 01. S/P. 11 10 01. OFDMA Symbol. 011011011010. SC-FDMA. 011011011010. FFT. SC-FDMA Symbol. SC-FDMA and OFDMA The most immediately apparent difference between the operation of the E-UTRA downlink and uplink technologies is that OFDM transmits data in parallel across multiple subcarriers, whereas SC-FDMA transmits data in series, but still employs multiple subcarriers. Using the basic E-UTRA numerology of 12 subcarriers occupying 180 kHz of bandwidth, during one symbol period on the downlink OFDM will transmit 12 modulation symbols, one on each subcarrier. On a corresponding uplink channel, SC-FDMA will transmit 12 modulation symbols in series during the same time period – each SC-FDMA modulation symbol in this example therefore has a duration 1/12th that of an OFDM modulation symbol. In the case of an OFDM channel employing 16QAM (16-state Quadrature Amplitude Modulation), if all subcarriers happened to modulate a high-amplitude symbol at the same time, the aggregate transmitted power of the channel would be high. If during the next symbol period all subcarriers modulated a low-amplitude symbol the aggregate transmitted power of the channel would drop – the corresponding ratio between the peak power transmitted and its long term average could therefore be high. This is an extreme example and is unlikely to occur often, if at all, when real data is being applied to parallel OFDM subcarriers, but the potential for a large difference between peak and average radiated power remains. SC-FDMA avoids such large differences by employing an additional processing stage in front of the IFFT process. This additional stage deals with the group of modulation symbols to be transmitted during one SC-FDMA symbol period in a series. An FFT process represents the changes made to the modulated signal during the symbol period as outputs on a set of subcarriers – each modulation symbol results in a set pattern of outputs across the 12 subcarriers that make up an individual uplink LTE channel. The subcarriers created by this process have a set amplitude, which should remain more or less constant between one SC-FDMA symbol and the next for a given modulation scheme, resulting in little difference between the peak power radiated on that channel and its long-term average. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.13.

(46) LTE/SAE Engineering Overview. Frequency component analysis of modulation symbols. Lower PAPR than OFDM. a. Modulation mapping, e.g. QPSK symbols 011101010100. F F T. I F F T. b c. Transmitter. d. a. b c. d. f. a. 001010110110. I F F T. Mod. b c. Receiver. F F T. Mod. d. SC-FDMA Operation The typical layout of a SC-FDMA transmitter and receiver is shown in the diagram. Despite its name, the transmitted radio signal for SC-FDMA is an orthogonal multicarrier transmission similar to OFDM. The term ‘single carrier’ refers to the pre-processing of the baseband data prior to its application to the IFFT. The data stream is presented in a serial fashion to the radio transmission process. The first stage is modulation symbol mapping, which produces blocks of complex-valued symbols. Modulation mapping may operate on pairs of baseband bits, as shown in the diagram, for QPSK (Quadrature Phase Shift Keying) but if 16QAM or 64QAM are in use then each modulation symbol will represent either four or six serial baseband bits respectively. The series of modulation symbols is then presented to the FFT, which produces an output representing the frequency components of the modulation symbols. It is then these frequency components that are mapped to the allocated inputs of the IFFT. Note that the FFT output size is always smaller than the IFFT input size. This is because a typical cell’s uplink capacity will generally be greater than 180 kHz, meaning that more than one uplink channel will be available. Individual UEs will be assigned one or more 180 kHz uplink blocks to use, which represents only a portion of the total uplink capacity in the cell. Other UEs will be assigned other groups of subcarriers to use across the uplink channel bandwidth. No two UEs will be assigned the same 180 kHz block to use simultaneously, unless Multi-User MIMO is in use. The output of the IFFT and modulator will be a multi-carrier transmission. However, unlike OFDM there is not a direct mapping of each baseband symbol onto individual transmitted subcarriers. Instead, the frequency components of each baseband symbol are now represented across all the transmitted subcarriers, hence the term ‘single carrier’. The result is a transmitted signal with an improved PAPR compared to an equivalent OFDM transmission.. Further Reading: 3GPP TS 36.211, 36.300 2.14. © Wray Castle Limited. LT3600/v1.1.

(47) OFDMA Principles. UE 1. UE 2. UE 3. F F T. I F F T. F F T. I F F T. F F T. I F F T. Mod. UE 1. Mod. Mod. Mod. F F T. I F F T. UE 2 UE 3. Combined signal occupying entire uplink channel bandwidth. SC-FDMA Multiple Access Again, in the example above, the channel is being shared by three UEs, each of which has been assigned a different set of subcarriers. From the receiver’s point of view, it receives one signal that occupies the whole channel but which is in reality the sum of three separate orthogonal signals generated by the three UEs. An overall receive FFT process, operating across the entire channel, recovers the separate subcarrier data streams and individual IFFT processes can then recover the original serial data streams transmitted by each UE.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 2.15.

(48) LTE/SAE Engineering Overview. 2.16. © Wray Castle Limited. LT3600/v1.1.

(49) LTE/SAE Engineering Overview. Section 3. The LTE Radio Interface. © Wray Castle Limited.


(51) The LTE Radio Interface. Contents. Physical Layer Functions ........................................................................................................................ 3.1 Channels Bandwidths ............................................................................................................................. 3.2 Physical Layer Parameters for LTE ........................................................................................................ 3.3 Timing Units ............................................................................................................................................ 3.4 LTE Frequency Bands ............................................................................................................................ 3.5 The Physical Layer Frame Structure ...................................................................................................... 3.6 Resource Descriptions ............................................................................................................................ 3.7 Error Protection and Modulation Schemes ............................................................................................. 3.8 Downlink Reference Signals ................................................................................................................... 3.9 Uplink Reference Signals......................................................................................................................3.10 Air Interface Synchronization ................................................................................................................3.11 Timing and Power Control.....................................................................................................................3.12 LTE Radio Measurement Types ...........................................................................................................3.13 Downlink Physical Channels .................................................................................................................3.14 Downlink Transmission Structure .........................................................................................................3.15 Uplink Physical Channels......................................................................................................................3.16 Uplink Transmission Structure ..............................................................................................................3.17 Advanced Antenna Options ..................................................................................................................3.18. LT3600/v1.1. © Wray Castle Limited. iii.


(53) The LTE Radio Interface. Objectives. At the end of this section you will be able to: . 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. . outline parameters of the versions of OFDMA and SC-FDMA employed by EUTRA. . describe the configuration of downlink and uplink frames and list the range of frame types employed, with particular reference to Frame Type 1. . 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 and the purpose of UE sounding. . describe the arrangements made for managing synchronization, timing and power control functions. . list the type of physical layer measurements taken by an E-UTRA-enabled UE. . outline the functions of the E-UTRA physical channels on both uplink and downlink. . outline the basic configuration of typical Type 1 downlink and uplink frames and the positioning of relevant data types such as control channel, reference signals and synchronization. . describe the uplink and downlink MIMO options available to E-UTRA. LT3600/v1.1. © Wray Castle Limited. v.


(55) The LTE Radio Interface. Physical channels Error coding Reference signals Random access OFDM and SC-FDMA Modulation and up conversion Synchronization and timing Power control Reporting and feedback UE sounding and HARQ Measurements Handover Bandwidth agnostic TDD and FDD SFN supported for MBMS. 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 ARQ) ACK/NACK detection reporting of measurement results to higher layers and the network handover measurements, idle-mode measurements, etc. . 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 (Frequency Division Duplex) and TDD (Time Division Duplex) modes are supported, as is a ‘half duplex’ mode, which is midway between the two. E-UTRA has also been designed to work as the bearer for MBMS (Multicast and Broadcast Multimedia Services) and therefore 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 LT3600/v1.1. © Wray Castle Limited. 3.1.

(56) LTE/SAE Engineering Overview. 20 MHz/1200 15 MHz/900 10 MHz/600 5 MHz/300 3 MHz/180 1.4 MHz/72. Channel bandwidths (bandwidth/data subcarriers). Channels Bandwidths E-UTRA OFDMA was designed to work in a variety of downlink 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 spacings, 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 for TDD in very narrow bandwidth channels. Fixing the subcarrier spacing reduces the complexity of a system that can support multiple channel bandwidths. On the downlink there is an additional set of parameters relevant to operation in narrow channels with 7.5 kHz bandwidth.. Further Reading: 3GPP TS 36.211 3.2. © Wray Castle Limited. LT3600/v1.1.

(57) The LTE Radio Interface. 1.4 MHz. 3 MHz. 5 MHz. Subframe Duration (ms). 1. Subcarrier Spacing (kHz). 15. 10 MHz. 15 MHz. 20 MHz. Sampling Rate (MHz). 1.92. 3.84. 7.68. 15.36. 23.04. 30.72. Data Subcarriers. 72. 180. 300. 600. 900. 1200. Symbols/slot CP Length. Normal CP = 7, extended CP = 6 Normal CP = 4.69/5.12 µsec, extended CP = 16.67 µsec. Physical Layer Parameters for LTE One parameter that does change between different bandwidths is the sampling rate. Based on the UMTS WCDMA chip rate of 3.84 Mcps, E-UTRA employs sample rates that range from 1.92 MHz in a 1.4 MHz wide channel to 30.72 MHz in a 20 MHz wide channel. The decision to use factors or multiples of 3.84 was taken to ensure backwards compatibility with WCDMA devices through the use of common clocking. The allocation of subcarriers between data and null functions varies with channel bandwidth, with a 20 MHz channel supporting 1200 data subcarriers and a 1.4 MHz channel supporting just 72. Other OFDMA systems employ sets of dedicated pilot subcarriers, whereas E-UTRA embeds ‘reference signals’ into the data stream across selected subcarriers. This reduces the amount of ‘pilot’ overhead, but possibly at the expense of reduced accuracy. E-UTRA employs a fixed frame duration period of 10 ms, which is itself created from a set of slots and subframes. A subframe encapsulates two slots and a slot consists of a varying number of symbol periods, depending upon which frame type and CP (Cyclic Prefix) scheme is in use. There are two frame types. Type 1 is designed for FDD full- and half-duplex implementations, while Type 2 is reserved for TDD operation. Type 2 frames are only ever used with systems that need to be backwards-compatible with the Chinese 3GPP TD-SCDMA (Time Division-Synchronous CDMA) standard. There are two CP options, normal and extended. The normal CP is designed to be used either in small cells or in those with a short multipath delay spread; the extended CP is designed for use in large cells or those with complex and long lasting delay spread profiles.. Further Reading: 3GPP TS 36.211 LT3600/v1.1. © Wray Castle Limited. 3.3.

(58) LTE/SAE Engineering Overview. Timing Unit (Ts) =. 1 second subcarrier spacing x max FFT size. 1. =. 15,000 x 2048. =. 32.5 nsec. Timing Units 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 underlying calculations for E-UTRA are based on a service that operates in a 20 MHz channel, with 2048 subcarriers set at 15 kHz spacing. E-UTRA deployments at all other bandwidths are based on these parameters. 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 = 0.0325 μsec Frame, subframe and slot lengths, cyclic prefix durations and many other key parameters are calculated as multiples of Ts. Crucially, the value of Ts does not vary even when E-UTRA operates in channel bandwidths that are smaller then 20 MHz. As Ts stays constant, all of the key parameters used to define E-UTRA services also stay constant. The 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 3.4. © Wray Castle Limited. LT3600/v1.1.

(59) The LTE Radio Interface. FDD. TDD. Band. Popular name. Frequencies (MHz). 1. IMT Core. 1920–1980/2110–2170. 2. PCS 1900. 1850–1910/1930–1990. 3. GSM 1800. 1710–1785/1805–1880. 4. AWS (US). 1710–1755/2110–2155. 5. 850 (US). 824–849/869–894. 6. 850 (Japan). 830–840/875–885. 7. IMT Extension. 2500–2570/2620–2690. 8. GSM 900. 880–915/925–960. 9. 1700 (Japan). 1750–1785/1845–1880. 10. 3G Americas. 1710–1770/2110–2170. Band. Popular name. Frequencies (MHz). 33. TDD 1900. 1900–1920. 34. TDD 2.0. 2010–2025. 37. PCS Centre Gap. (1915)1910–1930. 38. IMT Extension Centre Gap. 2570–2620. LTE Frequency Bands Preparation for the deployment of EPS systems has begun in many countries, with operators, vendors and regulators starting to discuss the spectrum requirements for these systems. The standards currently identify 13 bands for FDD operation, ranging from frequencies in the range 800 MHz through to frequencies in the range 2.5 GHz. There also eight bands identified for TDD operation ranging from 1900 MHz to 2.5 GHz. Considerable scope has been left in the standards to add more frequency bands as global requirements evolve.. Further Reading: 3GPP TS 36.211, 36.104 LT3600/v1.1. © Wray Castle Limited. 3.5.

(60) LTE/SAE Engineering Overview. 10 ms Frame. 0. 1. 2. 3. 4. 5. 6. 7. 8. 9 10 11 12 13 14 15 16 17 18 19 0.5 ms Slot. 1 ms Subframe (2 slots). 0 1 2 3 4 5 6 0 1 2 3 4 5 6. Cyclic Prefix. Modulation Symbol. Normal CP 7 symbols/slot. 1 ms Subframe (2 slots). 0 1 2 3 4 5 0 1 2 3 4 5. Cyclic Prefix. Modulation Symbol. Extended CP 6 symbols/slot. The Physical Layer 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. Type 1 frames last for 10 ms and are composed of 20 slots of 0.5 ms each. Two slots are combined to form a subframe, which lasts for 1 ms. For FDD, 10 subframes are available for downlink transmissions and 10 for uplink transmissions in each 10 ms interval. Uplink and downlink transmissions are separated in the frequency domain. Type 1 slots contain either 7 or 6 symbols, depending upon which CP type is used. The length of the CP prefixed to each symbol may vary depending upon where that symbol sits within the slot. With the normal CP, symbol 0 in each slot has a CP equal to 160 x Ts or 5.21 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 16.67 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. Type 2 frames are used by TDD systems, which are required to be backwards-compatible with the Chinese TD-SCDMA IMT-2000 standard; they last for 10 ms and consist of two 5 ms half-frames. Each half-frame carries six subframes and three specialized fields. Subframe 0 and the DwPTS (Downlink Pilot Time Slot) field are reserved for downlink services and subframe 1 and the UpPTS (Uplink PTS) field are reserved for uplink – the other fields can be assigned dynamically between uplink and downlink.. Further Reading: 3GPP TS 36.211 3.6. © Wray Castle Limited. LT3600/v1.1.

(61) The LTE Radio Interface Resource Grid 1 ms Subframe (2 slots) Subcarrier 1. 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6. 180 kHz. 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6 0 1 2 3 4 5 66 0 1 2 3 4 5 6. Subcarrier 12. 0 1 2 3 4 5 66 0 1 2 3 4 5 6. Resource Element. Resource Block. Equal to one modulation symbol. Resource Descriptions Capacity allocation in E-UTRA is based on a concept known as the RB (Resource Block). A PRB (Physical Resource Block) consists of 12 subcarriers (in the frequency domain) for one slot period (in the time domain). On both uplink and downlink channels, 12 subcarriers correspond to 180 kHz of bandwidth. The minimum possible capacity allocation period is the TTI (Transmission Time Interval) of 1 ms. This equates to the allocation of two consecutive resource blocks. For capacity allocation purposes, the resources available during a frame period are organized into resource grids, which consist of a number of consecutive resource blocks. The theoretical minimum definable capacity allocation unit is the resource element, which is defined as one subcarrier during one symbol period. Within each resource grid the resource elements that will be carrying reference signals are assigned first; the remaining elements are then available to have user data or control messages mapped to them. In data transfer terms, one resource element is the equivalent of one modulation symbol on a subcarrier, so if QPSK modulation was being employed, one resource element would be equal to 2 bits, with 16QAM 4 bits and with 64QAM 6 bits of transferred data. If MIMO is employed on the downlink then separate resource grids are created for each antenna port – each port maps to a different MIMO stream.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 3.7.

(62) LTE/SAE Engineering Overview. Modulation Schemes. BPSK. Signalling functions only. QPSK 16QAM Optional on uplink. 64QAM. Error Coding Schemes. 1/3 Turbo Coding 1/3 CC. CRC. Traffic and most control channels BCH only. Transport Block. 24 bit CRC. Error Protection and Modulation Schemes The range of modulation schemes used in E-UTRA comprises BPSK, QPSK, 16QAM and 64QAM. 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, an HSPA device. For shared and multiplex traffic channels, the only error coding option mentioned in the current version of the applicable 3GPP specification (TR 36.212) is 1/3 Turbo 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 BER (Bit Error Rate), 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, 36.300 3.8. © Wray Castle Limited. LT3600/v1.1.

(63) The LTE Radio Interface. 1 ms Subframe (2 slots) Subcarrier 1. 0 1 2 3 4 5 6 0 1 2 3 4 5 6 R. R. R. R. 180 kHz. R. Subcarrier 12. R. R. R. Reference Signal. Downlink Reference Signals In any mobile radio system it is necessary to provide mobile devices with a means of measuring and monitoring the strength and quality of the signal they receive and of calibrating their own output to ensure that the correct frequencies are being employed. In an OFDM-based system, where different UEs are assigned the use of different sets of closely packed subcarriers, the need for reliable frequency calibration is critical. E-UTRA’s version of OFDMA/SC-FDMA employs a physical reference signal, embedded in the main body of the transmitted signal to provide an opportunity for channel estimation and frequency calibration on the downlink. A process of UE sounding performs a similar function on the uplink. Reference signal symbols are placed in predetermined locations within each resource grid. An example of this for both uplink and downlink is shown in the diagram. On the downlink, three types of downlink reference signals are currently defined: cell-specific reference signals, MBSFN (Multicast/Broadcast Single Frequency Network) reference signals, associated with MBSFN transmission, and UE-specific reference signals. In most circumstances only the first of these reference signal types will be used. The reference signal takes the form of a modulated symbol chosen from a pseudo-random sequence of symbols, which is inserted into the transmitted signal following a predetermined sequence. Cell-specific reference signals, as well as providing a ‘known signal’ upon which to base channel estimations, are modulated to identify the cell to which they belong. When MIMO is employed, separate reference signals are transmitted by each air interface port. In Type 1 frames the downlink reference signal is transmitted during symbol periods 0 and 4, although the subcarrier on which it is transmitted varies following a predetermined pattern.. Further Reading: 3GPP TS 36.211, 36.300 LT3600/v1.1. © Wray Castle Limited. 3.9.

(64) LTE/SAE Engineering Overview. 1 ms Subframe (2 slots) Subcarrier 1. 0. 1. 2. Subcarrier 1. Subcarrier 12. D. 4. 5. 6. 0. 1. 2. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. 4. 5. 6. Demodulation Reference Signal. Uplink Reference Signals. Uplink Reference Signals A ‘demodulation’ reference signal is embedded in uplink user traffic and control transmissions. The demodulation signal provides the receiving eNB with a ‘known signal’ element upon which to perform channel estimations and from which it can calculate timing adjustments. In Type 1 frames, the uplink demodulation reference signal is always sent in symbol 3 of each slot. Channel estimations for received uplink signals are made by the eNB based on measurements taken of the reference signal symbols embedded in uplink transmissions. If there is no uplink transmission taking place, however, the eNB cannot take measurements. In these circumstances a UE may be instructed to perform uplink sounding, which consists of the UE transmitting a reference signal within an uplink resource allocation specifically set aside for the purpose. UEs may also be instructed to undertake sounding to enable the eNB to perform ‘frequency-specific scheduling’. This term describes a procedure whereby the eNB measures the sounding signal transmitted by a UE across some or all subcarriers and then chooses the resource block that consists of the best performing set of frequencies.. Further Reading: 3GPP TS 36.211, 36.300 3.10. © Wray Castle Limited. LT3600/v1.1.

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