Wray Castle - UMTS Air Interface

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(1)UMTS Air Interface. Wray Castle Limited Bridge Mills, Stramongate, Kendal, LA9 4UB, UK. [email protected] www.wraycastle.com © Wray Castle Limited all rights reserved.

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(3) UMTS Air Interface. UMTS AIR INTERFACE. First published 2000 Last updated December 2006 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) UMTS Air Interface. UMTS AIR INTERFACE. CONTENTS Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8. UMTS Structure and Aims CDMA Principles in Practice Engineering for CDMA Operation UTRAN Protocol Structure UMTS Physical Layer Layer 2 Operation RLC and MAC Radio Resource Control (RRC) Air Interface Procedures and NAS Interactions. © Wray Castle Limited. iii.

(6) UMTS Air Interface. iv. © Wray Castle Limited.

(7) UMTS Air Interface. SECTION 1. UMTS STRUCTURE AND AIMS. © Wray Castle Limited. i.


(9) UMTS Air Interface. CONTENTS 1. General Service Aims for UMTS 1.1 Introduction 1.2 General Aims 1.3 Fixed and Mobile Differentiation. 1.1 1.1 1.1 1.1. 2. Service Definition 2.1 Service Capabilities 2.2 Efficient Use of the Resource 2.3 UMTS Bit Rates 2.4 Factors Limiting Bit Rate. 1.3 1.3 1.3 1.5 1.5. 3. UMTS Main Elements 3.1 Core Network (CN) 3.2 UMTS Terrestrial Radio Access Network (UTRAN). 1.7 1.7 1.7. 4. UMTS Core Network Architecture 4.1 Required Connections 4.2 The Evolution of Core Architecture. 1.9 1.9 1.11. 5. UE Capabilities 5.1 Baseline Capabilities 5.2 UE Service Capabilities. 1.13 1.13 1.13. 6. Standard Voice Service 6.1 Adaptive Multi-Rate (AMR) Speech Codec 6.2 Wideband AMR Codec. 1.15 1.15 1.17. 7. Multimedia Service Capabilities 7.1 Introduction. 1.19 1.19. 8. UE Radio Characteristics 8.1 Radio Spectrum 8.2 Basic UE Transmitter Characteristics 8.3 Basic UE Receiver Characteristics. 1.21 1.21 1.23 1.23. © Wray Castle Limited. iii.


(11) UMTS Air Interface. OBJECTIVES At the end of this section you will be able to: • • • • • •. state the general service aims for UMTS define the service capabilities and factors affecting the bit rate used identify the network elements and interfaces within UMTS outline User Equipment (UE) requirements and characteristics outline the use and basic action of the Adaptive Multi-Rate (AMR) voice coder state the need for video coding standards. © Wray Castle Limited. v.

(12) UMTS Air Interface. 1. GENERAL SERVICE AIMS FOR UMTS 1.1. Introduction. The key aim for UMTS is that it should provide a platform to carry the services and features that are generally associated with fixed networks into the mobile and roaming environments. UMTS should provide an integrated telecommunication system that is able to support a wide range of applications. The throughput should be variable, to provide a range of capability from narrowband to wideband. The user should experience true personal communication, regardless of location. 1.2. General Aims. All aspects of service provision need to be considered, including the need for types of services to be applicable to probable applications, and the ease with which these services can be utilized. 3rd Generation Partnership Project (3GPP) Specification TS 22.101 ‘UMTS Service Principles’ details many of these considerations. In many cases they are not specified as requirements, but set out as design aims for manufacturers and operators. The reason for the relaxation of rigid service specification is to enable more flexibility for operators to differentiate their services from those of their competitors. Despite this, services should be accessed in a uniform and easy to understand way; this impacts on the design of both the service and the User Equipment (UE). It is also intended that a user should experience a constant level of service, irrespective of location. In particular, attention should be paid to the roaming environment. 1.3. Fixed and Mobile Differentiation. The user should be able to access a range of services in the mobile environment that offer similar rates and reliability to those normally associated with fixed networks. The practical limitations of the radio resource and radio characteristics will probably mean that this will not be possible in all environments. However, in the localized business and residential environments, full emulation of Private Branch Exchange (PBX) and Local Area Network (LAN)-type services should be available.. 1.1. © Wray Castle Limited. MB2002/S1/v8.

(13) UMTS Air Interface. neral Service A e G ims TS M U Integrated Telecommunication System Personal Communication Regardless of Location Differentiation of Operators’ Offerings Narrowband or Broadband Simple to Operate Continuity of Service while Roaming PBX and LAN Emulation. Fixed. Mobile. Figure 1 UMTS General Service Aims MB2002/S1/v8. © Wray Castle Limited. 1.2.

(14) UMTS Air Interface. 2. SERVICE DEFINITION 2.1. Service Capabilities. For most telecommunication systems, including the first- and second-generation mobile systems, it is common to define rigidly the bearer and teleservices that should be provided by an operator. However, it was felt that this approach would be too restrictive for third-generation systems. With this in mind, it is only service capabilities that have been defined for UMTS rather than a full set of teleservices. The teleservices that were defined for second-generation systems remain in place, and in addition to these are the service capabilities. These imply the definition of bearers, resource control mechanisms and Quality of Service (QoS) parameters. This allows operators to define more advanced teleservice types for themselves, based on the standard set of service capabilities. These non-standard services may include those used for alternative speech services: video, multimedia, messaging, and other data applications. 2.2. Efficient Use of the Resource. Most applications, and in particular multimedia applications, exhibit some degree of asymmetry and are discontinuous. For example, applications involving Internet access would be both discontinuous and asymmetric; streaming video or audio would be completely asymmetric, but continuous. It is intended that UMTS, both in the Core Network (CN) and in the Access Network (AN), should take full advantage of these characteristics to promote resource utilization efficiency.. 1.3. © Wray Castle Limited. MB2002/S1/v8.

(15) UMTS Air Interface. Video Streaming Multimedia. Customized Supplementary Services. Access to an Enterprise Server Audio/Video Messaging Database Access. Video Telephony. File Transfer. High Quality Audio. Undefined Services. Defined Service Capabilities Figure 2 Service Capabilities MB2002/S1/v8. © Wray Castle Limited. 1.4.

(16) UMTS Air Interface. 2.3. UMTS Bit Rates. An application requesting a bearer service will specify it with regard to the variables mentioned on the previous pages. It is possible for a single UE to have several active bearers in operation simultaneously: these may be a mixture of connection-oriented and connectionless services. The actual bit rate available for a particular application will depend on the radio environment and on operator-determined limitations. In general, the aims for UMTS based upon service aims set out by the International Telecommunication Union (ITU) in documents relating to International Mobile Telecommunications 2000 (IMT-2000) are summarized as: • at least 144 kbit/s – rural outdoor • at least 384 kbit/s – urban/suburban outdoor • at least 2,048 kbit/s – indoor/low range outdoor 2.4. Factors Limiting Bit Rate. Two important factors limit the ultimate bit rate available to the user. The first is related to the radio characteristics applicable to the user’s physical location, including factors such as interference, Doppler shift and fading characteristics. These affect the performance of the channel and, in general, more hostile radio conditions will limit the achievable throughput in the channel. The second limiting factor relates to radio carrier capacity. The number of channels available on a UMTS radio carrier is inversely proportional to the bit rate provided in each channel. Thus, the higher the bit rate allocated to a user, the fewer other users will have access to the cell. It is possible that one bearer at 2,048 kbit/s could represent the entire capacity of one radio carrier, which would represent a significant drain on network resources if allocated in a rural or suburban environment. However, it may be acceptable if allocated in the indoor pico cell environment. For Phase 1 of UMTS (Release 99), circuit-switched services are limited to 64 kbit/s due to Mobile-services Switching Centre (MSC) capability. Higher bit rates are only applicable to packet-switched services.. 1.5. © Wray Castle Limited. MB2002/S1/v8.

(17) UMTS Air Interface. Target. User. Environment. Bit Rate. Mobility. Rural Outdoor. 144 kbit/s. 500 km/h. 384 kbit/s. 100 km/h. 2,048 kbit/s. 10 km/h. Operating. Urban/Suburban Outdoor Indoor/Low Range Outdoor. Figure 3 Bearer Types for UMTS MB2002/S1/v8. © Wray Castle Limited. 1.6.

(18) UMTS Air Interface. 3. UMTS MAIN ELEMENTS. A UMTS network can be considered as three interacting domains. These are the CN, the UMTS Terrestrial Radio Access Network (UTRAN) and the UE. Interfaces are defined within and between these systems, providing standardization and, in many cases, inter-vendor compatibility. 3.1. Core Network (CN). The main function of the CN is to provide switching, routing and transit for user traffic. There are many possible implementations for the CN, but a general requirement is flexible high bandwidth capability, provided as real-time or non-realtime services. This may be initially provided by a combination of circuit and packet switching. By Release 5/6 the core network becomes wholly packet switched with QoS mechanisms to support real-time services. The CN also contains the databases and network management functions. 3.2. UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is concerned with the provision of radio coverage across the operating area and the provision of services across the air interface. Included in this is the handling of macro diversity through the provision of soft handover. Soft handover may take place within the UTRAN, or between UTRANs. The UTRAN contains the base stations, referred to as Node Bs, and Radio Network Controllers (RNCs), which provide the control functionality for one or more Node Bs. The description of Node B and RNC functions is logical rather than physical. Although there is a defined interface between Node B and RNC, the logical nature of their definition means that this does not necessarily correspond to a physical interface in a particular vendor’s equipment. A single RNC and its associated Node Bs is known collectively as Radio Network Subsystem (RNS). An UTRAN may contain one or more RNS.. 1.7. © Wray Castle Limited. MB2002/S1/v8.

(19) UMTS Air Interface. Core Network. Iu. Iu. UTRAN RNS. RNS Iur. Iub. Node B. Iub. Iub. Node B. Node B. Iub. Node B. ser wray castle Brow .. Internet Search: http//www. xXX XXXXXXXXxx X XXXxXXXX XX XXXXXXXXXX XX XXXXxxxxxXX XXX XxXX XXX X XXX XXX. UE. Figure 4 UMTS Main Elements MB2002/S1/v8. © Wray Castle Limited. 1.8.

(20) UMTS Air Interface. 4. UMTS CORE NETWORK ARCHITECTURE. The basic architecture for UMTS is based on that of the GSM incorporating GPRS. At R99 and Release 4, switching is a combination of circuit switching provided by the MSC/VLR, and packet switching provided by the Serving and Gateway GPRS Support Nodes (SGSN/GGSN). These GSM elements are modified for UMTS operation and service provision. A UMTS MSC will need to support a new interface in order to communicate and exchange traffic with the UTRAN. Similarly, a 3G-SGSN also needs to support the new interface. In both cases these elements need to be able to supply the bearer types required to provide UMTS multimedia services. The GSM databases Visitor Location Register (VLR) and Home Location Register (HLR) are present in UMTS. The VLR is a temporary distributed database assumed to be integrated into the MSC. The HLR is a permanent store for subscriber data within an operator’s system. 4.1. Required Connections. There are four defined logical interfaces that interconnect the functional elements of the UTRAN and connect the UTRAN to other network domains. Two of these interfaces, the lu and Uu, are external interfaces. The lub is internal only, and the lur will usually be internal, but could be external for some network architectures. The interfaces used are as follows: lu. – RNC to CN. Uu – Node B to UE lub – RNC to Node B lur – RNC to RNC. 1.9. © Wray Castle Limited. MB2002/S1/v8.

(21) UMTS Air Interface. Uu wray castle Browser Internet Search: http//www.. xxxXX XXXXXXXX X XXXxXXXX XXXX XXXXXXXX XX XX XXXXxxxxx. Radio Access Network. XXX XxXX XXXX XXX XXX. RNC. lu-CS. PSTN. UE lu-PS. C HLR. lu-CS lur. Gs. EIR. Gr. AuC. Gc. Uu lu-PS wray castle Browser Internet Search: http//www.. xxxXX XXXXXXXX X XXXxXXXX XXXX XXXXXXXX XX XX XXXXxxxxx XXX XxXX XXXX XXX XXX. UE. RNC Radio Access Network. Gn. Gi. IP Network or X.25 Network. Signalling Connection Traffic and Signalling Connection. Figure 5 General UMTS Architecture MB2002/S1/v8. © Wray Castle Limited. 1.10.

(22) UMTS Air Interface. 4.2. The Evolution of Core Architecture. From Release 4 onwards the GSM/GPRS based UMTS core network evolves into an all-packet-switched IP-based architecture. This provides both economy and flexibility for service delivery. 4.2.1. Circuit-Switched Core. The UMTS circuit-switched core network still provides circuit-switched connectivity for voice and circuit-switched data, but with alterations to the nodes. The traditional MSC, with its 64 kbit/s group switch, may be replaced with a soft switch. This new node will consist of an MSC server, in essence a Media Gateway Controller (MGC), and a Media Gateway (MG). This approach means minimal impact on the radio access network, as signalling towards the MSC server is unchanged. The traffic will be converted from circuit switched to packet switched by the media gateway. This means that the transport of traffic through the core can make use of the efficiencies of packet switching. Interworking with legacy networks such as the Public Switched Telephone Network (PSTN) can be facilitated by using media gateways and signalling gateways. 4.2.2. Packet-Switched Core. The packet-switched core will have fewer architectural changes, with connectivity still being provided by GPRS. The main additional architecture is defined at Release 5 of the standards. The IP Multimedia Subsystem (IMS) can be accessed via the packet switched core, providing a variety of services and applications based on a packet-switched bearer. These will include peer-to-peer applications such as Voice over Internet Protocol (VoIP), video and audio streaming and Push-to-talk over Cellular (PoC).. 1.11. © Wray Castle Limited. MB2002/S1/v8.

(23) UMTS Air Interface. IMS. CSCF. Other IMS or IP Networks. Mw Mm. HSS. Cx. CSCF Mg MGCF. Gr Gi. Gi. PS Domain Gn. TCP/IP. Mc Iu. UTRAN Gi r. wray castle Browse Internet Search: http//www.. XXXXXXXXxxxXX X XXXxXXXX XXXXXXXXXXXX xXX XX XXXXxxxx XXX XxXX XXXX XXX XXX. Iu. PSTN/ Legacy CS Domain MGW. Nb. Mc. MGW Mc. MSC Server. Nc. SGW. GMSC Server. HSS. Figure 6 Release 5 Architecture MB2002/S1/v8. © Wray Castle Limited. 1.12.

(24) UMTS Air Interface. 5. UE CAPABILITIES. UMTS is intended to be capable of supporting a wide range of services, either individually or in combination. It cannot be expected that all UEs will support all types of service. In order to allow for UE capability variation, while maintaining compatibility between UEs and all UMTS networks, many options are included in the 3GPP specifications. As a consequence, there are many different physical forms for the UE, depending on its intended function. 5.1. Baseline Capabilities. The baseline capability describes the basic capabilities required by a UE to enable worldwide roaming within all 3GPP networks. The baseline capabilities include functions relating to scanning, cell and Public Land Mobile Network (PLMN) selection and registration, including authentication. 5.2. UE Service Capabilities. To make a UE marketable, the manufacturer has to add UE service capabilities. There is no requirement for any particular combination of service capabilities to be supported but a number of standardized UE service capabilities are described in the 3GPP specifications. These provide for services such as speech and text messaging as well as bearers for video and data services. This does not preclude the provision of other service capabilities. There are six categories of standardized service capabilities: • teleservices • bearer services • supplementary services • service capabilities • system features • other UE service capabilities The UE may also support service capabilities suitable for additional nonstandardized teleservices such as multimedia services.. 1.13. © Wray Castle Limited. MB2002/S1/v8.

(25) UMTS Air Interface. Bearer services. Teleservices Voice calls Emergency calls SMS. Symmetry Point-to-point/multipoint Delay characteristics Bit Error rate Bit rate. Service Capabilities. Other UE service capabilities. USIM Application Toolkit MExE LCS. Multimedia services Fax service. Supplementary services. System Features. Standardized GSM Non-standardized. NITZ USSD. UE Service Capabilities Baseline Capabilities. Scanning PLMN selection Cell selection/reselection Registration Authentication LA/RA updating. Figure 7 UE Capabilities MB2002/S1/v8. © Wray Castle Limited. 1.14.

(26) UMTS Air Interface. 6. STANDARD VOICE SERVICE. 6.1. Adaptive Multi-Rate (AMR) Speech Codec. Because of bandwidth limitations on the air interface, a low-rate voice-coding scheme is required. In addition, the voice coder must offer the user good quality in a variety of situations. The AMR speech codec is designed to allow dynamic adaptation of the bit rate for source and channel coding. The overall speech quality is improved by increasing the amount of error protection while reducing the bit rate for source coding if the radio link quality degrades. To realize rate adaptation, the decoder needs to signal the mode it prefers to receive to the encoder. The AMR codec consists of eight source codec bit rate modes. For codecs that support variable rate operation, the UE can be allowed by Radio Resource Control (RRC) in the UTRAN to reduce transmission rate independently without requesting a new codec mode from the network, within limits defined by the network in the current transport format set for the impacted Radio Bearer (RB).. 1.15. © Wray Castle Limited. MB2002/S1/v8.

(27) UMTS Air Interface. Codec Mode. Source Codec Bit Rate. AMR_12.20. 12.20 kbit/s (GSM EFR). AMR_10.20. 10.20 kbit/s. AMR_7.95. 7.95 kbit/s. AMR_7.40. 7.40 kbit/s (IS-641). AMR_6.70. 6.70 kbit/s (PDC-EFR). AMR_5.90. 5.90 kbit/s. AMR_5.15. 5.15 kbit/s. AMR_4.75. 4.75 kbit/s. AMR_SID. 1.80 kbit/s. SID – Silence Descriptor Frame. Figure 8 Source Codec Bit Rates for the AMR Codec MB2002/S1/v8. © Wray Castle Limited. 1.16.

(28) UMTS Air Interface. 6.2. Wideband AMR Codec. Traditionally in telecommunications networks speech has been limited in bandwidth with the highest modulating frequency set to 3.4 kHz. This has always been considered good commercial or toll quality. The introduction of a wideband speech service in Release 5 will provide improved voice quality by allowing the highest modulating frequency to extend to 7 kHz. The Adaptive Multi-Rate – Wideband (AMR-WB) speech coder is a development of the existing AMR speech coder offering nine source rates from 6.6 kbit/s to 23.85 kbit/s. The coder also includes a low bit rate background encoding scheme to support Discontinuous Transmission (DTX). The bit rate can be changed under instruction from the network every 20 ms.. 1.17. © Wray Castle Limited. MB2002/S1/v8.

(29) UMTS Air Interface. Codec Mode. Source Codec Bit Rate. AMR_WB_23.85. 23.85 kbit/s. AMR_WB_23.05. 23.05 kbit/s. AMR_WB_19.85. 19.85 kbit/s. AMR_WB_18.25. 18.25 kbit/s. AMR_WB_15.85. 15.85 kbit/s. AMR_WB_14.25. 14.25 kbit/s. AMR_WB_12.65. 12.65 kbit/s. AMR_WB_8.85. 8.85 kbit/s. AMR_WB_6.60. 6.60 kbit/s. AMR_WB_SID. 1.75 kbit/s. Figure 9 Source Codec Bit Rates for the AMR-WB Codec MB2002/S1/v8. © Wray Castle Limited. 1.18.

(30) UMTS Air Interface. 7. MULTIMEDIA SERVICE CAPABILITIES. 7.1. Introduction. For Release 99, 3G-324M has been agreed as the default standard for UEs supporting multimedia capabilities. 3G-324M is based upon the ITU H.234 standard. H.234 was developed to support video telephony over fixed networks. 3G-324M can be viewed as an umbrella standard for the support of real-time, multimedia services over circuit-switched networks. The standard includes several other protocols that handle multiplexing and demultiplexing of speech, video, control data and in-band call control. 7.1.1. H.223 Multiplexing and Demultiplexing. Multimedia transmission will require mechanisms to mix different information streams together. This mixing process is one of the tasks of H.323. Additionally, H.223 provides a number of degrees of resilient transport ranging from Level 0 with limited protection through to Level 3 which defines the most robust delivery scheme including both Forward Error Correction (FEC) and Automatic Repeat Request (ARQ) mechanisms. 7.1.2. H.245 Call Control. When two devices need to exchange information they may have different H.223 multiplexing and demultiplexing capabilities as well as different audio and video codecs. H.245 supports mechanisms for exchanging capability information as well as negotiating which end is to be master or slave. The master–slave relationship is necessary to resolve any conflicts. To provide reliability H.245 employs a Simple Retransmission Protocol (SRP) which can optionally be Numbered (NSRP). Media and data flows are organized into logical channels and H.245 provides logical channel signalling allowing logical channels to open and close. Finally, H.245 provides a range of call control commands to support flow control, codec control and user input indications. 7.1.3. H.263 and MPEG-4. The 3G-324M standard specifies H.263 as mandatory and MPEG-4 as a recommended video codec. H.263 is a legacy codec that is used with existing H.323 systems and has been kept for compatibility. MPEG-4 is more flexible than H.263 and offers advanced error detection and correction schemes.. 1.19. © Wray Castle Limited. MB2002/S1/v8.

(31) UMTS Air Interface. 3G-324M. ITU H.324. H.223. H.245. Multiplexing speech, video control data. Call control Organizes data flows into logical channels, provides logical channel signalling. Provides different degrees or error resilient transport. Speech codecs AMR. G.723.1. Video codecs H.263. MPEG-4. Figure 10 Multimedia Capabilities MB2002/S1/v8. © Wray Castle Limited. 1.20.

(32) UMTS Air Interface. 8. UE RADIO CHARACTERISTICS. The physical layer of the UMTS air interface is very complex and thus there are many detailed requirements for the UE. These are divided into those that relate to transmitter performance and those that relate to receiver performance. Some of these requirements are described here. 8.1. Radio Spectrum. The 3GPP specifications describe the requirements for UMTS operation in nine bands for Frequency Division Duplex (FDD) operation and a further three bands for Time Division Duplex (TDD) operation. The key FDD bands are Band I, which represents UMTS operation in 3G spectrum in Europe, Africa and Asia, and Band II, which represents UMTS operation in the Personal Communications System (PCS) spectrum in North and South America. The key TDD band is Band A, for which many operating licenses have been awarded in Europe.. 1.21. © Wray Castle Limited. MB2002/S1/v8.

(33) UMTS Air Interface. UMTS FDD Defined Operating Bands (Rel-7). I. Uplink (MHz) 1920–1980. Downlink (MHz) 2110–2170 MHz. Nominal duplex spacing (MHz) 190 MHz. II. 1850 –1910. 1930–1990 MHz. 80 MHz. III. 1710–1785. 1805–1880 MHz. 95 MHz. IV. 1710–1755. 2110–2155 MHz. 400 MHz. V. 824–849. 869–894 MHz. 45 MHz. VI. 830–840. 875–885 MHz. 45 MHz. VII. 2500–2570. 2620–2690 MHz. 120 MHz. VIII. 880–915. 925–960 MHz. 45 MHz. IX. 1749.9–1784.9. 1844.9–1879.9 MHz. 95 MHz. Band. UMTS TDD Defined Operating Bands (Rel-7) Spectrum blocks Band (MHZ) a. 1900–1920 and 2010–2025. b. 1850–1910 and 1930–1990. c. 1910–1930. d. 2570–2620. Figure 11 UE Spectrum Capabilities MB2002/S1/v8. © Wray Castle Limited. 1.22.

(34) UMTS Air Interface. 8.2. Basic UE Transmitter Characteristics. There are four power classes defined for UMTS UEs. These are outlined in terms of maximum transmit power. The minimum transmit power for all classes needs to be better than –50 dBm. This suggests a dynamic range in the order of 70 dB. Power step size needs to be switchable between 1, 2 and 3 dB. When the transmitter has to ramp for packet mode transmission, compressed modes of operation and discontinuous transmission, the ramping is required to be completed within 50 μs centred on the transition point. The UE has to maintain a frequency stability of 0.1 ppm in respect of the received signal from the Node B (Node B stability is 0.05 ppm). Stability for generated radio frequencies and codes is derived from the same source, and is subject to the same stability requirement. 8.3. Basic UE Receiver Characteristics. The receiver characteristics are complex, and quoted in terms of performance in a range of different test conditions. Some of these tests are designed to assess absolute performance capability in sanitized conditions; some are designed to simulate more hostile and realistic radio conditions. The absolute sensitivity level for all classes of UE is defined in terms of energy per chip in the Dedicated Physical Channel (DPCH_Ec) when the Bit Error Rate (BER) is better than 0.001. Thus DPCH_Ec = –117 dBm/3.84 Mcps.. 1.23. © Wray Castle Limited. MB2002/S1/v8.

(35) UMTS Air Interface. Power Class. Max O/P Power. Tolerance. 1. +33 dBm. 2W. +1 dB / –3 dB. 2. +27 dBm. 0.5 W. +1 dB / –3 dB. 3. +24 dBm. 0.25 W. +1 dB / –3 dB. 4. +21 dBm. 0.125 W. ±2 dB. Minimum power better than –50 dBm Step size 1 dB, 2 dB and 3 dB. Receiver sensitivity for BER better than 0.001 DPCH_Ec = –117 dBm/3.84 Mcps. Figure 12 Radio Characteristics MB2002/S1/v8. © Wray Castle Limited. 1.24.

(36) UMTS Air Interface. 1.25. © Wray Castle Limited. MB2002/S1/v8.

(37) UMTS Air Interface. SECTION 2. CDMA PRINCIPLES IN PRACTICE. © Wray Castle Limited. i.


(39) UMTS Air Interface. CONTENTS 1. Basic Principles 1.1 Introduction 1.2 Multiple Access Schemes. 2.1 2.1 2.1. 2. The Spreading and Despreading Process 2.1 The Code Application 2.2 Direct Sequence (DS) Spread Spectrum. 2.3 2.3 2.5. 3. Spreading and Multiple Access 3.1 Shannon’s Equation 3.2 Application for Multiple Access 3.3 Processing Gain (Gp) and Spreading Factor (SF) 3.4 Processing Gain and Cell Capacity 3.5 Multiple Cells. 2.11 2.11 2.11 2.13 2.15 2.17. 4. Code Generation 4.1 Code Correlation 4.2 Choice of Codes. 2.19 2.19 2.21. 5. Pseudo-Random Codes 5.1 Maximal Linear Codes 5.2 Auto Correlation Properties of Pseudo-Random Codes 5.3 Cross Correlation Properties of Pseudo-Random Codes. 2.23 2.23 2.25 2.27. 6. Combinational Codes 6.1 Gold Codes. 2.29 2.29. 7. Orthogonal Code Sets 7.1 Orthogonal Code Derivation 7.2 Orthogonal Code Tree 7.3 Orthogonal Code Application 7.4 Exercise 2. 2.31 2.31 2.33 2.35 2.37. © Wray Castle Limited. iii.


(41) UMTS Air Interface. OBJECTIVES At the end of this section you will be able to: • • • • • • •. explain the basic principles and operation of different multiple access schemes including Code Division Multiple Access (CDMA) explain how spectral spreading can be achieved and state the advantages of using such a scheme describe the effects of spreading in terms of processing gain, spreading factor, number of users in a cell and interference margins list the types of codes used in CDMA and explain the meaning and relevance of the term correlation identify the use of pseudo-random codes describe the generation of gold codes by the use of combinational codes explain the meaning of the term orthogonality and describe its relevance in code generation. © Wray Castle Limited. v.

(42) UMTS Air Interface. 1. BASIC PRINCIPLES 1.1. Introduction. Spread spectrum techniques for radio transmission are not a new technology. They have many characteristics that are attractive to those designing military communications equipment, and it is here that they have previously found most applications. They were first tested shortly after the Second World War, but it is only in the last ten years that they have come into the public domain. Spread spectrum stands apart from all other developments in radio communication. Engineers in this field have always had two main goals. First, to remove noise from the system; second, to reduce bandwidth with a view to improving spectrum efficiency. It is now said that a system will allocate channel bandwidths perhaps 100 or 1,000 times greater than would normally be required. Perhaps more surprising even than that, is that noise and interference are tolerated at a level usually considered disastrous. This unconventional approach to system design results in many claims for benefits offered by spread spectrum. 1.2. Multiple Access Schemes. The frequency spectrum can be divided in two ways. There are two finite resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). Whilst these systems can be very complex, the principles on which they are based are simple to visualize. In Code Division Multiple Access (CDMA), however, every communicator will be allocated all of the spectrum all of the time. It is much harder to visualize how this could result in anything but unacceptable interference. Imagine a room full of people who are all simultaneously in conversation, and all speaking at about the same level. Each listener’s partner is at least matched in volume by the ambient background noise level in the room. If they are all speaking a different language, then it is reasonable to assume that despite the noise, communication will be possible. This is because of the uniqueness of each individual’s information compared to the noise.. 2.1. © Wray Castle Limited. MB2002/S2/v8.

(43) UMTS Air Interface. Sender. Receiver FDMA 1. 2. Frequency. 3. Time TDMA. 1. 2. 3. 1. 2. 3. 1. 2. 3. CDMA. 1, 2 and 3. Figure 1 Multiple Access Schemes MB2002/S2/v8. © Wray Castle Limited. 2.2.

(44) UMTS Air Interface. 2. THE SPREADING AND DESPREADING PROCESS 2.1. The Code Application. CDMA uses a unique spreading code to spread the baseband data before transmission. The signal is transmitted in a channel which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be despread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.. 2.3. © Wray Castle Limited. MB2002/S2/v8.

(45) UMTS Air Interface. ƒ bandwidth of spread signal on the air interface ƒ Recovered narrowband data spectrum. ƒ spectrum of narrowband data. Correlator. RF. spreading sequence generator. RF despreading sequence generator. ƒ. ƒ. spectrum of baseband spreading sequence. spectrum of baseband despreading sequence. Figure 2 Simplified Spreading/Despreading Concept MB2002/S2/v8. © Wray Castle Limited. 2.4.

(46) UMTS Air Interface. 2.2. Direct Sequence (DS) Spread Spectrum. The code may be used to spread the baseband information by a number of different methods, but Direct Sequence (DS) is perhaps the most common, especially amongst digital systems. It is called ‘Direct Sequence’ because the spreading process is achieved by directly combining a high bit rate (or chip rate as it is described) binary code to the baseband information, which is also in a binary format. Manipulating and spreading the information in this digital format greatly simplifies the Radio Frequency (RF) design. 2.2.1. Baseband Spreading in the Transmitter. The baseband information and the code are presented in a bipolar format. The code is multiplied with the baseband bit, resulting in a repeat of the code if the baseband data is a logic ‘1’, or the inverse of the code if the data is a logic ‘0’. The modulated code is then modulated onto the RF carrier for transmission across the radio interface.. 2.5. © Wray Castle Limited. MB2002/S2/v8.

(47) UMTS Air Interface. +1 Baseband Data. 1. 0. 1. 1. 0. –1. +1 Code. 1 0 1 1 0 1 00 1 0 1 1 0 1 0 0 1 0 1 1 0 10 0 1 0 1 1 0 1 0 0 1 01 1 01 0 0. –1. Resultant Spread Baseband Signal. +1 1 0 11 0 1 0 0 0 1 0 0 1 0 1 1 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 0 1 0 0 1 0 1 1. –1. Figure 3 Direct Sequence (DS) Baseband Spreading MB2002/S2/v8. © Wray Castle Limited. 2.6.

(48) UMTS Air Interface. 2.2.1. Baseband Recovery in the Receiver. A receiver will receive all the encoded channels as they are all on the same frequency simultaneously, but it will only despread the wanted channel whose code sequence is known. Not only must the correct code be used, it must also be synchronized with its application on the wanted channel component of the incoming encoded bit stream. The remultiplication of the aligned code results in the recovery of the original baseband information. The application of the same code to other encoded channels will not result in their despreading, as they will still have changes at the chip rate. Actual recovery of the baseband information is achieved by integrating the waveform over a bit interval after the code has been applied. The wanted channel will integrate to a positive value if the underlying bit is a logic ‘1’ or to a negative value if the bit is a logic ‘0’, with detection being made against a threshold. Unwanted channels will integrate to zero as they are still random. RF demodulation and channel distortion and noise have been ignored to aid clarity.. 2.7. © Wray Castle Limited. MB2002/S2/v8.

(49) UMTS Air Interface. +1 Received Signal. 1 0 11 0 1 0 0 0 1 0 0 1 0 1 1 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 0 1 0 0 1 0 1 1. –1. +1 Code. 1 0 1 1 0 1 00 1 0 1 1 0 1 0 0 1 0 1 1 0 10 0 1 0 1 1 0 1 0 0 1 01 1 01 0 0. –1. +1 Baseband Data. 1. 0. 1. 1. 0. –1. Figure 4 DS Receiver Despreading MB2002/S2/v8. © Wray Castle Limited. 2.8.

(50) UMTS Air Interface. 2.2.3. Exercise 1. The following chip sequence is the result of spreading by a factor of 4: 1101110100101101 You are given a choice of two spreading codes with which to attempt to despread the sequence and recover the original bit sequence. Code A 1101 Code B 1001 In the spaces provided, apply code A to the received chip sequence, then repeat the process with code B. Which is the correct spreading code, A or B? Assuming that the chip sequence was received at a rate of 3.84 Mcps, what are the rates of the following: • The bit sequence after multiplication with code A? • The bit sequence after multiplication with code B?. Assuming that the chip sequence was received with a bandwidth of 3.84 MHz, what are the bandwidths of the following: • The signal after multiplication with code A? • The signal after multiplication with code B?. 2.9. © Wray Castle Limited. MB2002/S2/v8.

(51) UMTS Air Interface. Chip Sequence. +1 1. 1. 0. 1. 1. 1. 0. 1. 0. 0. 1. 0. 1. 1. 0. 1 –1 +1. Code A. 1. 1. 0. 1. 1. 1. 0. 1. 1. 1. 0. 1. 1. 1. 0. 1 –1 +1. Result –1. Chip Sequence. +1 1. 1. 0. 1. 1. 1. 0. 1. 0. 0. 1. 0. 1. 1. 0. 1 –1 +1. Code B. 1. 0. 0. 1. 1. 0. 0. 1. 1. 0. 0. 1. 1. 0. 0. 1 –1 +1. Result –1. Figure 5 Exercise 1 MB2002/S2/v8. © Wray Castle Limited. 2.10.

(52) UMTS Air Interface. 3. SPREADING AND MULTIPLE ACCESS 3.1. Shannon’s Equation. Shannon’s equation for channel capacity describes the relationship between the ability of a channel to transfer error-free information, the Signal to Noise Ratio (SNR) in the channel and the bandwidth used to transmit the information: C = Blog2(1+SNR) Where: B is bandwidth in Hz C is capacity in bit/s This suggests that very low, or even negative, values of SNR can be tolerated providing that the bandwidth of the channel is wide enough. This is the principle that underlies the spread spectrum radio technique. The result is a channel with a very high tolerance to noise and interference. 3.2. Application for Multiple Access. The spread spectrum technique can be adapted for use as a multiple access scheme if the interference margin is used to tolerate the high levels of interference generated when multiple users in the same area transmit on the same frequency at the same time. In this case, allocating different spreading code sets to each user separates different users. The mutual interference is tolerated as long as there remains an acceptable positive level of SNR after the recovered channel had been despread and filtered. This resulting positive signal to noise ratio in the recovered channel is measured in terms of energy per bit relative to noise (Eb/No). It is because separation is by code that the technique is described as Code Division Multiple Access (CDMA). Each additional user adds to the overall noise level in the radio channel. The capacity limit for the number of users is reached when the Eb/No is no longer acceptable in each individual user’s recovered channel. This is not a hard limit since it is influenced by many factors, some of which are determined by system configuration and some of which are variable and instance specific. Because of this, CDMA systems are sometimes described as exhibiting ‘soft block’ characteristics.. 2.11. © Wray Castle Limited. MB2002/S2/v8.

(53) UMTS Air Interface. Low bandwidth baseband channel data.. Transmitter. Spread with channel code. Modulated onto radio carrier. High bandwidth, low power density.. Radio channel. Transmitted in channel on the same frequency as other channels. Complete receive signal including all channels is demodulated.. Receiver. Channel code applied to despread wanted channel. Filtering applied to remove the majority of wideband noise power.. Eb/No. Figure 6 CDMA Channel and User Noise MB2002/S2/v8. © Wray Castle Limited. 2.12.

(54) UMTS Air Interface. 3.3. Processing Gain (Gp) and Spreading Factor (SF). The most commonly used quantity in describing or specifying spread spectrum systems is processing gain. This is a readily available quantity if the system bandwidth and information rate are known. Processing gain is quoted in dBs and can be estimated by: BWRF. Gp =. BINFO. Where: Gp = processing gain BWRF = bandwidth of the transmitted signal BINFO = bandwidth of the information signal or: Gp =. chip rate data rate. In general terms, the processing gain of a system describes the improvement in SNR from the received signal to the output of the receiver. When a simple ratio of chip rate and data rate are used, the processing gain is usually just described as Spreading Factor (SF). The term processing gain is reserved for use when the ratio is expressed in terms of decibels. Example: For a system with an information rate of 30 kbit/s and a chip rate of 3.84 Mcps: SF =. 3.84 Mcps 30 kbit/s. = 128. Gp = 21 dB. 2.13. © Wray Castle Limited. MB2002/S2/v8.

(55) UMTS Air Interface. Processing gain =. Radio Bandwidth Baseband Bandwidth. ≈. Chip Rate Bit Rate. Limits: radio bandwidth limited by practical radio design issues and regulatory issues baseband bit rate depends on the channel capacity required. Figure 7 Processing Gain Limits MB2002/S2/v8. © Wray Castle Limited. 2.14.

(56) UMTS Air Interface. 3.4. Processing Gain and Cell Capacity. The capacity of a cell is essentially limited by the total noise level at the receiver and the system’s ability to tolerate this total noise. There are many variable factors that influence this in a particular cell instance, but two important system design parameters are the processing gain, Gp, and the minimum acceptable signal to noise ratio in the recovered channel, Eb/No. In general terms these are related to the total number of simultaneous users in the cell with the following approximation: Xmax ≈. Gp Eb/No. Where Xmax is the maximum number of users in the cell.. This suggests two approaches for increasing capacity. Firstly, processing gain can be increased, and secondly, the required value of Eb/No can be reduced. The chip rate for the FDD mode UMTS is 3.84 Mcps. This allows a radio carrier to operate in 5 MHz spectrum blocks. This can be considered fixed for the UMTS FDD mode of operation. Therefore the only way to increase processing gain would be to reduce the channel bit rate. This means that the number of users in a UMTS cell is significantly influenced by the bit rate being provided to each user. It is possible to support a large number or low bit rate connections or a much smaller number of high bit rate connections. The minimum acceptable level or Eb/No can be influenced through improvements in the performance of the channel forward error protection scheme. More advanced forms of forward error protection have been specified for UMTS, which mean significant potential increases in cell capacity. However, the increase in required processing speed to support these schemes prohibits their use for real-time traffic such as voice or video telephony.. 2.15. © Wray Castle Limited. MB2002/S2/v8.

(57) UMTS Air Interface. wray castle Browser. wray castle Browser. Internet Search: http//www.. Internet Search: http//www.. XXXXXXXXxxxXX XXXxXXXX X. XXXXXXXXxxxXX XXXxXXXX X. XXXXXXXXXXXX XXXXxxxxxXX XX. XXXXXXXXXXXX XXXXxxxxxXX XX. XxXX XXX XXX XXX XXX X. XxXX XXX XXX XXX XXX X. Gp Processing Gain. wray castle Browser Internet Search: http//www.. wray castle Browser Internet Search: http//www.. XXXXXXXXxxxXX XXXxXXXX X. XXXXXXXXxxxXX XXXxXXXX X. XXXXXXXXXXXX XXXXxxxxxXX XX. XXXXXXXXXXXX XXXXxxxxxXX XX. XxXX XXX XXX XXX XXX X. XxXX XXX XXX XXX XXX X. Node B. User contributes power P and has x – 1 interferers. wray castle Browser. wray castle. Browser. Internet Search: http//www. Internet Search:. XXXXXXXXxxxXX XXXxXXXX X. http//www.. XXXXXX XXxxxXX XXXxXX XX X. XXXXXXXXXXXX XXXXxxxxxXX XX. XXXXXX XXXXXX XXXXxxx xxXX XX. XxXX XXX XXX XXX XXX X. XxXX XXX XXX XXX XXX X. This suggests that: Gp xmax ≈ Eb / No. Figure 8 Processing Gain and Capacity MB2002/S2/v8. © Wray Castle Limited. 2.16.

(58) UMTS Air Interface. 3.5. Multiple Cells. The situation is more complex when multiple cell contiguous coverage is considered. As seen from a serving cell, users in neighbouring cells also contribute to interference in the serving cell. This extra interference will reduce the capacity of any given cell in a system when compared to the same cell operated in isolation. The result is that prediction of expected system capacity in an area is not simply the sum of the theoretical capacities of each individual cell. Useable capacity will depend on the amount, type and distribution of traffic in a region.. 2.17. © Wray Castle Limited. MB2002/S2/v8.

(59) UMTS Air Interface. Cell A. Cell B. Cell A in isolation 10 channels. Cell B in isolation 10 channels. Interference. Capacity of Cells A and B is less than 20 because of increased interference. Figure 9 Effect on Capacity with Contiguous Coverage MB2002/S2/v8. © Wray Castle Limited. 2.18.

(60) UMTS Air Interface. 4. CODE GENERATION 4.1. Code Correlation. This is important when designing codes to be used in spread spectrum systems. The code is not usually required to provide call security, but it must possess sufficient uniqueness to enable call identification and successful recovery. Correlation may be measured by the number of agreements minus the number of disagreements between two code sequences, against time in chips. 4.1.1. Auto Correlation. This is the amount of similarity between a given code sequence and a time-shifted version of itself. Auto correlation is relevant when considering the effects upon a receiver decoding a signal that is subject to time dispersion due to multipath effects. 4.1.2. Cross Correlation. This is the amount of similarity between two separate codes. For successful multiple communications, codes must be as different as possible for all time-shifted combinations. This is relevant when different codes are used to separate different communications. 4.1.3. Spectral Characteristics. It is important that the Power Spectral Density (PSD) of the code sequence in the frequency domain resembles ‘white’ noise (i.e is spectrally flat) in order to allow simultaneous multi-user access. That is to say, all other users are perceived as noise by any given user.. 2.19. © Wray Castle Limited. MB2002/S2/v8.

(61) UMTS Air Interface. Key code characteristics auto correlation cross correlation white-noise-like spectral characteristic. Figure 10 Key Code Characteristics MB2002/S2/v8. © Wray Castle Limited. 2.20.

(62) UMTS Air Interface. 4.2. Choice of Codes. As has been explained, there are several key characteristics that are required of codes used in a CDMA system. Unfortunately, there is no single code type which exhibits ideal characteristics in all areas. For channel separation on a cell, a set of codes is required to exhibit good cross correlation to increase capacity. For cell identification there are a number of options for code implementation but, in general, the requirement is usually a good balance between auto correlation and cross correlation. For optimal spreading with an even distribution of power across the transmitted bandwidth, the requirement is usually a noise-like pseudo-random nature. 4.2.1. Code Types. There are three main types of code which are commonly used: • pseudo-random codes • combinational codes • orthogonal code sets Since no one type of code is ideal, the coding and spreading process is typically multistage, incorporating several codes, each suited to one particular function.. 2.21. © Wray Castle Limited. MB2002/S2/v8.

(63) UMTS Air Interface. Pseudo-random codes Combinational codes Orthogonal code sets. Figure 11 Main Code Types MB2002/S2/v8. © Wray Castle Limited. 2.22.

(64) UMTS Air Interface. 5. PSEUDO-RANDOM CODES 5.1. Maximal Linear Codes. Linear codes are commonly used, since they are simple to produce and have good auto correlation properties. These codes are not intended to be cryptographically secure, but will be combined with baseband data before modulation. Maximal linear codes, or m-sequences, are the longest code sequences that can be generated by a given number of logically interconnected shift registers. In binary sequence shift register generators, the code length N is: N = 2n – 1 chips where: n = Number of stages The missing code state is the not-allowed all zeros. The number of possible codes generated in this fashion is approximately equal to √ N The necessary feedback connections for modulo 2 addition have been tabulated for shift registers of 3 to 100 stages. The circuit for a code length of 7 is shown in the diagram. Since these generators can be easily produced with up to 100 stages, code sequences can be very long indeed.. 2.23. © Wray Castle Limited. MB2002/S2/v8.

(65) UMTS Air Interface. A. B. C. Clock steps. A. B. C. 0. 1. 1. 1. 1. 0. 1. 1. 2. 1. 0. 1. 3. 0. 1. 0. 4. 0. 0. 1. 5. 1. 0. 0. 6. 1. 1. 0. 1. 1. 1. O/P. etc.. The output will be of code length 7, i.e. 1 1 1 0 1 0 0. Figure 12 Maximal Length Code Generator (Length 7) MB2002/S2/v8. © Wray Castle Limited. 2.24.

(66) UMTS Air Interface. 5.2. Auto Correlation Properties of Pseudo-Random Codes. It can be seen that when compared with a time shifted-version of itself, a code of this type shows almost complete orthogonality. This property can be utilized such that all users have the same code, but with a unique time offset. However, in order to function, time offset would need to be fixed in a fully synchronous system.. 2.25. © Wray Castle Limited. MB2002/S2/v8.

(67) UMTS Air Interface. Shift. Sequence. 0 1 2 3 4 5 6. 1110100 0111010 0011101 1001110 0100111 1010011 1101001. Agreements (A) Disagreements (D) 7 3 3 3 3 3 3. A–D. 0 4 4 4 4 4 4. 7 –1 –1 –1 –1 –1 –1. 7. 0 –1 0. –2 chip –1 chip. +1 chip. +2 chip. Time. Figure 13 Auto Correlation MB2002/S2/v8. © Wray Castle Limited. 2.26.

(68) UMTS Air Interface. 5.3. Cross Correlation Properties of Pseudo-Random Codes. The cross correlation properties of short pseudo-random codes are poor when the code length is short. The cross correlation properties of this type of code may be improved by increasing the code length. However, increasing the code length increases the complexity of the code generator and it is difficult to generate a large set of different codes.. 2.27. © Wray Castle Limited. MB2002/S2/v8.

(69) UMTS Air Interface. Code 1 = 1 1 1 0 1 0 0 Code 2 = 1 1 1 0 0 1 0 Shift. Code 1/shifted Code 2. A–D. 0. 1110100 1110010. 3. 1. 1110100 0111001. –1. 2. 1110100 1011100. 3. 3. 1110100 0101110. –1. 4. 1110100 0010111. –1. 5. 1110100 1001011. –5. 6. 1110100 1100101. 3. 7. 1110100 1110010. 3. 3. 3. 0 Time –1. –5. Figure 14 Cross Correlation MB2002/S2/v8. © Wray Castle Limited. 2.28.

(70) UMTS Air Interface. 6. COMBINATIONAL CODES 6.1. Gold Codes. Maximal linear code sequences have many combinational properties. Probably the most interesting of these occurs with the modulo 2 additions of certain pairs of m-sequences (referred to as preferred pairs) of the same length. The result is found to be a new code of the same length. More important is that each phase shift produces another new code. The new codes are not maximal codes, but this is clearly a simple method of producing a generator with a large number of code options. When two code generators are used in this way, the result is a Gold code. It can be seen that if the codes to be combined are of length 2n – 1, then the number of code options is also 2n – 1. When compared to m-sequence codes, combinational codes have the advantage that a simpler code generator may be used to produce more, longer codes which exhibit the desired cross correlation properties and spectral characteristics.. 2.29. © Wray Castle Limited. MB2002/S2/v8.

(71) UMTS Air Interface. Shift. Code 1/Code 2. Resulting Codes. 0. 1110100 1110010. 0000110. 1. 1110100 0111001. 1001101. 2. 1110100 1011100. 0101000. 3. 1110100 0101110. 1011010. 4. 1110100 0010111. 1100011. 5. 1110100 1001011. 0111111. 6. 1110100 1100101. 0010001. 7. 1110100 1110010. 0000110. Code Generator 1 Clock. Gold Code Code Generator 2. Figure 15 Gold Code Generation MB2002/S2/v8. © Wray Castle Limited. 2.30.

(72) UMTS Air Interface. 7. ORTHOGONAL CODE SETS 7.1. Orthogonal Code Derivation. Code generators are not used for this type of code. Rather, they are produced as a predetermined set of codes which are orthogonal from each other. There are various codes that fall into this category, including Walsh–Hadamard codes. Because of the good orthogonality within the code set, these codes are well suited to the task of channel separation. There are two main limitations associated with the use of this type of code. The first is that they are only truly orthogonal when in correct time alignment. Then they are only suited to channel separation in the downlink direction since there will not be time alignment between UEs in the same cell. They could be used to separate channels in the uplink that have been allocated simultaneously to a single UE, provided a second level of code is also used to separate UEs. The second limitation is that these codes are not noise-like and pseudo-random. The result is that different codes within the code set will spread by different amounts even at the same chip rate. Therefore, when codes of this type are used, they need to be followed by a suitable, more noise-like scrambling code.. 2.31. © Wray Castle Limited. MB2002/S2/v8.

(73) UMTS Air Interface. General rule: Wk =. Wk/2 Wk/2 Wk/2 Wk/2. Where k is the number and length of codes in set.. W1 = ( 1 ) W2 =. 1 1 1 –1. W4 =. 1 1 1 1 1 –1 1 –1 1 1 –1 –1 1 –1 –1 1. W8 =. 1 1 1 1 1 1 1 1 1 –1 1 –1 1 –1 1 –1 1 1 –1 –1 1 1 –1 –1 1 –1 –1 1 1 –1 –1 1 1 1 1 1 –1 –1 –1 –1 1 –1 1 –1 –1 1 –1 1 1 1 –1 –1 –1 –1 1 1 1 –1 –1 1 –1 1 1 –1. Figure 16 Walsh–Hadamard Codes MB2002/S2/v8. © Wray Castle Limited. 2.32.

(74) UMTS Air Interface. 7.2. Orthogonal Code Tree. In UMTS, channels for different users can operate at different bit rates. Since the length of the orthogonal code is related to baseband bit period, different bit rates will mean that different code lengths will need to be used simultaneously on one CDMA radio carrier. Not all Walsh code combinations are orthogonal when codes are taken from different matrices. The acceptable code combinations are more easily identified if a tree construction is used rather than the matrix construction. This is the approach used in UMTS, and the resulting codes are referred to as Orthogonal Variable Spreading Factor (OVSF) codes. Once a code is allocated to a user there are two rules that determine which other codes can be allocated to other users. No other code can be used that is in the path to the root of the tree for the allocated code; nor is it possible to use any code in the branches derived directly from the allocated code. This is illustrated in the lower part of the diagram with the allocation of a four-chip code.. 2.33. © Wray Castle Limited. MB2002/S2/v8.

(75) UMTS Air Interface. –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 +1 +1 +1 +1 –1 –1 –1 –1 +1 +1 –1 –1 +1 +1 –1 –1 +1 +1 –1 –1 +1 +1 +1 +1 –1 –1 –1. –1 +1 –1 +1 –1 +1 –1 +1. SF = 1. –1 +1 –1 +1. –1 +1 –1 +1 +1 –1 +1 –1. –1 +1. SF = 2. –1 +1 +1 –1 –1 +1 +1 –1 –1 +1 +1 –1. SF = 4. –1 +1 +1 –1 +1 –1 –1 +1. SF = 8 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 –1 +1 +1 +1 +1 –1 –1 –1 –1 +1 +1 –1 –1 +1 +1 –1 –1 +1 +1. X –1. Allocated. SF = 1. –1 +1 –1 +1. X. –1 +1. SF = 2. –1 –1 +1 +1 +1 +1 –1 –1. X X. –1 +1 –1 +1 –1 +1 –1 +1 –1 +1 –1 +1 +1 –1 +1 –1. X X. –1 +1 +1 –1 –1 +1 +1 –1 –1 +1 +1 –1. SF = 4. –1 +1 +1 –1 +1 –1 –1 +1. SF = 8. Figure 17 Orthogonal Code Tree MB2002/S2/v8. © Wray Castle Limited. 2.34.

(76) UMTS Air Interface. 7.3. Orthogonal Code Application. For UMTS FDD mode, orthogonal codes are used for channel separation within a cell or within a UE’s transmission. Each channel will be spread using a code from a set of OVSF codes, then channels can be combined using a summation process. In the example shown, three data streams are spread using different OVSF codes from a potential set of four. The resulting chip sequences are then combined to produce a single, multi-level waveform.. 2.35. Data stream A Data stream B Data stream C. 1 1 0 1 0 0 1 0 1. OVSF code 1 OVSF code 2 OVSF code 3. 1 –1 1 –1 1 1 –1 –1 1 –1 –1 1. © Wray Castle Limited. MB2002/S2/v8.

(77) UMTS Air Interface. +1 Data stream A. 1. 1. 0 –1 +1. OVSF code 1. 1 –1 1 –1 1 –1 1 –1 1 –1 1 –1 –1 +1. Chip sequence 1. 1 –1 1 –1 1 –1 1 –1 –1 1 –1 1 –1 +1. Data stream B. 1. 0. 0 –1 +1. OVSF code 2. 1. 1 –1 –1 1. 1 –1 –1 1. 1 –1 –1 –1 +1. Chip sequence 2. 1. 1 –1 –1 –1 –1 1. 1 –1 –1 1. 1 –1 +1. Data stream C. 1. 0. 1 –1 +1. OVSF code 3. 1 –1 –1 1 1 –1 –1 1 1 –1 –1 1 –1 +1. Chip sequence 3. 1 –1 –1 1 –1 1 1 –1 1 –1 –1 1 –1 3 2 1 0 –1 –2 –3. Summation of chip sequences 1, 2 and 3. Figure 18 OVSF Codes in Combination MB2002/S2/v8. © Wray Castle Limited. 2.36.

(78) UMTS Air Interface. 7.4. Exercise 2. In the following example you are presented with a received chip sequence, which has not been despread. You will be allocated one of the OVSF (4) codes in order to recover a channel. The chip sequence also contains a small number of errors in order to simulate noise in the channel. Using your allocated OVSF code, despread the chip sequence and recover the underlying data stream in the following stages: 1.. Perform a chip-wise multiplication of the received sequence and OVSF code.. 2.. Integrate over each bit-period to produce a positive or negative result.. 3.. For each bit-period use the following mapping rule to produce the data stream: Positive Negative. 2.37. → →. data 1 data 0. © Wray Castle Limited. MB2002/S2/v8.

(79) UMTS Air Interface. 3 2 1. Received chip sequence. 0 –1 –2 –3. 1 0. OVSF code. –1. 3 2 1 0. Result. –1 –2 –3. Integration. Data stream. Figure 19 Exercise 2 MB2002/S2/v8. © Wray Castle Limited. 2.38.

(80) UMTS Air Interface. 2.39. © Wray Castle Limited. MB2002/S2/v8.

(81) UMTS Air Interface. SECTION 3. ENGINEERING FOR CDMA OPERATION. © Wray Castle Limited. i.


(83) UMTS Air Interface. CONTENTS 1. Pilot Signals 1.1 Pilot Transmission 1.2 Common Pilot Signal 1.3 Channel Associated Pilot Signal. 3.1 3.1 3.1 3.1. 2. CDMA Receiver 2.1 Code Synchronization 2.2 Acquisition 2.3 Tracking. 3.3 3.3 3.3 3.3. 3. Rake Diversity 3.1 Multipath 3.2 Rake Receiver. 3.5 3.5 3.5. 4. The Near–Far Effect 4.1 Mobiles at Different Ranges 4.2 Dynamic Range 4.3 The Need for Fast Power Control 4.4 Open Loop Power Control. 3.7 3.7 3.7 3.9 3.11. 5. Soft Handover 5.1 The Need for Soft Handover 5.2 Power Control During Soft Handover 5.3 Soft Handover Regions 5.4 Power Budget and Coverage 5.5 Planning for Soft Handover Area. 3.13 3.13 3.15 3.17 3.19 3.19. 6. Noise and Interference Reduction Techniques 6.1 Introduction 6.2 Directional (Sectored) Antennas 6.3 Diversity Reception 6.4 Beamforming Antennas 6.5 Adaptive Antennas 6.6 Discontinuous Transmission 6.7 Mast Head Amplifiers 6.8 Multi-User Detection (MUD). 3.21 3.21 3.21 3.21 3.23 3.23 3.25 3.25 3.27. © Wray Castle Limited. iii.


(85) UMTS Air Interface. OBJECTIVES At the end of this section you will be able to: • • • • • • • •. identify the need for and usage of pilot signals in CDMA systems describe the implementation of a basic CDMA receiver and the process of initial synchronization, tracking and data despreading identify how multipath propagation can be used as an advantage when Rake reception is deployed explain the near–far problem and describe the requirements for fast power control explain why soft handover is used in CDMA-based systems describe the impact of cell breathing explain the considerations for system load and implementation of multimedia services explain the various techniques used to minimize noise and interference in a CDMA-based system. © Wray Castle Limited. v.

(86) UMTS Air Interface. 1. PILOT SIGNALS A pilot signal is a known sequence of bits, which allows a UE to acquire code rate and timing alignment. It also provides a phase reference for coherent demodulation and a means for signal strength comparisons between neighbouring cells in order to determine when a handover should occur. 1.1. Pilot Transmission. Broadly there are two types of downlink pilot signals: a continuous common pilot signal and a channel-associated pilot signal. 1.2. Common Pilot Signal. This involves the allocation of a channel on each radio carrier solely for the transmission of a continuous pilot signal. A common pilot is used to assist in initial synchronization. The UE’s receiver will also track the pilot to enable despreading of data-carrying channels. The disadvantage with a common pilot signal is that the pilot must be transmitted at a significantly higher power level than the other channels. This will reduce capacity in both the serving cell and neighbour cells. If in-fill cells are used to increase capacity, the extra radiated power in the pilots can cause a situation known as pilot pollution. 1.3. Channel-Associated Pilot Signal. This involves the transmission of pilot bits within a channel carrying traffic and signalling information to a specific UE. The pilot bits are used to aid radio channel estimation and maintain timing alignment.. 3.1. © Wray Castle Limited. MB2002/S3v8.

(87) UMTS Air Interface. Pilot Signall. ing/Tra. ffic er. wray castle Brows. er. wray castle Brows. Internet Search: http//www.. Internet Search: http//www.. X. XXXXXXXXxxxX X XXXxXXXX. X. XXXXXXXXxxxX X XXXxXXXX. XXXXXXXXXXXX XX XXXXxxxxxXX. XXXXXXXXXXXX XX XXXXxxxxxXX. XXX XxXX XXX X XXX XXX. XXX XxXX XXX X XXX XXX. wray castle Brows. wray castle Brows. er. er. Internet Search: http//www.. Internet Search: http//www.. X. XXXXXXXXxxxX X XXXxXXXX. X. XXXXXXXXxxxX X XXXxXXXX. XXXXXXXXXXXX XX XXXXxxxxxXX. XXXXXXXXXXXX XX XXXXxxxxxXX. XXX XxXX XXX X XXX XXX. XXX XxXX XXX X XXX XXX. all UEs use the same common pilot Figure 1a Common Pilot Signal. Traff. ic Sig nal. ling. Pilot. wray castle Brows. er. Internet Search: http//www.. X. XXXXXXXXxxxX X XXXxXXXX. XXXXXXXXXXXX XX XXXXxxxxxXX XXX XxXX XXX X XXX XXX. Pilot specific to a UE. Figure 1b Channel-Associated Pilot MB2002/S3v8. © Wray Castle Limited. 3.2.

(88) UMTS Air Interface. 2. CDMA RECEIVER Once a suitable modulation scheme has been decided upon, the appropriate demodulator may be implemented. The incoming RF signal will be down converted to a suitable Immediate Frequency (IF), analogue-to-digital converted, despread, filtered and passed for data detection. To achieve this, the receiver must synchronize to the carrier frequency and the spreading code. A top-level block diagram of a CDMA receiver is illustrated. To simplify the explanation it is assumed that carrier frequency synchronization has been obtained. 2.1. Code Synchronization. Usually the code synchronization process is performed in two phases, acquisition followed by tracking. In the acquisition phase a coarse alignment of the receiver clock to within one chip is achieved. Once acquisition is attained the tracking phase can begin. The tracking phase performs a fine alignment of the receiver clock to within a fraction of a chip. 2.2. Acquisition. This is often implemented using a matched filter, whose impulse response is matched to the time reversed impulse of a pilot signal. When the pilot waveform is received, the matched filter produces an output pulse. The output is averaged over several successive chip periods until the peak region is detected. This amounts to tracing the autocorrelation curve of the pilot code sequence. 2.3. Tracking. A Delay-Lock Loop (DLL) is often used to maintain fine code alignment. The DLL consists of two tracking correlators, denoted Early and Late, and a data correlator. The code applied to the Late correlator lags the code applied to the Early correlator by one chip period. The feedback is such that it attempts to equalize the output values of the two tracking correlators. The Early, Data and Late correlators perform a correlation on three successive chip phases of the input signal spaced one half-chip period apart, so when the Early and Late output amplitudes are equal, the Data correlator will be perfectly aligned to the input signal and will produce the peak output value. The despread signal is then passed for data detection.. 3.3. © Wray Castle Limited. MB2002/S3v8.

(89) UMTS Air Interface. Local Oscillator RF Mixer. Input signal. IF Analogue to Digital. CDMA Processes. Matched Filter. Detection and Error Correction. Threshold Detector. ACQUISITION Spreading Code Clock. Spreading Code Generator. late code Early/Late Correlators early code Data Correlators. data code despread user data. TRACKING. Figure 2 Basic CDMA Receiver Block Diagram MB2002/S3v8. © Wray Castle Limited. 3.4.

(90) UMTS Air Interface. 3. RAKE DIVERSITY 3.1. Multipath. Once the receiver is synchronized and locked, the process of extracting the optimum signal can begin. Most terrestrial radio systems suffer from multipath propagation, where signals arrive at the receiver from a number of paths of differing lengths. The consequence is a summation of randomly phased signals that can cause additive or destructive interference. Mobile subscribers in particular suffer from this problem as there is rarely a dominant path and the fading is Rayleigh based. However, CDMA systems, being based on high-chip-rate digital signals, offer the ability to use multipath to their advantage by employing rake diversity. 3.2. Rake Receiver. Time, hence distance, resolution in a pulse system is governed by pulse duration; in this case a chip period (Tc). By employing a number of correlation receivers with the same code but displaced by increasing time intervals of Tc, not only can the most direct received signal be detected, but also multipath signals of increasing additional path delay. With reference to the rake receiver illustrated, the output from the matched filter identifies the time delay positions at which significant energy arrives and allocates rake fingers to those peaks (selection combing). The correlators in each rake finger are now synchronized to one of the path delays. A pilot signal is employed to sound the air interface channel. The despread pilot symbols are averaged over several chip periods in order to improve estimation quality, and applied to an adaptive channel estimator. The output from the channel estimator corrects any phase or amplitude distortion (caused by the radio path) in the despread data. The delay equalizer compensates for the time offsets between rake fingers, allowing the outputs from each rake finger to be summed coherently. Phase and amplitude correction followed by coherent summation of the rake finger outputs is referred to as maximal ratio combing. The maximally combined despread data signal can then be passed on for data detection.. 3.5. © Wray Castle Limited. MB2002/S3v8.

(91) UMTS Air Interface. Down Converted Baseband Signal. Phase and Amplitude Correction. Data Correlator. Pilot Correlator. Delay Equalization. Maximally Combined Signal. Adaptive Channel Estimator Finger 1 Finger 2 Finger N Code Generator. Matched Filter. Path delay profile Tracking and Code Clock. Figure 3 Rake Receiver MB2002/S3v8. © Wray Castle Limited. 3.6.

(92) UMTS Air Interface. 4. THE NEAR–FAR EFFECT 4.1. Mobiles at Different Ranges. In direct sequence systems using multiple access, a problem arises when UEs using a Node B are at different ranges. Consider a Node B that is serving two UEs simultaneously. One UE is 3 km distant and the other is 30 m distant. Their distance ratio is: 3 0.03. = 100. Thus, if they are both transmitting the same power, then their power ratio at the Node B, assuming inverse square law propagation, is: 1002 = 10,000 In order to recover both signals successfully, the system must have an interference margin of 40 dB. In order to avoid this problem, very precise and frequently updated power control of the UEs is required. The aim is to ensure that all signals arriving at the Node B are at the same level (ideally within ±1 dB). 4.2. Dynamic Range. If power contributions from all UEs need to be within ±1 dB at the Node B, then dynamic range for the UE needs to reflect the likely variation in path loss across the cell area. Typically, this is as much as 80 dB. Downlink power control also offers benefits, but is less important. If it is used at all, dynamic range will probably be much less, perhaps less than 20 dB.. 3.7. © Wray Castle Limited. MB2002/S3v8.

(93) UMTS Air Interface. 3 km er. wray castle Brows. 30 m. Internet Search: http//www.. X. XXXXXXXXxxxX X XXXxXXXX. XXXXXXXXXXXX XX XXXXxxxxxXX XXX XxXX XXX X XXX XXX. er wray castle Brows. UE A. Internet Search: http//www.. X XXXXXXXXxxxX X XXXxXXXX XXXXXXXXXXXX XX XXXXxxxxxXX. Node B. XXX XxXX XXX X XXX XXX. UE B. Distance Ratio. 3 0.03. = 100. Power Ratio with Square Law Propagation = 1002 = 10,000 Interference Margin Required = 40 dB. Figure 4 Near–Far Problem MB2002/S3v8. © Wray Castle Limited. 3.8.

(94) UMTS Air Interface. 4.3. The Need for Fast Power Control. Dynamic power control is common in all cellular systems. In Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) systems, where frequency planning is used, power control commands can be infrequent, and coarse power steps can be used. This is because the transmission of too much power has less effect on other users in either the serving cell or in neighbour cells. In a CDMA system it is important that the power contribution from each user into the Node B is carefully balanced. If any user’s UE transmits too much power, even for a short period of time, then it will degrade the communications of all other users in the cell and probably those of some users in neighbour cells also. Corner effect and movement out of multipath fades are a particular problem. Very frequent power commands are required to control this, typically several hundred per second. More frequent power commands should increase capacity, particularly in urban areas.. 3.9. © Wray Castle Limited. MB2002/S3v8.

(95) UMTS Air Interface. As UE A comes out of shadow power must be reduced quickly to avoid degradation of UE B signal.. Building. Node B er. UE needs to transmit high power in shadow. wray castle Brows. wray castle Brows. Internet Search: http//www.. er. Internet Search: http//www.. B. X XXXXXXXXxxxX X XXXxXXXX XXXXXXXXXXXX XX XXXXxxxxxXX XXX XxXX XXX X XXX XXX. XXXXXXXXxxxX X XXXxXXXX. X. XXXXXXXXXXXX XX XXXXxxxxxXX XXX XxXX XXX X XXX XXX. A. er. wray castle Brows Internet Search: http//www.. X. XXXXXXXXxxxX X XXXxXXXX. XXXXXXXXXXXX XX XXXXxxxxxXX. A. XXX XxXX XXX X XXX XXX. Figure 5 Need for Fast Power Control MB2002/S3v8. © Wray Castle Limited. 3.10.

(96) UMTS Air Interface. 4.4. Open Loop Power Control. In a CDMA system, UEs attempting a random or packet access do not have access to exclusive frequency or time in the cell. The UE is separated only by code, and is contributing to interference. It has no closed loop power control commands, yet it is required to meet the criteria for power contribution at the Node B. It must transmit just enough power to get a response, but no more. The two main ways of achieving this are step increment and power estimation. The first method involves a UE stepping up its transmit power gradually until it gets a response. The second method involves a UE getting information from the cell and taking measurements which enable it to estimate the correct power. In practice, a combination of these two methods provides the best solution. The UE will start with a power estimation and then use step increment to find the correct power.. 3.11. © Wray Castle Limited. MB2002/S3v8.

(97) UMTS Air Interface. UE Transmit Power on RACH. step increment. step increment Initial estimated transmit power. First Random Access. Second Random Access. Third Random Access. Time. Figure 6 Example of Open Loop Power Control MB2002/S3v8. © Wray Castle Limited. 3.12.

(98) UMTS Air Interface. 5. SOFT HANDOVER 5.1. The Need for Soft Handover. Power-control CDMA (PCDMA) systems (in which the transmitted power from every user must reach a single receiver at an equal power level) have a particular problem, in that if in adjacent cells the Node B can hear a single user, then provision must be made for that user to be controlled by both Node Bs. The reason is that if the user, being controlled by Node B 1, moves away from 1 while in sight of Node B 2, then Node B 1 is forced to tell the user to increase the transmit power, which must cause cell 2 to be blocked. To alleviate the blocking, cell 2 will then be forced to increase the transmit powers of all its active UEs. This has a ripple effect, causing every cell in sight of cell 2 (including cell 1) to increase power also.. 3.13. © Wray Castle Limited. MB2002/S3v8.

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