TD-LTE 技术及其应用
By Yvonne Liu and Bai Ying, Agilent Technologies, Inc.
Third-generation (3G) wireless systems are deployed all over the world. W-CDMA maintains a mid-term competitive edge by providing high speed packet access (HSPA) in both downlink and uplink modes. Typical cell maximum data rate today is around 7.2 Mbps, and typical single-user data rates of around 1.5 Mbps can be expected. To ensure competitiveness into the future, the long-term evolution (LTE) of the 3rd Generation Partnership Project’s (3GPP) UMTS is first specified in release 8 of the 3GPP specification, and covers the emerging needs of “mobile broadband” into the next decade with cell data rates of over 300 Mbps expected when the system is fully functional.
第三代 (3G) 无线系统部署在世界各地。W-CDMA 通过在下行和上行模式下提供高速数据包访问 (HSPA) 来保持中期竞争优势。如今典型的电池最大数据速率约为 7.2 Mbps, 典型的单用户数据速率约为 1.5 Mbps。为确保未来的竞争力, 第三代伙伴关系项目 (3GPP) UMTS 的长期演变 (LTE) 首次在3GPP 规范的第8版中得到了规定, 并涵盖了未来十年 "移动宽带" 的新需求。当系统完全正常工作时, 预计可超过 300 Mbps 的单元数据速率。
The majority of work to date on LTE has focused on the frequency division duplex (LTE FDD) variant. Following the integration of the Chinese TD-SCDMA standard, based on time division duplex (TDD), into the 3GPP specifications for LTE, chipset and device designers are now working to include TDD capability. Now known as TD-LTE, the standard allows carriers to make use of the unpaired spectrum that many of them already own.
迄今为止, 关于 LTE 的大部分工作都集中在频分双工 (LTE FDD) 变种上。随着基于时间划分双工 (TDD) 的中国 TD-SCDMA 标准的集成, 集成到 LTE 的3GPP 规范中, 芯片组和设备设计人员现在正在努力将 TDD 功能包括在内。现在被称为 TD-LTE, 该标准允许运营商利用他们中的许多人已经拥有的未配对频谱。
Compared to previous standards such as GSM/EDGE and W-CDMA, the timescale from first-generation standards documents to commercial release for LTE in general is short, and for TD-LTE in particular is shorter, due to its later addition into the standards. For handsets and data cards, LTE’s maximum specified RF bandwidth of 20 MHz has driven a change in block diagram and the emergence of standard connections, while the requirement for multi-format devices which include compatibility with legacy systems may lead designers to the increased use of software-defined radios. New designs need more analog/digital cross-domain measurement and "digital-in, RF-out", meaning designers need new tools and measurement methods.
与以前的 GSM/EDGE 和 W-CDMA 等标准相比, 从第一代标准文件到 LTE 的商业发布的时间安排很短, 特别是 TD-LTE 的时间安排较短, 因为它后来被添加到标准中。对于手机和数据卡, LTE 指定的最大 RF 带宽为 20 MHz, 导致框图的变化和标准连接的出现, 而对包括与遗留系统兼容性在内的多格式设备的要求可能会导致设计人员更多地使用软件定义的收音机。新设计需要更多的模拟数字跨域测量和 "数字输入, RF-out", 这意味着设计师需要新的工具和测量方法。
TD-LTE is specified to operate in the frequency range 1850 to 2620 MHz, and uses the same MIMO scenarios and up- and down-link modulation formats as FDD: OFDMA (orthogonal frequency division multiple access) in the downlink and SC-FDMA (single carrier frequency division multiple access) in the uplink. There are two frameconfigurations, each with an overall length of 10 ms and divided into 10 subframes, as shown below.
The “5ms” version has two special synchronizing subframes rather than the one in the “10ms” version, and gives greater opportunity for uplink/downlink flexibility. The frame can be dynamically configured to any one of the preset configurations shown, depending on the instantaneous data transmission requirement.
TD-LTE 被指定在1850至 2620 MHz 的频率范围内运行, 并在下行链路和 SC-FDMA (单载波频率) 中使用与 FD:OFDMA (正交频率划分多重访问) 相同的 MIMO 方案和上下链路调制格式在上行链路中)。有两个框架, 每个框架的总长度为10毫秒, 并分为10个子帧, 如下所示。
"5ms" 版本有两个特殊的同步子帧, 而不是 "10ms" 版本中的子帧, 并为上行/下行链接的灵活性提供了更多的机会。根据瞬时数据传输要求, 可以将帧动态配置为所显示的任何一种预设配置。
Figure 1: TD-LTE Frame Structures
Each one millisecond downlink subframe contains blocks of data (“resource blocks”) destined for a number of different users, while uplink subframes contain blocks of data from the users to the base station (eNB). The specified latency target (time from information request to reply) is 5 ms, or only half a frame, for small data packets, so system timing, including timing offset to compensate for distance from the eNB, is critical. The current system is optimized for low speed (stationary or pedestrian) users, who will see the highest-speed operation, but may eventually be extended to users traveling at up to 500 kph.
每个1毫秒的下行链写包含面向多个不同用户的数据块 ("资源块"), 而上行子帧包含从用户到基站 (eNB) 的数据块。对于小数据包, 指定的延迟目标 (从信息请求到答复的时间) 是5毫秒, 或者只有半个帧, 因此系统计时 (包括计时偏移来补偿与 eNB 的距离) 非常关键。目前的系统针对低速 (固定或行人) 用户进行了优化, 他们将看到最高速度的操作, 但最终可能会扩展到以每小时500公里的速度行驶的用户。
The TD-LTE standards currently contain specifications and measurements methods for RF channels of 1.4, 3, 5, 10, 15 and 20 MHz (same as LTE FDD with scalable bandwidths). Most test methods and items are defined for single code data using a single transmit and receive part. Specifications for multiple codes and MIMO configurations are still under discussion. For up-to-date information, go to www.3gpp.org and check the latest version of document TS34.141.
TD-LTE 标准目前包含1.4、3、5、10、15和 20 MHz (与具有可扩展带宽的 LTE FDD 相同) 的 RF 通道的规范和测量方法。大多数测试方法和项都是使用单个传输和接收部分为单个代码数据定义的。多个代码和 MIMO 配置的规范仍在讨论中。有关最新信息, 请转到 www.3gpp.org 并查看 TS34.141 文档的最新版本。
Initial measurements aim to ensure un-impaired transmission and reception: uplink and downlink transmit masks, maximum and minimum power, power control, adjacent channel leakage and spurious emissions are defined to ensure minimum interference. An example transmit on/off mask is shown below.
初始测量的目的是确保传输和接收不受损害: 上行和下行传输掩码、最大和最小功率、功率控制、相邻通道泄漏和杂散发射被定义为确保最小干扰。下面显示了一个传输无掩码的示例。
Figure 2: Example transmission mask
The next series of measurements focuses on transmission quality, with the main metric being error vector magnitude (EVM). For the OFDM downlink, the measurement is defined over one subframe (1 ms) in the time domain and 12 subcarriers (180 kHz) in the frequency domain. Limits depend on the modulation complexity; the higher order the modulation, the tighter the limit. For the SC-FDMA uplink from the UE, transmission quality is defined in terms of allocated and unallocated resource blocks, and requires separately measuring the spectrum within the channel where the UE is transmitting and the remainder of the channel bandwidth where it is not. EVM and spectrum flatness are specified for allocated resource blocks, and in-band leakage and IQ offset (carrier leakage), which are interfering signals that degrade network performance, are specified for unallocated resource blocks.
接下来的一系列测量重点是传输质量, 主要指标是误差矢量幅度 (EVM)。对于 OFDM 下行链路, 测量是在时域中的一个子帧 (1 毫秒) 和频域中的12个子载波 (180 kHz) 上定义的。限制取决于调制的复杂度;调制的顺序越高, 限制就越严格。对于来自 UE 的 SC-FDMA 上行链路, 传输质量是根据分配和未分配的资源块定义的, 需要分别测量 UE 传输通道内的频谱和剩余的信道带宽在那里它不是。为分配的资源块指定了 EVM 和频谱平整度, 并为未分配的资源块指定了带内泄漏和 IQ 偏移 (载波泄漏), 它们是影响网络性能的干扰信号。
Figure 3: VSA screenshot showing uplink performance data
Basic receiver RF performance tests – reference sensitivity, dynamic range, in-channel selectivity, adjacent selectivity blocking and spurious emissions – are set up using normal call protocol until the UE is communicating on traffic channel. At the specified value, the block error rate (BLER) must not exceed a target value and maintain the target throughput value, normally 95%. Specified values depend on the test being performed, receiver bandwidth, and modulation complexity. The receiver is then checked for its ability to correctly demodulate the dedicated control channel from within the dedicated physical channel in both static and faded environments, and at all supported data rates and channel bandwidths.
基本接收器 RF 性能测试 (参考灵敏度、动态范围、通道内选择性、相邻选择性阻塞和杂散发射) 使用正常的呼叫协议进行设置, 直到 UE 在交通信道上通信。在指定的值, 块错误率 (BLER) 不得超过目标值并保持目标吞吐量值 (通常为 95%)。指定的值取决于正在执行的测试、接收器带宽和调制复杂性。然后检查接收器是否能够在静态和褪色的环境中从专用物理通道内正确解调专用控制通道, 并在所有受支持的数据速率和通道带宽。
TD-LTE devices must provide compatibility with legacy 3GPP systems, and a series of handover scenarios are specified to ensure conformance. These aim to ensure service continuity for the user, and check everything from idle mode and in-call intra-frequency TDD-TDD handovers, through inter-frequency changes and TDD-FDD handovers, handovers to 3G W-CDMA and HPSA systems, and finally to handing over from TDD to GSM.
TD-LTE 设备必须提供与传统3GPP 系统的兼容性, 并指定了一系列切换方案以确保一致性。这些目的是确保用户的服务连续性, 并检查一切从空闲模式和随叫随到的频率内 TDD-TDD移交, 通过频率间变化和 TDD-FDD 移交, 移交到 3G W-CDMA 和 HPSA 系统, 最后移交从 TDD 到 GSM。
While the specified RF environment for both LTE FDD and TD-LTE requires multiple input-multiple output (MIMO) operation, test and validation methods remain to be determined. Signal analysis of the individual data streams that make up a MIMO transmitter is straightforward. The multiple signals for MIMO receiver test include realtime fading, and require specialized test signal generation. Validation of correct MIMO receiver operation is still under discussion in both 3GPP and the test community. First LTE deployments will use 2 x2 MIMO (i.e. 2 separate transmitters and receivers) though the specification call for up to 4 x4 MIMO in the future. MIMO is designed to improve coverage and data transmission capacity, with each transmitter broadcasting its own unique data stream, and the receivers performing complex matrix demodulation to recover the initial data.
虽然 LTE FDD 和 TD-LTE 的指定 RF 环境都需要多个输入-多输出 (MIMO) 操作, 但测试和验证方法仍有待确定。对构成 MIMO 发射机的各个数据流进行信号分析非常简单。MIMO 接收机测试的多个信号包括实时衰落, 需要专门的测试信号生成。3GPP 和测试界仍在讨论正确的 MIMO 接收机操作的验证问题。第一次 LTE 部署将使用 2x2 MIMO (即2个单独的发射机和接收器), 尽管该规范要求在未来最多 4 x2 MIMO。MIMO 旨在提高覆盖范围和数据传输能力, 每个发射机广播自己独特的数据流, 接收机执行复杂的矩阵解调以恢复初始数据。
Figure 4: Example 2 x 2 MIMO configuration
These are only the beginning of system testing needs. From chipset design to network deployment, much work must be done to test out the end-user’s experience at all stages of the design process. In addition to ensuring interoperability, full testing will involve validating thousands of additional user-experience scenarios. Only by validating functionality early on will network operators be able to manage customer expectations and retain loyalty. Early experiences with WAP and W-CDMA have taught them the potential customer issues of technology deployment and roll out – from coverage issues, real data speed and battery drain time, to simultaneous interactions. Designers and service providers must be able to verify both the maximum design performance and the performance of devices under realistic network conditions using controlled and repeatable test scenarios, before deployment and after design changes. Protocol and conformance test tools, such as the Agilent 8960 and E6620, and systems based on them supplied in partnership with Anite, provide a feature-rich environment for comprehensive performance validation.
这些只是系统测试需求的开始。从芯片组设计到网络部署, 必须做大量工作来测试最终用户在设计过程的所有阶段的体验。除了确保互操作性外, 全面测试还需要验证数千个其他用户体验方案。只有尽早验证功能, 网络运营商才能管理客户期望并保持忠诚度。WAP 和 W-CDMA 的早期经验告诉他们技术部署和推广的潜在客户问题--从覆盖问题、实际数据速度和电池消耗时间到同时进行的交互。在部署之前和设计更改之后, 设计人员和服务提供商必须能够使用受控和可重复的测试方案, 验证设备在实际网络条件下的最大设计性能和性能。协议和一致性测试工具, 如安捷伦8960和 E6620, 以及系统
Agilent has developed several first-to-market TD-LTE test products to help ensure the success of TD-LTE deployment. The Agilent 3GPP LTE TDD Wireless Library for Agilent SystemVue and Advanced Design System (ADS) works directly with Agilent’s MXA Signal Analyzer to provide the world’s first fully coded BER solution for the TDD version of the LTE standard using 2 x 2 and 4 x4 MIMO technology. The solution allows fully coded BER measurements of a device under test, including simulation of channel impairments for multipath fading.
安捷伦开发了多种首次上市的 TD-LTE 测试产品, 以帮助确保 TD-LTE 部署取得成功。安捷伦 3GPP Lte TDD 用于安捷伦系统 Vue 和高级设计系统 (ADS) 的无线库直接与安捷伦的 MXA 信号分析仪合作, 使用 2 x 2 和 4 x4 MIMO 为 LTE 标准的 TDD 版本提供世界上第一个完全编码的 BER 解决方案技术。该解决方案允许对被测试设备进行完全编码的 BER 测量, 包括模拟多径衰落的通道损伤。
The Agilent N7625B Signal Studio for LTE TDD is a powerful, PC-based software application for creating standards-based TD-LTE signals using Agilent’s N5182A/62A MXG and E4438C ESG vector signal generators, and N5106A PXB MIMO receiver tester. The Signal Studio solution supports the 3GPP LTE TDD September 08 standard, offers multichannel capability for PDSCH, PHICH, PCFICH, PBCH, PDCCH, PUSCH, PUCCH, and has the ability to transmit DL and UL signals. These products provide basic capabilities well suited for testing components used in base stations and mobile handsets, such as power amplifiers and filters, and advanced receiver test capabilities that support transport layer coding, 4 x 4 MIMO pre-coding and fading.
用于 LTE TDD 的 Agilent N7625 信号工作室是一个功能强大的基于 pc 的软件应用程序, 用于使用安捷伦的 n5182a22222a mxg 和 E4438C ESG 矢量信号发生器以及 N5106A PXB MIMO 测试仪创建基于标准的 TD-LTE 信号。信号工作室解决方案支持 3GPP LTE TDD 08年9月标准, 为 PDSCH、PHICH、PCICH、PBCH、PDCCH、PUSH、PUCCH 提供多通道功能, 并具有传输 DL 和 UL 信号的能力。这些产品提供了非常适合于测试基站和移动手机中使用的组件 (如功率放大器和滤波器) 的基本功能, 以及支持传输层编码、4 x 4 MIMO 预编码和衰落。
Agilent 89600 VSA software provides RF and baseband engineers with a comprehensive set of TD-LTE signal analysis tools, physical layer testing, and troubleshooting of LTE transceivers and components. TD-LTE downlink (OFDMA), uplink (SC-FDMA), and MIMO analysis is a single option. The VSA software offers industry-leading performance with EVM of < -50 dB (hardware dependent) and bandwidths of 1.4 MHz to 20 MHz. Modulation formats included are BPSK, QPSK, 16 QAM, 64 QAM, CAZAC, OSxPRBS, TDD DL/UL allocation (0-6) and special subframe length (0-8), and 2 x 2 MIMO. This VSA software can be used with more than 30 Agilent products, including spectrum and signal analyzers, oscilloscopes, and logic analyzers to make LTE measurements anywhere in the block diagram -- from baseband to antenna, on digitized or analog signals. It supports 2 x 2 MIMO analysis in conjunction with Agilent’s EXA and MXA Signal Analyzers, VXI-based VSA analyzer, and several scopes. It also has connectivity with Agilent’s Advance Design System TD-LTE wireless library.
安捷伦 89600 VSA 软件为 RF 和基带工程师提供了一套全面的 TD-LTE 信号分析工具、物理层测试以及 LTE 收发器和组件的故障排除。TD-LTE 下行链路 (OFDMA)、上行链路 (SC-FDMA) 和 MIMO 分析是一个单一的选项。VSA 软件提供业界领先的性能, 具有 <-50 dB (硬件相关) 的 EVM 和 1.4 mhz 至 20 MHz 的带宽. 包括调制格式有 BPSK、QPSK、16 QAM、64 Qam、Cazac、OSXPRBS、TDD DL/2 ul 分配 (0-6) 和特殊的子帧长度 (0-6) 和 2 x 2 MIMO。该 VSA 软件可用于30多种安捷伦产品, 包括频谱和信号分析仪、示波器和逻辑分析仪, 可在框图中的任何位置 (从基带到天线、数字化信号或模拟信号) 进行 LTE 测量。它支持 2x2 mimo 分析, 结合安捷伦的 EXA 和 MXA 信号分析仪、基于 vsi 的 VSA 分析仪和多个范围。它还与安捷伦的
In summary LTE, and particularly TD-LTE, brings new challenges to developers and to the vendors of design and test tools. New RF modulation schemes, MIMO antenna configurations, higher system bandwidths and capacity, and lower latency are just some of them. New measurement methods, including crossing the digital and RF domains, combined with a reduction in test point accessibility mean there’s a steep learning curve for both system and test developers. Successful technical introduction is a must, with analysts predicting between 30 and 80 million LTE subscribers and well over $100 billion in operator revenues within 5 years.
总之, LTE, 特别是 TD-LTE, 给开发人员以及设计和测试工具的供应商带来了新的挑战。新的 RF 调制方案、MIMO 天线配置、更高的系统带宽和容量以及较低的延迟只是其中的一部分。新的测量方法, 包括跨越数字和 RF 域, 再加上测试点可访问性的降低, 意味着系统和测试开发人员都有一个陡峭的学习曲线。成功的技术介绍是必须的, 分析师预测 LTE 用户将在5年内达到 3, 000万至 8, 000万, 运营商收入将远远超过 1, 000亿美元。
About The Authors
Yvonne Liu works at the China Communication Operation (CCO) of Agilent Technologies in Beijing. She received a B.S. in Electrical Engineering from China Civil Aviation University in 1990. Since joining Hewlett-Packard/Agilent Technologies in 1993, Yvonne has had a variety of roles including field engineer, business development engineer, and marketing engineer. Yvonne started as a product marketing engineer where she supported the cdma2000 application one-box-tester. She is now responsible for application solutions on the Agilent X-series spectrum analyzers including cdma2000, 1xEV-DO, and TD-SCDMA. Yvonne’s current focus is supporting TD-SCDMA and TDLTE.
Bai Ying obtained his Master’s Degree in Communication and Information Systems from the Institute of Electronics, Chinese Academy of Sciences in 2006. He joined Agilent Technologies and has worked as an application engineer focusing on TD-SCDMA with his application expertise in LTE TDD signal studio planning, product marketing, and technical support.
Yvonne Liu在北京安捷伦科技的中国通信运营 (CCO) 工作。1990年毕业于中国民航大学, 获电气工程学士学位。自1993年加入惠普公司以来, Yvonne 担任过各种职务, 包括现场工程师、业务开发工程师和营销工程师。Yvonne 一开始是一名产品营销工程师, 在那里她支持 cdma2000 应用程序单箱测试仪。她现在负责安捷伦 x 系列频谱分析仪的应用解决方案, 包括 cdma2000、1xEV-DO 和 TD-SCDMA。Yvonne 目前的工作重点是支持 TD-SCDMA 和 TDTE。
Bai Ying于2006年在中国科学院电子研究所获得通信与信息系统硕士学位。他加入了安捷伦技术公司, 并在 TD-SCDMA 领域担任应用工程师, 在 LTE TDD 信号工作室规划、产品营销和技术支持方面拥有丰富的应用专长。