LTE中TDD和FDD的区别
是德科技与您一起探討TDD和FDD的区别。
LTE 概述
长期演进（LTE）是移动通信系统中使用的创新高性能空中接口的项目名称。LTE 由第三代合作伙伴计划（3GPP）开发，是通用移动电信系统（UMTS）朝向全 IP 宽带网络的演进。LTE 演进无线接入技术 ― E-UTRA 可以提供一个框架，以提升数据速率和整体系统容量，降低时延并改善频谱效率和信元边缘性能。
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LTE测试
LTE 的成功部署取决于系统各元素的兼容性和有效的互通。一致性测试可以确保这些元素满足 3GPP 规范中定义的最低性能水平。LTE一致性测试覆盖基站、用户设备和无线资源管理性能。这些也是当今开发 LTE 网络和用户设备的厂商的一大关注点。
复杂而灵活的 LTE空中接口支持调制格式、频段、资源分配和移动性的诸多选项。因此，可以测试的射频配置排列数量巨大。在选择用于 LTE一致性测试的射频配置时，3GPP 确定的参数组合代表了最困难的操作条件，因此，如果产品通过了测试，我们就可以认为该设备在许多其他挑战性更低的场景中能有效运行。
尽管一致性测试清单看起来很长，但仍然需要其他类型的测试。例如，由于一致性测试仅提供合格/不合格结果，没有显示产品对于特定限制的近似程度，因此有必要对性能裕量进行进一步调查。LTE一致性测试的设计主旨是确保网络的底层传输机制可以承载最终用户业务，因此，更高级别的应用仍然需要加以测试。运营商验收测试是这个过程中的另一步，它包含更多以用户为中心的测试。因此，一致性测试代表了迈向 LTE部署的一个重要步骤，但它们既不是测试过程的开始，也不是测试过程的结束。
LTE主要特性
"LTE/LTE-Advanced 射频多通道测试解决方案 ― 包括一组时间同步的 PXIe 硬件、世界级的应用软件和专业测量技术，为多信道 LTE/LTE-Advanced 测试解决方案提供了基本的器件。"
LTE-Advanced特性
1. 载波聚合 Carrier Aggregation
载波聚合是通过聚合 20 MHz 连续和非连续分量载波来创建更宽的带宽，以实现高达 100 MHz 的频谱。
2. 单载波频分多址（SC-FDMA）技术
3. 中继技术 Relaying
The relay method is the use of a repeater, which receives, amplifi es, and then retransmits the downlink and uplink signals to overcome areas of poor coverage.
FDD模式和TDD模式
TDD和FDD的区别与特点
TDD是一种通信系统的双工方式，在移动通信系统中用于分离接收和传送信道。移动通信目前正向第三代发展，中国于1997年6月提交了第三代移动通信标准草案（TD-SCDMA），其TDD模式及智能天线新技术等特色受到高度评价并成三个主要候选标准之一。在第一代和第二代移动通信系统中FDD模式一统天下，TDD模式没有引起重视。但由于新业务的需要和新技术的发展，以及TDD模式的许多优势，TDD模式将日益受到重视。
一、4G的TDD和FDD是什么意思？
TDD（Time Division Duplexing）时分双工技术，在移动通信技术使用的双工技术之一，与FDD相对应，是在帧周期的下行线路操作中及时区分无线信道以及继续上行线路操作的一种技术。
FDD 频分双工(Frequency Division Duplexing)是移动通信系统中使用的全双工通信技术的一种，与TDD相对应。FDD采用两个独立的信道分别进行向下传送和向上传送信息的技术。为了防止邻近的发射机和接收机之间产生相互干扰，在两个信道之间存在一个保护频。
TDD：时分双工(Time Division Duplexing)，收发共用一个射频频点，上、下行链路使用不同的时隙来进行通信
FDD：频分双工(Frequency Division Duplexing)，收发使用不同的射频频点来进行通信
TDD和FDD通信原理
TDD和FDD已经是比较成熟的技术了，这些拓扑广泛用于高级无线通信系统，例如WLAN，WiMAX（固定/移动），LTE等。
如图所示，在TDD系统中，收发系统在不同的时刻都使用了相同的频段，即Fc频段。而在FDD系统中，收发系统在同一时刻使用不同的频带Fc1和Fc2进行信号传输。
我们知道，在蜂窝/无线通信系统中，从基站到用户站的传输称为下行链路，而从用户站到基站的传输称为上行链路。
在TDD系统中，上行链路和下行链路的信号传输时间尺上按序排列，即上行链路在“ t1”时隙发送，而下行链路在“ t2”时隙发送，两个发送时隙直接有保护时间间隔。上行链路和下行链路传输都是在相同的RF载波频率（Fc）上进行。
在FDD系统中，上行链路和下行链路的信号在同一时隙（'t1'）上进行传输，但是上行链路和下行链路被调制到两个不同的频率Fc1和Fc2上。
FDD是一种出现较早的技术，比较适合于类似语言这样的对称流量的应用，而TDD适合于诸如Internet或其他以数据为中心的突发性，非对称流量的应用。
（1）在TDD中，发送器和接收器的工作频段相同，但分间工作。因此，TDD系统可共用滤波器，混频器，频率源和合成器，从而降低了隔离发射和接收天线之间的复杂性和成本。FDD系统使用一个双工器和/或两个需要空间分隔的天线，不能重复使用资源，导致FDD系统的硬件成本更高。
（2）TDD比FDD更有效地利用频谱。在服务提供商没有足够带宽的应用环境，发射和接收通道之间没有足够的保护带宽，FDD则不能满足应用需求。
（3）TDD比FDD更加灵活，可以满足动态地重新配置分配的上行和下行带宽以响应客户需求的需求。
（4）TDD允许通过适当的频率规划来减轻干扰。与FDD相比，TDD仅需要一个无干扰信道，而FDD则需要两个无干扰信道。
但是与FDD系统相比，TDD系统需要处理系统间精确的时间同步，从而导致MAC层相对复杂度更高。
TDD和FDD的区别
1. 双工方式
TDD：时分双工（Time Division Duplexing），收发共用一个射频频点，上、下行链路使用不同的时隙来进行通信。
FDD：频分双工（Frequency Division Duplexing），收发使用不同的射频频点来进行通信。
2. 速率
理论上讲，在相同的带宽条件下，比如FDD分配10M+10M，TDD分配20M，TDD的速率会低于FDD，这主要原因是TDD的帧结构中有个叫做特殊子帧的帧，这些帧会被浪费一部分（比如其中的保护时隙）并不传送任何数据，而FDD的帧不存在这种完全浪费掉的情况。
3. 区域覆盖
TD-LTE适合热点区域覆盖，FDD适合广域覆盖。 早年高通的一份报告显示，在相同频率相同功率的条件下，FDD比TDD能提供更好的覆盖，TDD覆盖比FDD小80%（DL/UL=2:1）/小40%（DL/UL=1:1）。这主要原因是TDD上行链路存在发射功率的时间（一个10ms帧中）要比FDD时间短。
4. 移动台移动速度
FDD是连续控制的系统，TDD是时间分隔控制的系统。在高速移动时，多普勒效应会导致快衰落，速度越高，衰落变换频率越高，衰落深度越深。 在目前芯片处理速度和算法的基础上，当数据率为144kb/s时，TDD的最大移动速度可达250km/h，与FDD系统相比，还有一定差距。一般TDD移动台的移动速度只能达到FDD移动台的一半甚至更低。
FDD frame type 1
In FDD mode, uplink and downlink frames are both 10ms long and are separated either in frequency or in time.
For full-duplex FDD, uplink and downlink frames are separated by frequency and are transmitted continuously and synchronously.
For half-duplex FDD, the only difference is that a UE cannot receive while transmitting.
The base station can specify a time offset (in PDCCH) to be applied to the uplink frame relative to the downlink frame.
TDD frame type 2
In TDD mode, the uplink and downlink subframes are transmitted on the same frequency and are multiplexed in the time domain. The locations of the uplink, downlink, and special subframes are determined by the uplink-downlink configuration. There are seven possible configurations given in the standard. The following is an illustration of a TDD frame with uplink-downlink configuration set to 2 and special subframe configuration set to 6.
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"查看 Keysight MXG 信号发生器、PXB 和 Signal Studio 如何满足 LTE FDD 和 TDD Rx 性能要求的 LTE 一致性测试。"
"为了支持尽可能多的频带分配，分别使用频分双工（FDD）和时分双工（TDD）技术支持成对和不成对频谱操作。成对频谱操作称为 FDD-LTE，不成对频谱操作称为 TD-LTE。"
LTE系统支持FDD和TDD两种双工方式。
"使用载波聚合和高序MIMO等高级功能分析LTE/LTE-Advanced FDD的上行链路和下行链路信号。"
FDD帧结构
FDD下行帧结构
FDD下行物理映射
FDD上行物理映射
"生成符合 LTE 和 LTE-Advanced FDD 通信标准的 NB-IoT 或 eMTC 信号，用于使用是德科技信号发生器进行的基站（eNB）测试"
TDD帧结构
DL、UL 和特殊子帧
TDD 5 ms 开关周期映射
"这是用于 O-RAN Studio 支持的 PathWave 信号生成订阅包，包括 5G NR、LTE FDD 和 LTE TDD。"
Uplink
Uplink user transmissions consist of uplink user data (PUSCH), random-access requests (PRACH), user control channels (PUCCH), and sounding reference signals (SRS).
FDD and TDD uplink transmissions have the same physical channels and signals. The only difference is that TDD frames include a special subframe, part of which can be used for SRS and PRACH uplink transmissions.
The following illustration shows part of an LTE uplink frame and contains an allocation for each type of uplink channel. The illustration is applicable to both TDD and FDD.
User 1 has a PUSCH allocation of [RB 20, slots 4-5], and User 2 has a PUCCH allocation of [subframe 2, PUCCH index 0]. User 3 has been given an SRS allocation of subcarrier 94 to 135 in subframe 2, and User 4 is transmitting in a PRACH allocation.
A user cannot transmit both PUCCH and PUSCH data in the same slot.
LTE FDD and LTE TDD Tests
LTE Clause 6 Tests
These tests are used to determine the performance of the transmitter and typically require the use of only one signal. The interferer signal is applied to the transmitter port.The tests are used do to determine a range of attributes such as power adjustment capability in the presence of an interferer, and the ability to inhibit undesired signals due to intermodulation. The tests are supported on only the N5172B EXG or the N5182B MXG signal generators.
The test case 6.2.6 Home BS output power for adjacent UTRA channel protection is different in that it uses a W-CDMA signal rather than an LTE signal as the interferer. Because it uses a W-CDMA signal, it requires having licenses for the N7600B Signal Studio for W-CDMA / HSPA+ as shown in the requirements.
The test case 6.2.8 Home BS output power for co-channel E-UTRA channel protectionhas the option to use one or two interferers. When using two interferers, there is the potential to set a wide offset between the two signals that it exceeds the instrument's capability. Because of this potential, it is recommended that a separate signal generator be used for each interferer.
LTE Clause 7 Tests
Most of these tests provide the flexibility to combine waveforms into a single waveform file so minimal instruments are required. For example, use only a single signal generator as a standalone instrument, or use only one of the BBGs in the PXB with the PXB connected to one signal generator. The exception to this flexibility is the 7.6 Blocking (Co-Located) test. This test always requires the use of a separate signal generator or PXB BBG with an additional signal generator for each signal. The other 7.6 blocking tests may also require separate signal generators for each signal if the combined signals exceed the instruments bandwidth limitations. If this condition arises with the other 7.6 tests, the software provides a message.
In addition with using the PXB, MIMO and fading are also considerations that can be added to the signals.
LTE Clause 8 Tests
Signal Combining and Instrument Use
A separate signal generator is required for each test signal, because the test signals cannot be combined into a single waveform file. But how these signal generators are controlled for the signals is dependent upon the Fading Simulator selection, PXB or External Fader.
With the PXB, the PXB controls the signal generators based on input from the software. The PXB generates the signals, then using the PXB configuration set in the PXB Setting window, outputs the signals to the signal generators for upconverting. Because the PXB controls the signal generators, the Signal Generator fields in the test setting window are grayed out.
With the external fader, the user must select a different signal generator for each signal. Each signal generator must be configured and show as connected in the Hardware Connections and Settings node. The software then controls, and downloads the signal and settings to each of the signal generators.
Test Cases
For the test case 8.2.3 HARQ-ACK Multiplexed on PUSCH with the wanted signal FRC selection of A3-1, this signal can take minutes to generate. This is due to the longer waveform length required to provide enough randomness in the payload data. The computer's CPU speed also influences this time. In order to mitigate the generation time on subsequent Executes, the software, for the initial generation, creates a local waveform file that has a unique name .Upon a subsequent Execute, provided that there were no setting changes that includes the Common Settings, the software downloads the existing waveform file. This saves time because the software does not have to regenerate the file for download. For the software to use this feature, the target instrument must have the required Basic/Advanced waveform playback licenses for the Signal Studio software and the Override Settings unchecked (disabled). Test Case Manager's feature of using the Signal Studio real-time license in place of the playback licenses cannot be used.
Upon a subsequent Execute, if any of the settings were changed since the initial waveform generation, the software creates a new waveform file with another unique name.
The unique waveform file name is comprised of settings that includes whether it is FDD or TDD. The following table shows an example of a unique name and parses that name to show how it is created.
To see whether a particular unique waveform file exists, use the Open Waveform Folder…function.
The following flowchart illustrates this unique named file creation process and reuse.
应用指南 - 推荐阅读
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