Microwave Journal
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Progress and Challenges of Test Technologies for 5G

January 12, 2018

The 5G test system must closely integrate pre-research results with practice at all stages, providing researchers and developers with flexible, efficient and reliable test platforms and test data to accelerate the commercialization of 5G products. In this article, we introduce the testing requirements and challenges running through different processes in the 5G industry. Combined with various test scenarios, the test industry’s current R&D status is summarized, technical challenges faced by instrument researchers and developers are highlighted and the potential development of 5G test is forecast.

The information and communications industry is facing great change due to the rapid development of applications, leading to an explosive growth in data traffic. Traditional transmission technologies and architectures for wireless communications are challenged by a variety of smart devices and different connectivity requirements.1-2 The fifth generation mobile communications system (5G) imposes diverse scenarios and extreme performance requirements. Its main operational scenarios include seamless wide area coverage, high capacity hotspots, low power massive connections and low latency with high-reliability.3-4

The 5G technology roadmap contains two parts: a new air interface and a 4G evolution air interface.5 In the 5G air interface technical framework, the key technology areas include massive MIMO, ultra-dense network (UDN), new multiple access and full spectrum access. As one of the most important enabling technologies for seamless wide area coverage, massive MIMO efficiently utilizes spatial dimension resources to dramatically increase system spectral efficiency and enhance the user experience. UDN significantly reduces cell interference through inter-microcell collaboration and expands network capacity in local hot spots. Novel multiple access technologies increase equipment connectivity and reduce signaling overhead, as well as user equipment (UE) power consumption through grant-free mechanisms. Full spectrum access, which supports a hybrid network integrating low and high frequency bands, can simultaneously meet the requirements of high data rates and large capacity.

Since 2013, the U.S., European Union, Japan, South Korea and other countries or regions have launched 5G R&D programs. Since 2014, the Chinese government has supported domestic 5G technology research via the National 863 plan, as well as major science and technology projects. These national or regional initiatives strive to establish favorable positions in future 5G technology and business competitions.6 To establish standards, 3GPP launched 5G research projects in early 2016 and plans to complete the first version of the 5G standard (3GPP Release 15) in 2018. The IMT-2020 group, which consolidates the efforts of industry-university research, was established to promote 5G research and international cooperation. It has released a series of white papers, including 5G Vision and Requirements,3 5G Concept,4 5G Wireless Technology Architecture5 and 5G Network Technology Architecture.7 In January 2016, the IMT-2020 Promotion Group launched 5G R&D experiments to evaluate key technology candidates and facilitate the formulation of technical standards. It plans to complete R&D trials of technologies and products in 2018 and 2020, respectively.

Test technology development has accompanied the development of enabling technologies for each generation of mobile communications, leading to collaborative development. All kinds of test instruments and systems support the multiple needs of the wireless communications industry, from research to verification and production. 5G test and measurement technologies are expected to appear before network and UE products, guiding product design and standard formulation. At present, the first and second phases of developing 5G wireless test specifications have been completed under the organization of the IMT-2020 Promotion Group.



In recent years, research institutions, operators, equipment, IC and instrument manufacturers around the world have carried out key 5G technology validation and prototype testing in succession.8 According to China’s 5G R&D test plan, the next step will be testing technical solutions introduced by manufacturers and, finally, system testing for typical 5G services based on small-scale networks.

Compared to traditional testing in the 3G/4G era, three technical characteristics of 5G impose enormous challenges on test instruments and methods:

  • the introduction of microwave and mmWave bands above 6 GHz
  • the generation, reception and storage of ultra-wideband signals with bandwidths of hundreds of MHz or even GHz; and
  • the design and application of large-scale antenna arrays, with 64, 128 or more channels.

5G TEST TECHNOLOGIES

The following sections explain the influences these characteristics have on 5G test instruments and test technology with respect to different test scenarios for 5G wireless communications. Existing solutions are introduced as well.

5G Channel Sounding and Modeling

The wireless channel is one of the core components of the wireless communications system. Physical properties of the wireless channel are characterized by a series of parameters, such as channel impulse response, path loss, Doppler delay, power delay profile and angle of arrival. Channel sounding can help extract these MIMO channel parameters and provide an important reference for subsequent 5G channel modeling and standardization. The basic structure of the channel sounding system is shown in Figure 1, which includes the signal transmission/receiving instruments and measurement/analysis software.

Figure 1

Figure 1 5G channel sounding system.

With the development of 5G technology, the traditional 3G/4G channel sounding system can neither cope with new test challenges nor be upgraded to achieve better performance. Characteristics of mmWave channels are not yet fully explored or understood, and the upgrade of traditional dedicated channel sounding equipment is likely to be costly, due to inadequacies dealing with flexible testing from 6 to 100 GHz. The ability to generate, receive and store ultra-high speed baseband signals will need to be greatly improved, as well. The introduction of large-scale antenna arrays significantly increases the required computational capacity of channel sounding instruments, making new multi-channel RF transceiver components the inevitable choice. Hardware and software platforms must support massive data analysis and channel parameter extraction.

In recent years, enterprises and academic institutions around the world have proposed a variety of solutions for 5G channel sounding based on a combination of existing products. Keysight Technologies proposed a mmWave MIMO channel sounding system.9 Rohde & Schwarz (R&S) proposed a scheme that supports fast measurement of both indoor and outdoor time domain channels with operating frequencies up to 100 GHz and bandwidths as wide as 2 GHz.10 Various research projects are investigating 5G mmWave channel measurements and modeling, including METIS, NYU WIRELESS,11 mmMAGIC, MiWEBA and 3GPP.



Several popular channel models, such as WINNER,12 COST 210012-13 and METIS 2020,13 have attracted more attention due to their scalability and reasonable complexity. These 5G channel models are adapted to specific scenarios and frequency bands. Although the mathematical methods used are not the same, these models are based on the analysis of a large number of channel sounding results. At the 3GPP RAN meeting held from June 13 to 16, 2016 in Bushan, Korea, the first standard for the mobile broadband 5G high frequency (6 to 100 GHz) channel model was approved. So far, there has been no unified 5G channel model integrating both low and high frequency bands, which requires the corresponding aspects of 5G wireless technology to be taken into account. Extensive work on theoretical and practical channel modeling has been done during the past few years, but most research results are constrained by spatial correlation and mutual coupling between adjacent antennas. Arrangement of antenna elements is relatively simple and assumptions about propagation conditions and antenna characteristics are too idealistic and limited to specific application scenarios. Therefore, it is difficult to accurately extract a variety of actual channel characteristics.

Figure 2

Figure 2 5G channel emulator.

Channel Emulation

When conducting field tests for wireless communications systems in the real channel environment, there are many shortcomings, such as climate effects, poor mobility, high cost and unrepeatable test processes. The MIMO channel emulator (see Figure 2) enables researchers to emulate typical wireless channel environments in the lab, flexibly controlling and changing channel parameters, to identify performance problems as early as possible, reduce test costs and significantly improve efficiency.14 Therefore, manufacturers have always included channel emulation as a critical part of the deployment of each generation of mobile communications technology, including 5G.

As a key technology for 5G, massive MIMO greatly reduces transmission power while improving channel capacity and spectral efficiency.15 Nevertheless, the required number of network equipment antennas is 10 to more than 100x that of existing MIMO system antennas, which becomes a major bottleneck in the upgrade and optimization of current 4G/LTE channel emulators. The inherent “pilot contamination” problem of massive MIMO technology directly affects the baseband channel estimation algorithm, feedback mechanism, interference control and synchronization scheme.16-18 Data throughput in the channel emulator increases sharply with expanding antenna array size, requiring extremely high-level computational resources, storage capacity and bus speed for the baseband processing unit. The RF system design must ensure isolation and amplitude/phase consistency among multiple channels, which greatly increases channel calibration complexity.

Currently, Propsim F32, an advanced channel emulator from Anite (acquired by Keysight in 2015), is only able to support 32 RF channels at most and realize 64 × 8 MIMO channel emulations by combing multiple instruments. Unfortunately, this only covers operating frequencies below 6 GHz and a maximum bandwidth 80 MHz; it is unable to cope with the high frequency and large bandwidth challenges of 5G test. Another channel emulator, Vertex, released by Spirent in 2016, is configured with 32 RF channels and 100 MHz bandwidth to meet the requirements of MIMO beamforming, MIMO over-the-air (OTA) and massive MIMO test, but the maximum operating frequency to 5.925 GHz merely satisfies low frequency 5G test demands.

Over the years, a handful of Chinese instrument manufacturers have launched 8 × 8 MIMO channel emulators for 4G testing, laying a hardware platform and algorithm architecture foundation for the development of 5G channel emulators. Future channel emulator technology must achieve bandwidths of hundreds of MHz, cover frequency bands beyond 6 GHz and contain multiple channel models.

RF Module and Antenna Array Test

Large-scale antenna arrays and RF front-ends are essential 5G subsystems, consisting of digital-to-analog converters (DAC), analog-to-digital converters (ADC), frequency synthesizers and transmit/receive (T/R) multi-beam antenna arrays. The T/R array contains RF components such as filters, mixers, power amplifiers and low noise amplifiers, each with its own set of performance specifications and corresponding test methods.



Figure 3

Figure 3 Instruments used to test RF T/R components.

Figure 3 shows the classes and functions of the excitation/source instruments and receiving/analysis instruments commonly used in RF test. Among them, the vector signal generator (VSG) and signal analyzer provide the most comprehensive measurement and analysis of a communication system’s overall performance. The operating frequency range must cover from DC to approximately 110 GHz, while supporting 200 MHz to several GHz of vector signal bandwidth. Regarding frequency coverage, the Keysight E8267D PSG and the R&S FSW85 signal analyzer have reached 44 and 85 GHz, respectively. A breakthrough was announced at Mobile World Congress in February 2016: R&S exhibited the world’s first VSG (SMW200A) with a maximum frequency of 40 GHz and a modulation bandwidth up to 2 GHz. The maximum signal analysis bandwidth of two well-known signal analyzers products, i.e., FSW from R&S and UXA from Keysight, is 2 and 1 GHz, respectively. Further bandwidth expansion requires the assistance of other components. To generate/analyze signals with ultra-large (GHz) bandwidths, the main technical difficulties include RF channel equalization, high sampling rate ADCs, high speed digital signal processing and high data rate transmission.

Figure 4

Figure 4 Using multiple VNAs to test a massive MIMO antenna array.

For antenna array testing, the vector network analyzer (VNA) is a key instrument. Due to the lack of a single, 64-port VNA, three methods are usually adopted. The first is a step-by-step test using a single, multi-port VNA, which is relatively inexpensive but sacrifices test speed and ignores the coupling characteristics between antenna elements. The second involves cascading several multi-port VNAs, e.g., a 64-element antenna array is tested with eight, 8-port VNAs cascaded (see Figure 4). This approach can accurately test the actual S-parameters of each antenna element after calibration and greatly increases test speed. Still, there are some technical difficulties: the crosstalk between ports restricts dynamic range, and calibration time impacts test efficiency. The third scheme uses the conventional dual-port VNA with a switch matrix, which is a compromise between the prior two options. Cost is relatively low, but the speed is somewhat slow and the switch matrix introduces measurement errors. A few manufacturers are developing a single multi-port VNA to provide new solutions that address crosstalk between channels, fast calibration, cost and other aspects.

OTA test19 is another important aspect of 5G antenna array testing, for two reasons. First, directional indicators of the antenna array, e.g., effective isotropic radiated power (EIRP) and effective isotropic sensitivity (EIS), must be tested by OTA, which is consistent with 4G MIMO OTA test principles. Second, since 5G will use the microwave and mmWave bands, the antenna array and T/R elements will likely be integrated to reduce loss and improve matching. In this situation, most T/R component characteristics cannot be evaluated without wired conduction tests, and measures of performance such as RF circuit transmit power and sensitivity may interact with the characteristics of the antenna, making individual assessment difficult.

IC, Network and User Equipment Test

Although 5G-related technologies and standards are not yet clear, IC, UE and network equipment manufacturers, as well as operators, are in full swing conducting R&D of 5G prototypes to launch competitive solutions. Among the existing 5G prototype UEs, some support high speed transmission of several Gbps and some support as low as millisecond latency. The battery life of certain UEs (particularly IoT terminals) has been extended to nearly 10 years. Qualcomm, Spreadtrum, MediaTek and other IC manufacturers are developing 5G chips, and Qualcomm has announced prototypes. Because of the emerging 128-channel integrated network equipment, the corresponding test technology has been placed on the agenda. Three features, including UE diversity, scenario complexity and massive connections, challenge the testing of ICs, UEs and network equipment,20 and available 4G/LTE test instruments can hardly fulfill these 5G tasks.



Figure 5

Figure 5 Integrated system for 5G terminal testing.

The integrated UE tester is used to emulate partial functioning of the network, then test the RF performance of the UE under network conditions or, with the signal generators, signal analyzers (including spectrum analyzer) and other conventional test instruments, perform conformance testing such as RF, protocol and radio resource management. How to emulate massive UEs is likely to be a huge challenge for UE emulator design. 5G UE diversity means that existing test instruments must have superior scalability and compatibility. Additionally, typical application scenarios of the IoT, such as intelligent water meters and smart parking call for low power consumption testing to assess battery self-discharge and sleep mechanisms. The industry is presently lacking a mature test methodology to quantify UE power consumption characteristics. Other test systems, such as the NV-IoT test system, 5G terminal card interface test system and signaling monitors, are all indispensable components of 5G UE test. Note that 5G UE test tends to be integrated; it will be a comprehensive test system to replace multiple sets of discrete systems, as shown in Figure 5.

Similar to UE test, the IC manufacturers’ desire for 5G test instruments is strong throughout all phases, from chip development to product certification and mass production. Specifically, test instruments are required to simulate network functions, verify and evaluate RF solutions, complete chip function/performance authentication and perform final production test. To facilitate the operation, further needs include installing software with a project configuration and results display, integrating other instruments to build test systems and supporting remote control. Also, driven by continuous module redesign, configuration changes and reduced IC R&D cycle times and costs, the traditional IC test system is faced with the need to be flexible, to reduce test costs and improve production efficiency.

Network equipment testing is used to verify compliance with a communications system’s quality specifications, interface requirements with other devices and electromagnetic compatibility, both intra-system and inter-system. A 5G network equipment test requires general instruments, such as a VSG, spectrum analyzer, power meter, UE emulator and channel emulator to build a massively connected test system with broad coverage. The aim is to test the load capacity limit and overload coordination capabilities of specific uplink and downlink service models and to evaluate system performance under different channel conditions. In the future, these test instruments must follow corresponding 5G test specifications with continuous optimization and upgrade capability, satisfying the performance needs of 5G and supporting a wider range of application scenarios.

CONCLUSION

Driven by considerable market demand, 5G test has made rapid progress. Top instrument suppliers around the world are working closely with universities and other research institutions. The entire industry is inspired by recent research achievements and products. Still, there are some fundamental problems to be solved, including the establishment of new test specifications, the exploration of 5G measurement principles and the development of new measurement platforms.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (61627801) and China National S&T Major Project (2015ZX03001011).

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