Practical Tips | Four Key Aspects of 5G Equipment Design and Testing

5G is coming to us with a trillion-level mobile industry chain and tens of millions of job opportunities. The communications industry is surging, and many people are flocking to it. As many as 192 operators in more than 81 countries have announced their investment in 5G.

5G Timeline – Key Milestones

At the specification level, since the freeze of 5G NSA in December 2017, the physical layer specifications have been gradually formed, and the entire R15 specification focuses on enhanced mobile broadband (eMBB) and ultra-reliable ultra-low latency communication (uRLLC). These new specifications bring many new challenges to device and component designers.

Let’s discuss the four most important aspects of 5G device design and testing:

R15 specifies a maximum carrier bandwidth of up to 400 MHz and up to 16 component carriers, which can be aggregated to a bandwidth of up to 800 MHz. At the same time, 5G NR provides flexibly configurable waveforms, parameter sets, frame structures, and bandwidth combinations, but also brings complex channel coding, signal quality challenges, and numerous test cases.

Compared with 4G, 5G NR allows for scalable OFDM parameter sets whose subcarrier spacing can be controlled by 2uX15kHz, and ultimately can provide different levels of throughput, latency and reliability services through scalable time slot spacing. The ultra-high reliability and ultra-low latency (uRLLC) part of one of the three major scenarios of 5G is implemented through mini-slots, which can provide shorter latency and load than standard time slots. The NR subframe structure also allows the dynamic allocation of OFDM symbol link direction and control within the same subframe.

By using this dynamic TDD mechanism, the network can dynamically balance uplink and downlink traffic demands and include control and acknowledgment in the same subframe.

Time slots and Mini-slots within a subframe and their associated durations

Sub-bandwidth part (BWP) is a new concept defined in the 5G NR standard, which means that the system bandwidth of a carrier can be divided into several sub-bandwidths according to different scenario requirements. Each BWP can have a different parameter set, and the signaling control is also independent.

A carrier can contain several mixed parameter sets to support different levels of services and support traditional 4G devices and new 5G devices.

BWP can support multiplexing of different services in the carrier

In summary, 5G NR has a flexible and scalable parameter set with different subcarrier spacing, dynamic TDD and BWP, which increases the complexity of creating and analyzing waveforms. Therefore, it is particularly important to generate waveforms with larger bandwidth for different test cases through a combination of software and hardware in the sub-6GHz and millimeter wave bands and analyze 5G NR waveforms from the time domain, frequency domain and modulation domain.

Analyze 5G NR and 4G LTE waveforms in adjacent frequency bands using vector signal analysis software

In order to achieve the ambitious data throughput target, 5G NR not only defines a new frequency band in Sub-6GHz, but also expands the operating frequency band to the millimeter wave band, thereby greatly broadening the available channel bandwidth. In the millimeter wave band, the impact of the channel on signal quality becomes more significant, making it more difficult to meet the signal quality requirements.

Some Sub-6GHz and millimeter wave frequency bands and expected commercialization time

Many factors affect signal quality, including baseband signal processing, modulation, filtering, and up-conversion. Although it brings greater continuous available bandwidth, the baseband and RF components in the millimeter wave band are also more susceptible to interference from common signals. Due to the inherent characteristics of the OFDM system, such as IQ modulation impairments, phase noise, linear and nonlinear distortion, and frequency errors can all cause distortion of the modulated signal.

In millimeter-wave OFDM systems, the impact of phase noise is particularly significant. Excessive phase noise will directly lead to mutual interference between subcarriers and ultimately cause serious signal distortion. In addition, any deviation in the broadband signal circuit, such as phase, amplitude or noise, will eventually be reflected in the system's EVM and other indicators. Therefore, performance optimization and problem solving can only be guaranteed by good design optimization of each device in broadband and millimeter-wave frequency bands.

Test verification equipment needs to have comprehensive functions and better performance to ensure the correct presentation of test results such as constellation diagram, EVM, spurious power, spurious leakage, occupied bandwidth and adjacent channel power ratio. In addition, in high-frequency broadband test systems, the performance and indicators of test fixtures, cables, filters, couplers, power dividers, preamplifiers and switches will have a crucial impact on the measurement results, so the system including accessories needs to be calibrated as a whole before testing.

5G NR device/equipment comprehensive test platform (including overall system calibration solution)

MIMO and Beamforming are the most talked-about technologies in 5G. IMT2020 hopes that their introduction will bring 100X data throughput and 1000X channel capacity.

To this end, the 5G NR standard provides a physical layer frame structure, new reference signals, and a new transmission model to support the data throughput of 5G eMMB, but it also brings new challenges to terminal design engineers. These challenges include:

3D Antenna Beam Pattern Design and Verification

3D beam performance verification includes building and verifying the 3D radiation pattern of the antenna and ensuring that the correct gain, sidelobes and nulls are generated across the entire operating frequency band and bandwidth. Since the construction of a millimeter wave prototype system is expensive, simulation is essential and major problems in the system are discovered in advance. Proper construction of the antenna system and simulation combined with channel models and base station connections can reduce duplication and repetitive work and costs during the prototype design phase.

SystemVue, a system-level simulation system, helps designers quickly iterate and verify during the prototype stage

After the design is ready, the designer can use the actual environment to evaluate and verify whether the terminal/device has the correct beam width, sufficient null depth and gain within the operating frequency band and bandwidth to achieve maximum radiation efficiency. This requires the use of OTA testing methods.

mmWave link integrity

In order to overcome the location confirmation problem caused by the use of narrow beams, R15 defines a new access initialization process. Since the base station transmits channel information in the synchronization channel by scanning, the terminal will judge and determine the most suitable channel and inform the base station, thus finally establishing a communication link.

5G access initialization and beam management process

Procedures such as beam synchronization, tracking, management, and failure recovery are defined here. When mixed parameter sets are used, connection establishment may take more time. Designers need to implement, verify, and optimize all these functions, otherwise the user end may experience poor experience such as dropped calls.

Terminal performance evaluation and optimization in real environment

Throughput and latency are key performance indicators for wireless communication systems. Different layers of the protocol stack must work together to achieve the latency and throughput targets of 5G systems. This requires understanding the performance of the terminal in various states of beam management, including beam synchronization, switching, and fallback to 4G.

One of the most effective ways to evaluate end-to-end throughput is to use a network simulator to control the DUT and measure its feedback signals. The network simulator can configure cell connections, change the power of synchronization and reference signals, set beamforming parameters, and control the resource blocks for transmission and reception. In addition, by inserting a channel simulator into the system, it is possible to simulate real signal transmission problems in a laboratory environment, including path loss and multipath fading, thereby characterizing system performance in a real environment.

The base station simulator UXM is combined with the channel simulator PropsimF64 to allow you to evaluate the terminal performance in real environments

With the introduction of technologies such as millimeter wave, MIMO, beam control and management, the evaluation and testing of the overall performance of equipment and terminals are inseparable from OTA solutions, which include: RF performance, throughput, RRM and signaling.

How to comprehensively consider path loss, test site size, etc. to choose a suitable OTA solution has also become a difficult problem. A typical OTA test solution includes: darkroom, probe or antenna, and test equipment. Currently, there are three main solutions for terminal OTA testing:

① Direct Far Field Method (DFF)

For the direct far-field method, the DUT is fixed on a turntable that can rotate horizontally and vertically, so that measurements can be performed at any angle on the 3D projection surface. Although the direct far-field method can obtain the most direct and comprehensive antenna far-field test results, it requires a darkroom of the largest size. For a 15cm DUT, a darkroom of up to 4.2m is required to support far-field testing in the 28GHz frequency band, resulting in unacceptable test path loss.

Direct far-field test environment

② Indirect Far Field Method (IDF)

The indirect far-field method is based on the test method of the compact field, using a parabolic reflector to convert the signal from the near-field spherical wave to the far-field plane wave, thereby creating a far-field test environment. This solution can provide a more compact and low-path loss test environment compared to the direct far-field method, and is currently the only OTA test solution recognized by 3Gpp R4.

Compact field test environment

③ Near Field to Far Field Method (NFTF)

The near-field to far-field solution is to collect the electromagnetic field phase and amplitude in the near-field environment, and predict the radiation pattern under far-field conditions through algorithms. Although this is a compact test solution, it is easily interfered by the transmitter under near-field conditions, which affects the test accuracy, and it can only support single-line-of-sight measurements.

Near-field to far-field environment

In summary, 5G's three extreme application scenarios, especially eMMB and uRLLC, will bring great challenges to our design and testing.

Source: Internet compilation. If copyright is involved, please contact us to delete.

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