Technical Insights > RF + Microwave

How to Make Accurate 5G NR Measurements

2020-10-22  |  8 min read 

The promise of 5G is faster and more reliable communications. To enable mobile broadband communications, 5G uses existing and new technologies to achieve extreme data throughputs. The introduction of these technologies leads to new testing demands, including operation in more frequency bands, wider channel bandwidths, and complex multi-antenna configurations.

With the standalone version (Release 15.2.0) of 5G New Radio (NR) approved by the 3rd Generation Partnership Project (3GPP) in December 2017, the cellular ecosystem is evolving from research to product development and early production. This evolution has sparked challenges for the wireless industry related to test costs and time to market as it transitions from research and design (R&D) to design verification and volume production. The global wireless standards consortium announced the freeze of Release 16 specifications in July 2020. Engineers need to adapt to the new standard faster and accelerate their 5G product development.

New Frequency Coverage

3GPP Release 16 is the second phase of the 5G NR standard. It adds more than 20 technical features to extend the range of 5G capabilities. For example, Release 16 adds support for 5G NR unlicensed spectrum bands to extend FR1 to 7.125 GHz. This extension gives mobile network operators more spectrum options for deploying 5G.

Table 1. Definition of frequency ranges in the 3GPP standard
Frequency range designation Corresponding frequency range
FR1 450 MHz–7,125 MHz
FR2 24,250 MHz–52,600 MHz

FR2 mmWave operating bands have wider channel bandwidths, up to 1.2 GHz contiguous or 1.6 GHz noncontiguous when aggregating multiple component carriers. This additional spectrum is essential for enabling 5G’s promise of extreme data rates of 20 Gbps in the downlink and 10 Gbps in the uplink. However, mmWave frequencies with wider bandwidths also expose signal propagation issues, such as excess path loss, delay spread, and blockage, resulting in a poor radio link.

To overcome these propagation issues, 5G NR uses multi-antenna techniques, such as phased-array antennas, to increase directivity and gain. The mmWave components are compact and highly integrated with no place to probe, requiring radiated tests, also known as over-the-air (OTA) tests.

Millimeter-Wave Test Challenges

Using mmWave frequencies and wide signal bandwidths poses challenges related to path loss and signal propagation. Both transmitter and receiver tests require radiated testing. Component tests can be either conducted or radiated, depending on the device under test (DUT).

Excessive Path Loss

At mmWave frequencies, the excess path loss between instruments and DUTs results in a lower signal-to-noise ratio (SNR) for signal analysis. The low SNR causes the transmitter measurements to deliver poor error vector magnitude (EVM) and adjacent channel power ratio (ACPR) performance, which does not represent the DUT’s actual performance, as shown in Figure 1.

Figure 1. SNR’s impact on transmitter measurements

 

 

 

 

 

 

 

 

 

 

 

To achieve accurate transmitter measurements for 5G NR, you require a signal analyzer with a low display average noise level (DANL) and a wider analysis bandwidth. Also, you need to optimize signal analyzer’s input level, phase noise, and, and IF gain to achieve the best EVM measurement results.

Figure 2. Keysight’s N9021B MXA x-series signal analyzer provides a DANL -138 dBm at 40 GHz and up to 510 MHz analysis bandwidth

For receiver and component tests, signal generators need higher-output power levels with less distortion to compensate for the excess path loss. In addition, connectorless radiated testing signal routing to OTA chambers requires longer cables (typically 2 to 4 meters) and switch matrixes, which introduce about 5 to 10 dB higher insertion loss.

Figure 3. Keysight M9384B VXG microwave signal generator delivers signals at 39 GHz with maximum power levels of +24 dBm

To overcome these challenges at higher frequency bands and wider bandwidths, high-performance signal generators and signal analyzers are essential to ensure errors are not generated by the test instruments. The test system integration and test costs create higher barriers from R&D to volume production.

Phase Noise

Phase noise describes the frequency stability of an oscillator. It is the noise spectrum around the oscillator’s signal in the frequency domain. Phase noise can cause errors in the phase component of an error vector. The phase noise performance of a signal analyzer or a signal generator contributes error to EVM measurements.

5G NR adopts the orthogonal frequency-division multiplexing (OFDM) modulation scheme. OFDM uses many closely spaced orthogonal subcarrier signals, each with its own modulation scheme, to transmit data in parallel. During frequency conversion with a poor phase noise local oscillator of a signal analyzer or a signal generator, the subcarrier with phase noise spreads into other subcarriers as interference as shown in Figure 4. The interference degrades measurement accuracy. For a higher-order modulation scheme (for example, 256QAM), the symbols are closer and the test requirement of EVM performance is higher.

The impact on OFDM subcarriers from poor phase noise LO of a signal analyzer
Figure 4. The impact on OFDM subcarriers from poor phase noise LO of a signal analyzer

Two microscopic electronic effects influence the phase noise performance: thermal noise from passive devices, which is broad and flat (the green line), as shown in Figure 5, and flicker noise from active devices, which is a 1/f shape (the purple line). The flicker noise, also known as pink noise, is 20 dB per decade. If you increase frequency by 10 times, the phase noise increases by 20 dB. Therefore, the phase noise performance of a mmWave test instrument is often a key factor in determining how well it fits an application.

Phase noise profile with thermal noise and flicker noise
Figure 5. Phase noise profile with thermal noise and flicker noise

Summary

The evolution of 5G technology introduces challenges and requirements that demand test instruments with sophisticated capabilities and performance. Implementing DVT systems for 5G requires multiple high-performance test instruments and complex system integration. The test requirements and complexity lead to high test costs across R&D and volume production.

Next post, we will discuss production test strategies for 5G NR. Stay tuned.