Maintain compliance with 5G NR base station transmitter testing: Part 1

As standards continue to evolve, test solutions must support higher frequencies, wider bandwidths and new physical layer capabilities. Thanks to the faster, more reliable, and near-instantaneous connectivity brought about by 5G, a variety of innovative and comprehensive mobile wireless communications applications improve our daily lives. To ensure base stations live up to their promises, they must now pass new conformance tests. Conformance testing is an important part of the base station life cycle and requires a deep understanding of the 3rd Generation Partnership Project (3GPP) specifications. Before a 5G New Radio (NR) base station or user equipment (UE) can be released on the market, it must pass all necessary tests. Unless products are 3GPP compliant, they cannot be deployed on the network.

In Release 15, 5G NR is included along with some new features for Long Term Evolution (LTE). In Release 16, as the first step in NR evolution, several important new enhancements have been made to NR, as well as LTE enhancements and extensions. Version 16 introduces two specific sets of features. The first set includes new verticals such as multi-radio access technology (multi-RAT), dual connectivity and carrier aggregation (CA) enhancements, industrial Internet of Things (IIoT), ultra-reliable low-latency communications (URLLC), vehicle-to-everything (V2X). This set includes unlicensed NR - NR-U, NR positioning and two-step random access channel (RACH). The second category is features designed to increase capacity and improve operational efficiency, such as multiple-input multiple-output (MIMO) enhancements.
In this blog series, we will discuss critical conducted and radiated emissions for frequency range (FR)1 and FR2 base stations. machine testing, test challenges, and state-of-the-art industry solutions to tackle them for accurate and reliable results. In Part 1, we will discuss transmit power testing and requirements.

Conformance testing of 3GPP base stations

3GPP defines radio frequency (RF) conformance test methods and requirements for NR base stations in TS 38.141, covering transmission, reception and performance testing. This technical specification consists of two parts depending on whether the test method has conducted or radiated requirements. Part 1 of TS 38.141-1 focuses on conducted compliance testing, while Part 2 of TS 38.141-2 covers radiated compliance testing in frequency range (FR) 1 and FR2 depending on the base station type. Table 1 summarizes base station compliance testing for conducted and radiated conditions.

The base station can be configured in one of four ways, depending on whether the test is conducted or radiated, and the configuration of the base station. Type 1-C refers to NR base stations operating on FR1 and requiring a single antenna connector. Type 1-H NR base stations operate in the first frequency range (FR1), whose requirements are defined at the individual transceiver array boundary (TAB) connectors and the over-the-air (OTA) requirements are defined at the radiating interface boundary (RIB). Both This type of O base station refers to an NR base station operating on FR1 or FR2, and only needs to meet the OTA constraints defined by the RIB. The main difference between conducted and radiated testing is the radiated testing of base station types 1-H, 1O and 2O.

Table 1. Release 16 Base Station Conducted and Radiated Compliance Tests

Testing requirements for base station transmitters

The 5G NR measurement application on your signal analyzer should be able to measure the required tests specified by the standard and the tests covered by the 5G NR application. Channel power and occupied bandwidth are the most common tests, along with adjacent channel leakage ratio (ACLR), operating band unwanted emissions (OBUE), spurious emissions, Tx on/off power, error vector magnitude (EVM), frequency error and Time Alignment Error (TAE). The standard also covers many transmitter characteristics and test measurements, such as output power, output power dynamics, transmit on/off power, transmitted signal quality, unwanted emissions and transmitter intermodulation.

Output power dynamic measurement

Perform an output power test to determine how accurate the power output is compared to the base station's declared value when transmitting at maximum power. The measurement results shown in Figure 1 (left) were obtained in channel power measurement mode on a 100 MHz bandwidth time division duplex (TDD) signal. The gate start and stop lines represent the portion of the frame used to measure power. Within a 100 MHz bandwidth, the measured power is -1.04 dBm, while the transmitted signal is 0 dBm. Cable loss is precalibrated to 0.64 dB. Power accuracy is estimated to be -0.4 dB, which is within test requirements.

Output power dynamics refers to the difference in power levels when the base station is transmitting at maximum and minimum levels. Measurements should be performed on Orthogonal Frequency Division Multiplexing (OFDM) symbols carrying only Physical Data Shared Channel (PDSCH) data, without any Synchronization Signal Block (SSB) or Demodulation Reference Signal (DMRS) within the symbol. OFDM Symbol Transmit Power Limit (OSTP) is measured only on data symbols and is a requirement for dynamic power measurements.

Keysight's UXA signal analyzer and PathWave X-Series measurement application were used to measure the output power dynamics in Figure 1 (right). For the base station's maximum transmit power, the OSTP measurement is -1.02 dBm. It can be measured using test model 3.1, which can be selected from a list of test models. After successfully demodulating the signal, OSTP and other measurements will appear on the display.

Test model 3.1 is the full resource block (RB) allocation with 64QAM modulation, as shown in the multi-trace display. You can also measure minimum power using a test model 2 signal with a single RB. Therefore, you can calculate the power dynamics of 25.48 dB and compare it with the standard requirements.

Figure 1. Example of conducted output power measurement test model (left) and output power dynamics (right)

Measurement of transmitter power on and off

By testing the transmit shutdown power, we can ensure that it is within the range defined by the standard. However, this test only applies to base station systems operating in TDD mode. Defined as the average power measured over 70/N µs centered on the assigned channel frequency during the transmit off period (N = SCS/15), filtered using a square filter with a bandwidth equal to the base station transmission bandwidth configuration, and SCS is in kHz subcarrier spacing in units.

This test requires verification of two technical aspects: One is to measure the power level when the transmitter is turned off and check against pass or fail requirements. The other is to measure the transient time, rise time and fall time of the TDD signal burst. The shutdown power value should be less than approximately -83 dBm/MHz for conducted testing and -102 dBm/MHz for radiated testing.

Transient time is the time it takes for a transmitter to go from off to on power level, or vice versa. In Figure 2, Keysight's UXA signal analyzer and PathWave X-Series measurement application are used to measure 5G NR transmit off/on power and transient time. In this case, the external trigger determines the burst boundaries. Using the frame structure of the test model, transient times can be measured using power rise and fall.

The power envelope template indicates the expected limits of off power and the ramp-up and ramp-down locations. In the meter table at the bottom are measurements of transmit power, power off, ramp up, and downtime. Results are displayed in the upper left corner as a pass/fail indicator so you can compare them to defined criteria. These limits are set by default to the 3GPP specifications, but you can modify them to meet your specific testing needs.

Figure 2. Conducted emission on/off power measurement example

As standards evolve, test solutions must support higher frequencies, wider bandwidths and new physical layer features. As you plan the next steps in 5G design and innovation, understanding standards and how they impact testing is critical to ensuring successful and accurate testing.