Solutions that facilitate the interpretation of 3GPP specifications and simplify the test setup help overcome conformance testing challenges.
5G New Radio (NR) is being deployed around the world to deliver faster data rates and unprecedented reliability to wireless communications. New conformance testing has emerged to ensure base stations can deliver on these promises.
Conformance testing is an important part of the base station lifecycle that requires a good understanding of 3rd Generation Partnership Project (3GPP) specifications. With investments in network infrastructure to the tune of $40 billion per year, mobile network operators (MNO) need to ensure the base stations they choose to implement in their networks meet the 5G standards.
Technology evolution further increases the importance of conformance testing for base stations. 5G opens a Pandora’s box of considerations because of the lack of antenna connectors with 5G millimeter-wave (mmWave) units. Over-the-air (OTA) testing creates many challenges for research and development (R&D), integration, and verification engineers at network equipment manufacturers (NEM) and MNOs dealing with small cells, macro cells, and open radio access network (O-RAN) components.
5G introduces new base stations
With 5G NR, new base station types have emerged, defined according to the frequency range and the antenna configuration of the equipment. The four types are denoted as: 1-C, 1-O, 1-H, and 2-O. The numeral denotes the frequency range. Frequency range 1 (FR1) covers 450 MHz to 7.125 GHz, while frequency range 2 (FR2) refers to mmWave spectrum from 24.25 GHz to 52.6 GHz.
‘C’ refers to base stations with antenna connectors. Type 1-C base stations are tested using a conducted approach, like legacy cellular base stations for 3G and 4G. ‘O’ refers to base stations with no antenna connectors. All the testing for these units must be done over the air in a radiated type of test. ‘H’ refers to a hybrid approach with some antenna connectors accessible in the system between modules and an integrated antenna in the base station assembly. 5G base stations need to support an increasing number of channels for applications like spatial multiplexing and beamforming. The levels of integration are also increasing, even in FR1. The market is moving toward more type 1-O base stations.
Figure 1 Type 1-H, 1-O, and 2-O are new base stations introduced by 5G NR.
3GPP provides the necessary conformance testing documents
Base station conformance testing starts with the 3GPP specifications. 3GPP technical specification (TS) 38.104 and 38.141 are essential documents for 5G base station conformance testing. 3GPP TS 38.104 provides the minimum requirements for both conducted and radiated base stations. The specification covers transmitter and receiver characteristics, as well as receiver performance.
3GPP TS 38.141 includes 38.141-1 for conducted base stations and 38.141-2 for radiated units. These documents define the test requirements, provide a relaxed specification for test tolerance, and include the test methodology for ensuring compliance to the requirements outlined in TS 38.104. Additionally, these documents reference other 3GPP documents that provide additional context and background related to conformance tests. Each of these documents can be hundreds of pages long and it is essential to understand all the details to ensure proper compliance to the 3GPP specifications.
Base station conformance tests are organized into chapters. Chapter 6 covers transmitter characteristics including typical parameters for transmitters like output power, signal quality, and out-of-band (OOB) emissions. Chapter 7 covers receiver aspects such as sensitivity, dynamic range, selectivity, and blocking characteristics among other parameters.
Chapter 8 focuses on receiver performance, including tests for the uplink channels going from the user equipment to the base station – physical uplink shared channel (PUSCH), physical uplink control channel (PUCCH), and physical random access channel (PRACH).
In addition, anyone involved in 5G base station conformance testing should also review Chapter 4. This chapter covers the manufacturer’s declarations that define the base station types, classes, and basic RF properties like frequency range, bandwidth, and output power, as well as provide the beam configurations for radiated base stations including beam width, direction, and number. Each test case has a unique signal configuration derived from the manufacturer’s declarations.
Different configurations address various test cases
Chapter 6, 7, and 8 tests apply to all base stations, regardless of their type, but the test method differs between conducted and radiated units.
You can perform most transmitter tests with a simple test setup made of a base station connected to a spectrum analyzer (Figure 2). Each port on the base station is tested individually for all characteristics whether it be error vector magnitude (EVM), adjacent channel power ratio (ACPR), or spurious emissions.
Figure 2 Most transmitter test cases use a simple test setup consisting of a base station and spectrum analyzer.
Certain transmitter test cases need a different configuration though. Time alignment, for example, a test case important for spatial multiplexing and carrier aggregation scenarios, requires simultaneously measuring all of the antenna ports. You can perform this test with a single spectrum analyzer by combining all the signals into a single cable. You can use this technique for time alignment measurement because the demodulation reference signals are orthogonal in time and frequency allowing accurate timing measurements. However, the data is not orthogonal in the case of spatial multiplexing, and recovery of the data payload is not possible with this technique. Transmitter intermodulation testing also involves a different configuration with a source to inject an interfering signal into the transmitter.
Testing the receiver of the base station requires a signal generator to provide a metrology-grade reference signal. Some tests require multiple signals of disparate frequency and amplitude configurations. This is the case for intermodulation and blocking tests where the goal is to determine if the receiver can correctly demodulate a very low power ‘wanted’ signal in the presence of much higher power blocking signal(s) with specific frequency relationships to the wanted signal (Figure 3). Typically, this requires two signal generators because of the large dynamic range between the wanted and interfering signals, and combining these signals at RF. Each port of the base station is tested separately from the others.
Figure 3 Receiver intermodulation testing uses multiple sources in the test setup to generate interfering signals.
Receiver spurious emissions testing requires a different setup though. It connects a spectrum analyzer to the receive port of the base station to measure the emissions from the receiver port and make sure they are under the 3GPP-specified levels.
When it comes to receiver performance testing, the test setup can be a lot more complex than for transmitter and receiver characteristics. The most complex configuration is for the PUSCH test case. The setup stimulates all the base station ports simultaneously to ensure the base station can recover highly-faded signals with a very low signal-to-noise (SNR), which are simulated by adding additive white Gaussian noise (AWGN).
This test is also difficult because the base station needs to demodulate each block of transmitted data and provide a hybrid automatic repeat request (HARQ) feedback to the test equipment, but there is no standard interface for this. In addition, the setup needs a reference and frame trigger to ensure alignment between the test equipment and the base station. Some of these tests also require 2-layer spatially multiplexed multiple-input/multiple-output (MIMO) signals.
Radiated testing brings more challenges
Radiated testing takes base station conformance testing to a whole new level, placing the base station in a chamber and replacing the cables with antennas. The probe antenna receiving the signal must be far enough from the base station to perform the measurements in the far field where the radiated wave becomes a planar wave. Measurements performed too close can cause inaccuracies for a variety of reasons. As a result, the test chamber can become quite large, depending on the size of the antenna and the frequency.
For example, an antenna size of 15 cm at a frequency of 28 GHz requires at least 4.2 m between the base station and the probe antenna to be considered in the far field (Figure 4). Path loss is another concern at mmWave frequencies. It is much higher than in FR1. Loss occurs over the distance between the base station and the probe antenna and there is additional cable loss from the probe antenna to the spectrum analyzer to contend with.
Radiated test for certain aspects like output power and ACPR also bring a new twist to base station conformance testing. Total radiated power (TRP) involves making measurements from all possible directions. Mounted on a positioner, the base station rotates through all azimuths and elevations creating the need for thousands of measurements. 3GPP provides a range of measurement grids on possible ways to perform the TRP measurement. Some methods are better suited to particular measurements, however 3GPP does not mandate a particular grid pattern for any measurement. Ultimately, it is up to the vendor to ensure compliance with the 3GPP specification.
OOB spurious emissions testing is particularly difficult to make over the air because the test setup needs to cover a wide range of frequencies from 30 MHz to 60 GHz or the 2nd harmonic, whichever is lower, in the case of FR2 devices. Spectrum analyzers can easily cover this frequency range but you will not find a probe antenna that covers the entire range with suitable antenna pattern and gain, which necessitates using multiple banded probe antennas and therefore also some sort of switching matrix. Switching matrices increase loss between the antennas and the analyzer requiring the use of low-noise amplifiers (LNA). OOB spurious emissions testing also needs filters to capture low-level spurious signals. Using all these components add complexity to the test setup, increasing the need for calibration too.
Although challenging, these issues are not insurmountable. Solutions exist but no one size fits all because of the various types of 5G base stations. Figure 5 provides the diagram of an OTA base station conformance test solution, the most complex case. The solution covers all 3GPP test cases across Chapters 6, 7, and 8, and uses the manufacturer’s declarations to define the configuration for each test. It features interfaces to communicate with the signal generators and signal analyzers and application programming interfaces (API) to communicate with the base station under test.
Simplifying 5G base station conformance testing
New base station types have emerged to deliver the promise of 5G, bringing new conformance tests along with them, as detailed in 3GPP specifications. Various test cases require different configurations ranging from simple conducted setups with just a base station, cables, and a spectrum analyzer to more complex configurations for receiver performance testing and OTA test cases. Solutions that facilitate the interpretation of 3GPP specifications, simplify the test setup, and automatically generate test plans help overcome these challenges faster.
For more information on 5G challenges and solutions for base stations, visit Keysight’s 5G Network Equipment Manufacturers page. The webinar 3GPP gNB Conformance Testing Overview, Challenges, New gNB Test Solutions provides a walk-through of a 5G base station conformance test setup.
This article was originally published on EDN.
Jessy Cavazos is part of Keysight’s Industry Solutions Marketing team.