Massive MIMO (mMIMO) is a key enabling technology of 5G networks. As the wireless industry advances to 5G-Advanced (5G-A) and 6G, more mMIMO radios will be deployed in arrays at millimeter wave (mmWave) frequencies for terrestrial and non-terrestrial networks (NTNs). These mmWave antenna arrays will play a key role in 5G-A and 6G mobile networks. The mMIMO radios, coupled with the active antenna arrays, enable beamforming. Although antenna arrays have been around for a few decades, the high volume, low-cost and carrier-grade quality requirements of mobile networks make mMIMO radio and antenna array production, particularly their testing, a difficult problem for electronics manufacturers. This article presents an efficient production test solution to address this salient challenge. The solution includes a compact over-the-air (OTA) test chamber design, an algorithm estimating far-field (FF) RF performance from near-field (NF) measurements and a software package supporting test automation and data analytics in the cloud.
Production testing at millimeter frequencies is a challenge. Compared to 4G radios, e.g., 4T×4R remote radio heads (RRHs), a mmWave antenna array contains hundreds or thousands more transceivers. All transceivers need to be tested and calibrated at a production test station. In addition, these transceivers need to be aligned in phase and magnitude for beamforming. This beamforming calibration procedure can take hours for each radio using a brute force method like rotating element electric field vector (REV).1 Adding to the challenge, RF test and calibration can only be done using OTA methods in an anechoic chamber since the RF front-ends and radiating elements are integrated. A direct FF chamber and an indirect compact antenna test range (CATR) chamber will be too large and expensive to support high volume production in automated factories. Finally, the RF performance of active antenna arrays is sensitive to temperature. Temperature is an important factor to consider in the test and calibration process. However, it is impractical to implement temperature control in production OTA test chambers for cost and volume reasons.
The remainder of this paper presents an efficient production OTA test solution for mmWave antenna arrays. This solution consists of four parts:
- An automated compact OTA test chamber with multiple probes in the NF
- A signal processing algorithm to compute FF RF performance key performance indicators (KPIs) from NF measurements in the chamber
- A software package supporting automated test execution, test KPI calculation and production data analytics
- An innovative, fast, beamforming calibration algorithm.
The NF-to-FF conversion algorithm exploits the restricted production test conditions not considered by general methods.5-8 Beamforming calibration is a complex subject.2,3 Our information theory based algorithm is described in a paper to be presented at EuMW 2024.9 This production test solution is used in Jabil factories to support the high volume production of mMIMO radios at the n257 and n258 5G bands.
PRODUCT VERIFICATION TEST (PVT) CHAMBER DESIGN
PVT Chamber Design Requirements
Once an antenna array is developed, it goes through a design verification test (DVT) in an R&D lab with a certified FF chamber or a CATR chamber. Antenna functions are verified according to design specifications and 3GPP standards. Product verification testing differs from design verification testing because its goal is verifying antenna manufacturing quality rather than its design. PVT test cases usually only cover the critical performance of an antenna, e.g., channel power and adjacent channel power (ACP), to avoid recalls after deployment. Manufacturers can design a customized PVT system for high volume products, like a base station antenna, for each production line.
To support high volume and low-cost production, a PVT chamber should:
- Be compact
- Have good isolation and minimized reflection
- Support automatic antenna under test (AUT) loading/off-loading and test execution
- Avoid moving the AUT to reduce the wear of the fixture and test time
- Meet factory safety standards
- Be scalable to support multiple chambers per production line.
PVT Chamber with Multiple NF Probes
Jabil designed an automated PVT chamber to support its mMIMO radio production. Figure 1 shows this patented chamber in a Jabil factory.4 The external dimensions of this chamber are 2 × 1.5 × 1 m. It has five probes mounted in the near field from the AUT, as shown in Figure 2. Test operation is automated through pneumatic executors and a programable logic controller (PLC) controlled by a test PC.
RF Measurement System
A Keysight S91xx 5G Multi-Band Vector Transceiver measures and generates mmWave RF signals. The main chassis is installed in an equipment rack for easy maintenance. The remote head is placed close to the probes on top of the chamber for low cable loss. All test KPIs are calculated from captured IQ baseband data.