Multi-Probe Anechoic Chamber

Description

A Multi-Probe Anechoic Chamber (MPAC) is an advanced measurement setup designed for Over-the-Air (OTA) testing of wireless devices, such as smartphones, tablets, and IoT modules. Its primary function is to characterize the radiated performance of a Device Under Test (DUT) in a controlled, isolated environment that simulates real-world radio propagation conditions. The core of an MPAC is a shielded anechoic chamber lined with radio-absorbent material (RAM) to eliminate external interference and internal reflections, creating a 'quiet zone' of uniform electromagnetic fields. The defining feature is an array of multiple fixed antenna probes (often 8, 16, or more) arranged on a circular or spherical surface around the DUT's position. Each probe can be individually activated to transmit or receive signals, simulating incoming radio waves from different directions of arrival.

The testing procedure involves placing the DUT on a positioning system at the chamber's center. A vector signal generator and analyzer are connected to the probe array via a switching matrix. To measure Total Radiated Power (TRP), the DUT transmits a signal, and the system sequentially activates each probe to measure the received power from every direction. The results are integrated over the sphere to calculate the total power radiated. Conversely, for Total Isotropic Sensitivity (TIS), each probe transmits a known signal to the DUT, and the receiver sensitivity of the DUT is measured for each direction; the results are integrated to find an average sensitivity. The multi-probe array allows for rapid, sequential sampling of the spatial sphere without needing to physically rotate the DUT for every angle, significantly speeding up tests for complex multi-antenna systems.

MPAC systems are critical for evaluating Multiple-Input Multiple-Output (MIMO) and beamforming performance, which are foundational to 4G LTE and 5G NR. They can create dynamic fading environments by applying complex weightings to the signals from different probes, simulating specific channel models defined in 3GPP (e.g., TDL, CDL). This allows for testing of receiver performance under realistic multipath conditions and for validating the efficacy of antenna diversity and MIMO spatial multiplexing schemes. The specifications for MPAC test methodologies, including chamber calibration, probe configurations, and measurement uncertainty, are detailed in 3GPP TS 37.544 and related 3GPP Radio Access Network (RAN) working group specifications. This ensures that performance tests are standardized, repeatable, and correlate with real-world network performance, providing a reliable benchmark for device certification and R&D.

Purpose & Motivation

The MPAC was developed to address the significant challenges in testing modern wireless devices, whose performance is increasingly defined by their integrated antennas and MIMO capabilities. Traditional conductive testing, where a cable is connected directly to the antenna port, became insufficient because it bypasses the antenna system—the very component that defines radiated power, sensitivity, and spatial characteristics. As devices shrank and used more integrated, non-removable antennas, OTA testing became mandatory. Early OTA methods used a single probe and a rotating positioner, which was time-consuming and could not accurately simulate the fast-fading, multi-path environments of MIMO.

The limitations of single-probe systems motivated the creation of the MPAC. For accurate MIMO and beamforming validation, it is necessary to stimulate the device with signals arriving from multiple spatial directions simultaneously or in rapid succession, emulating a realistic spatial channel. A single-probe system with a mechanical positioner is too slow to capture the channel's temporal coherence properties. The MPAC solves this by using a fixed array of probes, enabling rapid switching between spatial angles and the application of complex channel emulation. This allows for efficient testing of key performance indicators (KPIs) like MIMO throughput under standardized fading conditions.

3GPP standardized the MPAC methodology to ensure consistency and fairness in device performance evaluation, particularly for carrier acceptance and regulatory conformance. It provides a controlled and repeatable alternative to expensive and variable field testing. By defining precise test setups in specifications like TS 37.544, 3GPP enables device manufacturers, test labs, and network operators to have a common, accurate understanding of a device's real-world radiated performance, driving improvements in antenna design and overall user experience in cellular networks.