Total Radiated Multi-antenna Sensitivity

Description

Total Radiated Multi-antenna Sensitivity (TRMS) is a comprehensive performance metric defined in 3GPP specifications for evaluating the receive sensitivity of User Equipment (UE) equipped with multiple antennas. Unlike traditional single-antenna sensitivity measurements that test one antenna port in isolation, TRMS assesses the UE's ability to receive signals in a realistic, over-the-air (OTA) setup where all antenna branches and the entire receiver processing chain (including combining algorithms) are active. The metric is defined as the minimum average power received at the UE's antenna connectors (or equivalently, the minimum far-field incident power) required to achieve a specified throughput performance (e.g., 95% of maximum throughput) for a given reference measurement channel. It is typically measured in dBm.

The measurement of TRMS is conducted in an anechoic chamber using a setup that mimics real-world spherical wavefront illumination. A test system transmits a standardized reference signal (e.g., a specific PDSCH configuration) from one or more base station emulator antennas. The UE, placed on a positioning system, is rotated through multiple spatial angles (azimuth and elevation) to capture its performance across the entire sphere. At each angle, the input power is adjusted to find the threshold where the target throughput is just met. The TRMS value is then derived from a statistical aggregation (often a linear average) of these sensitivity power levels over all measured angles. This process inherently captures the effects of antenna gain patterns, mutual coupling between antennas, receiver noise figures, and the performance of the UE's diversity combining or MIMO reception algorithms.

Architecturally, TRMS evaluates the integrated performance of the UE's antenna subsystem, RF front-end, and baseband digital receiver. For multi-antenna UEs using techniques like receive diversity (e.g., selection combining, maximal ratio combining) or MIMO spatial multiplexing, the TRMS metric quantifies the actual sensitivity improvement gained from these techniques in a radiated environment. A lower (more negative) TRMS value indicates better receive sensitivity, meaning the UE can maintain a connection at lower signal strengths, which directly extends cell coverage and improves reliability at cell edges. TRMS is therefore a key figure of merit for network operators and device manufacturers to ensure consistent user experience, especially in challenging radio conditions.

Purpose & Motivation

TRMS was introduced to solve the inadequacy of traditional conducted sensitivity tests for evaluating modern multi-antenna UEs. Conducted tests, which inject signals directly into a single antenna port via a cable, fail to account for real-world effects like antenna efficiency, radiation patterns, and the interaction between multiple antennas. As UEs evolved to incorporate 2x2, 4x4 MIMO, and diversity reception to boost data rates and reliability, it became clear that the performance of the integrated antenna-receiver system could not be predicted from component-level tests alone. Poor antenna design could negate the benefits of an excellent baseband receiver.

Its creation was motivated by the need for a standardized, holistic metric that reflects true end-user experience in weak signal conditions. Network operators, particularly, required a way to benchmark different UE models on their ability to maintain service at the edge of coverage, which impacts network planning and customer satisfaction. TRMS provides this by measuring sensitivity in an OTA manner, considering the UE as a complete radiating and receiving entity. It addresses the problem of performance variability due to antenna placement, industrial design, and user handling (e.g., hand grip effects).

Historically, OTA testing for TRMS was formalized in 3GPP Release 13, building upon earlier work for TRP (Total Radiated Power). It represents a shift towards system-level performance verification. By defining a rigorous test methodology, TRMS ensures that UEs claiming multi-antenna capabilities actually deliver the promised coverage and reliability improvements in practice, which is essential for the successful deployment of advanced LTE and 5G networks where MIMO is a foundational technology.