Sensing Transmitter

Description

The Sensing Transmitter (STX) is a functional entity introduced in 3GPP Release 19 as part of the Integrated Sensing and Communication (ISAC) framework. It is defined in specifications 38.765 and 38.901. The STX is responsible for generating and transmitting sensing waveforms that are used to detect, localize, and track objects in the environment. Unlike traditional radar systems, the STX operates within the cellular network, leveraging the same infrastructure and spectrum as communication services. This integration allows for efficient use of resources and enables new applications such as autonomous driving, drone detection, and environmental monitoring.

Architecturally, the STX can be implemented in a base station (gNB) or as a standalone transmitter. It interfaces with the core network's sensing functions, such as the Sensing Management Function (SMF) or the Sensing Control Function (SCF), which coordinate sensing tasks and process the received echoes. The STX transmits specific reference signals, known as sensing reference signals (SRS-like), which are designed for high-resolution sensing. These signals are typically wideband and may use beamforming to focus energy in a particular direction. The STX may also cooperate with multiple receivers (RX) to form a distributed sensing network, improving coverage and accuracy.

The operation of the STX involves several steps: first, the network determines a sensing task based on a request from an application or internal trigger. The STX is then configured with the appropriate waveform parameters, such as bandwidth, duration, and beam direction. It transmits the sensing signal, which propagates through the environment and reflects off objects. The reflected signals are received by sensing receivers (SRX), which may be co-located with the STX or separate. The received signals are then processed to extract information about the objects, such as distance, velocity, and angle. The STX may also receive feedback from the receivers to adjust its transmission parameters for optimal sensing performance.

Key components of the STX include a waveform generator, a beamforming module, and a synchronization unit. The waveform generator creates the sensing signal based on parameters provided by the network. The beamforming module uses antenna arrays to steer the signal, which is crucial for achieving high angular resolution and gain. The synchronization unit ensures that the STX is synchronized with the network and with other sensing nodes, which is essential for coherent processing of reflected signals. The STX also supports multiple sensing modes, such as monostatic (where the same node transmits and receives) and bistatic (where transmitter and receiver are separated).

The STX is a key enabler for ISAC, which is considered a major feature of 5G-Advanced and 6G. By integrating sensing into the cellular network, operators can offer new services without deploying separate radar infrastructure. This is particularly valuable in spectrum-scarce environments, as it allows for dual use of spectrum. The STX also supports privacy-preserving sensing, as the network can control what information is extracted and shared. As the technology matures, the STX is expected to support higher frequencies (e.g., mmWave and THz) and more advanced waveforms, enabling even finer sensing resolution. The STX is defined in 3GPP TS 38.765 and TS 38.901, which specify the sensing architecture, procedures, and channel models.

Purpose & Motivation

The Sensing Transmitter (STX) was introduced in 3GPP Release 19 to enable Integrated Sensing and Communication (ISAC) in cellular networks. Traditionally, sensing (e.g., radar) and communication were separate systems operating in different frequency bands. This separation led to inefficient use of spectrum and infrastructure. The STX solves this problem by allowing the cellular network to transmit sensing signals using the same spectrum and hardware as communication signals. This integration is motivated by the growing demand for sensing applications in areas such as autonomous vehicles, smart cities, and industrial automation, where real-time environmental awareness is critical.

Previous approaches to sensing in cellular networks were limited to using existing communication signals (e.g., CSI-RS) for opportunistic sensing, which provided limited resolution and flexibility. The STX enables dedicated sensing waveforms that can be optimized for specific sensing tasks, such as high-resolution detection or long-range tracking. This is made possible by the flexibility of 5G NR, which supports a wide range of waveforms and beamforming capabilities. The STX also addresses the need for coordination among multiple sensing nodes, which is essential for achieving coverage and accuracy in complex environments.

The motivation for the STX also comes from regulatory and standardization efforts. Regulatory bodies are increasingly allocating spectrum for ISAC, and 3GPP is defining the necessary architecture and procedures to support it. The STX is a key component of this architecture, providing the transmit side of the sensing link. By standardizing the STX, 3GPP ensures interoperability among vendors and enables global deployment. In summary, the STX exists to provide a standardized, efficient, and flexible means of transmitting sensing signals within cellular networks, unlocking new services and applications.