How does a base station perform radar sensing?

The base station transmits its normal OFDM downlink waveform and simultaneously listens for echoes reflected from objects in the environment using either a separate receive antenna panel or the same array during guard periods. By correlating the received echoes with the known transmitted waveform, the system extracts range (from time delay), velocity (from Doppler shift across successive OFDM symbols), and angle (from beamforming across the antenna array). The OFDM waveform's large bandwidth (100-400 MHz in FR1, up to 2 GHz in FR2) provides range resolution from 0.15 to 1.5 meters, comparable to automotive radar.

What is ISAC and how does it relate to BS sensing?

ISAC (Integrated Sensing and Communication) is the 3GPP framework being developed for Release 19 and beyond that defines how base stations perform sensing alongside their primary communication function. BS sensing is the base station implementation of ISAC. The key technical challenge is managing self-interference: the base station's own transmit signal is 80-120 dB stronger than the received echoes. Solutions include full-duplex operation with analog/digital self-interference cancellation, bistatic sensing using neighboring base stations as receivers, and time-division sensing during downlink guard periods.

What is BS Sensing? | Base Station RF Sensing

Q: What applications does BS sensing enable?

BS sensing enables several use cases without deploying additional sensors: traffic monitoring and vehicle counting on roadways, pedestrian detection for smart intersections, drone detection and tracking in restricted airspace, weather sensing through precipitation echo analysis, and gesture/presence detection for smart building automation. The sub-6 GHz bands provide long-range sensing (hundreds of meters) with coarse resolution, while mmWave bands (24-71 GHz) provide centimeter-level resolution at shorter range, similar to dedicated automotive radar performance.

Definition & Concept

BS sensing (base station sensing) is an emerging radio technology where cellular base stations repurpose their existing transmit and receive antenna arrays to perform radar-like detection and tracking of objects in the environment. Rather than deploying separate radar infrastructure, the base station uses its OFDM communication waveform as a sensing signal, extracting range, velocity, and angle information from echoes reflected by vehicles, pedestrians, drones, and other objects within the cell coverage area.

This capability is a core element of ISAC (Integrated Sensing and Communication), being standardized in 3GPP Release 19 for 5G-Advanced. The technical foundation builds on the fact that a 5G NR base station already possesses all the hardware needed for radar: a large-aperture phased array (64-256 elements) for angular resolution, wide bandwidth (100-400 MHz) for range resolution, and powerful digital signal processors for matched filtering and Doppler analysis. The primary engineering challenge is managing the 80-120 dB self-interference between the transmitter and receiver operating simultaneously on the same platform.

Key Performance Parameters

Range Resolution:

ΔR = c / (2 × BW)

100 MHz BW: ΔR = 1.5 m | 400 MHz: 0.375 m | 2 GHz (FR2): 0.075 m

Maximum Unambiguous Range:

R_max = c / (2 × Δf)

where Δf = subcarrier spacing. 30 kHz SCS: R_max = 5 km

Velocity Resolution:

Δv = λ / (2 × T_obs)

3.5 GHz, 10 ms observation: Δv = 4.3 m/s

BS Sensing Architecture Comparison

Parameter	Monostatic	Bistatic	TDD Pulsed	Full Duplex
Tx/Rx Location	Same gNB	Different gNBs	Same gNB	Same gNB
Self-Interference	Critical (~120 dB)	None	Moderate (guard)	Managed (~100 dB SIC)
Coverage	360°	Between sites	360°	360°
Range Accuracy	High	Moderate	High	High
Complexity	Very High	Moderate	Low	High
Comm Impact	Minimal	Minimal	Some (guard time)	Minimal
3GPP Status	Study item	Rel-19 candidate	Rel-19 candidate	Research

Practical Application

A 5G-Advanced gNB at a smart intersection uses its 64-element n78 (3.5 GHz) massive MIMO array with 100 MHz bandwidth for simultaneous communication and vehicle sensing. During TDD downlink slots, the base station transmits OFDM and listens for echoes during the 20 µs guard period between DL and UL, achieving 1.5-meter range resolution and detecting vehicles within 300 meters. The 64-element array provides 5° angular resolution, sufficient to distinguish vehicles in adjacent lanes. Processing 14 OFDM symbols per slot yields velocity resolution of 4.3 m/s, enabling speed estimation for traffic flow optimization. No additional infrastructure is required beyond a software upgrade to the existing baseband unit.

Frequently Asked Questions

How does a base station perform sensing?

It transmits normal OFDM and correlates received echoes with the known waveform to extract range (delay), velocity (Doppler), and angle (beamforming). Bandwidth of 100-400 MHz provides 0.15-1.5 m range resolution, comparable to automotive radar.

What is ISAC?

Integrated Sensing and Communication, the 3GPP Rel-19 framework for BS sensing. The key challenge is 80-120 dB self-interference. Solutions include bistatic sensing between gNBs, TDD guard period sensing, and full-duplex with self-interference cancellation.

What applications does BS sensing enable?

Traffic monitoring, pedestrian detection, drone tracking, weather sensing, and presence detection. Sub-6 GHz gives hundreds-of-meters range with coarse resolution; mmWave provides cm-level resolution at shorter range.

BS Sensing