How Does Radar Presence Sensing Work?

A room can appear completely still while a person inside it is reading, working at a computer, watching a display or simply sitting quietly.

This is where conventional motion detection can reach its limits.

Traditional motion sensors generally rely on noticeable movement to determine whether a space is occupied. Radar-based presence sensing takes a different approach. By transmitting RF signals and analysing their reflections, a radar sensor can identify subtle human micro-movements that may remain detectable even when a person appears stationary.

For smart buildings, energy management systems, automation and intelligent equipment, this distinction can be important. A lighting or HVAC system that incorrectly assumes an occupied space is empty can affect user experience, system efficiency and energy performance.

How Radar Presence Sensing Works

Radar presence sensing begins with the transmission of radio-frequency energy.

A radar IC or module drives an antenna that emits RF energy into a defined sensing area. Objects within that area reflect part of the transmitted signal back towards the receiving antenna.

The radar receiver then analyses changes in the returned signal.

Depending on the sensing architecture, these changes can provide information associated with:

• movement

• distance

• direction

• target position

• subtle body micro-movements

The reflected signal contains information about how the environment changes over time. When a person moves, even slightly, the characteristics of the received RF signal can change.

With sufficient RF sensitivity and appropriate signal processing, these small variations can help distinguish genuine human presence from a completely static environment.

Detecting More Than Obvious Movement

Human presence does not always involve walking or large body movements.

A seated person may produce only very small movements through normal human activity, such as:

• body sway

• hand or arm movement

• posture changes

• small head movements

• subtle chest and body micro-movements

Radar sensing can detect these small variations in reflected RF energy.

This enables a properly designed presence-sensing system to continue recognising an occupant even when conventional motion detection may no longer receive a strong movement event.

This capability is particularly valuable in applications such as meeting rooms, offices, workstations, hotel rooms, residential spaces and other environments where people may remain relatively still for extended periods.

Motion Detection and Presence Detection Are Different

Motion detection and presence detection are often discussed as though they are the same function, but they solve different sensing problems.

A motion sensor primarily asks:

“Has something moved?”

A presence sensor asks:

“Is someone still here?”

This distinction is important for intelligent building and automation systems.

For example, a person working quietly at a desk may not create enough large-scale movement to continually trigger a conventional motion detector. A radar-based human presence sensor can instead look for smaller and more persistent patterns associated with an occupant.

This allows the system to make a more informed occupancy decision.

Radar Presence Sensing Compared with PIR

Passive infrared, or PIR, sensing is widely used in lighting, security and building-control applications.

PIR sensors detect changes in infrared radiation across their sensing zones. They are inexpensive, mature and effective when people are moving clearly within the detection area.

However, PIR sensors can become less effective when an occupant remains relatively stationary.

Radar takes a different approach.

Rather than passively measuring infrared changes, radar actively transmits RF energy and analyses the reflected signal. This allows the sensing system to detect small movement patterns that can be difficult for conventional motion sensors to recognise.

Radar may therefore be particularly useful where an application requires:

• stationary human presence detection

• detection of subtle human micro-movements

• configurable sensing areas

• operation under different lighting conditions

• more detailed spatial sensing information

• reliable occupancy information for automated systems

PIR remains suitable for many simple movement-triggered applications. Radar becomes particularly valuable when the system needs to understand continued human presence rather than only obvious motion.

From RF Reflection to a Presence Decision

A useful radar presence sensor is not defined by RF transmission alone.

Performance depends on the complete sensing system, including:

• RF front-end design

• antenna characteristics

• receiver sensitivity

• analogue and digital signal processing

• environmental filtering

• sensing algorithms

• presence-decision logic

Walls, furniture, equipment and other fixed objects can all reflect radar signals.

A well-designed system therefore needs to distinguish between static background reflections and meaningful changes associated with people.

Instead of reacting to a single strong signal, radar presence sensing typically analyses measurements over time.

This temporal behaviour helps the system determine whether a detected change is consistent with a genuine occupant rather than transient interference or environmental noise.

Why Micro-Movement Detection Matters

One of the most important advantages of radar-based presence sensing is its ability to recognise very small human movements.

In many real-world environments, people spend significant periods sitting relatively still.

Examples include:

• employees working at desks

• people attending meetings

• occupants reading or watching television

• hotel guests resting in a room

• operators monitoring equipment

• residents working or relaxing at home

A conventional motion detector may eventually interpret these environments as unoccupied if no sufficiently large movement occurs.

A radar presence sensor can continue analysing subtle changes associated with the person.

This makes radar particularly relevant to energy-saving systems, where reliable occupancy information can help lighting, HVAC and other building systems respond more appropriately to actual room usage.

Understanding Distance and Direction

More advanced radar sensing architectures can provide information beyond a simple occupied/unoccupied result.

Depending on the radar architecture, antenna configuration and signal-processing system, additional spatial information can include:

• target distance

• movement direction

• angular position

• detection zone

• target behaviour over time

This spatial information allows developers to create more intelligent sensing applications.

For example, rather than knowing only that movement exists somewhere in a room, a system may be able to determine whether activity is occurring within a defined area.

This can be useful in smart-building, automation and embedded sensing applications where the intended detection zone must be controlled carefully.

Why Antenna Design Matters

The antenna is a fundamental part of any radar sensing system.

Its characteristics directly influence where RF energy is transmitted and from which areas reflected signals are received.

Important considerations include:

• antenna beamwidth

• gain

• mounting position

• PCB layout

• product enclosure

• nearby metallic structures

• sensing direction

• required coverage area

A broader sensing pattern may cover a larger area but can also receive reflections from unwanted regions.

A more focused sensing area can improve spatial control but requires appropriate product placement and system configuration.

At 24 GHz, compact antenna dimensions can support highly integrated radar sensing products, but PCB design, enclosure materials and surrounding structures remain important to overall sensing performance.

For OEM developers, reference hardware and evaluation systems can therefore provide a useful starting point before moving into product-specific antenna and enclosure optimisation.

Managing False Detection

Every presence-sensing system must balance two types of error.

A false negative occurs when a person is present but the system reports the area as unoccupied.

A false positive occurs when the system reports occupancy even though no person is present.

The acceptable balance depends on the application.

For example, in an office lighting system, incorrectly declaring an occupied desk to be empty can cause lights to switch off unnecessarily.

In an automated system, persistent false-positive detection may cause equipment to remain active when it is no longer required.

Reliable sensing therefore requires more than simply increasing sensitivity.

A radar system may use a combination of:

• sensing-range configuration

• environmental filtering

• background-clutter suppression

• temporal analysis

• movement-pattern analysis

• spatial constraints

• application-specific detection parameters

The objective is to identify genuine human activity while reducing responses to unrelated environmental changes.

Radar Sensing in Real Environments

Radar behaviour depends strongly on the environment in which the sensor is installed.

A sensor used in an office will encounter different conditions from one installed in a corridor, meeting room, warehouse, hotel room or industrial environment.

Possible sources of unwanted RF signatures can include:

• moving machinery

• fans

• vibrating structures

• activity outside the intended sensing area

• highly reflective surfaces

• moving doors

• environmental movement

For this reason, presence sensing should be treated as a system-design problem rather than simply a component-selection exercise.

The required sensing distance, coverage area, installation position, enclosure design and target behaviour should all be considered during product development.

Selecting a Radar Solution for an Embedded Product

The right sensing architecture begins with clearly defined application requirements.

Product developers should consider:

• required detection distance

• sensing angle and coverage area

• minimum movement that should be detected

• required response time

• acceptable false-trigger rate

• power requirements

• installation position

• enclosure constraints

• objects or activity that should be ignored

There are also different approaches to product integration.

A radar module can simplify early evaluation and reduce RF integration requirements.

An IC-based design can provide greater control over PCB layout, antenna integration, form factor and bill of materials for high-volume products.

Neither approach is universally better. The correct choice depends on the technical requirements, production volume and engineering resources available to the product developer.

Keywave KW307 24 GHz Human Presence Sensing

Keywave Technology develops radar-based semiconductor solutions for embedded and OEM sensing applications.

The KW307 is a 24 GHz human presence sensing SoC designed to detect human presence, including stationary occupants and subtle human micro-movements.

By combining RF sensing and embedded detection intelligence within the sensing platform, KW307 is designed to simplify the development of intelligent presence-detection products.

Depending on system configuration and integration, KW307 can provide spatial sensing information including object distance, angle and direction, enabling developers to build applications that require more than conventional motion detection.

Potential applications include:

• smart lighting

• occupancy-based HVAC control

• building energy management

• smart offices

• home automation

• intelligent appliances

• room occupancy sensing

• industrial and embedded automation

• edge-intelligent sensing systems

Keywave provides ICs, modules and evaluation hardware to support developers from initial evaluation through product integration.

From Evaluation to Product Integration

Successful radar integration requires more than demonstrating detection in a laboratory environment.

Product developers should validate sensing performance using realistic installation conditions.

This includes evaluating:

• antenna behaviour

• product enclosure materials

• installation height and orientation

• actual room geometry

• expected occupant behaviour

• environmental interference

• detection-zone requirements

A module or evaluation kit can help development teams collect early field data and understand sensing behaviour before committing to a final hardware design.

As the product moves towards production, the antenna, enclosure, PCB and sensing configuration can then be optimised around the intended application.

Keywave supports this development process through sensing ICs, modules, evaluation hardware and application-specific configuration support.

Designing Presence Sensing Around the Application

Reliable radar presence sensing starts with the real user environment.

A sensor positioned above a doorway has different requirements from one installed above a workstation.

A meeting room, hotel room, smart home, warehouse or industrial system will each contain different reflectors, movement patterns and sensing requirements.

The most effective approach is therefore to define the sensing problem before selecting the final configuration.

Product developers should establish:

• where people may be located

• how stationary they may become

• which areas should be monitored

• which areas should be ignored

• how quickly occupancy changes should be reported

• what information the host system requires

Once these requirements are clear, the RF architecture, antenna configuration and sensing parameters can be optimised around the application.

Explore Keywave Human Presence Sensing

Keywave Technology develops RF-based sensing ICs and embedded sensing solutions designed to help OEMs integrate intelligent human presence detection into next-generation products.

The KW307 24 GHz Human Presence Sensing SoC is designed for applications requiring reliable detection of both moving and stationary occupants, with spatial sensing capabilities suitable for smart-building, automation and energy-management applications.

Explore the KW307, evaluation hardware and sensing solutions from Keywave Technology, or contact our team to discuss your application requirements.