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Tarek Hassan
Knowledge Hubintegrated sensing and communication1. What is Integrated Sensing and Communication (ISAC)?

integrated sensing and communication learning note

1. What is Integrated Sensing and Communication (ISAC)?

A detailed introduction to ISAC, why it matters for 6G, and how a radio signal can both communicate and sense.

At a glance

  • One signal, two jobsThe same transmission delivers bits to a user and reflects off objects, revealing their range, speed and direction.
  • Share more, gain moreCoexistence shares spectrum, cooperation shares information, full ISAC shares the waveform and the hardware.
  • Judge both sidesReport communication quality and sensing error under the same power, spectrum and hardware budget.

The Core Idea

ISAC uses one radio platform for two jobs:

  • Communication: deliver information bits to a user.
  • Sensing: infer physical properties from received or reflected waves.
  • What can be shared: spectrum, antennas, hardware, waveform, or infrastructure. Sharing does not mean both receivers run the same algorithm.

The two jobs ask different questions of the same wave:

  • Communication asks: what information should reach the receiver?
  • Sensing asks: what does the reflected signal reveal about objects, their distance, speed, direction and surroundings?

ISAC answers both with one radio system. The lab below shows the same transmission doing both jobs at once.

  • One transmitted wave: carries modulation and also interacts with objects.
  • At the user: a communication receiver detects symbols.
  • At the sensing receiver: echo delay, angle, and Doppler can reveal objects or motion.

One transmission, two jobs

1. Formula and symbols
τ=2R/c,λ=c/fc,fD=2vr/λ\tau=2R/c,\quad\lambda=c/f_c,\quad f_D=2v_r/\lambda
  • RR: monostatic target range
  • τ\tau: round-trip delay
  • cc: wave speed
  • fcf_c: carrier
  • λ\lambda: wavelength
  • vrv_r: signed radial speed
  • fDf_D: Doppler under the displayed sign convention
data symbolsecho: τ, f_DISAC BSUserapproaching
Carrier
2. Work with the current inputs
R=c τ2  ⇒  τ=2Rc=2×603×108=400 ns,fD=2vrλ=2×13.910.7×10−3=2.59 kHzR = \frac{c\,\tau}{2} \;\Rightarrow\; \tau = \frac{2R}{c} = \frac{2\times 60}{3\times10^{8}} = 400\ \mathrm{ns}, \qquad f_D = \frac{2v_r}{\lambda} = \frac{2\times 13.9}{10.7\times10^{-3}} = 2.59\ \text{kHz}
3. Result
At the userData decodedcommunication job
Round-trip delay400 nsgives R = 60 m
Doppler shift2.59 kHzgives v = 13.9 m/s
The base station sends one downlink signal. The user decodes the data it carries (teal). The same signal also hits a car, and part of it bounces back (orange): the echo's delay gives the car's range, and its frequency shift gives the car's radial speed. Positive speed means approaching; a receding car gives a negative Doppler shift.
References for this section3

Why ISAC is Important

  • Why integrate: spectrum and hardware are limited resources.
  • What may improve: reuse of a radio platform and faster use of sensing results for communication control.

Expected benefits:

  • Better spectrum utilization.
  • Lower hardware duplication.
  • Lower latency between sensing and communication decisions.
  • Environment-aware beamforming.
  • Network-based localization and tracking.
  • New services such as sensing-as-a-service.
References for this section3

Radar-Communication Relationship

Radar and communication both transmit and receive waves. The goal is different:

  • Communication wants to recover symbols/bits.
  • Radar wants to recover target parameters.

Target parameters include:

  • Range.
  • Angle.
  • Velocity.
  • Radar cross section.
  • Micro-Doppler signatures.

ISAC designs the shared resources around both goals rather than treating sensing as a free side effect.

References for this section3

Read a Sensing Echo

For a simple monostatic setup (transmitter and echo receiver at the same site), delay gives an idealized range:

R=cτ2R=\frac{c\tau}{2}
  • RR: target range in metres.
  • cc: propagation speed, approximately 3×1083\times10^8 m/s in free space.
  • τ\tau: measured round-trip echo delay in seconds; division by 2 removes the outbound-and-return double journey.

For a target moving along the line of sight, a simple two-way Doppler magnitude is:

∣fD∣≈2∣vr∣λ|f_D|\approx\frac{2|v_r|}{\lambda}
  • ∣fD∣|f_D|: magnitude of echo Doppler shift in hertz.
  • vrv_r: target radial speed relative to the radio
  • λ\lambda: wavelength in metres.
  • Limit: bistatic geometries, moving platforms, and multipath need a different geometry model.

The next note, Radar Basics for Communication Engineers, builds these measurements up from scratch, and the one after it shows how an ordinary OFDM signal performs them.

References for this section3

ISAC vs Coexistence

There are three levels of integration:

LevelMeaning
CoexistenceRadar and communication share spectrum but remain mostly separate
CooperationSystems exchange information to reduce interference
Full ISACA joint waveform/hardware/platform is designed for both functions
  • Deployment reality: coexistence and cooperation can precede a fully joint waveform and platform.

Three levels of sharing

1. Formula and symbols
qtask=Nusable,task/Ntotalq_{\rm task}=N_{\rm usable,task}/N_{\rm total}
  • qtaskq_{\rm task}: usable resource fraction for a task
  • NtotalN_{\rm total}: all time-frequency cells
  • Nusable,taskN_{\rm usable,task}: cells that task can use, including jointly used cells

Shared cells can count toward both task fractions.

time →frequency →sensingcommsboth
Integration level
2. Work with the current inputs
qsensing=128/128=100.0%q_{\rm sensing}=128/128=100.0\%
qcomms=128/128=100.0%q_{\rm comms}=128/128=100.0\%

Full ISAC. One waveform does both jobs in every resource element: every data symbol is also a sensing probe. Nothing is duplicated, but the waveform must satisfy both.

3. Result
Resources usable by sensing100.0%
Resources usable by comms100.0%
HardwareShared
Each square is one resource element: a slice of frequency (rows) during a slice of time (columns). Orange elements serve sensing, teal serve communication, and striped elements serve both at once. Switch between the levels to see what each one shares.
References for this section3

A Simple Example

Picture one base station sending an OFDM-like downlink:

  1. The user decodes data from the direct or multipath signal.
  2. A car reflects part of the energy; an echo receiver estimates delay, angle, and Doppler.
  3. The estimated car movement helps aim or update a later beam.

This is the closed loop at the heart of ISAC: communicate → sense → update the beam → communicate better.

References for this section3

Takeaway

  • Core idea: one platform can both carry data and measure the environment.
  • Performance check: report communication quality and sensing error under the same power, spectrum, and hardware budget.
References for this section3

Check your understanding

0 of 4 answered

  1. What does the user receiver extract, and what does the sensing receiver extract?

  2. A monostatic echo returns after 1 µs. How far away is the target?

  3. Which level of integration designs one waveform and platform for both tasks?

  4. Why is the communicate → sense → update-beam loop valuable?

Complete references and further reading