Motivation and Use Cases

From Separate Systems to a Unified Waveform

Radar and wireless communication have developed as independent disciplines for over a century. Radar systems transmit carefully designed waveforms and process the echoes to extract range, velocity, and angle information about targets. Communication systems transmit data-bearing waveforms and process the received signal to recover the information bits. Historically, these two functions occupied different frequency bands, used different hardware, and were designed by different engineering communities.

Several converging trends are now driving the unification of radar sensing and wireless communication into a single system:

Spectrum scarcity. Both radar and communication systems are migrating to similar frequency bands — sub-6 GHz bands (e.g., 3.5 GHz for 5G NR and weather radar), mmWave bands (e.g., 77 GHz for automotive radar and 5G FR2), and sub-THz bands for 6G. Sharing the same spectrum and waveform eliminates inter-system interference and improves spectral efficiency.
Hardware convergence. Modern phased arrays, high-speed ADCs, and digital signal processors can support both radar and communication signal processing on the same platform. The marginal cost of adding sensing to a communication system (or vice versa) is small compared to deploying two separate systems.
New applications. Autonomous driving requires simultaneous high-data-rate V2X communication and high-resolution radar sensing. Indoor environments benefit from joint WiFi communication and gesture recognition, device-free localisation, or vital-sign monitoring. UAV systems need communication links and radar-based detect-and-avoid capabilities.

The key insight of integrated sensing and communication (ISAC) is that a single waveform, a single hardware platform, and a single frequency band can serve both functions simultaneously, provided the waveform and signal processing are carefully co-designed.

Definition:
Integrated Sensing and Communication (ISAC) System

An integrated sensing and communication (ISAC) system is a wireless system that performs both radar sensing (target detection, ranging, velocity estimation, and/or imaging) and data communication using:

a shared waveform (fully unified) or co-existing waveforms (spectrally overlapped but separately designed);
a shared hardware platform including antennas, RF chains, and baseband processors;
a shared frequency band in which radar echoes and communication signals coexist.

The most tightly integrated variant is the dual-function radar-communication (DFRC) system, where a single transmitted waveform simultaneously carries communication data and provides radar illumination. The DFRC transmitter serves communication users in the forward (downlink) direction while processing the backscattered echoes from targets of interest.

Formally, an ISAC system transmits a signal $s(t)$ that is designed to satisfy two objectives:

Communication: The intended receiver decodes a message $W$ from the received signal $y_c(t) = h_c(t) * s(t) + n_c(t)$ at a rate $R \geq R_{\min}$ .
Sensing: The radar receiver estimates target parameters $\boldsymbol{\eta} = [\tau, f_d, \theta]^T$ (delay, Doppler, angle) from the echo $y_s(t) = \alpha \, s(t - \tau) e^{j2\pi f_d t} + n_s(t)$ with estimation accuracy bounded by the Cram'{e}r-Rao bound (CRB).

The fundamental design challenge is managing the trade-off between communication performance (rate, BER) and sensing performance (detection probability, estimation accuracy).

Alternative acronyms in the literature include JRC (joint radar-communication), RadCom (radar-communication), and JCAS (joint communication and sensing). The 3GPP Release 19 study item uses the term "integrated sensing and communication" (ISAC), which we adopt throughout this chapter.

Key ISAC Use Cases

Automotive radar and V2X communication. Modern vehicles employ 77 GHz FMCW radar for adaptive cruise control, collision avoidance, and lane-change assistance. Simultaneously, vehicle-to-everything (V2X) communication enables cooperative perception, platooning, and intersection management. An ISAC system at 77 GHz can use a single waveform for both functions, reducing hardware cost, size, weight, and power (SWaP) while eliminating mutual interference between the vehicle's own radar and communication subsystems.

Indoor localisation and activity recognition. WiFi access points operating at 2.4/5/6 GHz or 60 GHz (IEEE 802.11ad/ay) can extract channel state information (CSI) from communication signals to perform device-free localisation, gesture recognition, fall detection, and respiration monitoring. The 802.11bf standard ("WLAN Sensing") formalises the use of WiFi signals for sensing, turning every access point into a potential radar sensor.

Drone detection and counter-UAS. Small unmanned aerial systems (UAS) pose security threats in restricted airspace. A cellular base station operating as an ISAC node can detect, track, and classify drones using the uplink/downlink 5G NR waveforms while continuing to serve mobile users. The large bandwidth (up to 400 MHz at FR2) and massive MIMO antenna arrays of 5G provide sufficient range and angular resolution for drone detection at distances of several hundred metres.

Sub-THz sensing and communication for 6G. Frequencies above 100 GHz offer several GHz of contiguous bandwidth, enabling sub-centimetre range resolution. At these frequencies, the distinction between radar and communication signals blurs: the same wideband waveform can achieve both multi-Gbps data rates and high-resolution 3D imaging. ISAC at sub-THz is expected to be a cornerstone of 6G systems.

Historical Note: From Separation to Convergence

pre-2000s -- present

The evolution towards ISAC can be traced through several stages:

Separate systems (pre-2000s). Radar and communication systems were designed, deployed, and regulated independently. Military radar operated in dedicated bands (L, S, C, X), while communication systems used cellular and satellite bands.
Spectral coexistence (2000s--2010s). As both radar and communication systems expanded into shared bands (e.g., 3.5 GHz, 5 GHz), research focused on interference mitigation: how can radar and communication systems share spectrum without degrading each other's performance?
Cooperative design (2010s). Researchers began exploring joint waveform design: instead of treating interference as an enemy, the waveform is designed to serve both functions. The DFRC concept emerged, where the radar waveform is modulated with communication data.
Fully integrated systems (2020s--present). ISAC is now a major research and standardisation theme for 5G-Advanced and 6G. 3GPP Release 19 includes a study item on ISAC, and ITU-R has identified sensing as a key capability of IMT-2030 (6G).

ISAC

Integrated Sensing and Communication: a system that performs both radar sensing and data communication using a shared waveform, hardware platform, and frequency band. Also known as JRC (Joint Radar-Communication) or DFRC (Dual-Function Radar-Communication).

Related: DFRC, Ambiguity Function

DFRC

Dual-Function Radar-Communication: the most tightly integrated ISAC variant, where a single transmitted waveform simultaneously carries communication data and provides radar illumination.

Related: ISAC

Quick Check

What is the primary advantage of a dual-function radar-communication (DFRC) system over operating separate radar and communication systems in adjacent frequency bands?

The DFRC system achieves higher radar detection probability because communication signals have higher transmit power

The DFRC system uses a single waveform, hardware platform, and frequency band for both functions, improving spectral efficiency and reducing hardware cost while eliminating inter-system interference

The DFRC system can detect targets at longer range because it uses coherent integration over communication frames

The DFRC system eliminates the need for any radar signal processing at the receiver

Correction: