Why New Waveforms Matter for JCAS
The Challenge of Communication and Sensing in One System
6G networks will not only connect people but also sense the world around them. This is the vision of Joint Communication and Sensing (JCAS): a single system that can stream data and, at the same time, detect objects, measure distances, and estimate velocities.
But there is a catch — the waveforms we use today, like OFDM in Wi-Fi and 5G, were designed only for communication. When the channel becomes highly dynamic, such as with fast-moving vehicles, OFDM struggles. Interference and signal distortion make it hard to sense accurately and to communicate reliably.
Figure 1. Communications and sensing scenarios involved in an ISAC system.
Enter the Delay-Doppler Domain
Imagine describing a moving object not just by when the signal arrives (delay), but also by how fast it is moving (Doppler). This delay-Doppler view matches the way wireless channels behave in reality: each path has its own delay and Doppler.
By using this perspective, we can design waveforms that turn a complicated, fast-changing channel into something almost static and predictable. That is exactly what Orthogonal Time Frequency Space (OTFS) does. It places information symbols directly onto a delay-Doppler grid, making both communication and sensing tasks much easier. [1]
Figure 2. Doubly-dispersed channel representation in DD-Domain
Chirps to the Rescue
Another new idea is Affine Frequency Division Multiplexing (AFDM). Instead of sending pure tones like OFDM, AFDM uses chirps — signals whose frequency changes over time. [2] Different paths in the channel shift these chirps in a way that makes them easier to separate. The result: stronger resilience to motion, clearer sensing resolution, and more reliable communication.
Why This Matters
Both OTFS and AFDM represent a step forward from OFDM. They bring:
- Better Doppler resilience, meaning more reliable links at high speeds.
- Improved multipath separation, which boosts sensing accuracy.
- Support for full diversity, unlocking performance in rich scattering environments.
To better illustrate these differences, Figure 4. below compares how OFDM, OTFS, and AFDM represent the wireless channel of 3 resolvable paths in Frequency domain. Figure 4. (a) shows the case with integer delay and Doppler, while (b) illustrates the case with continuous parameters. [3]
- OFDM struggles in dynamic environments: Doppler shifts create interference across subcarriers, making both communication and sensing unreliable.
- OTFS transforms the time-varying channel into a nearly static representation in the delay-Doppler grid, which is excellent for tracking moving objects and maintaining communication quality.
- AFDM takes a different route, using chirps that shift in delay and frequency, allowing paths to be separated more effectively in high-mobility conditions.
Figure 4. Effective channel matrix structures in a doubly dispersive channel
This comparison highlights why OFDM alone is not enough for JCAS, and why new waveforms like OTFS and AFDM are promising alternatives.
But There Are Still Challenges
While OTFS and AFDM both outperform OFDM, each still comes with its own challenges:
- OTFS: The two-dimensional delay-Doppler grid offers great accuracy, but it comes at a cost. To keep the channel response separated from data symbols, OTFS requires large guard intervals. As the grid size grows (more subcarriers and longer transmission time), these guard intervals expand, reducing spectral efficiency and increasing complexity.
- AFDM: It is more flexible, since each symbol is independent and needs smaller guard regions. However, delay and Doppler in AFDM act jointly on chirps, rather than being naturally orthogonal as in OTFS. This makes separating them less straightforward, and requires careful pilot design and parameter selection.
These trade-offs motivate our ongoing research: how to design pilots, allocate carriers, and optimize resources so that future JCAS systems can fully exploit the strengths of both OTFS and AFDM.
For applications like autonomous driving, drone navigation, or smart factories, these features are game-changers. By rethinking waveform design, researchers are paving the way for JCAS systems that can truly handle the dual tasks of connecting and sensing in our 6G future.
References:
[1] Mohammed, Saif Khan, et al. “OTFS—A mathematical foundation for communication and radar sensing in the delay-Doppler domain.” IEEE BITS the Information Theory Magazine 2.2 (2022): 36-55.
[2] Bemani, Ali, Nassar Ksairi, and Marios Kountouris. “Affine frequency division multiplexing for next generation wireless communications.” IEEE Transactions on Wireless Communications 22.11 (2023): 8214-8229.
[3] Rou, Hyeon Seok, et al. “From Orthogonal Time–Frequency Space to Affine Frequency-Division Multiplexing: A comparative study of next-generation waveforms for integrated sensing and communications in doubly dispersive channels [Special Issue on Signal Processing for the Integrated Sensing and Communications Revolution].” IEEE Signal Processing Magazine 41.5 (2024): 71-86.
An article by Junjie Wu.