DFT-s-OFDM with Chirping for Integrated Sensing and Communications in 6G and Beyond

Abstract: The sixth generation (6G) of mobile communications and beyond is expected to enable advanced functionalities, such as integrated sensing and communication (ISAC), while involving diverse terminal/user equipment types from terrestrial to non-terrestrial networks. As waveforms are acknowledged as a fundamental technology driving 6G and beyond, this article presents a contribution in this technical domain. First, it provides an overview of several standardized communication waveforms, as well as chirp-based waveforms for radar sensing and Internet of Things (IoT) applications. This article then presents single-carrier chirping waveform: discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) with chirping. Its fundamental principles, key properties, performances, and advantages are examined from both communication and sensing perspectives. Finally, several future research directions are outlined to further explore its potential and opportunities for ISAC.

• The paper proposes chirped DFT-s-OFDM and DFT-s-OFDM with chirp modulation for ISAC, combining single-carrier efficiency with radar-compatible frequency sweeps and achieving approximately 3.5 dB PAPR at a CCDF of 10⁻⁴, or 0 dB with PSK.
• The paper shows that chirped variants achieve full frequency diversity in three-path channels, suppress FMCW ghost targets under interference, and maintain stronger sensing performance than AFDM and OTFS under power-amplifier clipping.
• The paper identifies reduced spectral efficiency, high-complexity detection, multiuser interference, synchronization and channel-estimation challenges, and limited NTN validation as priorities for future research and standardization.

Motivation and context

This magazine article, authored by researchers from Nanyang Technological University, AUMOVIO Germany, and Carleton University, positions a single-carrier chirping waveform—DFT-s-OFDM with chirping—as a candidate physical-layer technology for integrated sensing and communication (ISAC) in 6G. The motivation rests on three requirements that the authors argue will dominate 6G deployments: ubiquitous connectivity across terrestrial networks (TN) and non-terrestrial networks (NTN), support for high-mobility terminals ranging from pedestrians to orbiting satellites, and power efficiency for energy-constrained user equipment. The last requirement motivates the central design objective: low peak-to-average-power ratio (PAPR), which allows power amplifiers to operate near saturation.

The article is positioned relative to 3GPP activity: since June 2025, RAN1 has agreed to consider DFT-s-OFDM and CP-OFDM as baselines or benchmarks for 6G candidate waveforms, implying that enhanced or modified variants of these waveforms are within scope of standardization. The paper extends the authors' prior work, which proposed chirped DFT-s-OFDM and analyzed its communication performance [10897935] and introduced DFT-s-OFDM with chirp modulation (DFT-s-OFDM-CM) [liu2025dft], by adding a systematic ISAC evaluation.

Waveform landscape

The paper surveys two families of implemented waveforms. On the communication side, generations 1G through 3G used single-carrier schemes; 4G adopted OFDM in the downlink; 5G uses CP-OFDM bidirectionally and retains DFT-s-OFDM as the low-PAPR uplink waveform, chosen over SC-FDE for superior spectral efficiency. On the sensing side, chirp signals—constant-amplitude linear frequency sweeps invented in the 1950s—underpin FMCW automotive radar, while LoRa exploits chirp spread spectrum (CSS) for long-range IoT links, demodulating signals 20 dB below the noise floor at the cost of very low data rates.

Recent ISAC-oriented waveforms embed chirping into multicarrier structures: OCDM applies frequency-domain chirping around OFDM [10136610], AFDM applies time-domain chirping [10087310], and OTFS/ODSS operate in delay-Doppler or delay-scale spaces [10769778], [9772941]. The authors' key observation is that these multicarrier candidates generally suffer from high PAPR, whereas DFT-s-OFDM with chirping is constructed by multiplying a DFT-s-OFDM signal with either an unmodulated chirp (chirped DFT-s-OFDM) or a chirp whose starting frequency carries information bits (DFT-s-OFDM-CM). With interleaved subcarrier mapping, the resulting analog waveform remains single-carrier while acquiring the frequency-sweep property needed for radar-style processing.

Communication performance

Spectral efficiency. Because DFT-s-OFDM transmits over $M$ of $N$ subcarriers ($M < N$), its spectral efficiency $\frac{M\log_2 Q}{N}$ is lower than full-band OFDM, AFDM, and OTFS—a trade-off the authors acknowledge explicitly rather than hide. Chirp modulation recovers part of this loss, raising efficiency to $\frac{M\log_2 Q + \log_2 P}{N}$ with chirp modulation order $P$, and subcarrier index modulation offers further gains. In multi-access settings, the unused subcarriers can serve other users.

PAPR. This is the strongest quantitative claim in the paper. With interleaved mapping, the PAPR of DFT-s-OFDM with chirping is independent of DFT and IFFT sizes and matches that of the underlying constellation symbols: approximately 3.5 dB at a CCDF of $10^{-4}$, versus more than 6 dB for OFDM, AFDM, and OTFS under identical conditions. With PSK modulation, PAPR drops to 0 dB regardless of modulation order. The authors note that even under multi-access configurations using interleaved subcarriers or delay grids, OFDM, AFDM, and OTFS retain higher PAPR than the proposed waveform.

Complexity and receivers. Transmitter-side complexity, normalized to OFDM, sits between OFDM/DFT-s-OFDM and OTFS: roughly 4% above OTFS when $M_{\rm OTFS}=16$, but about 21% below OTFS configured with $M_{\rm OTFS}=128$, $N_{\rm OTFS}=2$. For unmodulated chirps, reception reduces to standard equalization (ML, LMMSE, message passing) on an equivalent channel matrix incorporating the known chirp. For DFT-s-OFDM-CM, the data and chirp streams mutually interfere; the current receiver is an exhaustive ML search borrowed from index-modulation literature, whose complexity the authors flag as a practical limitation motivating low-complexity designs.

Diversity. Under ML detection in a three-path channel, both chirped variants achieve full frequency diversity order $N$0, matching AFDM and OTFS, whereas plain DFT-s-OFDM does not. DFT-s-OFDM-CM outperforms chirped DFT-s-OFDM because splitting bits between constellation and chirp domains permits a lower-order constellation at equal spectral efficiency, improving noise robustness. These results are supported by pairwise error probability upper bounds derived in the prior work.

Sensing performance

A notable structural advantage is dual compatibility with both classical radar processing chains. Because the waveform is single-carrier and frequency-sweeping, it supports analog mixing à la FMCW: mixing transmit and echo signals produces a beat frequency from which range is estimated after low-pass filtering and an FFT. Unlike FMCW, however, the signal simultaneously carries communication data, and the data-dependent structure yields an interference-suppression benefit demonstrated in a two-vehicle scenario: when both vehicles transmit identical FMCW waveforms, each misinterprets the other's transmission as a ghost target, producing spurious peaks in the range ambiguity function. With distinct chirped DFT-s-OFDM signals carrying independent data, cross-correlation between vehicles is low, suppressing the ghost target. Quantitatively, at SNR $N$1 dB and ISR $N$2 dB, chirped DFT-s-OFDM exhibits higher peak-to-max-side-lobe ratio (PMSR) than FMCW, and as ISR grows, FMCW's PMSR degrades faster—evidence of superior interference robustness.

Under matched filtering, applicable to all compared waveforms, chirped DFT-s-OFDM eliminates a false-peak artifact inherent to DFT precoding: with spreading ratio $N$3, plain DFT-s-OFDM produces duplicated peaks due to symbol repetition, whereas chirping removes this ambiguity. At SNR $N$4 dB without clipping, its PMSR and detection probability match AFDM and OTFS. The practically important result concerns PA clipping: because AFDM and OTFS have higher PAPR, they are more sensitive to clipping, and their PMSR and detection probability fall below those of chirped DFT-s-OFDM as the clipping ratio decreases. Range and velocity resolution ($N$5 and $N$6 respectively) are waveform-independent, computed here as 3 m and 1.63 m/s.

Limitations and open problems

The authors are candid about several constraints. Spectral efficiency is inherently reduced by the subcarrier subset unless supplemented by chirp/index modulation, whose joint design may introduce ambiguity issues that remain unresolved. Multiuser operation breaks down under chirping: each user's sweep intrudes into other users' bands, creating multiuser interference that only high-complexity ML equalization currently handles. Receiver-side challenges include coupling of timing/frequency offsets induced by frequency sweeps, the need for dechirping before synchronization or joint parameter estimation, channel estimation models (element-wise or basis-expansion) that do not yet exist for the chirped waveform, and potential phase-noise penalties if chirping is realized in the analog domain via PLL sweeping. Sensing evaluation is limited to ambiguity functions, PMSR, and detection probability; RMSE-based analysis against CRLB, and high-resolution estimators such as MUSIC and ESPRIT, are left unexplored. Chirp shape optimization beyond linear sweeps, and AI/ML-assisted receiver design, are identified as open directions, as is validation in NTN channels with severe Doppler.

Conclusion

The article makes a coherent case that multiplying DFT-s-OFDM by a chirp combines the low-PAPR single-carrier character required for power-limited uplinks with the Doppler resilience and radar compatibility of chirp waveforms. Its most concrete results are the ~3.5 dB PAPR at CCDF $N$7 (0 dB with PSK), full frequency diversity under ML detection, ghost-target suppression relative to FMCW under interference, and clipping robustness relative to AFDM and OTFS. Whether these simulation-level advantages survive contact with practical synchronization, channel estimation, and multiuser scheduling—and whether 3GPP's baseline-centric 6G process accommodates such a modification—remain the decisive open questions.