Amalgamated CHIRP and OFDM for ISAC

TL;DR

This work introduces AAC-OFDM and CM-OFDM as ISAC waveforms that fuse OFDM with chirp signals to support simultaneous communication and sensing. AAC-OFDM adds a chirp in an affine fashion to OFDM, enabling pilot-free range-Doppler estimation and reduced PAPR, while CM-OFDM multiplies OFDM by a chirp to boost range resolution. The paper derives the ambiguity functions for both waveforms, analyzes PAPR, and designs sensing receivers with comparable complexity to conventional OFDM; it also demonstrates NR-compatible implementations at slot and symbol levels and shows sensing performance gains in RMSE and velocity resolution. Simulation results indicate AAC-OFDM can achieve higher spectral efficiency without pilot-based sensing and offers robust sensing when properly tuning the chirp weight $\alpha$, with CM-OFDM providing strong range resolution improvements. Overall, the proposed framework yields a practical ISAC waveform design that balances sensing accuracy, communication throughput, and implementation complexity for 5G NR and beyond.

Abstract

Integrated Sensing and Communication (ISAC) requires the development of a waveform capable of efficiently supporting both communication and sensing functionalities. This paper proposes a novel waveform that combines the benefits of both the orthogonal frequency division multiplexing (OFDM) and the chirp waveforms to improve both the communication and sensing performance within an ISAC framework. Hence, a new architecture is proposed that utilizes the conventional communication framework while leveraging the parameters sensed at the receiver (Rx) for enhancing the communication performance. We demonstrate that the affine addition of OFDM and chirp signals results in a near constant-envelope OFDM waveform, which effectively reduces the peak-to-average power ratio (PAPR), a key limitation of traditional OFDM systems. Using the OFDM framework for sensing in the conventional fashion requires the allocation of some resources for sensing, which in turn reduces communication performance. As a remedy, the proposed affine amalgam facilitates sensing through the chirp waveform without consuming communication resources, thereby preserving communication efficiency. Furthermore, a novel technique of integrating the chirp signal into the OFDM framework at the slot-level is proposed to enhance the accuracy of range estimation. The results show that the OFDM signal incorporated with chirp has better autocorrelation properties, improved root mean square error (RMSE) of range and velocity, and lower PAPR. Finally, we characterize the trade-off between communications and sensing performance.

Figures (17)

• Figure 1: System model for ISAC based on AAC-OFDM and CM-OFDM.
• Figure 2: Block diagram of the CM-OFDM and AAC-OFDM system architectures.
• Figure 3: 5G NR Frame structure. (a) Frame structure along with the demonstration of each sample in the symbol. (b) Slot-wise and symbol-wise chirp incorporation. (c) Hybrid incorporation of chirp in time domain for one subframe.
• Figure 4: Incorporation of the CM-OFDM waveform in the OFDM time-frequency resource block to improve the PRS performance.
• Figure 5: Flexibility of the AAC-OFDM implementation in OFDM time-frequency resources to improve the sensing task performance.