Introduction
Hello. This is Juan Liu from the DOCOMO Beijing Communications Laboratories. We presented our research paper on the 6G ISAC waveform design at the IEEE APCC 2024 (The 29th Asia-Pacific Conference on Communications) held in November 2024 *1. Our paper received the "Best Paper Award" at this conference. We will introduce this research work in this article.
About IEEE APCC 2024
IEEE APCC 2024 is jointly organized by IEEE-CS（The Institute of Electrical and Electronics Engineers-Communications Society）, CIC（China Institute of Communications）, IEICE-CS （The Institute of Electronics, Information and Communication Engineers-Communications Society）, and KICS (Korea Information and Communications Society）. APCC is a flagship conference in the Asia Pacific region to present extensive technological research results in the fields of wireless and wired communication. This year's APCC 2024 revolved around the theme “Sustainable Connectivity: Advancing Green Technologies for a Smarter Future”, which was held at “Truntum Kuta” in Bali, Indonesia, for a total of three days from November 5 (Tue) to 7 (Thur). This conference received 203 submissions from 715 authors centered around the Asian region, of which 105 were accepted (acceptance rate: 58.6%). The "Best Paper Award" is a special selection of 5 outstanding papers from these papers. The content of the presentations in this year's conference includes advanced wireless communication technologies, advanced wireless sensing technologies, Artificial Intelligence (AI) and machine learning (ML) for emerging technologies, quantum communication, optical wireless communication, etc. APCC received many research results on topics of high interest, including 6G ISAC, which is also a field of presentation from DOCOMO Beijing Communications Laboratories. The IEEE APCC 2024 was a very successful international conference.
Summary of the presentation
I gave a 20-minute presentation at the technical session "Parallel Session 1" held in APCC 2024, including Q&A time *2.
The following is an overview of this presentation.
Background
DOCOMO has released a "6G White Paper" that summarizes the various use cases, target performance, and elemental technologies expected for 6G *3, and aims to achieve Cyber-Physical Systems (CPS) for 6G. Wireless sensing based on communication radio waves is an important means to enable the vision of 6G CPS. With the development of higher frequency and wider bandwidth, the integrated sensing and communication (ISAC) can be realized by 5G-A and 6G. Waveform design is one of the fundamental topics for ISAC, which should be studied for 6G.
Different from conventional communication systems with communication capability only, ISAC system will need to provide both sensing and communication capabilities. Therefore, the ISAC waveform design must balance the requirements of sensing and communication. In this presentation, based on a comprehensive analysis of the technical means and advantages and disadvantages of three types of ISAC waveform schemes, namely radar-centric waveform, communication-centric waveform, and joint waveform optimization, a flexible waveform switching scheme to achieve OFDM for communication and Chirp for sensing based on a unified waveform framework is creatively proposed. This scheme can improve the power efficiency of sensing waveforms, thereby enhancing sensing coverage performance.
Problem to be solved
The sensing-centric waveform, e.g., chirp-based waveform *4 has good sensing performance due to low peak-to-average power ratio (PAPR) and good ambiguity function properties. And the complexity for self-interference cancellation (SIC) under a full-duplex system is very low. However, the low communication efficiency and poor backward compatibility with the communication waveform in 5G make it an unsuitable candidate waveform for 6G ISAC. The communication-centric OFDM-based waveform has advantages of high data rate and spectral efficiency. However, the sensing coverage of OFDM-based waveforms is limited by the high PAPR. Furthermore, the high complexity of SIC for OFDM-based waveforms under a full-duplex monostatic sensing system also limits the application scenarios for ISAC. How to design the 6G ISAC waveform to achieve a better tradeoff between sensing and communication performance, with better 5G backward compatibility and lower implementation complexity, should be studied.
Proposed method
In APCC 2024, we proposed a flexible waveform switching scheme to achieve OFDM for communication and Chirp for sensing based on a unified waveform framework. This scheme can achieve a better tradeoff between sensing and communication performance, with better 5G backward compatibility and lower implementation complexity. The transceiver structure of the proposed ISAC waveform is shown in Fig. 6.
There are two features of the proposed ISAC waveform
- Feature 1: Based on the communication OFDM transmitter structure, a frequency domain processing (FDP) model is designed before the Inverse Fast Fourier Transform (IFFT) to achieve the low PAPR chirp waveform. The FDP model includes two steps, shown in Fig. 6.
- Step 1: Implement the discrete Fourier transform (DFT) on the transmitted sensing symbol to transform the time domain symbol to the frequency domain.
- Step 2: Apply a designed frequency domain spreading (FDS) to spread the frequency domain signal to the total allocated bandwidth to achieve the chirp waveform.
- Feature 2: By designing the sensing symbol pattern, i.e., the number and location of the non-zero sensing symbol, to generate different patterns of orthogonal chirps flexibly, as shown in Fig. 7.
Computer simulation results
In the simulation, we assume a base station (BS) monostatic sensing mode, in which BS with two separate antenna sets is assumed to realize full-duplex and reduce self-interference between the received echo signal and the transmitted signal, as shown in Fig. 8. The frequency of 28 GHz and bandwidth of 400MHz are assumed for evaluation. Two-dimensional Fast Fourier Transform (2D-FFT) estimation algorithms are employed for sensing.
The complementary cumulative distribution function (CCDF) of PAPR performance related to different sensing symbol patterns is shown in Fig.9 (a). The root-mean-square-error (RMSE) of range estimation vs. signal-to-noise ratio (SNR) is shown in Fig.9 (b). The results demonstrate that, compared to the legacy OFDM waveform, the proposed single chirp pattern scheme achieves both lower PAPR and the same range-estimation accuracy while requiring a lower SNR.
The proposed ISAC waveform could be used for Unmanned Aerial Vehicle (UAV) detection to enhance the echo signal SNR to increase the coverage. As shown in Fig. 10, based on the proposed ISAC waveform, the coverage radius of UAV detection is expanded from 1.3 km to 1.9 km under mmW.
Furthermore, the proposed ISAC waveform with low PAPR could also be used for High Altitude Platform Station (HAPS）sensing. As shown in Fig. 11, based on the proposed ISAC waveform, the coverage range of HAPS sensing can be expanded from 8 km to 24 km at sub-6GHz.
After the presentation
In the Q&A session of the presentation, we discussed the more applicable use cases of the proposed scheme and the main challenges for 6G ISAC waveform design. I think the feasibility and effectiveness of the proposed ISAC waveform were widely recognized by audience at the venue. In addition, many of the presentations by other research organizations were diversified technologies for 6G, such as MIMO and NTN, etc., and we had a very in-depth discussion on the cutting-edge research topics. There are still remaining issues for the 6G ISAC research, and we would like to continue to provide innovative solutions to these issues while expanding our horizons.
*1:J. Liu, W. Liu, X. Hou, and L. Chen, "An OFDM Compatible Sensing Waveform Design for 6G ISAC System," 2024 IEEE 29th Asia Pacific Conference on Communications (APCC), BALI, Indonesia, 2024, pp. 71-76.
*2:APCC 2024, Technical Program: https://apcc2024.org/parallel-session-1/
*3:NTT DoCoMo, DoCoMo 6G White Paper Version 5.0, November 2022. https://www.docomo.ne.jp/binary/pdf/corporate/technology/whitepaper_6g/DOCOMO_6G_White_PaperJP_20200122.pdf
*4:M. Kronauge and H. Rohling, “New Chirp Sequence Radar Waveform,” IEEE Transactions on Aerospace and Electronic Systems, vol. 50, no. 4, pp. 2870–2877, Oct. 2014.
