Integrated communications and sensing at mmWave and sub-THz frequencies could add location detection that helps avoid accidents among pedestrians, bicycles, and vehicles.
With the 5G commercial rollout well underway, users can now use its capabilities. Even so, the telecom industry is laying the groundwork for the next generation of cellular communications. 6G will use many technologies to bridge the digital and physical worlds through ubiquitous connectivity. Integrated Communications and Sensing (ICAS) — also called Joint Communications and Sensing (JCAS) — looks to become a core part of 6G. Sensing within the network aims to leverage the large amount of network infrastructure already in place so sensing can come as a “free” utility that can improve device-to-device and device-to-human connectivity.
Figure 1. In this conceptual diagram, communications and sensing signals can travel between a base station, cars, and pedestrians.
Combining communication with sensing can provide a significant network benefit. Consider the diagram in Figure 1 and the autonomous driving use case. A radar signal can find the precise location of objects around the car, which can share this information with other autonomous vehicles as well as the networks. The combination of sensing and communications provides accurate positioning and lets a 6G network deliver spatial monitoring.
The idea of ICAS is simple: detect data about the physical world by adding sensing signals to traditional communications signals. In theory, communications signals can occupy part of the total allocated bandwidth. A sensing signal, such as a radar chirp or a sounding signal, can occupy the rest of the allocated bandwidth. Combining these two signals into one waveform lets different applications share antennas and transceiver hardware while occupying the same spectrum slice. Spectrum and physical space inside of user equipment are both finite resources and can lead to creative techniques that maximize both. Research continues into waveforms that work best for the sensing signal as well as for the ideal ratio of sensing versus communications bandwidth.
Design considerations and challenges
For signals below 6 GHz, reducing the bandwidth of the communications signal is unrealistic. Maximizing data throughput remains a priority at these frequencies because they reach a broad area with outdoor and indoor coverage. The total available bandwidth in the gigahertz range at higher bands will work, giving some of that bandwidth to a sensing signal. Regardless of frequency, a tradeoff will always exist between a sensing signal’s resolution and the communications signal’s throughput. For example, if a radar chirp is the sensing signal, 1 GHz of bandwidth is required to get 15 cm of range accuracy. Some applications may require this level of accuracy or tighter. Thus, a “one size fits all” solution to the sensing versus communications bandwidth challenge is unlikely. A flexible frame structure and reconfigurable numerology could address it.
Figure 2. Using TDD, a transmitter sends sensing and communications signals at different times where each gets the entire bandwidth.
Consider the following example of two different waveform configurations. In Figure 2, the full bandwidth of the signal is used for sensing by using alternating frames. This time-domain duplex (TDD) approach gives high sensing resolution. Unfortunately, it has a limited range.
In the second configuration (Figure 3), a narrow bandwidth sensing signal is placed in unused subcarriers of an OFDM signal. This frequency division duplex (FDD) approach allows for a longer sensing range but with lower resolution. Neither approach is wrong, but this example shows the sorts of tradeoffs that engineers will need to consider when designing transmitters and receivers for ISAC waveforms.
Figure 3. Using FDD, sensing and communications signals appear simultaneously but each occupies less bandwidth than with TDD.
Higher frequencies have sensing benefits in addition to available bandwidth. For high-resolution radar, large antennas in the K-band (18 GHz to 26.5 GHz) and Ka-band (26.5 GHz to 40 GHz) have been in place for many years. This overlap in frequencies and the growing number of larger antenna arrays in communications systems open the potential of reusing existing infrastructure and equipment for ICAS applications. Looking higher in the spectrum, automotive radar operating at 77 GHz has expanded in recent years, providing another opportunity to leverage and reuse existing infrastructure by adding sensing to wireless networks.
Testing and characterization
Engineers need testing to understand sensing performance at different frequencies better. For example, we need to compare and contrast sensing at 24 GHz and 77 GHz with different bandwidths. This sort of information will help to commercialize ICAS technology. Even more fundamentally, we need a better understanding of the transmission channel from 7 GHz to 24 GHz to know how sensing signals combined with communications signals in the so-called FR3 band will perform. The existing channel models standardized by 3GPP lack detail for this use case. The NextG Alliance is currently gathering sounding data and researching different frequency bands to better understand the channel in the context of sensing applications.
We need models that characterize the sensed object’s reflectivity because traditional communications channel models don’t account for reflection. We also need models that account for different signal altitudes, representing the complex physical world with sensing targets ranging from people on the ground to flying unmanned aerial vehicles (UAVs). Finally, the industry needs a discussion around using dynamic versus stochastic models to account for mobility and changing positions of targets in an ICAS use case.
Conclusion
If the goal is to bring sensing into the communication infrastructure, the communications industry must explore spectrum allocation and the amount of network resources that need to be dedicated to sensing. We need to minimize resources from the network and maximize sensing. Furthermore, we must consider novel waveform designs because waveform design can lend to the ease of sensing. Finally, we need testing at several frequencies to find the optimal frequency for this operation. Traditionally, sensing measurements occur at high mmWave or at terahertz frequencies. We need to establish feasibility within these bands for sensing. While we have many technical questions, adding sensing to 6G networks is a promising feature that will make future networks more intelligent and enable a host of new applications.
For more information
Video: Moving toward integrated sensing and communications in 6G, Nokia, https://www.youtube.com/watch?v=BCKQHQY7hMI.
Video: VTC2023-Spring Keynote: Integrated Sensing and Communications: It was Meant to Be, University College London, https://www.youtube.com/watch?v=Y8oCRAqtUCk
Integrated Sensing and Communication: Enabling Techniques, Applications, Tools and Data Sets, Standardization, and Future Directions, National Institute of Health, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10236932/
Joint communication and sensing in 6G networks, Ericcson, https://www .ericsson.com/en/blog/2021/10/joint-sensing-and-communication-6g
Sarah LaSelva leads the marketing efforts for Keysight in 6G. She has over a decade of experience in test and measurement concentrating on wireless communications, both studying and promoting the latest wireless technologies. Throughout her career she has spent time in marketing, test engineering, and applications engineering.
Sarah’s background is in microwave and millimeter wave technology. She has a BS in electrical engineering from Texas Tech University.
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