The electromagnetic spectrum is a fundamental resource for the development of our society. Radio signals are part of our daily lives - you use the electromagnetic spectrum when you send a photo with your smartphone (through wireless communications), when you navigate somewhere with your car (through GPS sensors that receive signals from satellites), when you check the weather forecast (which is based on sensing of electromagnetic emissions of molecules in the Earth atmosphere). In general, wireless communications provide a connected fabric that enables sharing of information among people, machines, and devices, acting as the nervous system of smart and digital organisms. Sensing is used to explore and measure a wide range of phenomena, in the Earth, on its atmosphere, and in space. For example, sensing techniques can gather the data that is fed as input to complex weather and climate models, can explore remote galaxies and stars, and can image the human body without invasive procedures.
Both wireless sensing and communications techniques manipulate and process radio signals in the electromagnetic spectrum, which is an extremely valuable asset, considering this wide range of applications and their relevance to the society and economy. However, it is also finite: communications, sensing, and their different use cases compete for a slice of the same pie. In addition, each application would benefit from using a large slice. For example, wireless communications can deliver higher data rates on larger chunks of spectrum (or bandwidth), and sensing techniques can be more precise with larger bandwidths. This introduces competing interests among the different stakeholders of the spectrum, which have led - so far - to rigid policies and inflexible allocations of different slices of pies to different spectrum applications. Spectrum allocations are regulated by international regulations, defined in plenaries of the International Telecommunications Union (ITU), and by national spectrum agencies, as, for example, the FCC and the NTIA in the US.
Recently, the explosion of connected communications devices in the bands below 6 GHz (typically used to connect your smartphone to Wi-Fi, cellular, and Bluetooth networks) has prompted communication technologies to move to higher frequencies, where future 6G wireless networks can exploit theoretically very large bandwidths. Indeed, while most communications today use bandwidths of few tens or hundreds of MHz in the portion of spectrum below 6 GHz, 5G is looking at using multi-gigahertz bandwidth at above 24 GHz, and 6G systems will likely expand into the spectrum above 100 GHz, as it could potentially provide enough bandwidth for links that can transfer one Terabit of data in one second.
However, the spectrum above 100 GHz features several narrow, yet numerous sub-bands that are exclusively allocated for key passive sensing applications, specifically, for climate and weather monitoring and radio astronomy. This prevents the allocation of large contiguous bands to active users of the spectrum, either being communications or radars.
In a paper that has been published in a journal of the Nature group, Communications Engineering, we explore a strategy for the coexistence of active and passive users of the spectrum through time sharing. Specifically, we consider the NASA Aura satellite, which features a sensing instrument - the Microwave Limb Sounder (or MLS) - that performs sensing of carbon monoxide and ozone by measuring radio frequency emissions in the spectrum around 230 GHz. We build a communication system that can operate in two different frequency bands (123.5–140 GHz and 210–225 GHz) so that, whenever the satellite orbits over the deployment area of the wireless link, we can use a band that does not interfere with the sensing system. By doing this, we enable wide-band transmissions in the spectrum above 100 GHz, while avoiding any harmful interference to satellite systems.
Thanks to safe methods for coexistence and spectrum sharing, both sensing and wireless communication technologies can share the same slice of pie. This results in a win-win for everyone, at the cost of an increased - but still manageable - complexity for the coordination of different systems. This study also comes at an important moment for the development of 6G networks, which are in a so-called pre-standardization phase. This means that the wireless community is working to select proper technologies and frequency bands that will be included in future standardization efforts and will result in commercial products in the next 5 to 10 years. Considering the long time it takes to update technology roadmaps and spectrum regulations, the next 12 to 24 months will be fundamental to shape the next decade of spectrum policy and coexistence.
This study reports the first spectrum sharing system in the spectrum above 100 GHz based with experimental results. Building the prototype has required us to address challenges that span from the design of the radio transmitter and receivers for the dual-band setup, to the dynamic control and integration of multiple systems that were not designed to work together. It is based on years of research into design and prototyping of radio technologies for communications above 100 GHz and on spectrum sharing. This is represented by the different backgrounds of the co-authors, who include researchers at the Institute for the Wireless Internet of Things at Northeastern University, in Boston MA, and at the Jet Propulsion Laboratory (JPL), in Pasadena, CA. This work has been developed as a result of a collaboration based on the NSF Spectrum Innovation Initiative (SII) planning grant AST-2037896, and on AFRL through Grant FA8750-20-1-0200, and ONR through grant N00014-20-1- 2132. The experiments have been conducted under the first dual-band FCC license for the spectrum above 100 GHz, with the equipment deployed over rooftops of Northeastern University buildings, to represent a realistic scenario for future 6G networks.
The fundamental results reported in the paper demonstrate how future regulations for this spectrum band may rely on sharing technologies to guarantee seamless coexistence of passive and active users, and foster future research on design of passive/active awareness and coexistence schemes in the spectrum above 100 GHz.
Michele Polese, Viduneth Ariyarathna, Priyangshu Sen, Jose V. Siles, Francesco Restuccia, Tommaso Melodia, Josep M. Jornet, "Dynamic Spectrum Sharing Between Active and Passive Users Above 100 GHz", Communications Engineering, May 2022. DOI: 10.1038/s44172-022-00002-x
The paper can be found at https://www.nature.com/articles/s44172-022-00002-x
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