Integration of communication and navigation technologies toward LEO-enabled 6G networks: A survey

First, authors provide an overview of ICAN, including framework, typical systems, and functions. The ICAN system supports both communication and navigation services with advanced network facilities and radio resource utilization efficiency, which is different from the separation between communication and navigation in conventional positioning systems. The link architecture between the satellite and the ICAN user equipment (UE) is illustrated in Fig. 2. On the one hand, by leveraging this system, an ICAN UE can receive integrated signals from multiple satellites in different orbits. The integrated signals provide advanced services like high-rate communication and high-precision positioning. On the other hand, to mitigate intersatellite interference, the signal broadcasting mode uses the multicolor reuse pattern, where each color corresponds to a particular combination of polarization and frequency. The ICAN architecture comprises 4 primary components (Fig. 3): spatial-temporal reference fusion as the foundation, hardware platform fusion as the basis, information protocol fusion as the core, and propagation waveform fusion as the key to achieving deep fusion. Typical systems are that (1) indoor positioning systems which demand submeter or even centimeter level accuracy and typically rely on ultra-wide-band signals, such as UWB, or positioning tags, such as radio frequency identification, (2) navigation augmentation systems which leverage communication signals to enhance navigation capabilities and enable wide coverage and high-precision PNT services, and (3) integration of communication and PNT which employs communication network as a PNT signal source with communication signals carrying PNT auxiliary and augmentation data, expanding the available spectrum range for navigation. Functions of ICAN include navigation augmentation, enabling intelligent devices, and high-precision positioning service.

Fig. 2. The ICAN framework with UE.

Fig. 3. The architecture of communication and navigation fusion.

Second, authors offer a comprehensive overview of the key technologies of ICAN, namely signal regime design, LEO satellite constellation design, clock design, channel modeling, receiver design, terrestrial station deployment, and precise orbit determination.

For signal regime design, carrier frequency and bandwidth should comply with frequency range specified by the International Telecommunication Union's Radiocommunication Bureau (ITU-R) and the frequency regulations of each country as well as should trade off among path loss, throughput, antenna size, and band utilization (as illustrated in Fig. 4.). The choice of modulation schemes varies between different satellite constellation designs, among which orthogonal time frequency space (OTFS) can achieve more robust signal processing with full diversity gain in the time-frequency domain and be adopted to suppress the doubly dispersive effect in the STL channel. Channel coding techniques need novel design according to the unique characteristics of LEO satellite channels. Multiple access techniques and beamforming should also be considered.

For satellite constellation design, the LEO-ICAN constellation is composed of 3 segments: the space segment, ground segment, and user segment (shown in Fig. 5). The LEO-ICAN system must ensure that at least 4 satellites are visible at any time and position on the ground. Global satellite systems prefer the Walker-Delta topology as it provides symmetric coverage for terrestrial users. In the future, thousands of LEO satellites will be deployed to build novel mega LEO constellations with desirable signal of opportunity (SoO) navigation characteristics.

For clock design, atomic clocks are utilized to define the space-time reference system to ensure stable space-based time and an appropriate formulation should be established at the LEO satellite altitudes to ensure precise clock synchronization for the LEO-ICAN system.

For channel modeling, various channel effects, including reflection, refraction, diffraction, path loss, etc., severely affect signal propagation. In addition, atmospheric gases, rainfall, and clouds can cause signal fading. Meanwhile, the rapid movement of LEO satellites produces severe Doppler effect that results in signal waveform aliasing. To cope with this issue, researchers have proposed various solutions.

For receiver design, external factors such as satellite vibration and space radiation must be considered. The reception scheme based on Rake receivers can effectively suppress multipath interference and improve the compatibility of integrated signals. The meta-signal concept has also provided new ideas for receiver design strategies.

For terrestrial station deployment, several metrics should be considered for determining the optimal location of ground stations, such as carrier-to-noise ratio, GDOP, link outage probability, and rainfall attenuation.

For precise orbit determination, the LEO-ICAN system requires a more accurate orbit description than traditional ones. More sophisticated satellite orbit determination techniques have emerged, such as satellite laser ranging, zero-difference observation, and double-difference observation.

Fig. 4. Frequency bands with tradeoffs in terms of path loss, throughput, antenna size, spectrum size, and bandwidth usage.

Fig. 5. The LEO-ICAN constellation system with the space, ground, and user segments.

Third, authors discuss challenges and opportunities in the combination of ICAN and LEO. (1) Communication signals and navigation signals differ in terms of service coverage and propagation manner. Navigation signals need to be integrated with communication signals with carefully designed multiplexing techniques and complex protocols to ensure efficient channel utilization, and the compatibility between communication and navigation signals should be considered in waveforms. (2) The LEO-ICAN system requires intelligent and dynamic architectural configuration and reconfiguration, scalable heterogeneous networking topology, and signal control procedures to support diverse services and enable massive collaborative information transmission under a highly dynamic environment. (3) The ICAN system requires the ability to propagate at least dual-band communication and navigation signals simultaneously, whereas there is competition for radio resources between the 2 signals. Whereas, available resources vary in dynamic network topology, and the complex spectrum compatibility and intelligent resource allocation of the ICAN system pose significant challenges during the preconstruction stage. (4) Mega LEO constellation program brings about exceptionally complex topology for ISLs and the accuracy of the acquired ephemeris data can be susceptible to fluctuations. (5) The LEO-ICAN system operates in a high-speed moving environment and needs to overcome severe Doppler effect. With the LEO-ICAN system, there are some new opportunities to upgrade conventional satellite communication and navigation systems, which are briefly summarized as follows. (a) Satellite-ground networking and interconnection: The objective of future communication technology is to achieve universal interconnectivity, with intelligent networking and space–air–ground integration. (b) Joint orbit determination: The optimized ISLs and STLs will further improve the orbiting accuracy. (c) Atmospheric monitoring: The LEO satellite constellation construction program is anticipated to include an increased number of small satellites, thereby expanding the range of atmospheric monitoring. (d) Indoor positioning with LEO satellites is a probable alternative due to less signal landed loss and can penetrate multiple layers of reinforced concrete materials.

Finally, authors put forward suggestions and prospects. For further work related to the LEO-ICAN system to achieve deep coupling of communication and navigation, authors propose some possible recommendations as follows:

• System architecture: It is recommended to improve the overall planning of system construction and ensure the simultaneous construction of base stations both in the air and on the ground.
• Cooperative research: International cooperation and collaboration between successful LEO systems and strategies should be encouraged, utilizing software-defined radio as a means of building the system load platform and further promoting the construction of the mega LEO constellation framework should be considered, and standardization and collaborative development across all aspects of the LEO-ICAN system should be promoted.
• Theoretical basis: It is crucial to stay up to date with the latest technology research trends (especially in fusion theory, satellite-ground networking, signal propagation, processing, and integrated network framework design), which ensures that theory and application remain in sync, allows for the full potential of new technologies to be realized, and provides strong theoretical guidance for the development of a deep-coupling system of LEO satellite communication and navigation.

In the future, the 6G LEO satellite networks will further expand ICAN-related functions and services to achieve unmanned intelligence. As for the prospects in the related field, some possible future extensions of technology routes or application areas are listed as follows.

• MIMO. The MIMO technology is now one of the feasible solutions for LEO satellite communication systems. The application of MIMO technology in LEO constellations can extend the spatial freedom of the system and significantly improve the spectrum and power efficiency.
• Cell-free network. The cell-free network is a type of communication network that does not depend on base station, instead relying on point-to-point connections and self-organizing networks. It can cover a wider range of communication areas when combined with LEO satellite networks.
• Broadband narrow-point multibeam management. Narrow-beam multiplexed antennas based on UWB technology have several advantages, such as high gain, wide coverage range, and the ability to realize frequency and polarization multiplexing. They have emerged as an important development direction in the field of future satellite communication.
• Ultradense networks. The deployment of ultradense networks can provide significant support for the ICAN paradigm, which is an attractive solution for improving the performance and reliability of LEO satellite communication and navigation systems.
• AI enabled approach. Intelligent resource allocation in the ICAN system requires the integration of AI-enabled techniques. Machine learning methods can effectively help manage multidimensional resources and achieve optimal matching of dynamic resources with diverse services. Cognitive and intelligent networks can be applied to topology optimization of LEO satellite networks as a way to improve resource scheduling and allocation, and enhance network performance. Advanced AI techniques can enable self-optimized maintenance of LEO satellite networks and improve the security and stability of the network.
• Optical atomic clocks. Bidirectional laser technology can improve the stability and accuracy of atomic clocks, making it useful for space clock synchronization in navigation missions.
• Deep space exploration. The ICAN system shows promise in meeting the growing demand for deep space exploration. Long-duration observations with high resolution and reliability can be achieved by utilizing x-ray flux signals.