One of the key issues facing Direct-to-Cellular (D2C) satellite communication systems is ionospheric scintillation on the uplink and downlink, which can significantly degrade link quality. This work investigates the spatial and temporal characteristics of amplitude scintillation at D2C frequencies by scaling L-band scintillation observations from Global Navigation Satellite Systems (GNSS) receivers to bands relevant to D2C operation, including the low-band, and 3GPP's N255 and N256. These observations are then compared to scaled radio-occultation scintillation observations from the FORMOSAT-7/COSMIC-2 (F7/C2) mission, which can be used in regions that do not possess ground-based scintillation monitoring stations. As a proof of concept, five years of ground-based GNSS scintillation data from Sharjah, United Arab Emirates, together with two years of F7/C2 observations over the same region, corresponding to the ascending phase of Solar Cycle 25, are analyzed. Both space-based and ground-based observations indicate a pronounced diurnal scintillation peak between 20--22 local time, particularly during the equinoxes, with occurrence rates increasing with solar activity. Ground-based observations also reveal a strong azimuth dependence, with most scintillation events occurring on southward satellite links. The scintillation occurrence rate at the low-band is more than twice that observed at N255 and N256, highlighting the increased robustness of higher D2C bands to ionospheric scintillation. These results demonstrate how GNSS scintillation observations can be leveraged to characterize and anticipate scintillation-induced D2C link impairments, which help in D2C system design and the implementation of scintillation mitigation strategies.
ONE of the key pillars of the sixth generation (6G) communications paradigm is ubiquitous connectivity [1]. The idea of being connected everywhere at all times is difficult to realize by terrestrial-bound base-stations. Yet, the recent emergence of the Direct-to-Cellular (D2C) approach, where standard cellular phones can connect directly to satellites, and vice versa, aims to address this issue [2].
As satellite signals propagate between the transmitter and ground receiver, they are affected by natural propagation effects or artificial sources of interference that reduce the quality of service. The fluctuation of the amplitude and phase of radio waves (scintillation) due to the signal’s propagation through the ionosphere is one of these sources [3]. Scintillations can occur naturally through interactions between the signal and ionospheric electron-density irregularities [4], or they can be caused by external interfering signals [5]. Ionospheric scintillation significantly attenuates signals under 3 GHz [6], and varies based on several spatial and temporal parameters, such as geographical location, time of day, season, and solar cycle progression.
Global Navigation Satellite Systems (GNSS)-based applications have long faced the issue of scintillation, and their effects on accuracy, availability, and integrity, especially at equatorial and high latitude regions [7]. Even so, the effect on D2C communications systems would be more prominent. This is due to the two-way nature of D2C, with standard users being a part of both uplink and downlink, compared to only downlink for standard GNSS users. Additionally, GNSS is broadcast-based and is resilient to short-term temporal fluctuations due to the usage of tracking techniques [8]. However, D2C communications, especially voice or broadband connectivity, are sensitive to abrupt signal fluctuations.
To better understand the risk of ionospheric scintillation on D2C, dedicated scintillation monitoring stations have to be installed, and long observation campaigns must be performed to study how the scintillation varies with the 11-year solar cycle. This is not feasible, especially since D2C deployment has already begun by some companies [9]. Therefore, this work proposes an alternative.
Ground-based GNSS ionospheric scintillation monitoring stations have long been established to monitor ionospheric irregularities and their impact on GNSS signals [10]- [12]. Furthermore, radio-occultation (RO) experiments onboard missions such as FORMOSAT-7/COSMIC-2 (F7/C2) [13] extend the coverage of observations to include regions with no established scintillation monitoring networks. This work proposes scaling widely available GNSS L-band amplitude-scintillation observations to D2C frequencies to assess scintillation risk to D2C links as a function of local time, season, signal The scaling of ionospheric scintillation from one frequency to another was investigated previously. For instance, L-band scintillation was scaled to very high frequency (VHF) in [14], and the different GNSS L-band frequencies (L1/L2/L5) in [15], [16]. Furthermore, using GNSS L-band scintillation observations as a proxy model for scintillation at UHF was presented in [17]. Yet, what is missing from the literature is a self-contained long-term study on the scaling of scintillation from L-band to D2C frequencies, including comparisons with space-based RO measurements, and the conclusions that can be derived as a result.
In this work, we utilize GNSS Ionospheric scintillation observations to assess how D2C signals might be affected. We rely on a PolaRx5S multi-frequency multiconstellation reference GNSS receiver located at Sharjah (Geographic Latitude: 25.28°, Geographic Longitude: 55.46°), United Arab Emirates, which provides data over the ascending phase of solar cycle 25 (2020-2024). The retrieved observations correspond to L1 scintillation from GPS L1 C/A and Galileo E1, both having a carrier frequency of 1 575.42 MHz. Additionally, to limit the influence of multipath on scintillation, this work utilizes observations with an elevation greater than 30°.
This work also utilizes RO amplitude scintillation observations from F7/C2 for the years 2023-20241 . The longitude and latitude values of the observations were limited to 54°≤ longitude ≤ 57°and 23°≤ latitude ≤ 27°to enable a comparison with observations from the groundbased receiver by matching the region of coverage. Additionally, only GPS observations were used in this work for F7/C2. Amplitude scintillation is represented by the standard deviation of the signal’s intensity (I) normalized to the average signal intensity over 60 seconds [18], i.e.,
The severity of scintillation can be inferred from the magnitude of S 4 , where weak, moderate, and strong scintillation are represented by S4 < 0.3, 0.3 ≤ S4 ≤ 0.6, and S4 > 0.6, respectively [18]. This work focuses on strong scintillation, since such events can cause peak-topeak power fluctuations greater th
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