[Paper Review] 5G NB-IoT via low density LEO Constellations

René Sørensen, Henrik Krogh Møller+1 more|arXiv (Cornell University)|Aug 13, 2021

IoT Networks and Protocols8 references4 citations

TL;DR

This paper proposes a viable 5G NB-IoT system over low-density low Earth orbit (LEO) satellite constellations using nano-satellites as base stations. It demonstrates that a 16 W satellite transmitter with a 560 km cell radius can support uplink link budgets 6.8 dB better than downlink, enabling global IoT coverage with minimal device complexity, though feeder link discontinuity requires on-board core network functions for continuity.

ABSTRACT

5G NB-IoT is seen as a key technology for providing truly ubiquitous, global 5G coverage (1.000.000 devices/km2) for machine type communications in the internet of things. A non-terrestrial network (NTN) variant of NB-IoT is being standardized in the 3GPP, which along with inexpensive and non-complex chip-sets enables the production of competitively priced IoT devices with truly global coverage. NB-IoT allows for narrowband single carrier transmissions in the uplink, which improves the uplink link-budget by as much as 16.8 dB over the 180 [kHz] downlink. This allows for a long range sufficient for ground to low earth orbit (LEO) communication without the need for complex and expensive antennas in the IoT devices. In this paper the feasibility of 5G NB-IoT in the context of low-density constellations of small-satellites carrying base-stations in LEO is analyzed and required adaptations to NB-IoT are discussed.

Motivation & Objective

• To evaluate the feasibility of deploying 5G NB-IoT over low-density LEO satellite constellations using small, low-cost nano-satellites as base stations.
• To analyze the uplink and downlink link budgets in a LEO NTN NB-IoT scenario, focusing on propagation loss, Doppler shift, and power constraints.
• To identify and propose necessary adaptations to NB-IoT protocols—such as RACH, scheduling timers, and paging mechanisms—for operation in highly dynamic, low-coverage LEO environments.
• To address challenges arising from discontinuous feeder links and satellite mobility in low-density constellations, particularly for power-constrained IoT devices.
• To validate the system design through link-level simulations and propose a path toward in-orbit demonstration and 3GPP standardization.

Proposed method

• A system model is developed with a 600 km altitude LEO orbit, Earth radius of 6357 km, and satellite velocity of 7.57 km/s to simulate orbital dynamics and Doppler spread.
• The NB-IoT uplink uses narrowband single-carrier SC-FDMA with bandwidths of 3.75–180 kHz, while the downlink uses OFDM on 180 kHz carriers, enabling high link-budget gains.
• Link budget analysis shows a 6.8 dB uplink advantage over downlink due to narrowband uplink transmission, allowing long-range LEO-to-ground communication.
• Random Access Channel (RACH) performance is simulated under AWGN, NCU, and NDH fading conditions, with repetition levels adjusted based on SNR to ensure reliable preamble detection.
• Scheduling timers are offset by an integer multiple of round-trip propagation delay to account for variable satellite-to-UE range, with dynamic or fixed configuration options proposed.
• Paging and DRX mechanisms are adapted for LEO by combining PSM during out-of-coverage periods with iDRX upon satellite acquisition, minimizing energy consumption.

Experimental results

Research questions

• RQ1 Can a nano-satellite with a 16 W transmission power and a 560 km cell radius support reliable NB-IoT uplink communications to ground IoT devices in a LEO NTN scenario?
• RQ2 What are the required adaptations to NB-IoT protocols—especially RACH, synchronization, and paging—to maintain connectivity in a low-density, high-mobility LEO constellation?
• RQ3 How does the uplink link budget advantage of 6.8 dB over downlink impact system design and coverage limitations in LEO NTN NB-IoT deployments?
• RQ4 What strategies can mitigate the impact of discontinuous feeder links in LEO constellations, particularly when ground stations are unavailable?
• RQ5 To what extent can on-board core network functions (e.g., MME) and store-and-forward mechanisms maintain service continuity during feeder link outages?

Key findings

• The uplink link budget exceeds the downlink budget by 6.8 dB due to narrowband single-carrier uplink transmission, enabling long-range communication from LEO to ground devices.
• A 16 W satellite transmission power with a 560 km cell diameter is sufficient to support NB-IoT connectivity, though a 60 W increase in downlink power would balance the link budgets.
• RACH performance simulations show that at SNR = -12 dB, 128 repetitions are required for reliable preamble detection in AWGN and NDH fading, with detection failure rates of 8% and 13%, respectively.
• The cell size is limited to approximately 500 km in diameter due to the DL synchronization range, which can be extended by increasing transmit power or optimizing antenna gain and beamwidth.
• Discontinuous feeder links necessitate on-board core network functions such as the MME and store-and-forward mechanisms to maintain connectivity and paging capability during outages.
• A hybrid PSM/iDRX approach is viable: UEs enter PSM when out of coverage and switch to iDRX upon satellite acquisition, reducing energy consumption while maintaining reachability.