GNSS signals are widely used for time synchronization purposes in many different application sectors, including telecom, finance and energy. Telecom service providers, in particular, consider GNSS-based synchronization well adapted to macro-cells whereas alternative synchronization technologies (e.g. wire-based solutions like Precise Time Protocol) are deemed more suitable for indoor small cells, achieving timing accuracies in the sub- range depending on network topology.

Precise indoor time synchronization is expected to be of particular importance in future terrestrial telecom networks, where small cells are expected to populate indoor areas to provide 5G services, and tight synchronization requirements are needed for transmission of new 5G signals (in the order of 50 ns or below). Additionally, accurate time synchronization might also be of interest to allow accurate positioning based on ranging measurements derived from 5G signals.

GNSS signals can still be used in mild indoor conditions for positioning and timing applications at the cost of degraded performance due to high impact of deep fading and strong multipath (from Non-Line-Of-Sight components). In the particular case of timing applications, mass-market timing receivers are able to operate even with only one GNSS signal being tracked (if the receiver’s position is a priori known) with expected accuracies in the order of 500 ns (1σ).

Lately, a new timing commercial service based on a low-earth orbit (LEO) constellation (i.e. Iridium) has become operational in order to provide an accurate alternative PNT in indoor conditions. In this case, the operator claims a timing accuracy of 100 ns (1σ) closer than GNSS to the most stringent requirements of telecom operators.

However, internal ESA activities have demonstrated that advanced antenna array processing techniques can achieve GNSS-based timing accuracies below 50 ns (1σ) in deep indoor conditions. The assessed techniques are fully compatible with low-cost antenna arrays that could be suitable for usage in small cells.

The objectives of the activity are to:

• design, develop and demonstrate innovative GNSS-based time synchronization techniques enabling the exploitation of GNSS signals in small cells operating in indoor conditions;
• target time synchronization accuracies below 50 ns (1σ) enabling usage of GNSS signals for future applications with tight time synchronization requirements in indoor conditions, like future 5G indoor small cells;
• implement a breadboard for demonstration in representative indoor conditions of achievable timing accuracies based on the developed techniques.

The tasks to be performed will include:

• consolidation of state-of-the-art on GNSS timing and indoor GNSS signal processing, including exploitation of advanced array processing techniques;
• consolidation of requirements for the targeted use cases, in particular those relating to 5G small cells;
• identification of promising techniques at receiver level, including trade-off analysis based on simulations;
• breadboard design, implementation, lab validation and field demonstration in representative indoor conditions.

The results of the activity will provide:

• a complete understanding of achievable capabilities of proposed techniques and related timing performance;
• a breadboard, with demonstration of performance in indoor small cells.

Results from other related ESA activities will be duly considered and assessed, as well as those from EC studies on a potential future G2G timing service.