May 3, 2017
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These past several months, our engineers have been busy testing Unmanned Aircraft Systems (UAS), or drones, on commercial 4G LTE networks at the Qualcomm UAS Flight Center in San Diego — testing key performance indicators (KPIs) such as coverage, signal strength, throughput, latency, and mobility under various scenarios on commercial LTE networks.
Today, we are pleased to present the results of the first the first* comprehensive, systematic study of cellular system performance in networks serving low-altitude (400 feet above ground level and below) drones. The analysis supports the viability of 4G LTE commercial mobile networks for drones operating beyond visual line of sight (BVLOS) at 400 feet above ground level (AGL) and below.
LTE provides superior coverage and handover performance
During the field trial, approximately 1,000 flights were performed to collect datasets that were post processed and analyzed. We also performed simulations to complement field trial results by allowing study of performance tradeoffs when the network is serving many mobile devices and LTE-connected drones simultaneously over a wide area. Simulations also enabled rapid testing of parameter and feature changes that are more difficult to study in a commercial network.
The field trial demonstrated that LTE networks can support safe drone operation in real-world environments. Our findings showed that existing commercial cellular networks can provide coverage to drones at low altitudes up to 400 feet AGL. Our test drones also showed seamless handovers between different base stations during flights. Below is a glimpse of these findings.
- Very strong signal availability at higher altitudes: Received signal strengths for LTE drones at altitude are very strong despite downtilted antennas in the network. In fact, signal strengths are statistically and significantly stronger for drones at altitude than for mobile devices on the ground (“ground mobile devices”) because the free space propagation conditions at altitude more than make up for antenna gain reduction.
- Successful handover and lower frequency of handover events: Handover performance for drones is superior to ground mobile devices. This is attributed to the increased stability of signals with free space propagation relative to those subjected to multipath, shadowing, and clutter experienced on the ground.
- Comparable coverage to ground mobile devices: The signal quality of the network-to-drone link (downlink) was statistically lower for drones at altitude compared to the quality at ground; we measured the signal-to-interference-plus-noise ratios (SINRs) with a median decrease of 5 dB due to neighbor-cell interference. However, the coverage outage probability (defined as SINR < -6dB) is very similar for drones and ground mobile devices. Given the downlink data rates required for drone use cases are mostly limited (e.g., command and control), commercial LTE networks should be able to support downlink communications requirements of initial LTE-connected drone deployment without any change.
Strong evolution path for enhanced drone support
As mentioned before, existing commercial LTE networks can support initial drone deployment. However, LTE evolution will take this to the next level, enabling our vision of wide-scale deployments of drones that are expected to reshape countless industries including construction, delivery, entertainment, insurance, mapping, news gathering, public safety, public utilities, railroads, real estate, agriculture, and wildlife conservation. To enable wide-scale operation of drones, we identified several optimization opportunities including:
- Interference mitigation: For uplink communications (drone-to-network link), a drone’s transmit power is significantly lower at altitude than ground mobile devices. However, drones at altitude produce more uplink interference in the network than ground mobile devices because free space propagation increases the interference energy received at neighbor cells. This effect should not be an issue for initial deployment of LTE-connected drones in limited numbers; however, interference mitigation techniques can be explored to enable wide-scale deployment of drones in the future.
- Power control optimization: In our simulations, we illustrated several effective interference mitigation techniques, including power control optimizations. For example, the interference issue is eliminated with the Optimized Open-loop Power Control (OLPC) approach that not only sets target signal strength at the serving cell, but also limits neighbor cell interference using downlink path loss estimation. Thus, many more drones with high uplink data rates can be supported without causing excess interference to the network or degradation to ground mobile device performance. This allows LTE networks to better support wide-scale deployment of connected drones with high-bandwidth uplink transmission (e.g., high-resolution video streaming).
- Serving cell selection optimization: We observed different cell selection characteristics for drones compared to ground mobile devices, where test drones were more likely than ground mobile devices to sub-optimally select a serving cell. Cell selection algorithms can be further optimized to better help ensure drones select the strongest serving cell, resulting in improved signal-to-noise ratio and optimized network capacity.
Benefiting from the fast-moving and established cellular ecosystem
This is just the beginning. The growing drone industry will benefit from the fast pace of innovation of the cellular ecosystem and continuous evolution of LTE technologies.
Qualcomm along with other cellular industry stakeholders have already started the next phase of research of further optimizing LTE for drones. A 3GPP study item was accepted this March to enhance LTE support for Aerial Vehicles (drones). Qualcomm will share its learnings from the trial with the cellular community to help shape the future optimizations in the standards. This is just another example of how Qualcomm is playing a leadership role in 3GPP by bringing our technology vision and supporting it with detailed technical designs and analysis created by our best-in-the-world communications system engineers. Also, as an active member of the Drone Advisory Committee Sub-Committee (DAC-SC) we are contributing our research and providing informed inputs to the industry and FAA about the viability of using commercial cellular networks for safe drone operation, particularly for BVLOS flights.