The first part of this article (Engenuity, July 2018) covered existing industry applications for unmanned aerial vehicle (UAV) communication links and the technological advancements driving solution implementation.
In this second part, we consider lower-level system requirements for UAV communication links (or “datalinks”), driven on the whole by a range of aircraft platform characteristics, ground system mobility, link capacity and networking requirements. These requirements affect the choice of modem, channel bandwidth and antenna configuration.
Modem type – line of sight (LOS) downlink
Two main classes of modulation are currently used for UAV-to-ground (downlink) communication, and the choice has a major impact on link range performance:
- Single-carrier, constant envelope modulation schemes are preferred for high performance (long range, efficiency of power amplification and on-board power). They include Gaussian Minimum Shift Keying (GMSK) and Constant Phase Modulation (CPM).
- Orthogonal Frequency Division Multiplexing (OFDM) multi-carrier schemes which stem from commercial applications for networking and broadband wireless access technologies. In those environments, they’ve delivered excellent performance in multipath domains such as mobile data and Wi-Fi applications, but with limited useable range, compared to constant envelope schemes. For shorter ranges and cluttered environments (such as urban sprawl), this modulation is preferred because of the inherent multipath capability.
Modem type – uplink
The ground-to-UAV (uplink) communication requirement is typically less than 1/10 of downlink capacity, allowing a reduction in channel bandwidth and an improvement in the link margin.
As the uplink is the primary control channel for the UAV, it puts higher demands on security, meaning the uplink modem scheme is usually based on Direct Sequence Spread Spectrum (DSSS) or an encrypted OFDM modulation.
Two factors determine channel bandwidth: The typical user requirement for compressed high definition (HD) video, putting typical throughputs in the 10-20 Mbps range; and the modem scheme in use. With GMSK, this capacity requires at least a 20 MHz channel.
MIMO and diversity
The abovementioned OFDM schemes generally use Multiple Input, Multiple Output (MIMO) antenna techniques to overcome excessive multipath in cluttered environments. MIMO offers improved link availability, but as the link distance increases, this benefit diminishes.
Diversity is another technique for improving availability of long-range LOS communication links. This technique works by receiving two non-identical transmissions from the transmitter – either through antenna separation or polarisation diversity using linear (cross polarised) or circularly polarised antennas.
Given all the choices available for UAS communication links, careful consideration is key whilst designing solutions that will perform adequately in high-end commercial and military requirements, especially when one considers operational ranges beyond 50 km.
Building on a successful 35-year track record as an innovator and supplier of LOS communication solutions for UAV applications, as well as their highly capable and cost-effective medium class UAS tactical surveillance system (ASTUS), Tellumat continues to develop leading-edge solutions in this domain.
* This is the second of a two-part article covering the technical requirements, technology developments and considerations for long-range UAV communication data links.