Before looking at why and what drones communicate, we must first understand how communication is achieved, and the type of communication required by the swarm.
At a basic level a drone swarm is a floating dynamic wireless network, commonly known as a wireless mesh network.
A mesh network is a network topology in which each node relays data for the network. All nodes cooperate in the distribution of data in the network. A mesh network whose nodes are all connected to each other is a fully connected network.
A mesh network can be designed using a routing technique or a Flooding technique.
When using a routing technique, the message is propagated along a path, by hopping from node to node until the destination is reached. To ensure all its paths’ availability, a routing network must allow for continuous connections and reconfiguration around broken or blocked paths, using self-healing algorithms.
The self-healing capability enables a routing based network to operate when one node breaks down or a connection goes bad. As a result, the network is typically quite reliable, as there is often more than one path between a source and a destination in the network.
However, in Flooding, instead of using a specific route for sending a message from one node to another, the message is sent to all the nodes in the network.
The Flooding technique is simplistic and highly reliable. There are no sophisticated routing techniques since there is no routing. No routing means no network management, no need for self-discovery, no need for self-repair algorithms, and because the message is the “payload”, no overhead for conveying routing tables or routing information.
Flooding technology has additional advantages related to propagation. Signals arriving at each node through several propagation paths benefit from the inherent space diversity, thus maximizing the network robustness of handling obstructions, interferences, and resistance to multipath fading, with practically no single point of failure. In other words, blocking one path or even a limited number of paths is usually of no consequence.
Furthermore, no routing means that the controller is extremely simple, requiring minimal computing power and memory and thus low power consumption, low PCB real estate, and low cost.
Despite these benefits, flooding the network with repeated messages has its own challenges. For transmitting data, the main questions are how data packet collisions “broadcast storms” are avoided, how the retransmitting process propagates the message efficiently toward its destination, and how the process ends, without an energy-wasting avalanche.
Fortunately, a synchronised-Flooding approach using a synergic combination of techniques enables us to answer these questions and solve the challenges. Incorporating time division multiple access combined with high-accuracy synchronization allows the retransmissions to occur simultaneously so that the message propagates one hop in all directions at precisely the same time and avoids collisions. At each hop, nodes retransmit only relevant information, and the number of retransmissions corresponds to the number of hops in the network, so there is no waste of retransmissions.
The diagram demonstrates Flooding a 24-node, three-hop network. The first hop signal propagation is blue, the second is purple, and the third is green.
In the Flooding-based scheme a signal obstruction will most likely not affect the operation at all because of the numerous redundant paths.
Considering the nature of the nodes in this network, being UAV drones, a wireless mesh network approach is the most effective network topology and communication technique for Drone Swarm Communication (DSC), however there are some network design considerations.
Challenges with Communication design
DSC design has several inherent restrictions compared to wired IP networks. The main restrictions are: limited bandwidth of the wireless links connecting drone nodes, limited computing power, and limited energy supply, especially in case of battery-operated drones.
One of the main design goals of swarm communication is to provide highly reliable and continuous communication of all the nodes in the network, all the time under the changing propagation conditions.
In a Flooding-based approach, networks always use all the available propagation paths. This precludes the need for self-organizing or human intervention, and large networks are handled with the same simplicity as small ones.
The same principle applies to a related challenge: scalability. In Flooding-based networks, a network size increase causes the network to be more robust. Whereas with the same size increase of routing-based networks this increases the complexity of the routing table and accordingly, the probability of network failure.
Energy consumption is one of the most important DSC design challenges. In routing-based networks, the total number of operating nodes at any moment (when the network is transmitting) is always lower than in Flooding-based networks; thus, routing consumes less energy. On the other hand, Flooding-based messages are much more efficient, as they do not need the overhead associated with transmitting routing tables and commands, which increases with the number of nodes and hops. In modern Flooding-based systems, the energy of the signals received from adjacent nodes adds up, so less power can be used for achieving the same range.
As a rule of thumb, routing technologies are expected to have lower energy consumption in smaller networks with very few hops. Flooding-based technologies are expected to have lower energy consumption in large networks with many nodes or hops, or when the average size of the message is small.
Range and coverage are among the most important DSC design challenges. In networks with simple topology, the range of the network is directly related to the range between two adjacent nodes and thus is affected by the quality of the physical layer circuitry and software.
In Flooding-based networks using mesh topology, the nodes sum up the energy from all the received nodes, creating a much better range of the whole network compared to the most sophisticated routing network. In addition, multiple propagation paths improve network coverage to a “no dead spots” quality.
Latency importance in DSC design depends heavily on the specific type of application. For example, in an application where drones with specific sensory for taking atmospheric readings, may only need to take a reading in tens of minutes, latency is of no consequence, On the contrary, in an application where drones on the outer rim of the swarm are sensing for collision detection and need to transmit to the swarm potential obstacles, the time in relaying the message to the swarm can be a deciding factor between crashing and avoiding collisions, a delayed feedback to an emergency situation can be very costly or even perilous.
When comparing the two techniques, Flooding-based networks with mesh topology have an inherent advantage because of their lower overhead. Furthermore, routing-based networks suffer from latency inconsistency caused by possible RF propagation problems in the designated route, an inconsistency that increases with the number of hops. In Flooding-based networks, propagation problems in one route usually do not affect latency at all because of the inherent space diversity.
Swarm Communication Design
Drone Swarm Communication utilising the Synchronised-Flooding Technique in a mesh network means that a signal is communicated to all drone nodes in the network. Each drone does not need to account for who its neighbours are in order to propagate the signal throughout the network. More so, the drone node itself does not require an ID or address for networking purposes. By utilizing Synchronised-Flooding, networks become more robust and there are no restrictions to increasing the size of the swarm, swarm deployments are simplified and therefor swarm pattern management is simplified, energy-efficiency is inherent, and the resulting bi-directional network becomes fully transparent to the data flowing through and the application at hand.
Swam communication networks can be extended to achieve advanced bi-directional communication between swarms and communication with the Overmind by utilising a bridge node in both swarms known as the Swarm-Mother. In Essence a bridge node is a higher powered node with a much larger receiver/antennae range than the standard drone node in the swarm.
Data is transmitted from the swarm drone nodes to the bridge node as it communicates as a standard node in the mesh network, the difference being in the processing and retransmission of data based on programmed logic/intelligence of the bridge node and ability to receive and transmit signals over long range and on different bands using different communication technologies.
The diagram demonstrates signal propagation across bridge nodes, as blue circles, joining two swarms into one conjoined mesh network. The signal propagation of the first swarm is green, while the signal propagation of the second swarm is purple, and the signal propagation between swarms is blue
The bridge node is a member of the Overmind group and also serves a purpose greater than simple relay of information between swarms however this will be discussed further in the Overmind control layer to demonstrate how the Overmind communicates with and can control swarms from an external sensory and overwatch control perspective.
In smaller scale cases, without the Overmind, an operator is attached to a swarm during times of relaying instructions or receiving feedback from a swarm. The Operator acts as one node in the network and bridge nodes are used to extend instruction to a swarm outside of the operators direct communication range.