The need for a high-bandwidth communication link to carry multiple video channels from a mobile robot (unmanned ground vehicle, or UGV) back to a control station requires using high-frequency RF communication links. These links are mostly line-of-sight, often limiting the flexibility of the robot’s movement. The Autonomous Mobile Communication Relays project demonstrated the capability of autonomous slave robots to provide communication-relaying capability for a lead robot exploring an unknown environment.

Figure 1. First-generation ADCR system on an iRobot® PackBot®.
Several generations of Automatically Deployed Communication Relay (ADCR) systems led to a robot-mountable Deployer module that deploys static relay nodes when and where needed. To meet specific in-theater requirements, the Manually Deployed Communication Relay (MDCR) system was developed. A fourth-generation ADCR system was developed in 2013 to automate the deployment of these fielded and proven MDCR relay nodes.

Radio Frequency Principles

One of the common weaknesses in other relay systems that causes them to fail in field tests is the use of high-gain antennas. While high-gain antennas may seem desirable for range extension, without the use of an electronic amplifier, this gain can only be achieved by focusing the radiation pattern.

For a relay system to be versatile and work well in various environments, low-gain antennas are generally best. Low-gain omnidirectional antennas should be used for a relay system to be versatile and able to work in a wide variety of environments with no a-priori terrain and placement information.

The height of the relay-node antenna when placed on the ground must be equal to or greater than the height of the antenna on the robot. Otherwise, the deployed relay node with a lower antenna would encounter lower received signal strength than the robot, and might be unable to join the network. When two nodes are in close proximity, the receiver front end of one node tends to get saturated by the strong signal emitted by the nearby node’s transmitter. This may lead to mutual jamming so that neither can enter the network. This often means that only one node should be on at a time while being transported by the robot.

Figure 2. Second-generation ADCR system on an iRobot® PackBot®.
MDCR relays and end-point radios use standard half-wave dipoles with 2.1- dBi gain. Active MDCR nodes (operating at 4.9 GHz) must be kept at least approximately 1 m (40”) from each other to ensure no mutual jamming. For this reason, in the first- and second-generation ADCR systems, only one stowed node inside the Deployer module was active at any given time. The system ensured that the active node had successfully joined the network before deploying it and activating the next node in the Deployer module. In the MDCR design, both relay nodes had to be active while being carried by the robot. Placing the two nodes on a level tabletop at the same distance apart prevented them from entering the network.

Input filtering is the best defense against unintentional radio frequency (RF) jamming to preserve the link between the operator control unit (OCU) and the robot. A commercial bandpass filter was used on the OCU-side MDCR end-point radio to mitigate jamming issues. The center frequency and bandwidth of the bandpass filter were chosen to match the relay network’s frequency characteristics. The bandpass filter helps to attenuate the noise outside the frequency band of interest, improving the overall signal-to-noise ratio of the received signal.

Wireless Networking

The high throughput required to transfer multiple streams of video data from a remotely controlled vehicle (often outfitted with multiple cameras) limits the practical number of relay nodes in the data route. This limit is about three for 802.11g wireless networks. Techniques for increasing this limit include reducing the video resolution, eliminating the color component, or using multi-frequency radios.

Due to the delay incurred in establishing a new route, the constant switching between two routes could bring the network to a halt. One way to prevent this is to use some measure of hysteresis and “good enough” metrics so that a new route is not selected as long as the current route can still carry the required network traffic.

The required throughput, type of data, mesh topology, and the data usage determine the maximum number of relay nodes that can be used in a data-traffic route. Problems with remotely controlling the vehicle in real time begin to appear after three relays are present in a linear route. This can be mitigated by reducing video resolution, number of cameras, and/or dropping color information from the video stream. Another solution is to use dual-frequency radios to allow simultaneous transmission and reception of data at each node, increasing the overall throughput capacity of the mesh network.