Cost effective communications to terrestrial and maritime field sensors, and industrial automation and control equipment, has the potential to deliver significant economic and environmental benefits in areas such as environmental monitoring for climate change, water, mining, agriculture, defence and national security. Many of these high-value applications have modest data rate requirements (kilobits per second), and can tolerate intermittent communications with latency up to several hours. Frequently, applications involve sensors in very remote areas where terrestrial communication solutions do not exist, are unreliable, are denied or insecure (e.g. in a defence context). These constraints mandate the use of communication systems in which multi-access (or multi-user) receivers are located in satellites or similar mobile platforms (e.g. unmanned aerial vehicles, or maritime vessels).
In such systems, the terminals may be stationary (at a fixed location), or they may be mobile (e.g. portable, or fitted to a vehicle, aircraft or vessel, or space vehicle, or carried by a person or animal). Thus, in such systems the number of terminals in the field of view can vary due to either movement of the receiver or terminals, or due to receivers only being intermittently in an active state (ready for transmission).
However, in the past, the combination of prohibitive cost and technical constraints of such satellite systems have typically limited the widespread use of such systems for communication with large numbers of remote field sensors. In particular, past systems have typically been one-way communications systems for very small amounts of data. Such one-way systems are also referred to as open-loop systems, as there is no feedback between the receiver and the terminals (i.e. terminals transmit, receivers listen). One problem with such open-loop systems is that each transmitting terminal does not know if its transmission was successful. Thus, the terminal remains active, and constantly repeats its transmission, whilst the receiver is in the field of view. This results in inefficient use of the available physical communication medium, and can significantly increase the number of packets which are not received due to multiuser receiver failures. Furthermore, the terminals may not even know when they are in the field of view of a receiver, and may unnecessarily transmit signals which cannot be received at all by any receiver. This reduces the energy efficiency of the terminals, which may be important when terminals are remotely located battery powered terminals.
There is thus a need to provide communications methods, components, and systems for improving system performance in such communication systems, or alternatively to at least provide users with a useful alternative.