CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national stage entry of International Patent App. No. PCT/EP2020/054680, filed on Feb. 21, 2020, which claims priority to European Patent App. No. 19166510.8, filed on Apr. 1, 2019, which are both hereby incorporated herein by reference as if set forth in full.

BACKGROUND

Field of the Invention

The present disclosure relates to industrial communication, such as communication methods and systems for industrial monitoring and control purposes (e.g. monitoring and controlling an electric power system, monitoring and controlling equipment in an industrial manufacturing facility, and/or for performing other industrial automation control and monitoring tasks), including scheduling of TDMA communication, and is relevant for wireless and wired communication.

Related Art

In industrial communication, a number of slave nodes, such as sensor and/or actuator nodes are connected to a central network controller or gateway, such as a master node. In many industrial applications, e.g. electric power substation automation, the slave nodes are arranged at different distances to the master node. A known method to avoid collisions is to use a wired communication system, and using the same length on an optical fiber for each communication link between slave node and master node, so that the transmission time for each slave node to the master node will be the same. Such a solution is effective in avoiding collisions, however lacks cost-efficiency. Using wireless communication is an alternative, which can provide comparably lower costs, however avoiding collisions cannot be provided by equalizing the communication distances.

One way to avoid collisions in wireless TDMA systems, is to utilize a guard time between packets from different nodes. The nodes can be scheduled to transmit in a slot and a guard time in each slot ensures that the transmission do not interfere with the transmission in the following slot. Such scheduling is comparably simple, however, using guard times may negatively affect the communication latency between the nodes. If the guard time is short compared to the slot, the negative effect of the guard time with respect to an overall latency will be small. If the guard time is comparable in size; e.g. 20% or larger of the duration of the transmission slot, the guard time will have a larger negative impact. In many instances the guard time used can be comparably small. In e.g. Wi-Fi™ communication systems that relate to short range transmissions of large packets the guard time will be comparably small, and for example a guard time for avoiding collisions can be less than 1% of the transmission time, such as having 100 μs (micro-seconds) long packets and using a guard time of 1 μs. For other applications, the use of guard times will have a larger negative impact.

The article “LDMAC: A propagation delay-aware MAC scheme for long-distance UAV networks”, by Chen et al. (E1) describes handling the problem of propagation delay in time-critical wireless communication. E1 specifically relates to wireless communication between a base station and a plurality of drones, or unmanned aerial vehicles (UAVs). The distances between the UAVs and the base station varies and thereby the propagation delay. In order to avoid communication collisions, E1 suggests using a “propagation delay-aware access scheme”, see abstract of E1. In short, see ch. 4.1 of E1, the drone (UAV) comprises a GPS unit and uses the GPS to determine its distance from the base station and the propagation delay. The drone then selects arrival time slot for the communication and calculates the corresponding transmission time.

The method of E1 is mainly intended for large distances, such as kilometers, and this “propagation delay-aware scheme” may be more complicated than using guard times in more narrow range communication.

Shorter range communication (than in E1) may suffer from similar collision problems. In industrial applications, e.g. manufacturing automation, or electric power substation automation, there is a need for transmitting comparably short packets, comparably often. Communication of packets that collects information from sensors and communicates control data or settings to actuators or control units, may be provided in packets comprising in the order of 100 bits wherein the transmission time, and thereby the slot duration, can be in the order of 1 μs. Radio waves travel about 300 m in one μs, and a guard time for avoiding collisions between nodes separated by 30 meters may therefore be in the order of one tenth of such a 1 μs long time slot duration.

In wired communication, different lengths of optical fiber can be used to equal out the transmission times. Wireless transmissions can be more economic in installation, and setting a guard time can be an effective way to avoid collisions, however, a guard time may introduce problems by negatively affecting the overall cycle time of the communication. A guard time extending about 30% of each time slot will result in an increased delay of the communication cycle by 30%.

In industrial applications having high demands for low latency, the guard time should be short; however, shortening the guard time increases the risks for collisions. Using delay-aware scheduling, as in the system of E1, can avoid collisions and keep the latency low, however, using a guard time to avoid collisions can contribute to a less complicated scheduling.

SUMMARY OF INVENTION

An aim of the present disclosure is to provide a method for wireless communication suitable for industrial appliances, such as industrial automation, which wireless communication can contribute further to low latency and high reliability, or at least provide an alternative to the wireless communication methods of the prior art.

For this purpose, a first aspect provides a method of scheduling in a TDMA based communication system, in which communication system a first node, or access node, communicates with a plurality of second nodes, or slave nodes using a respective communication link. The method includes scheduling the communication in time slots. Especially, the method of scheduling comprises:obtaining a propagation time for each communication link;determining the largest difference in propagation times between two communication links;comparing the largest difference to a threshold; andusing a first mode of scheduling when the largest difference is smaller than the threshold;using a second mode of scheduling when the largest difference is larger than the threshold.

The first mode of scheduling includes:selecting a guard time to be applied in each of the time slots, which guard time (GT) is based on the largest difference, and scheduling each second node for transmitting within a respective time slot of said time slots.

The second mode of scheduling includes:scheduling the transmission from each second node to arrive at the first node within a respective time slot of said time slots, wherein the scheduling of the transmission of each second node is based on the respective propagation time of the communication link of the second node in question. Preferably, the scheduling of the second mode includes scheduling each second node with a timing advance equal to the propagation time of the second node.

The guard time of the first mode should be large enough to accommodate the largest difference, whereas in the second mode the propagation times, or delays, are handled by scheduling the transmissions from the second nodes.

The first mode of scheduling regulates the guard time to avoid communication collisions.

The selection of operating mode is based on the largest difference in propagation times. In the first mode, the selection of guard time for the time slots is based on the largest difference in propagation times, wherein the scheduling is based on the largest difference. In the second mode, the scheduling is based on the respective propagation time, or propagation delay, of the communication link, wherein the scheduling of the transmissions from each (second) node is based on the propagation time between the node in question and the first node. Thus, the second mode of scheduling is propagation-delay-aware; using propagation-delay-awareness to avoid communication collisions.

In an embodiment of the first aspect, the first mode further includes scheduling transmissions from the first node to the second nodes, and switching from transmissions from the second nodes to transmissions from the first node, wherein the method further includes scheduling one, or more, empty time slots when switching from the transmissions from the second nodes to the transmissions from the first node.

In an embodiment of the first aspect, the second mode includes scheduling transmissions from the first node to the second nodes, and switching from transmissions from the first node to transmissions from the second nodes, wherein the method further includes scheduling one, or more, empty time slots when switching from the transmissions from the first node to the transmissions from second nodes.

In an embodiment of the first aspect, the second mode of scheduling includes scheduling the transmissions from each second node with a timing advance equal to the propagation time.

A second aspect provides a network for scheduling TDMA based communications in time slots. The network controller is configured to perform the method of scheduling according to any embodiment of the first aspect. The network controller comprises:communication interface for communicating with communication nodes to be scheduled,a propagation time obtainer for obtaining propagation times of communication links,a difference calculator for determining differences between obtained propagation times,an operation mode selector for selecting operation mode on the basis of a largest calculated difference between propagation times,a first mode scheduler for scheduling the TDMA based communication in accordance with a first mode of scheduling,a second mode scheduler for scheduling the TDMA based communication in accordance with a second mode of scheduling, anda node informer for informing each scheduled node about the schedule.

