The invention relates to a method for carrying out a Two Way Ranging Procedure between a blind node whose position is not known and is to be determined, and at least one reference or mobile node whose position is known in order to determine the location of the blind node.
This ranging method is explained with reference to
This method is called Symmetrical Double-Sided Two Way Ranging, because it is symmetrical in that the measurements from a first wireless transceiver station to a second wireless transceiver station are a mirror-image of the measurements from the second wireless transceiver station to the first wireless transceiver station, it is double-sided in that always two transceiver stations are used for ranging measurement, and it is two-way in that a data packet (called a data or standard packet) and an acknowledge packet (Ack) are transmitted between the wireless stations.
The data packet is transmitted from the first wireless transceiver station, e.g. blind node 10, to the second wireless transceiver station, e.g. one of the stations 20, 21, 22, 23. The time difference from when it was sent from the transmitter and received at the receiver is known as the signal propagation delay. The second wireless transceiver station will then acknowledge the reception by sending an acknowledgement packet back to the first wireless transceiver. The time required to process the incoming packet, generate the acknowledgement packet and prepare it for transmission is known as the processing delay Tprocessing. Upon reception of the acknowledgement packet, the first wireless transceiver will measure the time required between the transmission of the first data packet and the reception of the acknowledgement. This time is known as the two way process delay (Tround in
To increase the accuracy of the range calculation and to decrease the impact of crystal error on each station, the same procedure is repeated by the second wireless transceiver station sending a data packet to the first wireless transceiver station, and the first wireless station acknowledging the reception.
At the end of the procedure or during any transmission of a data packet, either the first wireless transceiver station either the second wireless transceiver station sends to the other station the time measured for the two way process delay and the processing delay. The two way process delay and the processing delay can then be used in an algorithm to calculate the range (distance) between the two stations.
At the end of the procedure, two range values are determined and an average of the two can be used to achieve a fairly accurate distance measurement between these two stations.
It is the object of the present invention to propose a method for carrying out a Two Way Ranging method which is energy saving, particularly on the side of the blind node, and only requires a simple infrastructure and less communication traffic.
This object is achieved by providing a method as claimed in the independent claim.
Preferred embodiments and advantageous features of the invention are disclosed in the dependent claims.
The subject of this invention is a node (blind node) in a real time location system whose position is not known in a certain area. This blind node will initiate a Two Way Ranging procedure preferably with several other nodes, reference or mobile nodes, whose positions are known in order to determine its location. In TWR methods known in the prior art the TWR procedure was always initiated by a reference node. Thus, the blind node had to be always in an active state all the time to listen to messages sent by the reference nodes. This led to high power consumption on the side of the blind node.
The method according to the present invention has several advantages compared to conventional TWR and SDS TWR methods known in the art. The advantages can be achieved only when the blind node initiates the TWR procedure as taught by the present invention.
First, the method increases battery lifetime, particularly on the side of the blind node. The blind node which is, in most of the times, the only battery power device, is only activated when it requires a TWR. Hence, considerably longer battery life time can be achieved compared to a solution where the blind node needs to listen periodically if TWR is necessary.
Second, there is no synchronization needed between the reference nodes. Using the blind node which requires TWR as an initiator combined with the process of TWR allows an operation without any synchronization between different reference nodes. This results in a simpler infrastructure without cabling between reference nodes and less wireless communication if the synchronization is done wirelessly.
Third, the method allows a TWR on demand. As the process is initiated by the blind node which requires TWR, it is possible to realize TWR on demand based on an event at any point in time. The event can be sensor based (push button, motion, temperature, humidity, etc others more . . . ), communication based (using other frequencies like LF, UHF or microwave or time based.
Forth, the blind node can display information based on distance. All the information is gathered at the blind node (initiator); therefore it is possible to provide information to a user using an embedded display. It is also possible to attract user attention using an indicator like LED or buzzer if a distance to a reference or mobile node reached a triggered value or if the node is accessed. It is mandatory for pick by light applications.
Fifth, the position of the blind node can be calculated “on board”. All the information is gathered at the initiator of the TWR (blind node). Therefore, it is possible to calculate the position of the initiator directly on it if it is required.
The TWR process is initiated by the blind node which requires or wants to perform a TWR or localization. For this, the initiating node (blind node) sends a broadcast packet to all the reference or mobile nodes in its range. The blind node will switch to reception mode just after the broadcast transmission to wait for an answer of the reference or mobile nodes. The initiator will wait for a defined amount of time. While the initiator is in receiving mode, the reference or mobile nodes are able to:
The interference between mobile or reference node packets can be avoided by using random time slots for sending the data packets or listening to the media before the transmission. It is also possible to assign time slots to the mobile or reference nodes. The assignment can be defined during configuration or during a synchronization process between reference or mobile nodes. The TWR could be complemented by a measurement of the RSSI or the Angle of Arrival.
A preferred embodiment of the invention using SDS TWR procedures is described below.
The method according to the invention is based preferably on a SDS TWR procedure to locate a blind node 10, as it is described above.
According to the invention, the blind node 10 initiates the ranging process rather than any of the reference or mobile nodes 20-23. As shown in
For the blind node 10 (initiator) point of view, the initialization is described hereafter:
Two possibilities are then available to perform a SDS TWR. The solution chosen must be defined before the start of the process, either by configuration or as a parameter in the broadcast packet.
In a first embodiment of invention, the reference or mobile nodes 20, 21, 22, 23 indicate their presence in response to an ID broadcast by the blind node 10.
In this mode, depicted in
The reference or mobile nodes 20, 21, 22, 23 have to send their packet only once and during a time slot. The time slot selected is chosen using a random function, which will not be defined here, or can be assigned during configuration of the system. Reference or mobile nodes 20, 21, 22, 23 are not allowed to transmit data packets to indicate their presence to the initiator after the last time slot.
At the end of the receiving period, the blind node 10 (initiator) is able to perform SDS TWR with the reference or mobile nodes 20, 21, 22, 23 which replied to its broadcast. A selection of the reference or mobile node can be realized on the initiator's side if it received more replies than defined in its configuration. To realize localization only three reference or mobile nodes 20, 21, 22, 23 are necessary but more can be used for redundancy or accuracy purpose.
The selection of reference or mobile nodes to be used for SDS TWR can be done on different metrics like RSSI, angle of arrival, etc. The whole list of metrics available will not be defined here.
After the selection of reference or mobile nodes to be used for SDS TWR, the blind node 10 will perform SDS TWR with the reference or mobile nodes 20-23 selected as described hereafter with reference to
The blind node 10 carries out a first SDS TWR with a first reference or mobile node 20. For this, the blind node 10 sends a first data packet 1 to the node 20. The difference in time from when it was sent from the blind node 10 and received at the reference node 20 is known as the signal propagation delay. The reference node 20 now will then acknowledge receipt by sending an acknowledgement packet R1 back to the blind node 10. The time to process the incoming data packet, generate the acknowledgement packet R1 (Ack packet), and prepare it for transmission is known as processing delay. Upon reception of the acknowledgement packet, the blind node 10 will measure the time required between the transmission of the first data packet and the reception of the acknowledgement packet R1. This time is known as the two way process delay Tround. The two way process delay can also be described as the sum of the signal propagation delays between both transceiver stations and the processing delay. To increase the accuracy of the range calculation and to decrease the impact of crystal error on each station, the same procedure is repeated by the reference node 20 sending a second data packet 2 to the blind node 10, and the blind node 10 sending a second acknowledgement packet R2 to the reference node 20. At the end of the procedure or during any transmission of a data packet, either the blind node 10 or the reference node 20 sends to the other station the time measured for the two way process delay and the processing delay. The two way process delay and the processing delay can then be used in an algorithm to calculate the range (distance) between the two nodes 10, 20. At the end of the procedure, two range values are determined and an average of the two can be used to achieve a fairly accurate distance measurement between these two stations.
The same procedure is repeated between the blind node 10 and the remaining reference nodes 21, 22, 23.
The blind node 10 (initiator) receives all the measurements required to calculate its distance to all the reference or mobile nodes 20, 21, 22, 23 it realized SDS TWR with. The initiator is able to calculate the distances to the reference nodes and might be able to define its location relative to the reference nodes.
The initiator is also able to broadcast or send all the information gathered (distance and/or measurement) to any reference of mobile node in its range.
Considering that collision may occur when the reference or mobile nodes 20, 21, 22, 23 send back their ID or that some reference or mobile nodes might not be selected based on the metrics, the reference or mobile nodes should be able to receive other broadcast from initiators.
If not enough reference or mobile node send their replies to the blind node 10, it can decide not to realize the SDS TWR.
In a second embodiment of the invention, the reference or mobile nodes start SDS TWR upon reception of a broadcast packet from the blind node 10 (initiator). This is shown in
The reference or mobile nodes 20-23 should select their time slot for transmission using a random time or it can be assigned during configuration. The random function will not be described here. The reference or mobile nodes 20-23 are not allowed to start a SDS TWR with the initiator after the last time slot.
One of the reference nodes 20-23, for example the reference node 20, carries out a first SDS TWR with the blind node 10. For this, the reference node 20 sends a first data packet 1 to the blind node 10. In consequence, as shown in
The same procedure is repeated between the other reference nodes 21-23 and the blind node 10.
During the SDS TWR procedure the blind node 10 (initiator) receives all the measurements required to calculate its distance to all the reference or mobile nodes 20, 21, 22, 23
To save battery (if the nodes are battery powered), the nodes should be able to go into low power mode during the main slot if they are not performing SDS TWR. The blind node 10 (initiator) should be able to go to low power mode if it does not receive the start of a SDS TWR until the end of the receiving time slot. The blind node 10 should go to active mode for the next time slot.
At the end or during the SDS TWR procedure, the reference or mobile nodes 20-23 must send back their measured delays to the initiator 10.
At the end of the process, the initiator 10 gathers all the measurement required to calculate its distance to all the reference or mobile nodes 20-23 it realized SDS TWR with. The initiator 10 (blind node) is able to calculate the distances to the reference nodes and might be able to define its location relative to the reference nodes.
The initiator 10 is also able to broadcast or send all the information gathered (distance and/or measurement) to any reference of mobile node in its range.
Precision on Mobile Nodes
Definitions
Number | Date | Country | Kind |
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08005945.4 | Mar 2008 | EP | regional |