GB2522409A

GB2522409A - Method, device, and computer program for selecting one communication mode among a plurality in a system comprising a mobile node and fixed nodes

Info

Publication number: GB2522409A
Application number: GB1400660.5A
Authority: GB
Inventors: Francois Thoumy; Mickaã L Lorgeoux; Alain Caillerie
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2014-01-15
Filing date: 2014-01-15
Publication date: 2015-07-29
Anticipated expiration: 2034-01-15
Also published as: GB201400660D0; GB2522409B

Abstract

Wireless transmission of data between fixed devices and a mobile device is performed using one of a first TDMA mode in which a copy of data is transmitted sequentially by the fixed devices to the mobile device and a second TDMA mode in which the data is transmitted by a selected subset of the fixed devices. The mode is selected on the basis of variations in communication conditions between the fixed devices and the mobile device. Evaluation signals are transmitted from the mobile device at a selected transmission rate when there is no data for transfer and a parameter of the signals determined at the fixed devices e.g. RSSI values. If the RSSI values are stable, indicating that the mobile device is stationary, the second mode may be selected and a subset of fixed devices chosen. The evaluation signals may represent dummy data. The system may be a Wireless Personal Area Network in which the mobile device is a camera situated on a robot arm (figure 1).

Description

METHOD, DEVICE, AND COMPUTER PROGRAM FOR SELECTING ONE

COMMUNICATION MODE AMONG A PLURALITY IN A SYSTEM COMPRISING A

MOBILE NODE AND FIXED NODES

FIELD OF THE INVENTION

The invention generally relates to the field of data communication between nodes of a communication system of the Personal Area Network (PAN) type. More particularly, the invention concerns a method, a device, and a computer program for selecting one communication mode among a plurality of communication modes, in a communication system comprising a mobile node and fixed nodes, for exchanging data between the mobile node and one of the fixed nodes, the communication system being of the Wireless Personal Area Network (W-PAN) type.

BACKGROUND OF THE INVENTION

Wireless audio and video applications are now increasingly numerous and require ever higher data bitrates of the order of some Gigabits per second (3bps) and an increasingly higher quality of service. Communication networks of the W-FAN type are particularly well suited to this type of application.

It is to be recalled that a communication network of the W-PAN type has the particularity of using millimeter wave frequencies, typically in the 60 GHz band, for interconnection of nodes connected to the communication network. At those frequencies, electromagnetic signals have a behavior close to that of light and so, preferably require a direct transmission, without reflection, called Line of Sight (LOS).

The spread of these waves is characterized by rapid attenuation of the signal strength.

The authorized band around a carrier frequency of 60GHz offers a wide bandwidth thus enabling the transport of a large quantity of data. Moreover, the radio range of such systems is limited to about ten meters, enabling re-utilization of the frequencies in different communication systems.

A particular application of a communication network of the W-PAN type is directed to a visual control system comprising at least one movable camera, for example a camera mounted on a robot arm, that is used to control an industrial process.

In such an application, a mobile emitting and receiving node, associated with the movable camera, is typically wirelessly communicatively coupled to one or more fixed nodes for receiving control commands and transmitting images (generally high definition images). After an image has been acquired, it is transmitted to a control system in relation with a "robot brain", from the mobile node via one or more fixed nodes. The fixed nodes are typically connected by wires to the control system.

It is to be noted that the transmission of an image from the mobile node to one or more fixed nodes preferably occurs when the robot arm is in a stable position.

After the image has been transmitted, the robot arm may move to a new position and acquire another image.

The use of several fixed nodes generally improves the reliability of the image transmission between the mobile node and the control system. Indeed, arms could cut the wireless data communication paths used and, for particular positions of the mobile node, the shape of the antenna used within the mobile and/or fixed nodes may not permit a given position of a fixed node to be reached. Therefore, increasing the number of fixed nodes increases the probability that at least one fixed node is enabled to receive a signal transmitted by the mobile node.

Regarding data transmission from the mobile node to the control system, all the fixed nodes which are in the transmission space covered by the antenna of the mobile node, and not shadowed by obstacles, can simultaneously receive signals from the mobile node and transmit the received signals to the control system.

Conversely, regarding data transmission from the control system to the mobile node, the same signals can be transmitted by several fixed nodes to provide a reliable transmission. However, the fixed nodes shall not wirelessly transmit signals simultaneously in order to prevent fading of the signals received by the mobile node due to combination of multiple emitted signals. Accordingly, as long as an effective path is not known, the signals have to be transmitted sequentially by each fixed node or set of fixed nodes.

To identify an effective communication path, a path discovery mechanism is advantageously used by the control system to select one or more fixed nodes that can be used to efficiently transmit data from the control system to the mobile node so as to avoid multiple attempts for transmitting the same data.

To make it possible to use such a communication mode according to which a single selected fixed node (or a limited number of selected fixed nodes) transmits data to the mobile node, it is necessary, during the setup of the communication system, to position the fixed nodes in such a way that at least one fixed node is always in line of sight with the mobile node, regardless of the position of the mobile node and of the other mobile elements (e.g. robot arms).

Wireless data communication between the mobile node and the fixed nodes is advantageously based on a Time Division Multiple Access (TDMA) scheme according to which a time slot is sequentially given to each data emitter of the communication system.

More precisely, wireless data communication between the mobile node and the fixed nodes is advantageously based on two different TDMA communication modes, a first TDMA communication mode being used for exchanging data when the mobile node is moving and a second TDMA communication mode being used for exchanging data when the mobile node is in a stationary state.

According to the first TDMA communication mode, each of the messages addressed by the control system to the mobile node is transmitted to all the fixed nodes and then, it is sequentially sent by each fixed node. Conversely, each item of the data addressed by the mobile node to the control system is simultaneously sent to all the fixed nodes which transfer them to the control system.

It is to be noted that a high latency results from the sequential transmission of the same message via several fixed nodes.

According to the second TDMA communication mode, each of the messages addressed by the control system to the mobile node is transmitted to a selected fixed node or to a set of selected fixed nodes (this or these node(s) correspond(s) to the fixed node(s) selected by the path discovery mechanism used) and then, it is sequentially sent by the selected fixed node(s). Conversely, each item of the data addressed by the mobile node to the control system is simultaneously sent to all the fixed nodes which transfer them to the control system (as done according to the first TDMA communication mode).

Therefore, the first TDMA communication mode can be referred to as a high latency TDMA communication mode in contrast to the second TDMA communication mode which can be referred to as a low latency TDMA communication mode.

One of the main difficulties encountered when implementing a communication network of the W-FAN type based on several TDMA communication modes is the determination of the optimal time and conditions to switch from one TDMA communication mode to another TDMA communication mode.

US Patent Application No. 2013/0100822 discloses a method for data communication between a first node and a plurality of second nodes in a communication network. The disclosed method is based on the use of a first uplink mode according to which a first message is sequentially transmitted from several second nodes to the first node and a second uplink mode according to which a second message is sent from a subset of nodes to the first node. A first receiving node evaluates communication path information based on messages received in the first uplink mode and returns this information via an acknowledgement to allow the subset to be selected.

This document does not provide any condition for switching from the first uplink mode to the second uplink mode.

US Patent Application No. 2009/0143024 discloses a method of determining the instant of consideration of a modification of at least one reception condition for signals in a communication system comprising at least two communication devices. The method comprises the steps, executed by a communication device, of detecting at least one modification of the reception conditions for signals in the communication system, determining the instant at which at least one other communication device will have detected the modification of the reception conditions for the signals, and of establishing an operating mode of the first communication device as a function of the instant determined.

Even if the solutions disclosed within US Patent Application No. 201 3/0100822 and 2009/0143024 can be used for determining the time and conditions to switch from one communication mode to another, there is a continuous need to improve such solutions.

Furthermore, there is a need to optimize the latency of the communication network and to take into account the energy consumption.

SUMMARY OF THE INVENTION

Faced with these constraints, the inventors provide a method, a device, and a computer program for optimizing the selection of a communication mode used for wireless transmission of data between fixed communication devices and a mobile communication device.

It is a broad object of the invention to remedy the shortcomings of the prior art as described above.

According to a first aspect of the invention there is provided a method for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, the method comprising: -continually evaluating variations of communication conditions between each of the fixed communication devices and the mobile communication device, the evaluation step comprising a step of estimating a transmission rate and a step of emitting at least one evaluation signal at the estimated transmission rate, the evaluation signal being emitted when no data are to be transferred between the mobile communication device and the fixed communication devices; and -selecting one of the first and the second communication modes as a function of evaluated variations of the evaluated variations of communication conditions.

According to the method of the invention, communication conditions are continuously monitored to allow selecting an appropriate communication mode, while reducing energy consumption. Since the invention enables switching and remaining in a low latency communication mode as long as the network conditions permit it, without loss of data and with optimization of the energy consumption, wireless network system latency is optimized.

Moreover, switching from one communication mode to another communication mode can be done efficiently, at the right time, without losing data, by detecting the mobile communication device motion based, for example, on the analysis of the variations of the received signal strength signals measured at the inputs of the fixed communication devices when the mobile communication device transmits data.

Analyzing the variations of received signal strength signals measured at the inputs of several fixed communication devices makes possible the motion detection whatever the movement direction is.

In an embodiment, the evaluation signal represents dummy data that may comprise iesponse messages to dummy received messages.

In an embodiment, the evaluation signal is emitted by the mobile communication device.

In an embodiment, the emitted evaluation signal is used to evaluate the variations of communication conditions between each of the fixed communication devices and the mobile communication device.

In an embodiment, the first communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a movement of the mobile communication device.

In an embodiment, the second communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a stationary state of the mobile communication device and when at least one reliable communication path can be set between at least one fixed communication device and the mobile communication device.

In an embodiment, the transmission rate is estimated as a function of the communication mode used.

In an embodiment, the transmission rate is estimated as a function of a movement of the mobile communication device, the movement of the mobile communication device being determined as a function of evaluated variations of the evaluated variations of communication conditions.

In an embodiment, the movement of the mobile communication device is determined as a function of evaluated variations of the evaluated variations of communication conditions associated with several fixed communication devices.

In an embodiment, the transmission rate is estimated as a function of a displacement speed of the mobile communication device, the displacement speed of the mobile communication device being estimated as a function of evaluated variations of the evaluated variations of communication conditions.

In an embodiment, the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise a variation of a received signal strength indicator.

In an embodiment, the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise an indication of stability of the received signal strength indicator.

A second aspect of the invention provides a computer program product for a programmable apparatus, the computer program product comprising instructions for carrying out each step of the method described above when the program is loaded and executed by a programmable apparatus.

According to the computer program product of the invention, communication conditions are continuously monitored to allow selecting an appropriate communication mode, while reducing energy consumption. Since the invention enables switching and remaining in a low latency communication mode as long as the network conditions permit it, without loss of data and with optimization of the energy consumption, wireless network system latency is optimized.

A third aspect of the invention provides a computer-readable storage medium storing instructions of a computer program for implementing the method described above.

According to the computer-readable storage medium of the invention, communication conditions are continuously monitored to allow selecting an appropriate communication mode, while reducing energy consumption. Since the invention enables switching and remaining in a low latency communication mode as long as the network conditions permit it, without loss of data and with optimization of the energy consumption, wireless network system latency is optimized.

A fourth aspect of the invention provides a device for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, the device comprising at least one microprocessor configured for carrying out the steps of: -continually evaluating variations of communication conditions between each of the fixed communication devices and the mobile communication device, the evaluation step comprising a step of estimating a transmission rate and a step of emitting at least one evaluation signal at the estimated transmission rate, the evaluation signal being emitted when no data are to be transferred between the mobile communication device and the fixed communication devices; and -selecting one of the first and the second communication modes as a function of evaluated variations of the evaluated variations of communication conditions.

According to the device of the invention, communication conditions are continuously monitored to allow selecting an appropriate communication mode, while reducing energy consumption. Since the invention enables switching and remaining in a low latency communication mode as long as the network conditions permit it, without loss of data and with optimization of the energy consumption, wireless network system latency is optimized.

In an embodiment, the at least one microprocessor is further configured so that the evaluation signal represents dummy data.

In an embodiment, the at least one microprocessor is further configured so that the evaluation signal is emitted by the mobile communication device.

In an embodiment, the at least one microprocessor is further configured so that the first communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a movement of the mobile communication device.

In an embodiment, the at least one microprocessor is further configured so that the second communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a stationary state of the mobile communication device and when at least one reliable communication path can be set between at least one fixed communication device and the mobile communication device.

In an embodiment, the at least one microprocessor is further configured so that the transmission rate is estimated as a function of the communication mode used.

In an embodiment, the at least one microprocessor is further configured so that the transmission rate is estimated as a function of a movement of the mobile communication device, the movement of the mobile communication device being determined as a function of evaluated variations of the evaluated variations of communication conditions.

In an embodiment, the at least one microprocessor is further configured so that the movement of the mobile communication device is determined as a function of evaluated variations of the evaluated variations of communication conditions associated with several fixed communication devices.

In an embodiment, the at least one microprocessor is further configured so that the transmission rate is estimated as a function of a displacement speed of the mobile communication device, the displacement speed of the mobile communication device being estimated as a function of evaluated variations of the evaluated variations of communication conditions.

In an embodiment, the at least one microprocessor is further configured so that the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise a variation of a received signal strength indicator.

In an embodiment, the at least one microprocessor is further configured so that the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise an indication of stability of the received signal strength indicator.

Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g. a microwave or RF signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will become apparent to those skilled in the art upon examination of the drawings and detailed description. It is intended that any additional advantages be incorporated herein.

Embodiments of the invention will now be described, by way of example only, and with reterence to the following drawings in which: Figure 1 illustrates an example of a system comprising one movable camera associated with a mobile node that can be wirelessly connected to fixed nodes that are in turn connected to a control system, wherein an embodiment of the invention may be implemented; Figure 2, comprising Figures 2a, 2b, and 2c, illustrates data streams for exchanging data between a mobile node and a control system, according to a first TDMA communication mode, in the system illustrated in Figure 1; Figure 3, comprising Figures 3a, 3b, and 3c, illustrates data streams for exchanging data between a mobile node and a control system, according to a second TDMA communication mode, in the system illustrated in Figure 1; Figure 4 illustrates an example of the use of the first and second TDMA communication modes for exchanging data between a control system and a mobile node according to an embodiment of the invention; Figure 5 illustrates an example of a flowchart of a method according to a first embodiment of the invention for determining optimal time and conditions for switching from a first TDMA communication mode to a second TDMA communication mode while optimizing the energy consumption; Figure 6 illustrates an example of a flowchart of a method according to a second embodiment of the invention for determining optimal time and conditions for switching from a first TDMA communication mode to a second TDMA communication mode while optimizing the energy consumption; Figure 7 illustrates advantages in using measurements (e.g. RSSI values) obtained from several fixed nodes to detect the motion of a mobile node; and Figure 8 represents a block diagram of a computer device in which steps of one or more embodiments may be implemented.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to a general embodiment of the invention, switching from one communication mode to another communication mode for exchanging data between a mobile node and one or several fixed nodes of a plurality of fixed nodes is done as a function of communication conditions of data communication between the mobile node and the fixed nodes and of variations of these communication conditions.

The communication modes can be TDMA modes. Communication conditions may be determined as a function of the detection of the motion of the mobile node, of the transmission of dummy data at a variable communication rate, and of the output of a path discovery mechanism allowing the identification of at least one fixed node for exchanging data with the mobile node.

Figure 1 illustrates an example of a system comprising a movable camera associated with a mobile node that can be wirelessly connected to fixed nodes that are in turn connected to a control system, wherein an embodiment of the invention may be implemented.

As illustrated, the system of the given example comprises an industrial robot having an arm 110 and one or more arms 120. One of the ends of arm 110 is equipped with at least one image capture sensor such as a video camera or a still image camera. Arm 110 further comprises a node 130, referred to as mobile node 130, connected to the image capture sensor. Mobile node 130 is preferably located near the image capture sensor. It allows wireless transmission of captured images to fixed nodes generically referenced 140 (according to a particular embodiment, the only communication means of mobile node 130 are wireless communication means). Power is supplied to mobile node 130 via robot arm 110.

The ability of robot arm 110 to move enables the image capture sensor associated with mobile node 130 to be placed at a good position for observation (preferably at the best position) of features to be observed (e.g. an operation carried out by arms 120).

As illustrated, several fixed nodes 140a to 140f are used so that the captured images can always be sent from the mobile node to one or several fixed nodes whatever the position of the mobile node. The positions of the fixed nodes in robot working space 100 are determined so that at least one fixed node is in the communication coverage area of mobile node 130, that is to say, typically, in line of sight of the mobile node.

For the sake of illustration six fixed nodes are used. However, the number of fixed nodes is not limited to six. Less or more fixed nodes can be used. The number of fixed nodes is preferably determined as a function of the cost of the fixed nodes, of communication efficiency regarding shadowing, of the size and of the configuration of the robot working space, and of the possible movements of the mobile elements.

Another advantage of using several fixed nodes is directed to the quality of data transmission: the global error rate is lower when the transmission can be performed with spatial diversity (it means that data are simultaneously transmitted to several destinations through different paths) than when data can only be sent along a single communication path.

As illustrated, all the fixed nodes are connected to a control system 150 via communication links 180a, 180b, 180c, 180d, 180e, and 180f respectively. According to a particular embodiment, a modulator/demodulator is included in each fixed node and the interconnection between the fixed nodes and the control system is performed thanks to digital high speed interfaces using differential signals. These high speed wired interfaces are set according to a point-to-point scheme or to a bus oriented scheme. They can be compliant with the l000BaseT standard or with any other standard. They can also be of proprietary type. They can use any physical medium allowing high data bitrates such as twisted pairs, optical fiber, coaxial cable, or wireless. Other embodiments can be envisaged where the interconnection physical medium is analogue and/or modulators/demodulators are not included in the fixed nodes but in the control system. Of course, such a list of interconnection possibilities is not exhaustive.

Control system 150 acts as a central unit and is in tight relationship with a robot brain 160.

Since an object of the system illustrated in Figure 1 can be directed to wirelessly transmitting high definition images, 60GHz millimeter waves can be used for this high bandwidth wireless transmission.

However, as mentioned above, one of the main drawbacks of the 60GHz is its line of sight propagation characteristics: obstacles can cut the radiofrequency beams (e.g. radiofrequency beam 170) between the emitter and the receiver.

For the sake of illustration, arm 120 that can be used to manipulate objects (and which is thus not directly involved in wireless communications) can cut the radiofrequency beam transmitted from mobile node 130 to one or more fixed nodes 140. In other words, arm 120 can be in the line of sight between the mobile node and one or more fixed nodes. Therefore, arm 120 can disturb the wireless communication between mobile node 130 and one of fixed nodes l4Oato 140f.

Figure 2, comprising Figures 2a, 2b, and 2c, illustrates data streams for exchanging data between a mobile node and a control system, according to a first TDMA communication mode, in the system illustrated in Figure 1.

Data exchanged in the first TDMA communication mode are typically control data (i.e. not image data).

More precisely, Figure 2a illustrates data streams when data are sent from the control system 150 to mobile node 130 via fixed nodes 140a to 140f. For the sake of illustration, solid arrows represent data streams transmitted over wire connections (i.e. between control system 150 and fixed nodes 140) while dotted arrows represent data streams transmitted over wireless connections (i.e. between fixed nodes 140 and mobile node 130).

In such a case, data are generated or received by control system 150 and transmitted by it to fixed nodes 140a, 140b, 140c, 140d, 140e, and 140f. As represented with aligned black circles 200a to 200f representing messages, data are almost simultaneously transmitted to fixed nodes 140. Next, each of these fixed nodes successively transmits the received data to mobile node 130 according to a time schedule given by control system 150, as represented with unaligned black circles 205a to 20Sf.

Figure 2b illustrates data streams when data are sent from mobile node to control system 150 via fixed nodes 140a to 140f. Again, for the sake of illustration, dotted arrows represent data streams transmitted over wireless connections (i.e. between mobile node 130 and fixed nodes 140) while solid arrows represent data streams transmitted over wire connections (i.e. between fixed nodes 140 and control system 150).

Data transmitted by mobile node 130 may represent, for example, an acknowledgement message confirming the correct reception of a previous message received by the mobile node or information characterizing the quality and/or level of a signal carrying received messages. They are transmitted only once and thus, they should be expected to be received successfully by at least one fixed node.

These data are transmitted by mobile node 130 to fixed nodes 140a, 14Db, 140c, 140d, 140e, and 140f. As represented with black circles 210 representing messages, data are indiscriminately simultaneously transmitted to all fixed nodes 140 according to a time schedule determined by control system 150 and previously transmitted via the fixed nodes. Upon reception by a fixed node, received data are transmitted to control system 150 as represented with black circle 215. It is to be noted that for the reasons set forth above, the data transmitted by mobile node 130 is typically received by a single fixed node (e.g. fixed node 14Db as illustrated) or by a few of the fixed nodes.

Figure 2c illustrates an example of time slots enabling wireless data exchanges between mobile node 130 and fixed nodes 140 according to the first TDMA communication mode.

Time slots referenced a, b, c, d, e, and f are used by fixed nodes 140a, 14Db, 140c, 140d, 140e, and 140f, respectively, to transmit data from control system to mobile node 130. The time slot comprising diagonal pattern is used by mobile node 130 to transmit data to control system 150.

Therefore, a data exchange cycle denoted SF (Super Frame) comprises as many time slots (T) as the number of fixed nodes (so that each fixed node can transmit an item of data per transmission cycle) plus one (so that the mobile node can transmit an item of data per transmission cycle), that is to say seven time slots (7T) in the given

example.

Figure 3, comprising Figures 3a, 3b, and 3c, illustrates data streams for exchanging data between a mobile node and a control system, according to a second TDMA communication mode, in the system illustrated in Figure 1.

Data exchanged in the second TDMA communication mode are typically control data and useful data (i.e. image data).

Figure 3a illustrates data streams when data are sent from control system to mobile node 130 via fixed nodes 140a to 140f. Again, for the sake of illustration, solid arrows represent data streams transmitted over wire connections (i.e. between control system 150 and fixed nodes 140) while dotted arrows represent data streams transmitted over wireless connections (i.e. between fixed nodes 140 and mobile node 130).

In such a case, data that are typically generated by control system 150 in response to a request received from robot brain 160 are transmitted to one or several of fixed nodes 140a, 140b, 140c, 140d, 140e, and 140f which have been selected according to a path discovery mechanism as the best fixed node(s) to transmit data from the control system to the mobile node. In the illustrated example, fixed node 140e is the selected fixed node to which data are transmitted from control system 150 (e.g. message 300) and from which data are transmitted to mobile node 130 (message 305).

Figure 3b illustrates data streams when data are sent from mobile node 130 to control system 150 via fixed nodes 140a to 140f. Again, for the sake of illustration, dotted arrows represent data streams transmitted over wireless connections (i.e. between mobile node 130 and fixed nodes 140) while solid arrows represent data streams transmitted over wire connections (i.e. between fixed nodes 140 and control system 150).

Data transmitted by mobile node 130 are typically image data. They are transmitted only once and thus, they should be expected to be received successfully by at least one fixed node.

These data are transmitted by mobile node 130 to fixed nodes 140a, 14Db, 140c, 140d, 140e, and 140f. As represented with black circles 310 representing data messages, data are indiscriminately simultaneously transmitted to all fixed nodes 140 according to a time schedule determined by control system 150 and previously transmitted via the fixed nodes. Upon reception by a fixed node, received data are transmitted to control system 150 as represented with black circle 315. Again, it is noted that for the reasons set forth above, the data transmitted by mobile node 130 is typically received by a single fixed node (e.g. fixed node 140e as illustrated) or by a few of the fixed nodes.

The second TDMA communication mode for transmitting data from the mobile node to one or several fixed nodes is similar to the first TDMA communication mode for transmitting data from the mobile node to one or several fixed nodes.

Figure 3c illustrates an example of time slots enabling wireless data exchanges between mobile node 130 and fixed nodes 140 according to the second TDMA communication mode.

Since a single fixed node transmits data to the mobile node, a single time slot is required for transmitting data from the control system to the mobile nodes (time slot referenced e in the given example, corresponding to the transmission of data by fixed node 140e). Another time slot (illustrated with diagonal hatching) is used by mobile node 130 to transmit data to the control system.

Therefore, a data exchange cycle denoted SF (Super Frame) comprises a first time slot (T) so that one fixed node can transmit an item of data per transmission cycle and a second time slot so that the mobile node can transmit an item of data per transmission cycle, that is to say two time slots (21) in the given example.

Figure 4 illustrates an example of the use of the first and second TDMA communication modes for exchanging data between a control system and a mobile node according to an embodiment of the invention.

The sequence of time slots represented comprises a sequence of super frames (SF) the structure of which depends on the TDMA communication mode used.

The latter can be switched at the end of a super frame (i.e. at the end of a transmission cycle).

As illustrated, the sequence example comprises a first set 400 of super frames corresponding to the first TDMA communication mode (comprising super frames SF(n-2), SF(n-1), and SF(n)) and a second set 405 of super frames corresponding to the second IDMA communication mode (comprising super frames SF(n+1), SF(n+2), and SF(n+3)).

According to a particular embodiment, super frame SF(n-2) corresponds to the transmission of data by fixed nodes 140a to 140f enabling the mobile node to make measurements on the related received signals. Next, during the time slot of the following super frame SF(n-1) allowing the mobile node to transmit data to the fixed nodes (i.e. the slot having a diagonal pattern), the mobile node transmits the measurements to the control system which, in turn, transmits these measurements to a path discovery mechanism which selects a fixed node to be used for transmitting data from the control system to the mobile node.

An identifier of the selected fixed node is then transmifted to the mobile node during the following super frame SF(n) along with data enabling switching from the first TDMA communication mode (IDMA mode 1) to the second TDMA communication mode (TDMA mode 2). As described by reference to Figures 5 to 7, the control system analyses the RSSI (Received Signal Strength Indicator) of signals received by the fixed nodes from the mobile node to check that the mobile node is stationary and allows switching to the second TDMA communication mode.

Next, data are exchanged according to the second IDMA communication mode (super frame SF(n÷1) and the following) using the selected fixed nodes.

Therefore, according to the example illustrated in Figure 3, all the data transmitted from control system 150 to mobile node 130 are transmitted via fixed node 140e.

Figure 5 illustrates an example of a flowchart of a method according to a first embodiment of the invention for determining optimal time and conditions for switching from a first TDMA communication mode to a second IDMA communication mode while optimizing the energy consumption.

A first step (step 510) is directed to the initialization of the system and in particular to setting all communication parameters to default values.

Next, in a following step (step 515), a transmission rate of dummy data is set to a low value in order to minimize energy consumption. For the sake of illustration, such a low transmission rate can be chosen between a transmission of dummy data every ten super frames and a transmission of dummy data every super frame.

It is to be noted here that the transmission rate of dummy data corresponds to the transmission rate at which dummy data (for example random data or data corresponding to a previous acquired image) are transmitted from the mobile node to fixed nodes when no control or useful data are to be sent from the mobile node to the fixed nodes in the time slots assigned to the mobile node.

The transmission of dummy data when no control or useful data are to be sent from the mobile node to the fixed nodes is used by the control system to estimate the quality of the communication paths between the mobile node and the fixed nodes and more precisely the variation of the quality of the communication paths between the mobile node and the fixed nodes, as described herein below, to determine whether or not the mobile node is stationary. It can also be used to estimate motion and displacement speed of the mobile node as described by reference to Figures 6 and 7.

According to a particular embodiment, dummy data comprise response messages to dummy messages previously received from the control system.

Next, in step 520, the communication mode is set to the first TDMA communication mode and a path discovery mechanism is launched to allow identification of fixed nodes that can be used in the second TDMA communication mode for transmitting data to the mobile node. Simultaneously, at step 545, all fixed nodes start the analysis of the strength and/or the quality of the signals received from the mobile node (typically by determining the corresponding RSSI) and provide the obtained values (e.g. the RSSI values) to the control system.

For the sake of illustration, the strengths and/or the quality of the signals received from the mobile node are represented, in the following description, by the corresponding RSSI. However, it is to be understood that other parameters can be used.

The control system checks the stability of the RSSI values, for each fixed node, at step 525. Accordingly, the control system checks the variation of each RSSI value with time (from previous received measurements) to determine whether or not each RSSI value is stable. To that end! the variation of RSSI values is compared to predetermined thresholds, an RSSI value being stable if, for example, its variation is smaller than the predetermined threshold and being unstable if it is greater than this threshold.

If all RSSI values are stable (which can be understood as meaning that the mobile node is stationary), the algorithm is directed to step 530.

On the contrary, if at least one of the RSSI values is unstable (which may result from a displacement of the mobile node or from shadowing of communication lines of sight by obstacles), the algorithm is directed to step 550.

In step 530, the path discovery mechanism is used to identify the nodes that can be used to establish a reliable communication between the control system and the mobile node. As it is well known for the one skilled in the art, the identification of the best communication paths can be based on various indicators comprising the SNR (Signal to Noise Ratio), an FEC (Forward Error Correction) metric measurement or the BER (Bit Error Rate).

If the path discovery mechanism is able to identify at least one fixed node enabling the expected communication reliability to be attained, the algorithm is directed to step 535.

On the contrary, if it is not possible to identify at least one fixed node enabling the expected communication reliability to be attained (which means that the second (low latency) TDMA communication mode cannot be used), the algorithm returns to steps 520 and 545 (which are carried out simultaneously).

In step 535, the transmission rate of dummy data is set to a high value in order to enable quick detection of the moment at which the mobile node changes its position. For the sake of illustration, such a high transmission rate can be chosen between a transmission of dummy data every ten super frames and a transmission of dummy data every super frame.

In a following step (step 540), the communication mode is set to the second TDMA communication mode (TDMA mode 2) and one or several of the fixed nodes identified at step 530 are selected to be used for transmitting data from the control system to the mobile node. The selected fixed nodes are preferably the identified nodes providing the most reliable communication paths.

According to a particular embodiment, only one of the identified fixed nodes is selected to transmit data to the mobile node.

Next, the algorithm returns to step 545 wherein the strength and/or quality of the received signals is analyzed by the fixed nodes in order to detect any possible change of the position of the mobile node.

Accordingly, the second TDMA communication mode is used as much as possible.

At step 550 (to which the algorithm is directed if at least one of the RSSI values is detected as being unstable at step 525), the stability of RSSI values is checked again to determine whether or not all the RSSI values are unstable.

If all RSSI values are unstable (which means that the mobile node is moving), the algorithm returns to steps 520 and 545 (which are carried out simultaneously) so as to use the first IDMA communication mode again and to monitor the RSSI values.

On the contrary, if at least one RSSI value is stable, the algorithm is directed to step 555 where the TDMA communication mode used is determined.

If the current TDMA communication mode is the first TDMJA communication mode (TDMA mode 1), the algorithm returns to steps 520 and 545 (which are carried out simultaneously) in order to continue to use that TDMA communication mode. On the contrary, if the current TDMA mode is the second TDMA communication mode (TDMA mode 2), the algorithm is directed to step 560.

In step 560, it is checked whether or not an unstable RSSI value is related to the selected fixed nodes which are used to transmit data from the control system to the mobile node according to the second IDMA communication mode. If no unstable RSSI value is related to the selected fixed nodes, the algorithm is directed to step 545 (the fading of the received signal does not have any effect on data transfer in the current TDMA communication mode).

On the contrary, if an unstable RSSI value is related to the selected fixed nodes, the algorithm is directed to steps 520 and 545 (that are carried out simultaneously) in order to switch back to the first IDMA communication mode (ensuring no loss of data).

Figure 6 illustrates an example of a flowchart of a method according to a second embodiment of the invention for determining optimal time and conditions for switching from a first TDMA communication mode to a second IDMA communication mode while optimizing the energy consumption. According to this embodiment, optimization of energy consumption takes into account the estimated motion of the movable device.

Most of the steps represented in this flowchart are similar to the one described by reference to Figure 5 excepted two steps that are directed to changing the transmission rate of dummy data when the first IDMA communication mode is used. Steps 510' to 560' of Figure 6 are similar to steps 510 to 560 of Figure 5, respectively. Accordingly, steps 510' to 560' of Figure 6 are described by reference to steps 510 to 560 of FigureS.

The first TDMA communication mode being used at least during the motion of the mobile node, the transmission rate of the dummy data is advantageously modified as a function of the displacement speed of the mobile node.

Various cases exist with regard to the movement scenario and the expected behavior of the system.

According to a particular embodiment, it can be considered that if the displacement speed of the mobile node is low, it takes a long time before a stationary position is reached and, as a consequence, it is not useful to determine frequently its position (or at least to determine frequently the variation of its position). In other words, this means that it is possible to transmit dummy data less often than in an initial step.

According to another particular embodiment, it can be considered that if the displacement speed of the mobile node is low, it means that the arm is slowing down before stabilization and thus, it is important to detect its stabilization quickly Accordingly, it can be considered that it is important to determine frequently its position (or at least to determine frequently the variation of its position). In other words, this means that it is desirable to transmit dummy data more often than in an initial step.

Adjusting the transmission rate of dummy data with regard to the displacement speed of the mobile node enables a decrease in the amount of dummy data to be transmitted and thus, to save energy.

By comparison to the flowchart illustrated in Figure 5, the one illustrated in Figure 6 is modified so that after carrying out step 550' or 555', the algorithm is directed to new step 565 which is followed by step 570 whose output is directed to steps 520' and 545' (which are carried out simultaneously).

In step 565, the displacement speed of the mobile node is estimated on the basis of the variations of the RSSI values of the signals received by at least one fixed node when the mobile node is transmitting data. Indeed, as the time period between two consecutive RSSI measurements is known by the system carrying out the steps illustrated in Figure 6, it is possible to determine the relative speed of the mobile node by analyzing the time period between two predetermined RSSI values.

Once the displacement speed of the mobile node has been estimated, the transmission rate of dummy data is adjusted as a function of the estimated displacement speed in step 595. As mentioned above, different functions can be used depending on the context and needs.

For the sake of illustration, it can be considered that the maximum speed of the robot arm (that is to say of the mobile node) is 8.3 mIs, that the duration of each time slot is 3Ops, and that RSSI values are measured at each time slot allowing the mobile node to transmit data.

Therefore, according to the example of Figure 1, RSSI values are measured every seven time slots (that is to say every 210 p5) in the first TDMA communication mode and every two time slots (that is to say every 60 ps) in the second IDMA communication mode.

As a consequence, at a maximum transmission rate of dummy data, the accuracy of measurements of the mobile node (that is to say the maximum possible displacement of the mobile node between two measurements) is equal 1.7 mm (8.3 x 210.10).

If the mobile node is moving at a speed of 2 mIs, RSSI measurements and thus the transmission rate of dummy data can be reduced by a factor of four to keep the same accuracy. This means that if there is no data to transmit, energy consumption can be divided by four.

Figure 7 illustrates advantages in using measurements (e.g. RSSI values) obtained from several fixed nodes to detect the motion of a mobile node.

It is to be noted that although the illustration is based on a two dimensional space, for the sake of clarity, similar advantages exist in the case of a three dimensional space (which corresponds to the real case for a robot scenario).

The system illustrated in Figure 7 comprises one mobile node 130 and three fixed nodes 140a, 140b, and 140c. Mobile node 130 is located at a first position at time t, referenced 130(t) and ata different position at time f4-1, referenced 130(t+1).

The distances between mobile node 130 and fixed nodes 140a, 14Db, and 140c are represented for the two positions 130(t) and 130(t+1) of the mobile node: dl, d2, and d3 represent the distance between mobile node 130 and fixed nodes 140a, 14Db, and 140c, at time t, respectively, and d'l, d'2, and d'3 represent the distance between mobile node 130 and fixed nodes 140a, 14Db, and 140c, at time t+l, respectively.

If the mobile node follows a portion of a circular orbit with regard to fixed node 140a during its displacement, the distance between nodes 130 and 140b will not change during the movement (i.e. d'l = dl). Accordingly, the RSSI values measured at time t and t+l will be roughly the same.

During this movement, the distance between nodes 130 and 140a will slightly change due to the geometrical configuration (d'2 d2) but the distance between nodes 130 and 140c will significantly change (d'3!= d3) as illustrated.

Therefore, using several RSSI measurements from several fixed nodes positioned all around the frames of the robot is helpful for estimating the displacement of the mobile node and thus, its displacement speed.

Accordingly, step 565 in Figure 6 is advantageously based on the variations of the RSSI values of the signals received by several fixed nodes when the mobile node is transmitting data.

3D Figure 8 represents a block diagram of a computer device 800 in which steps of one or more embodiments may be implemented.

Preferably, the device 800 comprises a communication bus 802, a central processing unit (CPU) 804 capable of executing instructions from program ROM 806 on powering up of the device, and instructions relating to a software application from main memory 808 after the powering up. The main memory 808 is for example of Random Access Memory (RAM) type which functions as a working area of CPU 804 via the communication bus 802, and the memory capacity thereof can be expanded by an optional RAM connected to an expansion port (not illustrated). Instructions relating to the software application may be loaded into the main memory 808 from a hard-disk (HD) 810 or the program ROM 806 for example. Such software application, when executed by the CPU 804, causes the steps described with reference to Figure 2, 3, 5 or 6 to be performed in the computer device.

Reference numeral 812 is a network interface that allows the connection of the device 800 to the communication network 814. The software application when executed by the CPU 804 is adapted to react to requests received through the network interface and to provide data and requests via the network to other devices.

Reference numeral 816 represents user interfaces to display information to, and/or receive inputs from, a user.

It should be pointed out here that, as a variant, the device 800 for processing data can consist of one or more dedicated integrated circuits (ASICs) that are capable of implementing the method as described with reference to Figures 5 and 6.

Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations all of which, however, are included within the scope of protection of the invention as defined by the following claims.

Claims

CLAIMS1. A method for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, the method comprising: -continually evaluating variations of communication conditions between each of the fixed communication devices and the mobile communication device, the evaluation step comprising a step of estimating a transmission rate and a step of emitting at least one evaluation signal at the estimated transmission rate, the evaluation signal being emitted when no data are to be transferred between the mobile communication device and the fixed communication devices; and -selecting one of the first and the second communication modes as a function of evaluated variations of the evaluated variations of communication conditions.
2. The method of claim 1 wherein the evaluation signal represents dummy data.
3. The method of claim 2 wherein the dummy data comprise response messages to dummy received messages.
4. The method of claim 1 or claim 3 wherein the evaluation signal is emitted by the mobile communication device.
5. The method according to claim 1 wherein the emitted evaluation signal is used to evaluate the variations of communication conditions between each of the fixed communication devices and the mobile communication device.
6. The method according to claim 1 or claim 5 wherein the first communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a movement of the mobile communication device.
7. The method according to claim 1 or claim 5 wherein the second communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a stationary state of the mobile communication device and when at least one reliable communication path can be set between at least one fixed communication device and the mobile communication device.
8. The method according to claim 1 wherein the transmission rate is estimated as a function of the communication mode used.
9. The method according to claim 1 or claim 8 wherein the transmission rate is estimated as a function of a movement of the mobile communication device, the movement of the mobile communication device being determined as a function of evaluated variations of the evaluated variations of communication conditions.
10. The method according to claim 9 wherein the movement of the mobile communication device is determined as a function of evaluated variations of the evaluated variations of communication conditions associated with several fixed communication devices.
11. The method according to claim 1, 8, or 9 wherein the transmission rate is estimated as a function of a displacement speed of the mobile communication device, the displacement speed of the mobile communication device being estimated as a function of evaluated variations of the evaluated variations of communication conditions.
12. The method according to claim 1 wherein the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise a variation of a received signal strength indicator.
13. The method according to claim 12 wherein the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise an indication of stability of the received signal strength indicator.
14. A computer program product for a programmable apparatus, the computer program product comprising instructions for carrying out each step of the method according to any one of claims 1 to 13 when the program is loaded and executed by a programmable apparatus.
15. A computer-readable storage medium storing instructions of a computer program for implementing the method according to any one of claims ito 13.
16. A device for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, the device comprising at least one microprocessor configured for carrying out the steps of: -continually evaluating variations of communication conditions between each of the fixed communication devices and the mobile communication device, the evaluation step comprising a step of estimating a transmission rate and a step of emitting at least one evaluation signal at the estimated transmission rate, the evaluation signal being emitted when no data are to be transferred between the mobile communication device and the fixed communication devices; and -selecting one of the first and the second communication modes as a function of evaluated variations of the evaluated variations of communication conditions.
17. The device of claim 16 wherein the at least one microprocessor is further configured so that the evaluation signal represents dummy data.
18. The device of claim 16 wherein the at least one microprocessor is further configured so that the evaluation signal is emitted by the mobile communication device.
19. The device according to claim 16 wherein the at least one microprocessor is further configured so that the first communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a movement of the mobile communication device.
20. The device according to claim 16 wherein the at least one microprocessor is further configured so that the second communication mode is selected when evaluated variations of the evaluated variations of communication conditions characterize a stationary state of the mobile communication device and when at least one reliable communication path can be set between at least one fixed communication device and the mobile communication device.
21. The device according to claim 16 wherein the at least one microprocessor is further configured so that the transmission rate is estimated as a function of the communication mode used.
22. The device according to claim 1 or claim 21 wherein the at least one microprocessor is further configured so that the transmission rate is estimated as a function of a movement of the mobile communication device, the movement of the mobile communication device being determined as a function of evaluated variations of the evaluated variations of communication conditions.
23. The device according to claim 22 wherein the at least one microprocessor is further configured so that the movement of the mobile communication device is determined as a function of evaluated variations of the evaluated variations of communication conditions associated with several fixed communication devices.
24. The device according to claim 16, 22, or 23 wherein the at least one microprocessor is further configured so that the transmission rate is estimated as a function of a displacement speed of the mobile communication device, the displacement speed of the mobile communication device being estimated as a function of evaluated variations of the evaluated variations of communication conditions.
25. The device according to claim 16 wherein the at least one microprocessor is further configured so that the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise a variation of a received signal strength indicator.
26. The device according to claim 25 wherein the at least one microprocessor is further configured so that the evaluated variations of communication conditions between each of the fixed communication devices and the mobile communication device comprise an indication of stability of the received signal strength indicator.
27. A method for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, substantially as hereinbefore described with reference to, and as shown in Figures 5 and 6.
28. A device for wireless transmission of data between a plurality of fixed communication devices and a mobile communication device using one of a first communication mode in which a copy of each item of data to be transmitted by the fixed communication devices is transmitted sequentially by a first number of fixed communication devices to the mobile communication device and of a second communication mode in which each item of data to be transmitted by the fixed communication devices is transmitted by a second number of selected fixed communication devices to the mobile communication device, the second number being smaller than the first number, substantially as hereinbefore described with reference to, and as shown in Figures 2, 3, and 8.