Method of transmitting data frames in a wireless communication system and wireless access device

The present invention is especially adapted for being used in an industrial automation system. Industrial automation systems usually comprise a multiplicity of automation devices networked to one another via an industrial communication network and, within the scope of production or process automation, is used to control or regulate installations, machines or devices. On account of time-critical boundary conditions in technical systems automated using industrial automation systems, real-time communication protocols such as Profinet, Profibus or real-time Ethernet are predominantly used in industrial communication networks for communication between automation devices. Wireless communications systems are becoming increasingly more important in industrial automation systems and are therefore expected to provide for reliable data transmission.

Interruptions in communication connections between computer units of an industrial automation system or automation devices may result in undesirable or unnecessary repetition of the transmission of a service request. This causes additional utilization of communication connections of the industrial automation system, which may result in further system faults or errors. In addition, messages which have not been transmitted or have not been completely transmitted may prevent an industrial automation system from changing to or remaining in a safe operating state, for example. This may finally result in failure of a complete production installation and costly production downtime. A particular problem regularly results in industrial automation systems from message traffic with a comparatively large number of, but relatively short, messages, thus inten ^¬ sifying the above problems.

US 2006/0221993 Al discloses a WLAN network with a multiplicity of access points and a multiplicity of radio stations in the radio coverage area of the access points. An access point selects a free frequency from a multiplicity of available frequencies, on which it then communicates with the radio stations located within its radio coverage area. In order to prevent interference between the different access points in the network, the different access points communicate with their respectively assigned radio stations on different WLAN channels, which are assigned to different radio frequencies. In order to locate a suitable access point as an access network node, the radio stations in the wireless network regularly scan the different WLAN channels, that is the different WLAN frequencies, in order to discover, via the signals received, where a suitable access network node is located or whether a better access network node than the currently assigned access network node is connected in the wireless network.

In EP 1 944 996 A2 , a method for use in wireless regional area network (WRAN) systems is described, in which a WRAN cell is operated by a base station to wirelessly communicate with at least one consumer premise equipment. The base station is operated to determine a channel available for use. The determined channel is initialized to provide an in-band channel for data transmission. Simultaneously, spectrum management and spectrum sensing on a plurality of out-of-band channels and data transmission on the in-band channel is performed.

EP 2 365 643 Al relates to a radio station system for a wireless network with at least two access network nodes, in which a first node is assigned to the radio station system as an access network node in the wireless network. The radio station system is connected to the first access network node via an active radio link for transmitting user data. Furthermore, the radio station system comprises a first radio station, which is connected to the first access network node via the active radio link for exchanging the user data. Moreover, the radio station system comprises a second radio station, which is connected to the first radio station via a communication interface. During the existence of the active radio link between the first radio station and the first access network node, the second radio station transmits or exchanges no user data via a radio link with an access network node in the wireless network and temporarily identifies available access network nodes in the wireless network for the radio station system.

In EP 2 421 297 Al, a method for switching channels in a wireless communication network comprising at least one access point and a number of clients is disclosed. In a predetermined frequency range, the communication network as secondary user uses one of a number of possible channels for data communication. Data communication takes place between the access point and the number of clients via a first transmit /receive interface of the access point on a first channel of the possible channels. In parallel to the data communication, a check of the possible channels is made via the transmit /receive interface by a second trans ^¬ mit/receive interface of the access point. Thereby, free channels are determined that are not being used by any primary user. Furthermore, a switch of channel from the first channel to one of the free channels is carried out if a prioritized use of the first channel by a primary user is established by the access point of the communication network.

Wireless communication systems for industrial automation systems are usually optimized for reliable communication. Generally, a method to avoid interference issues may be creating a guideline that prohibits using potential WLAN interferers in areas where WLAN is used for critical applications. However, such a guideline would be rather a weak protection, if there is no automatism for enforcing the guideline.

The present invention is based on the object of specifying a method for transmitting messages in an wireless communication system which allows for efficiently avoiding radio interference issues, and of providing a communication device suitable for carrying out the method.

This object is achieved, according to the present invention, by a method having the features stated in claim 1 and by a wireless access device having the features stated in claim 10. Advan ^¬ tageous embodiments of the present invention are stated in the dependent claims.

In accordance with the method according to the invention, for transmitting data frames in a wireless communication system between at least one wireless access point or client and at least one mobile station, in which the wireless access point or client comprises a single radio interface, the single radio interface of the wireless access point or client is employed both for communicating with the mobile station and for running a spectrum analysis relating to a selected radio channel. By means of the spectrum analysis, interferences on the selected radio channel are determined. Access to the selected radio channel is requested by sending a ready to send data frame to a selected mobile station connected to the wireless access point or client. The ready to send data frame indicates a time interval the access is requested for by the wireless access point or client. Besides, the selected mobile station the ready to send data frame is sent to may also be a wireless access point. Preferably, the access to the selected radio channel is requested by the wireless access point or client . The ready to send data frame is acknowledged by sending a clear to send data frame from the selected mobile station to the wireless access point or client. Preferably, the clear to send data frame comprises an indication or confirmation, respectively of the time interval the access is requested for. Upon receipt of the clear to send data frame at the wireless access point or client, the wireless access point or client runs the spectrum analysis relating to the selected radio channel during the time interval. Moreover, the wireless access point or client is switched from a communication mode to a spectrum analyzer mode during the time interval.

In comparison to existing approaches, the present invention allows for a significantly cheaper solution by using just a single radio access point, preferably a single radio WLAN fat access point, running spectrum analysis virtually concurrently with regular communication. Particularly, this is done without losing data frames. By virtually continuously monitoring, guidelines or policies for using radio interface resources can be enforced efficiently due to an early recognition of fault causes and a respective data logging of spectrum analysis results.

According to an advantageous embodiment of the present invention, the selected radio channel is switched, an alarm is generated and a warning message, respectively is sent to a supervisory station, if the interferences on the selected radio channel are higher than a specified threshold. By doing so, an automatic enforcement of guidelines for avoiding radio interferences can be achieved. Alternatively or additionally, interferences on the selected radio channel may be classified by the wireless access point or client running the spectrum analysis. Moreover, spectrum analysis data may also be transmitted to a network management station, e.g. for tracking or identifying a potential interferer . According to another advantageous embodiment of the present invention, every mobile station in range of the wireless access point or client stops transmitting data frames on the selected channel during the time interval the access is requested for. Furthermore, the selected mobile station the ready to send data frame is sent to may be a virtual mobile station. In this case, sending the clear to send data frame from the selected mobile station to the wireless access point or client is suspended.

Preferably, both the wireless access point or client and the mobile stations connected to the wireless access are RTS/CTS capable for doing so. Thereby, a falsification of the spectrum analysis can be avoided effectively. RTS/CTS capability means that access to the selected radio channel is generally requested by sending a ready to send data frame from a sender to a recipient. In doing so, the ready to send data frame indicates a time interval the access is requested for by the sender. Furthermore, the wireless access point or client may be the sender or the recipient. Accordingly, the ready to send data frame is acknowledged by sending a clear to send data frame from the recipient to the sender. Consequently, upon receipt of the clear to send data frame at the sender, the selected radio channel is accessed for transmitting data frames by the sender during the time interval.

According to a preferred embodiment of the present invention, the selected radio channel is also used for communicating with the at least mobile station. Thus, the spectrum analysis directly relates to the radio channel employed by the wireless access point or client. Of course, the spectrum analysis may also relate to all available radio channels. For doing so, the selected radio channel can be switched. The wireless access device according to the invention for carrying out a method as described above comprises a single radio interface and is embodied and configured for employing the single radio interface both for communicating with mobile stations and for running a spectrum analysis relating to a selected radio channel. Further, the wireless access device is embodied and configured for determining interferences on the selected radio channel by means of the spectrum analysis. Moreover, the wireless access device is embodied and configured for requesting access to the selected radio channel by sending a ready to send data frame to a selected mobile station. The ready to send data frame indicates a time interval the access is requested for. Besides, the wireless access device is embodied and configured for, upon receipt of a clear to send data frame from the selected mobile station, running the spectrum analysis relating to the selected radio channel during the time interval. This allows for avoiding radio interferences in wireless communications systems, es ^¬ pecially in industrial automation systems with a plurality of potential interferers like WLAN systems, RFID systems, frequency converters or electric drive systems.

According to an advantageous embodiment of the present invention, the wireless access device is a wireless access point or client and embodied and configured for carrying out a method of transmitting data frames in a wireless communication system as described above.

The present invention is explained in more detail below using an exemplary embodiment with reference to the drawing, in which the figure shows a schematic illustration of a wireless com ^¬ munication system with a selected radio channel monitored by means of spectrum analysis. The figure shows a wireless communication system of an industrial automation system comprising a wireless access point 1 and a plurality of mobile stations 201-203 located in a radio cell covered by the wireless access point 1. For establishing radio connections between the wireless access point 1 and the mobile stations 201-203, the wireless access point 1 comprises a single radio interface 11 for modulating and demodulating radio signals received or transmitted, respectively by means of an antenna 10. Furthermore, the wireless access point 1 comprises a controller 12 and a further transmit /receive unit 13 which is connected to an industrial automation network 2. In the present embodiment, a programmable logical controller 3 is also connected to the industrial automation network 2 and serves to control an automation device like a robot 4 or an electrical drive. The industrial automation network 2 may be an IP-based communication network .

The single radio interface 11 of the wireless access point 1 is primarily used for transmitting data frames 111 between the wireless access point 1 and the mobile stations 201-203. However, the single radio interface 11 is employed both for communicating with the mobile stations 201-203 during an active WLAN operation period 101 and for running a spectrum analysis relating to a selected radio channel during a spectrum analysis period 103. The spectrum analysis period 103 is comprised by a blocking period 102 in which the single radio interface 11 is not available for transmitting data frames 111. Instead of that, the single radio interface 11 is used for collecting measurement signals 112 during the spectrum analysis period 103. By means of the spectrum analysis, interferences on the selected radio channel are determined. If the interferences on the selected radio channel are higher than a specified threshold, the selected radio channel may be switched to another radio channel. Alternatively or additionally, an alarm may be generated or a warning message is sent to a supervisory station, respectively.

In the present embodiment, both the wireless access point 1 and the mobile stations 201-203 are RTS/CTS capable (ready to send, clear to send) . For requesting transmission access to the selected radio channel, a first ready to send data frame is sent from a sender to a recipient, in which the wireless access point 1 is either the sender or the recipient. The first ready to send data frame indicates an access time interval the transmission access is requested for by the respective sender. The first ready to send data frame is acknowledged by sending a first clear to send data frame from the recipient to the sender. If the first clear to send data frame is received by the sender, the selected radio channel can be accessed by the sender for transmitting data frames 111 on the selected channel during the respective access time interval. During the access time interval, except for the respective sender, every mobile station 201-203 and the wireless access point 1 stops transmitting data frames 111 on the requested channel. By doing so, collisions or data frame losses, re ^¬ spectively in the wireless communication system can be avoided.

For running the spectrum analysis, the wireless access point requests analysis access to the selected radio channel by sending a second ready to send data frame 121 to a selected mobile station 201-203. In this case, the second ready to send data frame 121 indicates an analysis time interval the analysis access is requested for by the wireless access point 1. As it can be taken from the figure, the blocking period 102 in which the single radio interface 11 is not available for transmitting data frames 111 already begins by sending the second ready to send data frame 121.

The second ready to send data frame 121 is acknowledged by sending a second clear to send data frame 122 from the selected mobile station 201-203 to the wireless access point 1. Accordingly, if the second clear to send data frame 122 is received by the wireless access point 1, the wireless access point 1 can access to the selected radio channel for running the spectrum analysis during the analysis time interval which equals the spectrum analysis period 103. During the analysis time interval, every mobile station 201-203 in range of the wireless access point 1 stops transmitting data frames 111 on the selected channel. Moreover, the wireless access point 1 is switched from a wireless transmission mode to a spectrum analyzer mode during the analysis time interval, e.g. by means of the controller 12.

By applying RTS/CTS channel reservation approaches, the col ^¬ lected measurement signals 112 relating to the spectrum analysis only contain signals that disturb WLAN traffic. With a tra ^¬ ditional spectrum analyzer this would be not possible because traditional spectrum analyzers are not capable of sending WLAN frames, especially RTS/CTS frames.