EP2394254B1

EP2394254B1 - A method of transmitting signals in a control system and a control system therefore

Info

Publication number: EP2394254B1
Application number: EP09776153.0A
Authority: EP
Inventors: Martin Sandal Nielsen; Jakob Trelborg
Original assignee: VKR Holding AS
Current assignee: VKR Holding AS
Priority date: 2009-02-05
Filing date: 2009-02-05
Publication date: 2017-08-09
Anticipated expiration: 2029-02-05
Also published as: EP2394254A1; WO2010088903A1

Description

Field of the invention

The invention relates to a method of transmitting signals, e.g. control signals, request signals, interrogation signals etc. in a control system comprising at least two units, wherein at least one of said units is designed to operate as a master unit and wherein at least one of said units is designed to operate as a slave unit,

whereby a plurality of channels are available for transmission,
whereby a master unit transmits a signal via one of said plurality of channels,
whereby at least one slave unit performs the step of scanning the channels for transmitted signals.

The invention also relates to a control system for transmitting signals.

Background of the invention

In radio communication control systems, it is well-known from the prior art for a master unit/transceiver unit to select a vacant channel from a plurality of channels when transmitting a signal to a slave unit/receiver, and for a slave unit/receiver to scan the plurality of channels to detect a signal from the master unit/transceiver. Such a system is disclosed in for example EP 1461790 B1 .
A similar system is disclosed in for example WO 03/056530 A . In relation to sending a signal from a master unit and receiving a signal on a slave unit using one or several channels, waiting periods may occur if the preamble of a data frame is transmitted for longer time than one scanning of the plurality of channels. This situation may occur when presetting a preamble signal time or when a slave unit performs channel shifting faster than the time allocated for that action. The situation may also occur due to possible latency in the master and/or slave. An idle time may be present which may be derived with respect to the number of channels, preamble length and scanning length of each channel. It is an object of the present invention to optimize this scanning procedure by means of taking advantage of possible idle time periods.

Summary of the invention

whereby a plurality of channels are available for transmission,
whereby a master unit transmits a signal via one of said plurality of channels,
whereby at least one slave unit performs the step of scanning the channels for transmitted signals,
whereby a received signal on a specific channel is analyzed to specify a preamble type,
whereby when said preamble type is a first type (PT1), said scanning is resumed by said at least one slave unit on the other channels than said specific channel, and
whereby said at least one slave unit returns to said specific channel after a predefined time interval (T1).

Hereby, an enhanced efficiency is achieved and furthermore a faster manner of scanning and analyzing signals on a plurality of channels is achieved. Further, an enlarged amount of data can be processed, hereby making the use of the available channels significantly more efficient.
According to an advantageous embodiment, as specified in claim 2, said at least one slave unit may perform a further analysis of said received signal when it returns to said specific channel after said predefined time interval (T1). Hereby, further information is achieved which allows the efficiency to be enhanced.
Preferably, as specified in claim 3, in a preceding/previous step, said at least one slave unit may have scanned said specific channel without detecting said first type of preamble.
Hereby, information is provided which facilitates a calculation of the time interval prior to the expected time of transmittal of the payload, e.g. since the duration of the first type of preamble is known.
Advantageously, as specified in claim 4, said further analysis may comprise the detection of a payload within a second predefined time interval (T2) and whereby said at least one slave unit may perform in accordance herewith.
Hereby it is achieved that the payload is processed after the resumed scanning on other channels, whereby it is possible for the slave unit to process more data.
According to a further embodiment, as specified in claim 5, said further analysis may comprise the detection of said first type of preamble (PT1) being received continuously through said second predefined time interval (T2).
Hereby it is possible for the slave unit to detect that the preamble is too long e.g. as compared with the expected duration, whereby the slave unit can process other data instead.
Preferably, as specified in claim 6, said at least one slave unit may resume the scanning of all available channels
Hereby it is achieved that the at least one slave unit can process data on other channels instead of waiting for the preamble to stop, i.e. it is avoided that communication with the slave unit is blocked by e.g. an erroneous signal.
According to a further embodiment, as specified in claim 7, when a signal is detected during said scanning performed by said at least one slave unit on the other than said specific channel, said signal may be analyzed to specify a preamble type.
Hereby the at least one slave unit can select different scenarios on the basis of the preamble type, e.g. it can scan other channels while waiting for the preamble to stop, it can go into a sleep mode to save power or the like. Likewise, the slave unit may process payloads corresponding to a preamble detected earlier, which facilitates that the slave unit may process more data, and enhance the speed with which preambles and payloads are detected and performed in accordance with.
Preferably, as specified in claim 8, when said preamble is a second type, said slave unit may receive the corresponding signal and perform accordingly.
Hereby the at least one slave unit can select to perform according to the received signal on the basis of the preamble type, and hereby a more dynamic choice of action is possible.
Preferably, as specified in claim 9, said first type preamble (PT1) may be a long preamble mode type, having a preamble length corresponding at least to the time required for said at least one slave unit to test said plurality of channels for a transmitted signal.
Hereby a more reliable and faster system is achieved.
According to a further preferable embodiment, as specified in claim 10, said first preamble type (PT1) may comprise at least one piece of information regarding the time to when the payload is transmitted.
Hereby it is achieved that the at least one slave unit can scan other channels more efficiently, and/or administer the time when to wake up or return to the channel for receiving the payload in an advantageous way, since the time to when the payload is transmitted is well defined due to the received information.
Advantageously, as specified in claim 11, said first preamble type (PT1) may comprise a plurality of preamble parts (PRP) each comprising information about the time to when the payload is transmitted.
Hereby it is achieved that the at least one slave unit can receive just one of the plurality of preamble parts PRP, and hereby relatively quickly determine the exact time to when the payload is transmitted. The at least one slave unit can then for example scan other channels for signals, go into sleep mode to save power or the like, and know when to return to the channel to receive the payload.
Advantageously, as specified in claim 12, each preamble part (PRP) may comprise a number or a time indicating the time left to when the payload is transmitted or expected to be transmitted.
Hereby the at least one slave unit can determine the exact time to when the payload is transmitted only by reading the number or time.
Advantageously, as specified in claim 13, said plurality of preamble parts (PRP) may be transmitted with a predetermined time interval e.g. 1 millisecond, 2 milliseconds, 5 milliseconds or the like.
Hereby it is possible to ensure that a whole preamble part PRP is received by the at least one slave unit during just one examination time duration of a channel, since it is possible to determine a suitable time interval for the examination time duration of the at least one slave unit.
In another embodiment the preamble parts PRP may be transmitted straight after each other and the slave unit could evaluate the preamble data to analyze the time left for the payload to be transmitted.
The invention also relates to a control system for transmitting signals, e.g. control signals, request signals, interrogation signals etc. comprising at least two units, wherein at least one of said units is designed to operate as a master unit and wherein at least one of said units is designed to operate as a slave unit,

wherein said units are designed to be able to use a plurality of channels available for transmission,
wherein a master unit is designed to be able to transmit a signal via one of said plurality of channels,
wherein at least one slave unit is designed to be able to perform the step of scanning the channels for transmitted signals,
wherein said slave unit is designed to be able to a analyze a signal received on a specific channel to specify a preamble type,
wherein when said preamble type is a first type (PT1), said control system is designed to be able to resume the scanning by said at least one slave unit on the other channels than said specific channel, and
wherein said at least one slave unit is designed for returning to said specific channel after a predefined time interval (T1).

According to an advantageous embodiment, said at least one slave unit may be designed for performing a further analysis of said received signal when it has returned to said specific channel after said predefined time interval (T1).
Advantageously, said least one slave unit may be designed to register, whether in a preceding step, where said specific channel has been scanned, a first type of preamble was detected.
Advantageously, said at least one slave unit may be designed to be able to perform a further analysis for detection of a payload within a second predefined time interval (T2) and perform in accordance herewith.
Advantageously, said slave unit may be designed to, when said preamble is a second type preamble, receive the corresponding signal and perform accordingly. Preferably, said first type preamble (PT1) may be a long preamble mode type, having a preamble length corresponding at least to the time required for said at least one slave unit to test said plurality of channels for a transmitted signal.
Advantageously, said at least one master unit and/or said at least one slave unit may comprise means for facilitating operation in a plurality of different frequency bands.
Hereby it is achieved that the system is more dynamic. Further this is a cost efficient solution since the same control system can facilitate operation on different frequency bands and hereby it is only necessary to produce one kind of control system. Further it is hereby possible to facilitate a system which can be compatible with earlier systems or future systems operating on various frequency bands.
According to a further preferable embodiment, said first preamble type (PT1) may comprise at least one piece of information regarding the time to when the payload is transmitted.
According to a still further preferable embodiment, said first preamble type (PT1) may comprise a plurality of preamble parts (PRP) each comprising information about the time to when the payload is transmitted.

The figures

The invention will be explained in further detail below with reference to the figures of which

fig. 1: shows a general overview of a system according to an embodiment of the invention,
fig. 2: shows a structure of a message and in particular a first frame of such a message utilized in an embodiment of the invention,
fig. 3: illustrates a more detailed example of a possible structure of a message according to an embodiment of the invention,
fig. 4: shows a flowchart illustrating an example of the receipt mechanism of a slave unit according to an embodiment of the invention, and
figs. 5a-c: show other structures of the preamble of a frame according to further embodiments of the invention.

Detailed description

Fig. 1 shows a general overview of a system according to a first embodiment of the invention. The system comprises a plurality of units 2, 4, 6, 8 (U1 - Un) comprising or linked to for example drive units for various devices 10, 12, 14, 16. These devices may for example be activators of various types and used in a wide variety of applications, as explained in further detail later on. The units may also or instead comprise or be linked to assorted apparatus, measuring devices, indicating devices, controllers etc. that need to receive information, control signals etc. at certain points of time. A common feature of the units 2, 4, 6, 8 is that it is necessary or advantageous to be able to send some sort of information, signal etc. to the units. Further, it may be necessary or preferable to be able to receive information or signals from the units, e.g. confirmation signals, acknowledgement signals, measurement signals etc.
In the embodiment shown in fig. 1, all units 2, 4, 6, 8 are equipped with receiving means for radio frequency signals, e.g. including antenna means 18, 20, 22, 24. Each unit comprises e.g. one or more motor drive units or control units and is connected to a driven or controlled member 10, 12, 14, 16. It will be understood that the means driven by, controlled by etc. the unit may be integrated with the unit. It will be understood that a system may comprise one or more units U1 - Un and that, when more than one unit are involved, these may be similar or dissimilar and may control one or more similar or different devices. The units U1 - Un will also be referred to as controllable units or slave units in the following.
It should further be noted that the units U1-Un could be connected to sensors and the like, and could comprise means for evaluating and sending signals to the master unit, the other units and the like, regarding the data from said sensors.
It will further be understood that at least one of the one or more units U1-Un comprises data storage means for storing received and/or transmitted data, as well as program data and the like. Further at least one of the one or more units Us-urn comprises data processing means for processing transmitted data, received data, program data and/or the like.
The system further comprises a remote control unit (C1) 26 designed for transmission of signals by means of radio frequency transmission means, e.g. including antenna means 28. This control unit may be used for transmitting control signals or other types of signals to one or more of the units U1 - Un in the system.
As illustrated the system may comprise at least one additional remote control unit (C2 - Cn) 30, 34 similar to the first remote control, e.g. designed for transmission of signals by means of radio frequency transmission means, e.g. including antenna means 32, 36 or dissimilar to the first remote control, e.g. designed for transmission by other means such as wired means, infra red transmission means etc (not illustrated in fig. 1).
The remote controls C1 - Cn will also be referred to as controllers or master units in the following.
The units U1 - Un and the controllers C1 - Cn will also be referred to as nodes in the illustrated system.
The method of performing a communication between the units or nodes comprised in the illustrated system, e.g. between a master unit, e.g. the remote control 26 (C1), and one or more of the other (controllable) units U1 - Un, which also will be described as slave units in the following, will now be described in further detail. It will be understood that in this context a master unit will imply a unit or a node in a communication system that initiates a transmission of data, and that a slave unit correspondingly will imply a unit that receives and responds to a transmission from a master unit. Thus, any of the units involved in the system illustrated in fig. 1 may in principle take the role as a master unit as well as a slave unit in accordance with this definition.
The transmission of data between the units involved will take the form of a transmission of messages comprising a number of frames, packages or the like that are being transmitted between the units, e.g. a first frame from a first unit to a second unit, a second frame from the second unit to the first unit etc. A frame or a package comprises a number of bits that are transmitted consecutively. In a suggested embodiment of the invention, a message will contain two or four frames, but it will be understood that the number of frames, e.g. one, two, three, four, five etc. may be arranged according to the actual application and needs.
In order to achieve optimal communication between the units, a number of communication channels or frequencies may be used for the communication. It will be understood that more than one master unit may need to communicate with a slave unit at a specific point of time in the system illustrated in fig. 1, and further it will be understood that a similar system comprising similar units may be situated nearby, e.g. whereby units from one system may occupy a channel needed for communication by units in the other system. In order to facilitate an optimal communication in these and other situations using a number of channels or frequencies, a frequency hopping technique is used in accordance with the invention. It is understood that a plurality of channels may be used, e.g. 2, 3,4 or 5 channels, or even more channels such as 10, 15 or 20 channels or any number there between.
In a preferred embodiment of the invention, each of the units U1-Un comprises channel shifting means for shifting between a plurality of channels. The channels could e.g. be in the 868 MHz area, the 2.4 GHz area or any other suitable communication band.
In an embodiment of the invention the units comprise means for operating on a plurality of different channels in different frequency bands, e.g. both the 868 MHz area, the 2.4 GHz band and the like.
In order to explain an embodiment of the invention, a possible structure of a message will be illustrated with reference to fig. 2.
Here a first frame, F1, of a message transmitted from a master unit is illustrated, and subsequent frames F2 and F3 etc. are shown as well. Each of the frames comprises a preamble part PREn and a payload data part PAYn. The preamble PRE1 of the first frame F1 is larger than the preambles of the rest of the frames in a message. This large preamble PRE1 will be referred to as a type 1 preamble PT1 with reference to figure 3. The subsequent frames will serve to contain the actual information, control signalling etc, that are to be transmitted to a slave unit, whereas the preamble PRE1 of the first frame of a message will serve to alert a slave unit that a message is transmitted on a given channel as explained in further detail in the following.
The illustrated message is transmitted on a given channel in accordance with a procedure which will be described later on. When a message is transmitted, the intended receiver, e.g. the slave unit in question, will need to detect that such a message is transmitted on a channel. In order to do this, each of the slave units will scan all of the obtainable channels, e.g. CH1, CH2, ... CHm. In a time interval A, each slave unit will as illustrated examine, e.g. listen to, each channel for a message or rather a preamble comprised in a first frame of a message. Optionally, each slave unit may also in addition examine each channel for a carrier wave. Since this examination has a time duration of A, a preamble of a first frame of a message from a master unit will have to have a duration at least equal to m (the number of channels) times A. Possibly, the duration of such a preamble PRE1 will also have to take into account the time needed for a slave unit to shift from one channel to the next.
Thus the illustrated first frame, F1, contains a preamble PRE1 serving the purpose of alerting the slave units, and further the preamble may contain particular information, e.g. system specific information giving the slave unit(s) the information that the message originates from a master unit related to the same system as the slave unit(s). For example, the preamble may contain the number 55h, corresponding to the bit sequence 01010101. Thus, if a slave unit detects a frame on a channel and detects the above bit sequence in the time interval A, it will stop the present scanning procedure and listen to the channel in question. It is understood that other types of information to indicate that the message originates from a master unit related to the same system as the slave unit(s) is possible, e.g. a signal with a predetermined sequence of altering duty cycle or the like.
In accordance with the present invention, the scanning procedure may under specific circumstances be resumed on other channels than the channel of which the signal is received. This scenario will be described in more detail with reference to figure 3.
Preferably, the method and the system according to the invention may be designed in such a manner that the bit sequence in question is repeated a number of times k in the preamble of a first frame of a message and that the slave units are designed in such a manner that the bit sequence in question will have to be detected a number of times 1 (≤k) before a slave unit stops the scanning procedure and listens to the channel on which the bit sequence is detected. Thus, a slave unit may be set up to require a specific symbol to be detected one, two, three, four or more times before it is determined that a master unit is trying to transmit a message on a given channel.
If a noise signal is present on a channel there will be a certain probability that the predefined symbol will be stochastically present one time, but the likelihood that the symbol will be repeated one or more times will be significantly lower. By having the predefined symbol, e.g. the bit sequence repeated a number of times, and by having the slave unit detect the bit sequence a number of times, before the slave unit decides that a message is transmitted via the channel in question, the risk of erroneous detection caused by noise signals on a channel is significantly reduced.
In an embodiment of the invention the preamble PRE1 contains information regarding which channel the payload is transmitted on, e.g. another channel than the channel on which the preamble is detected, so the slave units can scan this channel and receive and process the payload when it is transmitted. Further it is possible that the payload is split in parts and transmitted on different channels, e.g. to increase the security of the system.
Fig. 3 illustrates a more detailed example of a possible structure of a message according to an embodiment of the invention. Here, a frame F1 is illustrated which is transmitted from a master unit MU1. Frame F1 comprises a large type 1 preamble PT1 which has a duration that is longer than the time taken for a slave to scan all channels, i.e. in this example a duration longer than 3 times A. The frame is according to this example transmitted on channel CH2. As in figure 3, the intended receiver, in this example the slave unit SU1, will need to detect that such a message is transmitted on a channel.
The slave unit SU1 will scan all of the obtainable channels, where in this illustrated example the number of channels is 3, CH1, CH2, CH3. In a time interval A, each slave unit will as illustrated examine, e.g. listen to, each channel for a message or rather a preamble comprised in a first frame of a message. The transmission of the preamble of frame F1 on channel CH2 is according to the present example started, at a time so the signal is detected at the second scanning of channel CH2, referred to as SC2 in the figure. Subsequently, the received signal is analyzed to specify a preamble type. This analysis process AP may be a determination of the duration of the preamble, which in accordance with the present example is a preamble type PT1. Now, the slave unit SU1 will achieve information regarding the time that will pass before the preamble PT1 ends and the payload can be expected. This information may be achieved in a number of ways. The preamble PT1 may for example comprise information regarding the specific time, when the payload is expected, or the slave unit SU1 may estimate or calculate the time, since it has knowledge of the length of the preamble type PT1 and since it has the knowledge that such a preamble was not present on the particular channel at the preceding scan made by the slave unit SUI. Thus, the slave unit can in a predefined time interval T1 resume the scanning of the other channels, i.e. channel 1 and channel 3. Within the time interval T1 another master unit MU2 may for example transmit a frame to the slave unit SU1 on channel CH3, having a shorter preamble of the type PT2. The type 2 preamble PT2 is thus received at the slave unit SU1 at a slave unit scan, SC4, of channel CH3 whereby the preamble is detected and the payload PAY2 may be processed by the slave unit SU1 which may perform in accordance hereto.
Prior to the end of the time interval T1, the slave unit SU1 returns to channel CH2 where the payload PAY1 of frame F1 may be received and processed. The slave unit SU1 will hereinafter perform in accordance with the received payload PAY1.
It is a very advantageous feature according to an embodiment of the invention that the time period between detecting a type 1 preamble PT1 and receiving the payload of the frame is taken advantage of. This entails that instructions and queries may be handled by the slave unit in question in a more efficient and faster manner.
Fig. 4 shows a flowchart illustrating an example of a receipt mechanism of a slave unit according to an embodiment of the invention.
Initially, the slave unit scans all available channels for a received signal in step 41. If a signal or frame is received from a master unit on a specific channel at step 42 (e.g. "Signal detected?"), it is determined in step 43 if the preamble is a type 1 which according to an embodiment of the invention is understood as long preamble e.g. a preamble that last for more than for example 50 ms. If the preamble is a type 1 PT1, it is known, calculated or estimated that at least a predefined time interval T1 is available as long as the master unit transmits the preamble signal. If the signal is not a type 1 preamble PT1, the slave unit may in step 44 read a possible short preamble and payload and perform in accordance herewith. If the preamble is a type 1 preamble, the slave unit will scan the other channels for transmitted signals 45 and thereby take advantage of the available time until the payload may be transmitted in the specific channel. The scanning of the other channels will be performed in a predefined time interval T1, therefore it is in step 46 checked whether the time for scanning the other channels has reached or exceeded the time interval T1. If not, in step 47 the other than the specific channels are again scanned for a received signal. This step, 47, is in this way repeated either until the time has exceeded the time interval T1 in step 46 or until a signal is detected on a channel where a preamble possibly is detected and a payload is possibly received 48. The slave unit may hence in this step, 48, act pursuant to the payload.
Subsequently after a signal is detected and processed in step 48, the flow continues to step 46 and if the time interval T1 is exceeded, the slave unit in step 49, returns to the specific channel where the type 1 preamble PT1 was originally received. It is now decided whether a preamble signal PT1 is still detected at 50. If yes, the step 50 is repeated as long as the time passed is below a predefined time interval T2 in step 51, or until the preamble PT1 is no longer detected. If the time passed exceeds the time interval T2 and the same preamble PT1 is still detected, which may indicate an erroneous signal, the slave unit will resume the scanning of all available channels in step 52. Step 52 may hence be a restart procedure and send the flow back to step 41. If, within the time interval T2, a preamble is no longer detected in step 50, the slave unit will attempt to detect a payload on the specific channel in step 53. If no payload is detected within a predefined time limit, the slave unit will resume the scanning of all available channels in step 54 and may hence be a restart procedure and send the flow back to step 41. If a payload is detected, the slave unit may perform or act pursuant to the payload 55.
Fig. 5a illustrates an embodiment of the invention where the transmitted frame FX comprises a long preamble PRE1, which comprises a plurality of preamble parts PRPn - PRP0 and a payload PAY. Each preamble part PRP comprises a preamble number (e.g. a bit sequence) or the like, for information regarding the time period before the payload PAY is transmitted.

Pre. fr. Indic. Pre. no.
As illustrated in the table above a preamble part may comprise:

A preamble frame indication (Pre. fr. indic.) to indicate that the received frame is a preamble part
A preamble number (Pre. no.) to indicate the time left to when the payload is transmitted.

The preamble part could additionally comprise an indication of the system type (Sys. type) as shown in the table below.

Pre. fr. Indic. Sys. type Pre. no.
The slave unit can hereby recognize that the received signal is from and/or to a unit in its own system or from and/or to a unit in another system which the slave unit or its system is configured to communicate with. Hereby the slave unit can sort out signals which are not intended for itself, and if the slave unit is configured to read the preamble number in preambles of other system types than its own system type, the channel can restrict itself only to listen to other channels until it determines that the frame for the other system is communicated. It is understood that the system type in a preamble part could be used for other suitable purposes as well.
Further each preamble part PRP may comprise indication of the preamble type (Pre. type). See the table below.

Pre. fr. Indic. Sys. type Pre. type. Pre. no.
The preamble type may comprise information regarding the length of the preamble and/or the subsequent preambles, it may comprise information regarding if the preamble part and/or its payload is intended to be forwarded, or the like. This likewise facilitates that the slave unit advantageously may sort out signals which is not intended for itself.
A preamble part PRP is preferably transmitted with a predetermined time interval, e.g. each millisecond, each five milliseconds, each ten milliseconds or the like, and the preamble number is decremented or incremented each time. Likewise the preamble parts PRP could be transmitted continuously without a time interval between the preamble parts, i.e. when preamble part PRPn-1 has been transmitted, preamble part PRPn-2 is transmitted straight after, etc.
The at least one slave unit SUx listens to a plurality of different channels as described earlier. When a preamble part (in this case PRPn-1) is detected by the at least one slave unit SU on a channel, in the time interval A, the information regarding the time to when the payload PAY is transmitted is evaluated by the at least one slave unit SUx. Hereby, the at least one slave unit SUx can determine the time to when the payload PAY is transmitted (T3). The at least one slave unit SUx may hereby scan other channels as described earlier. Alternatively, the at least one slave unit SUx may go into a sleep mode in the time interval T3 to save power, which is advantageous if it is a low power slave unit e.g. with batteries, solar cells or the like as power supply. When the time interval T3 is expired or prior hereto, the at least one slave unit SUx wakes up to receive the payload PAY (if it was sleeping) or it returns to the channel on which the payload PAY is transmitted (if it was listening to other channels).
In an embodiment of the invention it is assured that the examination time duration A is long enough to ensure that a whole preamble part PRP is assured to be received. Alternatively, the at least one slave unit SU may stay on the channel to receive a whole preamble part PRP if a part of a preamble part is detected.
Fig 5b illustrates another embodiment, where the preamble comprises an indication of the time to when the payload PAY of the frame FX is transmitted. In this embodiment the preamble comprises a signal with the same duty cycle (e.g. 0.5 as illustrated), but with altering period time Tp. The period time Tp is altering according to a predefined function, e.g. by decreasing or increasing the period time.
When the received period time hereby is calculated by the slave unit, it is possible to determine the time to when the payload PAY of the frame is transmitted.
Fig 5c illustrates an embodiment of the invention, where the preamble comprises a signal with a period time which is kept constant, but the duty cycle is altered according to a predefined function, whereby it is possible to determine the time to when the payload PAY of the frame is transmitted. The duty cycle is determined by the function: $D = \frac{t}{T}$

where: D is the duty cycle,
t is the altering duration that the function is non-zero
T is the period time of the function.

As it is seen from the duty cycle function above, by changing t while keeping T constant, the duty cycle is altered.
The signal may be a square signal as illustrated, but it may also be a triangle signal or the like. Likewise, the preamble may comprise a signal with altering rise times, fall times, amplitudes or the like, to indicate the time left to the transmittance of the payload PAY.
The preamble may also be altered according to when the payload PAY of the frame is transmitted, by altering both the period time and the duty cycle, rise times, amplitudes or the like. Likewise a plurality of different signals may be incorporated in the preamble or preamble part(s) to comprise information regarding the preamble type, time to when the payload is transmitted, which channel (or channels) the payload or payloads is transmitted on or the like.
It should be understood that the invention is not limited to the particular examples above, but may be designed and altered in a multitude of varieties within the scope of the invention as characterized in the claims.

Claims

Method of transmitting signals, e.g. control signals, request signals, interrogation signals etc. in a control system comprising at least two units, wherein at least one of said units is designed to operate as a master unit and wherein at least one of said units is designed to operate as a slave unit,
- whereby a plurality of channels are available for transmission,

- whereby a master unit transmits a signal via one of said plurality of channels,

- whereby at least one slave unit performs the step of scanning the channels for transmitted signals,

- whereby a received signal on a specific channel is analyzed to specify a preamble type,

- whereby when said preamble type is a first type (PT1), said scanning is resumed by said at least one slave unit on the other channels than said specific channel, and

- whereby said at least one slave unit returns to said specif channel after a predefined time interval (T1).
Method according to claim 1 whereby said at least one slave unit performs a further analysis of said received signal when it returns to said specific channel after said predefined time interval (T1).
Method according to claim 1 or 2 whereby in a preceding/previous step, said at least one slave unit has scanned said specific channel without detecting said first type of preamble (PT1).
Method according to claim 2 or 3 whereby said further analysis comprises the detection of a payload within a second predefined time interval (T2) and whereby said at least one slave unit performs in accordance herewith.
Method according to claim 2 or 3 whereby said further analysis comprises the detection of said first type of preamble (PT1) being received continuously through said second predefined time interval (T2).
Method according to claim 5, whereby said at least one slave unit resumes the scanning of all available channels.
Method according to any of the preceding claims whereby, when a signal is detected during said scanning performed by said at least one slave unit on the other than said specific channel, said signal is analyzed to specify a preamble type.
Method according to any of the preceding claims, whereby, when said preamble is a second type, said slave unit receives the corresponding signal and performs accordingly.
Method according to any of the preceding claims, whereby said first type preamble (PT1) is a long preamble mode type, having a preamble length corresponding at least to the time required for said at least one slave unit to test said plurality of channels for a transmitted signal.
Method according to any of the preceding claims, whereby said first preamble type (PT1) comprises at least one information regarding the time to when the payload is transmitted.
Method according to according to any of the preceding claims, whereby said first preamble type (PT1) comprises a plurality of preamble parts (PRP) each comprising information about the time to when the payload is transmitted.
Method according to claim 11, whereby each preamble part (PRP) comprises a number or a time indicating the time left to when the payload is transmitted.
Method according to claim 11 or 12, whereby said plurality of preamble parts (PRP) are transmitted with a predetermined time interval e.g. 1 millisecond, 2 milliseconds, 5 milliseconds or the like.
Control system for transmitting signals, e.g. control signals, request signals, interrogation signals etc. comprising at least two units, wherein at least one of said units is designed to operate as a master unit and wherein at least one of said units is designed to operate as a slave unit,
- wherein said units are designed to be able to use a plurality of channels available for transmission,

- wherein a master unit is designed to be able to transmit a signal via one of said plurality of channels,

- wherein at least one slave unit is designed to be able to perform the step of scanning the channels for transmitted signals,

- wherein said slave unit is designed to be able to a analyze a signal received on a specific channel to specify a preamble type,

- wherein when said preamble type is a first type (PT1), said control system is designed to be able to resume the scanning by said at least one slave unit on the other than said specific channel, and

- wherein said at least one slave unit is designed for returning to said specific channel after a predefined time interval (T1).
Control system according to claim 14 wherein said at least one master unit and/or said at least one slave unit comprises means for facilitating operation in a plurality of different frequency bands.