Field of the invention

[1] The present invention relates to a wireless channel sensing technique and applications thereof, such as a medium access control, MAC, protocol for resolving wireless channel contention.

Background art

[2] Various fields of industry, such as aviation industry, logistics, Internet of Things, smart factories, are interested in using a large number of wireless transmitters that are configured to perform wireless transmissions on the same radio-frequency, RF, channel, so as to save resources of the wireless medium for other purposes. Frequently, a situation occurs when multiple transmitters attempt to perform wireless transmissions almost simultaneously in time, leading to potential collisions of packet transmissions and loss of data. Such a situation may be referred to as a channel congestion or channel contention.

[3] Communication protocols for reducing the number of collisions and for resolving channel contention are known as medium access control, MAC, protocols for communications, especially for wireless/radio communications. Examples are the ALOHA protocol, the carriersense multiple access, CSMA, protocol, the 1-persistent CSMA protocol, or the non-persistent CSMA. Such communication protocols are channel-access mechanisms that may coordinate access of multiple transmitters to a shared wireless channel, thereby resolving channel congestion.

[4] The transmitters of interest preferably use a minimum amount of energy and preferably perform wireless transmissions, so as to avoid additional wiring and to reduce maintenance costs. However, known state-of-the-art methods cannot efficiently resolve channel contention at the desired low energy costs while reliably transmitting the pending data transmissions, as these methods require too much energy, for example, on channel sensing, central scheduling, and/or performing multiple transmissions upon detection of collisions.

[5] Therefore, there is a need for wireless/radio channel-access mechanisms that can efficiently handle channel contention at low energy costs with reliable data transmission performance.

Summary of the invention

[6] The present invention relates to a radio-frequency, RF, channel sensing circuit, an apparatus comprising a RF channel sensing circuit and a communication unit configured to transmit data according to an indicator received from the RF channel sensing circuit, and a system including a plurality of such apparatuses.

[7] According to the present invention, a RF channel sensing circuit as defined in appended claim 1 is defined.

[8] An apparatus according to the invention is defined in an independent apparatus claim.

[9] A system according to the invention is defined in an independent system claim.

[10] The RF channel sensing circuit may comprise: a RF-harvesting circuit configured to harvest energy from a RF channel; a capacitor configured to be charged by the harvested energy; and an indicating-circuit configured to generate an indicator, wherein if a voltage level of the capacitor is below a threshold voltage level, the indicator indicates the RF channel being idle, and if the voltage level is not below the threshold voltage level, the indicator indicates the RF channel being occupied.

[11] In the RF channel sensing circuit, the voltage level of the capacitor may be continuously compared to the threshold voltage level.

[12] In the RF channel sensing circuit, the RF-harvesting circuit may be coupled to a RF- harvesting antenna configured to receive a RF signal on the RF channel, wherein the RF- harvesting circuit may comprise a rectifying circuit configured to rectify the received RF signal and to output the rectified RF signal to charge the capacitor.

[13] In the RF channel sensing circuit, the RF-harvesting circuit may further comprise a matching circuit configured to tune the RF channel sensing circuit to a specific RF range; and the rectifying circuit may comprise a log amplifier circuit or a voltage multiplier circuit.

[14] The RF channel sensing circuit may further comprise at least one of a resistor coupled to the capacitor; and a tuner configured to tune the threshold voltage level.

[15] In the RF channel sensing circuit, at least one of the threshold voltage level and a charging and/or discharging speed of the capacitor may be configured according to at least one of a density of a plurality of RF channel sensing circuits located in a communication system which comprises the said RF channel sensing circuit, and a noise level of the RF channel.

[16] An apparatus may comprise: a radio-frequency, RF, channel sensing circuit; and a communication unit configured to transmit data if the indicator indicates the RF channel being idle.

[17] The apparatus may further comprise a sensor configured to sense a trigger event and to generate the data from the trigger event, wherein the communication unit may be further configured to transmit the data in response to the trigger event when the indicator indicates the RF channel being idle. [18] The apparatus may further comprise an energy-harvesting, EH, unit configured to harvest energy from the trigger event, wherein the energy harvested from the trigger event may be used for powering the apparatus.

[19] The apparatus may further comprise an antenna for transmitting the data and the same antenna may be used by the RF channel sensing circuit for receiving the RF signal on the RF channel.

[20] A system may comprise a plurality of apparatuses.

[21] In the system, each of the plurality of apparatuses may be configured to sense a shared RF channel and to transmit data on the shared RF channel.

[22] In the system, at least two apparatuses may differ from each other by at least one of a threshold voltage level and a charging and/or discharging speed of their respective capacitors.

[23] The proposed circuit/apparatus/system may have at least one of the following advantages. a. The RF channel sensing circuit provides a channel sensing technique that takes advantage of the energy in the wireless medium (e.g., the channel) to detect channel activity with essentially no energy cost (e.g., not external power supply or “battery-less”). In other words, whether a RF channel is occupied or idle may be indicated without relying on external energy supply from for example a battery. Thus, the components of the RF channel sensing circuit may be arranged so as to automatically generate an indicator indicating whether the RF channel is occupied or not, without continuously collecting data (from sensing the channel) that needs to be processed by higher layers. b. A system comprising a plurality of apparatuses may provide an efficient way of resolving channel contention without requiring central scheduling and without additional signalling to coordinate access to the RF channel. Access to the RF channel may be coordinated automatically by the distribution of apparatuses. With the present invention, retransmission attempts may be reduced significantly, thus, the data throughput may be improved greatly by the higher first transmission successful ratio compared to the prior art MAC protocols. The total energy cost is also reduced since less retransmission is needed. c. The present invention provides a system with self-organized nodes for example in terms of communications, such that adding apparatuses to and/or removing apparatuses from the system may be easily done without adapting parameters (e.g., scheduling, changing configurations of other nodes, etc.), as the system may easily integrate further apparatuses without interrupting the remaining of the system.

Brief description of the drawings

[24] Embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. However, the embodiments of the present disclosure are not limited to the specific embodiments and should be construed as including all modifications, changes, equivalent devices and methods, and/or alternative embodiments of the present disclosure.

[25] The terms “have,” “may have,” “include,” and “may include” as used herein indicate the presence of corresponding features (for example, elements such as numerical values, functions, operations, or parts), and do not preclude the presence of additional features.

[26] The terms “A or B,” “at least one of A or/and B,” or “one or more of A or/and B” as used herein include all possible combinations of items enumerated with them. For example, “A or B,” “at least one of A and B,” or “at least one of A or B” means (1) including at least one A, (2) including at least one B, or (3) including both at least one A and at least one B.

[27] The terms such as “first” and “second” as used herein may modify various elements regardless of an order and/or importance of the corresponding elements, and do not limit the corresponding elements. These terms may be used for the purpose of distinguishing one element from another element. For example, a first element may be referred to as a second element without departing from the scope the present invention, and similarly, a second element may be referred to as a first element.

[28] It will be understood that, when an element (for example, a first element) is “(operatively or communicatively) coupled with/to” or “connected to” another element (for example, a second element), the element may be directly coupled with/to another element, and there may be an intervening element (for example, a third element) between the element and another element. To the contrary, it will be understood that, when an element (for example, a first element) is “directly coupled with/to” or “directly connected to” another element (for example, a second element), there is no intervening element (for example, a third element) between the element and another element.

[29] The expression “configured to (or set to)” as used herein may be used interchangeably with “suitable for” “having the capacity to” “designed to” “adapted to” “made to,” or “capable of’ according to a context. The term “configured to (set to)” does not necessarily mean “specifically designed to” in a hardware level. Instead, the expression “apparatus configured to...” may mean that the apparatus is “capable of...” along with other devices or parts in a certain context.

[30] The terms used in describing the various embodiments of the present disclosure are for the purpose of describing particular embodiments and are not intended to limit the present disclosure. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. All of the terms used herein including technical or scientific terms have the same meanings as those generally understood by an ordinary skilled person in the related art unless they are defined otherwise. The terms defined in a generally used dictionary should be interpreted as having the same or similar meanings as the contextual meanings of the relevant technology and should not be interpreted as having ideal or exaggerated meanings unless they are clearly defined herein. According to circumstances, even the terms defined in this disclosure should not be interpreted as excluding the embodiments of the present disclosure.

[31] For the purpose of determining the extent of protection conferred by the claims of this document, due account shall be taken of any element which is equivalent to an element specified in the claims.

[32] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[33] Figure 1 shows an example of a radio-frequency, RF, channel sensing circuit.

[34] Figure 2 illustrates, by means of example, how a RF channel sensing circuit may indicate an idle RF channel and an occupied RF channel, respectively, depending on the voltage level of the capacitor.

[35] Figure 3 shows an apparatus comprising a RF channel sensing circuit and a communication unit.

[36] Figure 4 shows an apparatus in which an antenna is shared by a RF channel sensing circuit and a communication unit.

[37] Figure 5 shows, by means of example, a system of apparatuses accessing a shared RF channel.

[38] Figure 6 shows a system of apparatuses.

Detailed description of the Figures

[39] The figures show different embodiments of the invention, which will be described in detail hereinafter. It is to be understood that elements / components shown in one or more of these embodiments and not in others may be used in those others too unless mechanical or other limitations prevent such an implementation. Moreover, describing features of different embodiments in a single paragraph does not automatically mean that those features are inextricably linked. They may be applied separately from one another. Furthermore, the embodiments of the present disclosure are not limited to the specific embodiments and should be construed as including all modifications, changes, equivalent devices and methods, and/or alternative embodiments of the present disclosure.

[40] In general, a radio-frequency, RF, channel may refer to a certain part or frequency band within the electromagnetic spectrum. For instance, the RF channel may be a frequency band with frequencies below 3 terahertz, THz. A RF wave may refer to an electromagnetic wave propagating through space within a certain frequency range of the electromagnetic spectrum (e.g., on the corresponding RF channel). A RF wave may also be referred to as a RF signal or a RF transmission. For example, a RF signal may refer to an electromagnetic wave that is generated by a transmitter, for instance a radio transmitter. When a transmitter transmits a RF signal on a RF channel, the transmitter or the transmission is using the RF channel, i.e., the transmitter is occupying the RF channel, and/or the RF channel is active. An active RF channel may also refer to the electromagnetic field being excited, i.e., an active RF channel may mean that energy is stored in the electromagnetic field because modes of the electromagnetic field are excited or the electromagnetic field is activated or is oscillating with frequencies within the RF channel.

[41] Figure 1 shows a RF channel sensing circuit comprising various optional components.

[42] Figure 1 shows a RF channel sensing circuit (100) comprising a RF-harvesting circuit (110) that may be configured to harvest energy from a RF channel (e.g., via an antenna tuned on the RF channel); a capacitor (120) that may be configured to be charged by the harvested energy; and an indicating-circuit (130) that may be configured to generate an indicator (131, 311), wherein if a voltage level of the capacitor is below a threshold voltage level, the indicator may indicate the RF channel being idle, and if the voltage level is not below the threshold voltage level, the indicator may indicate the RF channel being occupied.

[43] The RF-harvesting circuit (110) may be configured to harvest energy from a RF channel. That is, by coupling to the electromagnetic field in a frequency range corresponding to the RF channel, energy may be harvested from RF signals received on the RF channel. Harvesting energy from the RF channel may mean extracting or scavenging energy from the RF channel when a RF signal is transmitted on the RF channel.

[44] The capacitor (120) may be configured to be charged by the energy harvested from a RF channel, i.e., the energy harvested by the RF-harvesting circuit from the RF channel may be temporarily stored in the capacitor (120). Since capacitors may continually discharge, the energy may be temporarily stored.

[45] The indicating-circuit (130) may be coupled to the capacitor (120). The indicating- circuit (130) may be configured to generate an indicator (131, 311) indicating the RF channel to be idle or to be occupied.

[46] The indicator may be generated in various ways. For instance, if the voltage level (VRF) of the capacitor (120) is below a threshold voltage level (VTH), the indicating-circuit (130) may output a signal, thereby indicating an idle RF channel, and if the voltage level (VRF) of the capacitor is not below the threshold voltage level (VTH), the indicating-circuit may not output the signal, thereby indicating an occupied RF channel. Alternatively, vice versa, the indicating- circuit may indicate an idle RF channel by not outputting a signal and may indicate an occupied RF channel by outputting the signal. As a further example, the indicating-circuit may output a first signal for indicating an idle RF channel and may output a second signal for indicating an occupied RF signal.

[47] The voltage level of the capacitor (120) may be continuously compared to the threshold voltage level. For example, by the arrangement of the capacitor and the indicating-circuit, the voltage level of the capacitor may be automatically compared to the threshold voltage level.

[48] For example, the indicating-circuit (130) may comprise an inverting comparator. The inverting comparator may be configured to continuously compare the voltage level of the capacitor with the threshold voltage level. For example, in such a case, the indicator indicating the RF channel being idle may be a digital high signal and the indicator indicating the RF channel being occupied may be a digital low signal. Alternatively, the indicator indicating the RF channel being idle may be a digital low signal and the indicator indicating the RF channel being occupied may be a digital high signal. Alternatively, the indicator indicating the RF channel being idle may be a digital low signal or a digital high signal while the indicator indicating the RF channel being occupied may be not outputting a signal. Alternatively, the indicator indicating the RF channel being idle may be not outputting a signal while the indicator indicating the RF channel being occupied may be a digital low signal or a digital high signal. Further ways of generating the indicators may be contemplated.

[49] A RF channel being idle may be referred to as an idle RF channel or a clear RF channel or a clear channel, for example, no RF signal is transmitted on the RF channel and/or the power level or energy level on the channel is lower (or not higher than) a power/energy threshold level (e.g., background noise level, acceptable interference level, etc.). Accordingly, the indicator may indicate the RF channel being clear or may indicate a clear RF channel or may indicate a clear channel.

[50] The voltage level of the capacitor (120) may be referred to as a current voltage level or a momentary voltage level. The voltage level of the capacitor may thus be interpreted as to continuously represent a current state of the RF channel. In other words, the RF channel sensing circuit (100) may indicate whether the RF channel is occupied or is idle based on the voltage level of the capacitor. The voltage level may be used to represent the current occupancy state of the RF channel. Alternatively, the voltage level of the capacitor (120) may be referred to as an average voltage level during a certain time period, thus, the voltage level may be used to represent the average occupancy state of the RF channel during the certain time period.

[51] The capacitor (120) may be referred to as a reference capacitor. The capacitor’ s voltage level may serve as a reference value for representing the current state and/or average state of the RF channel.

[52] The RF channel sensing circuit (100) may be interpreted as to sense an activity level or occupancy of the RF channel by means of the capacitor (120) and to generate an indicator (131) accordingly. In other words, the activity level or occupancy of the RF channel may be represented as a function of the voltage level of the capacitor.

[53] The RF channel sensing circuit (100) may be referred to as a RF information harvester. That is, the RF channel sensing circuit (100) may be interpreted as harvesting energy from the RF channel primarily for gaining information about the state of the RF channel. In other words, the RF channel sensing circuit may harvest energy from the RF channel not for storing the energy for later use, but primarily for indicating a current and/or average occupancy state of the RF channel. The energy may only be temporarily stored due to the capacitor (120) automatically discharging when no transmission is on the RF channel. Thus, the RF channel sensing circuit (100) turns RF energy harvesting into a channel sensing mechanism, as the energy may be harvested from the RF channel primarily for sensing the RF channel and not for storing the energy for later use.

[54] The voltage level or current and/or average voltage level of the capacitor (120) may thus continuously change depending on the activity level on the RF channel. If no transmissions are performed on the RF channel, the capacitor’s voltage level may still fluctuate for instance due to noise.

[55] Before describing further optional components of a RF channel sensing circuit (100), an example is described with reference to Figure 2. [56] Figure 2 illustrates, by means of example, how a RF channel sensing circuit (100) may indicate an idle RF channel and an occupied RF channel, respectively, depending on the voltage level of the capacitor. In this example of Figure 2, a voltage level VRF( of a capacitor (120) and a corresponding indicator (131) are shown as a function of time (t). Therein, VTH denotes the threshold voltage level of a RF channel sensing circuit, and to<t i<t2<ts denote points in time.

[57] In the example of Figure 2, at time to, voltage level VRF(IO) may be below the threshold voltage level VTH. Accordingly, at time to, the indicating-circuit may generate an indicator indicating an idle RF channel.

[58] At time to, an external transmitter (not shown in Figure 2) may perform a RF transmission using the RF channel. Alternatively, another activity using the RF channel may be performed. The capacitor may thus be charged with energy harvested from the RF channel (e.g., via an antenna), such that the voltage level of the capacitor may rise.

[59] At time ti, voltage level VRF(II) may rise above the threshold voltage level VTH. That is, at time ti the voltage level is no longer be below the threshold voltage level VTH. Accordingly, the indicator-circuit generates an indicator indicating an occupied RF channel.

[60] The capacitor may be continuously charged with the energy harvested from the active RF channel until the voltage level of the capacitor reaches a certain voltage level. The reached voltage level of the capacitor may depend for example on a received signal strength. For example, the reached voltage level may depend on the distance of the RF channel sensing circuit to the transmitter performing the RF transmission using the RF channel and/or the transmission power of the RF signal. The reached voltage level may be the result of a balance between a charging speed and a discharging speed of the capacitor.

[61] At time t2, the external transmitter has finished performing the RF transmission using the RF channel. Accordingly, starting from time t2, the capacitor may discharge and voltage level VRF(I) may start dropping.

[62] At time ts, voltage level VRF^S) drops below the threshold voltage level VTH. Thus, at time ts, the indicating-circuit again generates an indicator indicating an idle RF channel.

[63] For times t such that ti<t<t3, the indicator generated by the indicating-circuit may indicate an occupied RF channel, because VRF( is not below the threshold voltage level VTH for ti<t<t3.

[64] As may become clear from the explanation hereinbefore, a RF channel sensing circuit (100) may function using passive electric components (e.g., the capacitor) that do not require powering by an energy source in order to indicate an activity level of the RF channel. In other words, the RF channel sensing circuits may work without a battery, i.e., battery-less. [65] Returning to Figure 1, a RF channel sensing circuit (100) may comprise further optional components that are described next. The further optional components may describe preferable configurations.

[66] A RF channel sensing circuit (100) may comprise a RF-harvesting antenna (140) that may be configured to receive a RF signal on the RF channel.

[67] For example, the RF-harvesting antenna (1 0) may receive a RF signal on a RF channel by capturing the electromagnetic waves into an alternating current. That is, the electromagnetic wave on the RF channel may resonate with charged particles in the RF-harvesting antenna and may induce an alternating current in the RF-harvesting antenna. In other words, an oscillating electromagnetic field may couple to the RF-harvesting antenna in such a way as to cause charged particles to oscillate, which may create an alternating current. Thereby, oscillations of the electromagnetic field originating from the electromagnetic waves may be transferred into oscillations of currents in the RF-harvesting antenna.

[68] In other words, the incoming RF signal on the RF channel is received by the RF- harvesting antenna by transforming energy in the form of electromagnetic oscillations into energy in the form of oscillations of currents.

[69] The RF-harvesting circuit (110) may be coupled to the RF-harvesting antenna (140) for harvesting energy from the RF channel. The RF-harvesting circuit may be configured to output energy from an alternating current to the capacitor (120). In other words, the RF-harvesting circuit (110) may be configured to receive an alternating current and to output energy stored in the alternating current to the capacitor.

[70] The RF-harvesting circuit (110) may also be considered to comprise the RF-harvesting antenna.

[71] The RF-harvesting circuit (110) may comprise a rectifying circuit (112) that may be configured to rectify a received RF signal and to output the rectified RF signal to charge the capacitor.

[72] The rectifying circuit (112) may rectify a RF signal from a RF channel and output the rectified RF signal to charge the capacitor. For example, an alternating current induced in a RF- harvesting antenna may be transformed by the rectifying circuit (112) into a directed current, and the directed current may be output to the capacitor, thereby charging the capacitor. Thus, the received RF signal may be rectified by the rectifying circuit before charging the capacitor.

[73] A RF-harvesting circuit (110) may comprise a matching circuit (111) configured to tune the RF channel sensing circuit (100) to a specific RF range. The matching circuit (111) may for example be a pi-matching network suitable for tuning the RF channel sensing circuit. [74] The matching circuit (111) may also be referred to as a radio tuner or as a RF tuner. For example, the RF channel sensing circuit may be tuned by the matching circuit from a first RF channel to a second RF channel. That may mean the RF channel sensing circuit is configured to sense the first RF channel and may be configured to sense the second RF channel after the matching circuit may have tuned the RF channel sensing circuit to the second RF channel.

[75] The rectifying circuit (112) may comprise a log amplifier circuit (113) or a voltage multiplier circuit (114) for rectifying a received RF signal.

[76] A voltage multiplier circuit can use passive electric components. That means, no external energy supply may be required, and the voltage multiplier circuit may function without relying on additional power supply.

[77] A log amplifier circuit may be using at least partly active electric components. That means, the log amplifier circuit may require additional powering from an energy source. An example of powering a log amplifier circuit is described below in the context of an apparatus.

[78] The choice of selecting a specific type of a rectifying circuit (142), such as a voltage multiplier circuit or a log amplifier circuit, may depend on the field of application of the RF channel sensing circuit. Considerations for choosing optional components for a RF channel sensing circuit are described later with reference to Figures 5 and 6.

[79] The RF channel sensing may comprise a resistor (150) coupled to the capacitor.

[80] The resistor may be configured to discharge the capacitor. In other words, the capacitor may be continually discharged through the resistor. The configuring of the resistor may be achieved by coupling the resistor to the capacitor. For example, by electrically coupling the resistor to the capacitor, the capacitor may be discharged through the resistor.

[81] A resistor may allow for controlling a discharging time/speed of the capacitor. That is, while capacitors may continually discharge by default due to, for example, electric resistance originating from electrical components of circuits, further installing a resistor may allow to control the discharging time/speed of the capacitor.

[82] The discharging time may be the duration, or the length of the time period, it takes for the capacitor’s voltage level to drop below a threshold voltage level when starting to discharge from an initial voltage level higher than the threshold voltage level. For example, the higher the resistance of the resistor, the larger may be the discharging time of the capacitor. In other words, a first discharging time arising from including a first resistor may be larger than a second discharging time arising from including a second resistor, when the resistance of the first resistor is larger than the resistance of the second resistor. Thus, the resistor may be configured with a resistance to achieve a desired discharging time and/or a discharging speed of the capacitor. The resistor may even be a tuneable resistor such that the discharging time and/or discharging speed may be tuned dynamically as desired, e.g., when a quick reacting/ sensing to the channel idle state is needed (e.g., quicker discharging speed/time such that the idle state is indicated quickly after the channel is idle), the resistor may have a lower resistance than the resistance when a slow reacting/ sensing to the idle state is needed (e.g., longer discharging time and speed, and waiting longer before indicating the channel is idle when the channel is actually idle).

[83] The RF channel sensing may comprise a tuner (160) configured to tune the threshold voltage level (VTH). In other words, the threshold voltage level may be changed or may be adjusted by the tuner. The tuner may also be referred to as a potentiometer.

[84] By increasing the threshold voltage level, the RF channel sensing circuit may be less susceptible to noise on the RF channel. For instance, if the threshold voltage level is configured too low, white/b ackground noise on the RF channel may increase the charge level/voltage level to cause the indicating-circuit to indicate an occupied RF channel while no RF transmissions are occupying the RF channel. In other words, a noisy environment may cause a higher minimum voltage level of the capacitor, which may affect the sensitivity of the RF channel sensing circuit. On the other hand, when the threshold voltage level is configured too high, the indicating-circuit may indicate an idle RF channel while still RF transmissions are occupying the RF channel. A tuner may allow to adapt the threshold voltage level depending on environmental circumstances to circumvent such issues.

[85] For a RF channel sensing circuit (100), at least one of the threshold voltage level and a charging and/or discharging speed/time of the capacitor may be configured based on at least one of a density of a plurality of RF channel sensing circuits located in a communication system which comprises the said RF channel sensing circuit (100) and a noise level. Such considerations of configuring at least one of the threshold voltage level and the charging and/or discharging speed, and/or further components of a RF channel sensing circuit (100), are described later with reference to Figures 5 and 6.

[86] Next, an apparatus is described with reference to Figures 3-4. The apparatus may also be referred to as a node, a transmitter node, a transmitter, a transmitter-and-receiver node, a transmission device, a transceiver and/or the like.

[87] Figure 3 shows the general concept of an apparatus comprising optional components and/or optional features. [88] The apparatus may comprise a RF channel sensing circuit (310) as described above, and a communication unit (320) configured to transmit data according to the indicator (311) received from the RF channel sensing circuit.

[89] In Figure 3, the RF channel sensing circuit (310) may be a RF channel sensing circuit (100) as described with reference to Figures 1 and 2. The RF channel sensing circuit may comprise any of the optional components described in Figures 1 and 2, and/or any combination of these optional components. The RF channel sensing circuit (310) may generate an indicator (311) indicating to the communication unit (320) whether a RF channel is idle or not, that is whether the RF channel is idle or is occupied.

[90] The communication unit (320) may be configured to transmit the data only when the indicator indicates the RF channel being idle. That means, a packet transmission may be performed by the communication unit only if the RF channel sensing circuit (310) indicates to the communication unit (320) an idle RF channel. For example, in case the communication unit has a packet transmission pending (that is, data to be transmitted) and the RF channel is indicated to be occupied, the communication unit may not perform the packet transmission, but may wait until the RF channel is indicated to be idle. When the indicating-circuit of the RF channel sensing circuit switches from indicating an idle RF channel to indicating an occupied RF channel, the communication unit may perform the packet transmission immediately. In other words, the transmission of the data may be postponed until the moment the indicator indicates the RF channel to be idle, such that the transmission of the data may be deferred.

[91] The communication unit may be configured to transmit data immediately if the channel is idle according to the indicator received from the RF channel sensing circuit. In other words, the data may be transmitted when the RF channel is indicated to be idle.

[92] The data to be sent/transmitted may comprise pre-stored data by the communication unit (320). The data to be sent/transmitted may, alternatively and/or in addition to pre-stored data, also comprise data generated by events or trigger events, e.g., sensed by the communication unit (320). The data to be sent/transmitted may be a notification message indicative of a trigger event being sensed by the apparatus or any other data.

[93] An example of an apparatus further sensing an event and transmitting data in response to the event is described next.

[94] The apparatus may comprise a sensor (340) configured to sense a trigger event and to generate the data from the trigger event, wherein the communication unit (320) may be configured to transmit the data in response to the trigger event when the indicator indicates the RF channel being idle. [95] For example, the sensor may comprise a mechanical switch, and a trigger event may comprise toggling or clicking the mechanical switch. The operation of toggling or clicking the switch may be sensed by the sensor (e.g., clicking the calling button in airplanes to ask attention from flight attendants). The sensor may also, additionally or alternatively, comprise a sensorantenna receiving RF signals on a RF channel that may be different than the RF channel sensed by the RF channel sensing circuit. An incoming RF signal received by the sensor-antenna may be a trigger event and may be sensed by the sensor. The sensor may also be configured to sense various other events, such as vibrations, audio signals, and the like.

[96] For example, when a sensor (340) senses a mechanical switch operation, the sensor may generate a corresponding notification in response. A sensor may be configured to generate data to be wirelessly transmitted that may be a notification of the trigger event. Similarly, a sensor may generate data in response to the sensor sensing an incoming RF signal and/or in response to sensing various other events, such as vibrations, audio signals, and the like.

[97] A trigger event may also be referred to as an event. The data generated by the sensor from an event sensed by the sensor may also be referred to as an event notification. A trigger event may activate a transmission procedure. That means, a trigger event may initiate to perform a packet transmission and/or the transmission of an event notification (if the RF channel is indicated as idle).

[98] The communication unit (320) may comprise various components for preparing the data for transmitting the data according to telecommunication standards, such as processor or a memory for storing data and for storing instructions for preparing data such that the data can be transmitted by the communication unit. For example, the memory may store instructions that, when executed by the communication unit, cause the communication unit to prepare the data to be wirelessly transmitted in a suitable form. The various components may comprise electronic components such as integrated circuits, ICs, a central processing unit, CPU, and the like.

[99] The communication unit (320) may comprise a wake-up element configured to receive the indicator from the RF channel sensing circuit (310), that is from an indicating-circuit of the RF channel sensing circuit. For example, the wake-up element may be an interrupt pin configured to receive the indicator from the RF channel sensing circuit. For example, in case the indicating-circuit of the RF channel sensing circuit comprises an inverting comparator, the interrupt pin may be configured to receive the signals from the inverting comparator.

[100] An apparatus may comprise an antenna (330) for performing data transmissions. Optionally, the antenna for performing the data transmissions may be the same as an antenna used by the RF channel sensing circuit for receiving RF signals on the RF channel. Optionally, the antennas may also be different from each other, that is one antenna may be for performing data transmissions, while another antenna may be for channel sensing.

[101] An apparatus may comprise an energy-harvesting, EH, unit (350) configured to harvest energy. The EH unit may for instance be configured to harvest energy from the environment in the form of solar energy, thermal energy, wind energy, energy from mechanical events (e.g., a switch click) and the like. The harvested energy may be used to power the apparatus, for example transmission of data or generating of data.

[102] An apparatus may comprise an energy-harvesting, EH, unit (350) configured to harvest energy from the trigger event, wherein the energy harvested from the trigger event may be used for powering the transmitting of the data. For example, when a passenger in an airplane clicks the calling button above his/her seat, the clicking energy is harvested by the EH unit (350) (may be stored before using, e.g., in a battery or capacitor) to transmit the data (i.e., the calling notification) via the communication unit (e.g., wirelessly) to a server (to notify the air attendants that the passenger needs attention).

[103] The EH unit (350) may be configured to harvest energy from a trigger event sensed by the sensor. For example, when the sensor comprises a mechanical switch, the EH unit may be configured to harvest energy from a clicking of the mechanical switch. That is, energy may be scavenged from an operation of the mechanical switch or from operating the mechanical switch. As a further example, when the sensor comprises a sensor-antenna, energy may be harvested by the EH unit from an incoming RF signal received by the sensor-antenna. That means, in general, energy may be extracted from an event and may be stored temporarily by the EH unit for later use. Energy extracted from an event may also be stored only for immediate use of transmitting a corresponding event notification as explained in the above airplane example.

[104] The energy harvested by the EH unit (350) may also be used to power the apparatus. For example, the harvested energy may power various components of the communication unit and/or one or more operations executed by one or more components of the communication unit. For example, the harvested energy may be used to power the process of preparing and wirelessly transmitting data, that is a packet transmission may be powered by the harvested energy. The harvested energy may also be used to power an antenna and/or operations of an antenna used for transmitting data. The harvested energy may also be used to power the RF channel sensing circuit and/or operations of and/or components of the RF channel sensing circuit. For example, a log amplifier circuit of the RF channel sensing circuit may be powered by the harvested energy. [105] The amount of energy harvested from an event may be minimal. That is, the amount of harvested energy may be enough only for performing one transmission. For example, an operation of a mechanical switch may deliver approximately 200 pj, or may deliver approximately 400 pj.

[106] The communication unit (320) may be configured to transmit the data in response to a trigger event as soon as the indicator indicates an idle RF channel. For example, the communication unit may be configured to initiate a packet transmission procedure in response to a trigger event, but may perform a packet transmission only once the RF channel is indicated to be idle by the RF channel sensing circuit.

[107] Figure 4 shows an apparatus (400) that may comprise a RF channel sensing circuit (410) as described above, and a communication unit (420) configured to transmit data according to the indicator (411) received from the RF channel sensing circuit.

[108] The apparatus (400) may be the same or may in part be the same as the apparatus (300) describe above with reference to Figure 3. The RF channel sensing circuit (410) and the indicator (411) may be the same or may in part be the same as a RF channel sensing circuit (100, 310) and the indicator (311), respectively, as described above with reference to Figures 1-3. The communication unit (420), the sensor (440), the EH unit (450), may be the same or may in part be the same as the communication unit (320), the sensor (340), and the EH unit (350), respectively, as described above with reference to Figure 3.

[109] The apparatus (400) may comprise an antenna (430) for transmitting the data. The antenna (430) may be the same or may in part be the same as the antenna (330) described with reference to Figure 3. The antenna (430) may be shared by the RF channel sensing circuit (410) and the communication unit (420). For example, the antenna (430) may also further be used by the RF channel sensing circuit for receiving RF signals on the RF channel. For example, the antenna (430) may be used both for transmitting RF signals as well as receiving RF signals.

[110] Further advantages of the apparatus may become apparent in the context of a system comprising a plurality of apparatuses, which is described next.

[111] A system comprising a plurality of apparatuses (300, 400) is described with reference to Figures 5-6.

[112] The system may comprise a plurality of apparatuses (300, 400), wherein each of the plurality of apparatuses may be configured to sense a shared RF channel and may be configured to transmit data on the shared RF channel. I.e., each apparatus may be configured to perform transmissions using the same RF channel and may be configured to sense the same RF channel by their respective RF channel sensing circuit. [113] A system comprising a plurality of apparatuses may be referred to as a plurality of transmitters that may share the same RF channel. The system may be used for efficiently resolving channel congestion, which may become apparent from the description herein below. In the system, the transmitters may be self-organized, i.e., no central resource allocation for transmitting data may be required and it is easy to add or remove the transmitters in the system.

[114] Figure 5 illustrates a detailed operation of an example system. The invention is not limited to the specifics of this example system. The specifics of this example system are chosen for ease of the description and for illustration, and are not supposed to limit the invention to these specifics. Further considerations of more a general system of apparatuses are also described later with reference to Figure 6.

[115] In the example system of Figure 5, a system of three apparatuses (Al, A2, A3) aligned in one direction is shown. The distance between a first apparatus Ai and a second apparatus A2 may be Im, that the distance between A2 and a third apparatus A3 may also be Im. The distance between Ai and A3 may accordingly be 2m. Each apparatus may be an apparatus as described above, and the three apparatuses may be assumed to share a same RF channel.

[116] In Figure 5, the voltage levels or charge levels (Vi,V2,V3) of the capacitors of each of the three apparatuses (Al, A2, A3) are shown as a function of time (t). Points in time are denoted by ti,. . .,t _n , wherein n >1. In this example, it is assumed that the threshold voltage level VTH is the same for each capacitor. The threshold level may be for instance VTH = 3 mV. For simplicity, one may refer to the voltage level of an apparatus Ai when referring to the voltage level of a capacitor of Ai. An upward-pointing arrow marked with DPi may indicate that a data packet of apparatus Ai is pending/is to be sent. In other words, an upward-pointing arrow may indicate that a data transmission is waiting to be sent and that the communication unit of the corresponding apparatus may intend to use the RF channel for sending the data. For example, the communication unit may have received a MAC frame from higher layers, so that a packet transmission is pending. Boxes marked with TXi indicate an ongoing transmission performed by the corresponding apparatus Ai using the RF channel.

[117] At time ti, a first arrival of a first packet transmission (DPi) may occur for apparatus Ai. Since the voltage level of the capacitor of Ai at time ti is below the threshold voltage level VTH, the RF channel sensing circuit of Ai may indicate an idle RF channel, and thus the communication unit of Ai may perform a data transmission (TXi). The communication unit of Ai may also be said to start transmitting data or to commence a packet transmission.

[118] As the ongoing data transmission (TXi) using the RF channel is progressing, the capacitors of each of the apparatuses may be charged and thus the voltage level of each of the capacitors may grow. The capacitor’ s voltage levels may each rise until they may reach a certain voltage level.

[119] The reached voltage level may depend on the distance of the capacitor to the source of the data transmission. In this example, apparatus A3 is further away from apparatus Ai than apparatus A2 is from Ai. Therefore, the reached voltage level of A3 may be lower as compared to the reached voltage level of A2. In the example, the apparatus A2 may thus reach a higher voltage level than A3 during the transmission TXi performed by Ai. In general, one may observe that a smaller distance to the source may lead to a higher reached voltage level, and a larger distance may lead to a lower reached voltage level. A reached voltage level may further depend on the capacitance of the capacitor, the transmission power of the data transmission, a sensitivity of the RF channel sensing circuit and/or environmental conditions that may affect a signal strength received at the RF channel sensing circuit.

[120] The charging speed of the capacitors may also depend on the distance of the capacitors to the source of the data transmission. A higher distance may lead to a lower charging speed, and vice versa, a smaller distance may lead to a higher charging speed. The charging speed and/or the reached voltage level may thus depend on the power or the energy transfer rate received at an apparatus, the capacitance of the capacitor, the transmission power of the data transmission, a sensitivity of the RF channel sensing circuit and/or environmental conditions.

[121] While the ongoing data transmission (TXi) is progressing, each of the apparatuses A2 and A3 may acquire/receive/schedule an upcoming transmission to be performed (DP2, DP3). For example, A2 and/or A3 may receive data to be sent, or may receive MAC frames arriving from higher layers, or may have generated data in response to sensing a trigger event.

[122] Since each of the capacitors of A2 and A3 may be charged above the threshold voltage level when acquiring/receiving/scheduling an upcoming transmission (DP2, DP3), it may be that neither A2 nor A3 perform a data transmission yet. Thus, A2 and A3 may instead postpone their data transmissions as they may each wait until their RF channel sensing circuit may indicate an idle RF channel. Thereby, their transmissions are deferred as their RF channel sensing circuits may indicate that the RF channel is occupied.

[123] At time t2, apparatus Ai may complete its data transmission (TXi). Upon completion of said data transmission (TXi), the voltage levels of the capacitors may start dropping.

[124] At time t3, the voltage level of A3 may drop below the threshold voltage level. The voltage level of A3 may be the first to drop below the threshold voltage level, since at time t2, the reached voltage level of A2 may be higher than the reached voltage level of A3, that is V2(t2)>V3(t2). As soon as the voltage level of A3 drops below the threshold voltage level, the RF channel sensing circuit of A3 may indicate the communication unit of A3 an idle RF channel and the communication unit may commence a data transmission (TX3) using the RF channel. As a consequence, the capacitors of all apparatuses may start being charged again. In particular, the capacitor of A2 may start charging again before the voltage level of A2 could drop below the threshold voltage level, which may further postpone the pending data transmission of A2.

[125] At time t4, apparatus A3 may complete its data transmission (TX3). Upon completion of the data transmission (TX3), the voltage levels of the capacitors may start dropping again.

[126] At time ts, the voltage level of A2 may drop below the threshold voltage level, which may trigger the communication unit to start a data transmission (TX2) using the RF channel.

[127] At time te, apparatus A2 may complete its data transmission (TX2), and the voltage levels of the capacitors may start dropping again.

[128] At time t?, the voltage level of apparatus A2 may drop below the threshold voltage level. The dropping below the threshold level may occur for apparatus A2 later than for apparatuses Ai and A3, as the reached voltage levels of apparatuses Ai and A3 at time te may be higher than the reached voltage level of A2 at time te.

[129] Various specifics of the above-described example may be different. For example, at least one of the number of apparatuses, the positions of the apparatuses, the positions of the apparatuses relative to each other, the distances between the apparatuses, the threshold voltage levels, the charging speeds, the discharging speeds, the components of each of the apparatuses, the length of packet transmissions, and the like, may be different from what is illustrated in the example.

[130] As may be appreciated from the example, the system comprising a plurality of apparatuses may provide an efficient way of resolving channel contention. Even though the plurality of apparatuses may all use the same RF channel, no central scheduling and no additional signalling may be required to coordinate access to the RF channel. Access to the RF channel may instead be coordinated automatically depending on at least one of the distribution of apparatuses, the environmental conditions affecting the charging/discharging speeds and the power of the RF signals received at the apparatuses.

[131] For a RF channel sensing circuit (100, 310, 410) of an apparatus, at least one of the threshold voltage level and a charging and/or discharging speed of the capacitor may be configured according to system characteristics and/or environmental system characteristics. For example, at least one of the threshold voltage level and a charging/discharging speed may be configured according to at least one of a density of a plurality of RF channel sensing circuits of the system which comprises the said RF channel sensing circuit (100, 310, 410), a noise level of the RF channel, and/or a density of RF-ab sorbing objects in the environment.

[132] Such above-described considerations are further described with reference to Figure 6.

[133] In Figure 6, a system comprising a number of apparatuses (AI, ... ,AN) is shown. Each apparatus may be an apparatus as described above with reference to Figures 3-5. At least two apparatuses may be different from each other. For example, while some of the apparatuses may be the same, some of the apparatuses may also be different from each other. The apparatuses may differ from each other in various ways. For example, the apparatuses may differ by which components they comprise and/or by the specifics of components they comprise. For example, one apparatus may comprise a resistor, while another apparatus may not. For example, one apparatus Ai may comprise a resistorthat has different characteristics than a resistor of a further apparatus Aj. For example, resistors of different apparatuses may have different resistances. For example, capacitors of different apparatuses may have different capacitances. For example, some apparatuses may have sensors comprising mechanical switches, and some apparatuses may have sensors comprising sensor-antennas. Various further combinations can be considered depending on the field of application.

[134] In Figure 6, the number N of apparatuses (Ai, . . . ,AN) may be arbitrary. The number may be changed, for example by adding at least one apparatus to the system, and/or by removing at least one apparatus from the system. Furthermore, the position of at least one of the apparatuses (Ai, . . . , AN) may be changed. For example, an apparatus Aw may be placed at a different location than shown in Figure 6, such as to be located at a location Aw’. The number of apparatuses and the positions of the apparatuses may be chosen depending on, for example, the environmental conditions under which the apparatuses are installed, and/or on the field of application.

[135] In Figure 6, each of the apparatuses may be configured to sense the same RF channel by their RF channel sensing circuits. For example, when a first apparatus Ai performs a packet transmission using the RF channel, in other words transmits a RF signal on the RF channel, the capacitors of each of the system of apparatuses may be charged with energy harvested from the active RF channel. Each of the apparatuses may thus receive the RF signal on the RF channel and may each charge their capacitor. During transmission, that is while the first apparatus Ai may keep transmitting, the capacitors may be charged with their individual charging speed until they each may reach a certain charge level.

[136] The individual charging speed and/or the charge level of a capacitor reached during transmission of the first apparatus Ai may depend on the capacitor’s distance to the first apparatus Ai transmitting the RF signal. For example, a second apparatus A2 may be closer to the first apparatus Ai than a third apparatus A3. As a consequence, the second apparatus A2 may be charged faster than the third apparatus A3, and may be charged to a higher charge level than the third apparatus A3. That is, a distance dn between Ai and A2 may be smaller than a distance di3 between Ai and A3. Thus, as a consequence of the smaller distance, the charge level of A2 reached during an ongoing transmission of Ai may be higher than the charge level of A3 reached during said ongoing transmission.

[137] The individual charging speed and/or the charge level of a capacitor may also depend on various environmental conditions. For example, an obstacle (not shown in Figure 6) located between the first apparatus Ai transmitting the RF signal and an apparatus As may cause the capacitor of As to be charged less quick than it would without the obstacle, and may cause the capacitor of As to be charged to a lower charge level than it would without the obstacle. For example, the obstacle may partly absorb RF signals passing through the obstacle, which may deteriorate the signal strength received at for example As and/or A7. For example, environmental conditions, such as weather conditions, may influence individual charging speeds and/or charge levels for apparatuses placed in open environments.

[138] A system of apparatuses may thus coordinate the access to the RF channel automatically based on the charge level of capacitors, as upcoming transmissions to be performed by the apparatuses may be deferred differently for different apparatuses as a consequence of each apparatus sensing the RF channel by their RF channel sensing circuit and deferring their transmissions until the RF channel is indicated to be idle. While the first apparatus Ai is transmitting, the RF channel sensing circuits of each of the plurality of apparatuses may indicate an occupied RF channel. Consequently, the communication units of the apparatuses may not commence or start transmissions, in accordance with their received indicators from their RF channel sensing circuits. For example, another apparatus having a pending transmission waiting to be sent out may defer its pending transmission until its RF channel sensing circuit indicates an idle RF channel. In Figure 6 (similarly as described in the example system with reference to Figure 5), as soon as the first apparatus Ai stops transmitting, the charge level of the apparatuses may start dropping, and an apparatus whose capacitor’s voltage level first drops below its threshold voltage level may be the next apparatus to perform a data transmission. The voltage level of a capacitor of an apparatus that may be furthest away from Ai, for example of apparatus A10 whose distance to Ai may be the largest, may thus first drop below the threshold voltage level. Continuing the procedure, each of the apparatuses may eventually gain access to the RF channel. [139] As a consequence, this procedure may result in positional unfairness for apparatuses deployed in a region with a higher density of apparatuses as compared to apparatuses deployed in a region with a lower density of apparatuses. That may be because capacitors surrounded by more apparatuses may be more likely to be charged than capacitors in a region with a low density of apparatuses, as capacitors in high density regions may more frequently encounter transmissions on the RF channel from close-by transmitters.

[140] For example, in Figure 6, capacitors of apparatuses in a region R may be more frequently charged to higher charge levels than the capacitor of apparatus A3. In region R, a density of apparatuses may be higher than in a region around apparatus A3. As a consequence, apparatuses in the region R may less frequently gain access to the RF channel.

[141] The above-described effects may be taken into account by configuring a system of apparatuses accordingly. For example, a system comprising a plurality of apparatuses (300, 400, AI,A2, ... ,AIO) may be considered in which wherein at least two apparatuses may differ from each other by at least one of a threshold voltage level and a charging and/or discharging speed of their respective capacitors.

[142] For example, to counter the effect caused by positional unfairness, at least one of the threshold levels and the charging and/or discharging speeds of the apparatuses may be configured in consideration of a density of apparatuses. For example, when the system of apparatuses is placed on a regular grid, apparatuses on the boundary of the grid may be considered to be in a low density region, and apparatuses away from the boundary (that is closer to the core of the grid) may be considered to be in a high density region. Accordingly, charging/discharging speeds of the apparatuses on the boundary of the grid may be decreased and/or charging/discharging speeds of the apparatuses away from the boundary may be increased. Choosing the charging/discharging speeds in such a way may balance the average time for each apparatus to gain access to the RF channel, and may even out the imbalances that may be caused by positional unfairness.

[143] Various other examples of grids and densities, and configuring for example the discharging speeds accordingly, may be considered. A charging and/or discharging speed of a capacitor of a RF channel sensing circuit may be adapted to balance positional unfairness. For example, increasing a discharging speed may increase the chance/probability of transmissions of the respective apparatus, and accordingly, decreasing a discharging speed may decrease the chance/probability of transmissions.

[144] A charging/discharging speed of an apparatus may be configured according to at least one of the threshold level configured for the apparatus, the reached voltage level from which the discharging may start, the noise level on the RF channel, the resistance R of a resistor comprised in the RF channel sensing circuit of the apparatus, and the capacitance of the capacitor comprised in the RF channel sensing circuit.

[145] A charging and/or discharging speed/time may be configured and/or manipulated for instance by adapting a RC value for the corresponding RF channel sensing circuit. For example, a resistor with a higher resistance and coupled to a capacitor may be used to decrease the discharging speed.

[146] A system comprising a plurality of apparatuses (300, 400, AI,A2, .. .,Aio) may be considered wherein at least two apparatuses may differ from each other by at least one of a threshold voltage level and a charging and/or discharging speed/time of their respective capacitors.

[147] For example, at least one of the threshold voltage level and a charging and/or discharging speed/time of a capacitor of a RF channel sensing circuit (100, 310, 410) may be configured according to at least one of a density of a plurality of RF channel sensing circuits located in the system which comprises the said RF channel sensing circuit, and a noise level of the RF channel. In general, components of each apparatus may thus be chosen in consideration of at least one of the distances between the apparatuses, the density of apparatuses, the sensitivity of at least one of the used RF channel sensing circuits, positional fairness of at least one of the apparatuses, desired threshold voltage levels and/or discharging speeds of the reference capacitors comprised in the RF channel sensing circuits, influence of noise, environmental effects.

[148] For example, a voltage multiplier circuit may be used when amounts of harvested energy from a RF channel are relatively low and/or when transmitting nodes are relatively nearby. A log amplifier circuit may assist is sensing activity of other transmitters using the RF channel over longer ranges/di stances than compared to a voltage multiplier circuit. That is, a log amplifier circuit may sense channel activity from transmitting nodes that are placed further away from the RF channel sensing circuit. A log amplifier circuit may for example be powered by energy harvested from a EH unit of the corresponding apparatus.

[149] A system comprising a plurality of apparatuses may thus perform an ultra-low power MAC protocol by taking advantage of RF information harvesting. That is, each apparatus may employ its RF channel sensing circuit to sense the RF channel to gain information about the occupancy of the RF channel. The provided method performed by such a system may be referred to as an ultra-low power MAC protocol, as the method of channel sensing via RF channel sensing circuits may be done at essentially no energy costs. [150] Multiple systems according to the present invention may be used. For example, a first system may be configured to work on a first RF channel, and a second system may be configured to work on a second RF channel.

[151] The present invention may find applications in various field of industry, comprising the fields of airline industry, logistics, Internet of Things, smart factories.

[152] For example, in air industry, the crew of an airplane is at risk during turbulences as they need to manually inspect if passengers have fastened their seat belts. The losses for airliners due to mid-air injuries from falling baggage during severe turbulences as seat belts were not fastened and lost equipment may be in the order of hundreds of millions of dollars. Event-driven signalling devices may improve safety automatically by checking whether seat belts are fastened or overhead bins are properly latched. For example, energy generated by fastening the belts or closing the bins may be harvested to power the generating of the notification messages and the transmitting of the messages (when the channel is sensed idle). Since the data rate requirement is low, the device density is also low and the transmitted distance is normally not long, the generated energy would be enough for such communications without additional energy sources.

[153] For example, a signalling device including a mechanical switch may be installed for each seat in an airplane. The event of toggling the mechanical switch may trigger a wireless signal indicating to the crew a fastened seat belt for the corresponding seat. During turbulences it is thus expected that a large number of such events occur almost simultaneously, leading to a large number of notifications being triggered almost simultaneously. For example, it may be the same as explained above in the safe belt and bin examples.

[154] The channel-access mechanisms according to the present invention can efficiently handle such channel contention at ultra-low energy costs (e.g., without being powered by batteries), so that additional wiring is avoided and maintenance costs are reduced.