Patent Description:
Optical cardiac activity measurement, such as measuring heart rate using a wrist device comprising an optical sensor, has become increasingly popular. At the same time, the quality of such measurements may be reduced by, for example, movement of the optical sensor with respect to body tissue. Hence, it may be beneficial to provide solutions which reduce effect of motion artefacts in cardiac activity signals. Relevant background prior art is provided in <CIT>, <CIT>, <CIT>, <CIT>.

According to an aspect, there is provided the subject matter of the independent claims. Some embodiments are defined in the dependent claims.

In the following embodiments will be described in greater detail with reference to the attached drawings, in which.

<FIG> illustrates a system to which embodiments of the invention may be applied. Said system may be used to monitor physical training, activity, and/or inactivity of a user <NUM>. Thus, the embodiments may not be limited to monitoring and/or measuring physical training of the user <NUM>, and thus said system may be used to monitor physical activity and/or inactivity during the day and/or night (e.g. <NUM> hours a day). Such may be possible using one or more devices described with respect to <FIG> and in the embodiments below.

Referring to <FIG>, the user <NUM> may wear a wearable device, such as a wrist device <NUM>, a head sensor unit 104C, a torso sensor 104B, and/or a leg sensor 104A. In another example, the wearable device may be and/or be comprised in glasses. In another example, the wearable device is comprised or configured to be coupled with a garment or garments (or apparel). Examples of such garments may include bra(s), swimming apparel, such as swimming suit or cap, and glove(s). The garment or apparel may be worn by the user. In some embodiments, the wearable device is integrated as a part of the garment or apparel. Due to simplicity reasons, let us now describe the wearable device as being the wrist device <NUM>. However, embodiments described in relation to wrist device <NUM> may be utilized by other types of wearable devices. the embodiments are not necessarily limited to wrist device or devices <NUM>.

The wrist device <NUM> may be, for example, a smart watch, a smart device, sports watch, and/or an activity tracking apparatus (e.g. bracelet, arm band, wrist band, mobile phone). The wrist device <NUM> may be used to monitor physical activity of the user <NUM> by using data from internal sensor(s) comprised in the wrist device <NUM>, data from external sensor device(s) 104A-C, and/or data from external services (e.g. training database <NUM>). It may be possible to receive physical activity related information from a network <NUM>, as the network may comprise, for example, physical activity-related information of the user <NUM> and/or some other user(s). Thus, the wrist device <NUM> may be used to monitor physical activity related information of the user <NUM> and/or the other user(s). Naturally, one or more of the external sensor device(s) 104A-C may be worn by the other user(s), and thus information received from said one or more sensor device(s) 104A-C may be monitored from the wrist device <NUM> by the user <NUM>. The network <NUM> may comprise the training database <NUM> and/or the server <NUM>. The server <NUM> may be configured to enable data transfer between the training database <NUM> and some external device, such as the wearable device. Hence, the database <NUM> may be used to store cardiac activity measurement data, for example.

It needs to be understood that the wrist device <NUM> may be used to monitor physical activity of the user <NUM> and/or to be used as a smart watch configured to enable communication with, for example, a portable electronic device <NUM>, the network <NUM>, and/or some other network, such as a cellular network. Thus, for example, the wrist device <NUM> may be connected (i.e. wirelessly connected) to the portable electronic device <NUM>, such as a mobile phone, smart phone, tablet and/or computer to name a few. This may enable data transfer between the wrist device <NUM> and the portable electronic device <NUM>. The data transfer may be based on Bluetooth protocol, for example. Other wireless communication methods, such as Wireless Local Area Network (WLAN) and/or Near Field Communication (NFC), may also be used.

In case of communicating directly with the cellular network, the wrist device <NUM> may comprise similar communication capabilities as mobile devices, such as <NUM>, <NUM>, LTE, LTE-A, <NUM> and/or <NUM> communication capabilities. Thus, for example, the wrist device <NUM> may comprise the communication circuitry capable of operating on said technologies, a Subscriber Identification Module (SIM) and/or a memory comprising a virtual SIM configured to provide a secured identification for the wrist device <NUM> when operating with the cellular network. It is also pointed out that, in general, the wearable device may comprise a communication circuitry capable of cellular, Bluetooth, NFC, WLAN, and/or LAN communication.

The wrist device <NUM> may be used to monitor activity and/or inactivity of the user <NUM>. Similarly, the portable electronic device <NUM> may be used to monitor the activity and/or inactivity of the user <NUM>. Such may require the portable electronic device <NUM> to acquire physical activity-related data from the wrist device <NUM>, some other wearable device, and/or from external sensor device(s) 104A-C. However, it may be that the portable electronic device <NUM> determines activity and/or inactivity of the user <NUM> by utilizing internal sensor(s), such as accelerometer or satellite positioning circuitry.

The wrist device <NUM> may comprise a cardiac activity circuitry configured to determine cardiac activity of the user <NUM>, such as heart rate, Heart Beat Interval (HBI) and/or Heart Rate Variability (HRV), for example. The cardiac activity circuitry may comprise an optical cardiac activity sensor unit configured to measure the cardiac activity of the user <NUM>. Example of such sensor may be a PPG (photoplethysmography) sensor. The optical cardiac activity sensor unit may detect the cardiac activity of the user <NUM> by optical measurement, which may comprise emitting light towards body tissue of the user <NUM> and measuring the bounced, reflected, scattered and/or emitted light from the body tissue of the user <NUM>. The emitted light may alter when travelling through veins of the user <NUM> and the alterations may be detected by the optical cardiac activity sensor unit. By using the detected data, the wrist device <NUM>, may determine cardiac activity of the user <NUM>, such as heart rate for example. The optical cardiac activity sensor unit may obtain via the measurement a cardiac activity signal characterizing or carrying the cardiac activity information on the user. As understood, similar cardiac activity circuitry may be comprised in some other wearable device also.

It also needs to be noted that the cardiac activity circuitry may produce raw measurement data of the cardiac activity and/or it may process the measurement data into cardiac activity information, such as heart rate for example. The sensor(s) in the cardiac activity circuitry may comprise data processing capabilities. Also, the wrist device <NUM> and/or some other wearable device may comprise a processing circuitry configured to obtain the cardiac activity measurement data from the cardiac activity circuitry and to process said data into cardiac activity information, such as a cardiac activity metric characterizing the cardiac activity of the user <NUM>. For example, the measurement data of the optical cardiac activity sensor unit may be used, by the processing circuitry, to determine heart rate, HRV and/or HBI of the user <NUM>. Further, the raw measurement data and/or processed information may be processed by the wrist device <NUM> or some other wearable device, and/or transmitted to an external device, such as the portable electronic device <NUM>.

The wrist device <NUM> (or more broadly, the wearable device) may comprise other types of sensor(s). Such sensor(s) may include a Laser Doppler-based blood flow sensor, a magnetic blood flow sensor, an Electromechanical Film (EMFi) pulse sensor, a temperature sensor, a pressure sensor, and/or a polarization blood flow sensor.

In an embodiment, the wearable device comprises a motion circuitry configured to measure motion induced by the user <NUM> to the wearable device, for example, by moving hand (if the wearable device is the wrist device). The motion circuitry may comprise one or more gyroscopes, one or more accelerometers and/or one or more magnetometers. The motion circuitry may use other motion data, such as location data of the user, to determine motion of the user <NUM>. For example, the motion circuitry may comprise a satellite positioning circuitry, such as a global navigation satellite system (GNSS) circuitry. The GNSS circuitry may comprise, for example, a Global Positioning System (GPS) and/or a GLObal NAvigation Satellite System (GLONASS). The satellite positioning circuitry may be used for receiving satellite positioning data. The satellite positioning data may be used, by the wearable device, to determine motion and/or location of the user <NUM>.

In an embodiment, the motion circuitry comprises at least one of the following: an accelerometer, a magnetometer, and a gyroscope.

In an embodiment, the motion circuitry comprises an accelerometer and a gyroscope. The motion circuitry may further comprise sensor fusion software for combining the accelerometer data and gyroscope data so as to provide physical quantities, such as acceleration data, velocity data, or limb trajectory data in a reference coordinate system having orientation defined by a predetermined gyroscope orientation.

In an embodiment, the motion circuitry comprises a gyroscope and a magnetometer. The motion circuitry may further comprise sensor fusion software to combine gyroscope data and magnetometer data so as to provide a reference coordinate system for the gyroscope based on the Earth magnetic field measured by the magnetometer. In general, the sensor fusion software described above may combine measurement data acquired from at least two motion sensors such that measurement data acquired from one motion sensor is used to establish the reference coordinate system for the measurement data acquired from at least one other motion sensor. Thus for example, the satellite positioning data may also be utilized in the sensor fusion.

Measuring cardiac activity of the user with the optical cardiac activity sensor unit (referred to simply as OHR), may be affected by motion artefacts. That is, motion artefacts may cause an effect on the measured cardiac activity signal. The effect may cause the information carried by the signal to be erroneous and/or incomplete. On the other hand, the OHR may not be in good contact with the body tissue (e.g. skin) of the user <NUM>, which may cause possible problems in the measurement. Therefore, there is provided a solution to reduce the effect of motion artefacts on a cardiac activity signal measured using the OHR. The solution may enable the users to receive even more accurate cardiac activity information to help them, for example, during physical training or to plan their future training sessions.

<FIG> illustrates an arrangement <NUM> (could be referred to as a measuring head <NUM>) for measuring cardiac activity of the user <NUM> according to an embodiment. Said arrangement <NUM> may be comprised in a wearable device. One example of such wearable device may be seen in <FIG>. Said wearable device <NUM> may be the wearable device discussed in relation to <FIG>. For example, the wearable device <NUM> may be the wrist device <NUM>, head sensor 104C, torso sensor 104B (e.g. physical activity measurement belt, such as a heart activity transmitter), and/or the leg sensor 104C.

Referring to <FIG>, the arrangement <NUM> comprises an optical cardiac activity sensor unit <NUM> configured to be placed in contact with a measurement area <NUM> and to enable cardiac activity measurement of the user <NUM> to obtain a cardiac activity signal. As explained, the cardiac activity signal may represent or carry information about cardiac activity of the user. The wearable device <NUM> may, based on the signal, cause transmission and/or displaying cardiac activity data to user.

Arrows <NUM> may indicate emitted light by the OHR <NUM> towards and/or into the measurement area <NUM>. Dotted arrows <NUM> may indicate the light that is detected and/or detectable by the OHR <NUM>. Based on these detections, the wearable device <NUM> or the OHR <NUM> may obtain and/or generate the cardiac activity signal.

The arrangement <NUM> further comprises a plurality of electrodes <NUM>, <NUM> configured to enable bioimpedance measurement on the measurement area <NUM> to obtain a bioimpedance signal. The wearable device <NUM> may thus obtain the cardiac activity signal and the bioimpedance signal which are both associated with the same measurement area <NUM>. The measurement area <NUM> may be comprised in a body tissue <NUM> (illustrated with a dotted pattern) of the user <NUM>. Hence, both the OHR <NUM> and the plurality of electrodes <NUM>, <NUM> may be placed in contact with the body tissue <NUM>, and in contact with the measurement area <NUM>.

Arrow <NUM> may indicate a bioimpedance measurement path between the electrodes <NUM>, <NUM>. The electrodes <NUM>, <NUM> may be arranged and placed such that they can be used to obtain, by the wearable device <NUM>, the bioimpedance signal representing and/or indicating bioimpedance of the measurement area. The path <NUM> may actually cross the emitted light <NUM> and/or the detected light <NUM> (or more generally the light path caused by the OHR <NUM>). However, for illustration purposes it has been drawn separate from the light arrows.

The wearable device <NUM> may be further configured to detect changes in the bioimpedance signal and to reduce a motion artefact effect on the cardiac activity signal based on the detected changes in the bioimpedance signal. Thus, the wearable device <NUM> may enhance the cardiac activity signal or form/generate a new cardiac activity signal that has less motion artefact effects compared with the originally measured signal. As shown in <FIG>, the body tissue <NUM> may form an uneven plane for the measurement arrangement <NUM>, thus complicating the measurement even further. Hence, the enhancing the cardiac activity signal as proposed in this solution may be even more useful.

In an embodiment, diameter of the arrangement of <FIG> is between <NUM> centimeter (cm) and <NUM> centimeters. In an embodiment, the arrangement of <FIG> may have even smaller diameter than <NUM>. Diameter may refer to distance between the two outermost electrodes. In the example of <FIG>, this may mean distance between the electrodes <NUM> and <NUM>.

Let us first discuss how the bioimpedance measurement can be performed by looking at an embodiment of <FIG> illustrating a circuit diagram of bioimpedance measurement. Bioimpedance may describe electrical properties of a biological tissue. Biological tissues and cells are conductive and can be modelled as resistive and/or capacitive elements. When current flows through biological tissue of the user <NUM>, the corresponding bioimpedance can be measured by the electrodes <NUM>, <NUM> (or some other electrodes) contacting the body tissue (e.g. skin) of the user <NUM>. Bioimpedance measurement, in general, can be used as means to measure, for example, body composition of the user. However, in the context of the present solution, it may provide some valuable information about the motion artefacts associated with the OHR <NUM> measurement.

Referring to <FIG>, the electrodes <NUM>, <NUM> may be arranged to measure impedance of an object <NUM>. In the context of the presented solution, the object <NUM> may refer to the measurement area <NUM>. the electrodes may be arranged to measure bioimpedance from or of the measurement area <NUM>. The measurement area <NUM> may be part of a human body or situated in a part of a human body (e.g. arm, wrist, leg, ankle, ear, head, forehead, chest). Hence, the bioimpedance measurement. The wearable device <NUM> may further comprise a voltage meter <NUM> (or similar voltage measurement means) coupled in parallel with a current source <NUM> (or similar electric current providing means).

As discussed, the current source <NUM> (e.g. alternative current (AC) source) may be connected in between the plurality of electrodes <NUM>, <NUM> (e.g. the two electrodes). Additionally, one or more biasing resistors (no shown in <FIG>) may be coupled in between the plurality of electrodes <NUM>, <NUM> (or in other words in parallel with the AC source <NUM>). An AC coupling capacitor (not shown in <FIG>) may be coupled in between one of the electrodes <NUM>, <NUM> and the AC source <NUM>. Another AC coupling capacitor (not shown in <FIG>) may be coupled in between another one of the electrodes <NUM>, <NUM> and the AC source <NUM>. Said capacitors may be used to remove direct current (DC) component from the AC source. The capacitors and/or the resistor(s) may be used to filter out low frequency noise from the bioimpedance signal. In general, the bioimpedance of the measurement area <NUM> may be obtained by the wearable device by dividing the measured voltage (e.g. voltage meter <NUM>) with the known current (e.g. current source <NUM>). In case at least one of the electrodes <NUM>, <NUM> (e.g. an electrode pair selected among a plurality of electrodes) is not in contact (or not sufficiently in contact) with the body tissue of the user <NUM>, the impedance signal may be based on measuring the impedance of the one or more biasing resistors (in case such resistors are used). The resistance of said biasing resistors can be selected to such that the non-contact or insufficient contact situation may be detected by the wearable device <NUM>. That is, as the resistance of said biasing resistor may be known, the measured impedance signal may indicate certain predictable or known values. Furthermore, if the resistance of the biasing resistors is configured to be substantially higher than the largest possible impedance of the measurement area <NUM>, the non-contact or insufficient contact situation may be detected.

In an embodiment, the frequency of the AC signal is lower than <NUM> (e.g. <NUM>-<NUM>). In one example, the frequency of the AC signal may be <NUM>-<NUM>. Different frequencies or frequency areas may be used to reveal different parameters of the user.

Using the shown arrangement of <FIG>, the impedance of the object <NUM> may be measured. It needs to be noted that the shown arrangement may be one suitable way to perform the measurement. Hence, other suitable measurement arrangements for measuring bioimpedance of the measurement area <NUM> may be used.

<FIG> illustrate some examples of a bioimpedance signal acquired using the plurality of electrodes <NUM>, <NUM> from the measurement area <NUM>. At least two electrodes at a time are used, by the wearable device <NUM>, to measure the bioimpedance signal. Referring to <FIG>, bioimpedance <NUM> may indicated as a function of time t. Bioimpedance signal <NUM> of <FIG> may indicate bioimpedance signal in case the contact of the OHR <NUM> with the body tissue of the user <NUM> is not sufficient or not good. For example, wrist strap of the wrist device <NUM> may be kept too loose, and thus the signal <NUM> indicates greater variation compared with the bioimpedance signal <NUM> representing a case where the strap is kept tighter.

In an embodiment, the wearable device <NUM> is configured to measure contact of the OHR <NUM> with the body tissue based on the bioimpedance signal (e.g. <NUM>, <NUM>). Based on the measuring, the wearable device <NUM> may be configured to output a control signal. For example, the wearable device <NUM> may be configured to output the control signal in case the OHR <NUM> is not in contact with the body tissue. The control signal may cause output of a visual (e.g. via display), haptic (e.g. via vibration element) and/or sound notification (e.g. via speaker) to the user <NUM>. Alternatively or additionally, the control signal may be transmitted, by the wearable device, to an external device (e.g. portable device <NUM>). For example, the control signal may cause output of a notification via said external device. The measuring of contact of the OHR <NUM> may be performed alternatively or additionally to the reducing the effect of motion artefacts on the cardiac activity signal. In some instances it may suffice that the user is indicated that the contact of the OHR <NUM> is good or not good. However, in some instances this may be performed together with enhancing the cardiac activity signal by removing or reducing the motion artefact effect. The determination whether the OHR <NUM> is in good or sufficient contact with the body tissue may be based on comparing the bioimpedance signal against one or more thresholds. For example, if the bioimpedance signal is between certain thresholds, the contact may be determined, by the wearable device <NUM>, to be good, and bad if the signal is not within said thresholds. However, in general, the wearable device <NUM> may acquire the bioimpedance signals via the electrodes <NUM>, <NUM>, and cause the output of the control signal in case the bioimpedance signal indicates a condition indicating insufficient body tissue contact by the OHR <NUM>. Such condition may be, for example, that the wearable device <NUM> may not be able to output a cardiac activity data of the user <NUM> (such as heart rate, HRV and/or HBI). Consequently, the control signal may be outputted by the wearable device <NUM>.

Let us then look at <FIG> illustrating some examples of bioimpedance signal and cardiac activity signal(s). Referring to <FIG>, as said the cardiac activity signal <NUM>, indicating cardiac activity <NUM> as a function of time t, may comprise some variations caused by motion artefacts. For example, at time period <NUM>, the cardiac activity signal <NUM> seems to first have a pulse with higher amplitude and then the signal drops well below average level (or DC level). Looking at bioimpedance signal <NUM> (signals <NUM>, <NUM> may be time synchronized with each other) indicating bioimpedance <NUM>, during the same period <NUM> similar observation may be made. It needs to be noted that the similarity of the pattern of the signals <NUM> and <NUM> during period <NUM> may as well be a coincidence. However, it may also be that such observation may be made depending on the motion artefact. Now, by processing the signals <NUM>, <NUM> in a certain manner, the effect of the motion artefacts on the cardiac activity signal <NUM>, during period <NUM>, may be reduced based on the bioimpedance signal <NUM>. There are several ways to perform the reduction.

In an embodiment, the wearable device <NUM> is configured to scale the cardiac activity signal and/or the bioimpedance signal, wherein the reducing the motion artefact effect on the cardiac activity signal is further based on performing, by the wearable device, a division operation, a subtraction operation and/or an adding operation between the cardiac activity signal and the bioimpedance signal. The division, subtracting and/or adding may be performed in time domain and/or in frequency domain. In the example of <FIG>, the bioimpedance signal <NUM> may be scaled such that an average of the signal during a certain time period substantially equals to an average of the cardiac activity signal <NUM>. Then the bioimpedance signal <NUM> may be subtracted from the cardiac activity signal <NUM>, thus reducing the motion artefact effect on the cardiac activity signal <NUM>.

In an embodiment, the wearable device <NUM> is configured to control (e.g. reduce, increase or repair) phase of the measured bioimpedance signal <NUM> and/or the cardiac activity signal <NUM> before reducing the motion artefact effect on the cardiac activity signal <NUM> (e.g. by processing the cardiac activity signal <NUM> based on the bioimpedance signal <NUM> to obtain an enhanced cardiac activity signal).

In an embodiment, the wearable device <NUM> is configured to determine a correlation factor between the bioimpedance signal (e.g. signal <NUM>) and the cardiac activity signal (e.g. signal <NUM>). In case the correlation factor indicates a correlation between the two signals exceeding a certain threshold, the wearable device <NUM> may trigger the motion artefact compensating. That is, for example, before proceeding to step <NUM> of <FIG>, the wearable device <NUM> may determine whether the correlation exceeds the threshold. If the correlation does not exceed the threshold, the wearable device may not perform step <NUM> of <FIG>. So, it may be useful to determine whether there is enough correlation between the two signals before trying to reduce the motion artefact effects. The correlation factor may be determined in time domain and/or frequency domain. It may also be possible that the signals (e.g. <NUM>, <NUM>) are filtered before determining the correlation factor. In general, there are multiple ways to do the reduction of the effect of the motion artefacts on the cardiac activity signal, such as on the signal <NUM>, based on the measured bioimpedance signal, such as the signal <NUM>. In the context of the present solution, the effect of the motion artefacts may cause the cardiac activity signal to comprise errors during period(s) where cardiac activity data or information (e.g. heart rate, HRV, HBI) may not be derivable. Such effects may be caused by, for example, movement between the OHR <NUM> and the body tissue of the user <NUM>. For example, the OHR <NUM> may move with respect to the body tissue, and thus the signal obtained via the measurement may cause errors. Such movement may occur, for example, if the OHR <NUM> is not attached firmly enough against the body tissue. Another example, is that body tissue deformation(s) and/or change in body tissue characteristics may cause the described motion artefact effect. That is, if the user <NUM> squeezes his hand or fingers into a fist, such deformations may occur at the measurement area <NUM>, which may be situated at the wrist area of the user <NUM>. The OHR <NUM> may move with respect to the measurement area <NUM> along the skin that is being deformed. Hence, the describe effects may be caused by multiple different things, such as body tissue deformation and/or movement of the OHR <NUM> with respect to the body tissue.

Before discussing <FIG> in more detail, let us look closer on <FIG> illustrating the wearable device <NUM> according to some embodiments. Referring to <FIG>, the wearable device <NUM>, such as the wrist device <NUM>, comprises the plurality of electrodes <NUM>-<NUM>. The plurality may comprise two or more electrodes. In the example of <FIG>, there are four electrodes <NUM>-<NUM>. In some instances, the plurality of electrodes <NUM>-<NUM> may be referred to as bottom electrodes, as they may be situated at the bottom face or side of the wearable device <NUM>. The electrodes <NUM>-<NUM> may comprise bioimpedance electrodes, electrocardiography (ECG) electrodes, and/or galvanic skin response (GSR) electrodes, for example.

In an embodiment, the wearable device <NUM> comprises the OHR <NUM> comprising a plurality of light emitting elements (LEEs) <NUM> and/or a plurality of light detectors <NUM> (e.g. photodiodes (i.e. configured for cardiac activity measurement) and/or matrix detectors). In one example, the wearable device <NUM> comprises one LEE <NUM> and four light detectors <NUM>. Each LEE <NUM> may comprise one or more Light Emitting Diodes (LEDs), for example. The LEDs may be of same or different color. Different colors may comprise green (about <NUM>), red (about <NUM>), yellow (about <NUM>), and blue (about <NUM>), for example. Thus, in an embodiment, the OHR <NUM> may be multicolor OHR configured to use a plurality of different light wavelengths to perform the optical cardiac activity measurement.

With respect to using a plurality of different colors (i.e. different wavelengths) it is noted that the detected motion errors may be different for different wavelengths. Hence, the correlation between a cardiac activity signal and the bioimpedance signal may be different depending on the used color. For example, the bioimpedance signal may correlate better with a cardiac activity signal obtained using red color compared with a cardiac activity signal obtained using green color. Hence, the wearable device <NUM> may be configured to determine respectively the correlation between the bioimpedance signal and a plurality of cardiac activity signals obtained using different colors. The wearable device <NUM> may further be configured to select the cardiac activity signal with highest correlation with the bioimpedance signal based on the determined correlation. The selected cardiac activity signal may further be processed as described herein after and/or above to reduce the motion artefact effect on the selected signal. Similar process may be used to select a cardiac activity signal amongst a plurality of different cardiac activity signals obtained from different locations.

It may be possible to utilize different frequencies in the bioimpedance measurement (e.g. see source <NUM> of <FIG>) to obtain as relevant biompedance signal as possible. That is, different AC frequencies may correlate better with different colors. For example, for a first color based OHR <NUM> measurement, the bioimpedance measurement may be configured to be performed using a first frequency, and for a second color based OHR <NUM> measurement, the bioimpedance measurement may be configured to be performed using a second frequency, wherein said frequencies or frequency areas are different and/or do not overlap with each other. Hence, in general, the wearable device <NUM> may be configured to perform bioimpedance measurement on at least two different frequencies. Each frequency may be associated with a certain selected wavelength used in OHR <NUM> measurement. For example, bioimpedance measurement between electrodes <NUM> and <NUM> may be performed using a first frequency and a bioimpedance measurement between electrodes <NUM> and <NUM> may be performed using a second frequency. Similar logic may be used with other electrode pairs formed from the group of electrodes <NUM>-<NUM> and/or electrodes <NUM>, <NUM>.

In an embodiment, the optical cardiac activity sensor unit (i.e. the OHR <NUM>) comprises at least one light emitting element <NUM> and at least one light detector <NUM>, wherein at least one of said at least one light emitting element <NUM> and said at least one light detector <NUM> is positioned partially or fully between first and second electrodes of the plurality of electrodes <NUM>-<NUM>. For example, the LEE <NUM> may be situated between the electrodes <NUM> and <NUM>, in the example of <FIG>. For example, the light detector <NUM> may be situated between the electrodes <NUM> and <NUM>, in the example of <FIG>. However, in some cases both the LEE <NUM> and the detector <NUM> may situated between the same electrodes. Such arrangement may enable the electrodes, e.g. <NUM>, <NUM>, to measure bioimpedance on the propagation route of the light emitted by the LEE(s) <NUM> and detected by the light detector(s) <NUM>. Hence, the changes in the bioimpedance signal may reveal effect of motion artefacts on the propagation route(s) of light between source(s) and detector(s). By forming different pairs from the plurality of electrodes <NUM>-<NUM>, the wearable device <NUM> may measure bioimpedance signal(s) from plurality of different measurement areas. Such measurement area or areas (e.g. area <NUM>) may refer to area(s) through which a light beam is transmitted by the OHR <NUM>.

Referring to <FIG>, the plurality of electrodes <NUM>-<NUM> may be electrically connected (e.g. galvanic connection) to a switch <NUM> or switches of the wearable device <NUM>. For simplicity reasons, the solution is described using only one switch <NUM>, but more than one may still be used. The switch <NUM> may be used, by the wearable device <NUM>, to select at least two electrodes (e.g. two) at a time from the plurality of electrodes <NUM>-<NUM> to perform the bioimpedance measurement. Hence, for example, a first electrode pair may comprise electrodes <NUM> and <NUM>, a second electrode pair may comprise electrodes <NUM> and <NUM>, a third electrode pair may comprise electrodes <NUM> and <NUM>, a fourth electrode pair may comprise electrodes <NUM> and <NUM>, a fifth electrode pair may comprise electrodes <NUM> and <NUM>, and a sixth electrode pair may comprise electrodes <NUM> and <NUM>. In some instances, at least one of the plurality of electrodes may be used a grounding electrode in addition to the pair used in the bioimpedance measurement. For example, this may reduce errors in the measurement caused by static electricity. The bioimpedance measurements by the plurality of electrodes <NUM>-<NUM> may thus be time interleaved or simultaneous. Thus, one or more bioimpedance signals may be obtained by the wearable device <NUM>. For example, a first bioimpedance signal may be associated with a first measurement area and a second bioimpedance signal may be associated with a second measurement area. The first measurement area may be subject to optical cardiac activity measurement by a first OHR sub-unit and the second measurement area may be subject to optical cardiac activity measurement by a second OHR sub-unit, wherein each sub-unit comprises one or more LEEs <NUM> and/or one or more detectors <NUM>. Hence, the different cardiac activity signals may be enhanced in the describe manner using associated bioimpedance signals. Furthermore, the enhanced (i.e. motion artefact effect reduced) different cardiac activity signals may further be used, by the wearable device <NUM>, to obtain needed cardiac activity data or information.

The switch <NUM> may be connected to one or more circuitries of the wearable device <NUM>, wherein said one or more circuitries may be configured to obtain the bioimpedance signal based on the measurements by the electrodes. For example, a BIAOHR circuitry <NUM> (sometimes referred to as cardiac activity bioimpedance measurement circuitry <NUM>) may comprise the voltage meter <NUM> and/or the current source <NUM>, or may be at least connected to said element <NUM> to obtain the bioimpedance signal. The wearable device <NUM> may comprise the BIAOHR circuitry <NUM>.

According to an aspect, the wearable device <NUM> comprises the arrangement <NUM> (or measurement head <NUM>) comprising the OHR <NUM> and the electrodes <NUM>-<NUM>. The wearable device <NUM> may further comprise a casing enclosing one or more of elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

In an embodiment, the wearable device <NUM> comprises an optical cardiac activity switch <NUM> (in short OHR or PPG switch). Said switch may be connected to the OHR <NUM> similarly as the electrodes are connected to the switch <NUM>. Hence, the switch <NUM> may be used to control the OHR <NUM>. Controlling may comprise, for example, controlling which of the LEEs the OHR <NUM> are on at a time (e.g. sequencing the light emitting) or which of the detectors <NUM> are detecting at a time. The switch <NUM> may thus be connected to at least one of the elements of the OHR <NUM> (e.g. to all elements of the OHR <NUM>).

In an embodiment, the switch <NUM> is comprised in the OHR <NUM>.

The wearable device <NUM> may further comprise an optical cardiac activity circuitry <NUM> (can be referred to as OHR/PPG <NUM>) electrically coupled with the switch <NUM> or directly with the OHR <NUM> in case there is no switch <NUM>. The optical cardiac activity circuitry <NUM> may be configured obtain the one or more cardiac activity signals from the OHR <NUM>. The optical cardiac activity circuitry <NUM> may be communicatively connected to a controller <NUM> (CTRL).

In an embodiment, the OHR/PPG <NUM> is comprised in the OHR <NUM>. The wearable device <NUM> may comprise the CTRL <NUM> connected to the BIAOHR <NUM> and to the optical cardiac activity circuitry <NUM>. Hence, the CTRL <NUM> may obtain the cardiac and bioimpedance signals and process them to obtain the cardiac activity signal with errors caused by the motion artefacts.

In some instances the operations of the BIAOHR <NUM> and/or OHR/PPG <NUM> are carried by the CTRL <NUM>. Hence, the BIAOHR <NUM> and/or OHR/PPG <NUM> may not be necessary, and the CTRL <NUM> may thus be directly connected to the switch(es) <NUM>, <NUM>, to the electrodes <NUM>-<NUM> and to the OHR <NUM>, depending on the implementation. In any case the CTRL <NUM> may be arranged such that it may receive the cardiac and bioimpedance signals, and further process them according to the embodiments described herein (i.e. obtain motion compensated/corrected optical cardiac activity signal), and to output the corrected cardiac activity signal.

The CTRL <NUM> may comprise at least one processor or one or more processing circuitries configured to perform the one or more operations of the wearable device <NUM> described above and hereinafter. For example, the CTRL <NUM> may be configured to cause performing, alone or together with program instructions comprised in a memory <NUM> of the wearable device <NUM>, the cardiac activity measurement to obtain the cardiac activity signal and the bioimpedance measurement to obtain the bioimpedance signal.

In an embodiment, with reference to <FIG>, the OHR <NUM> comprises a first light detector and a second light detector, the first light detector situated at least partially between two electrodes (e.g. <NUM> and <NUM>) of said plurality of electrodes, the second light detector situated at least partially between one of said at least two electrodes (e.g. <NUM> or <NUM>) and another electrode (e.g. <NUM> or <NUM>) of said plurality of electrodes.

In an embodiment, the LEE <NUM> is situated between two or more light detectors <NUM>.

Still referring to <FIG>, the wearable device may further comprise a communication circuitry (TRX) <NUM> and/or a user interface <NUM>. The user interface <NUM> may comprise input element for inputting information to the wearable device (e.g. controlling the wearable device) and/or output element for outputting information (e.g. audio, visual and/or haptic output elements, such as speaker, display, vibration member). The TRX <NUM> may be configured to enable wireless data transfer (e.g. the cardiac activity data or information), and/or wired communication via the interface <NUM> of the wearable device <NUM>. Data and/or control information may be transmitted by the wearable device <NUM> and/or received from external device(s). Suitable communication technologies were discussed in reference to <FIG>. One especially interesting may be Bluetooth which may be a wireless technology standard for exchanging data over short distances (using short-wavelength Ultra high frequency (UHF) radio waves in the industrial, scientific and medical (ISM) radio bands from <NUM> to <NUM>). In general, short range wireless communication may be suitable for transferring the cardiac activity data to external sources. There are other options in addition or as alternatives to Bluetooth.

According to an aspect, there is provided an interface <NUM> electrically connected to the switch <NUM>, and inherently to the plurality of electrodes <NUM>-<NUM>. The interface <NUM> may be configured to enable charging a device (i.e. device comprising at least the plurality of electrodes <NUM>-<NUM>, interface <NUM>, and the switch <NUM>) from an external source via the plurality of electrodes <NUM>-<NUM> and/or transferring data between said device and an external device via the plurality of electrodes <NUM>-<NUM>. Said device may comprise a power source, such as a rechargeable battery. Said external source and/or device may depicted as external apparatus <NUM>. For example, such apparatus <NUM> may comprise a power cable, external power source and/or an external electronic device (e.g. portable apparatus <NUM>) depending on how the interface <NUM> is used. Said device may be the wearable device <NUM>. Hence, the plurality of electrodes <NUM>-<NUM> (two or more) used to measure the bioimpedance, may be used as an interface to transfer power and/or data.

In an embodiment, the plurality of electrodes <NUM>-<NUM> are comprised in the interface <NUM>.

In an embodiment, the interface <NUM> is a Universal Serial Bus (USB) interface.

In an embodiment, at least some of the plurality of electrodes <NUM>-<NUM> are used to transfer power and/or data (i.e. are comprised in the interface <NUM>). However, all electrodes do not necessarily need to be used to both bioimpedance measurement and data/power transfer. However, using the same electrodes for both actions may provide some cost savings via material savings and/or make the device (e.g. wearable device <NUM>) more robust as there may be less through-holes in the device. Hence, for example, there may be less apertures via which water or moist may get within the device.

In an embodiment, the interface <NUM> utilizes at least four electrodes of the plurality of electrodes <NUM>-<NUM>. In an embodiment, the plurality of electrodes <NUM>-<NUM> comprises or consist of four electrodes. For example, USB interface may utilize four connection points, i.e. four electrodes in this case.

In an embodiment, the plurality of electrodes <NUM>-<NUM> (e.g. at least one of the electrodes) comprises magnetic material for magnetically coupling the device with the external apparatus <NUM>. That is, by including magnetic material to the electrodes or to at least one of them may enable the connection between the external apparatus <NUM> and the interface <NUM> to be more stable. For example, a power cable and/or a data cable may then be more firmly connected to the interface. In an embodiment, the plurality of electrodes <NUM>-<NUM> comprises both the magnetic material and are also configured to be used a connection elements for the interface <NUM> to connect the device (e.g. wearable device <NUM>) to the external apparatus <NUM>. However, in some instances, the plurality of electrodes <NUM>-<NUM> are not used as electrical connection elements, but simply as magnetic connection elements. The power transfer and/or data transfer may then happen, for example, wirelessly (e.g. induction coil(s), TRX <NUM>).

In an embodiment, the interface <NUM> is connected (e.g. galvanic connection) to the switch <NUM>. Hence, the interface <NUM> may be used to control the operation of the switch <NUM>.

Referring now to <FIG>, examples of two cardiac activity signals <NUM> and <NUM> are shown. Both signals <NUM>, <NUM> may represent cardiac activity <NUM> as a function of time t. Thus, the wearable device <NUM> may be configured (or more precisely the OHR <NUM>) measure two or more cardiac activity signals of the user <NUM>. The different signals may be spatially separated and/or may be acquired using different wavelengths. For example, spatially separated signals may mean that the each cardiac activity signal is measured from a different measurement area or location of the user. For example, looking at <FIG>, light detectors 214A-214D may each measure light at a different locations. Similarly, one or more of the detectors 214A-D may measure or detect light having different wavelengths, wherein each wavelength may be processed as a cardiac activity signal. For example, a LEE 212A may transmit light having more than one wavelength (e.g. green and blue). Spatially and/or wavelength-wise separated cardiac activity signals may reveal different things. For example, the wearable device <NUM> may determine the cardiac activity signal based on a plurality of detected signals.

Referring to <FIG> and <FIG> as examples, in an embodiment, the optical cardiac activity sensor unit <NUM> comprises a first light emitting element 212A configured to emit light having a first wavelength and a second light emitting element 212B configured to emit light having a second wavelength, the optical cardiac activity sensor unit <NUM> configured to detect a first signal <NUM> caused by the emitted light having the first wavelength and a second signal <NUM> caused by the emitted light having the second wavelength, wherein the reducing the motion artefact effect on the cardiac activity signal is further based on performing a subtracting or addition operation between the first signal <NUM> and the second signal <NUM>. For example, such operation may further reduce the motion artefact effect during the period <NUM> (which may be the same period as period <NUM>). Such operation(s) may require scaling the signals <NUM>, <NUM>. Further, such operations may be performed in time domain and/or in frequency domain, for example.

According to the invention, the optical cardiac activity sensor unit <NUM> comprises a third light emitting element 212C configured to emit light having a third wavelength, the optical cardiac activity sensor unit configured to detect a third signal caused by the emitted light having the third wavelength, the reducing the motion artefact effect on the cardiac activity signal is further based on at least halving amplitudes of the second and third signals, obtaining a sum signal of the second and third signals having the at least halved amplitudes, and performing a subtracting operation between the first signal and the obtained sum signal. That is, if there are more than two obtained signals, it may be useful to scale the signals (e.g. by adjusting DC level of the signals) before adding or subtracting the different signals with each other.

<FIG> illustrates a generalization of the describe embodiments of utilizing more than one cardiac activity signal. Referring to <FIG>, the wearable device <NUM> may obtain the first signal (block <NUM>) and one or more other signals (block <NUM>). Each signal may be detected by the OHR <NUM>, and thus used in the cardiac activity signal determination. Each signal may, at least in some embodiments, represent cardiac activity depending on the used wavelengths of emitted light. In block <NUM>, the wearable device <NUM> may perform one or more operations to the signals obtained in the blocks <NUM> and <NUM>. Examples of such operations include, subtraction (block <NUM>), addition (block <NUM>), and/or scaling (<NUM>). Based on these operations performed on the signals, the wearable device <NUM> may obtain the cardiac activity signal (or enhanced signal) (block <NUM>).

Still referring to <FIG>, in one example, the first signal is obtained from emitted substantially green light. The other signals (block <NUM>) may be obtained from emitted substantially red and/or blue light (e.g. one is red and one is blue). The wearable device <NUM> may determine DC level (i.e. average) of the first signal for certain time period, and adjust the DC levels of other signal(s) according to the determined DC level of the first signal. For example, if there are two other signals in addition to the first signal, the DC levels of the two other signals may be halved. For example, if there are three other signals in addition to the first signal, the DC levels of the two other signals may be scaled to one third of the original DC level of a signal. If there is only one other signal, there may be no need to adjust the DC level. Then, sum signal of said other signals may be formed (which were scaled previously). Alternatively or additionally, the scaling (e.g. halving amplitudes and the like) may be achieved by scaling the sum signal to have same DC level as the first signal. Hence, the sum signal may be formed before scaling the other signals (obtained in block <NUM>), and the sum signal and/or the first signal may then be scaled such that their DC level is substantially the same. Then, the sum signal may be reduced from the first signal or the first signal may be reduced from the sum signal (block <NUM>). The result may be a signal from which DC level has been removed, and further motion errors may be removed or reduced from the signal. This may provide even better cardiac activity signal. So, in general, the effect of the motion artefacts on the cardiac activity signal or the cardiac activity measurement may be reduced based on the bioimpedance signal or signals and/or based on a plurality of cardiac activity signals measured using different wavelengths and/or different measurement locations.

Let us then look some other aspects of the provided solution first by referring to embodiment illustrated in <FIG>. The wearable device <NUM> may further comprise at least one further electrode <NUM>, <NUM> physically separate from the measurement area of the OHR <NUM> (e.g. measurement area <NUM>) when said plurality of electrodes <NUM>-<NUM> are in contact with said measurement area. That is, said further electrode(s) <NUM>, <NUM> may be arranged on a different face or side of the wearable device <NUM>. What this may mean is that said electrode(s) <NUM>, <NUM> may be configured to be used for some other measurement than measuring the bioimpedance signal of the measurement area <NUM>. Hence, the wearable device <NUM> may further comprise a switch <NUM> for enabling said at least one further electrode <NUM>, <NUM> together with at least one of said plurality of electrodes <NUM>-<NUM> to perform a further measurement on the user <NUM>. Said further measurement may comprise another bioimpedance measurement, such as a measurement for determining body composition of the user <NUM>. Additionally or alternatively, said further measurement may comprise an electrocardiography (ECG) measurement.

It first needs to be noted that although some blocks of the wearable device <NUM> visible in <FIG> are not shown in <FIG>, similar or the same features and/or elements may also be used in the embodiment of <FIG>. However, due to simplicity reasons, some aspects are not drawn into <FIG>.

The wearable device <NUM> may further comprise a BIOZ circuitry <NUM> connected to the at least one further electrode <NUM>, <NUM> via the switch <NUM>, and also connected to the switch <NUM>. Connection may refer to electrical (e.g. galvanic connection). The BIOZ circuitry <NUM> and/or the CTRL <NUM> may be configured to perform the further bioimpedance measurement (e.g. body composition), and cause an output of a signal representing the measurement results. Outputting may refer to outputting the results via the user interface <NUM> and/or transmitting the results to an external device via the TRX <NUM>.

The wearable device <NUM> may further comprise an ECG circuitry <NUM> connected to the at least one further electrode <NUM>, <NUM> via the switch <NUM>, and also connected to the switch <NUM>. Connection may refer to electrical (e.g. galvanic connection). The ECG circuitry <NUM> and/or the CTRL <NUM> may be configured to perform the ECG measurement, and cause an output of a signal representing the measurement results. Outputting may refer to outputting the results via the user interface <NUM> and/or transmitting the results to an external device via the TRX <NUM>.

<FIG> illustrates yet another embodiment showing one example of ECG measurement arrangement. For example, the shown arrangement may comprise the ECG circuitry <NUM> comprising a detector <NUM> and a differential amplifier <NUM> connected to the electrodes <NUM>, <NUM> via the switches <NUM>, <NUM>. That is, the electrode <NUM> may depict one of the electrodes <NUM>-<NUM> and the electrode <NUM> may depict one of the electrodes <NUM>, <NUM>. It is yet again noted that the use of the switches <NUM>, <NUM> enables forming more than one pair from the electrodes <NUM>-<NUM> and electrodes <NUM>-<NUM>.

In an embodiment, the ECG measurement arrangement is configured to measure a cardiac activity signal of a subject, the cardiac activity signal comprising ECG or a part thereof, such as P, Q, R, S, or T waves. A first signal line from a first electrode <NUM> may be applied to a first input of the differential amplifier <NUM>, and a second signal line from a second electrode <NUM> may be applied to a second input of the differential amplifier <NUM>. The differential amplifier <NUM> may operate as a front stage of the signal detection circuitry, e.g. as a first operational component counted from an input of the signal detection circuitry and carrying out pre-processing of received signals, and amplify the received biosignals differentially and apply the amplified biosignal to the detector <NUM> configured to detect a determined waveform in the differentially amplified biosignal, e.g. one or more of the above-mentioned P, Q, R, S, and T waves. In an embodiment, the further measurement comprises a blood pressure measurement performed by measuring a first cardiac activity signal with the OHR <NUM> and a second cardiac activity signal with the electrodes (e.g. electrodes <NUM>, <NUM>), and determining a pulse transit time (PTT) of a blood pulse(s) based on the two signals. Thus, blood pressure of the user may be determined. As described, the first and second cardiac activity signals may be measured from different locations (e.g. wrist optical heart rate measurement and finger-to-wrist ECG measurement), and thus the PTT of the blood pulse(s) may be determined. As with other measurements, the results may be displayed and/or transmitted to another device by the wearable device <NUM> performing said blood pressure measurement.

Using the switches <NUM>, <NUM> (or simply one switch comprising functions of both switches <NUM>, <NUM>) may enable the selection of at least one of the electrodes <NUM>-<NUM> and at least one of the electrode(s) <NUM>, <NUM> to perform the further measurement. In one example, the user <NUM> may wear the wearable device <NUM> in his/her wrist. Hence, the electrodes <NUM>-<NUM> may be brought into contact with body tissue of a first arm. The user <NUM> may then select to touch said further electrode(s) <NUM>, <NUM>. This enables electrical current to travel via a longer way or route in the user's <NUM> body, i.e. from one arm to another. Hence, the body composition measurement may be more reliable. Such arrangement may enable the electrodes <NUM>-<NUM> to be even more suitable for multiuse situations, e.g. motion artefact compensation, provide part of an interface (charging and/or data transfer), and/or enable further electrode based measurements on the user. Hence, using the additional further electrode(s) <NUM>, <NUM> (e.g. only one electrode) actually may even further enhance the inventive merit of the provided solution in which the electrodes <NUM>-<NUM> are used.

According to an aspect, there is provided a wearable device comprising only one of said electrodes <NUM>-<NUM> and at least one further electrode <NUM>, <NUM>. Such may enable at least the body composition and/or ECG measurements.

In an embodiment, the at least one further electrode <NUM>, <NUM> is comprised in a bezel of the wrist device <NUM>. Said bezel may be a multipurpose bezel configured to be rotated between at least two positions. Each position may cause the wrist device <NUM> to enter a certain mode. For example, one mode may be a normal mode (e.g. training mode). For example, one of said modes may be the ECG and/or bioimpedance measurement mode. When in said ECG mode or said bioimpedance mode, the wrist device <NUM> may be configured to perform the ECG measurement and/or the body composition measurement. When in the normal mode, the wrist device <NUM> may perform the cardiac activity measurement, wherein the electrodes <NUM>-<NUM> may be used to reduce the motion effects. The bezel may be a part of the user interface <NUM>, for example. In an embodiment, the at least one electrode <NUM>, <NUM> is situated around a display of the wearable device <NUM>.

<FIG> illustrates a flow diagram according to yet another embodiment. Referring to <FIG>, the wearable device <NUM> may obtain cardiac activity data (block <NUM>). In an embodiment, the wearable device further obtains motion data (block <NUM>). Blocks <NUM> and <NUM> may happen concurrently, for example. The cardiac activity data may be obtained based on the measured cardiac activity signal. The motion data may be obtained using one or more motion sensors (e.g. accelerometer, gyroscope, magnetometer), wherein the motion data may represent or characterize physical motion by the user <NUM>.

In block <NUM>, the wearable device <NUM> may determine whether or not to initiate the ECG measurement based on the obtained cardiac activity data and/or motion data. If ECG measurement is needed, said measurement may be initiated (block <NUM>). Initiation may mean, for example, that the switch or switches <NUM>, <NUM> are caused to form an electrode pair comprising one of the electrodes <NUM>-<NUM> and one of the electrodes <NUM>, <NUM>. Initiation may comprise indicating, to the user <NUM>, that the ECG measurement is initiated or needed. The ECG measurement may then be performed accordingly. Based on the ECG measurement, the wearable device <NUM> may perform an action. For example, if arrhythmia is detected, it may be indicated to the user <NUM>.

<FIG> illustrates yet another example of the wearable device <NUM> according to an embodiment. Referring to <FIG>, the wearable device <NUM> may further comprise an attachment element <NUM> configured to enable the attachment of the wearable device <NUM> to the user or at least the detachable placing of the OHR <NUM> and/or the electrodes <NUM>-<NUM> against the body tissue of the user. The attachment element <NUM> may comprise a strap and/or a garment, such as shirt, bra, or trousers. At least some of the connections between the different elements of the wearable device <NUM> are shown in <FIG>. It needs to be noted that at least in some embodiments, the OHR <NUM> further comprises the OHR/PPG switch <NUM> and/or the OHR PPG circuitry <NUM>. The interface <NUM> may be connected to the CTRL <NUM> (although the connection is not shown in <FIG>). Hence, the possible data and/or power transfer via the interface <NUM> may be controlled by the CTRL <NUM>, for example. In an embodiment, the attachment element <NUM> comprises silicone based material. In an embodiment, the wearable deivce <NUM> is detachably attachable to the attachment element <NUM>. For example, the device <NUM> may be an electronics module that may be detachably attached to the element <NUM>, the pair thus forming the wearable device <NUM>. Let us then look closer on <FIG> illustrating some embodiments. Said embodiments may relate to different layout structures for the OHR <NUM> and/or the plurality of electrodes <NUM>-<NUM>. Referring to <FIG> which was already briefly discussed, the OHR <NUM> may comprise a plurality of light detectors 214A-D and plurality of LEEs 212A-E. In an embodiment, the light detector 214A is configured to mainly detect light originated from the LEE 212A. Similarly, the light detector 214B may be configured to mainly detect light originated from the LEE 212B, the light detector 214C may be configured to mainly detect light originated from the LEE 212C, and the light detector 214D may be configured to mainly detect light originated from the LEE 212D. In addition, the plurality detectors 214A-D may be configured to detect light from the LEE 212E. In an embodiment, LEEs 212A-D are configured to emit substantially blue and/or green light and LEE 212E are configured to emit substantially red and/or yellow light.

Each of the light detectors, such as detectors 214A-214D, may comprise one or more photodiodes. Similarly, each of the LEEs, such as LEEs 212A-212E, may comprise one or more LEDs or similar light source. The LEDs may be of same or different colour. As shown in <FIG>, the different OHR <NUM> elements may be situated between at least two of the electrodes <NUM>-<NUM>. This may enable bioimpedance measurement from different OHR <NUM> measurement points or areas.

Referring to <FIG>, the one or more LEEs 212A-D may be arranged to be situated between a first electrode <NUM> and a second electrode <NUM>. Said electrodes may be, for example, hollow electrodes, such as hollow rectangle electrodes and/or hollow circle electrodes (as shown in <FIG>). In an embodiment, one or more light detectors <NUM> are situated within the second electrode <NUM> being a hollow electrode. In the example of <FIG>, the detector <NUM> may be configured to detect light emitted by each of the LEEs 212A-D. The bioimpedance measurement may be performed using the two electrodes <NUM>, <NUM>, wherein the measurement signal is both fed and sampled from said two electrodes <NUM>, <NUM>.

Referring to <FIG>, the electrodes may be similar as in <FIG>, but the second electrode <NUM> may be arranged not to be hollow. Hence, it may be shaped as a circle or a rectangle, for example. The OHR <NUM> elements <NUM>-<NUM> may be arranged between the first and second electrodes <NUM>, <NUM>. Each of said elements <NUM>-<NUM> may comprise on or more LEEs <NUM> and/or one or more light detectors <NUM>.

Referring to <FIG>, an arrangement comprising one light detector (e.g. one photodiode) <NUM> and a plurality of LEEs (e.g. LEDs) 212A-H is shown. As discussed earlier, the switches <NUM> and/or <NUM> may be used to select a needed electrode pair and/or needed OHR <NUM> elements to perform the needed measurements. For example, the switch <NUM> may be configured to time multiplex the burning LEEs 212A-H. That is, different LEEs may be on (i.e. emitting light) or off (not emitting light) according to the configuration by the switch <NUM>. Hence, for example, different LEEs 212A-H may be on at different times during the measurement. In one example, LEEs 212A-H may be on according to round robin or some other similar sequence, such that after the sequence, all LEEs 212A-H have been on at least once. In some examples, more than one LEE 212A-H may be on at a time.

In one example, with reference to <FIG>, LEE 212C and 212D are configured to be on at a first time period, LEE 212A and 212B are configured to be on at a second time period, LEE 212E and 212F are configured to be on at a third time period, and LEE <NUM> and <NUM> are configured to be on at a fourth time period. Said first, second, third and fourth time periods may form the measuring sequence. For example, said periods may be consecutive. Similarly, bioimpedance electrodes <NUM>-<NUM> may be used to form pairs with each other. Referring to <FIG>, OHR <NUM> symmetry axes <NUM>, <NUM> are shown. That is, the symmetry axis <NUM> may be formed by arranging the LEEs 212A-B on two opposite sides of the light detector <NUM>. Similarly, the symmetry axis <NUM> may be formed by arranging the LEEs 212C-D on two opposite sides of the light detector <NUM>. The electrodes <NUM>-<NUM> may be arranged to measure bioimpedance on each of said symmetry axes <NUM>, <NUM> (or simply axes). Number of said axes may be increased by including further LEEs to the arrangement. In such case, further electrodes may be needed in order to measure bioimpedance of each of said axes. One way to measure said bioimpedance may be to arrange the OHR <NUM> elements performing a measurement (e.g. 212C-D and <NUM>; 212A-B and <NUM>) between an electrode pair (e.g. <NUM> and <NUM>; <NUM> and <NUM>).

Referring to <FIG>, the OHR <NUM> may further comprise one or more light blocking elements <NUM>. The light blocking element(s) (e.g. light wall) <NUM> may be arranged between the LEE(s) <NUM> and the light detector(s) <NUM>, and configured to block travelling of light directly between the LEE(s) <NUM> and the light detector(s) <NUM>. Hence, the detector(s) <NUM> may detect light that has travelled via the body tissue of the user <NUM>, whereas leakage of light may at least substantially be reduced by the wall <NUM>. In an embodiment, the light detector <NUM> is formed as a substantially hollow element, such as a hollow circle or a hollow rectangle. The light detector <NUM> may be formed from a plurality of photodiodes arranged in a hollow circle or hollow rectangle form, for example.

Referring to <FIG>, the OHR <NUM> comprises a first light detector <NUM> and a second light detector <NUM>. These detectors may be similar to the detector <NUM>, for example. The OHR <NUM> may further comprise a plurality of LEEs <NUM>-<NUM>. The OHR <NUM> may further comprise the wall <NUM> between the LEEs and the detectors, for example. The LEEs <NUM>-<NUM> and the detectors <NUM>, <NUM> may be arranged and dimensioned such that a distance L1A (between the first LEE <NUM> and the second detector <NUM>) substantially equals to a distance L1B (between the fourth LEE <NUM> and the first detector <NUM>); and a distance L2B (between the first LEE <NUM> and the first detector <NUM>) substantially equals to a distance L2A (between the fourth LEE <NUM> and the second detector <NUM>). Hence, the effective distance travelled by emitted light before detection by the detectors <NUM>, <NUM> may be substantially same. Similar logic may apply to distances between other LEEs <NUM>, <NUM> and the detectors <NUM>, <NUM>. In an embodiment, the LEEs <NUM> and <NUM> emit light having substantially the same wavelength (e.g. green). In an embodiment, the LEEs <NUM> and <NUM> emit light having substantially the same wavelength (e.g. yellow). In an embodiment, the light emitted by the LEEs <NUM> and <NUM> has a different wavelength compared with the light emitted by the LEEs <NUM> and <NUM>. In an embodiment, the OHR <NUM> comprises a further LEE arranged between the LEEs <NUM>-<NUM>. Said LEE may be arranged to emit light having a different wavelength (e.g. red) compared with light emitted by the LEEs <NUM>-<NUM>. Said LEE may have equal distance to both detectors <NUM>, <NUM>, and thus the effective distance travelled by the light emitted by said LED may be the same to before the light reaches the detectors <NUM>, <NUM>.

The OHR <NUM> may further comprise one or more analog-to-digital converters (ADCs) and/or more or more amplifiers. An ADC may be electrically connected to an amplifier and the amplifier may be electrically connected to the light detector or detectors <NUM>. The amplifier may amplify the cardiac activity signal and the ADC may convert said signal into a digital cardiac activity signal. The digital cardiac activity signal may further be processed by the CTRL <NUM>. The processing may include obtaining cardiac activity data based on the digital cardiac activity signal. The CTRL <NUM> may further cause outputting said data (e.g. display via the user interface <NUM> or transmit via TRX <NUM> to an external device).

It needs to be understood that LEEs <NUM> and 212A-H may refer to same or similar LEEs. Similarly, detectors <NUM> and 214A-D may refer to same or similar light detectors.

<FIG> illustrates a flow diagram of a method according to an embodiment. Referring to <FIG>, the method comprises: obtaining a cardiac activity measurement signal from an optical cardiac activity sensor unit configured to be placed against a body tissue of the user (block <NUM>); obtaining a bioimpedance measurement signal utilizing a plurality of electrodes configured to be placed against the body tissue (block <NUM>); detecting changes in the bioimpedance measurement signal (block <NUM>); and reducing a motion artefact effect, caused by a movement between the optical cardiac activity sensor unit and the body tissue, on the cardiac activity measurement signal based on the detected changes in the bioimpedance measurement signal (block <NUM>). The method may further comprise other steps described above or hereinafter (e.g. performed by the wearable device <NUM>).

According to an aspect, there is provided a solution in which an optical cardiac activity sensor, such as the OHR <NUM>, is configured to measure cardiac activity using sampling frequency of <NUM> (i.e. Hertz) or below. Based on such measurement, said optical cardiac activity sensor may be used to determine breathing intervals of the user by, for example, determining variation of mean heart rate. On basis of the breathing intervals, said optical cardiac activity sensor may be used to determine sleep stages of the user, for example. For example, said optical cardiac activity sensor may comprised in a wearable device, such as the device <NUM>, wherein said wearable device is configured to determine the breathing intervals and/or sleep stages. Further, said wearable device may be configured to transmit and/or display data indicating the breathing intervals and/or sleep stages. For example, said data may transmitted to a server for storing and further use (e.g. monitoring the user).

In an embodiment, said optical cardiac activity sensor is configured to measure cardiac activity of the user using sampling frequency of <NUM> or below. Said optical cardiac activity sensor may be communicatively connected to a processor and/or controller of said wearable device. Said processor and/or controller may acquire cardiac activity data of the user on the basis of the measurement using sampling frequency of <NUM> or below. Said processor and/or controller may further be configured to trigger said optical cardiac activity sensor to increase the sampling frequency to over <NUM>. Hence, a more accurate measurement (e.g. HRV measurement) may be performed. The triggering may happen, for example, on the basis of user input, on the basis of processing the measured cardiac activity data, on the basis of measurement performed by a one or more motion sensors (e.g. accelerometer and/or gyroscope) and/or on certain time intervals (e.g. every hour or every <NUM> minutes). That is, the optical cardiac activity sensor may continuously measure cardiac activity to the user using the sampling rate of <NUM> (e.g. to save battery) and increase the sampling rate (e.g. repetitively or periodically). One further example of the triggering the higher sampling frequency may be that the wearable device detects a certain sleep phase. That is, for example, the wearable device may be configured to detect that the user enters a certain sleep phase (e.g. Rapid eye movement sleep (REMS)) and to trigger the increased sampling frequency to perform some measurement (e.g. HRV). It may be beneficial to measure HRV when the user is sleeping as it may be used to determine stress of the user or quality of sleep, for example. In an embodiment, the wearable device is configured to trigger the increased sampling frequency when the user is determined to sleep. The triggering may happen periodically during sleep.

One example of the motion sensor triggered sampling rate change may be that the wearable device detects a physical activity or motion change exceeding a threshold. For example, if the user starts to run, the higher sampling rate may be triggered. In an embodiment, the triggering of the higher sampling rate may be performed in response to detecting a physical activity change (e.g. from lower activity to higher activity) associated with the user. The physical activity change may be determined on the basis of the motion sensor measurement(s) and/or optical cardiac activity measurements using the lower sampling rate. If both are used, the triggering may be performed even more accurately and timely. In an embodiment, triggering of the lower sampling rate (i.e. back to the lower sampling rate from the higher sampling rate) may be performed in response to detecting a physical activity decrease associated with the user.

<FIG> illustrates an embodiment. Referring to <FIG>, the operation modes <NUM>, <NUM> may refer to operation mode of the optical cardiac activity sensor (e.g. OHR <NUM>). In the first mode <NUM>, the optical cardiac activity sensor may be configured to utilize the sampling frequency of <NUM> or below, whereas in the second mode <NUM> the sampling frequency may be higher (e.g. suitable for measuring HRV, i.e. at least over <NUM>). The optical cardiac activity sensor may be associated with a mode controller <NUM> configured to change the mode according to one or more conditions (e.g. user input or detecting that the user is sleeping or is in certain sleep phase). In an embodiment, the mode controller <NUM> comprises one or more processors. In an embodiment, the mode controller <NUM> is comprised in the CTRL <NUM> or some other part of the wearable device <NUM>. Using a certain sampling frequency may further mean that the light emitted by the LEE(s) of the optical cardiac activity sensor is transmitted in pulses, i.e. in accordance with the sampling frequency. This may further save power.

It is further noted that using the bioimpedance measurement to cancel movement artifact effect from the cardiac activity signal may be especially purposeful and useful for the measurements performed using the lower sampling rate (i.e. <NUM> or below). This is due to the fact that fewer samples than in normal cardiac activity measurements are received, and thus the fewer samples may be beneficial to be enhanced in the described manner.

In an embodiment, at least some of the processes described in connection with <FIG> may be carried out by an apparatus comprising corresponding means for carrying out at least some of the described processes. Some example means for carrying out the processes may include at least one of the following: detector, processor (including dual-core and multiple-core processors), digital signal processor, controller, receiver, transmitter, encoder, decoder, memory, RAM, ROM, software, firmware, display, user interface, display circuitry, user interface circuitry, user interface software, display software, circuit, antenna, antenna circuitry, and circuitry. In an embodiment, the at least one processor, the memory, and the computer program code form processing means or comprises one or more computer program code portions for carrying out one or more operations according to any one of the embodiments of <FIG> or operations thereof.

According to yet another embodiment, the apparatus carrying out the embodiments comprises a circuitry including at least one processor and at least one memory including computer program code. When activated, the circuitry causes the apparatus to perform at least some of the functionalities according to any one of the embodiments of <FIG>, or operations thereof.

The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus(es) of embodiments may be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chip set (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by processors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and/or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with regard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art.

Embodiments as described may also be carried out in the form of a computer process defined by a computer program or portions thereof. Embodiments of the methods described in connection with <FIG> may be carried out by executing at least one portion of a computer program comprising corresponding instructions. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. The computer program medium may be a non-transitory medium, for example. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art. In an embodiment, a computer-readable medium comprises said computer program.

Claim 1:
A wearable device (<NUM>) for measuring cardiac activity of a user, the wearable device comprising:
an optical cardiac activity sensor unit (<NUM>) configured to be placed in contact with a measurement area and to enable cardiac activity measurement of the user to obtain a cardiac activity signal, wherein the optical cardiac activity sensor unit comprises a first light emitting element (212A) configured to emit light having a first wavelength, a second light emitting element (212B) configured to emit light having a second wavelength and a third light emitting element (212C) configured to emit light having a third wavelength, wherein
the optical cardiac activity sensor unit is configured to detect a first signal (<NUM>) caused by the emitted light having the first wavelength, a second signal (<NUM>) caused by the emitted light having the second wavelength and a third signal caused by the emitted light having the third wavelength;
a plurality of electrodes (<NUM>-<NUM>) configured to enable bioimpedance measurement on the measurement area to obtain a bioimpedance signal;
means for detecting changes in the bioimpedance signal,
means for reducing a motion artefact effect on the cardiac activity signal based on the detected changes in the bioimpedance signal, wherein the means for reducing the motion artefact effect on the cardiac activity signal is further based on at least halving amplitudes of the second and third signals, obtaining a sum signal of the second and third signals having the at least halved amplitudes, and performing a subtracting operation between the first signal and the obtained sum signal.