Power supply control in a wireless receiver

Description of equivalent WO 2009095869 A2

DESCRIPTION

POWER SUPPLY CONTROL IN A WIRELESS RECEIVER

FIELD OF THE INVENTION

0004The invention relates to a wireless receiver and method of operating a wireless receiver, and in particular to controlling power supply in a wireless receiver. The invention is particularly, but not exclusively, applicable to a magnetic induction receiver.

BACKGROUND TO THE INVENTION

0006Magnetic induction technology is based on the quasi-static magnetic component of the field generated by a coil through which a sinusoidal current is flowing. When a second coil is introduced within that field, the magnetic flux passing through that second coil induces a modulated current in it. This phenomenon has been used for communication between a magnetic induction transmitter and magnetic induction receiver using near-field propagation where the magnetic field is much greater than the electric field and is inversely proportional to the cube of the distance from the transmitting coil. The coils are tuned to resonate at a desired frequency by employing programmable capacitance. The coils may be mounted on a ferrite core to increase the transmission efficiency. Typical operating parameters for a magnetic induction communication system, such as may be employed for an ultra low power hearing aid, are a carrier frequency of 10.579MHz and a modulation rate of 298 kbits^"1 using FSK modulation at a modulation index of 1.0 and having a bandwidth of about 60OkHz. A receiver in such a low power communication system has a sleep mode in which power is conserved by powering-up the receiver only intermittently to monitor for a received signal, and maintaining the receiver powered-up to synchronise only if a signal is present.

0007An object of the invention is to enable improved power conservation in a wireless receiver. SUMMARY OF THE INVENTION

0008According to a first aspect of the invention there is provided a method of operating a wireless receiver, comprising: measuring a level of energy received in each of intermittent time periods; generating a first value dependent on the levels of energy in a first plurality of the time periods; and enabling, dependent on the first value, the supply of power to a circuit .

0009According to a second aspect of the invention there is provided a wireless receiver, comprising: a measurement circuit for measuring a level of energy received in each of intermittent time periods; an evaluation circuit for generating a first value dependent on the levels of energy in a first plurality of the time periods; and a controller for enabling, dependent on the first value, the supply of power to a circuit .

0010Thus, the wireless receiver measures the energy received in individual intermittent time periods and controls the power supply to a circuit, depending on a function of a plurality of the energy measurements. In this way, power can be saved when wanted signals are unlikely to be received, as indicated by the value of the function. When the average energy level changes with time, the value of the function will vary, and so the control of power saving can be adapted to the current average energy level. This can enable the level of variable unwanted signals or noise to be taken into account when exercising power control.

0011In one embodiment the receiver may generate a second value dependent on the levels of energy in a second plurality of the time periods, the second plurality including at least one time period occurring later than the time periods of the first plurality; and enable the supply of power if the second value exceeds the first value by more than a first margin. This enables the control of the power to be dependent on the change in average energy level, and power to be conserved if the increase in average energy is insufficient to enable reliable reception of a wanted signal in the presence of unwanted signals, including noise.

0012At least one of the time periods may be common to the first plurality of time periods and the second plurality of time periods. This enables successive ones of the first and second values to be calculated in a sliding window or a window stepped through overlapping positions, or calculated for a recursive function stepped through successive positions.

0013In another embodiment the receiver may enable the supply of power if the level of energy in at least one of the time periods exceeds the first value by more than a second margin. This enables power to be conserved if the short term energy level is not greater than the average energy level by a sufficient margin for reliable reception of a wanted signal in the presence of unwanted signals, including noise.

0014Optionally, the first and/or the second value may be generated based on a recursive function of the levels of energy in the respective plurality of time periods. In particular, the first and/or the second value may be generated according to the function A_n = (1 -α)A_n-i+α.E_n, where A_n is the value generated for the n^th time period, E_n is the level of energy in the n^th time period, and α is a constant less than unity. This can provide an indication of average energy level whilst requiring only a small amount of storage.

0015The wireless receiver may be a magnetic induction receiver. The invention also provides: a computer program adapted to perform the method according to the first aspect of the invention; a wireless transceiver comprising the wireless receiver according to the second aspect of the invention; and a wireless system comprising the wireless receiver according to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

0017The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

0018Figure 1 is a block schematic diagram of a magnetic induction transmitter and receiver;

0019Figure 2 is a timing diagram illustrating the operation of an embodiment of the invention; and Figure 3 is another timing diagram illustrating the operation of another embodiment of the invention.

0020DETAILED DESCRIPTION OF EMBODIMENTS Referring to Figure 1 , there is illustrated a magnetic induction transmitter 200 and magnetic induction receiver 300. The magnetic induction transmitter 200, for brevity referred to below as a transmitter 200, comprises an encoder 10 for encoding signals to be transmitted. An output of the encoder 10 is coupled to an input of a modulator 20 for modulating the encoded signals onto a carrier signal. An output of the modulator 20 is coupled to an input of an upconverter 30 for mixing the modulated carrier signal up to a higher frequency for transmission. An output of the upconverter 30 is coupled to an input of a coil driver 40 which supplies power to a coil 50 which is coupled to an output of the coil driver 40. The coil driver 40 is tuned to a desired transmission frequency by means of an inductor-capacitor tuned circuit (not illustrated).

0021The magnetic induction receiver 300, for brevity referred to below as a receiver 300, comprises a coil 60 for receiving signals transmitted by the coil 50. The coil 60 is coupled to an input of a tuner 70 which is tuned to the desired transmission frequency by means of an inductor-capacitor tuned circuit (not illustrated). An output of the tuner 70 is coupled to an input of an amplifier 80 for amplifying received signals. An output of the amplifier 80 is coupled to an input of a downconverter 90 for mixing received signals to a lower frequency. An output of the downconverter 90 is coupled to an input of a demodulator 100 for demodulating received signals. An output of the demodulator 100 is coupled to an input of a synchroniser 110 for detecting the occurrence of a wanted signal in demodulated signals. An output of the synchroniser 110 is coupled to an input of a decoder 120 for decoding a wanted signal that has been detected.

0022The receiver 300 further comprises a measurement circuit 130 coupled to the output of the amplifier 80 for generating an indication of the level of energy in received signals. The measurement circuit 130 is coupled to a controller 150 which controls the time periods over which the measurement circuit 130 generates the indication; in particular the indication is generated in each of intermittent time periods.

0023The receiver 300 further comprises an evaluation circuit 140 coupled to an output of the measurement circuit 130 for generating a value indicative of an average level of energy in received signals over a plurality of the intermittent time periods. The evaluation circuit 140 is coupled to the controller 150 which dictates the intermittent time periods over which the evaluation circuit 140 performs averaging. Preferably, the averaging is performed for successive positions of a sliding window or a window stepped through overlapping positions, in which, in successive positions, the energy level in the oldest of the time periods is discarded from the average and replaced by the energy level of the most recent of the time periods.

0024The value indicative of average level of energy in received signals over the plurality of the intermittent time periods is provided to the controller 150. The controller 150 is coupled to each of the downconverter 90, the demodulator 100, the synchroniser 110 and the decoder 120, for controlling the supply of power to these stages. Dependent on the indicated average level of energy, the controller 150 enables or inhibits the supply of power to these stages.

0025The operation, according to first and second embodiments, of the receiver 300 illustrated in Figure 1 is described below with reference to Figures 2 and 3 respectively. In these examples, the controller initially inhibits the supply of power to the downconverter 90, the demodulator 100, the synchroniser 110 and the decoder 120.

0026Figure 2, graph a) illustrates the power P at the desired operating frequency that reaches the coil 60 of the receiver 300 as a function of time t. The low initial power represents unwanted power, that is power emitted by sources other than the transmitter 200. At time t_tx, the power increases because the transmitter 200 commences transmission of a wanted signal to the receiver 300.

0027Figure 2, graph b) illustrates the energy E measured by the receiver 300 in intermittent time periods occurring at times t₀ to t₆. The measured energy level increase at time t₂, which is the first measurement following the commencement of transmission at time t_tx by the transmitter 200.

0028Figure 2, graph c) illustrates the average energy A measured over a plurality of the time periods illustrated in graph b); in this example the plurality is two, the averaging being performed over a window encompassing two of the time periods and stepped by increments equal to the interval between the intermittent time periods. The initial level of the average energy A, preceding time ti, is determined by the energy preceding time t₀ but which is not represented in graphs a) or b). The initial level of the average energy is maintained until time t₂, at which point the average energy A increases by an amount X₂ due to including in the average the energy E measured at time t₂ and discarding the energy measured at time t₀; the increased average value is the average of the energy measured at time ti and time t₂. The average energy A increases again at time t₃ by an amount X₃ to a value which is the average of the energy measured at time t₂ and time t₃. The measurements proceed in subsequent time periods in the same manner.

0029In the first embodiment, the controller 150 assesses the size of each successive increase, xi and x₂, in the average energy A. If such an increase exceeds a predetermined margin, the receiver is deemed to have possibly detected the transmission of the transmitter 200, and the controller 150 enables the supply of power to all stages of the receiver 200. Otherwise, the power supply remains inhibited. Thus, depending on the level of unwanted signals, the level of the wanted signal, the number of time periods contributing energy to the averaging, and the value of the predetermined margin, the power may be enabled at time t₂ or time t₃, in which case the receiver 300 can process the wanted signal received from the transmitter 200.

0030In a variation of the operation described above with reference to Figure 2, the controller 150 may take account of the rate of change of the average energy level A in determining whether to enable the supply of power, for example by assessing the combined value xi+x₂, which represents the increase over two successive time periods.

0031Figure 3, graph a) illustrates the power P at the desired operating frequency that reaches the coil 60 of the receiver 300 as a function of time t. The low initial power represents unwanted power, that is power emitted by sources other than the transmitter 200. At time t_n an additional source of unwanted power starts transmitting or comes into range of the receiver 300, causing an increase in the power P. At time t_tx, the power increases again because the transmitter 200 commences transmission of a signal to the receiver 300.

0032Figure 3, graph b) illustrates the energy E measured by the receiver 300 in intermittent time periods occurring at times to to t₆. The measured energy level increases at time t₂ from Ei to E₂, which is the first measurement following the commencement of the unwanted transmission at time t_n, and again at time t₃ from E₂ to E₃, which is the first measurement following the commencement of transmission at time t_tx by the transmitter 200.

0033Figure 3, graph c) illustrates the average energy A measured over a plurality of the time periods illustrated in graph b); in this example the plurality is two, the averaging being performed over a window encompassing two of the time periods and stepped in increments equal to the interval between the intermittent time periods. The initial level of the average energy, preceding time ti, is determined by the energy preceding time t₀ but which is not represented in graphs a) or b). The initial level of the average energy Ai is maintained until time t₂, at which point the average energy A increases to a level A₂ due to the inclusion in the average of the energy E₂ measured at time t₂ and the discarding of the energy Ei measured at time t₀; the increased average value A₂ is the average of the energy Ei and E₂ measured at, respectively, time ti and time t₂. The average energy A increases again at time t₃ to a level A₃ which is the average of the energy E₂ and E₃ measured at, respectively, time t₂ and time t₃.

0034In the second embodiment, the controller 150 assesses the level of each successive measured energy E relative to the average energy level A. If the level of such energy E exceeds the level of the average energy A by a predetermined margin, the receiver 300 is deemed to have possibly detected the transmission of the transmitter 200, and the controller 150 enables the supply of power to all stages of the receiver 200. Otherwise, the power supply remains inhibited. In the example of Figure 3, in successive assessments in successive time periods, the energy level Ei is compared with the current average energy level Ai, the energy level E₂ is compared with the current average energy level A₂, and the energy level E₃ is compared with the current average level A₃. In a variation of this scheme, the energy level may be compared with the preceding average energy level, thereby excluding the effect of the latest energy measurement from the average; thus the energy level Ei is compared with average energy level Ai, the energy level E₂ is also compared with the average energy level Ai, and the energy level E₃ is compared with the average energy level A₂. The measurements and comparisons proceed in subsequent time periods in a similar manner.

0035Thus, depending on the level of unwanted signals, the level of the wanted signal, the number of time periods contributing to the averaging, and the value of the predetermined margin, the power may be enabled at time t₂ in response to noise, or time t₃ in response to the wanted signal, in which case the receiver 300 can process the wanted signal received from the transmitter 200.

0036In a variant of the second embodiment, instead of the criterion for deeming the receiver 300 to have possibly detected the transmission being whether the energy E in one of the intermittent time periods exceeds the average energy A by a predetermined margin, the criterion may be whether the energy E in more than one of the intermittent time periods, preferably consecutive, exceeds the average energy A by a predetermined margin.

0037Suitable values for the number of time periods contributing energy to the averaging, and the value of the predetermined margin for either embodiment, may be determined for different environments by experimentation, to provide a receiver 300 that provides a balance between: the reliability of detection of wanted signals in the presence of unwanted signals; the failure to detect wanted signals in the presence of unwanted signals; the waste of power due to enabling the supply of power in the presence of only unwanted signals; and the waste of power due to enabling the supply of power in the presence of wanted signals that are too small, relative to the unwanted signals, for reliable decoding. In particular, the invention enables the receiver 300 to adapt to changes in the level of unwanted signals, and can reduce the likelihood that changing levels of unwanted signals result in increased power consumption.

0038By way of example, the interval between intermittent time periods may be 15ms, the measurement duration for each of the intermittent time periods may be 20μs, the number of intermittent time periods over which averaging is performedmay be 20, and for the first embodiment the predetermined margin for the increase in average energy may be 2dB, while for the second embodiment the predetermined margin for the energy E exceeding the average energy A may be 8dB.

0039In a further embodiment, the controller 150 may implement the assessment criteria of both the first and second embodiments described above, and enable the supply of power if either criteria are met, that is if the average energy A increases by more than a predetermined margin or the energy level E in one or more of the intermittent time periods exceeds the average energy A by more than a predetermined margin.

0040In the embodiments described, the averaging of energy has been performed, for the purpose of illustrating the concept, over two of the intermittent time periods. In practice, the averaging may be performed over more than two of the intermittent time periods, although the speed of detecting a wanted signal may be reduced as the number of time periods is increased.

0041In the embodiments described, the averaging has been performed in a window that is stepped through overlapping positions. Alternatively, the window may be stepped through non-overlapping positions. In the embodiments described, the levels of energy in a plurality of the time periods have been averaged. The term average is not intended to be limited to any particular mathematical function, such as a arithmetic average or geometric average, but is intended to include any function of a plurality of measured energy levels. Furthermore, the function is not intended to be time limited, but may be a recursive function dependent on the level of energy measured in an indefinite number of the intermittent time periods. For example, the average may be evaluated according to the function A_n = (1 - α)A_n-i+α.E_n, where A_n is the average value generated for the n^th time period, A_n- 1 is the average value generated for the (n-1 )^th time period, E_n is the level of energy in the n^th time period, and α is a constant less than unity. The value of α may be, for example, 1/16. Such a function requires only a small storage.

0042In the examples illustrated in Figures 2 and 3, the intermittent time periods are periodic. However, this is not an essential requirement and alternatively the intermittent time periods may be unequally spaced in time. Furthermore, the intermittent time periods need not all have the same duration.

0043Optionally, the measurement circuit 130 may be coupled to alternative or additional points in the receiver 300 for measuring the level of received energy, such as an output of the coil 60, the tuner 70, the downconverter 90 or the demodulator 100. The level of received energy in each of the intermittent time periods may be determined, for example, as a sum or an average of samples of received signal level. Optionally, after the receiver 300 has enabled the supply of power to circuitry, it may retune to a different frequency in order to detect a further signal transmitted by the transmitter 200 on the different frequency and to engage in communication with the transmitter 200. In this way, one frequency, for example 8MHz, may be used for initially invoking a communication link, and a different frequency, for example 10MHz may be used to continuing the communication.

0044Optionally, the signal transmitted by the transmitter 200 may comprise both frequency modulation and amplitude modulation, and the receiver 300 may be adapted to demodulate the amplitude modulation and to remain powered-up to demodulate subsequent frequency modulation only if the amplitude modulation conveys a predetermined code. In this way, power saving may be improved as the receiver 300 will refrain from decoding signals in which the amplitude modulation conveys an inapplicable code.

0045The receiver 300 may be part of a transceiver that also includes a transmitter.

0046Additional power saving measures may be included. For example, if the synchroniser 110 fails to synchronise to a received signal, the supply of power to those stages of the receiver 300 that are power controlled may be inhibited. As another example, the supply of power may be terminated after a received message has been decoded, and acknowledged if required.

0047Although the invention has been described with reference to a magnetic induction receiver 300, the invention may be used with other types of receivers, particularly where it is desirable to minimise power consumption, such as in a portable devices.

0048Portions of the receiver 300 may be implemented in analogue or digital hardware, or, particularly the measurement circuit 130, the evaluation circuit 140 and the controller 150, in computer software or computer program code executed by a processor, such as a digital signal processor. The computer software or computer program code can be carried by a computer readable medium. The medium may be a physical storage medium such as a Read Only Memory (ROM) chip. Alternatively, it may be a disk such as a Digital Versatile Disk (DVD-ROM) or Compact Disk (CD-ROM). It could also be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

0049From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of wireless communications, and which may be used instead of, or in addition to, features already described herein.

0050Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub- combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. For the sake of completeness it is also stated that the term

0051"comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single processor or other unit may fulfil the functions of several elements recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.