WO2010146490A1

WO2010146490A1 - Signal-detection with band-pass spectral shaping

Info

Publication number: WO2010146490A1
Application number: PCT/IB2010/052420
Authority: WO
Inventors: Sotir Ouzounov; Alberto Fazzi
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-06-18
Filing date: 2010-05-31
Publication date: 2010-12-23

Abstract

The present invention relates to an apparatus, method and computer program product, wherein a power and area efficient approach for detection and reception of communication signals is proposed, in which a variable band-pass spectral shaper configuration is used. The proposed configuration attenuates significantly the low-frequency content of the input signal and spectrally shapes it outside the desired communication band, with the help of an inherent self-oscillation mechanism.

Description

DESCRIPTION

Signal detection with band-pass spectral shaping

FIELD OF THE INVENTION

The present invention relates to an apparatus, method, and computer program product for detecting a communication signal, such as - but not limited to - a body-coupled communication signal.

BACKGROUND OF THE INVENTION

Body-coupled communication (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks (BANs). BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electric fields onto the body surface. Signals are conveyed over the body instead of through the air. As such, the communication is confined to an area close to the body in contrast to RF communications, where a much larger area is covered. Therefore, communication is possible between devices situated on, connected to, or placed close to the body. Moreover, since lower frequencies can be applied than typically applied in RF-based low range communications, it opens the door to low-cost and low-power implementations of BANs or personal area networks (PANs). Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for BANs (e.g. ZigBee or Bluetooth systems). Since in BCC the signals are limited to the proximity to the body and strongly attenuated if the distance increases, it can be used to realize new and intuitive body-device interfaces based on contact or proximity. This creates possibilities for many applications in the field of identification and security.

BCC can be technically realized by generating electric fields with a small body-worn tag, e.g., being integrated into a credit card, mobile phone, sensor or another suitable device attached to or worn in close proximity to the body. This tag capacitively or galvanically couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as "near-field intra- body communication". BCC is thus a wireless technology that allows electronic devices on and near the body to exchange digital information through capacitive or galvanic coupling via the human body itself. Information can be transmitted by modulating an electric field to either capacitively or galvanically coupling tiny currents onto the body. The body conducts the tiny signal that can be then detected by body mounted receivers or transceivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal. The specific characteristics of the body channel allow for robust and efficient transmission of wide band digital signals on the body. BCC communication can provide high bit-rate with very low power consumption. However, it requires a receiver with a sufficiently large bandwidth for correct reception of the BCC signals. As a result, the receiver is open to environmental noise and interference.

Fig. 1 shows an exemplary body communication system structure, where data signals are transmitted via couplers placed near or on the body. These couplers transfer the data signal either galvanically or capacitively to the body. In example of Fig. 1, one coupler or electrode provides ground potential GND and the other coupler or electrode is used for transmitting/receiving a signal S. More specifically, a transmission from a transmitter (TX) 100 to a receiver (RX) 200 of a human arm is depicted. Generally, every node can in principle act both as transmitter and receiver, i.e., as a transceiver (TRX), and communication can take place from everywhere on the body.

For capacitively coupled signals, the transfer characteristic of the body channel is especially suitable for transmission of signals with frequencies from about 100 kHz up to about 30 MHz, as described for example in T. Schenk et al., "Experimental Characterization of the Body- Coupled Communications Channel", ISWCS 2008, October 2008, or N. Cho et al., "The Human Body Characteristics as a Signal Transmission Medium for Intrabody Communication", IEEE Transactions on Microwave Theory and Techniques, Vol. 55, No. 5, pp: 1080-1086, May 2007. Frequency bands outside that range are affected from significant electrostatic interference. Given the characteristic of the body channel and the bandwidth of interest, BCC can be realized by directly coupling wideband digital signals to the human body without any kind of modulation or up-conversion. Examples of such techniques can be found in Seong-Jun Song et al., "A 0,2mW 2Mb/s Digital Transceiver Based on Wideband Signalling for Human Body Communications", JSSC, Vol. 42, Issue 9, Sept. 2007 pp: 2021-2033, Seong-Jun Song et al., "Energy-Efficient Human Body Communication Receiver Chipset Using Wideband Signaling Scheme", EMBS, pp: 2292-2295, Aug. 2007, Seong-Jun Song et al., "A 0,9V 2,6mW Body-Coupled Scalable PHY Transceiver for Body Sensor Applications," ISSCC Dig. Tech. Papers, pp: 366-367, Feb. 2007 or Fazzi et al., "A 2.75mW Wideband Correlation-Based Transceiver for Body-Coupled Communication" ISSCC Dig. Tech. Papers, Feb 2009.

When digital signals are transmitted via the body, the information may be encoded in the transitions between high and low levels. The receiver 200 has to extract this information from the strongly attenuated signal which is propagated via the body and to produce a locally synchronized digital signal that correctly replicates the transmitted signal.

Due to envisioned application areas, immunity for interference, low power consumption and active chip areas are important requirements for successful deployment of the BCC technology. Conventional receiver architectures are supposed to overcome the above problems by performing a significant high-pass filtering and detecting those peaks that correspond to signal transitions. However, this approach is not optimal because a simple high-pass filtering also attenuates the wanted wideband signal and because the receiver chain is open for interference at higher frequencies, where they are most likely to appear due to the specific characteristics of the body channel.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved detection processing with less attenuation of the wanted wideband signal and less interference at higher frequencies.

This object is achieved by an apparatus as claimed in claim 1, a method as claimed in claim 10, and a computer program product as claimed in claim 11.

Accordingly, a power and area efficient approach for detection and/or reception of communication signals, such as BCC communication signals, is proposed, which provides an active, easily programmable band-pass filtering effective for time and frequency characteristics of body channels at the reception electrode. The proposed apparatus or method serves to significantly attenuate low- frequency content of an input signal and to spectrally shape it outside a desired communication band, with the help of the inherent self-oscillation mechanism.

The low frequency band can be strongly suppressed without chopping, such that charge injection and charge feed through can be avoided completely. In contrast to correlation approaches, the proposed reception or detection apparatus and method do not require time for synchronization and can react much faster to channel activity. Moreover, due to the low number of required active components, a very low power implementation can be achieved.

According to a first aspect, the high-pass filter may comprise an asynchronous sigma- delta modulator (ASDM). In the ASDM, amplitude-time transformation is achieved using an inherent self-oscillation denoted as a limit cycle. Information in the amplitude of the input signal is transformed into time information in the output signal without introducing quantization noise.

According to a second aspect which can be combined with the above first aspect, the communication signal may be a body-coupled communication signal received via a body channel. Immunity for interference combined with low power consumption and less active chip area makes the proposed detection/reception approach especially suitable for BCC technology. According to a third aspect which can be combined with the above first and second approaches, the high-pass filter may be programmable to select a sensed frequency band by controlling the self-oscillation mechanism. Thereby, the desired frequency band can be made variable so as to process a wanted signal band with minimal attenuation.

According to a fourth aspect which can be combined with any one of the above first to third aspects, the low-pass filter outside the ASDM loop may be adapted to detect and accumulate an output power of the output bit stream in a desired frequency band. The power detection result can be accumulated to obtain an indication of channel activity. Thereby, a programmable band-pass channel sensing system can be provided. The channel sensing system can be enhanced by further adding an optional comparator for comparing the accumulated output power with a predetermined threshold value to indicate sufficient channel activity. Thereby, an indication for sufficient channel activity can be provided when the accumulated power detection value has reached the threshold or reference value. According to a fifth aspect which can be combined with any one of the above first to fourth aspects, the low-pass filter may be adapted to produce a predetermined band-limited signal so as to receive the communication signal. Now, a complete receiver can be provided which, in addition to amplification and band-pass filtering also recovers the received data. The receiver can be enhanced by adding a digitizer for digitizing the received communication signal at Nyquist frequency. It is noted that the above apparatus can be provided in a receiver or transceiver for any kind of data signal and may be implemented as a discrete hardware circuit with discrete hardware components, as an integrated chip, as an arrangement of chip modules, or as a signal processing device or a chip controlled by a software routine or program stored in a memory, written on a computer readable medium, or downloaded from a network, such as the Internet. Further advantageous embodiments are defined below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings, wherein:

Fig. 1 shows a schematic electrode arrangement of a BCC system;

Fig. 2 shows a schematic block diagram of a high-pass filtering circuit with ASDM;

Fig. 3 shows a frequency characteristic of a first order ASDM;

Fig. 4 shows a time diagram with ASDM input and output waveforms; Fig. 5 shows a time diagram with an ASDM output bit stream and a low-pass filtered bit stream;

Fig. 6 shows a schematic block diagram of a channel sensing circuit according to a first embodiment; and

Fig. 7 shows a schematic block diagram of a receiver circuit according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

Various embodiments of the present invention will now be described based on a detection processing in a BCC receiver. Of course, the present invention can also be applied to other types of communications and communication channels not related to the specific BCC applications. In the present embodiments, a power and area efficient approach for detection and/or reception of signals transmitted via the body is proposed based on a high-pass filter for quantizing amplitudes of a received signal in a non-linear closed loop which transforms amplitude information into time information using an inherent self-oscillation mechanism. An example of such a high-pass filter is an ASDM in a variable band-pass spectral shaper configuration. This configuration acts as an active, easily programmable band-pass filter which is especially effective for time and frequency characteristics of BCC signals at a reception electrode. The proposed configuration attenuates significantly low-frequency content of the input signal and spectrally shapes it outside a desired communication band with the help of the inherent self-oscillation mechanism.

The exemplary ASDM is a closed-loop non-linear system (i.e. high-pass filter) that transform information in the amplitude of the input signal into time information in the output signal without introducing quantization noise. In such loop systems, amplitude-time transformation can be achieved using an inherent self-oscillation denoted as limit cycle. The oscillation frequency determines the spectral properties of the system and the quality of the amplitude-time transformation. More specifically, the input/output transfer characteristic of a closed loop system depends on the point at which the input signal is injected in the loop. Traditionally, ASDMs are used to process low frequency band signals and the input signal is injected at the input of the loop filter. In contrast thereto, according to the present embodiments, the input signal is injected after the loop filter, so that the loop system (e.g. ASDM) can act as a high-pass filter.

Furthermore, the digitization of a received (BCC) signal may require quantization in amplitude and quantization in time. Both operations can introduce significant errors in the signal processing path. Due to the fact that the proposed high-pass loop filter incorporates amplitude quantization in a closed loop, the introduced errors can be spectrally shaped outside the signal band around the inherent self-oscillation frequency. The added high frequency content can be removed by subsequent low-pass filtering. The time quantization can then be performed at Nyquist frequency on a strong band- limited signal.

Fig. 2 shows a schematic block diagram of an exemplary high-pass filter circuit based on an ASDM loop in a high-pass filter configuration. The ASDM comprises a binary quantizer 30 with hysteresis, a low-pass loop filter (L(s)) 10 and an amplifier or gain stage (A) 20 incorporated in a closed-loop system. An input signal x(t) is inserted in the ASDM loop in front of the quantizer 30 which can be specified by its input partitions and output levels. The input range may be divided into levels of equal or non-equal spacing. The quantizer 30 thus partitions its input signal i(t) which is obtained as the sum of the input signal x(t) and the output signal of the gain stage 20, and outputs amplitude discrete levels as an output signal y(t). The hysteresis in the quantizer 30 is required for the operation of a first order ASDM. For higher order loop filter is not required but still might be present.

Fig. 3 shows an exemplary input-output transfer function of the high-pass filter circuit of Fig. 2 in case it is a first order loop filter. The transfer function of the closed loop has a high-pass character with respect to the input which suppresses low frequencies. In view of the exemplary first- order configuration, the steepness of the realized filter is 20 dB/dec. In general, filter steepness depends on the order of the ASDM loop filter 10. In this way, the ASDM functionality suppresses low frequency interferences which are a source of noise in the body channel and processes the desired signal band with minimal attenuation. Optionally, the desired signal band can be made variable via realization of control means for a programmable loop gain and self-oscillation frequency.

The cut-off frequency of the ASDM loop filter 10 depends on the self-oscillation frequency of the ASDM and can be regulated by changing the hysteresis level of the quantizer 30, the loop frequency response or the gain A of the gain stage 20 in the loop. Thus, input signals with higher frequency (above 20 MHz for the example of Fig. 3) are not attenuated by the high-pass loop filter of Fig. 2 and appear directly at the output. As far as such high-frequent input signals do not overload the high-pass loop filter, they are not harmful because they can be filtered without deterioration of the desired signal.

In addition to the above high-pass filtering, the proposed high-pass filter can also perform noise shaping. The impact of circuit noise and quantization errors is shifted to the high- frequency band around the limit cycle frequency of the ASDM. As a result, the quantization in amplitude obtained by the quantizer 30 no longer deteriorates the useful signal band. The performed spectral shaping can be easily interpreted in the time domain, as explained in the following.

Fig. 4 shows a time diagram with ASDM input and output waveforms. The upper waveform of Fig. 4 corresponds to the input signal and the lower waveform corresponds to the output signal. In the present example, the input signal is a Manchester encoded digital signal which may be attenuated by 20 dB with two binary values referred to as "+/- 1". Because the ASDM transforms information in amplitude into information in time, it reacts to digital-like or binary input signals only at the edges when there is a change from one state to the other. At an incoming input edge, the idle limit cycle is disturbed and the duty cycle of the limit cycle oscillations changes to accommodate this amplitude change. The high-pass loop filter transforms the amplitude information of the input signal into time information in its output bit stream without introduction of quantization noise. Therefore, the received signal can be easily reconstructed from the output bit stream of the high-pass filter (y(t) in Fig. 2) with a simple low-pass filtering. The proposed cascade of the proposed high-pass filter, used as a high-pass noise shaper, and a subsequent (passive) low-pass filter, e.g. with corner frequency at the edge of the desired signal band, leads to a simple power and area efficient band-pass filter. The quality of the performed signal processing can be determined to a large extent by the order of the low-pass loop filter 10 and the self-oscillating frequency of the high-pass filter ASDM.

It is noted that the proposed self-oscillation mechanism can be obtained in any loop configuration with an amplifier, at least one filter circuit and a non- linear element, and is not restricted to ASDM-type filters. The change of the output duty cycle depends on the amplitude change at the input and on the frequency of the limit cycle oscillations (e.g. time resolution of the high pass filter). As already mentioned, the useful component of the input signal can be extracted from the output bit stream y(t) with a simple passive low-pass filter. Fig. 5 shows an output signal (lower waveform) which has been obtained by filtering the output bit stream with a second-order low-pass filter with cut-off frequency at e.g. 40 MHz in the present example. In general, the low-pass filter should be adapted to suppress only high frequency content around the limit cycle frequency without strict band limitation and can be implemented as a passive filter.

As can be gathered from Fig. 5, suppressions of high frequency content of the output signal lead to a signal consisting in a series of pulses in the low-pass filtered bit stream. These pulses represent the input signal to which the high pass filtering of the loop is applied. As it can be seen, the self-oscillation frequency component is largely removed from the output.

Thus, a high-pass filter with a loop arrangement is provided for filtering and quantizing a signal derived from the received signal, so as to introduce frequency selectivity and spectrally shape quantization noise, wherein a forced or self-induced oscillation is generated in the loop arrangement so as to transform amplitude information into time information. In the following, two application examples are described based on first and second embodiments. In the first embodiment, the proposed detection scheme is used for realization of a channel sensing circuit which establishes a pre-programmed level of channel activity (e.g. in the body channel) and indicates e.g. to a processor unit which in turn can decide to wake-up a main (BCC) receiver. The second embodiment relates to a receiver circuit which also performs digitalization of the received data.

Fig. 6 shows a schematic block diagram of a channel sensing circuit according to the first embodiment. A BCC signal received via a signal electrode 40 and coupling capacitor is amplified by a low noise amplifier (LNA) 50 which increases overall sensitivity and provides a sufficiently large input signal for the high-pass filter 100 which in the present example is implemented based on an ASDM circuit. The LNA 50 can be band-limited in order to prevent ASDM overload from high frequency interference. As already mentioned above, the high-pass filter 100 which consists of a controllable low-pass filter 15 and a quantizer 30 with hysteresis in a feedback loop is used for high- pass filtering and spectral shaping. The output bit stream of the high-pass loop filter 100 is a binary signal (i.e. an amplitude-discrete time-continuous signal) which allows simple detection of signal components in the desired frequency or signal band. This output signal is supplied to a power detector and accumulator circuit 60 which is adapted to provide an output that corresponds to the accumulation of the detected signal power in the band of interest. The accumulation can be continuous or limited to a predetermined time interval. Consequently, the power detector and accumulator circuit 60 performs a kind of low-pass filtering at very low resolution to achieve a simple channel sensing functionality: if the power in band of interest is large enough for certain duration of time we can infer that BCC channel is in use and BCC device is transmitting information. The output of the power detector and accumulator circuit 60 may then optionally be supplied to a comparator circuit 70 to which a threshold value or reference value V_ref can be supplied as a comparison input. When the accumulated output signal of the power detector and accumulator circuit 60 reaches the reference value V_ref, the optional comparator 70 can give an indication for sufficient channel activity, which may be a signal transition, an output pulse, or the like.

The limit cycle frequency of the high-pass filter 100 can be made programmable e.g. via the low-pass filter 15, so that the desired and sensed frequency band can be selected. In addition, the accumulator or integrator time constant of the power detector and accumulator circuit 60 can also be made programmable or controllable, so that different types of input signals can be detected and the effect of interference further suppressed.

Fig. 7 shows a schematic block diagram of a (BCC) receiver circuit according to the second embodiment. It is noted that components 15, 30, 40 and 50 correspond to the same components of Fig. 6 and are not described again in the second embodiment.

In addition to input amplification and band-pass filtering, the receiver circuit according to the second embodiment also performs digitization of the received data. Low-pass filtering in a (controllable) low-pass filter 80 is adapted to produce a well defined band-limited signal which can be digitized at Nyquist frequency by supplying a corresponding clock signal Cl to the digitizer circuit 90. Thus, the high-pass loop filter 100 in combination with the controllable low-pass filter 80 can be used as a programmable band-pass spectral shaper. The embodiments described above can be used in a large number of potential applications in the field of capacitive sensing, identification and security, wireless sensor networks and medical applications, such as for example wireless patient monitoring and identification. However, the proposed channel sensing and receiver circuit may as well be used in any other communication technology to achieve the above mentioned advantages. Moreover, the invention is not intended to be restricted to an ASDM-based implementation of the high-pass filter 100. Any filter within a feedback non-linear configuration with self, or forced oscillation mechanisms can be used to provide the intended functionality.

In summary, the present invention relates to an apparatus, method and computer program product, wherein a power and area efficient approach for detection and reception of communication signals is proposed, in which a variable band-pass spectral shaper configuration is used. The proposed configuration attenuates significantly the low- frequency content of the input signal and spectrally shapes it outside the desired communication band, with the help of an inherent self- oscillation mechanism.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modification may involve other features which are already known in the art and which may be used instead of or in addition to features already described therein.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. A single processor or other unit may fulfill at least the functions of the detecting, channel sensing, or receiving procedure, e.g. as described in connection with Figs. 6 and 7, based on corresponding software routines. The computer program may be stored and/or distributed on a suitable medium such as an optical storage medium or a solid-state medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. The mere fact that the measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claim should not be construed as limiting the scope thereof.

Claims

1. An apparatus for detecting a communication signal, said apparatus comprising:

- a high-pass filter (100) having a loop arrangement for filtering and quantizing a signal derived from said communication signal, so as to introduce frequency selectivity and spectrally shape quantization noise, wherein a forced or self-induced oscillation is generated in said loop arrangement so as to transform amplitude information into time information; and

- a low-pass filter (60; 80) for filtering an output bit stream of said high-pass loop filter (100) to detect said communication signal in said received signal.

2. An apparatus according to claim 1, wherein said high-pass filter (100) comprises an asynchronous sigma-delta modulator.

3. An apparatus according to claim 1, wherein said high-pass filter (100) comprises a sigma-delta modulator synchronized to an external clock signal.

4. An apparatus according to claim 1, wherein said communication signal is a body-coupled communication signal received via a body channel.

5. An apparatus according to claim 1, wherein said high-pass filter (100) is programmable to select a sensed frequency band by controlling said self-oscillation mechanism.

6. An apparatus according to claim 1, wherein said low-pass filter (60) is adapted to detect and accumulate an output power of said output bit stream in a desired frequency band.

7. An apparatus according to claim 6, further comprising a comparator (70) for comparing said accumulated output power with a predetermined threshold value to indicate sufficient channel activity.

8. An apparatus according to claim 1, wherein said low-pass filter (80) is adapted to produce a predetermined band- limited signal so as to receive said communication signal.

9. An apparatus according to claim 8, further comprising a digitizer (90) for digitizing said received communication signal at Nyquist frequency.

10. A receiver device for receiving a body-coupled communication signal, said receiver device (200) comprising an apparatus according to claim 1.

11. A method of detecting a communication signal, said method comprising:

- quantizing amplitudes of a received signal in a closed loop which transforms amplitude information into time information using an inherent self or forced oscillation mechanism; and

- detecting said communication signal in said received signal by low-pass filtering an output bit stream of said closed loop.

12. A computer program product comprising code means for producing the steps of method claim 10 when run on a computing device.