Abstract:
A system for communicating with an implantable medical device via RF telemetry is disclosed which mitigates the effects of nulls caused by, e.g., multi-path distortion. In one embodiment, signals transmitted by the implantable device to an external device are simultaneously received with a pair of separate spaced apart first and second antennas. The antennas may provide spatial and/or polar diversity. The presence of nulls in the implantable device&#39;s transmission pattern can be determined by detecting an error rate in the signals received from the implantable device with each antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to commonly assigned, U.S. patent application Ser. No. 11/068,476, entitled “METHOD AND APPARATUS FOR OPERATING A DIVERSITY ANTENNA SYSTEM COMMUNICATING WITH IMPLANTABLE MEDICAL DEVICE” filed on Feb. 28, 2005, now issued as U.S. Pat. No. 7,392,092, which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     This invention pertains to implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators. 
     BACKGROUND 
     Implantable medical devices (IMDs), including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with an external device (ED) via a radio-frequency telemetry link. One such external device is an external programmer used to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data that may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations. An external device may also be a remote monitoring unit which collects data from the implantable device and transmits it over a network to a data gathering center. 
     External programmers are commonly configured to communicate with an IMD over an inductive link. Coil antennas in the external programmer and the IMD are inductively coupled so that data can be transmitted by modulating a carrier waveform which corresponds to the resonant frequency of the two coupled coils. An inductive link is a short-range communications channel requiring that the coil antenna of the external device be in close proximity to the IMD, typically within a few inches. Other types of telemetry systems may utilize far-field radio-frequency (RF) electromagnetic radiation to enable communications between an MD and an ED over a wireless medium. Such long-range RF telemetry allows the IMD to communicate with an ED, such as an external programmer or remote monitor, without the need for close proximity. 
     Communications via far-field RF telemetry, however, can be hindered by the effects of multi-path distortion which result in nulls in the transmission pattern of either the external device or the implantable device. In a typical environment, reflections of a transmitted wave caused by walls and other objects result in a standing wave pattern. Areas where the standing wave pattern results in a low amplitude signal below the noise floor are referred to as nulls or null areas. When an external programmer antenna is in a null with respect to an implantable device antenna, the RF link is lost and further communications are not possible. Because a patient may typically be moving around during telemetry sessions, such nulls may be transient and of short duration. Even short-duration nulls, however, cause difficulties when collecting certain types of data from an implantable device such as real-time electrograms. 
     SUMMARY 
     The present disclosure relates to a system for communicating with an implantable medical device via RF telemetry which mitigates the effects of nulls caused by, e.g., multi-path distortion. In one embodiment, signals transmitted by the implantable device to an external device are simultaneously received with a pair of separate spaced apart first and second antennas. The antennas may provide spatial and/or polar diversity. The presence of nulls in the implantable device&#39;s transmission pattern can be determined by detecting an error rate in the signals received from the implantable device with each antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the basic components of an external device. 
         FIG. 2  illustrates an exemplary antenna configuration for an external programmer which provides spatial diversity. 
         FIG. 3  illustrates an exemplary antenna configuration for an external programmer which provides both spatial and polar diversity. 
         FIG. 4  illustrates an exemplary scheme for operating a diversity antenna with one receive path. 
         FIG. 5  illustrates an exemplary scheme for operating a diversity antenna with a dual receive path. 
         FIG. 6  illustrates an exemplary scheme for operating a diversity antenna which provides polar transmission diversity. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a basic block diagram of an external device configured for communicating with an implantable medical device, where the external device may be either an external programmer or a remote monitoring device. In this exemplary embodiment, the external device includes a microprocessor  10 , memory  11 , and a hard disk  12  for data and program storage that supervises overall device operation as well as telemetry. Code executed by the microprocessor may be used to control the operation of the various telemetry components to be described below. User input/output devices  13 , such as a keyboard and display, are interfaced to the microprocessor in order to enable a user such as a clinician to direct the operation of the external device. To provide telemetry, a long-range RF transceiver  180  which includes components for transmitting and receiving RF signals is interfaced to the microprocessor  10 . The transmitter and receiver components are coupled to an antenna array  100  through one or more transmit/receive switch. The transmit/receive switches are controlled by the microprocessor and either passes radio-frequency signals from the transmitter to the antenna or from the antenna to the receiver to establish an RF link. As explained below, the antenna array  100  may comprise two or more antennas to form a diversity antenna. To effect communications between the devices over the RF link, a radio-frequency carrier signal modulated with digital data is transmitted wirelessly from one antenna to the other. A demodulator for extracting digital data from the carrier signal is incorporated into the receiver, and a modulator for modulating the carrier signal with digital data is incorporated into the transmitter. The interface to the microprocessor for the RF transmitter and receiver enables data transfer by the microprocessor.  FIG. 1  also shows an inductively coupled transmitter/receiver  140  and antenna  150  by which communication may take place over an inductive link when the external and implantable devices are in close physical proximity to one another. 
     A diversity antenna is an array of two or more antennas separated in space to resulting spatial diversity and/or differing in polarity to result in polar diversity.  FIG. 2  illustrates one embodiment of a diversity antenna configuration for an external programmer. External programmers are typically designed as laptop computers which incorporate the necessary telemetry equipment for communicating with an implantable device. The external programmer in  FIG. 2  includes a housing  210  for containing the electronic components such as those illustrated in  FIG. 1 . The housing  210  is connected to a hinging display screen  215  and has a keyboard  211  mounted thereon. In this embodiment, two dipole antennas designated  20  and  30  are shown as mounted on the sides of the display screen extending vertically from its top edge. Because the two antennas  20  and  30  are separated by the width of the display screen, this antenna configuration provides spatial diversity. That is, should one antenna be located in a null area of the implantable device&#39;s transmission pattern, it is likely that the other antenna is not so that reception can continue. Similarly, when transmitting to the implantable device, if the implantable device should be in a null area of one antenna, it is likely that it is not in a null area of the other antenna. 
       FIG. 3  illustrates another embodiment in which the antenna  20  extends vertically as in  FIG. 2 , but the other antenna  40  is embedded in the top edge of the display screen and oriented horizontally. This antenna configuration provides spatial diversity due to the separation of the two antennas and also provides polar diversity because the two antennas are oriented differently. A monopole or dipole antenna is a linearly polarizing antenna which radiates electromagnetic waves which are polarized in the direction of the antenna&#39;s orientation. Such an antenna also most sensitively receives electromagnetic radiation which is polarized in the same direction as the antenna&#39;s orientation and is not sensitive at all to radiation polarized orthogonally to it. The antenna of an implantable medical device is usually a dipole or monopole antenna which, for example, extends from the implantable device housing or is incorporated into an intravenous lead. The waveform transmitted by the implantable device is therefore polarized in a direction which depends upon the position of the patient. Furthermore, the polarization of the transmitted waveform can change as the patient moves. The vertically oriented antenna  20  and the horizontally oriented antenna  40 , being orthogonal to one another, provide polar diversity since an arbitrarily polarized waveform will be sensitively received by at least one of the antennas. Similarly, at least one of the antennas  20  or  40  will be capable of transmitting a waveform with a polarization that can be sensitively received by the implantable device antenna regardless of the latter&#39;s orientation. 
     Different schemes may be employed to operate a diversity antenna. In one embodiment, as illustrated by  FIG. 4 , an antenna selection switch  140  operated by the microprocessor  10  is located between the transmit/receive switch  130  and two antennas  100   a  and  100   b , where the transmit/receive switch is connected to a transmitter  110  and a receiver  120 . The antennas  100   a  and  100   b  may, for example, correspond to either the antennas  20  and  30  or to antennas  20  and  40  in  FIGS. 2 and 3 , respectively. The antenna selection switch  140  is operated by the microprocessor so that only one of the antennas  100   a  or  100   b  is active at a time by being connected to either the transmitter  110  or the receiver  120 . In an exemplary scheme, the microprocessor selects one of the antennas for use in both transmission and reception and connects that antenna to the transmit/receive switch through the antenna selection switch. The microprocessor is then programmed to perform error detection on the signal received from the implantable device to determine if the currently used antenna may be in a null area. For example, a cyclic redundancy check applied to each received frame after digitization of the received signal. If the error rate exceeds a specified threshold, the microprocessor switches to the other antenna for both reception and transmission. 
       FIG. 5  illustrates another scheme for operating a diversity antenna in which both of the antennas  100   a  and  100   b  are active during reception by being connected to separate receivers  120   a  and  120   b . In this embodiment, the receivers  120   a  and  120   b  are connected to antennas  100   a  and  100   b  by transmit/receive switches  130   a  and  130   b , respectively. The two transmit/receive switches allow the device to receive signals from both antennas simultaneously but to use only one antenna at a time for transmission. During reception, the dual receive path allows the signal from both antennas to be monitored simultaneously in order to derive an error rate and determine if one of the antennas may be in a null area. For example, the signal from each antenna may be digitized, and a cyclic redundancy check then applied to each received frame from both signal paths. The device may use the data derived from either signal if no errors are present. For example, the device may simply use the data received from the antenna presently selected for transmitting unless an error is discovered in that signal. If the error rate exceeds a specified threshold in the antenna currently being used for transmission, the microprocessor switches to the other antenna for subsequent transmissions. Simultaneous reception from both antennas thus eliminates the need for retransmission of data by the implantable device should one antenna be in a null while the other is not. Also, since whether or not the alternate antenna is in a null is determined at the same time, switching to the alternate antenna may be performed more rapidly without requiring a retry for that antenna. Although two separate transmit/receive switches are used to allow only one antenna to be selected for transmitting, there is no need for an antenna selection switch as in the previously described embodiment. 
       FIG. 6  illustrates another scheme for operating a diversity antenna which provides polar diversity such as illustrated in  FIG. 3 . The configuration of the transmitter  110 , receivers  120   a  and  120   b , transmit/receive switches  130   a  and  130   b , and antennas  100   a  and  100   b  is identical to that of  FIG. 5  except that a 90 degree phase shifter  150  is inserted in the transmission paths between the transmitter  110  and the transmit/receive switch  130   a . Such a phase shifter may simply be a delay element which results in a 90 degree phase lag at the transmitting frequency. If both transmit/receive switches are switched on so that both antennas are driven at the same time by the transmitter, the result is a circularly polarized waveform. A circularly polarized waveform comprises two orthogonally polarized waveforms with a 90 degree phase difference between them. It may be desirable to transmit such a waveform when it is not known what the orientation of the implantable device&#39;s antenna is, such as when the external device is initiating communications, so that the transmitted waveform will have a component which is assured to be received by the implantable device&#39;s antenna. Although ideally a circularly polarized waveform is desired to provide this type of transmission diversity, an elliptically polarized waveform produced by a phase shift in one of the transmitted waveforms which is greater or lesser than 90 degrees may provide the same benefits. Such elliptical polarization may result, for example, should the transmission frequency change such as occurs with frequency-hopping spread spectrum techniques. 
     The embodiments described above have been discussed primarily with reference to an external programmer. It should be appreciated, however, that those embodiments could be incorporated into any kind of external device which is configured to communicate with an implantable medical device, including a remote monitor. 
     Although the invention has been described in conjunction with the foregoing specific embodiment, many alternatives, variations, and modifications will be apparent to those of ordinary skill in the art. Such alternatives, variations, and modifications are intended to fall within the scope of the following appended claims.