Patent Publication Number: US-11660455-B2

Title: Tissue conduction communication using ramped drive signal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 16/204,172, filed Nov. 29, 2018, granted as U.S. Pat. No. 11,045,654, which claims priority to, and the benefit of, U.S. Provisional Patent Application Ser. No. 62/591,813, filed on Nov. 29, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to devices, systems and methods for communicating using tissue conduction communication. 
     BACKGROUND 
     Communication between two or more devices associated with a person, e.g., implanted within the person and/or attached to or otherwise contacting the person, may be desirable in a number of applications, such as for monitoring or managing health of a patient. Communication between these devices may, for example, enable the exchange of information, coordinated monitoring of a health condition and/or coordinated therapy to treat health conditions. Such systems, some examples of which are described below, may communicate using tissue conduction communication (TCC). TCC uses the human body as the medium of communication. TCC may sometimes be referred to as human body conduction (HBC) or intrabody communication. 
     A wide variety of implantable medical devices (IMDs) for delivering a therapy to or monitoring a physiological condition of a patient have been used clinically or proposed for clinical use in patients. Examples include IMDs that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other tissue. Some therapies include the delivery of electrical stimulation to such tissues. Some IMDs may employ electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic physiological electrical signals within the patient, which may be propagated by such organs or tissue, and/or other sensors for sensing physiological signals of a patient. 
     Implantable cardioverter defibrillators (ICDs), for example, may be used to deliver high energy defibrillation and/or cardioversion shocks to a patient&#39;s heart when atrial or ventricular tachyarrhythmia, e.g., tachycardia or fibrillation, is detected. An ICD may detect a tachyarrhythmia based on an analysis of a cardiac electrogram sensed via electrodes, and may deliver anti-tachyarrhythmia shocks, e.g., defibrillation shocks and/or cardioversion shocks, via electrodes. An ICD or an implantable cardiac pacemaker, as another example, may provide cardiac pacing therapy to the heart when the natural pacemaker and/or conduction system of the heart fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient to sustain healthy patient function. ICDs and cardiac pacemakers may also provide overdrive cardiac pacing, referred to as anti-tachycardia pacing (ATP), to suppress or convert detected tachyarrhythmias in an effort to avoid cardioversion/defibrillation shocks. 
     Some IMDs are coupled to one or more of the electrodes used to sense electrical physiological signals and deliver electrical stimulation via one or more leads. A medical electrical lead carrying sensing and/or electrical therapy delivery electrodes allow the IMD housing to be positioned a location spaced apart from the target site for sensing and/or stimulation delivery. For example, a subcutaneously or sub-muscularly implanted housing of an ICD or implantable cardiac pacemaker may be coupled to endocardial electrodes via one or more medical electrical leads that extend transvenously to the patient&#39;s heart. Other ICD systems, referred to as extracardiovascular ICD systems, are not coupled to any transvenous leads, and instead sense and deliver shocks via electrodes implanted away from the patient&#39;s heart, e.g., implanted subcutaneously or substernally. The extra-cardiovascular electrodes may be provided along the housing of the subcutaneous ICD and/or coupled to the housing via one or more leads extending subcutaneously, submuscularly or substernally from the housing. 
     Leadless IMDs may also be used to deliver therapy to a patient, and/or sense physiological parameters of a patient. In some examples, a leadless IMD may include one or more electrodes on its outer housing to deliver therapeutic electrical stimulation to the patient, and/or sense intrinsic electrical signals of patient. For example, a leadless pacemaker may be used to sense intrinsic depolarizations or other physiological parameters of the patient, and/or deliver therapeutic electrical stimulation to the heart. A leadless pacemaker may be positioned within or outside of the heart and, in some examples, may be anchored to a wall of the heart via a fixation mechanism. 
     In some situations, two or more IMDs are implanted within a single patient. It may be desirable for the two or more IMDs to be able to communicate with each other, e.g., to coordinate, or cooperatively provide, sensing for monitoring the patient and/or therapy delivery. Although some IMDs communicate with other medical devices, e.g., with external programming devices, using radio-frequency (RF) telemetry, TCC allows for communication between two or more IMDs by transmitting signals between the electrodes of two IMDs via a conductive tissue pathway. Likewise, TCC may be utilized to communicate between an IMD and an external device having electrodes proximate to or in contact with the skin of the patient or between two external devices having electrodes proximate to or in contact with the skin of the patient. 
     SUMMARY 
     The techniques of this disclosure generally relate to TCC signal transmission techniques performed by a device. The techniques of this disclosure are described in the context of an IMD. However, the techniques can be utilized by any device, medical or non-medical, implanted or external, that communicates using TCC. 
     A TCC transmitter included in a transmitting IMD is configured to generate at least one beacon signal during a wakeup mode of the TCC transmitter and transmit data signals during a data transmission mode of the TCC transmitter. A TCC transmitter operating according to the techniques disclosed herein is configured to transmit a ramp on signal to charge an alternating current (AC) coupling capacitor prior to transmitting a TCC signal, e.g., at the beginning of a TCC transmission session, to minimize interference of the TCC signal with electrical signal sensing circuitry in the IMD system. The TCC transmitter may be controlled to produce a ramp off signal to control discharge of the coupling capacitor after a TCC signal, e.g., at the end of a TCC transmission session. 
     In one example, the disclosure provides a device comprising a housing and a tissue conductance communication (TCC) transmitter enclosed by the housing. The TCC transmitter includes a coupling capacitor for coupling TCC signals to a transmitting electrode vector. The TCC transmitter is configured to generate a TCC ramp on signal having a peak-to-peak amplitude that is stepped up from a starting peak-to-peak amplitude to an ending peak-to-peak amplitude according to a step increment and a step up interval, transmit the TCC ramp on signal via the coupling capacitor coupled to the transmitting electrode vector, and transmit a second TCC signal after the TCC ramp on signal. 
     In another example, the disclosure provides a method comprising generating, with a tissue conduction communication (TCC) transmitter, a TCC ramp on signal having a peak-to-peak amplitude that is stepped up from a starting peak-to-peak amplitude to an ending peak-to-peak amplitude according to a step increment and a step up interval, transmitting, with the TCC transmitter, the TCC ramp on signal via a coupling capacitor coupled to a transmitting electrode vector, and transmitting, with the TCC transmitter, a second TCC signal after the TCC ramp on signal. 
     The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a conceptual diagram of an IMD system capable of TCC according to one example. 
         FIG.  2    is a conceptual diagram of an IMD system configured to communicate using TCC techniques disclosed herein according to another example. 
         FIG.  3 A  is a conceptual diagram of a leadless intracardiac pacemaker according to one example. 
         FIG.  3 B  is a schematic diagram of circuitry that may be included in the pacemaker of  FIG.  3 A  according to one example. 
         FIG.  4    illustrates a perspective view of a leadless pressure sensor according to one example. 
         FIG.  5    is a schematic diagram of an ICD capable of transmitting TCC signals according to one example. 
         FIG.  6    is a conceptual diagram illustrating an example configuration of a TCC transmitter that may be included in the ICD of  FIG.  5    or in the pacemaker of  FIG.  3 B  or pressure sensor of  FIG.  4   . 
         FIG.  7    is a conceptual diagram of a TCC transmission session that may be executed by the TCC transmitter of  FIG.  6   . 
         FIG.  8    is a diagram of operations performed by an IMD system during the wakeup mode of the TCC transmitter of  FIG.  6   . 
         FIG.  9    is a diagram of a TCC ramp on signal, beacon signal, and ramp off signal according to one example. 
         FIG.  10    is a diagram of one example of a transmission session performed by a transmitting device of an IMD system. 
         FIG.  11    is a diagram of a data packet that may be transmitted during the data transmission mode of  FIG.  10    by the transmitting device according to one example. 
         FIG.  12    is a conceptual diagram of a portion of a data field that may be included in the data packet of  FIG.  11    followed by a ramp off signal. 
         FIG.  13    is a flow chart of a method for transmitting TCC signals that may be performed by an IMD system according to one example. 
         FIG.  14    is a conceptual diagram of a method for transmitting TCC signals during multiple transmission sessions according to one example. 
     
    
    
     DETAILED DESCRIPTION 
     Wireless communication between two or more medical devices may be desired for a number of reasons, including to exchange data and/or to coordinate, or cooperatively provide, sensing of physiological signals and/or therapy delivery. TCC signals may be wirelessly transmitted from one IMD to one or more IMDs co-implanted within a patient and/or to an external medical device having skin or surface electrodes coupled to the patient for transmitting and/or receiving TCC signals. Some IMDs and external medical devices may be configured to sense an electrophysiological signal via sensing electrodes and/or monitor electrical impedance such as transthoracic impedance signals. Examples of electrophysiological signals include a cardiac electrical signal produced by the patient&#39;s heart, an electromyogram signal produced by skeletal muscle tissue, and other electrophysiological signals produced by the brain, nerve or muscle tissue. Transmission of a communication signal through body tissue may cause interference with electrical signal sensing circuitry and/or may unintentionally cause electrical stimulation of muscle or nerves depending on the amplitude and frequency of the transmitted signal. 
     An IMD or an external medical device that includes electrical signal sensing circuitry configured to receive an electrophysiological signal or monitor impedance may be a TCC transmitting device, an intended TCC receiving device, or an unintended receiving device that is coupled to electrodes within the tissue conduction pathway of a TCC signal being transmitted between two other devices. In each case, a transmitted TCC signal may be received by sensing electrodes coupled to the transmitting or receiving IMD or external device and interfere with the sensing circuitry. In other examples, a transmitting or receiving device may be configured to monitor the electrical impedance of one or more medical electrical leads or the tissue impedance between one or more electrode vectors coupled to the device. A TCC transmitter and transmission techniques are disclosed herein for enabling reliable communication of multi-byte streams of encoded data between medical devices while minimizing the likelihood of a TCC signal interfering with electrophysiological signal sensing circuitry, impedance monitoring, or other monitoring of electrical signals performed by a system. 
       FIG.  1    is a conceptual diagram of an IMD system  10  capable of TCC according to one example.  FIG.  1    is a front view of a patient  12  implanted with IMD system  10 . IMD system  10  includes an ICD  14 , an extra-cardiovascular electrical stimulation and sensing lead  16  coupled to ICD  14 , and an intra-cardiac pacemaker  100 . ICD  14  and pacemaker  100  may be enabled to communicate via TCC for transmitting a variety of data or commands. For example, ICD  14  and pacemaker  100  may be configured to communicate via TCC to confirm detected cardiac events or a detected heart rhythm and/or coordinate delivery of cardiac pacing pulses for bradycardia pacing, ATP therapy, cardioversion/defibrillation (CV/DF) shocks, post-shock pacing, cardiac resynchronization therapy (CRT) or other electrical stimulation therapies in response to an abnormal heart rhythm being detected by one or both of the IMDs  14  and  100 . 
     IMD system  10  senses cardiac electrical signals, such as R-waves attendant to ventricular depolarizations and/or P-waves attendant to atrial depolarizations, for detecting abnormal heart rhythms with high sensitivity and specificity to enable IMD system  10  to deliver (or withhold) appropriate therapies at appropriate times. Transmission of TCC signals by an IMD, e.g., by ICD  14  or pacemaker  100 , may cause interference with the sensing circuitry of the transmitting IMD, resulting in false sensing of a cardiac event. Such false sensing of cardiac events due to TCC interference with a cardiac event detector included in electrical signal sensing circuitry may lead to withholding a pacing pulse when a pacing pulse is actually needed or contribute to false detection of a tachyarrhythmia event. The TCC signal transmission techniques disclosed herein reduce the likelihood of a TCC signal being falsely detected as a cardiac event by a cardiac electrical signal sensing circuit of the transmitting device. 
     The TCC signal transmission techniques may also reduce the likelihood that another IMD implanted in patient  12  that is configured to sense electrophysiological signals, such as R-waves and/or P-waves, falsely senses TCC signals as physiological signals. Another IMD implanted in patient  12  may be the intended receiving device of the transmitted TCC signals, e.g., pacemaker  100  receiving signals from ICD  14  or vice versa. In other cases, another IMD co-implanted in patient  12  may not be the receiving device of transmitted TCC signals but may be configured to sense electrophysiological signals via electrodes coupled to the co-implanted IMD. A voltage signal may develop across sensing electrodes of the intended or unintended receiving device and interfere with electrophysiological sensing and event detection. The TCC signal transmission techniques of the present disclosure may reduce or eliminate the incidence of TCC signals being sensed as electrophysiological signals or events by any other IMD implanted in patient  12  or an external device having electrodes coupled to the patient externally. 
       FIG.  1    is described in the context of an IMD system  10  including ICD  14  and pacemaker  100  capable of sensing cardiac electrical signals produced by the patient&#39;s heart  8  and delivering cardioversion and/or defibrillation (CV/DF) shocks and cardiac pacing pulses to the patient&#39;s heart  8 . In some examples, the TCC communication may be “one-way”communication, e.g., transmission only from ICD  14  to pacemaker  100  or transmission only from pacemaker  100  to ICD  14 . In other examples, the TCC communication may be “two-way” communication between ICD  14  and pacemaker  100  such that each of pacemaker  100  and ICD  14  can receive and transmit information. It is recognized that aspects of the TCC signal transmission techniques disclosed herein may be implemented in a variety of IMD systems which may include an ICD, pacemaker, cardiac monitor or other sensing-only device, neurostimulator, drug delivery device or other implantable medical device(s). The TCC signal transmission techniques disclosed herein may be implemented in any IMD system that requires communication between one IMD and at least one other medical device, implanted or external. Moreover, the techniques described herein may be utilized by two external devices that communicate using TCC. The techniques may also have non-medical applications as well for devices that are implanted and/or external and communicate using TCC. 
     ICD  14  includes a housing  15  that forms a hermetic seal that protects internal components of ICD  14 . The housing  15  of ICD  14  may be formed of a conductive material, such as titanium or titanium alloy. The housing  15  may function as an electrode (sometimes referred to as a “can” electrode). In other instances, the housing  15  of ICD  14  may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing  15  functioning as an electrode(s) may be coated with a material, such as titanium nitride for reducing post-stimulation polarization artifact. Housing  15  may be used as an active can electrode for use in delivering CV/DF shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing  15  may be available for use in delivering relatively lower voltage cardiac pacing pulses and/or for sensing cardiac electrical signals in combination with electrodes carried by lead  16 . In any of these examples, housing  15  may be used in a transmitting electrode vector for transmitting TCC signals according to the techniques disclosed herein. 
     ICD  14  includes a connector assembly  17  (also referred to as a connector block or header) that includes electrical feedthroughs crossing housing  15  to provide electrical connections between conductors extending within the lead body  18  of lead  16  and electronic components included within the housing  15  of ICD  14 . As will be described in further detail herein, housing  15  may house one or more processors, memories, transceivers, cardiac electrical signal sensing circuitry, therapy delivery circuitry, TCC transmitting and receiving circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm and for transmitting TCC signals to pacemaker  100  and/or receiving TCC signals from pacemaker  100 . 
     Lead  16  includes an elongated lead body  18  having a proximal end  27  that includes a lead connector (not shown) configured to be connected to ICD connector assembly  17  and a distal portion  25  that includes one or more electrodes. In the example illustrated in  FIG.  1   , the distal portion  25  of lead body  18  includes defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 . In some cases, defibrillation electrodes  24  and  26  may together form a defibrillation electrode in that they may be configured to be activated concurrently. Alternatively, defibrillation electrodes  24  and  26  may form separate defibrillation electrodes in which case each of the electrodes  24  and  26  may be selectively activated independently. 
     Electrodes  24  and  26  (and in some examples housing  15 ) are referred to herein as defibrillation electrodes because they are utilized, individually or collectively, for delivering high voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). Electrodes  24  and  26  may be elongated coil electrodes and generally have a relatively high surface area for delivering high voltage electrical stimulation pulses compared to pacing and sensing electrodes  28  and  30 . However, electrodes  24  and  26  and housing  15  may also be utilized to provide pacing functionality, sensing functionality, and/or TCC signal transmission and receiving in addition to or instead of high voltage stimulation therapy. In this sense, the use of the term “defibrillation electrode” herein should not be considered as limiting the electrodes  24  and  26  for use in only high voltage cardioversion/defibrillation shock therapy applications. For example, electrodes  24  and  26  may be used in a sensing vector used to sense cardiac electrical signals and detect and discriminate tachyarrhythmias. Electrodes  24  and  26  may be used in a TCC signal transmitting electrode vector in combination with each other, collectively with housing  15 , or individually with housing  15 . In the case of ICD  14  being configured to receive TCC signals from pacemaker  100 , electrodes  24 ,  26  and/or housing  15  may be used in a TCC receiving electrode vector. The transmitting and receiving electrode vectors may be the same or different vectors. 
     Electrodes  28  and  30  are relatively smaller surface area electrodes which are available for use in sensing electrode vectors for sensing cardiac electrical signals and may be used for delivering relatively low voltage pacing pulses in some configurations. Electrodes  28  and  30  are referred to as pace/sense electrodes because they are generally configured for use in low voltage applications, e.g., delivery of relatively low voltage pacing pulses and/or sensing of cardiac electrical signals, as opposed to delivering high voltage cardioversion defibrillation shocks. In some instances, electrodes  28  and  30  may provide only pacing functionality, only sensing functionality or both. Furthermore, one or both of electrodes  28  and  30  may be used in TCC signal transmitting and/or receiving in some examples, e.g., in conjunction with electrodes  24 ,  26  and/or housing electrode  15 . 
     ICD  14  may obtain cardiac electrical signals corresponding to electrical activity of heart  8  via a combination of sensing electrode vectors that include combinations of electrodes  24 ,  26 ,  28 ,  30  and/or housing  15 . Various sensing electrode vectors utilizing combinations of electrodes  24 ,  26 ,  28 , and  30  may be selected by sensing circuitry included in ICD  14  for receiving a cardiac electrical signal via one or more sensing electrode vectors. 
     In the example illustrated in  FIG.  1   , electrode  28  is located proximal to defibrillation electrode  24 , and electrode  30  is located between defibrillation electrodes  24  and  26 . Electrodes  28  and  30  may be ring electrodes, short coil electrodes, hemispherical electrodes, or the like. Electrodes  28  and  30  may be positioned at other locations along lead body  18  and are not limited to the positions shown. In other examples, lead  16  may include none, one or more pace/sense electrodes and/or one or more defibrillation electrodes. 
     A TCC transmitting electrode vector may be selected from defibrillation electrodes  24 ,  26 ,  28 ,  30  and housing  15  for transmitting TCC signals produced by a TCC transmitter included in ICD  14 . Electrodes, such as defibrillation electrodes  24  and  26  and housing  15 , having a relatively large surface area may be used to transmit TCC signals to minimize the impedance of the transmitting electrode vector. A low impedance of the transmitting electrode vector maximizes the injected current signal. 
     The TCC transmitting electrode vector may be selected to both minimize impedance of the transmitting electrode vector and maximize transimpedance from the transmitting electrode vector to the intended receiving electrode vector. As used herein, the term “transimpedance” refers to the voltage received at a TCC signal receiving electrode vector divided by the transmitted current (voltage out divided by current in). As such, the transimpedance for a given TCC communication electrode vector for each of two IMDs configured to communicate bidirectionally is the same for communication in both directions for a given set of transmitting and receiving electrode vectors. By maximizing transimpedance, the voltage signal at the intended receiving electrodes is maximized for a given current signal injected into the tissue conductance pathway. As such, a low impedance of the transmitting electrode vector and high transimpedance of the TCC pathway increases the received TCC signal strength (voltage signal) at the receiving electrode vector. 
     Among the factors that may contribute to a maximized transimpedance of the TCC pathway are a substantially parallel electrical configuration of the transmitting and receiving electrode vectors, relatively wide spacing of the transmitting electrodes, relatively wide spacing of the receiving electrodes, and close proximity of the transmitting electrode vector to the receiving electrode vector. A transmitting electrode vector closer in proximity to the receiving electrode vector improves the strength of the TCC signal compared to a larger separation of the transmitting and receiving electrode vectors. The optimal orientation for the receiving electrode vector is parallel to the conductive tissue pathway of the current flow. A transmitting electrode vector that is substantially electrically parallel to the receiving electrode vector improves the strength of the TCC signal compared to the receiving electrode vector being orthogonal to the pathway of the current flow through the body tissue, which may result in a null signal. 
     A parallel electrical configuration between the transmitting and receiving electrode vectors may coincide with physically parallel electrode pairs. The physical electrode vectors may be viewed in some cases as the line the extends from one electrode of the vector to the other electrode of the vector to determine orientation of the transmitting and received vectors relative to one another. In some instances, however, physically parallel electrode pairs may not be electrically parallel depending on the electrical conduction properties of the intervening tissues. For example, a body tissue having relatively low electrical conductance, such as lung tissue, compared to other surrounding tissues, may require a physical electrode configuration that is not necessarily parallel in order to achieve an electrical configuration that is substantially parallel. 
     The TCC transmitting electrode vector may be selected to include electrodes that are not coupled to ICD sensing circuitry, e.g., a cardiac event detector configured to sense R-waves and/or P-waves from an electrical signal received by a sensing electrode vector. Use of an electrode for TCC signal transmission that is also coupled to a cardiac electrical event detector or other electrical signal sensing circuitry may increase interference with cardiac event detection or other electrical signal monitoring. The transmitting electrode pair may be selected to include at least one or both electrodes that are not coupled to the cardiac electrical event detector of ICD  14  so that TCC signals that are unintentionally received by the cardiac event detector are received via a transimpedance pathway from the transmitting electrode vector to the sensing electrode vector rather than directly through the sensing electrode impedance. 
     In other examples, however, the TCC transmitting electrode vector may include one or more electrodes coupled to a cardiac electrical event detector included in ICD  14 . A transmitting electrode vector may include electrodes coupled to the ICD sensing circuitry when the resulting transmitting electrode vector is optimal in other ways, e.g., low impedance and high transimpedance. Transmission of TCC signals using one or both electrodes included in a sensing electrode vector coupled to a cardiac event detector may be selected in a trade-off for optimizing other considerations in achieving reliable TCC signal transmission and reception. TCC signal transmission techniques disclosed herein may reduce or eliminate interference of the TCC signal transmission with cardiac event (or other electrophysiological signal) sensing as well as other sensing functions such as electrical impedance monitoring of a medical electrical lead or body tissue. 
     In one example, defibrillation electrode  24  may be selected in combination with housing  15  for transmitting TCC signals to pacemaker  100 . In other examples, TCC signals may be transmitted by ICD  14  using defibrillation electrode  26  and housing  15  or using two defibrillation electrodes  24  and  26 . The transmitting electrode vector impedance (delivered voltage divided by delivered current) may be up to hundreds of ohms. The transimpedance of the TCC pathway that includes a transmitting electrode vector including one defibrillation electrode  24  or  26  paired with housing  15  may be less than 10 ohms and even less than 1 ohm. A high transimpedance at the TCC signal transmission frequency is desired to produce a relatively high voltage on the receiving electrodes for a given injected current of the TCC signal. 
     The electrode pair selected for transmitting TCC signals may include one or both of pace/sense electrodes  28  and  30  in some examples. For example, the pace/sense electrode  28  or  30  may be paired with housing  15 , defibrillation electrode  24  or defibrillation electrode  26  for transmitting TCC signals. The impedance of the transmitting electrode vector may be increased due to the relatively smaller surface area of pace/sense electrodes  28  and  30 , which may have the effect of lowering the injected current during TCC signal transmission and thereby lowering the received voltage signal at the receiving electrode vector. 
     ICD  14  may be configured to select a TCC transmitting electrode vector from among multiple possible vectors using electrodes  24 ,  26 ,  28 ,  30  and housing  15  to achieve the best TCC signal strength at the receiving electrodes of pacemaker  100  and/or minimize TCC signal interference with cardiac event detection, impedance monitoring, or other functions performed by the ICD sensing circuit and/or by a sensing circuit of pacemaker  100 . In some examples, multiple vectors may be used to transmit TCC signals to cover different angles in three-dimensional space to achieve at least one TCC transmitting electrode vector that is substantially electrically parallel to the receiving electrode vector. The electrical configuration of a single transmitting vector relative to the TCC receiving electrode vector may be time-varying due to heart motion when the receiving electrode vector is within or coupled to the patient&#39;s heart, as in the case of pacemaker  100 . 
     In the example shown, lead  16  extends subcutaneously or submuscularly over the ribcage  32  medially from the connector assembly  27  of ICD  14  toward a center of the torso of patient  12 , e.g., toward xiphoid process  20  of patient  12 . At a location near xiphoid process  20 , lead  16  bends or turns and extends superior subcutaneously or submuscularly over the ribcage and/or sternum or substernally under the ribcage and/or sternum  22 . Although illustrated in  FIG.  1    as being offset laterally from and extending substantially parallel to sternum  22 , the distal portion  25  of lead  16  may be implanted at other locations, such as over sternum  22 , offset to the right or left of sternum  22 , angled laterally from sternum  22  toward the left or the right, or the like. Alternatively, lead  16  may be placed along other subcutaneous, submuscular or substernal paths. The path of extra-cardiovascular lead  16  may depend on the location of ICD  14 , the arrangement and position of electrodes carried by the lead body  18 , and/or other factors. 
     Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body  18  of lead  16  from the lead connector at the proximal lead end  27  to electrodes  24 ,  26 ,  28 , and  30  located along the distal portion  25  of the lead body  18 . The elongated electrical conductors contained within the lead body  18  are each electrically coupled with respective defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 , which may be separate respective insulated conductors within the lead body  18 . The respective conductors electrically couple the electrodes  24 ,  26 ,  28 , and  30  to circuitry of ICD  14 , such as a signal generator for therapy delivery and TCC signal transmission and/or a sensing circuit for sensing cardiac electrical signals and/or receiving TCC signals, via connections in the connector assembly  17 , including associated electrical feedthroughs crossing housing  15 . 
     The electrical conductors transmit therapy from a therapy delivery circuit within ICD  14  to one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  and transmit sensed electrical signals from one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  to the sensing circuit within ICD  14 . The electrical conductors also transmit TCC signals from a TCC transmitter to electrodes selected for transmitting the TCC signals. In some examples, ICD  14  may receive TCC signals from pacemaker  100  in which case the TCC signals are conducted from a receiving pair of electrodes of ICD  14  to a TCC signal receiver enclosed by housing  15 . 
     The lead body  18  of lead  16  may be formed from a non-conductive material and shaped to form one or more lumens within which the one or more conductors extend. Lead body  18  may be a flexible lead body that conforms to an implant pathway. In other examples, lead body  18  may include one or more preformed curves. Various example configurations of extra-cardiovascular leads and electrodes and dimensions that may be implemented in conjunction with the TCC transmission techniques disclosed herein are described in pending U.S. Publication No. 2015/0306375 (Marshall, et al.) and pending U.S. Publication No. 2015/0306410 (Marshall, et al.), both of which are incorporated herein by reference in their entirety. 
     ICD  14  analyzes the cardiac electrical signals received from one or more sensing electrode vectors to monitor for abnormal rhythms, such as bradycardia, tachycardia or fibrillation. ICD  14  may analyze the heart rate and morphology of the cardiac electrical signals to monitor for tachyarrhythmia in accordance with any of a number of tachyarrhythmia detection techniques. ICD  14  generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia, e.g., ventricular tachycardia (VT) or ventricular fibrillation (VF), using a therapy delivery electrode vector which may be selected from any of the available electrodes  24 ,  26 ,  28   30  and/or housing  15 . ICD  14  may deliver ATP in response to VT detection, and in some cases may deliver ATP prior to a CV/DF shock or during high voltage capacitor charging in an attempt to avert the need for delivering a CV/DF shock. If ATP does not successfully terminate VT or when VF is detected, ICD  14  may deliver one or more CV/DF shocks via one or both of defibrillation electrodes  24  and  26  and/or housing  15 . ICD  14  may generate and deliver other types of electrical stimulation pulses such as post-shock pacing pulses or bradycardia pacing pulses using a pacing electrode vector that includes one or more of the electrodes  24 ,  26 ,  28 , and  30  and the housing  15  of ICD  14 . 
     ICD  14  is shown implanted subcutaneously on the left side of patient  12  along the ribcage  32 . ICD  14  may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient  12 . ICD  14  may, however, be implanted at other subcutaneous or submuscular locations in patient  12 . For example, ICD  14  may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead  16  may extend subcutaneously or submuscularly from ICD  14  toward the manubrium of sternum  22  and bend or turn and extend inferiorly from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD  14  may be placed abdominally. 
     Pacemaker  100  is shown as a leadless intracardiac pacemaker configured to receive TCC signals from ICD  14  via housing-based electrodes in the examples presented herein and may be configured to transmit TCC signals via housing-based electrodes to ICD  14 , examples of which are illustrated in  FIG.  3 A  and the associated description. Pacemaker  100  may be delivered transvenously and anchored by a fixation member at an intracardiac pacing and sensing site. For example, pacemaker  100  may be implanted in an atrial or ventricular chamber of the patient&#39;s heart. In further examples, pacemaker  100  may be attached to an external surface of heart  8  (e.g., in contact with the epicardium) such that pacemaker  100  is disposed outside of heart  8 . 
     Pacemaker  100  is configured to deliver cardiac pacing pulses via a pair of housing-based electrodes and may be configured to sense cardiac electrical signals for determining the need and timing of a delivered pacing pulse. For example, pacemaker  100  may deliver bradycardia pacing pulses, rate responsive pacing pulses, ATP, post-shock pacing pulses, CRT pacing pulses, and/or other pacing therapies. Pacemaker  100  may include a TCC receiver that receives and demodulates TCC signals transmitted from ICD  14  and received by pacemaker  100  via housing-based electrodes. Pacemaker  100  may include a TCC transmitter that transmits TCC signals to ICD  14  via the housing-based electrodes. Pacemaker  100  is described in greater detail below in conjunction with  FIG.  3   . An example intracardiac pacemaker that may be included in an IMD system employing TCC is described in U.S. Pat. No. 8,744,572 (Greenhut et al.) incorporated herein by reference in its entirety. 
     In some examples, pacemaker  100  may be implanted in the right atrium, the right ventricle or the left ventricle of heart  8  to sense electrical activity of heart  8  and deliver pacing therapy. In other examples, system  10  may include two or more intracardiac pacemakers  100  within different chambers of heart  8  (e.g., within the right atrium, the right ventricle, and/or left ventricle). ICD  14  may be configured to transmit TCC signals to one or more pacemakers implanted within the patient&#39;s heart  8  to coordinate electrical stimulation therapy delivery. For example, ICD  14  may transmit command signals to cause pacemaker  100  to deliver a cardiac pacing pulse, ATP therapy, or request confirmation of sensed cardiac electrical events or a tachyarrhythmia detection. 
     An external device  40  is shown in telemetric communication with ICD  14  by a wireless communication link  42  and pacemaker  100  via a wireless communication link  44 . External device  40  may include a processor, display, user interface, telemetry unit and other components for communicating with ICD  14  and/or pacemaker  100  for transmitting and receiving data via communication link  42  and  44 , respectively. Communication link  42  or  44  may be established between ICD  14  or pacemaker  14 , respectively, and external device  40  using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF or communication frequency bandwidth. In some examples, ICD  14  or pacemaker  100  may communicate with an external device  40  using TCC, e.g., using receiving surface electrodes coupled to external device  40  are placed externally on patient  12 . 
     External device  40  may be embodied as a programmer used in a hospital, clinic or physician&#39;s office to retrieve data from ICD  14  and to program operating parameters and algorithms in ICD  14  for controlling ICD functions. External device  40  may be used to program cardiac event sensing parameters (e.g., R-wave sensing parameters), cardiac rhythm detection parameters (e.g., VT and VF detection parameters) and therapy control parameters used by ICD  14 . Data stored or acquired by ICD  14 , including physiological signals or associated data derived therefrom, results of device diagnostics, and histories of detected rhythm episodes and delivered therapies, may be retrieved from ICD  14  by external device  40  following an interrogation command. External device  40  may alternatively be embodied as a home monitor or hand-held device, such as a smart phone, tablet or other hand-held device. 
     In some examples, pacemaker  100  is not capable of bidirectional communication with external device  40 . ICD  14  may operate as a control device and pacemaker  100  as a responder. Pacemaker  100  may receive TCC communication signals from ICD  14  that include operating control data and commands (which may be transmitted from external device  40  to ICD  14 ) so that RF telemetry circuitry need not be included in pacemaker  100 . Pacemaker  100  may transmit data, such as information related to delivered pacing therapy and/or acquired cardiac electrical signals on command from ICD  14  via TCC transmissions, and ICD  14  may transmit data received from pacemaker  100  to external device  40  via RF communication. Alternatively, pacemaker  100  may periodically transmit data to ICD  14 , which stores it until receiving a request from external device  40 . 
       FIG.  2    is a conceptual diagram of an IMD system  200  configured to communicate using TCC transmission techniques disclosed herein according to another example. The IMD system  200  of  FIG.  2    includes an ICD  214  coupled to a patient&#39;s heart  8  via transvenous electrical leads  204 ,  206 , and  208 . IMD system  200  may include a leadless pacemaker  100  and/or a leadless sensor  50 . Sensor  50  is shown as a leadless pressure sensor positioned in the pulmonary artery for monitoring pulmonary arterial pressure. Leadless pressure sensor  50 , also referred to herein as “pressure sensor”  50 , may be positioned at other intracardiac or arterial locations for monitoring blood pressure. In other examples, the IMD system  200  (or IMD system  10  of  FIG.  1   ) may include other wireless sensors performing sensing-only or monitoring-only functions configured to send and/or receive TCC signals to/from ICD  214  (or ICD  14  of  FIG.  1   ) and/or pacemaker  100 . Other wireless sensors may include, for example, an electrogram (EGM) monitor, electrocardiogram (ECG) monitor, an oxygen monitor, acoustical monitor, accelerometer, bioimpedance monitor, pH monitor, temperature monitor, insulin monitor, or other sensing device including one or any combination of sensors. 
     ICD  214  includes a connector block  212  that may be configured to receive the proximal ends of a right atrial (RA) lead  204 , a right ventricular (RV) lead  206  and a coronary sinus (CS) lead  208 , which are advanced transvenously for positioning electrodes for sensing and stimulation in three or all four heart chambers. RV lead  206  is positioned such that its distal end is in the right ventricle for sensing RV cardiac signals and delivering pacing or shocking pulses in the right ventricle. For these purposes, RV lead  206  is equipped with pacing and sensing electrodes shown as a tip electrode  228  and a ring electrode  230 . RV lead  206  is further shown to carry defibrillation electrodes  224  and  226 , which may be elongated coil electrodes used to deliver high voltage CV/DF pulses. Defibrillation electrode  224  may be referred to herein as the “RV defibrillation electrode” or “RV coil electrode” because it may be carried along RV lead  206  such that it is positioned substantially within the right ventricle when distal pacing and sensing electrodes  228  and  230  are positioned for pacing and sensing in the right ventricle. Defibrillation electrode  226  may be referred to herein as a “superior vena cava (SVC) defibrillation electrode” or “SVC coil electrode” because it may be carried along RV lead  206  such that it is positioned at least partially along the SVC when the distal end of RV lead  206  is advanced within the right ventricle. 
     Each of electrodes  224 ,  226 ,  228  and  230  are connected to a respective insulated conductor extending within the body of RV lead  206 . The proximal end of the insulated conductors are coupled to corresponding connectors carried by proximal lead connector  216 , e.g., a DF-4 connector, for providing electrical connection to ICD  214 . It is understood that although ICD  214  is illustrated in  FIG.  2    as a multi-chamber device coupled to RA lead  204  and CS lead  208  in addition to RV lead  206 , ICD  214  may be configured as a dual-chamber device coupled to only two transvenous leads or a single-chamber device coupled to only one transvenous lead. For example, ICD  214  may be a single-chamber device coupled to RV lead  206  and may be configured to perform the TCC techniques disclosed herein using electrodes  224 ,  226 ,  228 , and  230  and/or housing  215  in addition to receiving cardiac electrical signals from heart  8  and delivering electrical stimulation therapy to heart  8 . 
     RA lead  204  is positioned such that its distal end is in the vicinity of the right atrium and the superior vena cava. Lead  204  is equipped with pacing and sensing electrodes  220  and  222 , shown as a tip electrode  220  and a ring electrode  222  spaced proximally from tip electrode  220 . The electrodes  220  and  222  provide sensing and pacing in the right atrium and are each connected to a respective insulated conductor within the body of RA lead  206 . Each insulated conductor is coupled at its proximal end to a connector carried by proximal lead connector  210 . 
     CS lead  208  is advanced within the vasculature of the left side of the heart via the coronary sinus (CS) and a cardiac vein (CV). CS lead  208  is shown in  FIG.  2    as having one or more electrodes  232 ,  234  that may be used in delivering pacing and/or sensing cardiac electrical signals in the left chambers of the heart, i.e., the left ventricle and/or the left atrium. The one or more electrodes  232 ,  234  of CS lead  208  are coupled to respective insulated conductors within the body of CS lead  208 , which provide connection to the proximal lead connector  218 . 
     Any of electrodes  220 ,  222 ,  224 ,  226 ,  228 ,  230 ,  232 ,  234  may be selected by ICD  214  in a TCC electrode vector for transmitting and/or receiving TCC signals. In some examples, housing  215  is selected in a TCC transmitting electrode vector along with a lead-based defibrillation electrode, e.g., RV coil electrode  224  or SVC coil electrode  226 , to provide a low impedance and high transimpedance TCC transmitting electrode vector. In other examples, TCC transmission is performed using the RV coil electrode  224  and the SVC coil electrode  226 . In still other examples, an electrode  232  or  234  carried by the CS lead  208  may be selected in combination with housing  215 , RV coil electrode  224 , or SVC coil electrode  226 . It is recognized that numerous TCC transmitting electrode vectors may be available using the various electrodes carried by one or more of leads  204 ,  206  and  208  coupled to ICD  214 . In some examples, multiple vectors may be selected to promote transmission via a vector that is substantially parallel to the housing-based electrodes of pacemaker  100  or to receiving electrodes of leadless pressure sensor  50  for transmitting signals to the respective pacemaker  100  or pressure sensor  50 . 
     Housing  215  encloses internal circuitry generally corresponding to the various circuits and components described in conjunction with  FIG.  5    below, for sensing cardiac signals from heart  8 , detecting arrhythmias, controlling therapy delivery and performing TCC with pacemaker  100  and/or pressure sensor  50  using the techniques disclosed herein. It is recognized that these TCC transmission techniques may be practiced in conjunction with alternative lead and electrode configurations other than those depicted in the examples of  FIG.  1    and  FIG.  2   . 
     Pressure sensor  50  may be implanted in the pulmonary artery of the patient for monitoring the pulmonary arterial pressure as an indication of the hemodynamic status of the patient  12 . One example of pressure sensor  50  is described below in conjunction with  FIG.  4   . Pressure sensor  50  may be configured to receive pressure signals via a pressure sensor and receive TCC signals via a TCC receiver coupled to electrodes carried by pressure sensor  50 . 
     In the examples of  FIGS.  1  and  2   , two or more IMDs may be co-implanted in a patient and communicate via TCC to enable a system level of functionality such as sharing the detection of arrhythmias between devices, synchronized timing of anti-tachyarrhythmia shocks, ATP, and/or post-shock pacing, optimization of the resources (e.g., battery capacity or processing power) available to each device, or sharing or coordination of physiological signal acquisition. In some examples, communication between the ICD  14  or ICD  214  and pacemaker  100  may be used to initiate therapy and/or confirm that therapy should be delivered. Communication between ICD  14  or ICD  214  and pressure sensor  50  may be used to initiate pressure signal acquisition and/or retrieval of pressure signal data from pressure sensor  50 . One approach is for ICD  14  or ICD  214  to function as a control device and pacemaker  100  and/or sensor  50  to function as responders. For instance, a TCC signal from ICD  14  or  214  may cause pacemaker  100  to deliver a cardiac pacing pulse or therapy. 
     In another example, ICD  214  may transmit a TCC command signal to pressure sensor  50  for causing pressure sensor  50  to begin acquiring a pressure signal. Pressure sensor  50  may be configured to transmit pressure signal data via TCC to ICD  214  or to external device  40  (shown in  FIG.  1   ). ICD  214  may transmit a TCC command to pressure sensor  50  to cause pressure sensor  50  to transmit a pressure signal in real time, transmit a pressure signal previously acquired and stored by pressure sensor  50 , or transmit pressure data derived from a pressure signal received by pressure sensor  50 . In other examples, pressure sensor  50  may be configured to transmit pressure signal data via RF telemetry to ICD  214  and/or to an external device, such as device  40  shown in  FIG.  1    in response to a TCC command signal received from ICD  214 . 
     During TCC signal transmission, current is driven through the patient&#39;s body tissue between two or more electrodes of the transmitting IMD (e.g., ICD  14  or  214 ). The current spreads through the patient&#39;s body, e.g., through the thorax, producing a potential field. The receiving IMD (e.g., pacemaker  100  or sensor  50  or other implanted or external device) may detect the TCC signal by measuring the potential difference between two of its electrodes, e.g., two housing-based electrodes of pacemaker  100  or sensor  50 . Optimally, the receiving electrodes are parallel to the tissue conduction pathway of the injected current to maximize the potential difference developed on the receiving electrode vector. The current injected to transmit the TCC signal is of sufficient amplitude to produce a voltage potential that can be detected by an intended receiving IMD but should at the same time not capture excitable body tissue, e.g., causing unintended stimulation of nerve or muscle tissue, possibly leading to muscle contraction, pain or even cardiac capture. Any unintended stimulation of nerve or muscle tissue also likely increases noise received on the sensing electrodes of a device of system  10  or  200 . 
     In some cases, a co-implanted IMD may be an unintended receiver of the TCC signal. If a co-implanted IMD includes electrodes or is coupled to electrodes for receiving electrical signals, but is not the intended receiver of a TCC signal, a voltage potential may develop across the electrodes of the unintended receiver leading to interference with the normal signal detection functions of the unintended receiver. For example, in system  200 , ICD  214  and pressure sensor  50  may be configured to communicate using TCC. Pacemaker  100  may be co-implanted with ICD  214  and pressure sensor  50  but not configured to send or receive TCC signals. A TCC signal transmitted by ICD  214  to pressure sensor  50  may result in voltage developed across the housing-based electrodes of pacemaker  100 . Pacemaker  100  may be an unintended receiver of the transmitted TCC signal. The voltage developed across the housing-based electrodes of pacemaker  100  may interfere with a cardiac event detector included in pacemaker  100 . In other examples, a subcutaneous cardiac electrical signal monitor having housing-based electrodes for monitoring a subcutaneously-acquired electrocardiogram (ECG) signal, such as the REVEAL LINQ™ Insertable Cardiac Monitor (available from Medtronic, Inc., Minneapolis, Minn., USA) may be implanted in a patient having two other IMDs configured to communicate via TCC, such as ICD  214  and pressure sensor  50 . The cardiac electrical signal monitor may be an unintended receiver of TCC signals transmitted between ICD  214  and pressure sensor  50 . The methods disclosed herein for transmitting TCC signals may eliminate or minimize interference of TCC signals with electrical signal sensing circuitry of other IMDs or external devices in or on the patient, which may be intended or unintended receivers. 
     While particular IMD systems  10  and  200 , including an ICD  14  or ICD  214 , respectively, pacemaker  100  and/or pressure sensor  50  are shown in the illustrative examples of  FIGS.  1  and  2   , methodologies described herein for TCC transmission may be used with other IMD systems including other types and locations of IMDs as well as other lead and electrode arrangements. For example, an implantable cardiac monitor, such as the REVEAL LINQ™ Insertable Cardiac Monitor, may be utilized as a relay device for leadless pacemaker  100  and/or pressure sensor  50  by receiving data from those devices via TCC and transmitting that data to an external device  40  via RF communication, such as BLUETOOTH™ communication. Generally, this disclosure describes various techniques for transmitting TCC signals by an IMD that includes sensing circuitry for sensing a cardiac electrical signal. The TCC signal transmission techniques reduce the likelihood that a TCC signal is oversensed as a physiological event by the sensing circuitry of the transmitting device. The TCC transmission techniques may also reduce the likelihood of TCC signal oversensing by sensing circuitry included in another IMD co-implanted with the transmitting device. Another IMD co-implanted with the transmitting device may be the intended receiving device of the TCC signal transmission or another IMD that is not the targeted recipient and may not even be configured to receive and detect TCC communication signals. 
       FIG.  3 A  is a conceptual diagram of pacemaker  100  according to one example. As shown in  FIG.  3 A , pacemaker  100  may be a leadless pacemaker including a housing  150 , housing end cap  158 , distal electrode  160 , proximal electrode  152 , fixation member  162 , and a delivery tool interface member  154 . Housing  150 , sealed with end cap  158 , encloses and protects the various electrical components within pacemaker  100 . Pacemaker  100  is shown including two electrodes  152  and  160  but may include two or more electrodes for delivering cardiac electrical stimulation pulses (such as pacing pulses or ATP), sensing cardiac electrical signals for detecting cardiac electrical events, and for receiving and/or transmitting TCC signals. 
     Electrodes  152  and  160  are carried on the housing  150  and housing end cap  158 . In this manner, electrodes  152  and  160  may be considered housing-based electrodes. In other examples, one or more electrodes may be coupled to circuitry enclosed by housing  150  via an electrode extension extending away from housing  150 . In the example of  FIG.  3 A , electrode  160  is disposed on the exterior surface of end cap  158 . Electrode  160  may be a tip electrode positioned to contact cardiac tissue upon implantation and fixation at a pacing site by fixation member  162 . Electrode  152  may be a ring or cylindrical electrode disposed along the exterior surface of housing  150 . Both housing  150  and housing end cap  158  may be electrically insulating. In some examples, housing  150  is an electrically conductive material, e.g., a titanium alloy or other biocompatible metal or metal alloy. Portions of housing  150  may be coated with a non-conductive material, e.g., parylene, polyurethane, silicone or other biocompatible polymer, to insulate portions of housing  150  not functioning as electrode  152 . 
     Electrodes  160  and  152  may be used as a cathode and anode pair for cardiac pacing therapy and receiving and/or transmitting TCC signals. In addition, electrodes  152  and  160  may be used to detect intrinsic electrical signals from the patient&#39;s heart  8 . In other examples, pacemaker  100  may include three or more electrodes, where any two or more of the electrodes may be selected to form a vector for delivery of electrical stimulation therapy, detecting intrinsic cardiac electrical signals from the patient&#39;s heart  8 , transmitting TCC signals, and receiving TCC signals. In some examples in which pacemaker  100  includes three or more electrodes, two or more of the electrodes may be selected, e.g., via switches, to form a vector for TCC. Pacemaker  100  may use multiple vectors for TCC transmission or receiving, for example, to promote a substantially parallel electrical configuration with a TCC transmitting electrode vector of ICD  14  or ICD  214 , which may increase the transimpedance and increase the received voltage signal. 
     Fixation member  162  may include multiple tines of a shape memory material that retains a preformed curved shape as shown. During implantation, fixation member  162  may be flexed forward to pierce tissue and elastically flex back towards housing  150  to regain their pre-formed curved shape. In this manner, fixation member  162  may be embedded within cardiac tissue at the implant site. In other examples, fixation member  162  may include helical fixation tines, barbs, hooks or other fixation features. 
     Delivery tool interface member  154  may be provided for engaging with a delivery tool used to advance pacemaker  100  to an implant site. A delivery tool may be removably coupled to delivery tool interface member  154  for retrieving pacemaker  100  back into a delivery tool if removal or repositioning of pacemaker  100  is required. 
       FIG.  3 B  is a schematic diagram of circuitry that may be enclosed by pacemaker housing  150  according to one example. Pacemaker housing  150  may enclose a control circuit  170 , memory  172 , pulse generator  176 , sensing circuit  174 , and a power source  178 . Control circuit  170  may include a microprocessor and/or other control circuitry for controlling the functions attributed to pacemaker  100  herein, such as controlling pulse generator  176  to deliver signals via electrodes  152  and  160  and controlling sensing circuit  174  to detect signals from electrical signals received via electrodes  152  and  160 . Power source  178  may include one or more rechargeable or non-rechargeable batteries for providing power to control circuit  170 , memory  172 , pulse generator  176  and sensing circuit  174  as needed. Control circuit  170  may execute instructions stored in memory  172  and may control pulse generator  176  and sensing circuit  174  according to control parameters stored in memory  172 , such as various timing intervals, pacing pulse parameters and cardiac event sensing parameters. 
     Pulse generator  176  generates therapeutic pacing pulses delivered via electrodes  152  and  160  under the control of timing circuitry included in control circuit  170 . Pulse generator  176  may include charging circuitry, one or more charge storage devices such as one or more capacitors, and switching circuitry that couples the charge storage device(s) to an output capacitor coupled to electrodes  160  and  152  to discharge the charge storage devices via electrodes  160  and  152 . In some examples, pulse generator includes a TCC transmitter (standalone or as part of a transceiver), such as the transmitter described below in conjunction with  FIG.  6   , for generating TCC signals transmitted via electrodes  160  and  152 . Power source  178  provides power to the charging circuit of pulse generator  176  and the TCC transmitter when present. 
     Pacemaker  100  may be configured for sensing cardiac electrical signals, e.g., R-waves or P-waves, and include a cardiac event detector  173 . Intrinsic cardiac electrical events may be detected from an electrical signal produced by the heart and received via electrodes  152  and  160 . Cardiac event detector  173  may include filters, amplifiers, an analog-to-digital converter, rectifier, comparator, sense amplifier or other circuitry for detecting cardiac events from a cardiac electrical signal received via electrodes  152  and  160 . Under the control of control circuit  170 , cardiac event detector  173  may apply various blanking and/or refractory periods to circuitry included in event detector  173  and an auto-adjusting cardiac event detection threshold amplitude, e.g., an R-wave detection threshold amplitude or a P-wave detection threshold amplitude, to the electrical signal received via electrodes  152  and  160 . 
     Sensing circuit  174  may further include a TCC signal detector  175  for detecting a TCC signal from ICD  14  (or ICD  214 ). A voltage potential develops across electrodes  152  and  160  in response to current conducted via a tissue pathway during TCC signal transmission from ICD  14  or ICD  214 . The voltage signal may be received and demodulated by TCC signal detector  175  and decoded by control circuit  170 . TCC signal detector  175  may include amplifiers, filters, analog-to-digital converters, rectifiers, comparators, counters, a phase locked loop and/or other circuitry configured to detect a wakeup beacon signal from a transmitting device and detect and demodulate the modulated carrier signal transmitted in data packets including encoded data. For example, TCC signal detector  175  of pacemaker  100  (and other TCC signal detectors referred to herein) may include a pre-amplifier and a high-Q filter tuned to the carrier frequency of a carrier signal that is used to transmit beacon signals and data signals during a TCC transmission session. The filter may be followed by another amplifier and a demodulator that converts the received signals to a binary signal representing coded data. 
     The circuitry of TCC signal detector  175  may include circuitry shared with cardiac event detector  173  in some examples. The filters included in TCC signal detector  175  and cardiac event detector  173 , however, are expected to operate at different passbands, for example, for detecting different signal frequencies. The TCC signals may be transmitted with a carrier frequency in the range of 33 to 250 kHz, in the range of 60 to 200 kHz, or at 100 kHz as examples. Cardiac electrical signals generated by heart  8  are generally less than 100 Hz. The TCC signal transmission techniques disclosed herein may reduce or eliminate oversensing of a received TCC signal, e.g., transmitted from ICD  14  or ICD  214 , as a cardiac electrical event by cardiac event detector  173 . In examples that include a TCC transmitter in pacemaker  100 , the TCC signal transmission techniques disclosed herein may reduce or prevent oversensing of a TCC signal produced by the TCC transmitter and transmitted via electrodes  152  and  160  from being detected as a cardiac event by cardiac event detector  173 . In some instances, the TCC transmitter or transceiver may include circuitry shared with pulse generator  176 , such that the TCC signals are transmitted using the pacing circuitry of pacemaker  100  and/or transmitted as sub-threshold pacing pulses or pacing pulses that occur during the refractory period of the heart. 
     In other examples, pacemaker  100  may include fewer or more components than the circuits and components shown in  FIG.  3 B . For instance, pacemaker  100  may include other physiological sensors and/or an RF telemetry circuit for communication with external device  40  instead of or in addition to TCC signal detector  175  and a TCC transmitter (if included). 
       FIG.  4    illustrates a perspective view of leadless pressure sensor  50  according to one example. Leadless pressure sensor  50  may generally correspond to the IMD disclosed in U.S. Pat. Publication No. 2012/0323099 A1 (Mothilal, et al.), incorporated herein by reference in its entirety. As shown in  FIG.  4   , pressure sensor  50  includes an elongated housing  250  having a pressure sensitive diaphragm or window  252  that exposes a pressure sensitive element within housing  250  to the surrounding pressure. Electrodes  260  and  262  may be secured to opposite ends of housing  250  and may be electrically insulated from housing  250  to form an electrode pair for receiving TCC signals. Electrodes  260  and  262  may be coupled to a TCC signal detector (corresponding to the TCC signal detector  175  described above) enclosed by housing  250 . The TCC signal detector is configured to detect and demodulate TCC signals received from ICD  14  or ICD  214 . 
     Housing  250  may enclose a battery, a pressure sensing circuit, a TCC signal detector, control circuitry, and memory for storing pressure signal data. In some examples, the pressure sensing circuit includes an air gap capacitive element and associated circuitry, which may include temperature compensation circuitry, for producing a signal correlated to pressure along window  252 . The pressure sensing circuit and window  252  may correspond to a pressure sensor module as generally disclosed in U.S. Pat. No. 8,720,276 (Kuhn, et al.), incorporated herein by reference in its entirety. The pressure sensing circuit may include a micro electro-mechanical system (MEMS) device in some examples. A fixation member  270  extends from housing  250  and may include a self-expanding stent or one or more self-expanding loops  272  that stabilize the position of pressure sensor  50  along an artery, such as the pulmonary artery, by gently pressing against the interior walls of the artery. When deployed in an arterial location, pressure sensor  50  produces and stores pressure signals correlated to arterial blood pressure. 
     In some examples, pressure sensor  50  includes a TCC transmitter or transceiver, such as the transmitter shown in  FIG.  6   , for transmitting TCC signals to another medical device, such as ICD  14  or ICD  214 , pacemaker  100  or external device  40 . Pressure sensor  50  may transmit a pressure signal, data extracted from a pressure signal or other communication data in a TCC signal via electrodes  260  and  262 . For instance, pressure sensor  50  may include a TCC transmitter or transceiver for at least producing acknowledgment and/or confirmation signals transmitted back to a transmitting device, e.g., ICD  14  or ICD  214 , in response to receiving a TCC signal to confirm detection of a beacon signal and/or reception of transmitted data packets. 
       FIG.  5    is a schematic diagram of an ICD capable of transmitting TCC signals according to one example. For illustrative purposes, ICD  14  of  FIG.  1    is depicted in  FIG.  5    coupled to electrodes  24 ,  26 ,  28 , and  30 , with housing  15  represented schematically as an electrode. It is to be understood, however, that the circuitry and components shown in FIG.  5  may generally correspond to circuitry included in ICD  214  of  FIG.  2    and adapted accordingly for single, dual, or multi-chamber cardiac signal sensing and therapy delivery functions using electrodes carried by transvenous leads. For instance, in the example of the multi-chamber ICD  214  of  FIG.  2   , signal generator  84  may include multiple therapy delivery output channels and sensing circuit  86  may include multiple sensing channels each selectively coupled to respective electrodes of RA lead  204 , RV lead  206  and CS lead  208 , corresponding to each cardiac chamber, e.g., the right atrium, the right ventricle, and the left ventricle. 
     The ICD circuitry may include a control circuit  80 , memory  82 , signal generator  84 , sensing circuit  86 , and RF telemetry circuit  88 . A power source  89  provides power to the circuitry of the ICD, including each of the circuits  80 ,  82 ,  84 ,  86 , and  88  as needed. Power source  89  may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source  89  and each of the other circuits  80 ,  82 ,  84 ,  86  and  88  are to be understood from the general block diagram of  FIG.  5   , but are not shown for the sake of clarity. For example, power source  89  may be coupled to charging circuits included in signal generator  84  for charging capacitors or other charge storage devices included in therapy circuit  85  for producing electrical stimulation pulses such as CV/DF shock pulses and pacing pulses. Power source  89  is coupled to TCC transmitter  90  for providing power for generating TCC signals. Power source  89  provides power to processors and other components of control circuit  80 , memory  82 , amplifiers, analog-to-digital converters and other components of sensing circuit  86 , and a transceiver of RF telemetry circuit  88 , as examples. 
     Memory  82  may store computer-readable instructions that, when executed by a processor included in control circuit  80 , cause ICD  14  to perform various functions attributed to ICD  14  (e.g., detection of arrhythmias, communication with pacemaker  100  or pressure sensor  50 , and/or delivery of electrical stimulation therapy). Memory  82  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog media. 
     Control circuit  80  communicates with signal generator  84  and sensing circuit  86  for sensing cardiac electrical activity, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals. The functional blocks shown in  FIG.  5    represent functionality included in ICD  14  (or ICD  214 ) and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to ICD  14  herein. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern IMD system, given the disclosure herein, is within the abilities of one of skill in the art. 
     Sensing circuit  86  may be selectively coupled to electrodes  24 ,  26 ,  28 ,  30  and/or housing  15  in order to monitor electrical activity of the patient&#39;s heart  8 . Sensing module  86  may include switching circuitry for selecting which of electrodes  24 ,  26 ,  28 ,  30  and housing  15  are coupled to sense amplifiers or other cardiac event detection circuitry included in cardiac event detector  85 . Switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple sense amplifiers to selected electrodes. The cardiac event detector  85  within sensing circuit  86  may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), or other analog or digital components configured to detect cardiac electrical events from a cardiac electrical signal received from heart  8 . 
     In some examples, sensing circuit  86  includes multiple sensing channels for acquiring cardiac electrical signals from multiple sensing vectors selected from electrodes  24 ,  25 ,  28 ,  30  and housing  15 . Each sensing channel may be configured to amplify, filter, digitize and rectify the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for sensing cardiac events, e.g., P-waves attendant to atrial depolarizations and/or R-waves attendant to ventricular depolarizations. For example, each sensing channel in sensing circuit  86  may include an input or pre-filter and amplifier for receiving a cardiac electrical signal developed across a selected sensing electrode vector, an analog-to-digital converter, a post-amplifier and filter, and a rectifier to produce a filtered, digitized, rectified and amplified cardiac electrical signal that is passed to a cardiac event detector included in sensing circuit  86 . The cardiac event detector  85  may include a sense amplifier, comparator or other circuitry for comparing the rectified cardiac electrical signal to a cardiac event sensing threshold, such as an R-wave sensing threshold amplitude, which may be an auto-adjusting threshold. Sensing circuit  86  may produce a sensed cardiac event signal in response to a sensing threshold crossing. The sensed cardiac events, e.g., R-waves and/or P-waves, are used for detecting cardiac rhythms and determining a need for therapy by control circuit  80 . ICD  214  of  FIG.  2    may include a sensing circuit having a separate atrial sensing channel for sensing P-waves using atrial electrodes and a ventricular sensing channel for sensing R-waves using ventricular electrodes. 
     Control circuit  80  may include interval counters, which may be reset upon receipt of a cardiac sensed event signal from sensing circuit  86 . The value of the count present in an interval counter when reset by a sensed R-wave or P-wave may be used by control circuit  80  to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals, which are measurements that may be stored in memory  82 . Control circuit  80  may use the count in the interval counters to detect a tachyarrhythmia event, such as atrial fibrillation (AF), atrial tachycardia (AT), VF or VT. These intervals may also be used to detect the overall heart rate, ventricular contraction rate, and heart rate variability. 
     Signal generator  84  includes a therapy circuit  92  and a TCC transmitter  90 . The therapy circuit  92  is configured to generate cardiac electrical stimulation pulses, e.g., CV/DF shock pulses and cardiac pacing pulses for delivery to heart  8  via electrodes carried by lead  16  (and in some cases housing  15 ). Signal generator  84  may include one or more energy storage elements, such as one or more capacitors, configured to store the energy required for a therapeutic CV/DF shock or pacing pulse. In response to detecting a shockable tachyarrhythmia, control circuit  80  controls therapy circuit  83  to charge the energy storage element(s) to prepare for delivering a CV/DF shock. Therapy circuit  83  may include other circuitry, such as charging circuitry, which may include a transformer and/or a charge pump, to charge the energy storage element, and switches to couple the energy storage element to an output capacitor to discharge and deliver the CV/DF shock and change the polarity of the shock to provide a bi-phasic or multi-phasic shock. Therapy circuit  83  may include a variety of voltage level-shifting circuitry, switches, transistors, diodes, or other circuitry. Therapy circuit  83  may include switching circuitry for selecting a shock delivery vector and delivers the shock therapy to the patient&#39;s heart  8  via the shock delivery vector, e.g., two or more electrodes such as defibrillation electrode  24  or  26  and housing  15 . 
     In some examples, therapy circuit  83  may include both a low voltage therapy circuit for generating and delivering relatively low voltage therapy pulses, such as pacing pulses, and a high voltage therapy circuit for generating and delivering CV/DF shocks. Low voltage pacing pulses may be delivered via a pacing electrode vector selected from electrodes  24 ,  26 ,  28 ,  30  and housing  15 . Pacing pulses may be delivered when a pacing escape interval set by a pace timing circuit of control circuit  80  times out without a sensed cardiac event causing the escape interval to be reset. The pace timing circuit may set various escape intervals for timing pacing pulses, e.g., to provide bradycardia pacing or post-shock pacing, or in response to detecting a tachyarrhythmia by delivering ATP. In some examples, pacemaker  100  is provided for delivering at least some low voltage pacing therapies, e.g., when signaled to do so by a TCC signal transmitted from ICD  14 . A low voltage therapy circuit included in ICD  214  of  FIG.  2    may include multiple pacing channels, including an atrial pacing channel, a right ventricular pacing channel, and a left ventricular pacing channel, to provide single, dual or multi-chamber pacing in addition to the high voltage therapy circuit used for delivering CV/DF shocks. 
     In some examples, ICD  14  (or ICD  214 ) is configured to monitor the impedance of an electrode vector. For example, signal generator  84  may apply a current drive signal to a pair of electrodes coupled to ICD  14 . Sensing circuit  86  may detect the resulting voltage developed across the pair of electrodes. Impedance monitoring may be performed for detecting a lead or electrode issue and for selecting a therapy delivery electrode vector, a TCC transmitting electrode vector, or a sensing electrode vector based at least in part on the lead/electrode impedance. In other examples, ICD  14  or ICD  214  may be configured to monitor bioimpedance in a tissue volume, e.g., thoracic impedance or cardiac impedance, for monitoring a patient condition. 
     TCC transmitter  90  is configured to generate TCC signals for transmission from a transmitting electrode vector selected from the electrodes  24 ,  26 ,  28 ,  30  and housing  15  via a conductive tissue pathway. TCC transmitter  90  is configured to generate and transmit a TCC signal, e.g., to communicate with pacemaker  100 , sensor  50  or another IMD, or an external device  40 . In some examples, signal generator  84  includes switching circuitry for selectively coupling TCC transmitter  90  to a selected transmitting electrode vector, e.g., using any two or more of electrodes  24 ,  26 ,  28   30  and housing  15 , e.g., housing  15  and defibrillation electrode  24 , for transmission of a TCC signal. 
     The TCC signal may be transmitted having a carrier signal with a peak-to-peak amplitude and carrier frequency selected to avoid stimulation of excitable tissue of patient  12 . In some examples, the carrier frequency of the TCC signal may be 100 kilohertz (kHz) or higher. A TCC signal emitted or received, for example by electrode  24  and housing  15 , at a frequency of at least approximately 100 kHz may be less likely to stimulate nearby tissue, e.g., muscles or nerves, or cause pain than lower frequency waveforms. Consequently, a TCC signal having a frequency of at least approximately 100 kHz may have a higher amplitude than a lower frequency signal without causing extraneous nerve or muscle stimulation. A relatively higher amplitude signal may increase the likelihood that pacemaker  100 , pressure sensor  50  or another implanted or external device, may receive the TCC signal from ICD  14  (or ICD  214 ). The peak-to-peak amplitude of the TCC signal may be within a range from approximately 100 microamps to 10 milliamps (mA) or more, such as within a range from approximately 1 mA to approximately 10 mA. In some examples, the amplitude of the TCC signal may be approximately 3 mA. A TCC signal having a frequency of at least approximately 100 kHz and an amplitude no greater than approximately 10 mA may be unlikely to stimulate nearby tissue, e.g., muscles or nerves, or cause pain. For a transmitting electrode vector having an impedance of 200 ohms injecting a current signal having an amplitude of 10 mA peak-to-peak, the voltage signal at the transmitting electrode vector may be 2 Volts peak-to-peak. The voltage developed at the receiving electrode vector may be in the range of 0.1 to 100 millivolts peak-to-peak. 
     The modulation of the TCC signal may be, as examples, amplitude modulation (AM), frequency modulation (FM), or digital modulation (DM), such as frequency-shift keying (FSK) or phase-shift keying (PSK). In some examples, the modulation is FM toggling between approximately 150 kHz and approximately 200 kHz. In some examples, the TCC signal has a frequency of 150/200 kHz and is modulated using FSK modulation at 12.5 kbps. In the illustrative examples presented herein a TCC signal having a carrier frequency of 100 kHz is modulated to encode data using binary phase shift keying (BPSK). Balanced pulses of opposite polarity may be used to shift the phase of the TCC signal, e.g., by 180 degrees positively or negatively, and balance the charge injected into the body tissue during the phase shift to minimize the likelihood of interfering with cardiac event sensing operations of the cardiac event detector  85 . Techniques for BPSK modulation of the TCC carrier signal using charge balanced phase shifts are disclosed in U.S. patent application Ser. No. 16/202,418 (Roberts, et al.) incorporated herein by reference in its entirety. The data modulated on TCC signals, e.g., being sent to pacemaker  100  or pressure sensor  50 , may include “wakeup” commands, commands to deliver a therapy, and/or commands to collect or send physiological signal data, as examples. 
     The configuration of signal generator  84  including TCC transmitter  90  illustrated in  FIG.  5    may provide “one-way” or uni-directional TCC. Such a configuration may be used if, for example, the ICD  14  is configured as a control device to transmit a command or request to another IMD configured as a responder, e.g., to pacemaker  100  or sensor  50 , to provide commands for pacing delivery or pressure signal acquisition, for instance. In some examples, sensing circuit  86  may include a TCC receiver  87  to facilitate “two-way” TCC between the ICD and another IMD. ICD  14  or ICD  214  may be configured to receive confirmation signals from the intended receiving, slave device to confirm that a transmitted TCC signal was successfully received. In other examples, ICD  14  or ICD  214  may receive commands via TCC receiver  87  from another IMD or external device. The TCC receiver  87  may have more sensitivity than an RF telemetry circuit  88 , e.g., to compensate for lower signal-to-noise ratio signals from a transmitting device such as pacemaker  100  or sensor  50 . For instance, pacemaker  100  may generate relatively low signal-to-noise ratio signals by generating relatively small amplitude signals due to its smaller power source, and/or to avoid stimulation of adjacent cardiac tissue. A modulated or unmodulated carrier signal may be received by TCC receiver  87  via electrodes selectively coupled to sensing circuit  86 . TCC receiver  87  may include an amplifier, filter and demodulator to pass the demodulated signal, e.g., as a stream of digital values, to control circuit  80  for decoding of the received signal and further processing as needed. 
     In other examples, TCC receiver  87  and/or TCC transmitter  90  may be distinct components separate from sensing circuit  86  and signal generator  84 , respectively. For example, ICD  14  may include a TCC transceiver that incorporates the circuitry of TCC receiver  87  and/or TCC transmitter  90 . In this case, the functionality described with respect to TCC receiver  87  and/or TCC transmitter  90  may be performed via a distinct TCC component instead of being part of sensing circuit  86  and signal generator  84 . 
     Memory  82  may be configured to store a variety of operational parameters, therapy parameters, sensed and detected data, and any other information related to the monitoring, therapy and treatment of patient  12 . Memory  82  may store, for example, thresholds and parameters indicative of tachyarrhythmias and/or therapy parameter values that at least partially define delivered anti-tachyarrhythmia shocks and pacing pulses. In some examples, memory  82  may also store communications transmitted to and/or received from pacemaker  100 , pressure sensor  50  or another device. 
     ICD  14  may have an RF telemetry circuit  88  including an antenna and transceiver for RF telemetry communication with external device  40 . RF telemetry circuit  88  may include an oscillator and/or other circuitry configured to generate a carrier signal at the desired frequency. RF telemetry circuit  88  further includes circuitry configured to modulate data, e.g., stored physiological and/or therapy delivery data, on the carrier signal. The modulation of RF telemetry signals may be, as examples, AM, FM, or DM, such as FSK or PSK. 
     In some examples, RF telemetry circuit  88  is configured to modulate the TCC signal for transmission by TCC transmitter  90 . Although RF telemetry circuit  88  may be configured to modulate and/or demodulate both RF telemetry signals and TCC signals within the same frequency band, e.g., within a range from approximately 150 kHz to approximately 200 kHz, the modulation techniques for the two signals may be different. In other examples, TCC transmitter  90  includes a modulator for modulating the TCC signal and/or TCC receiver  87  includes a demodulator for modulating the TCC signal rather than RF telemetry circuit  88 . 
       FIG.  6    is a conceptual diagram of TCC transmitter  90  (or transmitter portion of a transceiver) according to one example. TCC transmitter  90  may include a controller  91 , drive signal circuit  92 , polarity switching circuit  94 , alternating current (AC) coupling capacitor  96 , protection circuit  97  and voltage holding circuit  98 . In other examples, TCC transmitter  90  may include fewer or more components than the circuits and components shown in  FIG.  6   . ICD power source  89  is shown coupled to TCC transmitter  90  to provide power necessary to generate TCC signals. While the controller  91 , drive signal circuit  92 , polarity switching circuit  94 , AC coupling capacitor  96 , protection circuit  97  and voltage holding circuit  98  are shown as discrete circuits by the blocks in  FIG.  6   , it is recognized that these circuits may include common components or a common circuit may perform the functions attributed to the separate circuit blocks shown in  FIG.  6   . For example, generating a carrier current signal having a carrier frequency and a peak-to-peak amplitude may be performed by drive signal circuit  92  and polarity switching circuit  94  under the control of controller  91 . 
     Controller  91  may include a processor, logic circuitry, data registers, a clock circuit and/or other circuitry or structures for providing the functionality attributed to controller  91  herein. Controller  91  may include a dedicated clock circuit  93  for generating clock signals used to control the frequency of the transmitted TCC signals. In other examples, controller  91  may be implemented within control circuit  80 . The clock circuit  93  may be configured to provide a clock signal that may be used to transmit the TCC signal during a transmission session using more than one frequency. For example, TCC transmitter  90  may be configured to provide a clock signal that may be used to transmit the TCC signal using at least three different frequencies, the TCC signal being modulated using FSK during a wakeup mode (e.g., modulating the signal using two different frequencies) and switch to a data transmission mode that includes transmitting data packets using a carrier signal at a third frequency (e.g., modulated using BPSK or other modulation technique). For example, during a wakeup mode a beacon signal may be transmitted using high and low alternating frequencies, which may be centered on the frequency of the carrier signal. The beacon signal may indicate the proximity or location of IMD  14  and/or its readiness to communicate. The beacon signal may be followed by a request to establish a communication, sometimes referred to as an “OPEN” request or command, transmitted at the carrier frequency. A clock signal generated by clock circuit  93  may be required to enable generation of at least three different frequencies of the TCC signal produced by drive signal circuit  92  and polarity switching circuit  94  and passed to AC coupling capacitor  96  in this particular example. 
     After switching from the wakeup mode to the data transmission mode, e.g., after receiving an acknowledgement signal from the other device, the TCC transmitter  90  may be configured to transmit subsequent TCC signals at the carrier frequency, different than the distinct high and low frequencies used during the beacon signal transmission. The carrier signal is modulated using BPSK in one example such that the TCC signals are transmitted using a single frequency during the data transmission mode. 
     The clock circuit  93  may operate at one clock frequency during the wakeup mode and at another clock frequency during the data transmission mode. For example, clock circuit  93  may be controlled to operate at the lowest possible clock frequency that can be used to generate the high frequency and low frequency cycles of the beacon signal during the wakeup mode to conserve power provided by power source  89 . The clock circuit  91  may be configured to operate at a higher frequency for controlling drive signal circuit  92  and polarity switching circuit  94  to generate the carrier signal during signal transmission. The clock circuit frequency may be changed between the wakeup and transmission modes under the control of controller  91  using digital trim codes stored in hardware registers. 
     TCC transmitter  90  is shown coupled to a transmitting electrode vector  99  including defibrillation electrode  24  and housing  15  (of  FIG.  1   ) in this example. It is to be understood that TCC transmitter  90  may be coupled to one or more TCC transmitting electrode vectors selected from any of the available electrodes coupled to the transmitting device as described above via switching circuitry included in signal generator  84 . Controller  91  may be configured to switchably connect a transmitting electrode vector  99  to TCC transmitter  90  for transmission of TCC signals, e.g., by controlling switches included in signal generator  84 , which may be included in TCC transmitter  90  between AC coupling capacitor  96  and transmitting electrode vector  99 , e.g., in protection circuit  97 . Controller  91  may select a transmitting electrode vector from among multiple electrodes coupled to the transmitting device, which may include electrodes carried by the housing of the transmitting device, a transvenous lead, e.g., any of leads  204 ,  206  or  208  shown in  FIG.  2   , or a non-transvenous lead, e.g., extra-cardiovascular lead  16  shown in  FIG.  1   . 
     Drive signal circuit  92  may include a voltage source and/or a current source powered by power source  89 . In one example, drive signal circuit  92  may be an active drive signal circuit generating a balanced, bi-directional drive current signal to balance the return current with the drive current for a net zero DC current injected into the body tissue via transmitting electrode vector  99 . In another example, the drive signal circuit  92  may include a charge pump and a holding capacitor that is charged by the charge pump to generate a current signal that is coupled to the transmitting electrode vector  99 . In yet another example, drive signal circuit  92  may include a current source that is used to charge a holding capacitor included in drive signal circuit  92 . 
     The drive signal generated by drive signal circuit  92  may be a voltage signal in some examples. In the illustrative examples presented herein, the drive signal circuit  92  generates a current signal to deliver TCC signal current through the transmitting electrode vector  99  having a desired peak-to-peak amplitude, e.g., high enough to produce a voltage signal on receiving electrodes of a receiving device that is detectable by the receiving device, which may be pacemaker  100 , sensor  50  or another intended receiving medical device, implanted or external. The peak-to-peak current amplitude is low enough to avoid or minimize the likelihood of stimulation of tissue. A carrier signal that may be generated by drive signal circuit  92  and polarity switching circuit  94  may have a peak-to-peak amplitude in a range from approximately 1 mA to approximately 10 mA, such as approximately 3 mA peak-to-peak, as discussed above. The voltage developed at the receiving electrode vector may be in the range of 0.1 to 100 millivolts peak-to-peak. 
     Polarity switching circuit  94  receives the drive signal from drive signal circuit  92  and includes circuitry configured to switch the polarity of the drive signal current at a carrier frequency of the TCC signal. For example, polarity switching circuit  94  may include transistors and/or switches configured to switch the polarity of the drive current signal at the frequency of the TCC signal. In some examples, polarity switching circuit includes a respective one or more transistors and/or switches coupled to each of electrode  24  and housing  15 , and the on-off states of the respective transistor(s) and/or switch(es) are alternated to switch the polarity of the TCC signal current between the electrodes at the carrier frequency. As discussed above, the carrier frequency may be approximately 100 kHz. For example, the carrier frequency may be within a range from approximately 33 kHz to approximately 250 kHz. 
     In some examples, RF telemetry module  86  may include a mixed signal integrated circuit or other circuitry configured to provide a digital version of the modulated TCC signal to controller  91 . In other examples, controller  91  is configured to produce the digital input signal for modulating the TCC carrier signal to encode communication data in the transmitted signal. Controller  91  controls one or both of drive signal circuit  92  and polarity switching circuit  94  to modulate the TCC carrier frequency signal to generate the modulated TCC signal with an amplitude, phase shifts and/or frequency according to the encoding. For example, controller  91  may control polarity switching circuit  94  to toggle the frequency of the carrier signal according to FSK modulation to encode the communication data. In another example, controller  91  may control polarity switching circuit  94  to switch the polarity of the current signal after a desired portion of the carrier frequency cycle length to shift the phase of the AC current signal by 180 degrees according to BPSK modulation. 
     Polarity switching circuit  94  is capacitively coupled to the transmitting electrode vector  99  (e.g., electrode  24  and housing  15  in the example shown) via AC coupling capacitor  96 . AC coupling capacitor  96  couples the current signal output from polarity switching circuit  94  to the transmitting electrode vector  99  to inject the current into the conductive body tissue pathway. AC coupling capacitor  96  may include one or more capacitors coupled in series with one or each of the electrodes included in electrode vector  99 . The AC coupling capacitor  96  is charged to a DC operating voltage at the beginning of a TCC signal. AC coupling capacitor  96  is selected to have a minimum capacitance that is based on the frequency and the peak-to-peak current amplitude of the carrier signal being used to transmit beacon and data signals. As examples, AC coupling capacitor  96  may have a capacitance of at least one nanofarad and up to ten microfarads for coupling a carrier signal having a frequency between 25 kHz and 250 kHz and peak-to-peak current amplitude of 100 microamps to 10 milliamps. Larger capacitances may be used but may increase the time required to charge the AC coupling capacitor to a DC operating voltage. 
     During a “cold start,” e.g., at the beginning of a TCC transmission session when AC coupling capacitor  96  is uncharged, the charging of AC coupling capacitor  96  to the DC operating voltage may result in a low frequency current being injected into the body through the transmitting electrode vector. This low frequency current is more likely to interfere with the operation of cardiac event detector  85  or other electrophysiological signal sensing circuits included in co-implanted IMDs or external devices coupled to the patient. Cardiac event detector  85  and other electrophysiological signal sensing circuits of intended or unintended receiving devices may operate in a low frequency band, e.g., 1 to 100 Hz. As such, low frequency artifact at the start of TCC signal transmission, during charging of the AC coupling capacitor  96 , may interfere with cardiac event detector  85 . After the DC operating voltage is established on AC coupling capacitor  96 , the high frequency carrier signal, e.g., 100 kHz, is typically above the operating bandwidth of cardiac event detector  85  and other electrophysiological sensing circuitry of an IMD system and unlikely to cause interference or false event detection. 
     TCC transmitter  90  may include a voltage holding circuit  98  coupled to AC coupling capacitor  96 . Voltage holding circuit  98  is configured to hold the AC coupling capacitor  96  at the DC operating voltage between transmitted TCC signals during a TCC transmission session and/or between TCC transmission sessions. By holding the AC coupling capacitor at a DC voltage during time intervals between TCC signal transmissions, interference with sensing circuitry that may otherwise occur due to the low frequency artifact injected during charging of the AC coupling capacitor  96  to the DC operating voltage is minimized or avoided. 
     Examples of circuitry included in voltage holding circuit  98  are described in U.S. Patent Application No. 62/591,806 (Peichel, et al.), incorporated herein by reference in its entirety. In some examples, voltage holding circuit  98  may include circuitry for floating AC coupling capacitor  96  at the DC voltage between TCC signal transmissions. In other examples, voltage holding circuit  98  may include circuitry to actively hold the AC coupling capacitor  96  at a DC voltage between TCC signal transmissions. A variety of circuitry may be conceived for preventing or minimizing discharging of AC coupling capacitor  96  between TCC signal transmissions. In this way, at the start of transmitting the next TCC signal the AC coupling capacitor  96  is already at or near the DC operating voltage. Without having to re-establish the DC voltage on the AC coupling capacitor  96 , low frequency artifact injected into the TCC tissue pathway at the onset of the next TCC signal transmission is avoided or minimized. It is recognized that leakage currents may still exist within TCC transmitter  90  and may cause some discharge of AC coupling capacitor  96  between signal transmissions. Voltage holding circuit  98  may be used to minimize any discharge of AC coupling capacitor  96  between transmitted TCC signals to minimize low frequency interference with sensing circuit  86  ( FIG.  5   ) of the transmitting device as well as sensing circuits of other co-implanted IMDs and/or external device coupled to the patient. 
     The TCC transmitter  90  may include protection circuit  97  that allows the delivery of the TCC signal via electrodes coupled to other ICD circuitry but protects the TCC transmitter  90  and other circuitry of the ICD  14  from voltages that may develop across the electrodes, e.g., during a CV/DF shock delivered by therapy circuit  83  or an external defibrillator as well as high voltages that may develop across the TCC transmitting electrode vector during other situations such as an electrocautery procedure or magnetic resonance imaging. The circuitry within housing  15  of ICD  14  protected by protection circuit  97  may include circuitry of any of the components of ICD  14  illustrated in  FIG.  5   , such as control circuit  80 , memory  82 , sensing circuit  86 , signal generator  84 , and RF telemetry circuit  88 . 
     Protection circuit  97  may be coupled between drive signal circuit  92  and the transmitting electrode vector  99 , e.g., between AC coupling capacitor  96  and electrode  24  and housing  15  as shown. In some examples, protection circuit  97  may include circuitry before and/or after AC coupling capacitor  96 . Protection circuit  97  may include, as examples, capacitors, inductors, switches, resistors, and/or diodes. Examples of TCC signal generation and protection circuitry that may be utilized in conjunction with the signal transmission techniques disclosed herein are generally described in U.S. Pat. No. 9,636,511 (Camey, et al.), incorporated herein by reference in its entirety. 
     In some examples, TCC transmitter  90  may be controlled by control circuit  80  to transmit data via TCC multiple times throughout a cardiac cycle. In some cases, multiple transmissions at different times during the cardiac cycle increase the likelihood that the data is sent during both systole and diastole to make use of cardiac motion to increase the chance that the intended receiving electrode vector, such as housing-based electrodes of pacemaker  100  or pressure sensor  50 , is orientated in a non-orthogonal position relative to the transmitting electrode vector. Multiple transmissions at different times during the cardiac cycle may thereby increase the likelihood that that the packet is received. While TCC transmitter  90  is shown coupled to a transmitting electrode bipole (vector  99 ) in  FIG.  6   , it is to be understood that multiple transmitting electrode vectors may be coupled to TCC transmitter  90  for transmitting a TCC current signal along multiple conductive tissue pathways for reception by multiple receiving electrode vectors or to increase the likelihood of being received by a single receiving electrode vector. 
       FIG.  7    is a conceptual diagram of a transmission session  300  that may be executed by transmitter  90  under the control of control circuit  80 . The challenges of transmitting encoded information in a TCC signal include avoiding unintentional electrical stimulation of nerve and muscle tissue, including myocardial tissue, and avoiding or minimizing interference with sensing circuitry included in one or more devices of the IMD system performing TCC while still successfully transmitting information in a time efficient and power efficient manner. Techniques disclosed herein include a method for transmitting TCC signals including at least one ramped carrier signal to minimize low frequency artifact at the beginning of a transmission session followed by transmitting a wakeup signal (e.g., beacon signal) and encoded data packets having a frequency and amplitude that reduces the likelihood of stimulating excitable tissue. 
     Transmission session  300  may include a wakeup mode  310  followed by data transmission mode  311  that may include transmission of one or more data packets  330 . In other instances, transmission session  300  does not include a wakeup mode. In the illustrative examples described herein, a group of bits of encoded data is referred to as a data “packet.” In some uses, the term “packet” may imply that transmitted data is guaranteed to be received along a communication pathway without error and a confirmation signal indicating receipt without error may be returned from the intended receiving device. In some applications, a group of bits of encoded data may be referred to as a “datagram” when transmission of the encoded data occurs without guarantee that the data reaches the intended receiver and without certainty that transmission errors did not occur. Groups of bits of encoded data  330  are referred to as “packets” herein, however, it is recognized that in some clinical applications the groups of bits  330  may be transmitted as datagrams, without guarantee that the receiving device actually received the data error-free. 
     Each transmission session  300  may, in some instances, begin with a wakeup mode  310 , as further described in conjunction with  FIG.  8   , followed by at least one data packet  330 . Multiple data packets  330  may be transmitted and assembled into a stream of data by the receiving device. In examples that include bi-directional communication, the transmitting device may toggle between data transmission, during which one data packet  330  is transmitted, and a receiving window  350  between data packets, during which the transmitting device waits for a response from the intended receiver, e.g., a signal confirming receipt of the transmitted packet, requested data sent back to the transmitter or other requested response to the received data packet. Examples of the structure of each data packet  330  are described below, e.g., in conjunction with  FIG.  11   . 
       FIG.  8    is a diagram of one example of operations performed during the wakeup mode  310  by an IMD system, e.g., system  10  of  FIG.  1    or system  200  of  FIG.  2   , according to one example. Functions performed by the transmitting device (TRN) are represented above the dashed line. Functions performed by the receiving device (RCV) are performed below the dashed line. In the example of ICD  14  (or ICD  214 ) being the transmitting device, control circuit  80  controls TCC transmitter  90  to transmit an alternating current TCC ramp on signal  366  during wakeup mode  310 , prior to the first beacon signal  312 . Beacon signal  312  may indicate to another device the proximity or location of IMD  14  and/or its readiness to enter into a communication session. 
     At the start of a transmission session, the early cycles of the carrier signal establish a DC voltage across the AC coupling capacitor  96 . During this time, a low frequency current may be injected into the body tissue conductive pathway via the TCC transmitting electrode vector. The low frequency current is more likely to cause interference with cardiac event detector  85  of sensing circuit  86  (or other electrical signal sensing circuits of other implanted devices) than the relatively high carrier frequency of the TCC signal. By starting each transmission session with a ramp on signal  366 , the gradual charging of the AC coupling capacitor  96  to the DC operating voltage is controlled in a manner that minimizes potential interference with cardiac event detector  85  and/or other electrophysiological sensing circuits co-implanted IMDs. 
     The TCC ramp on signal  366  may be transmitted as an unmodulated carrier signal and may be up to 200 ms long in some examples. The TCC ramp on signal  366  may be digitally controlled to step up the peak-to-peak amplitude of the AC carrier signal in a manner that minimizes any low frequency current that may be received at a sensing electrode vector to avoid interfering with electrical signal sensing. As described below in conjunction with  FIG.  9   , the TCC ramp on signal  366  may be stepped up in amplitude according to a step increment and step up interval that results in step changes in the voltage potential developed at a sensing electrode vector that are below the sensitivity of the electrical signal sensing circuit  86 . The duration of the ramp on signal  366  may be selected based on the time required to reach the peak-to-peak amplitude of the carrier signal used to transmit the beacon signal  312  given a selected step increment and the step up interval. The maximum size of the step increment and minimum step up interval that can be used without causing interference with electrophysiological signal sensing circuitry of the IMD system may depend on the programmed sensitivity of electrophysiological sensing circuitry, e.g., cardiac event detector  85 , the proximity of the transmitting electrode vector and the sensing electrode vector and associated transimpedance and other factors that influence the size of the DC voltage shift at a sensing electrode vector. 
     If the transmitting device includes a sensing circuit, such as sensing circuit  86 , the ramp on signal  366  may optionally be started during a blanking period  304  applied to the sensing circuit  86  following a cardiac event  302 . By starting ramp on signal  366  during a blanking period  304  applied to the sensing circuit  86  of the transmitting device, the DC voltage is established on the AC coupling capacitor  96  mostly or entirely during the blanking period  304  when the cardiac event detector  85  is blanked and relatively immune to the low frequency artifact. 
     The blanking period  304  may be an automatic blanking period that the control circuit  80  applies to the cardiac event detector  85  following an intrinsic or paced cardiac event  302 . Cardiac event  302  may be an intrinsic cardiac event sensed by the cardiac event detector  85 , and blanking period  304  may be a post-sense blanking period set in response to detecting the intrinsic cardiac event, e.g., an R-wave or P-wave. For example, a post-sense blanking period may be applied to a sense amplifier or other cardiac event detection circuitry of sensing circuit  86  in response to a cardiac event sensing threshold crossing. At other times, cardiac events  302  may be pacing pulses delivered at a pacing interval  306 , in which case blanking period  304  is a post-pace blanking period automatically applied to the sensing circuit  86  upon delivery of the pacing pulse by therapy circuit  83 . A post-pace or post-shock blanking period may be applied to prevent saturation of the sense amplifier(s) of sensing circuit  86  during delivery of a pacing pulse or cardioversion/defibrillation shock. An automatic post-sense or post-pace blanking period may be in the range of 50 to 200 ms, for example 150 ms. 
     Control circuit  80  may alternatively apply a communication blanking period to cardiac event detector  85  that is independent of the timing of cardiac electrical events, sensed or paced. In some cases, a communication blanking period may be applied during the cardiac cycle between sensed or paced events. The communication blanking period may be applied by control circuit  80  to the cardiac event detector  85  to enable TCC signal transmission to be initiated with ramp on signal  366  at any time during the cardiac cycle, without waiting for an automatic post-sense or post-pace blanking period. 
     A communication blanking period may be shorter or longer than the automatic post-sense or post-pace blanking period. For example, a communication blanking period may be in the range of 10 ms to 200 ms and may depend on the programmed sensitivity of the cardiac event detector  85 . The communication blanking period may be applied for only a starting portion of ramp on signal  366 , e.g., the first one or more step up increments. The maximum duration of the communication blanking period may be limited based on the particular clinical application. For example, in the cardiac monitoring and therapy delivery IMD systems  10  and  200  disclosed herein, the maximum time that cardiac event detector  85  is blinded to detecting cardiac events may be 200 ms or less. In non-cardiac applications, e.g., monitoring muscle or nerve signals, longer or shorter communication blanking intervals may be applied. 
     Starting ramp on signal  366  during a blanking period  304  may allow ramping up of the carrier signal peak-to-peak amplitude more quickly in that any low frequency artifact is not detected as a cardiac event by cardiac event detector  85  during a blanking period  304 . Furthermore, during a post-sense, post-pace or post-shock blanking period, myocardial tissue is in a state of physiological refractoriness such that any low frequency signal injected at the beginning of a TCC signal started during a blanking period  304  is highly unlikely to capture the myocardial tissue. 
     In the example of the receiving device being pacemaker  100  having sensing circuit  174 , control circuit  170  may apply a post-sense or post-pace blanking period in response to detecting an intrinsic cardiac event or delivering a pacing pulse. The blanking period applied to sensing circuit  174  by control circuit  170  is applied to cardiac event detector  173  to prevent oversensing of non-cardiac events during the blanking period. The blanking period is not applied to TCC signal detector  175 , which may be operating in a polling mode including beacon search periods  320  and enabled to detect a beacon signal, even during a blanking period applied to cardiac event detector  173 . Since both pacemaker  100  and ICD  14  (or ICD  214 ) may be configured to sense cardiac electrical signals from heart  8 , and may be configured to detect pacing pulses delivered by another co-implanted device, both cardiac event detectors  85  and  173  of the transmitting and receiving devices, respectively, may be in a blanking period at the same time or at least during overlapping time periods. As such, by starting transmission of at least the ramp on signal  366  of a new transmission session during a blanking period  304 , sensing circuitry of other co-implanted devices configured to detect cardiac electrical signals may also be in a blanking period, reducing the likelihood of low frequency interference with cardiac event detection by other sensing circuits during AC coupling capacitor charging. 
     Depending on the duration of the TCC ramp on signal  366  and the blanking period  304 , the first beacon signal  312  may be transmitted or at least started during the blanking period  304 . A DC operating voltage is at least partially established on the AC coupling capacitor  96  during the TCC ramp on signal  366 . In some examples, the AC coupling capacitor  96  may continue to be charged to the DC operating voltage during the first cycles of the beacon signal  312 . In other instances, the AC coupling capacitor  96  is fully charged to the DC operating voltage during ramp on signal  366 . While ramp on signal  366  is shown during a blanking period  304 , it is understood that ramp on signal  366  is provided to eliminate or minimize low frequency artifact injected into the TCC pathway during charging of the AC coupling capacitor  96  to the DC operation voltage. As such, the TCC transmission session beginning with ramp on signal  366  may be started independent of the timing of blanking periods  304 , with ramp on signal  366  occurring at least partially or entirely outside a blanking period  304 . Starting the TCC transmission session during a blanking period  304  is not required. By controlling the rate of charging the AC coupling capacitor  96  to the DC voltage during the ramp on signal  366 , low frequency artifact interference may be avoided. 
     One or more beacon signals  312  may be transmitted consecutively after the ramp on signal  366  to wake up the receiving device. In the example of pacemaker  100  being the receiving device, control circuit  170  may power up TCC signal detector  175  (shown in  FIG.  3 B ) periodically for a beacon search period  320  to detect the beacon signal  312 . The beacon signal  312  may be transmitted multiple times as needed until a response is received from the receiving device. In the example shown, the beacon signal  312  is sent four times, each time followed by a receiving period  314  for waiting for acknowledgement signal  328  transmitted from the receiving device to confirm detection of the beacon signal  312 . In response to receiving the acknowledgement signal  328  as indicated at arrow  316 , the transmitter  90  stops transmitting the beacon signal  312  and switches from the wakeup mode  310  to the transmission mode  311  as shown in  FIG.  7   . 
     A single ramp on signal  366  may be applied at the beginning of the wakeup mode  310 , prior to the first beacon signal  312 . Leakage current that may cause AC coupling capacitor  96  to discharge between beacon signals  312  may be minimal, particularly when the periods  314  between beacon signals  312  are relatively short. Discharge of the AC coupling capacitor  96  due to leakage current between TCC signals may be negligible for up to one minute or even up to two minutes or more in some examples. The receiving periods  314  between beacon signals may be less than 10 seconds or even less than 1 second, such that negligible discharge of the AC coupling capacitor  96  occurs between beacon signals  312 . In other examples, transmitter  90  may be configured to maintain the DC voltage established on the AC coupling capacitor  96  during the ramp on signal  366  between beacon signals  312 . For example, voltage holding circuit  98  may be controlled by controller  91  to float or actively hold AC coupling capacitor  96  at the DC voltage that was established during the ramp on signal  366  during the receiving period  314 . 
     After the ramp on signal  366  and the first beacon signal  312 , the AC coupling capacitor  96  is already at (or near) the DC operating voltage at the start of the next beacon signal  312  such that low frequency artifact during the early cycles of the next beacon signal is minimized or avoided. Transmission of any subsequent beacon signals  312  does not require any additional ramp on signals immediately preceding each additional beacon signal, and the timing of the additional beacon signals is not limited to the timing of cardiac events  302  and blanking periods  304 . 
     The beacon signal  312  may be shorter or longer than the ramp on signal  366 . In one example, the TCC ramp on signal  366  is up to 200 ms long and the beacon signal  312  is up to 120 ms long. Beacon signal  312  may include a single tone at the unmodulated carrier signal frequency, e.g., 100 kHz and may be transmitted for 100 ms, 200 ms, 500 ms, 1 second, 2 seconds, or even up to 8 seconds. In other examples the beacon signal  312  may vary between two or more tones within a range of the carrier signal. For instance, the beacon signal  312  may be an FSK signal modulated between two different frequencies to transmit beacon signal  312  having a pre-defined frequency signature that is detected by the TCC signal detector. 
     The TCC signal detector of the receiving device, e.g., TCC signal detector  175  included in pacemaker  100  or in pressure sensor  50 , is configured to detect the beacon signal frequency and compare the frequency to detection criteria. The TCC signal detector may include a comparator and counter configured to count pulses, e.g., by counting zero crossings, edges or other features of the voltage signal received at the receiving electrode vector, and comparing the count to a beacon detection threshold value. In other examples, the TCC signal detector of the receiving device may include a phase locked loop (PLL) that detects the frequency of the voltage signal at the receiving electrode vector. The frequency signal output of the PLL may be compared to the expected beacon signal frequency or frequency pattern. 
     As described below, during the TCC ramp on signal  366  the peak-to-peak amplitude of the carrier signal may be ramped up from a starting peak-to-peak amplitude to the maximum peak-to-peak amplitude of the beacon signal  312  using digitally controlled, charge balanced steps. By slowly ramping the peak-to-peak amplitude of the carrier signal, the rate of charging the AC coupling capacitor  96  to the DC operating voltage is controlled and any injected current signal during the ramp on signal  366  is at a frequency that is substantially attenuated by a filter included in cardiac event detector  85  and/or produces an amplitude shift that is less than the sensitivity of the cardiac event detector  85 . As used herein, the maximum peak-to-peak amplitude of a signal refers to the maximum peak-to-peak amplitude selected for transmitting the carrier signal of a beacon signal  312  or a data packet  330  and is not necessarily the maximum available peak-to-peak amplitude that the transmitter  90  is capable of generating. The maximum peak-to-peak amplitude of the carrier signal used to transmit a beacon signal  312  may be greater than the maximum peak-to-peak amplitude of the carrier signal during transmission of data packets  330 . The greater peak-to-peak amplitude of the beacon signal  312  may increase the likelihood of the beacon signal being detected by the receiving device. In other examples, the maximum peak-to-peak amplitude of the carrier signal transmitted as a beacon signal is the same as the maximum peak-to-peak amplitude of the modulated carrier signal during data packet transmission. 
     The receiving device controls the TCC signal detector, e.g., TCC detector  175 , to operate in a polling mode until a beacon signal  312  is detected. The polling mode includes beacon search periods  320  scheduled at polling interval  322 . Polling interval  322  may be a pre-determined time interval, e.g., from 0.5 seconds to 8 seconds. In other examples, the polling interval  322  may be variable, for example as generally disclosed in the above-incorporated U.S. Pat. Application No. 62/591,810 (Reinke, et al.). 
     The duration of the beacon search period  320  may be less than, equal to or greater than the beacon signal  312  in various examples. For instance, with no limitation intended, the beacon signal  312  may be approximately 8 ms to 150 ms long. The beacon search period  320  may be 0.4 to 4 ms long. In other examples, the beacon signal  312  may be up to one second long, up to four seconds long, or even up to eight seconds long. The beacon signal transmission may be suspended if a therapy such as a pacing pulse is scheduled for delivery, e.g., by therapy circuit  83 . Transmission of a suspended beacon signal may be resumed after delivery of the pacing pulse. The duration of the beacon search period  320  may be any portion of the duration of the beacon signal  312 . 
     In  FIG.  8   , the first, earliest beacon search period  320  only partially overlaps with the first beacon signal  312  of wakeup mode  310 . If the overlap of beacon search period  320  and beacon signal  312  is too short, beacon detection criteria applied by the receiving device may not be met, and beacon signal  312  may go undetected. The first beacon search period  320  is shown overlapping in time with the TCC ramp on signal  366 . The TCC ramp on signal  366  may be undetected by the receiving device. The initial, low peak-to-peak amplitude of ramp on signal  366  may be too low to be detected by the TCC signal detector. The ramp on signal  366  may not meet the beacon signal detection criteria applied by the receiving device. For example, the controller  91  may control the drive signal circuit  92  and polarity switching circuit  94  to produce the TCC ramp on signal  366  at the carrier signal frequency without modulation. The beacon signal  312  may be a modulated signal, e.g., using FSK or PSK modulation of the carrier signal. The unmodulated frequency and phase of the carrier signal transmitted during the TCC ramp on signal  366  is not detected as a beacon signal by the receiving device configured to detect a modulated beacon signal. 
     In other examples, beacon signal  312  may be transmitted as an unmodulated carrier signal having a carrier frequency and maximum peak-to-peak amplitude approached or reached during the TCC ramp on signal  366 . In this case, the TCC signal detector  175  of the intended receiving device is configured to detect the unmodulated carrier signal to wake up and switch from the polling mode to the receiving mode. As the amplitude of the ramp on signal  366  rises, carrier signal cycles of the ramp on signal  366  may be detected by the TCC signal detector  175  of the receiving device during beacon search period  320 . Beacon signal detection may occur relatively early, e.g., within or even before the first beacon signal  312 . 
     In the example shown, the first (leftmost) beacon signal  312  goes undetected. The second beacon search period  320  occurs during a later beacon signal  312 . Beacon signal detection criteria may be reached during the beacon search period  320 , e.g., a threshold number of carrier frequency cycles, a threshold number of paired intervals of alternating frequencies of an FSK modulated beacon signal (as further described below), or a threshold number and/or pattern of phase shifts of a BPSK modulated beacon signal. The receiving device TCC signal detector  175  may generate a beacon detection interrupt signal  324  that is passed to the control circuit of the receiving device, e.g., control circuit  170 . The control circuit may end the polling mode of the receiving device and switch to a communication receiving mode to enable reception of data packets  330  by the TCC signal detector. 
     In the example shown, the receiving device may include a TCC transmitter that is controlled to transmit an acknowledgement signal  328  back to the transmitting device to confirm beacon signal detection and that the receiving device is waiting to receive data packet transmissions. The acknowledgement signal  328  may be transmitted after a delay period  326  to ensure that the transmitting device is no longer transmitting the beacon signal  312  and has switched to a receiving period  314  and is capable of receiving the acknowledgement signal. Acknowledgement signal  328  may be the carrier signal transmitted for a predetermined time interval, e.g., 10 ms or less. The TCC signal transmission techniques disclosed herein which include a ramp on signal are described in the context of a controlling device transmitting beacon signals and data packets to a responder (receiving device). However, it is to be understood that the TCC signal transmission techniques including a ramp on signal may be used by the receiving device in generating and transmitting the acknowledgment signal  328  as well. 
     During the receiving period  314 , the transmitting device enables TCC receiver  87  to detect the acknowledgement signal  328 , e.g., by powering the TCC receiver  87  to enable the various filters, amplifiers, comparators, phase locked loops, or other circuitry to receive and detect the acknowledgement signal  328 . TCC receiver  87  may generate an acknowledgement detect signal  316  to control circuit  80 . Control circuit  80  switches the transmitting device from the wakeup mode  310  to a data transmission mode  311  during which the data packets  330  are transmitted. 
     In the example shown, not all beacon signals  312  and the data packet  330  are started during a blanking period  304 . Using the TCC ramp on signal techniques for charging the AC coupling capacitor disclosed herein, TCC signals may be transmitted outside of the blanking periods  304 , enabling TCC signal transmission independent of cardiac event timing. All or a portion of the first ramp on signal  366  of a TCC transmission session may be started during a blanking period  304  to ensure that any small low frequency artifact is blanked. Subsequent TCC signals transmitted during the same TCC transmission session, such as multiple beacon signals  312  and/or one or more data packets  330 , may or may not start during a blanking period  304 . 
     Beacon signal  312  may be transmitted multiple times during a cardiac cycle or over more than one cardiac cycle. In other examples, each beacon signal  312  may be followed by an OPEN command signal transmitted to the receiving device. The OPEN command signal may, for example, be a request to initiate a TCC communication session and, in some instances, may include some communication parameters of the session. The receiving device may detect the beacon signal and switch to a data receiving mode. Upon receiving the subsequent OPEN command signal, the receiving device may transmit an acknowledgement signal back to the transmitting device to confirm to the transmitting device that the TCC signal detector is powered on and ready to receive data transmissions. 
       FIG.  9    is a diagram of TCC ramp on signal  366 , beacon signal  312 , and a ramp off signal  368  according to one example. During the ramp on signal  366 , controller  91  controls the drive signal circuit  92  to step up the peak-to-peak amplitude of the carrier signal from a starting peak-to-peak amplitude  370  (which may be starting from zero) to an ending peak-to-peak amplitude  371 . During the ramp off signal  368 , the controller  91  may control the drive signal circuit  92  to step down the peak-to-peak amplitude of the carrier signal from the maximum peak-to-peak amplitude  372  of the beacon signal  312  to an ending peak-to-peak amplitude  380 , which may be zero. 
     Ramp on signal  366  may be the first signal transmitted during a transmission session, prior to the first beacon signal  312  as shown in  FIG.  8   . During ramp on signal  366 , adjustments of the peak-to-peak amplitude may be controlled digitally by controller  91  to produce a charge-balanced ramped signal. In other examples, controller  91  may control a large resistor having variable resistance to be gradually stepped down to control the rate that the peak-to-peak amplitude ramps up and the rate that the DC voltage is developed on the AC coupling capacitor  96 . The large resistor may be gradually stepped up to control the rate of discharge of the AC coupling capacitor  96  during ramp off signal  368 . The large resistor may be included in voltage holding circuit  98  to provide controlled discharge of AC coupling capacitor  96 . In another example, the ramp on signal  366  is controlled digitally and the ramp off signal  368  is a passive discharge of the AC coupling capacitor  96  using a resistance included in voltage holding circuit  98  that is selected to provide a long RC time constant producing a slow exponential discharge of the AC coupling capacitor  96  during the ramp off signal  368 . 
     Controller  91  may control drive signal circuit  92  to step up the peak-to-peak amplitude during ramp on signal  366  according to a step increment  374  made after each step up interval  376 . The step increment  374  and step up interval  376  are selected to minimize low frequency current injected into the conductive tissue pathway, which may be result in a low frequency voltage signal at a receiving electrode vector intended to receive the subsequent beacon signal  312  or unintended receiving electrode vector(s) coupled to electrical signal sensing circuitry included in the IMD system. The maximum peak-to-peak amplitude  372  and frequency of the carrier signal of beacon signal  312  are expected to be ineffective in stimulating cardiac or other muscle or nerve tissue. The ramp on signal  366 , however, is provided to minimize or avoid low frequency electrical interference in the IMD system at the beginning of the TCC transmission session. By digitally controlling each step increment  374  and holding each stepped up peak-to-peak amplitude for a predetermined number of carrier signal cycles (corresponding to step up interval  376 ), the ramp on signal  366  is a charge balanced signal. 
     The step increment  374  may be 1 microamp or less, e.g., 0.25 microamps as one example. The step up interval  376  may be 5 ms or less, e.g., 4 ms. The step increment  374 , step up interval  376  and total ramp on signal duration  367  may be selected according to time constraints of a particular application. For example, if the peak-to-peak amplitude of the ramp on signal  366  is to be adjusted from the starting peak-to-peak amplitude  370  to the ending peak-to-peak amplitude  371  within a post-sense blanking period of 150 ms, the step increment  374  and step up interval  376  may be selected to minimize low frequency artifact (by minimizing each step size and maximizing each step increment) within the time limitation of the blanking period while still reaching or approaching the maximum peak-to-peak amplitude  372  of the carrier signal during the beacon signal  312 . Ending peak-to-peak amplitude  371  may be equal to or less than maximum peak-to-peak amplitude  372  of beacon signal  312 . For instance, ending peak-to-peak amplitude  371  may be one step increment  374  less than maximum peak-to-peak amplitude  372 . 
     The step increment  374 , step up interval  376 , and ramp on signal duration  367  are selected to minimize the amplitude and/or frequency of low frequency artifact during charging of the AC coupling capacitor  96  to the DC operating voltage within any given time constraints. A more gradual, longer ramp on signal  366  may be required depending on the programmed sensitivity and a high pass pole of the cardiac event detector  85  (or other sensing circuits included in the IMD system) and the proximity of the receiving electrode vector to the transmitting electrode vector. Higher sensitivity, lower high pass pole, and closer proximity may increase the likelihood of low frequency artifact interfering with electrical signal sensing circuitry and may be avoided by using a slower ramp rate (smaller step up interval  376  and/or longer ramp on signal  366 ) and overall longer duration  367  of the ramp on signal  366 . 
     The duration  367  of ramp on signal  366  may be, without limitation, at least 50 ms, at least 100 ms or at least 200 ms in some examples. The peak-to-peak current amplitude may be increased from 0 mA up to an ending peak-to-peak amplitude  371  of 3 mA, 5 mA, 8 mA, or 10 mA as examples. The frequency of any low frequency current signal injected during the ramp on signal  366 , which may produce a voltage signal at a receiving electrode vector, may be limited to be less than 10 Hz, less than 5 Hz or even less than 2.5 Hz by selecting an appropriate step increment  366  and step up interval  376 . In one example, a high pass pole of a filter included in cardiac event detector  85  is 5 Hz, and the ramp on signal duration  367  is at least 100 ms during which the peak-to-peak current amplitude is increased from 0 up to the maximum peak-to-peak amplitude  371  to limit any injected low frequency current signal to be less than 5 Hz. 
     In some cases, the DC operating voltage is established on the AC coupling capacitor  96  within the ramp on signal  366 . In other cases, charging of the AC coupling capacitor  96  to the DC operating voltage may be ongoing at the expiration of the ramp on signal  366 . The control circuit  80  of the transmitting device may monitor low frequency artifact at the sensing electrode vector, e.g., during ramp on signal  366  and/or at the start of beacon signal  312 . Control circuit  80  may be configured to signal the TCC transmitter controller  91  to adjust one or more parameters controlling ramp on signal  366 , e.g., step increment  374 , step up interval  376 , starting peak-to-peak amplitude  370 , ending peak-to-peak amplitude  371 , ramp on signal duration  367 , and/or the carrier signal frequency to decrease the low frequency artifact if detected. In particular, if a cardiac event is sensed by cardiac event detector  85  during ramp on signal  366  or within a predetermined time interval after ramp on signal  366  (e.g., at the start of beacon signal  312 ), one or more control parameters may be adjusted to minimize the likelihood of low frequency artifact interfering with sensing circuit  86  (and/or other electrical signal sensing circuit(s) included in the IMD system). 
     The step increment  374  and the step up interval  376  are shown as fixed values during ramp on signal  366 . In other examples, the step increment  374  and/or the step up interval  376  may be variable intervals. For example, if the beacon signal  312  starts during a post-sense, post-pace or communication blanking period but the ramp on signal duration  367  extends later than the expiration of the blanking period, the step increment  374  may start at a first value and may be decreased to a second value and/or the step up interval  376  may start at a shorter value and be increased to a longer value in response to the blanking period ending. A lower ramp speed after the blanking period may reduce the likelihood of a low frequency current signal being injected into the body tissue that causes interference with electrical signal sensing circuitry included in the IMD system. In other examples, the step increment  374  may be a higher value at the start of ramp on signal  366  and be gradually decreased during the ramp on signal  366  so that ending steps occurring near the maximum peak-to-peak amplitude  371  are smaller steps than at the beginning of the ramp on signal  366 . The step up interval  376  may be adjusted one or more times during the ramp on signal. For instance, the step up interval  376  may be shorter initially and longer near the end of the ramp on signal  366 . Adjustments to the step increment  374  and step up interval  376  may be made by drive signal circuit  92  under the control of controller  91 , for example, such that multiple settings of each of the step increment  374  and/or the step up interval  376  are used during the ramp on signal  366 . 
     The ramp off signal  368  may follow beacon signal  312  but does not necessarily follow every (or any) beacon signal  312 . The ramp off signal  368  may be provided after a last beacon signal at the end of a wake up mode or only after a last data packet  330  at the end of a transmission session as further described below. If the ramp off signal  368  is a transmitted signal coupled to transmitting electrode vector  99 , ramp off signal  368  may be provided as a digitally controlled, charge balanced ramp off signal to discharge the AC coupling capacitor  96  at a stepped rate that is slow enough to avoid or minimize low frequency artifact that may interfere with electrical signal sensing circuits included in the IMD system, including sensing circuit  86  of the transmitting device. Ramp off signal  368  is provided so that the DC voltage across the AC coupling capacitor  96  is in a known state at the start of the next TCC signal transmission. If the DC voltage established on the AC coupling capacitor  96  during the ramp on signal  366  is held by voltage holding circuit  98  or not ramped down to control the discharge of the AC coupling capacitor  96 , the DC voltage across the AC coupling capacitor is likely to drift to an unknown voltage between transmitted signals or between transmission sessions due to leakage currents inherent in the IMD circuitry. If the voltage change is large enough when the next TCC signal is transmitted, a large artifact may develop across a sensing electrode vector, even if the TCC ramp on signal  366  is provided prior to the next TCC signal. 
     The ramp off signal  368  may be provided only after the last beacon signal during the wake up mode in some examples. The time between beacon signals  312  may be relatively short such that the AC coupling capacitor remains at the DC operating voltage between beacon signals. In some cases, the DC voltage established on the AC coupling capacitor during the ramp on signal  366  may be held between beacon signals  312  by voltage holding circuit  98 . In these situations, the ramp off signal  368  is not provided immediately and consecutively following every (or any) beacon signal  312 . In other examples, a ramp off signal  368  is not provided during the wake up mode  310  at all and is only provided consecutively following the last data packet  330  of the data transmission mode  311  (shown in  FIG.  7   ). 
     When ramp off signal  368  is provided, controller  91  may control the drive signal circuit  92  and polarity switching circuit  94  to generate the ramp off signal  368  as the AC carrier signal that steps down from the maximum peak-to-peak amplitude  372  of beacon signal  312  according to a step decrement  384  and a step down interval  386 . The step decrement  384  and step down interval  386  may or may not match the step increment  374  and step up interval  376 . In some examples, the rate of stepping down the peak-to-peak amplitude is different than the rate of stepping up the peak-to-peak amplitude, which may result in different durations  367  and  369  of ramp on signal  366  and ramp off signal  368 , respectively. For instance, the step decrement  384  may have a larger absolute value than the step increment  374  and/or the step down interval  386  may be shorter than the step up interval  376 . 
     As described above with regard to the ramp on signal control parameters, the ramp off signal control parameters, e.g., the step decrement  384  and/or the step down interval  386 , may be fixed or variable control parameters which may increase or decrease between the beginning and end of the ramp off signal  368 . The ramp off control parameters, e.g., ending peak-to-peak amplitude  380 , step decrement  384 , step down interval  386 , and ramp off signal duration  369  may be adjustable to achieve ramping down of the current signal after beacon signal  312  within a predetermined time interval and/or within maximum low frequency artifact conditions as determined by monitoring a voltage signal developed on a sensing electrode vector of the transmitting device. 
     In the example shown in  FIG.  9   , ramp off signal  368  may be a transmitted signal that is the stepped down carrier signal produced by the drive signal circuit  92  and polarity switching circuit  94  coupled to the transmitting electrode vector  99  via AC coupling capacitor  96  to gradually discharge the AC coupling capacitor. In other examples, the ramp off signal  368  may be a non-transmitted signal. Controller  91  may uncouple AC coupling capacitor  96  from the transmitting electrode vector  99  at the end of the beacon signal  312  and couple the AC coupling capacitor  96  to a resistor, which may be included in voltage holding circuit  98 , to allow for a gradual discharge of the AC coupling capacitor  96 . In this case, ramp off signal  368  may be a continuous, e.g., exponentially decreasing, signal produced as the AC coupling capacitor  96  is discharged through a resistor coupled to the AC coupling capacitor rather than through a tissue pathway. 
     The beacon signal  312  shown in  FIG.  9    is an FSK modulated beacon signal that alternates between a high frequency  360  and a low frequency  362 . A TCC signal detector configured to detect a single tone beacon signal, e.g., at the carrier signal frequency, may make false beacon signal detections at an unacceptably high rate. EMI or other baseline noise may cause false beacon signal detections when no beacon signal is being transmitted. False wakeups unnecessarily use battery power of the receiving device. In order to avoid false wakeups, transmitter  90  may be controlled to transmit an FSK modulated beacon signal. An FSK modulated beacon signal may be discriminated from other noise or EMI that the receiving device may be subjected to. 
     In the example of a 100 kHz carrier signal frequency, the high frequency  360  may be in the range of 102 kHz to 120 kHz, and the low frequency  362  may be in the range of 85 to 98 kHz. For example, the beacon signal  312  may alternate between 98 kHz and 102 kHz, 95 kHz and 105 kHz, or 92 kHz and 108 kHz. Keeping the high and low frequencies within a bandpass filter range of the carrier signal frequency may enable the TCC signal detector  175  to use a common bandpass filter for detecting the FSK modulated beacon signal and a PSK modulated carrier signal transmitted as a data packet or datagram. In another example, the FSK modulated beacon signal  312  alternates between a low frequency  362  of 85 kHz and a high frequency  360  of 115 kHz. The ramp on signal  366  may be transmitted as the unmodulated carrier signal with ramped amplitude prior to the FSK modulation of the beacon signal  312 . In other examples, FSK modulation using the high and low frequencies  360  and  362  included in the beacon signal  312  may be applied from the beginning of the ramp on signal  366 . In this case, the FSK modulation may be detected early by the receiving device promoting an early transition from the wakeup mode  310  to the data transmission mode  311 . 
     The beacon signal  312  may include an end-of-beacon signature  364  in some examples to enable positive detection of the end of the beacon signal  312  by the receiving device and promote appropriate timing of the receiving device waking up and powering up the TCC signal detector for receiving TCC data transmissions. The relative orientation of the transmitting and receiving electrode vectors may vary over a cardiac cycle and over a respiration cycle. As a result, the voltage signal amplitude at the receiving electrode vector may vary over time and may even drop out due to positional changes of the transmitting and receiving electrode vectors caused by cardiac, respiratory or other body motion. To promote highly reliable positive wakeups, the beacon signal  312  may be terminated with an end-of-beacon signature. The FSK beacon signal  312  may be transmitted with a fixed number of cycles at each high and low frequency  360  and  362 , e.g., 8 cycles, 12 cycles, 16 cycles, 24 cycles, 32 cycles or more. In one example, the beacon signal is transmitted with 16 cycles of the high frequency  360  followed by 12 cycles of the low frequency  362 . The number of cycles transmitted at each frequency may be selected so that each high and low frequency  360  and  362  is transmitted for the same time interval to facilitate detection of the FSK beacon signal and the end-of-beacon signature  364 . 
     The end-of-beacon signature  364  may include any combination of high frequency  360  and/or low frequency  362  intervals that is distinct from the beacon signal transmitted prior to the end-of-beacon signature  364 . It is recognized that numerous variations of an end-of-beacon signature may be used that includes a distinct number of cycles of the high frequency  360  and/or the low frequency  362  that is different than the number of cycles of each respective frequency delivered during the FSK modulated beacon signal  312  leading up to the end-of-beacon signature  364 . In one example, each of the high frequency  360  and the low frequency  362  may be delivered for twice the number of cycles during the end-of-beacon signature  364  compared to prior to the end-of-beacon signature. In the example given above including 16 cycles of high frequency  360  alternating with 12 cycles of low frequency  362 , the end-of-beacon signature may include 32 cycles of the high frequency  360  alternating with 24 cycles of the low frequency  362 . 
     While shown with only one pair of alternating high and low frequencies  360  and  362  for the sake of illustration, it is to be understood that the end-of-beacon signature  364  may include multiple sequential pairs of high and low frequency intervals, for example eight sequential pairs. In this example, using the high and low frequencies of 115 kHz and 85 kHz given above, the end-of-beacon signature  364  may be 56 ms in duration. Similarly, while only two pairs of alternating high and low frequency intervals prior to the end-of-beacon signature  364  are shown for the sake of illustrating beacon signal  312  in  FIG.  9   , the number of paired alternating high and low frequency intervals that precede the end-of-beacon signature  364  may be more than two and is selected to achieve a desired beacon signal duration. 
     For instance, the total duration of beacon signal  312  may be 50 ms to 1000 ms after the ramp on signal  366  and before the ramp off signal  368 . In some examples, ramp on signal  366  is up to 200 ms long, followed by a beacon signal  312  including FSK modulation of the carrier signal at the maximum peak-to-peak amplitude  372  for at least 8 ms and up to 120 ms or more, and an end-of-beacon signature that may be 60 ms or less. If the ramp off signal  368  is provided consecutively following beacon signal  312 , the ramp off signal  368  may be applied after the end-of-beacon signature  364  so that the end-of-beacon signature  364  is transmitted at the maximum peak-to-peak amplitude  372  to promote reliable detection of the beacon signal  312  by the receiving device. It is to be understood that the ramp off signal  368  at the end of the beacon signal  312  is omitted in some examples since the next TCC signal, which may be another beacon signal or the first data packet during the data transmission mode, is expected to occur relatively soon, e.g., within one minute or less. 
       FIG.  10    is a diagram of one example of a transmission session  400  performed by a transmitting device of an IMD system, such as ICD  14  of system  10  or ICD  214  of system  200 , shown in  FIG.  1    and  FIG.  2    respectively. The control circuit  80  controls transmitter  90  to start transmission session  400  by signaling controller  91  to transition the transmitter  90  from a sleep (minimized power) mode to the wakeup mode  410 . Controller  91  controls drive signal circuit  92  and polarity switching circuit  94  to generate a ramped signal coupled to AC coupling capacitor  96  during ramp on signal  466 . The ramp on signal  466  may be transmitted as the carrier signal having a selected carrier frequency with a starting peak-to-peak amplitude that is stepped up to an ending peak-to-peak amplitude according to a step increment and step up interval as described above. In some examples, the ramp on signal  466  may include FSK modulated pairs of intervals of high and low frequencies. In other examples, the ramp on signal  266  is modulated using other modulation techniques. In yet other examples, the ramp on signal  466  includes only the single-tone carrier signal. 
     The ramp on signal  466  is followed by a beacon signal  412 . If a short time interval exists between ramp on signal  466  and the first beacon signal  412 , the AC coupling capacitor  96  may be held at the DC voltage developed on the AC coupling capacitor during the ramp on signal  466  during the time interval between ramp on signal  466  and the first beacon signal  412 . Any delay between ramp on signal  466  and the first beacon signal  412  may be minimal such that any leakage current that might occur during the delay results in minimal passive discharge of the AC coupling capacitor  96 . In some examples, ramp on signal  466  may be substantially continuous with beacon signal  412  with no time gap in between. 
     The ramp on signal  466  includes a ramped carrier signal that reaches the ending peak-to-peak amplitude as shown in  FIG.  9   , which may be the maximum peak-to-peak amplitude of the carrier signal used during beacon signal transmission. The ramp on signal  466  may be up to 200 ms in duration or more. The beacon signal  412  may be an FSK modulated signal including a predetermined number of pairs of alternating intervals of high and low frequencies as described above, delivered at the beacon signal maximum peak-to-peak amplitude. The beacon signal  412  may be terminated with an end-of beacon signature. If the ramp on signal  466  is delivered at the carrier signal frequency and the beacon signal  412  is an FSK modulated signal, the TCC signal detector  175  of the receiving device (e.g., pacemaker  100  or pressure sensor  50 ) is configured to detect the beacon signal by searching for the expected alternating pairs of high and low frequency intervals. The carrier signal delivered during the ramp on signal  466  may not be recognized by the TCC signal detector  175  as the start of the beacon signal since the carrier frequency is different than the high and low frequencies of the FSK modulated signal. The beacon signal  412  is not terminated with a ramp off signal in the transmission session  400 . 
     The controller  91  may control the drive signal circuit  92  and polarity switching circuit  94  to wait for a post-beacon interval  413  after beacon signal  412  before transmitting an OPEN command  415 . The post-beacon interval  413  is provided to allow time for the receiving device to detect the beacon signal, including detecting the frequency pattern of alternating intervals of high and low frequencies and the end-of-beacon signature for an FSK modulated beacon, and power up the TCC signal detector  175  to enable searching for the OPEN command  415 . The beacon signal  412  may be 50 ms to 1 second in duration and is 80 to 120 ms in some examples. The beacon signal  412  may be followed by a post-beacon interval  413  that is 100 ms and 200 ms in duration. The voltage holding circuit  98  may hold the AC coupling capacitor  96  at the DC voltage developed during the ramp on signal  466  during the post-beacon interval  413 . The OPEN command  415  may be 1 ms to 25 ms, e.g., 8 ms in duration. The OPEN command may be transmitted at the single-tone carrier frequency, e.g., a 100 kHz signal, for a predetermined duration, e.g., 8 ms at the beacon signal maximum peak-to-peak amplitude. 
     Relatively short beacon signals, e.g., 8 to 100 ms, may be repeated at multiple times during the cardiac cycle to promote transmission at a time that the receiving electrode vector is parallel to the tissue conductance pathway of the injected current. In the example shown, each beacon signal  412  is followed by an OPEN command  415 . In other examples, the beacon signal  412  may be transmitted repeatedly, e.g., two or more times during a cardiac cycle, separated by post-beacon intervals  413  and a single OPEN command  415  is transmitted after multiple short beacon signals  412  to increase the likelihood of the beacon signal being detected by the receiving device in advance of the OPEN command  415 . 
     The transmitting device control circuit  80  may enable the TCC receiver  87  to search for an acknowledgement signal from the receiving device during an acknowledgement receiving period  414  following each OPEN command  415 . Receiving period  414  may have a maximum duration for waiting for an acknowledgment signal. If the acknowledgement signal is not detected by the transmitting device by the expiration of the receiving period  414 , the transmitting device remains in the wakeup mode  410  as indicated by the curved, dashed arrow. The beacon signal  412  may be repeatedly delivered, followed by post-beacon intervals  413 , OPEN commands  415  and receiving periods  414  until an acknowledgment detect signal  416  is generated by the TCC receiver  87 . In some cases, if a predetermined number of beacon signals  412  are delivered and the acknowledgment signal is not received, the controller  91  may control transmitter  90  to wait for a beacon control interval  422  before sending another beacon signal  412 . The voltage holding circuit  98  may be enabled to hold the AC coupling capacitor  96  at the DC voltage established during ramp on signal  466  during the beacon control interval  422 . 
     If beacon control interval  422  is relatively long, however, voltage holding circuit  98  may not be enabled to hold the AC coupling capacitor  96  at the DC voltage during the beacon control interval  422 . If the receiving period  414  expires without detection of an acknowledgement signal from the receiving device, controller  91  may control transmitter  90  to provide a ramp off signal  468  after the last beacon signal  412  is transmitted and an acknowledgement signal is not received. Controller  91  may control the drive signal circuit  91  to digitally step down the peak-to-peak amplitude of a carrier signal from the maximum peak-to-peak amplitude of the beacon signals  412  according to a step decrement and step down interval. In other examples, a variable resistor included in voltage holding circuit  98  may be coupled to the AC coupling capacitor  96  and the resistance may be gradually adjusted to control a slow discharge rate of the AC coupling capacitor. In some examples, a large fixed resistor included in voltage holding circuit  98  is coupled to AC coupling capacitor  96  during ramp off signal  468  to control the gradual discharge of AC coupling capacitor  96 . The ramp off signal  468  may occur during the beacon control interval  422  and may not be a distinctly separate time interval. When a ramp off signal  468  is applied during the wake up mode  410 , a ramp on signal  466  may be re-applied after beacon control interval  422  to re-establish the DC voltage on the AC coupling capacitor  96  prior to the next beacon signal transmission. 
     Upon detection of the acknowledgement signal transmitted from the receiving device during the receiving period  414 , an acknowledgement detect signal  416  may be generated by the TCC receiver  87  and passed to control circuit  80  of the transmitting device. Control circuit  80  switches TCC transmitter  90  from the wakeup mode  410  to the data transmission mode  411  to begin transmitting data packets  430 . The voltage holding circuit  98  may hold the AC coupling capacitor  96  at the DC voltage established during the ramp on signal  466  to maintain the DC voltage during the wakeup mode  410 , between beacon signals  412  and from the time of the last OPEN command  415  until the first data packet  430 . 
     In some examples a ramped charge adjustment period  452  may be applied prior to the first packet  430 . The ramped charge adjustment period  452  may include a ramp on signal or a ramp off signal. The charge adjustment period  452  may include a ramp on signal if the AC coupling capacitor  96  is partially or wholly discharged during the time delay between the last OPEN command  415  and the start of the first packet  430 . For example, if the delay until the start of the first packet  430  is relatively long, AC coupling capacitor  96  may at least partially discharge before the first data packet  430  due to leakage currents. During the charge adjustment period  452 , controller  91  may determine the remaining charge on the AC coupling capacitor  96  to determine a starting amplitude of the ramp on signal and control drive signal circuit  92  and polarity switching circuit  94  to pass the carrier signal to the AC coupling capacitor  96  while stepping up the peak-to-peak amplitude of the carrier signal to the maximum peak-to-peak amplitude of the data packets  430  according to a step increment and step up interval. 
     For example, controller  91  may include an analog-to-digital converter to sample the voltage signal across the AC coupling capacitor  96  at the end of the most recently transmitted TCC signal (e.g., the last beacon signal  412  or OPEN command  415 ) to determine a target DC voltage. The AC coupling capacitor voltage may be sampled during the charge adjustment period  452  until the AC coupling capacitor  96  is adjusted to the target voltage. Since the AC coupling capacitor  96  may be at least partially charged, the ramp on signal applied during charge adjustment period  452  may be shorter than the ramp on signal  466  applied at the “cold start” at the beginning of transmission session  400 . In other examples that include a ramp off signal  468  terminating the wakeup mode  410 , a ramp on signal  466  may be applied during charge adjustment period  452  at the start of data transmission mode  411 . 
     In some cases, the beacon signals  412  are transmitted at a higher peak-to-peak amplitude than the data packets  430 . In this case, the charge adjustment period  452  may include a ramp down signal that includes an AC carrier signal having a current amplitude that is ramped down from the maximum peak-to-peak amplitude of the beacon signals  412  to the maximum peak-to-peak amplitude of the data packets  430 . 
     Each data packet  430  is transmitted during the data transmission mode  411  using a modulated carrier signal, e.g., an FSK or PSK modulated signal. Each data packet  430  may include a number of fields as described below in conjunction with  FIG.  11   . In one example, controller  91  may be configured to control the drive signal circuit  92  and polarity switching circuit  94  to generate BPSK modulated signals during the data transmission mode  411 , e.g., by producing 180 degree phase shifts in the carrier signal to encode digital signals in the modulated carrier. Methods for transmitting a BPSK signal by TCC transmitter  90  disclosed in the above-incorporated U.S. Patent Application No. 62/591,800 (Roberts, et al.) and U.S. Patent Application No. 62/591,806 (Reinke, et al.) may be used in conjunction with the various examples of the ramp on signal  466 , ramp off signal  468  and charge adjustment period  452 . 
     Packets  430  may be separated by receiving windows  450  during which TCC receiver  87  may be enabled to detect signals transmitted by the receiving device, such as a confirmation signal or other data requested by the transmitting device. Voltage holding circuit  98  may be enabled to hold the AC coupling capacitor at the DC voltage originally established during ramp on signal  466  during the receiving windows  450 . In other examples if the time between packets  430  is relatively long, each data packet  430  may be followed by a ramp off signal  468  and the next data packet may be preceded by a ramp on signal  466  to re-establish the DC operating voltage on the AC coupling capacitor  96  prior to transmission of each packet  430 . 
     The TCC transmission session  400  is terminated with a ramp off signal  468  following the last packet  430 . Controller  91  may control drive signal circuit  92  to digitally step down the carrier signal amplitude from the maximum peak-to-peak amplitude of the data packets  430  to complete discharge of the AC coupling capacitor  96  during the ramp off signal  468 . The carrier signal amplitude may be stepped down according to a step decrement and step down interval as described above in conjunction with  FIG.  9   . In other examples, controller  91  may couple AC coupling capacitor  96  to a discharge load, e.g., including a large resistor included in voltage holding circuit  98 , which may have an adjustable resistance, to slowly discharge the AC coupling capacitor  96  according to the RC time constant of the discharge load and AC coupling capacitor  96 . Transmission session  400  may include a single ramp on signal  466  consecutively followed by the first beacon signal  412  and a single ramp off signal  468  consecutively following the last packet  430 . 
     In other examples, one or more ramp on signals and one or more ramp off signals may be applied to enable charging and discharging of the AC coupling capacitor  96  to occur at controlled rates to minimize low frequency current being injected into the conductive body tissue pathway. The one or more ramp on signals  466  are each transmitted as AC signals, which may be at the carrier frequency used to generate the beacon signals  412  and data packets  430 . The one or more ramp off signals  468  may be transmitted AC signals at the carrier frequency or a non-transmitted, exponentially decaying signal produced by connecting the AC coupling capacitor  96  to a high impedance resistive load with the drive signal circuit  92  and polarity switching circuit  94  powered down. 
       FIG.  11    is a diagram of a data packet  430  that may be transmitted during the data transmission mode  411  by the transmitting device according to one example. Data packet  430  may include multiple fields  470 ,  472  and  474  transmitted using a carrier signal and BPSK modulation. A synchronization field  470  is transmitted as the first field at the unmodulated carrier signal frequency, e.g., 100 kHz, to provide a carrier lock for the demodulation by the TCC signal detector of the receiving device. The synchronization field  470  may include a predetermined number of carrier frequency cycles or bits, e.g., with eight cycles per bit. The synchronization field  470  may be between 128 and 256 cycles long in some examples. A ramp on signal  466  may be provided prior to synchronization field  470  in some examples. Depending on how often data packets  430  are being sent, a ramp on signal  466  may be provided to charge the AC coupling capacitor to the DC voltage before synchronization field  470  of the data packet  430  is transmitted at the carrier frequency and maximum peak-to-peak amplitude of the data packet. 
     In other examples, ramp on signal  466  is omitted at the start of data packet  430 . The leakage current may be negligible between TCC signal transmissions within a transmission session such that AC coupling capacitor  96  does not significantly discharge between the beacon signal(s) and data packets. In some examples, voltage holding circuit  98  may be enabled to hold the AC coupling capacitor at the DC voltage established during a preceding transmitted TCC signal, e.g., during the immediately preceding beacon signal or preceding packet. The DC voltage may be initially established during the first ramp on signal  466  applied prior to beacon signal transmission during the wakeup mode  410  as shown in  FIG.  10   . The DC voltage initially established during a ramp on signal  466  applied prior to the first beacon signal may be maintained throughout the transmission session by enabling voltage holding circuit  98  to hold the AC coupling capacitor  98  at the DC voltage between TCC signal transmissions such that ramp on signal  466  is not required at the beginning of packet  430 . 
     The carrier signal transmitted during the synchronization field  490  is modulated during the preamble and data fields  492  and  494  according to a binary coded input signal, which may be produced by a modulator included in controller  91 . The preamble field  472  may follow the synchronization field  470  and may be encoded to communicate the type of packet being transmitted and the packet length, e.g., the number of data fields. The preamble field  472  may also include a key code to provide bit sample timing to the receiving device, an acknowledgment request bit, source and/or destination address bits, or other bits or bytes that may normally be included in a header or preamble field  472 . 
     Data fields  474  include the information being communicated to the receiving device, which may include commands to perform therapy delivery or signal acquisition, requests for data, control parameter settings to be used by the receiving device for sensing physiological signals and/or delivering a therapy, or numerous other types of encoded information that enable coordination of the IMD system in monitoring the patient and delivering therapy. Each data field  474  may include one byte and each byte may be a predetermined number of bits, e.g., 8 bits, 9 bits, 13 bits or other predetermined number, representing a stream of digital values. Each data packet  430  may include 1 to 256 data fields or bytes. In some examples, the data packet  430  may be terminated with a cyclic redundancy check (CRC) field to enable the receiving device to perform an error check. While a particular example of a data packet  430  (or datagram) is shown in  FIG.  11   , numerous data frame structures including various fields may be conceived according to a particular clinical application and IMD system that utilize ramp on and ramp off signals as described herein. 
     If packet  430  is the last packet being transmitted in the transmission session, the last data field  474  may be followed consecutively by a ramp off signal  468 . The ramp off signal  468  is provided to step down the DC voltage charge of the AC coupling capacitor  96  in decrements that do not cause a detectable low frequency signal or DC voltage shift on a sensing electrode vector coupled to an electrical signal sensing circuit of the transmitting device or another co-implanted device, which may be the intended receiving device or an unintended receiver. Depending on how frequently a data packet is transmitted, each data packet may be preceded by a ramp on signal  466  and followed by a ramp off signal  468  to minimize the likelihood of TCC signal interference with electrical signal sensing circuitry of the IMD system. For example, if each data packet  430  is transmitted at least once per second or even at least once per minute, leakage current from the AC coupling capacitor may be small enough to not require a ramp on signal  466 . If leakage current is relatively high and/or time between transmitted signals is relatively long, controlled discharge of AC coupling capacitor  96  by applying ramp off signal  468  after and controlled recharging by applying ramp on signal  466  before each data packet  430 . 
       FIG.  12    is a conceptual diagram of a portion of a data field  474  that may be included in data packet  430  of  FIG.  11   , followed by ramp off signal  468 . Each data field  474  (or byte) of a packet  430  may be transmitted as the carrier signal  501  modulated using BPSK. The carrier signal  501  has a maximum peak-to-peak amplitude  572  and a carrier frequency that is defined by the length of one carrier frequency cycle  504 . The carrier signal  501  has a positive polarity reaching half the peak-to-peak amplitude  572  during one half of the carrier frequency cycle  504  and negative polarity reaching half the peak-to-peak amplitude  572  during the other half of the carrier frequency cycle  504 . 
     An input digital signal  520  may be generated by transmitter controller  91  to control drive signal circuit  92  and/or polarity switching circuit  94  of transmitter  90  (all shown in  FIG.  6   ) to control modulation of the carrier signal  501  during transmission of packet  430  ending with data field  474  and followed by ramp off signal  468 . Each bit  505 - 509  of the transmitted data field  474  is transmitted with a predetermined number of carrier frequency cycles. Each bit value is encoded by controlling the phase shift between bits according to input digital signal  520 . In the example shown, each bit  505 - 509  includes eight carrier frequency cycles  504 . The bit value is encoded by controlling drive signal circuit  92  and/or polarity switching circuit  94  to produce either a zero phase shift between bits or a phase shift between bits. No phase shift may correspond to a digital “0” in the bit stream, and a phase shift may correspond to a digital “1” in the bit stream. 
     To illustrate, the first bit  505  may be a digital “0” and is followed by a phase shift  510  leading into the next bit  506 . The phase shift is a positive 180 degrees in this example, but other phase shifts, between ±360 degrees may be used. The TCC signal detector  175  of the receiving device that is locked into the frequency of the carrier signal  501  is configured to detect the phase shift  510 . In response to detecting the phase shift  510 , the TCC signal detector outputs a digital “1” in digital output signal  524  that is passed to the control circuit of the receiving device for decoding. Bit  506  is followed by no phase shift  512  according to the digital input signal  520  which changes from a digital “1” to a digital “0.” The TCC signal detector  175  of the receiving device detects no phase shift and produces a digital “0” for bit  507  in response to detecting no phase shift after the eight cycles of bit  506 . 
     The next bit  507  is followed by a 180 degree positive phase shift  514  in accordance with the change from a digital “0” to a digital “1” in the input digital signal  520 . The phase shift  514  is detected by the TCC signal detector  175  and the bit value in the output digital signal  524  changes from “0” to “1” for bit  508 . The eight cycles of bit  508  are followed by no phase shift  516 , in accordance with a change from “1” to “0” in the input signal  524 . In response to no phase shift detection, the TCC signal detector  175  produces a digital “0” in the output digital signal  524  corresponding to the last bit  509 . The last five bits of data byte  500  are represented in  FIG.  12   , however it is recognized that data byte  500  may include 2, 4, 8, 16, or other predetermined number of bits. The last four bits  506 - 509  may represent a nibble (four bits) of byte  500  including at least 8 bits (an octet) in a hexadecimal encoding scheme. As can be seen in  FIG.  12   , the encoded data is transmitted during a data packet by continuously transmitting the carrier signal without interruption while controlling the drive signal circuit  92  and the polarity switching circuit  94  to shift the phase of the carrier signal. 
     As described above, each data packet  430  may include a header or preamble that includes one or more bytes to provide various header information including the number of data fields  474 . As such, the receiving device may be configured to ignore a ramp off signal  468  for the purposes of producing digital output signal  524  after the expected number of data fields  474  has been received. The controller  91  may control the drive signal circuit  92  and polarity switching circuit  94  to step down the peak-to-peak amplitude of the carrier signal  501  during the ramp off signal  468  according to a step decrement  484  and step down interval  486 . In other examples, controller  91  may disable the drive signal circuit  92  and polarity switching circuit  94  during the ramp off signal  468 . The AC coupling capacitor  96  may be coupled to a resistor in voltage holding circuit  98  to gradually discharge, e.g., as shown by the continuous exponential decay signal  469 . In some examples, each data packet  430  is terminated with a ramp off signal  468 . In other examples, only the last data packet  430  of a transmission session is terminated with a ramp off signal  468 . As such, a single ramp off signal  468  may occur during a transmission session, such as transmission session  300  of  FIG.  7   . 
     The ramp on signal  466  and ramp off signal  468  have been described in conjunction with an FSK modulated beacon signal and a BPSK modulated data packet. It is to be understood that the ramp on signal  466  and ramp off signal  468  may be applied at least at the beginning and end, respectively, of a transmission session that utilizes other modulation techniques and the ramp on and off signals  466  and  468  applied to the carrier signal, modulated or unmodulated, are not limited to use with a particular modulation scheme. 
       FIG.  13    is a flow chart  600  of a method for transmitting TCC signals that may be performed by an IMD system, e.g., system  10  of  FIG.  1    or system  200  of  FIG.  2   , according to one example. The control circuit of the transmitting device, e.g., control circuit  80  of ICD  14  or ICD  214 , determines that pending data are ready for TCC transmission at block  601 . As described above, the transmitting device may be ICD  14  or ICD  214  operating as a controlling device and the receiving device may be pacemaker  100  or pressure sensor  50  operating as a responder. The receiving device may be a reduced function device with a smaller power supply. For example, the receiving device may be configured to operate in a polling mode to be woken up by another device but may not be configured to operate in a wakeup mode to initiate TCC transmission sessions. In other examples, the transmitting device performing the method of flow chart  600  may be any IMD included in an IMD system that is configured to initiate a transmission session, which may include pacemaker  100  or pressure sensor  50  in some examples. 
     Control circuit  80  powers up transmitter  90  at block  602  to switch transmitter  90  to the wakeup mode from a sleep state, in which power supplied to the circuitry of transmitter  90  is minimized. At block  604  the ramp on signal is transmitted at the start of TCC signal transmission. The ramp on signal may be transmitted via the transmitting electrode vector  99  coupled to AC coupling capacitor  96  as the unmodulated, AC carrier signal having a ramped peak-to-peak amplitude. The AC ramp on signal is transmitted prior to transmission of the first beacon signal of the transmission session. In some examples, the ramp on signal may be a modulated carrier signal, e.g., FSK modulation may be applied to the ramp on signal corresponding to the FSK beacon signal modulation as described in conjunction with  FIG.  9   . Performing FSK modulation of the carrier signal during the ramp on signal may allow early detection of the alternating frequencies of the beacon signal by a receiving device if the peak-to-peak amplitude of the TCC signal during the ramp on signal causes a detectable FSK modulated voltage signal to be developed across the receiving electrode vector before the maximum peak-to-peak amplitude of the beacon signal is reached. 
     The ramp on signal may be controlled according to a step increment, step up interval, and total ramp duration as described above. In some examples, control circuit  80  may be configured to control sensing circuit  86  to monitor a voltage signal developed across a sensing electrode vector at block  606 . If the voltage signal meets artifact detection criteria, the ramp control parameters may be adjusted at block  608 . For example, if the voltage signal crosses a threshold during the ramp on signal, artifact detection criteria may be met. In some examples, the ramp on signal is applied during a blanking or refractory period applied to cardiac event detector  85  so that if the voltage signal crosses a cardiac event detection threshold, it is ignored for the purposes of determining a cardiac rate or detecting a cardiac rhythm. The voltage signal received by a sense amplifier or other component of sensing circuit  86  from the sensing electrode vector during the ramp on signal may be monitored, however, for detecting a voltage change induced by the TCC ramp on signal. The control circuit  80  may control transmitter  90  to decrease the step increment and/or increase the step up interval at block  608  to decrease any low frequency voltage signal that may be occurring at a sensing electrode vector coupled to sensing circuit  86 . In some examples, the adjustment at block  608  is applied immediately during the ongoing ramp on signal. In other examples, the ramp on signal may be completed using the unadjusted control parameters and the adjusted control parameters are applied during the next ramp on signal produced by transmitter  90 , e.g., at the start of the next transmission session. 
     After the ramp on signal, the beacon signal is transmitted at block  610 , e.g., using FSK modulation of the carrier signal as described in conjunction with  FIG.  9    and terminated with an end-of beacon signature. The beacon signal may be followed by an OPEN command as described above in conjunction with  FIG.  10   . The transmitting device waits for an acknowledgement (ACK) signal from the receiving device at block  612 . If the acknowledgement signal is not received before the receiving window times out, as described in conjunction with  FIG.  10   , the beacon signal may be re-transmitted by returning to block  610 . In the example of flow chart  600 , a ramp on signal is applied a single time during the transmission session, at the beginning of the wakeup mode. In other examples, if a ramp off signal is applied at the end of a beacon signal, the transmitter  90  may return to block  604  to apply another ramp on signal before the beacon signal is transmitted again. 
     In response to receiving the acknowledgment signal at block  612 , the controller  91  switches the operation of transmitter  90  from the wakeup mode to the data transmission mode at block  614 . In other examples, the transmitter  90  may transmit the beacon signal and switch to the data transmission mode without detecting an acknowledgement signal. The transmitter  90  may switch to the data transmission mode after a delay interval to allow the receiving device time to detect the beacon signal and switch from the polling mode to the receiving mode. 
     The transmitter  90  is controlled to transmit each data packet during the data transmission mode, e.g., using BPSK modulation of the carrier signal, at block  616 . Multiple data packets may be transmitted in a single transmission session. While not shown explicitly in  FIG.  13   , it is understood that between data packets, the transmitting device may enable a TCC receiver  87  for a receiving window, e.g., receiving window  350  shown in  FIG.  7   , to detect and demodulate TCC signals requested from and transmitted by the receiving device. In some cases, a return signal from the receiving device is not requested by the transmitting device so that the receiving window may not be required. 
     After all pending data packets of the TCC transmission session are sent, as determined at block  618 , the ramp off signal is produced at block  620 . The maximum peak-to-peak amplitude of the last data field may be stepped down according to a step decrement and step down interval to allow a controlled, gradual discharge the AC coupling capacitor  96  through the transmitting electrode vector and tissue pathway. In some examples, the voltage signal received at a sensing electrode vector may be monitored by sensing circuit  86  to detect artifact during the ramp off signal. The ramp off signal control parameters may be adjusted by the control circuit  80  in response to detecting a threshold level of voltage artifact by the sensing circuit  86 . The step decrement may be decreased and/or the step down interval increased to reduce the artifact that may be caused during discharge of the AC coupling capacitor. 
     In other examples, the ramp off signal is produced at block  620  by uncoupling the AC coupling capacitor  96  from the transmitting electrode vector  99  and coupling the AC coupling capacitor  96  to a resistor included in voltage holding circuit  98 . The AC coupling capacitor  96  is slowly discharged through the resistor according to the RC time constant. The controller  92  may disable the drive signal circuit  92  and the polarity switching circuit  94  during the ramp off signal, e.g., by powering down the drive signal and polarity switching circuits  92  and  94 . 
     The example of flow chart  600  includes a single ramp off signal applied after the last data field of the last data packet of the transmission session. In other examples, a ramp off signal may be applied at the end of each data packet, in which case a ramp on signal may be applied at the beginning of each data packet. Monitoring a voltage signal at a sensing electrode vector by sensing circuit  86  may be performed during any one or more of the ramp on and/or ramp off signals applied during a TCC transmission session to allow adjustments of ramp control parameters as needed to minimize interference of the TCC signal with electrical signal sensing circuits of the IMD system. 
     The transmission session is completed at block  622 . Transmitter  90  may be switched back to a low power, sleep state at block  622 , until the next pending TCC transmission session. In some instances, if the next TCC transmission session is expected to occur relatively soon, e.g., within up to one minute or within up to ten minutes as examples, the ramp off signal is not applied at block  620 . The next transmission session may be started without applying a ramp on signal. The leakage currents may be minimal such that minimal discharge of the AC coupling capacitor  96  occurs between transmission sessions. The voltage holding circuit  98  may be enabled by controller  91  to hold the AC coupling capacitor  96  at the DC voltage established during the most recently completed transmission session until the next transmission session. As such, not all transmission sessions may be required to include a ramp on signal and a ramp off signal. 
       FIG.  14    is a conceptual diagram of a method for transmitting TCC signals during multiple transmission sessions  700   a - n  according to one example. If multiple transmission sessions  700   a - n  are expected to be scheduled to occur sequentially within a relatively short period of time, e.g., within several minutes or within one hour of each other, control circuit  80  may control transmitter  90  to apply a ramp on signal  766  only at the beginning of the first transmission session  700   a  and apply the ramp off signal  768  only at the end of the last transmission session  700   n . Each transmission session  700   a - n  may include a wakeup mode  710  during which at least one beacon signal is transmitted and a data transmission mode  711  during which at least one data packet or datagram is transmitted. Voltage holding circuit  98  may optionally be controlled to hold the DC voltage initially established (at least partially) during the ramp on signal  766  in between beacon signals and data packets within a transmission session and during time intervals  720  between consecutive transmission sessions  700   a - n . When the intervals  720  between successive transmission sessions  700   a - n  are relatively short, e.g., 10 minutes or less, 5 minutes or less, 2 minutes or 1 minute or less, a ramp on signal  766  may be transmitted only at the start of the first transmission session  700   a , and a ramp off signal  768  may be produced only at the end of the last transmission session  700   n . When relatively long intervals  720  are expected between transmission sessions, e.g., more than ten minutes or more than one hour, a ramp on signal and a ramp off signal may initiate and terminate each transmission session  700   a - n.    
     Thus, various examples of a method and apparatus for TCC performed by a medical device system have been presented in the foregoing description with reference to specific embodiments. It is appreciated that various modifications to the referenced embodiments, including combining various aspects of the TCC signal transmission and detection methods in different combinations than the specific combinations described here, may be made without departing from the scope of the disclosure and the following claims. It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or component for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of circuits or components associated with, for example, a medical device and/or a single circuit or component may perform multiple functions that are represented as separate circuits or components in the accompanying drawings. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other non-transitory computer-readable medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer). 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     Thus, a system has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.