Communications: electrical – Continuously variable indicating – Phase variation
Reexamination Certificate
1998-12-21
2001-12-11
Horabik, Michael (Department: 2735)
Communications: electrical
Continuously variable indicating
Phase variation
C128S903000, C375S340000, C607S032000
Reexamination Certificate
active
06329929
ABSTRACT:
FIELD OF THE INVENTION
This invention lies in the field of telemetry systems and, in particular, telemetry systems having a transmitter characterized by an unstable carrier which is modulated with symbol data at a crystal-referenced symbol rate to provide pulsatile RF signals; and a receiver which demodulates by generating a carrier phase-locked symbol signal which is used for demodulating the symbol data.
BACKGROUND OF THE INVENTION
In many telemetry system applications, and in particular in the field of medical devices, the system must ensure the ability to detect data signals in the presence of significant noise. Often the noise may have components within the frequency band of the telemetry signal, making the detection process difficult. It is known in telemetry systems to use window tracking to detect pulses. In such systems, a detection window is created centered around the next expected pulse, to time discriminate against noise and thereby enable examination of the incoming signal. However, generally in such systems the time of the detected pulse is not sharply defined, and the window needs to be long enough to both “see” the pulse and allow for drift in the pulse position. Consequently, it is very difficult to separate out noise from signal in such time-based systems. Other systems have been employed with varying success, but it remains difficult to accurately and reliably receive pulsatile data in a noisy environment. An acceptable receiver, e.g., for frame-based uplink telemetry, using DSP or any other embodiment, must provide a simple yet very reliable method of discriminating the noise likely in the environment in which the system operates.
A problem which comes into play in telemetry systems involving implanted devices is that the carrier is frequently of an unstable and inaccurate nature. In many such systems the carrier is a continuous wave, i.e., a sinusoidal carrier, such that the phase information of the carrier can be retrieved by multiplying it with sine waves and cosine waves (complex demodulation). However, if the type of carrier is a complex multi-frequency wave form, e.g., monopolar chirps, etc., the necessary phase information is not easily retrieved, and an improved form of phase detection is required. Generally, where the telemetry system uses pulsatiles that can be regarded as short spread spectrum RF bursts with wide band signal properties, the receiver must also obtain information about the characteristics of the signal in order to effectively detect it in the presence of noise.
In view of the above, it is seen that what is needed in the art is an improved telemetry system, and in particular, a telemetry system with an improved noise-suppressing telemetry receiver. In particular, the need is to provide demodulation of pulsatile high frequency signals of various forms, e.g., multi-frequency wave forms such as BPSK signals, exponentially decaying sinusoidal signals, etc. In such telemetry systems, pulsatile RF signals are modulated in the transmitter by a data-carrying symbol signal with an accurate symbol rate. This invention uses the inherently accurate symbol rate as a basis for deriving the phase and other characteristics of the transmitted signal, for use in demodulating the RF signals and obtaining the transmitted data.
Further, telemetry receivers for uplinking data for implanted devices such as cardiac pacemakers, can utilize the efficiency and reliability inherently provided by DSP implementation. Examples of such inherent power are seen in cross correlation detection implemented by a finite impulse response (FIR) digital filtering structure, and quadrature demodulation. The potential of DSP based processing in fields such as cardiac pacing systems has been demonstrated. See U.S. Pat. Nos. 5,448,997 and 5,446,246. This invention may utilize the processing power of DSP to enable an improved time discrete system design for suppressing noise and reliably detecting data uplinked from, e.g., an implanted medical device, but also embraces other state-of-the-art embodiments.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a telemetry system having a receiver for noise free reception of uplink telemetry signals, suitably utilizing DSP-based technology. It is a more specific object to provide such a receiver to reliably receive uplink telemetry signals in an environment such as is present with a cardiac pacing system where there is inband noise generated by monitors and other sources, and where the transmitted carrier is unstable but is modulated by a stable symbol signal.
In accordance with the above objects, there is provided a telemetry receiver which utilizes a digital signal processor or other processing circuitry, and provides a technique for developing in the receiver a phase-synchronized replica of the transmitted data symbol signal for use in demodulating the RF signals. In particular, for pulsatile high frequency signals, a synchronized symbol phase signal is utilized for carrier replica detection of the uplinked data, thereby providing data detection even in the presence of noise having frequencies within the receiver bandwidth.
Most current modulator/demodulator telemetry systems use sinusoidal carriers. In such situations that are characterized by moderate or light noise, the phase information of the carrier can be retrieved by complex demodulation, i.e., multiplying the carrier with sine waves and cosine waves. However, the problem becomes more difficult when the carrier is not continuous, but is pulsatile, and even multi-frequency. In continuous carrier spread-spectrum systems, the demodulation is carried out by an early carrier replica signal and a late replica signal to get phase-related information, with a technique called early/late synchronization. In the system of this invention, the frequency, or rate of the incoming pulsatile RF signals is known, but the character of the incoming wave shape may be relatively unknown. Particularly for telemetry signals sent from an implanted device, such as used in biomedical devices such as pacemaker, neurostimulators and the like, the signals have a pulsatile nature. The telemetry uplink signal can be regarded as a short burst of a multi-frequency signal, and is effectively detected in the presence of noise by generating and storing replicas of the pulsatile signals, and providing a correlator or matched filter demodulator. Once the replica has been obtained, the modulation is performed by obtaining a phase locked symbol signal in the receiver, and using this to time the demodulation of the pulsatile symbols.
In a specific preferred embodiment, a free running quadrature oscillator is synchronized with the demodulated RF signal such that the phase difference with respect to each symbol is reduced to zero. When this is achieved, the receiver system is locked, and the locked oscillator signal is representative of the symbol clock in the transmitting device. With the symbol phase available, succeeding data symbols, or pulses, are predicted with great accuracy, and a very narrow detection window, or slot, is generated for detection of the next symbol. Alternately, the locked phase signal is used to generate a symbol replica signal for use in demodulation. By this means, other interfering signals are suppressed, regardless of their frequency. Another advantage that is obtained is the ability to reduce the DSP load to a minimum between predicted symbol detection slots, as the DSP can be turned off outside these slots.
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Introduction to Radar Systems (2ndedition)—Merril I. Skolnik), 1980, pp. 176-178 & 388-392.
Houben Richard
Weijand Koen J.
Berry Thomas G.
Edwards, Jr. Timothy
Horabik Michael
Jaro Michael J.
Medtronic Inc.
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