Continuous glucose monitoring system and methods of use

Description of equivalent WO 2004061420 A2

RELATED APPLICATION This application claims priority under 35 USC §119 to Provisional Patent Application No 60/437,374 filed on December 31, 2002, entitled "Continuous Glucose Monitoring System and Methods of Use", the disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

The present invention relates to continuous glucose monitoring systems. More specifically, the present invention relates to an in- vivo continuous glucose monitoring system which detects glucose levels continuously and transfers the detected glucose level information at predetermined time intervals to data processing devices for monitoring, diagnosis and analysis.

SUMMARY OF THE INVENTION

A continuous glucose monitoring system in accordance with one embodiment of the present invention includes a sensor configured to detect one or more glucose levels, a transmitter operatively coupled to the sensor, the transmitter configured to receive the detected one or more glucose levels, the transmitter further configured to transmit signals corresponding to the detected one or more glucose levels, a receiver operatively coupled to the transmitter configured to receive transmitted signals corresponding to the detected one or more glucose levels, where the transmitter is configured to transmit a current data point and at least one previous data point, the current data point and the at least one previous data point corresponding to the detected one or more glucose levels.

The receiver may be operatively coupled to the transmitter via an RF communication link, and further, configured to decode the encoded signals received from the transmitter.

In one embodiment, the transmitter may be configured to periodically transmit a detected and processed glucose level from the sensor to the receiver via the RF data communication link. In one embodiment, the transmitter may be configured to sample four times every second to obtain 240 data points for each minute, and to transmit at a rate of one data point (e.g., an average value of the 240 sampled data points for the minute) per minute to the receiver.

The transmitter may be alternately configured to transmit three data points per minute to the receiver, the first data point representing the current sampled data, and the remaining two transmitted data points representing the immediately past two data points previously sent to the receiver. In this manner, in the case where the receiver does not successfully receive the sampled data from the transmitter, at the subsequent data transmission, the immediately prior transmitted data is received by the receiver. Thus, even with a faulty connection between the transmitter and the receiver, or a failed RF data link, the present approach ensures that missed data points may be ascertained from the subsequent data point transmissions without retransmission of the missed data points to the receiver.

The transmitter- may be configured to encode the detected one or-m re glucose levels received from the sensor to generate encoded signals, and to transmit the encoded signals to the receiver. In one embodiment, the transmitter may be configured to transmit the encoded signals to the receiver at a transmission rate of one data point per minute. Further, the transmitter may be configured to transmit the current data point and the at least one previous data points in a single transmission per minute to the receiver. In one aspect, the current data point may correspond to a current glucose level, and where the at least one previous, data pQint,rn&y include at least two previous data points corresponding respectively to at least two consecutive glucose levels, the one of the at least two consecutive glucose levels immediately preceding the current glucose level. In a further embodiment, the receiver may include an output unit for outputting the received transmitted signals corresponding to one or more glucose levels. The output unit may include a display unit for displaying data corresponding to the one or more glucose levels, where the display unit may include one of a LCD display, a cathode ray tube display, and a plasma display. The displayed data may include one or more of an alphanumeric representation corresponding to the one or more glucose levels, a graphical representation of the one or more glucose levels, and a three-dimensional representation of the one or more glucose levels. Moreover, the display unit may be configured to display the data corresponding to the one or more glucose levels substantially in real time. Further, the output unit may include a speaker for outputting an audio signal corresponding to the one or more glucose levels.

In yet a further embodiment, the receiver may be configured to store an identification information corresponding to the transmitter.

The receiver may be further configured to perform a time hopping procedure for synchronizing with the transmitter. Alternatively, the receiver may be configured to synchronize with the transmitter based on the signal strength detected from the transmitter, where the detected signal strength exceeds a preset threshold level.

The transmitter in one embodiment may be encased in a substantially watertight housing to ensure continuous operation even in the situation where-the transmitter is in contact with water.

Furthermore, the transmitter may be configured with a disable switch which allows the user to temporarily disable the transmission of data to the receiver when the user is required to disable electronic devices, for example, when aboard an airplane. In another embodiment, the transmitter may be configured to operate in an additional third state (such as under Class B radiated emissions standard) in addition to the operational slate arid the disable sJate„discussed above, so as to allow limited operation while aboard an airplane yet still complying with the Federal Aviation Administration (FAA) regulations. Additionally, the disable switch may also be configured to switch the transmitter between various operating modes such as fully functional transmission mode, post-manufacture sleep mode, and so on. In this manner, the power supply for the transmitter is optimized for prolonged usage by effectively managing the power usage.

Furthermore, the transmitter may be configured to transmit the data to" the receiver in predetermined data packets, encoded, in one embodiment, using Reed Solomon encoding, and transmitted via the RF communication link. Additionally, in a further aspect of the present invention, the RF communication link between the transmitter and the receiver of the continuous glucose monitoring system may be implemented using a low cost, off the shelf remote keyless entry (RKE) chip set. The receiver in an additional embodiment may be configured to perform, among others, data decoding, error detection and correction (using, for example, forward error correction) on the encoded data packets received from the transmitter to minimize transmission errors such as transmitter stabilization errors and preamble bit errors resulting from noise. The receiver is further configured to perform a synchronized time hopping procedure with the transmitter to identify and synchronize with the corresponding transmitter for data transmission. Additionally, the receiver may include a graphical user interface (GUI) for displaying the data received from the transmitter for the user. The GUI may include a liquid crystal display (LCD) with backlighting feature to enable visual display in dark surroundings. The receiver may also include an output unit for generating and outputting audible signal alerts for the user,, or placing the receiver in a vibration mode for alerting the user by vibrating the receiver.

More specifically, in a further aspect, the receiver may be configured to, among others, display the received glucose levels on a display section of the receiver either real time or in response to user request, and provide visual (and/or auditory) notification to the user of the detected glucose levels being monitored. To this end, the receiver is configured to identify the corresponding transmitter from which it is to receive data via the RF data link, by mitially storing, the identification information of the transmitter, and performing a time hopping procedure to isolate the data transmission from the transmitter corresponding to the stored identification information and thus to synchronize with the transmitter. Alternatively, the receiver may be configured to identify the corresponding transmitter based on the signal strength detected from the transmitter, determined to exceed a preset threshold level.

A method in accordance with one embodiment of the present invention includes the steps of receiving an identification information corresponding to a transmitter, detecting data within a predetermined RF transmission range, determining whether the detected data is transmitted from the transmitter, decoding the detected data, 'and generating an output signal corresponding to the decoded data. In one embodiment, the step of determining whether the detected data transmission is transmitted from the transmitter may be based on the received identification information. In another embodiment, the step of determining whether the detected data transmission is transmitted from the transmitter may be based on the signal strength and duration of the detected data within the predetermined RF transmission range.

In a further embodiment, the step of decoding may also include the step of perforating error correction on the decoded data. Moreover, the step of decoding may include the step of performing Reed-Solomon decoding on the detected data. In the manner described, the present invention provides a continuous glucose monitoring system that is simple to use and substantially compact so as to minimize any interference with the user's daily activities. Furthermore, the continuous glucose monitoring system may be configured to be substantially water-resistant so that the user may. freely bathe, swim, or enjoy other water related activities while using the monitoring system. Moreover, the components comprising the monitoring system including the transmitter and the receiver are configured to operate in various modes to enable power savings, and thus enhancing post-manufacture shelf life.

INCORPORATION BY REFERENCE Applicants herein incorporate by reference Application No. 09/753,746 filed on January 2, 2001 entitled "Analyte Monitoring. Device anal Methods of Use" assigned to the Assignee of the present application for all purposes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a continuous glucose monitoring system in accordance with one embodiment of the present invention;

FIG. 2 is a block diagram of the transmitter of the continuous glucose monitoring system shown in FIG. Tin accordance with one embodiment of the present invention; FIG. 3 is a block diagram of the receiver of the continuous glucose monitoring system shown in FIG. 1 in accordance with one embodiment of the present invention; FIG. 4 illustrates a data packet of the transmitter of the continuous glucose monitoring system shown in FIG. 1 in accordance with one embodiment of the present invention;

FIGS. 5A-5C illustrate a data packet table for Reed-Solomon encoding in the transmitter, a depadded data table, and a link prefix table, respectively, in accordance with one embodiment of the continuous glucose monitoring system of FIG. 1 ; and

FIG. 6 is a flowchart illustrating the time hopping procedure for the receiver of the continuous glucose monitoring system shown in FIG. 1 in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a continuous glucose monitoring system 100 in accordance with one embodiment of the present invention. In such embodiment, the continuous glucose monitoring system 100 includes a sensor- L01, a transmitter 102 coupled to the sensor 101, and a receiver 104 which is configured to communicate with the transmitter 102 via a communication link 103. The receiver 104 may be further configured to transmit data to a data processing terminal 105 for evaluating the data received by the receiver 104. Only one sensor 101, transmitter 102, communication link 103, receiver 104, and data processing terminal 105 are shown in the embodiment of the continuous glucose monitoring system 100 illustrated in FIG. 1.

However, it will be appreciated by one of ordinary skill in the art that the continuous glucose monitoring system 100 may include one or more sensor 101, transmitter 102, communication link 103, receiver 104, and data processing terminal 105, where each receiver 104 is uniquely synchronized with a respective transmitter 102. In one embodiment of the present invention, the sensor 101 is physically positioned on the body of a user whose glucose is being monitored. The term user as used herein is intended to include humans, animals, as well as any other who might benefit from the use of the glucose monitoring system 100. The sensor 101 is configured to continuously sample the glucose level of the user and convert the sampled glucose level into a corresponding data signal for transmission by the transmitter 102. h one embodiment, the transmitter 102 is mounted on the sensor 101 so that both devices are positioned on the user's body. The transmitter 102 performs data processing such as filtering and encoding on data signals, each of which corresponds to a sampled glucose level of the user, for transmission to the receiver 104 via the communication link 103. In one embodiment, the continuous glucose monitoring system 100 is configured as a one-way RF communication path from the transmitter 102 to the receiver 104. In such embodiment, the transmitter 102 transmits the sampled data signals received from the sensor 101 without acknowledgement from the receiver 104 that the transmitted sampled data signals have been received. For example, the transmitter 102 may be configured to transmit the encoded sampled data signals at a fixed rate (e.g., at one minute intervals) after the completion of the initial power on procedure. Likewise, the receiver 104 may be configured to detect such transmitted . encoded sampled data signals at predetermined time intervals.

As-discussedin further detail below, in one-embodiment of -the resent invention the receiver 104 includes two sections. The first section is an analog interface section that is configured to communicate with the transmitter 102 via the communication link 103. In one embodiment, the analog interface section may include an RF receiver and an antenna for receiving and amplifying the data signals from the transmitter 102, which are thereafter, demodulated with a local oscillator and filtered through a band-pass filter. The second section of the receiver 104 is a data processing section which is configured to process the data signals received from the transmitter 102 such as by performing data decoding, error detection and correction, data clock generation, and data bit recovery.

In operation, upon completing the power-on procedure, the receiver 104 is configured to detect the presence of the transmitter 102 within its range based on the strength of the detected data signals received from the transmitter 102. For example, in one embodiment, the receiver 104 is configured to detect signals whose strength exceeds a predetermined level to identify the transmitter 102 from which the receiver 104 is to receive data. Alternatively, the receiver 104 in a further embodiment may be configured to respond to signal transmission for a predetermined transmitter identification information of a particular transmitter 102 such that, rather than detecting the signal strength of a transmitter 102 to identify the transmitter, the receiver 104 may be configured to detect transmitted signal of a predetermined transmitter 102 based on the transmitted transmitter identification information corresponding to the pre-assigned transmitter identification information for the particular receiver 104.

In one embodiment, the identification information of the transmitters 102 includes a 16-bit ID number. In an alternate embodiment, the ID number may be a predetermined length including a 24-bit ID number or a 32-bit ID number. Further, any other length ID number may also be used. Thus, in the presence of multiple transmitters 102, the receiver 104 will only recognize the transmitter 102 which corresponds to the stored identification information. Data signals transmitted from the other transmitters within the range of the receiver 104 are considered invalid signals.

Referring .again- to FIG. 1, -where the receiver 1044etermines he corresponding transmitter 102 based on the signal strength of the transmitter 102, when the receiver 104 is initially powered-on, the receiver 104 is configured to continuously sample the signal strength of the data signals received from the transmitters within its range. If the signal strength of the data signals meets or exceeds the signal strength threshold level and the transmission duration threshold level, the receiver 104 returns a positive indication for the transmitter 102 transmitting the data signals. That is, in one embodiment, the receiver 104 is configured to positively identify the transmitter 102 after one data signal transmission. Thereafter, the receiver 104 is configured to detect positive indications for three consecutive data signals transmissions for a predetermined time period. At such point, after three consecutive transmissions, the transmitter 102 is fully synchronized with the receiver 104.

Upon identifying the appropriate transmitter 102, the receiver 104 begins a decoding procedure to decode the received data signals.. In one embodiment, a sampling clock signal may be obtained from the preamble portion of the received^* data signals. The decoded data signals, which include fixed length data fields, are then sampled with the sampling clock signal. In one embodiment of the present invention, based on the received data signals and the time interval between each of the three data signal transmissions, the receiver 104 determines the wait time period for receiving the next transmission from the identified and synchronized transmitter 102. Upon successful synchronization, the receiver 104 begins receiving from the transmitter 102 data signals corresponding to the user's detected glucose level. As described in further detail below, the receiver 104 in one embodiment is configured to perform synchronized time hopping with the corresponding synchronized transmitter 102 via the communication link 103 to obtain the user's detected glucose level.

Referring yet again to FIG. 1, the data processing terminal 105 may include a personal computer, a portable computer such as a laptop or a handheld device (e.g., personal digital assistants (PDAs)), and the like, each of which is configured for data communication with the receiver via a wired or a wireless connection. Additionally, the data processing terminal 105 may further be connected to a data network (not <img file="WO2004061420A2_D0001.tif" /> updating data corresponding to the detected glucose level of the user.

FIG. 2 is a block diagram of the transmitter 102 of the continuous glucose monitoring system 100 in accordance with one embodiment of the present invention. The transmitter 102 includes an analog interface 201 configured to communicate with the sensor 101 (FIG. 1), a user input 202, and a temperature detection section 203, each of which is operatively coupled to a transmitter processor 204 such as a central processing unit (CPU). Further shown in FIG. 2 are a transmitter serial communication section 205 and an RF transmitter 206, each of which is also operatively coupled to the transmitter processor 204. Moreover, a power supply 207 is also provided in the transmitter 102 to provide the necessary power for the transmitter 102. Additionally, as can be seen from the Figure, clock 208 is provided to, among others, supply real time information to the transmitter processor 204.

In one embodiment, a unidirectional input path is established from the sensor 101 (FIG. 1) and/or manufacturing and testing equipment to the analog interface 201, while a unidirectional output is established from the output of the RF transmitter 206. In this manner, a data path is shown in FIG. 2 between the aforementioned unidirectional input and output via a dedicated link 209 from the analog interface 201 to serial communication section 205, thereafter to the processor 204, and then to the RF transmitter 206. As such, in one embodiment, through the datapath described above, the transmitter 102 is configured to transmit to the receiver 104 (FIG. 1), via the communication link 103 (FIG. 1), processed and encoded data signals received from the sensor 101 (FIG. 1). Additionally, the unidirectional communication data path between the analog interface 201 and the RF transmitter 206 discussed above allows for the configuration of the transmitter 102 for operation upon completion of the manufacturing process as well as for direct communication for diagnostic and testing purposes. Referring back to FIG. 2, the user input 202 includes a disable device that allows the operation of the transmitter 102 to be temporaiily disabled, such as, by the user wearing the transmitter 102. In an alternate embodiment, the disable device of the user input 202 may be configured to initiate the power-up procedure of the transmitter 1-02. As discussed above, the transmitter processor 204 is configured to transmit control signals to the various sections of the transmitter 102 during the operation of the transmitter 102. In one embodiment, the transmitter processor 204 also includes a memory (not shown) for storing data such as the identification information for the transmitter 102, as well as the data signals received from the sensor 101. The stored information may be retrieved and processed for transmission to the receiver 104 under the control of the transmitter jprocessor 204. Furthermore, the power supply 207 may include a commercially available battery pack.

The physical configuration of the transmitter 102 is designed to be substantially water resistant, so that it may be immersed in non-saline water for a brief period of time without degradation in performance. Furthermore, in one embodiment, the transmitter 102 is designed so that it is substantially compact and light-weight, not weighing more that a predetermined weight such as, for example, approximately 18 grams. Furthermore, the dimensions of the transmitter 102 in one embodiment includes 52 mm in length, 30 mm in width and 12 mm in thickness. Such small size and weight enable the user to easily carry the transmitter 102. The transmitter 102 is also configured such that the power supply section 207 is capable of providing power to the transmitter for a minimum of three months of continuous operation after having been stored for 18 months in a low-power (non- operating) mode. In one embodiment, this may be achieved by the transmitter processor 204 operating in low power modes in the non-operating state, for example, drawing no more than approximately 1 μiA. Indeed, in one embodiment, the final step during the manufacturing process of the transmitter 102 places the transmitter 102 in the lower power, non-operating state (i.e., post-manufacture sleep mode). In this manner, the shelf life of the transmitter 102 may be significantly improved. Referring again to FIG. 2, the analog interface 201 of the transmitter 102 in one embodiment includes a sensor interface (not shown) configured to physically couple to the various sensor electrodes (such as, for example, working electrode, reference electrode, counter electrode, (not shown)) of the sensor 101 (FIG.l) of the monitoring system 100. -The analog interface section 2Θ4 further includes Ά potentiostat circuit (not shown) which is configured to generate the Poise voltage determined from the current signals received from the sensor electrodes. In particular, the Poise voltage is determined by setting the voltage difference between the working electrode and the reference electrode (i.e., the offset voltage between the working electrode and the reference electrode of the sensor 102). Further, the potentiostat circuit also includes a transimpedance amplifier for converting the current signal on the working electrodpinto a corresponding voltage signal proportional to the current. The signal from the potentiostat circuit is then low pass filtered with a predetermined cut-off frequency to provide anti-aliasing, and thereafter, passed through a gain stage to provide sufficient gain to allow accurate signal resolution detected from the sensor 101 for analog-to-digital conversion and encoding for transmission to the receiver 104.

Referring yet again to FIG. 2, the temperature detection section 203 of the transmitter 102 is configured to monitor the temperature of the skin near the sensor insertion site. The temperature reading is used to adjust the glucose readings obtained from the analog interface 201. As discussed above, the input section 202 of the transmitter 102 includes the disable device which allows the user to temporarily disable the transmitter 102 such as for, example, to comply with the FAA regulations when aboard an aircraft. Moreover, in a further embodiment, the disable device may be further configured to interrupt the transmitter processor 204 of the transmitter 102 while in the low power, non-operating mode to initiate operation thereof. The RF transmitter 206 of the transmitter 102 may be configured for operation in the frequency band of 315 MHz to 322 MHz, for example, in the United States. Further, in one embodiment, the RF transmitter 206 is configured to modulate the carrier frequency by performing Frequency Shift Keying and Manchester encoding. In one embodiment, the data transmission rate is 19,200 symbols per second, with a minimum transmission range for communication with the receiver 104.

FIG. 3 is a block diagram of the receiver 104 of the continuous glucose monitoring system 100 in accordance with one embodiment of the present invention. Referring to FIG. 3, the receiver 104 includes a blood glucose test strip interface 301, an RF receiver 302, an-inout 303, a temperature detection section 304.. and a clock 305, each of which is operatively coupled to a receiver processor 307. As can be further seen from the Figure, the receiver 104 also includes a power supply 306 operatively coupled to a power conversion and monitoring section 308. Further, the power conversion and monitoring section 308 is also coupled to the receiver processor 307. Moreover, also shown are a receiver serial communication section 309, and an output 310, each operatively coupled to the receiver processor 307.

In one embodiment, the test strip interface 301 includes a glucose level testing portion to receive a manual insertion of a glucose testing strip, and thereby determine and display the glucose level of the testing strip on the output 310 of the receiver 104. This manual testing of glucose can be used to calibrate sensor 101. The RF receiver 302 is configured to communicate, via the communication link 103 (FIG. 1) with the

RF transmitter 206 of the transmitter 102, to receive encoded data signals from the transmitter 102 for, among others, signal mixing, demodulation, and other data processing. The input 303 of the receiver 104 is configured to allow the user to enter information into the receiver 104 as needed. In one aspect, the input 303 may include one or more keys of a keypad, a touch-sensitive screen, or a voice-activated input command unit. The temperature detection section 304 is configured to provide temperature information of the receiver 104 to the receiver processor 307, while the clock 305 provides, among others, real time information to the receiver processor 307.

Each of the various components of the receiver 104 shown in FIG. 3 are powered by the power supply 306 which, in one embodiment, includes a battery.

Furthermore, the power conversion and monitoring section 308 is configured to monitor the power usage by the various components in the receiver 104 for effective power management and to alert the user, for example, in the event of power usage which renders the receiver 104 in sub-optimal operating conditions. An example of such sub-optimal operating condition may include, for example, operating the vibration output mode (as discussed below) for a period of time thus substantially draining the power supply 306 while the processor 307 (thus, the receiver 104) is turned on. Moreover, the power conversion and monitoring section 308 may additionally be configured to include a reverse polarity-pjrpteetion circuit such as a field effect transistor (FET) configured as a battery activated switch.

The serial communication section 309 in the receiver 104 is configured to provide a bi-directional communication path from the testing and/or manufacturing equipment for, among others, initialization, testing, and configuration of the receiver 104. Serial communication section 104 can also be used to upload data to a computer, such as time-stamped blood glucose data. The communication link with an external device (not shown) can be made, for example, by cable, infrared (IR) pr RF link. The output 310 of the receiver 104 is configured to provide, among others, a graphical user interface (GUI) such as a liquid crystal display (LCD) for displaying information. Additionally, the output 310 may also include an integrated speaker for outputting audible signals as well as to provide vibration output as commonly found in handheld electronic devices, such as mobile telephones presently available. In a further embodiment, the receiver 104 also includes an electro-luminescent lamp configured to provide backlighting to the output 310 for output visual display in dark ambient surroundings. Referring back to FIG. 3, the receiver 104 in one embodiment may also include a storage section such as a programmable, non-volatile memory device as part of the processor 307, or provided separately in the receiver 104, operatively coupled to the processor 307. The processor 307 is further configured to perform Manchester decoding as well as error detection and correction upon the encoded data signals received from the transmitter 102 via the communication link 103. In conjunction with FIGS. 4 and 5 A-5C, a description is provided of a data packet from the transmitter 102 to the receiver 104 via the communication link 103. FIG. 4 illustrates a data pack from the transmitter 102 (FIG. 1) in accordance with one embodiment of the present invention. Referring to FIG. 4, each data packet from the transmitter 102 includes 13 bytes as shown in the Figure. For example, the first byte (zero byte) includes the transmitter 104 identification information ("Tx

ID"), while the third byte (byte two) provides transmitter status information, where a high nibble (byte) indicates an operating mode status, while a low nibble indicates a non-operating mode. In this manner, the signals received from the sensor 101 are packed into 13-hyte data acks,-for transmission to the receiver 104. FIGS. 5A-5C illustrate a data packet table for Reed-Solomon encoding in the transmitter, a depadded data table, and a link prefix table, respectively, in accordance with one embodiment of the continuous glucose monitoring system of FIG. 1. Referring to FIG. 5A, it can be seen that the Reed Solomon encoded data block contents include 13 bytes of packed data (FIG. 4), one byte of the middle significant bit of the transmitter identification information (Tx ID), one byte of the most significant bit of the transmitter identification information, 232 bytes of zero pads, 8 bytes of parity symbols, to comprise a total of 255 bytes. In one embodiment, the Reed Solomon encode procedure at the transmitter 102 uses 8 bit symbols for a 255 symbol block to generate 8 parity symbols. Thereafter, the transmitter 102 is configured to remove the 232 bytes of zero pads, resulting in the 21 bytes of depadded data block including the 13 bytes of packed data as well as the 8 bytes of the parity symbols as shown in FIG. 5B.

Thereafter, a link prefix is added to the depadded data block to complete the data packet for transmission to the receiver 104. The link prefix allows the receiver 104 to synchronize with the transmitter 102 as described in further detail below.

More specifically, as shown in FIG. 5C, the transmitter 102 is configured to add 4 bytes of link prefix (0x00, 0x00, 0x12, and 0x34) to the 21 bytes of depadded data block to result in 24 bytes of data packet. Once powered up and enabled in operational mode, the transmitter 102 is configured to transmit the 24 byte data packet once every minute. More specifically, the transmitter 102 is configured to Manchester encode the data at 2 bits per data bit (0 = 10; 1 = 01), and transmit the

Manchester bits at 19,200 symbols per second. The transmitter 102 is configured to transmit the data packets with the most significant bit of byte zero first.

FIG. 6 is a flowchart illustrating the time hopping procedure for the receiver of the continuous glucose monitoring system shown in FIG. 1 in accordance with one embodiment of the present invention.

Referring to FIG. 6, upon completing the power up procedure as discussed above, the receiver 104 listens for the presence of a transmitter within the RF communication link range. When the transmitter 102 is detected within the RF communication link range, the receiver 1- 04 is configured to- receive and store the identification information corresponding to the detected transmitter 102. Thereafter, at step 602, the receiver 104 is configured to detect (or sample) data transmission within its RF communication range. In one aspect, the receiver 104 is configured to identify a positive data transmission upon ascertaining that the data transmission is above a predetermined strength level for a given period of time (for example, receiving three separate data signals above the predetermined strength level from the transmitter 102 at one minute intervals over a period of five minute_s).

At step 603, the receiver 104 is configured to determine whether the detected signals within the RF communication range is transmitted from the transmitter 102 having the transmitter identification information stored in the receiver 104. If it is determined at step 603 that the detected data transmission at step 602 does not originate from the transmitter with the stored transmitter identification information, then the procedure returns to step 602 and awaits for the detection of the next data transmission.

On the other hand, if at step 603 it is determined that the detected data transmission is from the transmitter 102 corresponding to the stored transmitter identification information, then at step 604, the receiver proceeds with decoding the received data and performing error correction thereon. In one embodiment, the receiver is configured to perform Reed-Solomon decoding, where the transmitted data received by the receiver is encoded with Reed-Solomon encoding. Furthermore, the receiver is configured to perform forward ercor correction to minimize data error due to, for example, external noise, transmission noise and so on.

Referring back to FIG. 6, after decoding and error correcting the received data, the receiver 104 at step 605 generates output data corresponding to the decoded error corrected data received from the transmitter 102, and thereafter, at step 606, the receiver 104 outputs the generated output data for the user as a real time display of the output data, or alternatively, in response to the user operation requesting the display of the output data. Additionally, before displaying the output data for the user, other pre-processing procedures may be performed on the output data to for example, smooth out the output signals. In one aspect, the generated output data may include a visual graphical-output displayed n the graphical -user interface of the receiver. Alternatively, the output data may be numerically displayed representing the corresponding glucose level.

Referring now to FIGS. 4 and 6, the time hopping procedure of one embodiment is described. More specifically, since more than one transmitter 102 may be within the receiving range of a particular receiver 104, and each transmitting data every minute on the same frequency, transmitter units 102 are configured to transmit data packets at different times to avoid co-location collisions (that is, where one or more receivers 104 cannot discern the data signals transmitted by their respective associated transmitter units 102 because they are transmitting at the same time.) In one aspect, transmitter 102 is configured to transmit once every minute randomly in a window of time of plus or minus 5 seconds (i.e., it time hops.) To conserve power, receiver 104 does not listen for its associated transmitter 102 during the entire 10 second receive window, but only at the predetermined time it knows the data packet will be coming from the corresponding transmitter 102. In one embodiment, the 10 second window is divided into 400 different time segments of 25 milliseconds each. Before each RF transmission from the transmitter 102 takes place, both the transmitter 102 and the receiver 104 is configured to recognize in which one of the 400 time segments the data transmission will occur (or in which to start, if the transmission time exceeds 25 milliseconds.) Accordingly, receiver 104 only listens for a RF transmission in a single 25 millisecond time segment each minute, which varies from minute to minute within the 10 second time window.

Moreover, each transmitter 102 is configured to maintain a "master time" clock that the associated receiver unit 104 may reference to each minute (based on the time of transmission and known offset for that minute.). A counter also on the transmitter 102 may be configured to keep track of a value "Tx Time" that increments by 1 each minute, from 0 to 255 and then repeats. This Tx Time value is transmitted in the data packet each minute, shown as Byte 1 in FIG. 4. Using the Tx Time value and the transmitter's unique identification information (TX ID, shown as Byte 0 in FIG. 4), both the transmitter 102 and the receiver 104 can calculate which of the 400 .time segments will be used for -the subsequent transmission, hi one embodiment_s the function that is used to calculate the offset from the master clock 1 -minute tick is a pseudo-random number generator that uses both the Tx Time and the TX ID as seed numbers. Accordingly, the transmission time varies pseudo-randomly within the 10 second window for 256 minutes, and then repeats the same time hopping sequence again for that particular transmitter 102. hi the manner described above, in accordance with one embodiment of the present invention, co-location collisions may be_. ayoided with the above.described time hopping procedure. That is, in the event that two transmitters interfere with one another during a particular transmission, they are not likely to fall within the same time segment in the following minute. As previously described, three glucose date points are transmitted each minute (one current and two redundant/historical), so collisions or other interference must occur for 3 consecutive data transmissions for data to be lost. In one aspect, when a transmission is missed, the receiver 104 may be configured to successively widen its listening window until normal transmissions from the respective transmitter 102 resume. Under this approach, the transmitter listens for up to 70 seconds when first synchronizing with a transmitter 102 so it is assured of receiving a transmission from transmitter 102 under normal conditions. Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.