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REVIEW: Body Area Network gets under your skin

Zarlink Introduces Industry's Highest Performance Implantable Grade Radio Chip for In-Body Communication Systems

* Highly integrated ultra low-power RF (radio frequency) chip with high-performance MAC (media access controller) delivers data rates up to 800 kbps, operates in MICS (Medical Implant Communication Service) 402-405 MHz band

* Applications include implanted pacemakers, defibrillators, neurostimulators, implantable drug pumps and physiological monitors

OTTAWA, CANADA—Zarlink Semiconductor Inc. introduces the ZL70101 transceiver chip, an ultra low-power RF system-on-a-chip solution for use in implanted medical devices, programmers, and monitoring base stations.

Building on Zarlink's MICS technology platform, the ZL70101 transceiver chip delivers high data rates, low power consumption and unique wake-up circuitry. Using Zarlink's MICS technology, medical device manufacturers can design in-body communication systems that will improve patient care, lower healthcare costs, and support new monitoring, diagnostic and therapeutic applications.

Previous home health-monitoring systems required the patient to accurately position an inductive wand over the implanted device. In comparison, using Zarlink MICS technology, patient health and implanted device performance data can be stored in the implanted medical device's memory and wirelessly transmitted to a base station, without requiring patient intervention.

Data can then be forwarded over the telephone or Internet to a physician's office. If a problem is detected, the physician will schedule a patient follow-up visit where the two-way RF link can be used to interrogate and adjust implanted device performance.

During surgery to implant medical devices, the longer operating range of the ZL70101 chip allows the base station/programmer to be located outside the sterile environment. This potentially shortens surgery times and reduces healthcare costs, as programming equipment does not have to be sterilized for use in the operating room.

Ultra low-power RF technology is also enabling a range of new diagnostics and therapies, including implanted devices used to monitor and treat diabetes, neurostimulators that alleviate chronic pain or lessen the debilitating effects of Parkinson's disease and dystonia, and gastric stimulators that may offer a viable alternative in the treatment of obesity.

“As in-body communication systems evolve to support advanced diagnostics and therapies, it's critical that radio performance does not impact the battery life of an implanted medical device,” said Steve Swift, senior vice president and general manager, Ultra Low-Power Communications, Zarlink Semiconductor. “The ZL70101 transceiver offers unparalleled data rates and ultra low-power consumption performance in a highly integrated package, backed by Zarlink's established expertise in meeting the unique quality requirements for devices intended for human implant.”

Wake-up circuitry supporting ultra low-power performance

To help conserve implanted medical device battery life, in-body communication systems transmit data on a scheduled or as-required basis.

The ZL70101 transceiver incorporates a unique “wake-up” receiver that allows the integrated circuit to operate in an extremely low current 250 nA (nanoamp) “sleep” mode. Communication is then initiated using a specially coded wake-up signal from the base station transmitter. The implanted medical device can also wake up the ZL70101 radio on detection of an emergency medical event. An emergency signal could then be sent to the base station, which in turn could directly alert paramedics.

When in full operation the ZL70101 typically consumes 5 mA (milliamps) of supply current. By using the high data rate with heavy duty-cycling, the average power consumed by the ZL70101 can be very small. This conserves overall implanted medical device battery life.

Highly integrated solution with on-chip MAC

The highly integrated ZL70101 system-on-chip includes a MAC that implements a communication protocol specifically designed for the requirements of high-reliability implanted medical devices and is fully compliant to current MICS standards. The MAC protocol includes Reed-Solomon forward error correction together with CRC (cyclical redundancy check) error detection and retransmission to achieve an extremely reliable data link. The chip requires just three external components, excluding antenna matching, allowing device manufacturers to use board space savings to increase battery size and support advanced functionality while lowering overall system BoM (bill of material)
cost.

Availability and packaging

The ZL70101 transceiver chip is available as implantable-grade wire-bondable die or in a 48-pin QFN (quad flat no-lead) package for the non-implanted base station applications. The chip is fully supported by a reference system and application development kit. Full ZL70101 product information, including complete data sheet, design manual and pricing, is available for qualified customers. For more information please visit: http://products.zarlink.com/product_profiles/ZL70101.htm

Some implantable medical devices, such as pacemakers, neuro-stimulators, drug pumps, and monitors, rely on wireless links to provide two-way communications between the device and doctors. These devices arguably improve patient care and sometimes lower healthcare costs, but their designers often have to sacrifice performance because of limited battery life.

Zarlink Semiconductor 's rather sophisticated ZL70101 packet transceiver promises to change that picture. Not a run of the mill RF SoC (system-on-a-chip), the ZL70101 implements a robust architecture that can survive bodily implantation, provide very long run times, and be immune to failure. The latter is especially significant for medical systems OEMs.

Right now, Zarlink's device can be ordered as an implantable-grade bare die, or as a 48-pin QFN -packaged surface-mount part.

RF range is rated at greater than two meters (6.6-ft).

Special Allocation

A word about the MICS (Medical Implant Communication Service) band that's used for Zarlink's transceiver. It's an FCC (Federal Communications Commission) authorized band of frequencies specifically set aside for ultra-low power, unlicensed operation. The FCC permits transmitting data in this band for diagnostic or therapeutic functions associated with implanted medical devices.

MICS permits individuals and medical practitioners to use implanted devices, such as pacemakers and defibrillators, without causing interference to other users of the spectrum. No licensing is required, but MICS equipment must be operated by a duly authorized healthcare pro. What's more, each device must be registered for compliance and spectrum conflict avoidance.

MICS transmitters are similar to those used in FCC-designated Personal Radio Services equipment. However, the 402-MHz through 405-MHz band is available for MICS on a shared secondary basis. Note that the 402-405-MHz band is also compatible with international frequency allocations, so these frequencies pose a limited risk of interference to other radio operations in the band.

In addition to FCC regulations, Zarlink's all-CMOS ZL70101 also meets ETSI (European Telecommunications Standards Institute), and IEC (International Electrotechnical Commission) requirements. It can be used in base stations as well as implantable devices.

Battery-Conserving Modes

As mentioned in Zarlink's press release (on the left), the flea-power ZL70101 transceiver chip supports several battery-conserving wake-up modes. For one, low-power operation is ensured by using a 2.45-GHz ISM (industrial, scientific, medical) band wake-up receiver option.

Communication between implanted and base station transceivers is initiated using a coded wake-up signal from a 2.45-GHz transmitter. Once the system is started by receiving the 2.45-GHz wakeup signal, data is exchanged using the 402-405-MHz MICS-band circuitry.

In use, periodically an implanted transceiver would listen for a base station that wants to begin communication. This sniffing operation is operated frequently enough to provide reasonable start-up latency. It also ensures very low current draw, since it occurs regularly. It also offers a degree of noise immunity that might invoke an erroneous startup.

No LO

OOK (on-off keyed) modulation is used, since it removes the need for an LO (local oscillator) and synthesizer in this portion of the receiver (see block diagram). Operating at 2.45-GHz, and at 100-mW effective radiated power, that's also up to 36-dB higher in power than the 25-uW maximum permitted in the MICS band.


Click for larger simplified block diagram

The wakeup system uses an ultra low-power RF receiver to read the OOK transmitted data. The OOK receiver looks for a specific data packet that's transmitted from a base station. If found, it switches on power to the rest of the chip.

Alternative wake-up mechanisms can use 400-MHz signals. Direct wake-up by the implanted medical device is also supported. In operation, the ZL70101 transceiver has a peak current demand of less than 5-mA, operating from supply voltages between 2.1-V and 3.5-V. Note that the chip's 5-mA current demand not only includes the device's RF stages, but also the MAC (media access controller).

Error Correction

In addition to its power-savings modes, the device's MAC provides encoding and de-coding of digital RF signals, together with the Reed-Solomon error correction. According to Zarlink, Reed-Solomon codes are especially good at correcting burst errors.

The MAC ensures high integrity data, and it automatically performs much of the required link maintenance. Furthermore, the MAC protocol offers a power-save timer that turns off the IC's receiver in the implant for a programmable time after transmitting a packet. This helps conserve power if the implant momentarily has no information to send.

The MAC is complemented by CRC (cyclical redundancy check) error detection. Error handling mechanisms include re-transmissions of lost or corrupted data packets, as well as flow control, FEC (forward-error correction) and error detection. An SPI (serial peripheral interface) is used for access in base station applications or during development.

Higher Data Rates

On the RF side, the system supports data rates from 200-kbits/s to 800-kbits/s. With the higher data rates, patient events can be captured in an implanted device's memory and then rapidly uploaded to a base station for analysis.

This a speed and range advantage over predecessor low-frequency inductive links that have been the prevalent method of medical device communication. Inductive link systems typically operate in 10-kHz to 100-kHz range, with much slower data rates of about 1-kbit/s to perhaps 30-kbits/s.

Such low-power systems, using small coiled antennas in the implantable device, are robust and reliable, but antenna size and power limitations often result in low magnetic field-strengths. External readers/programmers sometimes actually have to make contact with the skin of a patient in order to communicate with the implanted device.

Buffering Data

Zarlink's 405-MHz-band device also uses duty-cycling, buffering low data-rate information and then transmitting in bursts at a higher data rate. This reduces average current demand. Sending data in short bursts also reduces the potential time window for interference from other RF emitters.

High data rates also minimize the probability of interference in burst-noise cases that are often observed in a medical RF environment. In addition, power supply decoupling requirements are more forgiving in systems with high battery impedance.

The ZL70101 uses either 2FSK or 4FSK modulation, achieving 200-ksymbol/s or 400-ksymbol/s data rates using varying frequency deviations. Lower data rates and correspondingly higher receiver sensitivity can also be attained using off-chip digital filtering.

Finally, the device has a MAC bypass mode of operation in which the radio is fully accessible. In this configuration, you can develop custom protocols and data rates.

Low BER

If you use Zarlink's protocol, it will give you error detection and correction that ensures an effective BER (bit error rate) is better than 1.5 x 10-10 given a raw radio BER of 10-3 . Zarlink's protocol is also capable of sending MICS emergency command and high priority messages.

The protocol also serves as a link watchdog to ensure a link is shut down after five seconds without successful communication. Finally, the protocol has provision for link quality diagnostics and control of automatic calibration.

Built-In Impedance Matching

An intriguing aspect of Zarlink's chip is its built-in antenna matching capacitors. Although the company is rather tight-lipped about its design (except to actual customers), Zarlink notes that the impedance matching capacitors are used in both the chip's receiver front-ends (at MICS and ISM frequencies) and the chip's transmitter output stages.

The antenna matching capacitor banks on all of the IC's RF ports support fine-tuning of the matching network for maximum output power for a given power setting, and optimum receiver noise-figure. The antenna tuning is an automatic calibration that uses a peak-detector coupled to an A/D (analog-to-digital converter), along with a state-machine for calibration control.

The output power of the chip's transmitter PA (power amplifier) is also register-programmable in less than 3-dB steps from -4.5-dBm to -17-dBm.

On the receiver side, the chip's sub-system amplifies 400-MHz incoming MICS-band signals, down-converting to the system's IF (intermediate frequency). A LNA (low-noise amplifier) is provided. The LNA's gain is also programmable from 9-dB to 35-dB. Higher gain settings are for implanted medical device transceivers. Zarlink recommends lower gain settings for base station transceivers that can use an external LNA.

Programmable Mixer Bias

The chip's programmable LNA is abetted by programmable mixer bias-current as well. This provides flexibility in optimizing linearity for best third-order intercept point operation, as well as dissipation and noise-figure.

At the IF, a poly-phase filter suppresses interference at the image frequency, and limits noise bandwidth. The device also includes appropriate limiters and a RSSI (received signal strength indicator) block. The RSSI measurement is converted by a 5-bit A/D, and can be read out of the SPI interface. This is useful for performing a MICS clear-channel assessment procedure.

Although Zarlink Semiconductor will only share details with qualified customers at this time, the ZL70101 transceiver chip shapes up as an intriguing device with a low-power wakeup method that could be used as a standalone wakeup system for other communication applications.

For example, it could serve in an IEEE- 802.11 wireless LAN battery-operated system requiring very long battery life in an application where battery replacement would be difficult or costly. The chip's 2.45-GHz wakeup system could be used to minimize sleep/sniffing current and only operate the much higher power 802.11 protocol when a communication session is desired.

Click here for a datasheet (in Adobe Acrobat .PDF format).

For more details contact Zarlink Semiconductor Inc., 400 March Rd., Ottawa, Canada K2K 3H4. Phone: 613 592-0200 or 1-800-325-4927. Fax: 613 592-1010. E-mail: corporate@zarlink.com

Zarlink Semiconductor , 613 592-0200, www.zarlink.com

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