RF Telemetry for Medical Implants

Kirk Snodgrass

Presented at

MEPTEC/SMTA Medical Electronics Symposium Sept 27 & 28, 2011

RF Telemetry Advantages

MICS MEDS   MICS=Medical Implant

Communications Service (402 – 405 MHz)

  Implant to Instrument   Tx/Rx   25 uW (-16 dBm) EIRP   Max 300 kHz per channel   Listen Before Talk, Clear

Chanel Assessment

  MEDS=Medical Data Services (401 – 402 MHz and 405 – 406 MHz)

  Implant to Implant, Implant to Instrument, or Instrument to Instrument

  Allows Tx-only at 250 nw (-36 dBm) EIRP or 25 uW if LBT, CCA

  Max 100 kHz per channel   Facilitates WBANs &

sensors

MICS and MEDS

MEDS

404.85

404.55

404.25

403.95

403.65

403.35

403.05

402.75

402.45

402.15

401-402 MHz

MICS (402-405 MHz)

405-406 MHz

MEDS

MICS= Medical Implant Communication Service (Implant to Instrument)

MEDS= Medical Data Service (WBAN)

Frequency Considerations   ISM bands offer many frequency choices, but

are heavily used. Interference is an issue.   Lower frequencies are less attenuated in the

body. Losses are proportional to frequency.   Higher frequencies allow smaller antenna size.

(Dk of muscle and water are approx 64—shortens antenna 8x from air)

  MICS/MEDS band has low interference (Shared services are Wx balloons and satellites) and employs Clear Channel Assessment and Listen Before Talk.

MICS/MEDS Frequency Band

MICS/ MEDS Allocation (401 - 406 MHz)

A Little under the Weather!

7

AM Broadcast Monopole

Monopole in the Header of an ICD (402 – 405 MHz MICS application)

Signal Characteristics

  Bandwidth requirements for telemetry applications.   Control functions are slow, and only need a few kHz.   Real Time monitor functions can take up to 100 kHz.   Factory test, code maintenance, etc. Need all the

speed they can get! Hundreds of kHz are common.   Typical Modulation types are FSK, BPSK,

QPSK, etc. Constant amplitude conserves bandwidth and eases transmitter linearity requirements.

QPSK Constellation (2 Bits per Symbol)

QPSK signal with noise

Error Vector Magnitude (EVM)

QPSK signal with Phase Noise

Forward Error Correction  Corrects randomly distributed errors.  Greatly reduces error rate with low overhead.  Reed-Solomon is used on CD’s, DVD’s, satellite

communications, deep space probes.....

Data Security

 AES (Advanced Encryption Standard) is a robust block cipher found in common applications such as Zigbee, 802.11, etc.

Direct Conversion Receiver

 Direct conversion to base band— results in a Zero IF which simplifies filtering

 Uses Local Oscillator operating at the receive RF frequency

 Requires careful management of the LO leakage as well as DC offsets

SAW Filter   Piezoelectric device that launches transverse Surface

Acoustic Waves across a substrate (Lithium Tantalate or Lithium Niobate are common)

  Interdigital combs determine the frequency response.   Special structures suppress spurious responses and

minimize insertion loss.   The SAW filters for 400 MHz operation are as small as 2

square mm!

400 MHz SAW Filter Broadband Response

Range and Power   Range needs to be arm’s length to room size.

This requires an implanted transmitter output of 0.1 mW to 1 mW (400MHz).

  Battery energy is limited and battery impedances are high. Low current and low voltage circuits are a must!

  Duty Cycling is needed to further reduce current drain. Wake-up strategy syncs the implant with the programmer/instrument.

Early Biomed Engineer

Gonna be tough to implant!

Crosstalk

  RF testing of telemetry can require shielding or screen room isolation to avoid crosstalk between testers. This is especially important for receiver sensitivity measurements.

Telemetry Standards   IEEE 802.15.6 Wireless Body Area Networks   ETSI TR 102 343 (V1.1.1)   ETSI EN 301 839-1 "Electromagnetic compatibility and Radio

spectrum Matters (ERM);Radio equipment in the frequency range 402 MHz to 405 MHz for Ultra Low Power Active Medical Implants and Accessories

  FCC Rules and Regulations, “MICS Band Plan”, Part 95, Jan. 2003   47 CFR 95.601-95.673 Subpart E, Federal Communications

Commission, 1999   ITU-R Recommendation SA.1346 (1998) (revised to RS.1346)

(MICS)

Questions?