Medical Device Link.

Originally Published MEM Fall 2004

WIRELESS TECHNOLOGY

Designers of wireless devices can be sure to select the right components if they first do a little math.

Carl Falcon

Wireless communications is defined as the technology of transmitting digital data from a transmitter to a receiver without the use or necessity of a physical connection. Of primary importance to the medical device designer trying to assemble a wireless link is an estimation of the data rate and data range of a low-power, low-data-rate radio. Fortunately, there are a few simple equations that can help provide a practical first-pass estimate of how quickly data can be transmitted and over what range.

This article provides a brief overview of calculations of data rate and data range that are useful for estimating the link rate and range for a low-power, low-data-rate wireless medical application.

Data Rate Estimation

The data transmission rate of a wireless network can be estimated using two factors: the volume of data that must be transmitted and the additional overhead that is required by the given protocol to perform error detection and correction.

Multipath and interference effects will cause the occasional transmission to need to be retransmitted because of errors in the data. A 90% success rate can be used as a conservative estimate for a well-designed system, which means that 10% of the transmissions must be resent.

For a point-to-point wireless system, the data rate can be estimated by applying the formula

(1)

where PD is the payload data in bits, AO is the application overhead in bits per payload transmission, PO is the radio overhead in bits per payload transmission, retry% is the percentage of failed transmissions that must be resent (10%), and transmission time is the time required to send the data.

By way of illustration, assume that a remote sensor unit is required to send 10 bytes of sensor data in response to a 2-byte transmit command within 100 milliseconds every 10 seconds. The hypothetical application uses a format consisting of a start byte, an address byte, the 10 bytes of payload data, and a 2-byte cyclic redundancy code (CRC), resulting in an application overhead of 4 bytes (start, address, and CRC), which is to say, 32 bits. Assume also that the protocol overhead is 64 bits and that two transmissions actually must take place during that 100-millisecond interval—transmit and receive.

The required data rate then can be estimated as follows:

This estimated rate can be compared with the data rate capabilities provided in the transceiver specifications.

Components of the Data Range Estimation

The effective data range of a wireless network can be estimated using a simple calculation based on signal-power loss, an estimation of antenna gain, the height above ground of antennas, and knowledge of the transmitter power and receiver sensitivity.

Transmitter Power and Receiver Sensitivity. Transmitter power is the energy used to broadcast a signal. It follows that the larger the transmitter power, the longer the transmitter range will be. However, upper limits are imposed on transmitter power by both regulatory and power-consumption issues.

In the 868–870 MHz band, the European Telecommunications Standards Institute (ETSI) allows just 100 mW (20 dBm) of effective isotropic radiated power (EIRP), while the Federal Communications Commission (FCC) permits up to 500 mW (27 dBm) in the 902–928 MHz band.

Most other nations follow either the U.S. or the ETSI rules. Therefore, to create a product that can be used in this band anywhere in the world, most manufacturers limit the transmitter power to 100 mW.

On the power-consumption side, it is important to note that even the most efficient power amplifiers have efficiencies only slightly better than 50%. That means that transmitting 100 mW of signal requires at least 200 mW of power for the power amplifier alone.

Transceivers offset the energy consumption of the power amplifier by turning the amplifier off when no data are to be transmitted. Except in point-to-point links with very heavy traffic, any one radio will rarely transmit more than 25% of the time. In applications where data streams are intermittent, the entire transceiver can be placed in a sleep mode.

The sensitivity of the receiver indicates the level of signal strength that must be present at the receiver in order to obtain data correctly at a specified bit-error rate.

The receiver sensitivity depends on the data rate, the carrier modulation technique, and the low-level environmental frequency noise. A theoretical receiver sensitivity can be estimated through the formula

(3)

where N_t is the thermal noise floor, N_s is the system noise figure, R_data is the rate of data transfer, and SNR is the minimum signal-to-noise ratio required for a given bit-error rate.

The important thing to note is that a doubling of the data rate reduces the receiver sensitivity by 3 dB (10 log₂ = 3 dB). Therefore, the higher the data rate, the lower the data range.

Figure 1. Dipole-antenna-gain radiation patterns. (click to enlarge)

Antenna Gain. Antenna selection is guided by antenna size and cost, followed by desired coverage area and gain. Antenna gain results from focusing transmitted energy into a cross-sectional area. The simplest antenna is the omnidirectional dipole antenna. The ideal signal-radiation pattern of a dipole antenna in free space would have a doughnut shape, as shown in Figure 1(a).

An antenna cannot create power, but it can focus energy into a more useful pattern. For example, when the radiated transmitter power is doubled in the direction of an intended receiver, the antenna provides a 3-dB increase in signal intensity, or gain, as shown in Figure 1b. Doubling the radiated energy density yet again yields a 6-dB gain (see Figure 1c). Limiting the radiated energy to some portion of the doughnut pattern increases signal intensity in the focal area, as illustrated in Figure 2. The smaller the coverage area, the higher the antenna gain.

Figure 2. Focused antenna gain. (click to enlarge)

For indoor applications, directional antennas can provide better performance than omnidirectional antennas. This performance is not due to energy concentration, but rather is the result of a reduction in the off-axis dispersion, which decreases multipath cancellation effects.

EIRP is the sum of the antenna gain and the transmitter power. Regulatory rules may limit radiated power to 30 dBm EIRP in a given band, for example. That power level might be attained by adding a 6-dBi antenna to a 24-dBm transmitter or by using an 18-dBi antenna with a 12-dBm transmitter.

Signal-Power Loss. Data range is the most difficult parameter to estimate simply because of all of the multipath effects that occur in indoor environments. However, if the data range of the wireless device link turns out to be inadequate, the radio may not work, or may require additional transmitters and receivers.

As a first step in estimating data range, a parameter known as free-space loss must be estimated. This path loss is the loss of signal power through the free space between transmitter and receiver.

The plane earth propagation model is commonly used to calculate signal-power loss. The model reflects the average signal attenuation over distance for a stationary transmitter and receiver with a clear line of sight. The signal loss for radios in the ultra-high-frequency bands between 200 and 5000 MHz may be roughly estimated using the formula

(4)

Figure 3. Signal pass loss using plane earth model. (click to enlarge)

where D is the distance between transmit and receive antennas, H_T is the height of the transmit antenna above ground, and H_R is the height of the receive antenna above ground. (It is assumed that D is much greater than H for either antenna.) For two antennas 1.5 m above ground, the plane earth model estimates the signal loss over distance as shown in Figure 3.

In order for a signal to propagate from the transmitter to the receiver, the sum of the transmitter power, receiver sensitivity, and antenna gains must be larger than the signal-power loss. The radio-frequency (RF) link budget can be expressed as

(5)

where P_T is the transmitter power, S_R is the receiver sensitivity, G_T is the transmit antenna gain, and G_R is the receive antenna gain. If the calculated link budget is not at least equal to the estimated signal-path loss, the system cannot be expected to function.

Calculating a Data Range Estimate

At this point, all the values are available to provide a quick data range estimation. Using the RF link budget and signal-power loss formulas, the estimated data range distance can be calculated as

(6)

Consider an RF system using the transceiver from Table I, operating at 915 MHz. As can be seen in the Data Range portion of the specifications, the transceiver has a transmitter power of +6 dBm and a receiver sensitivity of –103 dBm. The transmit and receive antennas will be assumed to have 0 dBi of gain. The link budget would be the sum of 6 – (–103) + 0 + 0, which is 109 dB.

Interior radio practice generally assumes 15–20 dB of fade margin, which provides for differences in multipath phenomena, shadows, reflections, system losses, and other divergences from ideal transmission. A 15-dB fade margin yields about 94 dB of usable link budget. Assuming that each antenna is about 1 m off the ground, then solving for the distance yields an estimated maximum line-of-sight range of about 223 m.

It is important to remember that as the data rate doubles, the receiver sensitivity drops 3 dB because of increased signal-to-noise requirements, which in turn reduces the effective data range.

A site survey aimed at identifying shaded areas and determining the obtained range is recommended as an important step toward ensuring reliable system operation in a variety of indoor locations. The data gathered should be used to plan RF-equipment deployment.

Conclusion

With the advent of frequency bands defined specifically for medical applications, the use of wireless technology in medical devices is growing rapidly. Medical device designers who are thinking about adding wireless communications to their medical application now find several application-specific low-power, low-data-rate radio integrated circuits available to simplify the task.

The benefits of a wireless link accrue to the patient, the physician, and the healthcare network. Wireless technology provides everyone involved in healthcare delivery with greater freedom of mobility and, it facilitates real-time patient monitoring.

Bibliography

Bensky, A. "Range Estimation for Short-Range Event Transmission Systems." RF Design 25, no. 11 (November 2002): 30–39.

Callaway, E., et al. "Home Networking with IEEE 802.15.4: A Developing Standard for Low-Rate Wireless Personal Area Networks." IEEE Communications 40, no. 8 (2002): 70–77.

Code of Federal Regulations, 47 CFR 15.

EN 300 220-1, European Standard (Telecommunications series). V1.3.1. (Sophia Antipolis, France: ETSI, September 2000).

Falcon, C. "Adding a Low Data Rate Radio ASSP to an ISM Application." Microwave Journal 46, no. 10 ( 2003).

Sklar, B. Digital Communications, Fundamentals and Applications. (Englewood Cliffs, NJ: Prentice Hall, 1998).

Carl Falcon is a strategic marketing manager for AMI Semiconductor (Pocatello, ID). He holds a BSEE and MSEE from Drexel University (Philadelphia) and an MBA from St. Joseph's University (Philadelphia).

Copyright ©2004 Medical Electronics Manufacturing