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Medical Instrumentation

Wireless Technology Choices Abound for Medical Monitoring

There's a plethora of wireless radio technologies that are applicable to medical monitoring applications. Technical, regulatory and compatibility issues drive designers choices.

DR. TIM MOORE, THE GENERICS GROUP

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Over the next 25 years, a number of factors—such as the global increase in the over-65-aged population—will have pushed medical resources to the limit. Efficiency of healthcare professionals is in turn becoming ever more vital. This can be achieved through development of new healthcare services, particularly wireless patient monitoring.

Clearly wireless monitoring can help, but today there is a plethora of wireless communication technologies and protocols, and there are a number of companies pushing different technologies and often giving conflicting advice. Choosing the right technology and implementation strategy for a medical monitoring application is critical.

A medical monitoring system can be split into a number of easily defined blocks, as shown in Figure 1. The sensor measures the desired parameter and transmits this data to the sensor processing unit. The unit processes the output from the sensor and produces a data stream compatible with the bedside monitoring station. The monitoring station will typically be connected to a number of other sensor units. The combined data from this unit can also be transmitted over a network to a nurses station as well as to a central server for data storage and information distribution. To take advantage of such a system, the correct wireless technology and strategy must be chosen.

When implementing a wireless system, an independent, flexible and objective view should be taken. Many selection criteria need to be taken into account and these differ depending upon the organization. The sidebar “Selection Criteria for Wireless Technologies” outlines the most important points to consider.

In the medical context, wireless communication encompasses a broad area of applications, including audio, free space optics (diffuse IR and laser), inductive and radio frequency. While these each play an important role, this article explores the choices in wireless radio technologies specifically.

Variety of Wireless Radio Schemes

Radio frequency regulation varies across the globe. In the U.S., the wireless standards available are the original VHF, land mobile and UHF bands (which are being phased out), WMTS and ISM—such as 2.4 GHz, wireless LAN -802.11 b, and Bluetooth. While in Europe the current wireless standards being used in hospitals for transport of medical data are ISM and DECT. All the radio systems are standards except the current medical telemetry bands and WMTS, which are effectively bespoke. The WMTS bands have service rules that support flexible product solutions and technologies. These rules allow either simplex or duplex communications modes as well as fixed frequency or spread spectrum modulation schemes. Bespoke development is allowed within the ISM bands, however we believe there is little economic advantage, and it is likely that the same disadvantages, discussed later, will exist.

Selecting the frequency band and standard is only one part of a radio-based wireless system development. Consideration to other elements, such as power consumption, application software and antenna design must also be given. Antenna design is particularly important since it must be able to transmit and receive a signal from a base unit at all times and ideally transmit radiation away from the body if the unit is worn by a patient. Software design is also important since it must be able to cope with potentially unreliable communications links and out of range situations.

The various wireless radio technology choices are compared in Table 1, and each is examined in more detail as follows:

Current U.S. Medical Telemetry Bands: This is for radio-based communication. Equipment approval may no longer be obtained for medical telemetry equipment operating in this band; it is therefore not recommended for use in new applications. The medical applications in these bands are also a secondary user and there have been some instances where TV stations have blocked medical data transmission. It is therefore an unsafe band and its usage is being phased out.

Wireless Medical Telemetry Service (WMTS) – U.S.: These are the new FCC bands and are specifically designed for medical telemetry over radio. The FCC ruling states: “physiological parameters and other patient-related information” are allowed. A WMTS station may transmit any biomedical emission appropriate for communications, except voice and video. Waveforms, such as ECGs, are not considered video. The following is a list of advantages and disadvantages in order of importance. Medical telemetry in these bands is a primary user and therefore interference is low from other types of equipment. The 608-614 MHz band also has co-primary status with certain radio astronomy observatories. On the downside, these bands support U.S. and Canada operation only. No standards of interoperability exist and hence the hospital has to manage this situation.

ISM (802.11): An alternative spectrum for medical telemetry use is the 2.4 GHz Industrial, Scientific, and Medical (ISM) band. In the U.S., the FCC specifically states that it will continue to allow medical telemetry equipment to operate in the ISM band, but caution must be used. Other organizations across the globe have adopted a similar position. The 2.4 GHz ISM band is available worldwide on a secondary use basis as long as spread spectrum modulation techniques are used at relatively modest power levels. The international standards bodies have identified this band for wireless local area network applications and the Institute of Electrical and Electronics Engineers (IEEE) has developed an open standard for two-way wireless communications (IEEE 802.11). The 802.11 specification is actually the wireless extension of the hard-wired LAN communication strategy, 802.3 and is therefore ideally suited to computer networking.

The 802.11 standards allow voice and video transmissions and multiple use, including PDAs for patient data. And as an industry standard there are a substantial selection of tools and software to help with maintenance and implementation. They integrate easily with current hospital infrastructure and are designed for use with TCP/IP. Security in 802.11 can be weak, so careful consideration to the network is required and should be treated as though it were an Internet connection.

Bluetooth / 802.15 (ISM): Bluetooth is a low-bandwidth, short-range wireless networking technology designed primarily to replace cables for communication between small personal computing/communication devices, such as desktop computers, laptops, PDAs and cell phones. This network is often called a Personal Area Network (PAN) or piconet. Each PAN is limited to 8 concurrently active devices, usually with a maximum range of 30 feet. Bluetooth technology transmits both voice and data on the 2.4 GHz ISM band. It uses FHSS technology (1600 hops/s) to increase the reliability of the communication channel. The IEEE 802.15 working group for PANs has announced that it will adopt Bluetooth as the IEEE 802.15 standard. Bluetooth and IEEE 802.11-based wireless LANs are complementary, rather than competing, technologies. However, research has shown that operating Bluetooth and 802.11 closely together can affect the performance of both networks; in fact Bluetooth operating in voice link mode can quite easily block an 802.11 b network.

Like 802.11, Bluetooth allows voice and video transmissions and multiple use, including PDAs for patient data. It provides better security than 802.11 and can be HIPAA-compliant. A disadvantage is that other radiators in the ISM band, including 802.11 devices and microwave ovens could cause interference problems

DECT and DPRS: DECT (Digital Enhanced Cordless Telecommunications) is a flexible digital radio access standard for cordless communications in residential, corporate and public environments. DECT provides for voice and multimedia traffic, and contains many forward-looking technical features that allow DECT-based cordless systems to play a central role in important new communications developments such as Internet access and interworking with other fixed and wireless services such as ISDN, GSM and recently 3G.

The DECT standard makes use of several advanced digital radio techniques to achieve efficient use of the radio spectrum; it delivers high speech quality and security with low risk of radio interference and low power technology. TDMA (Time Division Multiple Access) radio access, with its low radio interference characteristics, provides high system capacity to handle up to 100,000 users per km floor space in an office environment. DCS/DCA (Dynamic Channel Selection/Allocation) is a unique DECT capability that guarantees the best radio channels available to be used. This happens when a cordless phone is in stand-by mode, and throughout a call. This capability ensures that DECT can coexist with other DECT applications and with other systems in the same frequency, with high-quality, robust and secure communications for end-users. Other features of the DECT standard include encryption for maximum call security and optimized radio transmission for maximum battery life.

The original DECT specification was excellent but suffered from low bandwidth, now DMAP, an addition to the DECT standard, removes that problem. DMAP provides support for voice and high bandwidth data (DPRS) and is still compatible with current DECT systems. Current bandwidth is 512 Kbits/s with 2 Mbits/s coming in the future. It is interesting to note that in controlled broadcast tests, an 802.11 b-style system lost 30 times more data than a DECT-style system. Ideally when designing a WMTS system, some of the features, particularly TDMA and dynamic channel allocation from DECT should be used.

DECT offers excellent reliability and security along with high capacity and spectrum efficiency. But DECT does not work in the U.S., although there is a 2.4 GHz version (PWT). Some forecasters think Bluetooth will replace DECT. However, DECT is longer range, has better reliability characteristics and is part of the UMTS specification.

Other Radio Bands

There are other radio bands that can be used for short-range medical telemetry: 173-216 MHz (U.S.), 173-174 MHz (Europe), 433 MHz (Europe), 868 MHz (Europe) 900 MHz band (U.S.) and 5 GHz bands (U.S. and Europe). The 900 MHz band has seen some use in the U.S., however, in general, medical telemetry is a secondary user and these bands are overcrowded, hence they are not suitable for safety of life applications. They could be considered for non-critical applications, however we believe the advantages of using 802.11 b provide a strong argument for not using these radio bands. As discussed above, the 5 GHz bands are going to be used by the 802.11 a (U.S.) and 802.11 h (Europe) wireless LAN standard. It is anticipated that like 802.11 b, both these standards will be useful for non-critical medical telemetry, but not recommended for use in an ITU.

Selection Criteria for Wireless Technologies:

Ten Points to Consider

1. Resistance to interference—Where data is important, the resistance to interference and reliability is important. This can be achieved in a number of ways, for example, data can be resent, a suitable modulation scheme (the way the data is sent) can be chosen or the design of the antenna.

2. Risk of interference with medical equipment—While all medical devices should meet the regulations and licensing requirements, it is possible for a radio system to cause interference particularly in old equipment. Medical establishments should develop consistent guidelines for the use of wireless systems.

3. Power consumption—If a system is to be truly wireless then the system must be powered by a battery pack. Clearly the advent of battery technologies such as Lithium Ion, have dramatically increased the energy capacity of a battery and as a result made many wireless applications feasible. However, consideration must still be given to designing a wireless system such that the optimum power consumption and range is achieved. Modulation scheme, range and antenna can all have an effect on power consumption. Power management by hardware or software must also be investigated.

4. Data rate—This is the speed at which data is transmitted. Usually the higher the better, however sometimes it is necessary to have a lower data rate but improved reliability.

5. Security—This is particularly important in the medical world and represents whether data can be obtained through eavesdropping or access gained to servers by a hacker. In the U.S., HIPAA makes security a legal requirement.

6. Latency—This is important in real-time applications. It is the length of time between transmitting data and receiving an exact representation of that data. Ideally this should be as low as possible, however in environments where there is a lot of electrical noise, data might need to be sent multiple times and hence latency increases.

7. Dedicated use of spectrum—This is currently a growing issue. There are a number of standards that use the same frequency bands as other equipment, which causes interference in those bands. This means that if you use one of these standards your system has to be able to cope with the use of other standards. This problem predominantly affects radio systems in the 2.4 GHz bands.

8. Regulations and licensing—The use of wireless equipment in a medical environment varies across the globe. These regulations split into two parts—the regulation for radio spectrum allocation and medical device compliance, and the medical safety and privacy regulation. In the UK, the actual approval for use of wireless equipment in a medical environment depends on the hospital or local authority. In the U.S. there is also the HIPAA regulation. The area of HIPAA of most relevance when designing a wireless system is security and privacy of transmitted data.

9. Protocol—A protocol is essentially a format for the data sent over a communications link. There are usually a number of levels to a protocol and a protocol can dictate the reliability, performance and cost of a system. The most important protocol is TCP/IP, which is the standard Internet protocol. Any new local area wireless system should be able to carry TCP/IP so that it is future proof, since it is likely that all medical monitoring equipment will become TCP/IP-enabled in the future.

10. Link budget and propagation—A poor link margin will increase the number of errors and latency and so on.

The Generics Group Ltd.
Cambridge, UK.
+44 1223 875200.
[www.genericsgroup.com].