RF Emissions and Interference Aspects

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Most Mote systems use the unlicenced Industrial Scientific and Medical band ISM Band. For wireless communications, the FCC Federal Communications Commission has allocated the frequency range of 402-405MHz for medical implant communication services (MICS), and the frequency ranges of 608-614MHz, 1395-1400MHz and 1427-1432MHz for medical telemetry. It is crucial that interference does not occur and this will become more of an issue as wireless sensor networks grow in popularity.

Note: A major issue in the roll out of RFID - Radio Frequency-identification devices in the retail/logistic sectors has been the wireless interference form sources as benign as flourescent lighting and cordless phones, causing false readings, loss of data packets and corrupt packets being received.

Wireless sensor networks are expected to comply with strict EMC (radiating and interfering) guidelines and these are detailed by two main IEC documents.

• The first is concerned with radiated emissions and is titled - IEC/EN 61000-4-3: Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques. Section 3: Radiated, radio-frequency, electromagnetic field immunity test” ^[1]. This standard is applicable to the immunity requirements of electrical and electronic equipment to radiated electromagnetic energy. It establishes test levels and the required test procedures. The test method documented here describes a consistent method to assess the immunity of equipment or systems against immunity to RF electromagnetic fields from any source. Particular considerations are devoted to the protection against radio-frequency emissions from digital radiotelephones and other RF emitting devices.

• The second standard deals with electromagnetic immunity and is titled - “IEC EN 55024: Information technology equipment, immunity characteristics, limits and methods of measurement” ^[2]. This standard establishes uniform requirements for the electromagnetic immunity of information technology equipment. It defines the immunity test requirements for equipment defined in the scope in relation to continuous and transient, conducted and radiated disturbances, including electrostatic discharges (ESD).

The mechanism that most wireless networks (802.11 and 802.15.4) use to allow multiple users to share the same frequency band is called Carrier Sense Multiple Access with Collission Aviodance (CSMA/CA). The use of CSMA/CA obliges network designers to take into account interference issues among near operating networks or from electronic and telecommunication devices working in the surrounding environment. In particular, the signals radiated by such sources may occupy the channel assigned to a wireless network, which is forced to wait until the end of interference, causing delays or even worse, packet losses.

Bertocco et al ^[3] performed detailed interference and EMC measurements of CSMA/CA based networks (802.15.4)and found no significant effects for radiating interference for the wireless sensor network. Secondly they performed tests around the effects of in-channel Additive White Gaussian Noise (AWGN) interference. They concluded that network operation is successful until the received interference power does not overtake the CCA (Clear Channel Access) threshold. They also concluded that the overall performance of the wireless network is completely a function of interfering signal level and CCA level (and not dependent on the ratio of interfering signal to original signal level).

There is a surprising lack of information dealing with RF emmissions/immunity of body sensor networks in particular although much work has been done on wireless sensor PHY and MAC layers. For medical grade applications in which multiple network nodes may be deployed in to for example hospital environments, much further work focussing on the issues or reliability and RF compatability is needed. Most prototype solutions currently look at issues almost in isolation i.e. reliability, packet loss, latency, power management etc but not the system as a whole. Therefore more work is needed on end to end solutions and the analysis of overall operating parameters.

References

1. ↑ http://webstore.iec.ch/Webstore/webstore.nsf/0/0532BCCE36AEEEA3C125742C000B2E45
2. ↑ http://www.bsigroup.com/en/Shop/Publication-Detail/?pid=000000000030162817
3. ↑ M. Bertocco, G. Gamba1, A. Sona1, S. Vitturi. "Performance Measurements of CSMA/CA-Based Wireless Sensor Networks for Industrial Applications". IMTC 2007 – IEEE Instrumentation and Measurement Technology Conference, Warsaw, Poland, May 1–3, 2007. http://www.dei.unipd.it/~giogamba/pdf/IMTC_2007.pdf

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