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Interference and Electro-Magnetic Compatability


Is there an interference problem between mobile phones and hearing aids? If so, how can it be solved? This commentary explores some of the non-technical issues.

Introduction

Interference is one of the more intransigent problems affecting communication devices; intransigent because its effects are far more recognisable than its causes. Now that most of the elementary sources of interference, such as rotating electrical machines and motor vehicle ignition systems, have been identified and suppressed, the sources that remain are more subtle, or more difficult to identify, or both. The user of a communication device who suffers interference is only too well aware of the effects, as the performance of the equipment is degraded even to the point that it becomes unusable. The user of the device that is producing the interference is - usually - quite unaware of the havoc being caused and is very likely to be using that device in a proper manner and for its legitimate intended purpose. Who then is at fault when two devices, each being used legally and properly, interact so adversely that the performance of one of them is negated? The answer is neither obvious nor straightforward.


Sharing Environments

Interference may be more than just a nuisance. If the device interfered with is part of a control system, or carries messages in emergency situations, the potential for serious injury or damage is clear. The consequences of communication breakdown in Air Traffic Control, to take one example, could be dramatic and disastrous. Less spectacular, although no less disastrous for the participants, is a recent case where a radio-controlled model aircraft flew into the crowd of spectators, killing one of them, because another transmitter was jamming its allocated frequency. Unlike the classic example of the motor car ignition producing white flashes on near-by television screens, interference has increasingly become a by-product of energy emissions which are legitimate and desirable - to some users if not to all. The problem is then one of incompatibility between different devices, meaning that they cannot share the same environment without one or the other suffering degradation. There are two ways of enabling potentially incompatible devices to share the same physical and radiation space without suffering serious problems of interference, namely radio spectrum management and a framework of electro-magnetic compatibility (EMC).


Managing the Radio Spectrum

Radio spectrum management is a system that operates at global, European and national levels to ensure that frequency allocations are regulated in an organised manner. Radio channels are allotted for specific purposes, with maximum power and permitted transmission modes being defined for each one, so that clashes are avoided and Air Traffic Controllers do not have to compete with Taxi Cabs. However, there is a practical limit to the specificity of this channel allotment. Users of commonly available equipment, like CB radios, cordless telephones or model aircraft controllers, do not have personal channel allocations and must make use of a pool set aside for their purpose. If they cannot find a free channel, they must either wait until one is available or change location. Conversely, they cannot be forced to change channel or location if they are found to be the cause of interference to others. Provided that their equipment is operating within the permitted specification, nothing can be done.


Electro-Magnetic Compatibility

The basis of electro-magnetic compatibility is that every type of equipment which might cause interference is assigned limits for levels of emitted radiation, and is also given target immunity levels for radiation from other sources. It covers those instances where unwanted radiation is emitted, whereas spectrum management deals with that which is intentionally produced. In Europe, the EMC Directive (89/336/EEC) sets out the basic principles that apply to all devices which might give rise to, or be affected by, electro-magnetic radiation.


The EMC Directive

The Directive is basically very simple - it requires apparatus to be so constructed that any disturbance it produces is at a level which allows radio, telecommunications and other apparatus to operate as intended, and to have a sufficient degree of immunity to disturbance so that it too can operate as intended, in normal environments.

Understanding what is meant by these requirements of the EMC Directive is far from simple. If there is a level of transmitted signal - ie a field strength - at which analogue television receivers, for example, can readily pick up broadcasts from a transmitter, then disturbances which exceed this level are very likely to interfere with reception. It would be unreasonable to expect television receivers to be immune to interference within the frequencies on which they operate and at levels approximating to the normal signal strength. Devices which might emit radiation at these frequencies are therefore bound by limits that will protect television reception. Even so, there may be some interference if the emitting device is placed close to the receiver or its aerial, for this would not be regarded as the normal environment.


Interpreting the EMC Directive

The above example illustrates the complexity of the issues and highlights a particular problem in applying the EMC Directive. Setting the limits on emissions and the immunity levels can, in practice, only be done through the medium of standards. A pragmatic approach has to be adopted, in which each type of apparatus subject to the Directive has its specific emission and immunity parameters set out in European standards, together with the formal testing method to be used to prove compliance. Although the use of standards to support EU Directives in this way is commonplace, it takes on a particular importance where EMC is concerned.

Standards are regarded as voluntary, that is, they have no legal force unless they are given such by legislation or by contract. In order to show conformity with EU Directives it is usually convenient - but not obligatory - to refer to standards; it is acceptable instead to work to the broad requirements in the Directive itself. In the case of the EMC Directive, its broad requirements (which are paraphrased above) are so general that they can be consistently interpreted only through the medium of standards. Any EMC situation which is not addressed by standards is therefore likely to be difficult to unravel.


Limitations of EMC Standards

The example above used the environment of a television receiver surrounded by domestic appliances and subject also to interference sources outside the home. This is a very familiar 'normal environment' which can be adequately catered for in the standards for broadcast receivers, electrical consumer products, motor vehicles and various types of machinery. Other environments, for other types of equipment, are readily appreciated by the sector specialists who are responsible for preparing the standards. There are exceptions, however. The normal environment in which more unusual combinations of equipment might operate will probably not have been taken into consideration when drafting standards. In some extreme cases, the only way to maintain compatibility will be to keep different types of apparatus well separated. Whether the separation is by kilometres, as in the case of a radio Telescope and an electric railway, or by a metre or less, as for a cordless phone and a computer, depends upon the nature of the devices concerned. Clearly the latter case is the more likely to have been covered by standards. Where no relevant standard exists, users who experience equipment performance degradation due to interference will have little chance of redress


The Structure of Standards

Most apparatus standards are defined vertically by equipment types and classes. Even the so-called horizontal standards usually apply across a range of similar equipments within the same general category, such as medical equipment or domestic electrical apparatus. Standards such as these can include EMC requirements by setting out limits that apply across all examples of that type or class. What they cannot do, however, is to establish emission limits for other types and classes which may act as the sources of interference.


Mobile Phones and Hearing Aids

Although EMC standards usually work well within each equipment category, there are problems when equipment of one category interferes with that in another. A commonly quoted example is the interference caused to hearing aids by mobile phones. This can arise even when the phone is operating correctly and legally within the regulatory rules, and the hearing aid is fully compliant with the appropriate EU Directive. Both devices are working as intended, yet there may be serious interference if the two devices are brought into close proximity.

It is often claimed that there is no interference problem here because, if the two items are simply separated by a short distance, the interference ceases. In the culture of electro-magnetic compatibility this argument has validity, for the defined immunity levels tend to assume that there is some separation between potentially interfering devices. In fact, this argument remains valid until the user of a hearing aid wishes to use a mobile phone and then, not unreasonably, also wishes to listen to it through the hearing aid. This obvious situation has not been foreseen in the drafting of the relevant standards and the unavoidable conclusion is that interference problems will continue to arise unless and until the standards are amended. Technical answers to the problem are available, but will not be universally adopted until it is possible to prove that a piece of equipment is breaching EMC or other requirements, using a standard as evidence.


What Sort of Standard

If the mobile phone is performing as it should, and its emitted radiation is within the spectrum management limits, it might seem that it is the hearing aid which is at fault when interference is experienced. Mobile phones emit radiation by design, whereas hearing aids receive it by accident. The practical problem is of course that it is not easy to build in immunity in a product as small as a hearing aid. Some forms of radiation are more intrusive than others, so one solution might be to draft the spectrum management rules so as to prohibit the more troublesome modes. Another solution could be to adopt a definition of 'normal environment' for EMC purposes which assumed the use of a 'hands free' kit or some other form of coupling which distanced the hearing aid from the phone.

A third option would be to acknowledge that the normal environment in an age of mobile personal communication will inevitably embrace significant levels of radiation across various frequency bands and transmission modes, with the result that immunity levels need to be raised. The consequence of this third approach could be serious for many types of equipment - for aircraft navigation devices, pacemakers, alarms and monitors, as well as hearing aids, have all allegedly been affected by mobile phones - but the logic of it is becoming apparent. The real issue at the present would not seem to be that of which of the available solutions should be adopted, but rather the question of which body is competent to consider the matter and determine the answer.


Collaboration in Standards

The problems of electro-magnetic compatibility between mobile phones and other types of device - such as hearing aids - is one which standards ought to be able to resolve. The standards bodies overall have access to the necessary technical competence, and the availability of a unifying standard would meet their imposed objective of facilitating trade. There are safety issues to be examined, although perhaps not in the hearing aid situation, with a prospect of lengthy litigation if one product has caused another to malfunction disastrously. These are all matters which lie within the province of standards, but the structure of the standards-making bodies does impose an obstacle to achieving this resolution.

The three vertical hierarchies in the standards-making process create separations between electrical and non-electrical subjects, with telecommunications as the third entity and, because of the complexity of the organisations, cross-sector working is difficult to achieve.Standards for electrically powered equipment, including hearing aids and other active medical devices, are produced by the IEC and CENELEC while, for non-electrical equipment, the ISO and CEN are the relevant bodies, but telecommunications matters are handled in the ITU and ETSI. Within each sector a high degree of collaboration usually exists, but effective cross-sector working depends upon common membership in committees of different organisations. Finding an existing standards committee equipped to consider the interaction between mobile phones and hearing aids would be an extremely difficult task, compounded by the historical fact that hearing aid standards are produced by a group which is isolated from those which handle all other active medical devices.


Single Market Directives - Presumption of Conformity

The problem of the lack of an appropriate standard is thrown into greater prominence by a fundamental procedure built into the mechanism of the EU Single Market Directives. Although compliance with standards is usually voluntary, there is a presumption that a product conforms to the Essential Requirements of a Directive if it can be shown to meet the relevant harmonised standards. Lists of harmonised standards - linked to particular Directives - are published by the Commission and it is convenient for manufacturers to use these as if they represented obligatory requirements.

The EMC requirements attached to the Directive on medical devices (which hearing aids must meet) have been separated from the more general EMC Directive - the Medical Devices Directive is cited as a 'specific Directive' for EMC purposes. A somewhat similar situation applies to telecommunications terminals through the RTTE Directive. As a result, hearing aids will be designed to meet the harmonised EMC standards listed against the MDD, mobile phones will be subject to those cited alongside the RTTED, and the harmonised standards associated with the EMC Directive itself will apply to any other equipment not covered by a specific Directive.


Conclusion

The problems of compatibility between hearing aids and mobile phones are unlikely to be resolved if the de facto EMC standards which both have to meet are drafted without the benefit of any hearing aid expertise. Standards for hearing aids need to be developed within the context of medical devices, which is a matter for the IEC and CENELEC to consider. The much larger question of the interaction between mobile phones and hearing aids requires, at the European level, active collaboration between CENELEC and ETSI if a way forward is to be found. EMC is such a complex subject that its standards reflect a situation of 'more and more about less and less', and each committee drafting mandated harmonised standards must have access to the appropriate expertise. The absence of inputs - in the places that matter - on the EMC problems with hearing aids and the scope that newer technology offers to improve matters must be seen as a major contributory factor to the overall problem.

Much work has been done by the hearing aid industry and by organisations representing hearing aid users to demonstrate that the interactions between mobile phones and hearing aids can be expressed quantifiably. The data from which emission and immunity levels can be derived is largely available. This has to be translated into standards - especially harmonised standards in Europe - if there is to be any chance of world-wide recognition of the situation by equipment manufacturers. If this is seen as a part of a wider move towards compatibility between mobile communications devices and the many types of radiation-sensitive equipment with which they potentially interfere, it should be possible to convince the standards bodies and those who fund them that there is an urgent need to devote resources to this question.


This commentary has been prepared by Tony Shipley for the Scientific Research Unit of the Royal National Institute of the Blind and for the COST219 UK Group (PhoneAbility). The information and comments are presented in good faith but readers intending to act upon them are advised to obtain independent confirmation of critical points before doing so.


 



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