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Developments in Wireless Systems

Dr. John Gill
April 2002

In recent years there has been a dramatic increase in portable electronic devices for individual use. These include mobile phones, laptop computers, digital cameras and PDAs (personal digital assistants). PDAs started as electronic calculators which also incorporated facilities for storing addresses and telephone numbers; they have now become much more sophisticated and incorporate features which are more commonly associated with a personal computer.

However there has been a problem in that these devices have not easily communicated with each other. So one may store an address and telephone number on a laptop computer, but it has to be keyed in again on the mobile phone. To overcome this problem manufacturers provided cables to connect systems together. Unfortunately, with no single standard interface, each pair of systems needed a separate cable.

For some applications the USB (universal serial bus) became the standard interface, thus reducing the number of cables required. Since it was still fiddly to interconnect devices, wireless systems were developed as a cable replacement.

The first widely used wireless system was infra-red (as used in a television remote control). This had a range of about 3 metres and required line of sight between the two devices. However a significant problem was that there were slight differences between manufacturers so one could not be certain that a device from one manufacturer would work reliably with a device from another.

The laptop computer industry then developed a short-range radio system (IEEE 802.11b) primarily for connecting portable computers to local or metropolitan area networks. This means that a businessman could move about his office building and always be connected to the office computer network. A metropolitan area network is an extension of this concept to shopping centres and airports.

This system was unsuitable for mobile phones since it used too much electrical power, and so would drain the relatively small battery too quickly. Also it was not perceived to be sufficiently secure for applications involving financial transactions. So the industry developed the Bluetooth wireless system.

The manufacturers of domestic electronic equipment were also interested in using radio signals to interconnect systems from audio systems to heating controllers. So they developed HomeRF which was seen as an inexpensive method of installing smart housing features in an existing house. For instance, the action of locking the front door from the outside could trigger an audio message reminding you that a window has been left open or the cooker is switched on.

All three of these systems work in the same frequency band (see table) but are incompatible. In practice the maximum data rate is likely to significantly less than the figures shown.

IEEE 802.11b
Home RF
850 nm
24 GHz
2.4 GHz
2.4 GHz
Basic data rate bits/sec
4 M/115 K
11 M
1 M
0.8/1.6 M
Range - metres

These radio systems are of interest to people with disabilities since they greatly ease the problems of interconnecting assistive devices to mainstream equipment. In addition they open up the possibility of a range of new services to help disabled people.

For instance at a cash dispenser, the user's card could instruct the machine to send speech output to a particular mobile phone handset. Although Bluetooth has a range of 10 metres, it can be easily reduced to 1 metre. So the text on the screen of the cash dispenser would be presented as speech in a specific mobile phone handset. This does not involve a phone call, but just uses the handset which includes a Bluetooth interface.

Another potential application is to use the Bluetooth-enabled mobile phone handset at a light-controlled pedestrian crossing. Disabled persons, from their handset, could indicate that they want to cross the road and need more than the standard time to complete the crossing. Related applications include audio information about road junctions or the destination of buses.

As mentioned earlier the main technological contenders all work in the same radio band at 2.4 GHz. This band is getting congested since it is already used by some microwave ovens. Therefore there are plans for the next generation to operate at 5 GHz which will also give the ability to transmit at higher data rates.

The push for higher data rates comes from companies involved in digital television. The government wants to turn off analogue television broadcasts sometime between 2006 and 2010; the urgency is that they want the radio frequencies for the mobile phone networks. Before the analogue broadcasts are turned off, the consumers must be able to receive digital television. This is not just the television set in the sitting room, but also the ones in the bedrooms (the UK has an average of 3 television sets per household). Rather than having to purchase multiple set-top boxes, the consumer would find it advantageous to have one set-top box and a short-range radio connection to the other television sets in the house.

There are numerous wireless systems under development, and those which will dominate will not necessarily be the best technological solutions.

For smart housing applications, the requirements are very low cost and low power consumption when the device spends most of its time in 'sleep' mode; security is not a major issue. The successor to HomeRF is likely to be ZigBee which operates at 2.4 GHz and gives a maximum data rate of 250 kbit/s at a range up to 75 metres. ZigBee can be configured with up to 254 nodes in a network. The unit cost is predicted to be less than half that for Bluetooth.

For the personal area network, Bluetooth may be superseded by Bluetooth version 2 operating in the 5 GHz band. There is also a proposal for a low power version, Bluetooth Lite, to compete with ZigBee.

For the local area network, IEEE 802.11b (Wi-Fi) can operate under ideal conditions at speeds up to 11 Mbit/s. But with error correction, the data rate is decreased so much that it is insufficient to transmit a DVD-quality video image. Another system, also operating in the busy 2.4 GHz band, is IEEE 802.11g which has a theoretical capacity of 54 Mbit/s. Another possible option will be IEEE 802.11a which has a capacity of 54 Mbit/s operating at 5 GHz; there is uncertainty concerning the ability to transmit 5 GHz signals reliably through internal walls.

However all these systems could be overtaken by UWB (ultra wide band) which differs from all the previous systems in that it does not employ a carrier; instead specially shaped base-band pulses with a duration of less than one nanosecond are transmitted. Data rates of up to 1 Gbit/s have been predicted but 100 Mbit/s may be more realistic for ranges of a few metres. There are a number of problems, including regulatory ones, to be overcome before UWB can become generally available. If UWB lives up to a fraction of the hype, then it is likely to quickly supersede most of the other systems except for specialist applications.

For the mobile handset manufacturers, product shelf life is measured in months not years. So they are not too concerned about changing to another frequency in a few years' time. However for the pedestrian crossing application mentioned earlier, it will be a serious flaw if after a few years one could no longer purchase a compatible telephone handset.

So we could end up, yet again, with technological systems of significant potential benefit to people with disabilities, but which are not implemented for unrelated commercial reasons.

Further information

Bluetooth: Can It Help Disabled People?

Requirements for the Interconnection of Assistive Technology Devices and Information and Communication Technology Systems



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