Guidelines

Wireless Systems

1 Introduction

Various systems are under development for a range of applications but they have developed differently depending on what has been viewed as the primary application area. The systems either use infra-red or wireless signals to communicate short distances.

Feature	Home RF	802.11a	802.11b	Bluetooth	DECT	IrDA
Technology	Radio	Radio	Radio	Radio	Radio	Optical
Frequency	2.4 GHz	5 GHz	2.4 GHz	2.4 GHz	1.9 GHz	850 nm
Basic data rate bits/sec	0.8/1.6 M	54 M	11 M	1 M	552 K	4 M/115 K
Range - metres	50	150	30	10	300	3
Max terminals	128	128+	128	8	12	10

The above figures should be treated with caution since they indicate maximum figures under ideal conditions, which may not be realised in real situations. The unlicensed ISM band at 2.4 GHz is used by a number of systems including microwave ovens. In an environment, such as a conference hall, where there may be many notebook computers, mobile phones and personal digital assistants the data rate may be very low.

Home RF
This system operates in the 2400 MHz band with 100 mw transmit power at 50 hops/sec which gives a range of about 50 metres in a typical home environment. It supports 1 Mbit/sec data throughput with integrated TCP/IP networking protocols and a packet structure for use with Ethernet. It can provide up to four voice connections of a quality comparable to a wireline. It is based on 32 Kbit/sec ADPCM and DECT call processing. It includes power management for ultra-portable devices.

The primary application areas are for home automation, interconnecting computers, computer peripherals, computer network, and cordless phones. There are plans for making this system capable of communicating with Bluetooth systems. Future versions of HomeRF are predicted to be capable of speeds up to 20 Mbits/sec and could be available within 2 years.

Home RF has proved expensive to implement so Home RF Lite was developed as a simpler alternative. A further development has been ZigBee which it is claimed will be the dominant wireless system for smart housing with a couple of years.

IEEE 802.11
Developed primarily for local and metropolitan area networks. Version 11b is available now in the form of cards for plugging into a personal computer or laptop. Version 11a is more for the future. The low level of security could be problematic in some applications.

IEEE 802.11g works at 2.4 GHz but has a much greater capacity than 1lb, and is likely to supercede 1lb within a couple of years. The 802.11 series are also referred to as WiFi, but care should be taken as to which version applies.

Bluetooth
It operates in the band 2.402 to 2.480 GHz with frequency hopping at 1600 times per second between the 79 channels. It can transmit data both symmetric and asymmetric with a bandwidth of 1 Mbits/sec (although the maximum asymmetric data rate is 721 kbits/sec in one direction). It can be used to give three voice channels with 64 kbits/sec synchronous links. Each piconet can connect up to 8 devices, but each device can be a member of more than one piconet at the same time. Each terminal can be aware of up to 255 other terminals. The frequency hopping system means that the system is reasonably secure. The range is about 10 metres but that will depend on the environment (within a building it will depend on the construction of the building); the range can be extended to 100 metres with additional amplifiers.

The primary application areas have been considered to be interconnecting mobile phones, computers and PDAs. Secondary applications include smart housing and CANs (car area networks). There is considerable hype surrounding Bluetooth with promises of very low prices (some newspapers mention figures as low as $5 per unit). Bluetooth 2 is likely to operate at about 5 GHz and have similarities to IEEE 802.11a.

DECT
This was developed for cordless telephones where it can give a range of 300 metres. About 50 million units have been sold. It aims to provide speech transmission at a quality comparable to the wired telephony service. The standard is defined in ETS 300 175.

IrDA
The infra-red spectrum is unregulated world-wide which overcomes one problem faced by the radio-based systems, but has the disadvantage it is directional and requires both ends to be in the same room. However in can provide symmetric two-way communication at up to 16 Mbits/sec. Another significant advantage is that it is available now and is inexpensive. A problem is that the standards are not uniquely defined so equipment from one manufacturer is not always compatible with that of another.

2 Scenarios

Up to a few years ago, the usual interface between an assistive device and an ICT system was hard wire, frequently using RS232 or more recently USB. However wireless systems offer the potential for new types of services; some possible examples are given here.

2.1 Public Access Terminals

There are an increasing number of self-service terminals such as cash dispensers (ATMs) and ticket selling machines for public transport. These terminals give a number of problems for disabled users which could be alleviated using wireless technologies:

(a) A wheelchair user may not be able to reach the buttons on the terminal. A hand-held terminal (such as a PDA or a mobile phone) could be connected to the terminal via a wireless system.

(b) A blind person may have difficulty in locating the terminal; a wireless signal to the terminal could trigger an audible location signal from the terminal.

(c) A blind person might want speech output of the information on the terminal's screen; this could be transmitted via Bluetooth to a mobile phone handset.

2.2 Domestic Equipment

The user interfaces on many domestic devices (from washing machines to mobile phones) are difficult to use by people with visual, hearing, physical or cognitive impairments. So the ability to have an alternative user interface could make all the difference to their ability to use the equipment.

2.3 Navigation and Orientation Systems

Visually impaired people often have difficulty in determining whether it is safe to cross the road at traffic lights or the destination of a bus. If the traffic lights and buses had wireless transmitters, the blind person could receive an audible message in a hand-held device such as a mobile phone handset. Such a system could be extended to giving road names at road junctions. It could also be used in indoor environments such as shopping centres or railway stations.

2.4 Hearing Aids

Hearing aid users have particular problems in noisy environments, and a radio-based system would permit the connection of public address systems to their hearing aids; this would be useful in public places such as railway stations, theatres and at sports events.

2.5 Audio Description

Visually impaired people often have difficulties in following the plot on television or in the cinema. One solution is to insert an audio description in the gaps in the dialogue. However such a commentary can be annoying to other people, so it is desirable that only the visually impaired person hears the audio description. One possibility would be to use a wireless system such as Bluetooth to transmit to a headset worn by the visually impaired person.

2.6 Mobile Telephones with Remote Control Facility

Mobile telephones can already include short messaging, location functions, text telephone facilities and speech control. The next generation mobiles will include Internet compatability and picture transmission modes. Combined with Bluetooth the same mobile could be used for remote control of public terminals and domestic appliances. The devices to control and their protocols would be automatically identified thanks to Bluetooth and services built on top of Bluetooth. When the distress button is pressed, the mobile could send an alarm via the mobile net or via the local home bus. In both cases the location information is available, and communication with the service centre is opened automatically.

2.7 Convergence and Diversity

Although the mobile telephone could be used as a remote control or the other way round, it does not mean that we will not find them as separate devices. The remote control at home could be bigger, have bigger buttons and a better display and still offer the same facilities as the mobile telephone with its wireless interface module.

For the personal area network, Bluetooth may be superseded by Bluetooth version2 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.

Relevant standards

• C.63/ANSI ANSI C.63.19, American National Standard for Methods of Measurement of Compatibility between Wireless Communication Devices and Hearing Aids, Measurements of wireless telephone emissions and hearing aid immunity, with predicted performance based on measures. (Now in use in an FCC order).
• TIA TSB-121, 2.5 mm Audio Interface For Mobile Wireless Handsets - Text Telephones (TTY).
• TR 45 TSB-121, 2.5 mm Audio Interface For Mobile Wireless Handsets - Text Telephones (TTY), Connector standard for wireless telephones and TTYs.
  

Further information