An introduction to Optical Wireless

The provision voice data and visual communications to mobile users (‘mobility') has become a key area of research and product development.

Mobility embraces wide area ‘roaming' at one end of the spectrum, and users within a room demanding extremely high bandwidths and mobility at the other. In this regime there is an increasing mismatch between fixed and mobile networks: Fibre optic LANs will be carrying traffic at data rates of Gbits/s in the near future, whereas data rates of 10s of Mbits/s are difficult to provide to mobile users. We believe that optical channels, offering TeraHertz of bandwidth may offer a means to break this bottleneck, allowing in building wireless connections at upwards of 100Mb/s per channel.

Optical vs Radio

There is little doubt that RF communications will provide mobility outside and over large coverage areas. In this environment the data transfer rates to individual users are likely to be limited and can be met with spectrum that is available or is likely to be so.

In a building where both highly mobile and fixed terminals may be present the situation is less clear. Wired LANS are predominantly 10Mb/s ethernet, with 100Mb/s and 1000Mb/s standards being developed, and these are used to connect fixed terminals. Mobile connections are available using the traditional cellular networks.

The radio wireless LANs available at present use unregulated 'free' spectrum regions. Networks operating at up to 34Mb/s are available in the 2.4GHz ISM band.

At lower data rates RF is excellent at providing coverage, due to the scattering and the diffraction of the radio waves, and the sensitivity of the receivers that can be constructed. Channels are robust to being blocked by obstacles and coverage can be achieved between rooms.

Higher data rates require higher frequencies where spectrum is available. At these frequencies the radio signal propagation becomes line of sight, and problems become similar to that using light. Components operating at these frequencies are expensive, and the advantages of radio (coverage, and receiver sensitivity) become less clear.

Optical LANs uses two approaches, outlined in Figure 1. Diffuse networks use wide angle sources and scatter from surfaces in the room to provide an optical 'ether' similar to that which would be obtained using a local radio transmitter. This produces coverage that is robust to blocking, but the multiple paths between source and receiver cause dispersion of the channel, thus limiting its bandwidth. The optical transmitters required are also extremely high power, and dynamic equalisation is required for high bandwidth.

The commercial networks that have been demonstrated largely use this approach (see for instance www.spectrixcorp.com ) and provide approximately 10Mb/s ethernet connections. Multipath dispersion is not a problem at these data rates.






Figure 1 Approaches to optical wireless LANS: Diffuse or Line of sight network

The alternative approach is to use directed Line of Sight paths between transmitter and receiver. These can provide data rates of hundreds of Mbits/s and above, depending on particular parameters. However, the coverage area provided by a single channel can be quite small, so that providing area coverage, and the ability to roam presents the major challenge. Line of sight channels can be blocked, as there is no alternative scattered path between transmitter and receiver, and this presents a major challenge in network design. Multiple base stations within a room would provide coverage in this case, and optical or fixed connection could be used between the stations.A commercial line of sight system is offered JVC , and this offers 10Mb/s ethernet connections.

In general optical channels are subject to eye safety regulation, and this is difficult to meet, particularly for line of sight channels. Typically Optical LANS work in the near infrared regions (between 700-1000nm) where optical sources and detectors are low cost, and regulations are particularly strict in this region. At longer wavelengths (1500nm and above) the regulations are much less stringent, although sources of the type we require are not presently available.

As previously mentioned the other major problem for optical channels is that of blocking. Line of sight channels are required for high speed operation and these are necessarily subject to blocking. Within a building networks must be designed using appropriate geometries to avoid blocking, and using multiple access points to allow complete coverage.

Despite these problems we believe that optical networks have the potential to offer extremely significant advantages over radio approaches, within buildings or limited coverage spaces. To this end we are currently involved in the demonstration of high speed wireless transceiver components, that will operate at 155Mbits/s, at least an order of magnitude faster than commercial optical LANS.