the link, including provision for applications that demand
exclusive use of the link.
IrDA-standard transceivers are now an integral feature
of numerous portable and fixed information appliances, in-
cluding laptop computers, personal digital assistants, print-
ers, and wireless access points to wired networks. It is also
envisioned that IrDA transceivers will be incorporated into
cellular and desktop telephones, pagers, watches, digital
cameras, automobiles, public telephones, automatic teller
machines, information kiosks, and industrial machinery,
enabling new applications of short-range wireless commu-
nication.
Fig. 3(a)–(c) illustrate three general ways that IrDA-
standard (or similar) links can be utilized (these three usage
models are not mutually exclusive). In Fig. 3(a), a portable
device (e.g., a laptop computer or personal digital assistant)
establishes an infrared link to another portable device or to
a fixed device (e.g., a desktop computer or printer). Typical
applications include printing, file system synchronization,
and “business card” exchange. In Fig. 3(b), a portable
device establishes an infrared connection to an access point
to a wired network (e.g., a networked desktop computer or a
dedicated infrared access point). At present, both Extended
Systems and Hewlett-Packard offer dedicated access points
that bridge between IrDA links and Ethernet LAN’s. Such
access points make a wide range of networked applications
available to portable devices. In the future, infrared access
points in public telephones may enable wireless access
to the Internet, while infrared links in automatic teller
machines might allow one to download “digital cash.”
As previously mentioned, links using directional, LOS
transmitters, such as current IrDA links, cannot easily
achieve full connectivity between more than two nodes,
making them unsuitable for forming multiple-access net-
works. However, it might be possible to build a hub
capable of establishing simultaneous point-to-point links
with several portable devices, as illustrated in Fig. 3(c).
Such a hub could be equipped with an internal switching
fabric, buffering and control circuitry to interconnect the
portables in a multiple-access LAN. At the same time,
the hub could serve as a bridge to a wired network.
For example, such a hub might be used for information
exchange among several portables in a conference room.
One technical challenge to building this hub is cochannel
interference between different inbound transmissions. Since
these transmissions will arrive from different directions,
it might be possible to separate them using an angle
diversity receiver. Angle diversity receivers are discussed
in Section V below.
As mentioned previously, among all infrared link designs,
diffuse links (nondirected-non-LOS links) are the most
easy-to-use and robust, since no aiming of the transmitter
or receiver is required, and since no LOS path between the
transmitter and receiver is required. However, diffuse links
have a higher path loss than their LOS counterparts, requir-
ing higher transmit power and a receiver having a larger
light-collection area. Typical diffuse transmitters employ
several 850–900-nm LED’s, which are sometimes oriented
in different directions, to provide a diversity of propagation
paths. When transmitting, they typically emit an average
power in the range of 100–500 mW, making their power
consumption higher than a typical IrDA transmitter. Diffuse
receivers typically employ silicon p-i-n detectors encapsu-
lated in hemispherical or plano-cylindrical lenses, which
provide some light concentration while maintaining a wide
FOV. Often they employ several detectors, in which case
each is oriented in a different direction.
When several diffuse transceivers are located in prox-
imity to each other, they naturally form a shared bus
topology, making diffuse links suitable for multiple-access
LAN’s. However, “hidden nodes” may be present, i.e.,
each receiver cannot receive from, or even detect the pres-
ence of, each transmitter. When hidden nodes are present,
random medium-access control (MAC) protocols that rely
upon collision avoidance or detection, such as carrier-
sense multiple access with collision detection (CSMA/CD)
or with collision avoidance (CSMA/CA), do not always
work reliably. Wireless LAN’s using diffuse infrared can
be formed in two different ways, which are illustrated in
Fig. 3(d).
In the first technique, diffuse infrared links are used
to achieve access to resources on a wired LAN. Clearly,
this architecture also permits communication among the
portable terminals via the wired backbone. This wireless
LAN architecture is well suited for wireless data communi-
cation in offices, hospitals, schools, factories, restaurants,
financial trading centers, or other heavily used environ-
ments, in which the cost of installing a backbone and
wireless access points can be justified. An example of
this type of wireless LAN is SpectrixLite
TM
, made by the
Spectrix Corporation. This system utilizes a base station to
connect together up to 16 wireless access points, forming
a LAN having an aggregate capacity of 4 Mb/s. The
base station also bridges to a wired network (Ethernet or
Token Ring). Portable terminals equipped with wireless
LAN interfaces connect to the access points via 4 Mb/s
links employing OOK with RZ pulses. These diffuse links
are intended to achieve a BER of 10
over a range
of 15 m. Transmission over the wireless LAN is con-
trolled by the centralized, deterministic CODIAC protocol.
Uplink bandwidth-reservation requests, uplink data and
downlink data employ a single wavelength, and are time-
division multiplexed together within a superframe interval.
All transmissions occur at times scheduled by the CODIAC
protocol, permitting portable transmitters and receivers to
“sleep” at other times, thus saving power.
In the second technique, diffuse infrared links are em-
ployed to achieve direct, peer-to-peer communication be-
tween a number of portable and/or fixed terminals. This
type of ad hoc interconnection is well suited to new or
temporary work groups, for collaboration while traveling
or at off-site meetings, or for setting up LAN’s in a home
or office environment in which all nodes are located within
a single room. IBM supplies a diffuse infrared ad hoc LAN
operating at 1 Mb/s using 16-PPM. It is intended to achieve
coverage of a 10 m
10 m region. The transceivers are
KAHN AND BARRY: WIRELESS INFRARED COMMUNICATIONS 269