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Optical wireless ilution

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Optical wireless ilution Powered By Docstoc
					             Short-Range Optical Wireless
                   Communications
                                        Dominic C O’Brien,
                    University of Oxford, UK, dominic.obrien@eng.ox.ac.uk
                                           Marcos Katz,
           4G Standards Research Lab, Samsung, Korea, marcos.katz@samsung.com
         Peter Wang, Kari Kalliojarvi, Nokia Communications, kari.kalliojarvi@nokia.com
 Shlomi Arnon, Electrical and Computer Engineering Department, Ben-Gurion University of the
                              Negev, Israel (shlomi@ee.bgu.ac.il)
  Mitsuji Matsumoto, Graduate School of Global Information and Telecommunications Studies,
                           Waseda, Japan, mmatsumoto@waseda.jp
      Roger Green, School of Engineering, University of Warwick, Coventry, CV4 7AL, UK,
                                  roger.green@warwick.ac.uk,
 Svetla Jivkova, Central Laboratory of Optical Storage and Processing of Information, Bulgarian
              Academy of Sciences, 1113 Sofia, Bulgaria, sjivkova@optics.bas.bg.




   Abstract— It is commonly agreed that the
next generation of wireless communication
systems, usually referred to as 4G systems,
will not be based on a single access technique
                                                    INTRODUCTION AND MOTIVATION
but it will encompass a number of different           As     the   third    generation      mobile
complementary access technologies. The              communication system (3G) is being
ultimate goal is to provide ubiquitous              deployed, manufacturers and scientific
connectivity,       integrating     seamlessly      community are increasingly turning their
operations in most common scenarios,                research interests toward future wireless
ranging from fixed and low-mobility indoor
                                                    communication systems. It is commonly
environments in one extreme to high-mobility
cellular systems in the other extreme.              agreed that the next generation of wireless
Surprisingly, perhaps the largest installed         communication systems, usually referred to
base of short-range wireless communications         as 4G systems, will not be based on a single
links are optical, rather than RF, however.         access technique but it will encompass a
Indeed, ‘point and shoot’ links corresponding       number of different complementary access
to the Infra-Red Data Association (IRDA)
                                                    technologies. Future systems will not only
standard are installed in 100 million devices a
year, mainly digital cameras and telephones. In     connect users and their personal equipment
this paper we argue that optical wireless           but also access to independent (stand-alone)
communications (OW) has a part to play in the       equipment will be provided. Ultimately one
wider 4G vision. An introduction to OW is           would expect that everybody and everything
presented, together with scenarios where            will be wirelessly connected. This vision
optical links can enhance the performance of
                                                    places short-range communications in a
wireless networks.
                                                    position of preponderance, as one could
  Index      Terms—Optical         wireless         argue that most of the wireless links in future
communications, wireless communications             wireless communication networks will be

                                                                                      Page 1 (22)
established over relatively short distances. In
addition, a significant proportion of these
links will be characterized by high data
throughputs. Probably the largest portion of       Optical            Wireless
practical      applications     of   short-range   Communications     as     a
communications take the form of WLAN,
WPAN and WBAN (Wireless Local, Personal            complementary   Technology
and Body Area Networks), covering ranges           for            Short-Range
from a few tens of meters down to sub-meter        Communications
communications.                                       In this section an overview of Optical
   In      the     context     of    short-range   Wireless     Communication        systems     is
communications, two techniques have                presented. The main emphasis in this paper
received increasingly attention in the last        is put on OW for indoor environments.
years, namely multicarrier (MC) and Ultra          Another important approach to OW, free-
Wideband (UWB). These fundamental                  space optics (FSO), a point-to-point optical
technologies for the physical layer have been      connection supporting very high rates in
extensively studied in the literature, for a       outdoor environments, will not be considered
comprehensive introduction and initial             in this paper. We start with a brief
pointers[1, 2] [3, 4]. Application of these        introduction and classification of OW systems
techniques in short-range environments will        and then continue with different engineering
be considered in detail within the framework       aspects, including transmitters, receivers, the
of WWRF [5].                                       optical channel and other related issues. This
   In this paper we argue that optical wireless    introduction is based on the studies and
communications (OW) has a part to play in          reviews presented by [6],[7-9]. Comparisons
the wider 4G vision. The optical wireless          to conventional radio systems are presented
channel has THz of unregulated bandwidth,          to give the reader a broader perspective of
and characteristics that are distinct from that    the possible baseband technologies. An up-
of radio. It should be noted that our aim is to    to-date account of different techniques,
show that the optical channel has                  practical systems and standards related to
complementary characteristics and can, in          OW as well as future research issues
certain situations, add to, but certainly not      complete the paper.
replace the capability of a RF 4G wireless
                                                   Basic system configuration
system. Together these media might provide
                                                   Figure 1 shows a number of different OW
a broad spectrum of channel characteristics
                                                   configurations. There are two basic
and capabilities that radio alone would find it
                                                   configurations; communications channels
difficult to meet.
                                                   either use diffuse paths (Figure 1 (a)) or Line
   The aims of this paper are to;
                                                   Of Sight (LOS) paths (Figure 1(b)) between
   (i) Introduce OW and the components and
                                                   transmitter and receiver. In a diffuse system
systems used
                                                   an undirected source (usually Lambertian)
   (ii) Summarise the state of the art, and rich
                                                   illuminates the coverage space, much as it
research community that exists
                                                   would be illuminated with artificial lighting.
   (iii) Compare the characteristics of OW
                                                   The high reflectivity of normal building
with radio
                                                   surfaces then scatters the light to create an
   (iv) Identify particular areas where OW can
                                                   optical ‘ether’. A receiver within the coverage
contribute to the 4G vision, and areas of
                                                   space can detect this radiation, which is
future research
                                                   modulated in order to provide data
                                                   transmission. Diffuse systems are robust to
                                                   blocking and do not require that transmitter
                                                   and receiver are aligned, as many paths exist
                                                   from transmitter to receiver. However,

                                                                                      Page 2 (22)
                     (a)                         (b)                           (c)




                                    (d)                           (e)


                                    System configurations
                                                        Power from
                                                        different directions


                    Optical system to
                    collect power
  Photodetector
                           Rx                               Rx

                  Receiver electronics                                                           Rx

                     (f)                         (g)                                 (h)


                                   Receiver configurations

  Figure 1. Optical wireless configurations. (a) Diffuse system. (b) Wide LOS system. (c)
Narrow LOS system with tracking. (d)Narrow LOS system using multiple beams to obtain
coverage. (e)Quasi diffuse system. (f) Receiver configuration: single channel receiver. (g)
Receiver configuration: angle diversity receiver. (h) Receiver configuration: imaging
diversity receiver.

multipath interference at the receiver can             wide beam ensures coverage. As the beams
cause Inter Symbol Interference (ISI) and the          are narrowed path loss reduces and the
path loss for most systems is high.                    allowed bit rate increases, albeit at the cost
   The alternative approach is to use directed         of coverage. Narrow beam systems therefore
Line of Sight paths between transmitter and            either require tracking to allow user mobility
receiver. Wide LOS systems such as that                (Figure 1 (c)), or some sort of cellular
shown in Figure 1 (b) use ceiling mounted              architecture to allow multiple narrow beams
transmitters that illuminate the coverage              to be used (Figure 1 (d)). A third class of
area, but minimize reflections from walls,             system also exists; quasi diffuse systems
ensuring that a strong LOS path exists. The            minimize the number of multipaths by limiting
                                                                                           Page 3 (22)
                                                          160

                                                          140
                       Allowed class 1 source power(mW)
                                                          120

                                                          100

                                                                                                     1500 nm
                                                          80

                                                          60
                                                                                                        Diam.= 2mm, 850nm
                                                          40
                                                                                                       Diam.= 1mm, 850nm

                                                          20                                           Diam.= 0.5mm, 850nm
                                                                                                       Point source, 850nm
                                                           0
                                                                0   5   10      15     20      25     30     35   40     45
                                                                             Divergence half-angle (degrees)


Figure 2. Allowed emitted power for class 1 eye safe operation of transmitter as a function
of beam divergence.

                                                                                            850nm point source, different diffuse source
the surface reflections, but allow robust                                                   diameters at 850nm, and for a 1500nm point
coverage by directing radiation to a number                                                 source. Increasing the source diameter
of surfaces so that a suitable receiver may                                                 increases the size of the image on the retina
select a path from one surface only (Figure 1                                               of the eye, thus reducing the possibility of
(e)). Variants of this use structured                                                       thermal damage. At 1500nm water
illumination (perhaps using arrays of spots)                                                absorbtion in the eye protects the retina, so
[10, 11]. There is a large amount of                                                        the hazard is one of corneal damage, and
simulation and more limited amount of                                                       therefore independent of source size. As
measurement data that describes these                                                       might be expected more divergent sources
channels in some detail. In contrast with                                                   are less hazardous as the eye, or an optical
radio frequencies simulation of indoor                                                      instrument such as binoculars or magnifier
coverage spaces generally gives a good                                                      cannot collect all the radiation. The results in
estimate of the channel characteristics.                                                    the graph are calculated using the test
                                                                                            procedures and limits laid out in [12].
System components                                                                              Sources of small diameter can be made
                                                                                            diffuse using a ground glass plate, or more
                                                                                            sophisticated engineered diffuser elements,
Transmitter
                                                                                            including holographic [13] and reflective [14]
   The transmitter consists of a single, or a                                               examples. This latter device has been
number of sources, and an optical element to                                                incorporated in a commercial optical link [15].
shape the beam and also render it eyesafe if                                                The effect of a diffuser is to increases the
required. The main element of the transmitter                                               apparent size of the source, and the graph
is the optical source. Light Emitting Diodes                                                shows the benefit of this.
(LED) and laser diodes are employed as the                                                  Most systems use laser diodes, due to their
optical radiating     element,    and their                                                 higher modulation bandwidth and efficiency.
transmission power is limited by eye safety                                                 IR LEDs are also important optical sources
regulation. Figure 2 shows a plot of the                                                    being considered for establishing optical
allowed emitted power for class 1 (the most                                                 links. In addition, there is a small and growing
stringent eye safety regulation) operation vs.                                              interest in using visible LEDs that would be
beam divergence. This is shown for an
                                                                                                                               Page 4 (22)
installed in a building to provide solid-state       receiver to be increased [19] (Figure 1 (g)).
lighting for optical communications [16]. In         Imaging receivers, as developed at Oxford
such cases multiplexing of the low bandwidth         [20]and Berkeley [21] can also carry out this
devices might be used to increase data rates.        function (Figure 1 (h)). These use a large-
                                                     area pixellated detector array and an optical
Receiver                                             imaging system. Light from narrow range of
    A typical OW receiver consists of an optical     directions is collected by a single pixel, and
system to collect and concentrate incoming           together the array of pixels offers a large
radiation, an optical filter to reject ambient       overall field of view. It also allows multipaths
illumination, and a photodetector to convert         from different directions to be resolved as
radiation      to      photocurrent.    Further      they are imaged to different pixels on the
amplification, filtering and data recovery are       array. The array also allows the large
then required (Figure 1(f)).                         detection area to be segmented, reducing the
                                                     capacitance on each of the receiver front
Optical systems
                                                     ends. Both of these topologies can to some
  Receiver optical systems can be
                                                     extent resolve multipaths, and this may offer
characterised in terms of their angular Field
                                                     some means to reduce the effect of
of View (FOV) and their collection area.
                                                     shadowing, by selecting an alternative non
These are linked to the detection area by the
                                                     shadowed path. It is also possible to use a
constant radiance theorem. This states that;
                                                     combiner/equaliser to maximise the received
                                                     signal and BER [21].
                         ⎛ FOV ⎞
             Acoll sin 2 ⎜     ⎟ ≤ Adet ,            Optical filtering
                         ⎝ 2 ⎠                           Ambient light is the most important source
                                                     of interference and it may greatly deteriorate
  where Acoll is the collection area and Adet        link performance [22]. Constant ambient
is the photodetector area. This is important         illumination will generate a DC photocurrent,
as it limits the collection area that is available   and this will normally be blocked by the AC
for a given FOV and photodetector. For a             coupling of the receiver [23]. However the
truly diffuse channel the detector area sets         shot noise from the detection of this
how much power will be received. Any                 illumination cannot be filtered and can be
collection optical system changes the                large when compared with the noise from the
balance between field of view and collection         preamplifier. Artificial illumination, particularly
area subject to the constraints above;               modern         high     frequency      fluorescent
however if the system is receiving light from a      illumination induces electrical harmonics in
Lambertian source such as a wall or ceiling          the received signal, with components up to
the amount of optical power that is collected        1MHz [24] and this can greatly effect link
remains approximately constant for a given           performance. Various studies of this have
detector area.                                       been undertaken, including [25, 26].
   Both imaging and nonimaging optics [17]               Optical filtering can be used to reject out of
can be used to collect and focus radiation           band ambient radiation and reduce the
onto single element detectors. Recent                intensity reaching the detector. Various
designs of optical antenna [18] show good            different filter types have been demonstrated;
performance in compact form, although this           a longpass filter in combination with a silicon
cannot exceed that predicted by constant             detector provides a natural narrowing of the
radiance.       Various     different     receiver   bandwidth and absorbtion filters can be used
topologies have been investigated in order to        to reject solar and illumination [27]. Bandpass
circumvent these constraints. In angle               interference filters can be used, although
diversity systems a number of single channel         care has to be taken to allow sufficient
receivers are combined, so that each faces in        bandwidth to allow for passband shifting with
a different direction. This allows multipaths to     the varying angle of incidence. It is also
be resolved and collection areas for each            possible to filter by incorporating appropriate
                                                                                          Page 5 (22)
layers into the photodetector . Holographic        channel response, even in the presence of
receiver front-ends also allow ambient light       chairs and other objects. Depending on the
noise to be rejected [28].                         balance of LOS and diffuse paths within a
   Electrical filtering can be used to reduce      space channels can be modeled as Rician
the effect of the illumination harmonics, but at   [37] or Rayleigh, with exponential impulse
the cost of inducing baseline wander. Work         responses. Various measurements have also
on the optimal placement of the filter cut-offs    been made [38, 39]. Recent high-resolution
for particular modulation schemes is reported      data indicate that transparent ‘unlimited’
in [29].                                           bandwidth diffuse channels are available in
Detector/preamplifiers                             particular directions for most diffuse
   The detector and preamplifier together are      environments [40]. One of the major
the main determining factor in the overall         advantages of the OW channel is that there
system performance. Both PIN structures            is no coherent fading, and the channel is
and APDs have been used in single detector         therefore extremely stable when compared
systems, whilst array receivers have tended        with its RF counterpart. Even though sources
to use PIN devices. Most of the detectors are      are coherent the size of detectors, and the
designed for optical fibre systems, where          scattering environment mean that any effects
capacitance per unit area is relatively            are removed by the spatial integration that
unimportant, as areas can be small, and            occurs at the receiver.
hence commercial devices are highly                Modulation schemes
capacitive. Devices for OW should be                  Unlike in conventional RF systems, the
optimized for low capacitance per unit area        optical channel uses intensity modulation and
by increasing the width of the I-region, until     direct detection. The optical power output of
this effect is balanced by the increasing          the transmitting source is controlled
carrier transit time. Detectors partially          according to some characteristics of the
optimized for this application have been           information bearing signal. The transmitted
demonstrated [30] but further work is              signal is thus always positive and its average
required in this area.                             amplitude is limited [41]. Analog and digital
Various approaches to mitigating the effect of     optical modulation is possible but, due to the
input capacitance on bandwidth have been           intensity modulation, common modulation
taken. Bootstrapping [31], equalization [32]       schemes employed in the radio frequency
and capacitance tolerant front-ends [20, 33,       domain will perform differently when applied
34] have all been investigated.                    to optical systems.
The Optical Channel                                   The changes in optical power produced by
   LOS optical channels are subject to path        intensity modulation are detected by direct
loss, and this can be modeled using either         detection, that is, a current proportional to the
ray-tracing or analytical techniques. The          incident optical power is induced in the
diffuse channel has both high path loss            photodetector. As pointed out in [7] two
(>40dB typically) and is subject to multipath      criteria should be used to evaluate the
dispersion. Both of these characteristics are      feasibility of an optical modulation system;
dependent on the orientation of the source         the average optical received power required
and receiver within the space.                     to achieve a given target BER performance
   There has been extensive work on                and the required receiver electrical
predicting the characteristics of the diffuse      bandwidth.
channel, including [26, 35] [36] as well as           Three basic modulation schemes are
analytical models of the channel impulse           usually used in OW systems, namely On-Off
results. Most building materials are found to      Keying (OOK), Pulse-Position Modulation
have a high reflectivity (0.4-0.9) and they can    (PPM) and Subcarrier Modulation (SCM). An
be approximately modeled as Lambertian             extensive account of these and other
reflectors. Ray tracing techniques therefore       techniques can be found in [41]. Other issues
allow generally good predictions of the            to take into account when considering optical
                                                                                       Page 6 (22)
modulation schemes are their robustness to       time regulation of the RF spectrum limits
multipath propagation and, in networks, their    available bandwidth to several orders of
suitability to multiple access environments.     magnitude below this. The vision of a highly
PPM is very well suited to work in low signal-   connected world is likely to require
to-noise ratio scenarios, quite typical in       unaffordable amounts of the already scarce
optical channels due to blocking effects         radio frequency spectrum. OW occupies fully
(shadowing) and ambient noise. However,          unlicensed spectrum bands, and the
multipath propagation induces intersymbol        possibility of using unregulated and
interference and PPM is particularly sensitive   unlicensed bandwidth is one of the most
to these dispersive effects of the optical       attractive characteristics of OW.
channel [41].                                       Unlike radio communications, the nature of
   Many techniques have been considered in       the optical radiation is such that the
order to combat the deleterious effects of       transmitted signal is obstructed by opaque
dispersive optical channels, among them the      objects, and the radiation can have high
use of equalizers, angle-diversity and spread    directivity using sub-millimetre scale beam
spectrum techniques. Different equalization      shaping elements. This combination of high
approaches at chip or symbol rate have been      directivity and spatial confinement gives
studied for PPM based systems, including         optical channels an unmatched advantage in
linear and decision-feedback equalizers [41].    terms of security. Furthermore, these
   Spread-spectrum modulation techniques         characteristics allow exploiting wavelength-
can also be used to combat multipath             reuse at room level, without taking special
distortion as well as to reduce the effects of   provisions for interference from and to
interference, in a similar fashion as they are   neighboring rooms. Since the optical
exploited with radio systems. Direct-            radiation produces no interference to
sequence techniques are usually used in          electrical equipment, OW can be used in
conjunction with optical links. Since bipolar    sensitive environments where conventional
spreading sequences cannot be used to            radio wave transmission is not allowed.
modulate an always positive optical signal, a       Another unique characteristic of wireless
unipolar sequence is formed by biasing to the    optical links is in the channel itself, and it is
bipolar sequence with a fixed DC offset. This    the fact that these links are not affected by
unipolar sequence preserves the correlation      multipath fading. This is because the
properties of the original sequence and it can   dimensions of the receiver’s photodetector
be correlated with a bipolar sequence at the     are many orders of magnitude larger that the
receiver. Several direct-sequence spread-        wavelength of the optical radiation and thus,
spectrum approaches specially designed for       the spatial fluctuations in signal strength due
optical systems have been proposed and           to multipath are averaged over the large
studied, including sequence inversion keying     detector area, which acts as an integrator.
modulation (SIK), complementary SIK (CSIK)       For most of the cases, and as an essential
and M-ary bi-orthogonal keying modulation        advantage, optical components are small in
(MBOK) [42, 43]. Sequences with low auto-        size, low-cost and they have low power
and cross-correlation sidelobes are preferred    consumption. Furthermore, transceivers are
in order to minimize the degrading effects of    relatively simple compared with their radio
intersymbol interference.                        frequency counterparts.
Optical wireless              vs.      radio        There are several drawbacks, however;
communications                                   since IR radiation can reach the retina and
  Over the past decade the capacity of an        eventually cause thermal damage, the
optical fibre link has increased by several      maximum power that can be transmitted is
orders of magnitude, showing almost              limited by eye safety regulations and extra
‘Moore’s Law’ growth, largely due to the         optical elements are required to render high
availability of optical spectrum. At the same    power sources safe.
                                                    In diffuse optical communication systems,
                                                                                     Page 7 (22)
multipath propagation caused by the                 have transmitter and receiver gains of unity.
dispersive optical channel will introduce pulse     In this case
spreading and intersymbol interference (ISI),       the link loss Llink can be approximated by;
much as would be experienced by a radio
                                                              ⎛ λ ⎞ 1
                                                                     2
channel, although the process is due to
                                                    Llink    =⎜    ⎟ 2                        (1)
incoherent, rather than coherent fading.                      ⎝ 4π ⎠ r
Systems above 50Mb/s or so might typically
                                                    where r is the link distance and λ is the
require some form of equalisation.
                                                    wavelength of the radiation. The minimum
    Perhaps the major difference for the optical
                                                    link distance at which this occurs is the
wireless channel is the detection process is
usually incoherent, so that the detector            Fraunhofer distance     d f and can be
responds linearly with power, rather than           estimated as;
amplitude, as is the case with a radio                       2D 2
receiver. Receiver sensitivity is therefore         df =                                      (2)
substantially lower than for radio channels,                   λ
and therefore systems are more susceptible          where D is the largest dimension of the
to path loss, especially in the case of diffuse     antenna.
systems. More complex receiver and                  In a real environment the situation is more
transmitter structures can be used to reduce        complicated however. At distances greater
this, and the effect of noise from ambient          than a reference distance d ref from the
illumination. As the detection process is           antenna ( d ref is greater than the Fraunhofer
incoherent there is no inherent rejection in
the     detection    process,     so    filtering   distance) multiple paths from transmitter to
mechanisms must be introduced, as                   receiver interfere and cause a path loss that
                                                                     −γ
mentioned earlier.                                  varies as r where γ is the path loss
    The wavelength of optical radiation makes       exponent. This gives rise to a ‘dual-slope’
directive channels easy to implement, and                                     −2
                                                    path loss, with an r loss at distances less
system design often leads to asymmetric
                                                    than d ref (but greater than d f ) and an
channels [44]. Such directive channels are
necessarily subject to blocking, which is           r −γ loss beyond this.
again distinct from radio applications [9,          The link loss then becomes
44]and [45].
                                                               ⎛ λ ⎞ 1
                                                                          2
Link budget models                                  L link    =⎜    ⎟ 2                                   for
In this section simple models of the RF and                    ⎝ 4π ⎠ r
optical channels are developed, in order to                    d f ≤ r ≤ d ref     (3)
compare the performance (A similar analysis
is undertaken in[44], albeit with different         and
                                                                                          γ
                                                              ⎛ λ ⎞ ⎛ 1 ⎞⎛ d ref
emphasis).
                                                                                         ⎞
                                                                    2

                                                    Llink    =⎜    ⎟ ⎜       ⎟⎜
                                                                            2 ⎜
                                                                                         ⎟ for
                                                                                         ⎟          d ref < r
Radio communications                                          ⎝ 4π ⎠ ⎜ d ref ⎟⎝ r
                                                                      ⎝      ⎠           ⎠
There are many models of the path loss of a         (4)
radio link, depending on environment, and on
link distance. In this case a simple set of         Both the position of the break-point and the
limiting cases is considered.                       slope beyond it can therefore vary widely,
If (i) the transceivers lie in each others far-     and the model above is at best an indication
field, so that Fraunhofer diffraction can be        of the loss. The methods used to estimate
assumed, and (ii) a line of sight exists            d ref and γ are detailed in each of the
between transmitter and receiver the link loss
can be estimated using the standard Friis’          scenarios described later in the paper.
equation. In most indoor environments the           Assuming the receiver antenna is at the
antenna will be approximately isotropic, and        standard temperature, and feeds a matched

                                                                                               Page 8 (22)
preamplifier with noise figure F then for a                Eb Rb Pt Llink
transmitted signal power Pt the signal to
                                                                =                               (6)
                                                            No    FKT
noise ratio S / N at the receiver is;                      This expression allows the bit-rate available
 S   PL                                                    Rb to be related to the range for a given
   = t link                  (5)
N FKTB                                                     required    Eb / N o .       The        value    of
where K is Boltzmann’s constant and T is
the temperature in Kelvin. For a bit-rate                  Eb / N o required for a particular Bit Error
 Rb average energy per bit Eb and Noise                    Rate (BER) is determined by the modulation
                                                           and detection scheme used.
power density N o



                                                                                     Transmitter
          Receiver



                    Emission
                    half-angle θh            range    Receiver acceptance
           Transmitter                                                               Receiver
                                             r        angle ψ(=θh)


       (a) Link geometry for 'point and shoot' optical link, showing 'worst case' alignment
       for correct operation.Transmitter and receiver are both oriented at θh to 'boresight'
       alignment.


                                                       Transmitter
                                                                 Base station
             h=3m
                                                      θh




                                                                 range r

                                                                      Downlink


                                                                            ψ(=θh)
                                                                                       Terminal

                                                                             Receiver


                                                         Hotspot radius d
                  (b). Link geometry for optical hotspot for worst case alignment


                        Figure 3. Optical communications link geometries


                                                                                                   Page 9 (22)
                 Rsource



                           Electro-optic
                                                                   Opto-electronic
                           conversion
                                                                   conversion
                                                                                     Rload
                                                 Free space                                          Preamplifier
       Vsource                                   propagation                                 Vload




                                  Figure 4. Communications channel model

                                                                              Ps
Optical communications                                         Pcoll =                    Acoll cosψ                       (8)
                                                                       2πr (1 − cos θ h )
                                                                                 2

Figure 3 shows the geometry of the optical
links and Figure 4 shows an electrical model                   defining the optical loss, Loptical as
of the communications channel. Applying the                                 Pcoll
Friis’ formula for optical links creates results               Loptical =          leads to
that are unrealistic in real situations, in that                             Ps
the diffraction limited transmitter beam that                                        1
these simulate is very narrow, creating links                  Loptical   =                    Acoll cosψ                  (9)
that require very precise alignment. Two link                               2πr (1 − cos θ h )
                                                                                 2

geometries, for point and shoot links and a
‘hotspot’ geometry are shown in Figure 3.                      In both cases shown in the figure the
The Intensity profile of the source is                         transmitter launches power within an
 I ( r,θ ) where r is the link distance and θ is               emission cone with half-angle θ h and
the angle measured from the optical axis, as                   receiver optical systems have acceptance
show in Figure 3. In this case it is assumed                   angle ψ = θ h . The beam half-angle and
that this is a constant value I o so that
                                                               receiver acceptance angles are matched as
I ( r,θ ) = I o for θ varying from 0 to the                    this is optimum for systems with paired
                                                               uplinks and downlinks as shown in the figure.
beam half-angle        θh .    This is beneficial,             Any larger receiver acceptance angle is not
compared with the more normal Lambertian                       useful as if light is entering the receiver at
pattern as the off-axis fall of in intensity is                this orientation the transmitter within the
reduced. Such a distribution might be                          same transceiver unit will then miss the
achieved by using a holographic diffuser to                    receiver within the distant transceiver unit.
modify the source emission profile.                            At the receiver the electrical power delivered
Normalising this by the power emitted by the                   to the load S is given by;
source Ps yields;                                              S = ioptical Rload
                                                                    2
                                                                                                                    (10)
                    Ps
I ( r, θ ) =                               (7)                 where ioptical is the photocurrent. This is
             2πr (1 − cos θ h )
                2
                                                               given by;
This power is collected by a receiver with                     ioptical = RPoptical              (11)
area Acoll , which in the worst case is
                                                               where       R is the responsivity of the
oriented at an angle       ψ   from its optical axis,
                                                               photodetector and Poptical is the optical power
so the minimum power collected, Pcoll is
                                                               incident on it.
given by;

                                                                                                                    Page 10 (22)
                                  160


                                  140



                                  120
               10log10(EbRb/No)




                                  100                                                                           1000Mb/s OOK
                                                                                                                100Mb/s OOK
                                                                                                                10Mb/s OOK
                                   80                                                                           1Mb/s OOK


                                             Radio
                                   60        Optical link 2.5 degree half-angle
                                             Optical link 10 degree half-angle
                                             Optical link 20 degree half-angle
                                   40 -1                                      0                              1
                                    10                                     10                              10
                                                                    Link distance(m)

      Figure 5. Comparison of RF and optical link performance for point and shoot links

                                                                                       Eb Rb
                                                                                             =
                                                                                                           (RPoptical )   2


                                                                                               (2e(RPoptical + iAmbient ) + iAmplifier )
The noise at the receiver consists of shot
                                                                                                                             2
noise from the signal, shot noise from any                                              No
DC photocurrent caused by ambient light and                                                              (14)
amplifier noise. The noise power delivered to
the load is;                                                                           This expression allows a direct comparison
N = (2e(RPoptical + i Ambient )B + i                   2
                                                       Amplifier   B )Rload            with the radio link to be made. In the case of
                                                                                       RF the channel is subject to fading and both
                                                  (12)                                 RF and the Non Line of Sight (NLOS) optical
                                                                                       channel are subject to multipath dispersion.
where i Ambient is the photocurrent due to                                             These      expressions    are   therefore    a
                                                                                       simplification of the channel conditions, but
ambient illumination and i Amplifier is the input                                      are instructive as they allow a ‘best-case’
referred noise of the amplifier.                                                       comparison. In the next section these
The ambient light current can light current                                            expressions are used to compare the two
can vary over several orders of magnitude                                              media in typical scenarios.
depending on the field of view and optical
filtering[22]. The amplifier noise ranges from                                         Applications areas for optical
1 − 10 pA / Hz [46]                        with    a       value        of             wireless
5 pA / Hz typical for amplifiers with several
GHz of bandwidth.                                                                      ‘Point and shoot’ and narrow FOV
The overall signal to noise ratio is given by;                                         applications
S
  =
                (RPoptical )                  2
                                                                                                                        Eb Rb
N (2e(RPoptical + i Ambient )B + i Amplifier B )
                                   2                                                   Figure 5 shows a plot of
                                                                                                                         No
                                                                                                                              vs. range r
                                  (13)                                                 for both RF and OW links with different
                                                                                       optical link half-angles. The link geometry is
which leads to;                                                                        that shown in Figure 3 (a).

                                                                                                                               Page 11 (22)
The radio link model assumes an emitted               125 µA is used for the 10 degree FOV and
power of –3.8dBm at a frequency of
                                                      scaled appropriately with for different FOV.
3.975GHz, which is broadly representative of
                                                      This is equivalent to the maximum quoted in
a UWB signal operating in the ‘lower band’ of
                                                      [44] when scaled for FOV and assumes
the US frequency allocation[4]. This band
extends from 3.1 GHz to 4.85 GHz, and the             coarse optical filtering. The required values of
maximum emitted power is permitted is–                Eb Rb
                                                            for different bit-rates and incoherent
41dBm/MHz. The amplifier noise figure is               No
assumed to be 6dB.
Values of path loss γ and reference distance          OOK detection with a Bit-Error-Rate (BER) of

d ref are required for the RF model. There            10−6 are plotted on the graph also.
                                                         The optical link is superior for narrow FOV
have been numerous measurements [47-49],              applications up to approximately 10m, where
and these show that γ ≈ 2 for LOS                     for a FOV less than 2.5 degrees (half–angle)
environments and γ ≈ 4 in Non-LOS (NLOS)              performance exceeds or meets that available
situations for UWB systems. As the                    using RF. For wider FOV the power density
comparison is with an optical link the NLOS           available at the receiver drops and the
case is not relevant, so at the break point,          available range for a given bit rate reduces,
there is no change of slope. The value of             whilst for the isotropic RF link this is not
 d ref is therefore not required. (Measurements       relevant, and performance depends only on
                                                      range.     Assuming       that   a    FOV     of
in [50] reinforce this assumption; a reference        approximately 10 degrees (half-angle) or so
distance of 1m was chosen, and                        is required for a ‘point-and-shoot’ application
measurements at this distance closely                 a range of 1m or so can be achieved with an
matched the free-space Friis equation.
                                                      optical link operating at 1Gb/s. It should be
Beyond this point a value of γ = 1.91 was             noted that slightly better results-perhaps a
found, making the assumption of γ ≈ 2 for all         10% improvement in range- are obtained for
distances from the source valid).                     an optical link operating at 1500nm. As can
The optical link model assumes operation at           be seen from Figure 2 the allowed power
850nm, using a source 2.5mm in diameter.              emitted from such a source is similar (though
The optical link emits power levels allowed           slightly lower) to that from an 850nm source
for class 1 emission, at each specific beam           2.5mm in diameter for all beam half-angles.
half-angle. The source aperture dimensions            This is more than compensated by the
are large enough that it is classed as diffuse,       increased responsivity at the receiver
thus allowing greater than point source               however (1A/W compared with 0.6A/W)
emission. In addition at larger divergences           leading to the increase in range.
greater power is allowed; for the 2.5 and 10          In this analysis 850nm is as an operating
degree beams this is 12.2mW, which                    wavelength as components are available at
increases to 25.8mW for 20 degree half-               low cost. It should also be noted that this
angle.                                                calculation does not include detailed analysis
The receiver collection aperture is assumed           of the effects of filtering and other
to be 5mm in diameter. This is chosen as the          impairments, but this is the same for the RF
maximum practical area for a small portable           link budget, where effects such as fading are
terminal. A detector responsivity of 0.6A/W is        ignored, so the two are broadly comparable.
assumed, and calculation suggests that a              In addition the results show marked
compatible receiver should be capable of              differences, so that several dB is not of great
Gb/s performance.                                     importance to the overall conclusions.
   The amplifier noise density is assumed to          From this analysis it seems that the most
be 5 pA /    Hz . The current due to ambient          promising area of application, where optical
                                                      links can offer a distinct channel with greater
light i Ambient is a difficult quantity to estimate
due to widely varying conditions. A value of
                                                                                        Page 12 (22)
                                   Power                               Normalised      energy
 Standard                          consumption(W)   Bit rate(Mb/s)     consumption(J/Mb)
 IEEE802.11(g)[51]                 1.25             54                 2.31E-02
 Ultra-WideBand(UWB)[52]           0.75             114                6.58E-03
 Bluetooth[53]                     0.1              0.72               1.39E-01
 Optical link[46]                  0.3              150                2.00E-03

             Table 1. Power consumption for different communications standards

                                                    takes place between a handheld and a retail
                                                    terminal, albeit at low data rates [54]. Such a
range than the equivalent RF channel is             concept might be extended to retailing of high
where narrow FOV LOS channels are                   bandwidth content such as DVDs and CDs to
available.                                          portable players. In the future this might
Most of the links considered here will be used      require several Gb/s in order to achieve
to connect portable terminals to data               reasonable download times (Samsung have
infrastructure,    so    power    consumption       recently introduced a telephone with a 3Gb
becomes a consideration. Table 1 shows the          disk). Theft of content would be limited by the
power consumption of several short-range
                                                    confined nature of the optical signal, and
RF wireless communications standards
                                                    extremely high spatial bandwidth could be
normalised by the bit rate, and that for an
                                                    achieved within the retail environment. This is
optical link. The data shown are for particular
products, although are broadly representative       relatively straightforward to achieve with a
for examples of the same type.                      directed OW link used in a ‘point and shoot’
The optical link is the most efficient, by a        manner, or in a booth in which the link
substantial margin. This is largely due to the      environment can be controlled.
baseband nature of the optical channel and          Telematic applications
the resulting simple transmitter and receiver          Over longer distances the superior link
architecture. In the case of the RF systems         budget of narrow FOV systems makes OW
the complex transceiver structures dissipate        attractive for telematic applications, such as
substantial amounts of power. In all cases          road pricing and navigation. The German
(both optical and RF) the link losses are a         government has adopted an optical
small part of the overall power consumption.        communications system for its tolling system
Within the 4G framework UWB is considered           for freight vehicles [55], and an International
a likely solution for ad-hoc networks over          Standards Organisation (ISO) standard (ISO
distances <10m, and the example in table 1          CALM 204) has been defined for such
offers data rates of 114Mb/s at ranges of up
                                                    systems. Several train-operating companies
to 10m. However its energy consumption is a
                                                    are      also     investigating    FSO      for
factor of three higher than the example
                                                    communications with trains, in order to
optical link.
At distances <1m where there is a line of           provide broadband ‘to the seat’, and IRDA
sight very high data rate (1Gb/s) optical links     has formed The Travel Mobility Special
appear competitive with UWB, offering lower         Interest Group (IrTM).
power consumption and better intrinsic              Hotspots
security. Within       the   Infra-Red    Data      In general, optical wireless suffers from low
Association (IRDA) the IrBurst Special              receiver sensitivity when compared with RF,
interest group has identified this area, and        but has the advantage of good spatial
have specified links broadly in line with the       confinement, with the ability to maintain
results shown here.                                 sufficient power density for good data
   The IRDA is promoting ‘Financial                 reception over relatively small areas.
Messaging’, where a secure transaction              Optical wireless ‘hotspots’ that offer localised
                                                    high bandwidth connectivity have been
                                                                                      Page 13 (22)
suggested[8] and the 4G model of
heterogeneous standards allows this to be
included,     with   the   optical   capacity                    Under conditions of high blocking (the optical
augmenting that provided by RF. Figure 6                         link is blocked 10% of the time) the combined
shows an example of this approach. Low                           system delivers a lower delay (by a factor of
bandwidth coverage is ubiquitous, and this is                    six) in packet delivery compared with the
augmented at places where people                                 radio LAN alone, and the system is a factor
congregate or lobbies of buildings so that                       of nine times as efficient.
large file transfers and tasks requiring high                    In this case the optical link offers increased
bandwidth can be undertaken. In ‘regular’                        efficiency and capacity for the wide area
environments such as open plan offices the                       coverage RF LAN by augmenting capacity
hotspots may provide complete coverage,                          when it is needed. Within the framework of
with steps taken to ensure Optical LAN                           4G systems the optical link can be
coverage.                                                        considered to be another channel with
                                                                 specific distinct characteristics.
                          Transmitter                            There are several routes to Gb/s coverage in
                                                                 a hotspot. Perhaps simplest is the use of
 FOV of antenna                         Coverage provided by
                                                                 tracking narrow FOV links, as sold by
 matches 'cone angle'
 of hotspot to ensure
                                        low-bandwidth standard
                                                                 JVC[60], but this requires mechanical
 reception                                                       steering mechanisms. A potential alternative
                                                                 is the multiplexing of narrow LOS channels
                                                                 using multibeam transmitters. Further
                                                                 potential gains are available using imaging,
                        High bandwidth 'Hotspot'
                        coverage area
                                                                 multichannel receivers[20, 21, 61]. These
                                                                 approaches use the spatial multiplexing
                                                                 available due to the high gain optical
Figure 6. High bandwidth ‘Hotspot’                               antennas (lenses) and the ability to do very
                                                                 precise beamforming and steering when
   Very simple hybrid approaches combining                       compared with RF approaches. An
optical and radio frequency links have been                      architecture that uses wavelength to steer
proposed recently for short-range (indoor)                       narrow beams to terminals within the
communications [56, 57]. Reference [58]                          coverage area is presented in[62]. This offers
describes protocols that use RF signalling                       the potential for multi-Gb/s transmission to
                                                                 terminals using a simple passive base
and reallocate the optical sources under
                                                                 station. In this case there are formidable
blocking conditions.
                                                                 challenges in providing an uplink from
Studies by Hou et al [59]have shown the                          terminal to base station, and this architecture
advantage of this, especially in environments                    is well suited to a broadcast ’hotspot’ model.
where the required capacity is asymmetric                        As well as the fine control of narrow power
and the link capacity matches this. In the                       limited channels an alternative approach is to
study a 100Mb/s optical downlink (from                           increase the power transmitted, and a
infrastructure to portable terminal) was                         possible method may be enabled by a switch
combined with a 10Mb/s RF LAN. The optical                       to solid-state lighting. The efficiency of solid
link is subject to blocking, and depending on                    state lighting using LEDs is increasing
the duration of the blocking the system                          rapidly, and particularly in Japan there is an
performs a vertical handover to an RF LAN.                       interest       in      using       these      for
Downlink traffic is four times that of the                       communications[63-65]. If illumination power
uplink, a number that is typical within the                      is     available    for    data     transmission
Internet, and the decision to switch between                     approximately 30-40dB more power is
standards is taken using a fuzzy inference                       available than the case considered in Figure
engine.                                                          5 (assuming LED lighting units of the type



                                                                                                   Page 14 (22)
                                   180

                                   160

                                   140
               10log10(EbR b/No)




                                   120


                                   100                                                                1000Mb/s OOK
                                                                                                      100Mb/s OOK
                                   80                                                                 10Mb/s OOK
                                                                                                      1Mb/s OOK
                                   60
                                           Radio dref =7m
                                   40      Radio dref =96m
                                           Optical
                                   20 -1                   0                            1          2
                                    10                10                           10            10
                                                               Hotspot radius(m)


Figure 7. Comparison of RF and Optical communications for ‘Hotspot’ using solid-state
lighting for communications.
                                                 and that one may dominate leads to two
in[63]). This would increase the available       separate two-ray estimations, depending on
optical range        for   high  data rates      the reflectivity of the surfaces. For the
considerably.                                    geometry shown these yield d ref in the range
Figure 6 shows a ‘hotspot’ geometry with a
base station placed above a receiver plane       of 7-96m. In [66] a three-ray model is
within a room. The coverage area of the          proposed, and this simultaneously takes into
hotspot is of radius d, as shown in the          account reflections from ceiling and floor in
diagram, and together with the height of the     order to estimate d ref , but requires the base
base station above the receiver plane h (in      station to be mounted a distance from the
this case 3m) this sets the half angle of the    ceiling that is equal to the terminal antenna
hotspot θ h .                                    height from the floor. Assuming this value is
The optical transmitter is assumed to be a       0.7m d ref is 30m. In order to present
10W source, as would be provided by solid-       ‘bounds’ to the breakpoint in the figure curves
state lighting, with an emission profile that is
constant with angle, as was described            are        plotted        for       d ref = 7m and
previously. The beam half-angle is θ h . The      d ref = 96m with        γ = 4 beyond        these
receiver aperture is 15mm in diameter and all    distances. This assumption agrees with [48]
other receiver parameters are as before.         in that it is unlikely the effect of a breakpoint
The RF channel is considered to be provided      close to 10m would be observed in real
by some ‘future’ LAN, transmitting in the        measurements that were made up to a
2.4GHz ISM band, at a power level of             maximum link distance of this value.
+15dBm with no antenna gain.                                          Eb Rb
Values for d             and γ are required;
                         ref
                                                 Figure 7 shows                vs. the distance the
                                                                                                        No
measurements in [48] suggest that for link
                                                                                    receiver is from a base station situated 3m
distances of up to 10m γ = 1.91 at this                                             above the receiver plane. The geometry is
frequency in a LOS environment, so that an                                          that shown in Figure 3 (b).
estimate of γ ≈ 2 is appropriate. Assuming                                          The RF system shows superior coverage, but
reflections can occur from ceiling and floor                                        the optical system would provide sufficient

                                                                                                                     Page 15 (22)
                               200

                               180

                               160

                               140
            10log10(EbRb/No)




                               120

                                                                                                   1000Mb/s OOK
                               100
                                                                                                   100Mb/s OOK
                                                                                                   10Mb/s OOK
                               80
                                                                                                   1Mb/s OOK

                               60

                               40       Radio
                                        Optical
                               20
                                   -1              0                         1                 2
                                10                10                       10                10
                                                       Hotspot radius(m)

                       Figure 8. Comparison of 60GHz and solid-state illumination hotspot

coverage until the next lighting unit (as these                            ‘mostly LOS’ environments are required to
must be placed frequently enough to provide                                guarantee operation [69].
illumination). In the case of the RF system                                One of the major problems with the
the challenges are to provide sufficient                                   acceptance of OW has been the need for
bandwidth within the allocated spectrum,                                   geometric control of the transmission paths in
whereas for the optical link it is to modulate                             LOS systems and quasi-diffuse systems.
the LEDs at sufficient data rates. Research in                             There are a number of quasi-diffuse systems
this area is at a relatively early stage.                                  [72-74]that use novel schemes to mitigate
                                                                           this, but the general arrangement of RF
High capacity ‘future’ LANs                                                access points has always been a minor
It seems likely that RF LANs will migrate to                               consideration in indoor systems when
higher frequency regions of the spectrum, as                               compared with OW. As RF LANs move to
more bandwidth is required and the                                         higher frequencies this distinction is unlikely
implementation cost falls, so it is instructive                            to remain, with the optical and radio channels
to compare high frequency RF approaches                                    becoming ‘closer’ in nature.
with OW.                                                                   Figure 8 shows a comparison between a
There has been interest in 60GHz wireless                                  60GHz RF channel with a solid-state
systems over perhaps a decade or so or                                     illumination provided OW channel (emitting
longer, with several demonstrators [67] and                                10W as previously). The base station is 3m
recently funded projects[68]. The prospects                                above the receiver plane, and the geometry
and challenges for this medium are                                         is that shown in Figure 3(b). For the RF link
discussed in[69]. The availability of a large                              10mW transmitted power[69] is assumed.
region of spectrum is a major attraction, and                              The transmitter gain is set to match the half-
the improvement in semiconductor device
performance,      and     potential   fall    in                           angle of the hotspot θ h (as shown in Figure
infrastructure cost are seen as an important                               3) and the receiver gain is set to the same
enabling factor in its future widespread use.                              value (This is different from the previous 2.4
There have been a number of propagation                                    GHz example, where RF antenna gain is
studies that have investigated various                                     much more difficult to achieve). The receiver
environments and the effect of shadowing of                                noise figure is 6dB.
humans and other obstacles[70, 71]. The                                    The path loss exponent and reference
conclusions from these are generally that                                  distance must be determined for this

                                                                                                                  Page 16 (22)
configuration. Assuming either a two or three        the alternative has a similar requirement, and
ray model the values of d ref far exceeds the        OW may offer a simpler solution with low
                                                     power consumption.
range of interest, so the free-space equation
                                                     Longer term
holds and γ = 2 for the LOS environments             The results here show that the major long-
considered.                                          term challenge for OW is to improve the link
The optical model used is the same as for the        budget to that provided by RF systems, so
previous example, with a 10W emitter into a          that obtaining LOS geometries is less critical.
half-angle that is set by the hotspot                Coherent systems offer a potential long term
geometry.                                            solution, albeit with formidable challenges in
The optical and RF links are relatively close        providing a stable low-cost geometry[75].
in performance, with the RF providing slightly       Vertical cavity amplifiers using modified laser
improved coverage. The need to illuminate            structures have been demonstrated[76],
the room should mean that continuous                 although these usually operate with very
coverage using a series of hotspots may be           small etendue, which is unsuitable for OW
available in the optical case, however.              applications without modification. Substantial
Both systems are affected severely by                parametric gain [77] has been demonstrated
shadowing, and require some means to                 over     limited     bandwidths,     (although
provide a predominantly LOS path for a high          broadband operation is possible), as well as
capacity link. It therefore seems likely that a      the use of Avalanche Detectors [78]. Detector
60GHz LAN will be used to augment that               geometry and optimized devices also offer
provided      by     the     installed   802.11      potential for increased antenna size and
infrastructure, as is likely to be the case for      hence link margins[20]. The optimum solution
optical links. Despite the inferior link budget      may well be a combination of these
the optical system has the virtue of simplicity,     techniques, and further work is required to
being a baseband channel. It is also not             compare each of these approaches and
susceptible to the fading of the RF channel,         determine the best future approach.
and does not require complex coding and
modulation.
                                                     Research    directions                   and
Outlook for optical wireless                         conclusions
                                                     The distinct properties of the OW channel
Short term                                           can add to the 4G vision, with the possibility
The wide range of RF standards and the               of a future terminal having a number of
rapid increase in bit-rate available makes the       interfaces, both radio and optical. In order to
adoption of large area coverage OW LANs              achieve this work in the following areas is
unlikely in the short to medium term.                proposed, although this is not an exhaustive
However, within the broad range of strategies        list.
for 4G systems there is a common theme of            Link budget improvement: the major barrier
heterogeneous wireless links, and there are          to non-LOS systems is the power required at
situations where OW links can offer higher           the receiver, and work to improve this should
performance, or lower power consumption, or          be a major focus.
both than their RF counterparts. This is most        More       comprehensive        performance
likely where there are lines of sight or quasi       comparisons between short-range optical
line of sight. Links are likely to be narrow field   systems and their counterpart based on
of view and short range, or when the                 conventional RF approaches; there is a need
geometry can be controlled, such as in               to understand the properties of both channels
optical hotspots.                                    between the same points, so that alternative
Medium term                                          data paths can be modeled, and the
The use of higher frequency RF approaches            performance of a network that chooses the
to   obtain   bandwidth    requires    path          optimum path be determined.
management, and makes the use of OW in
‘managed’ situations more likely given that

                                                                                      Page 17 (22)
Network modelling: understanding how                 [5] K. Gosse, "New Radio Interfaces for Short
optical and radio communications might co-           Range Communications," presented at
exist.                                               WWRF9, 2003.
Signal processing: examination of radio              [6] J. D. Barry, Wireless infrared
processing techniques such as Multisensor            communications. Netherlands: Kluwer, 1994.
(MIMO) optical systems exploiting space and          [7] J. M. Kahn and J. R. Barry, "Wireless
angular diversity. Space-time coding for             infrared communications," Proceedings of the
optical wireless channels. Some work in this         IEEE, vol. 85, pp. 265-298, 1997.
field has already been recently introduced by        [8] D. J. T. Heatley, D. R. Wisely, I. Neild,
[79], where space-time codes are designed            and P. Cochrane, "Optical wireless: The story
specifically for optical channels, specifically in   so far," IEEE Communications Magazine, vol.
the      context     of    free-space      optics    36, pp. 72-+, 1998.
communications. A MIMO channel model                 [9] D. J. T. Heatley, D. R. Wisely, I. Neild,
applied to diffuse WOC has been recently             and P. Cochrane, "A review of optical
presented by [80].                                   wireless - What is it and what does if offer?,"
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of the optimum method of using the                   17, pp. 251-261, 1999.
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conditions, and the resulting performance            and fly-eye receivers for indoor infrared
improvement.                                         wireless communications," presented at 25
Visible light communications: fundamental            26 June 1992 Vancouver, BC, Canada, 1992.
capabilities and limitations of communication        [11] M. Kavehrad and S. Jivkova, "Indoor
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with illumination).                                  optical subsystems designs and their impact
                                                     on channel characteristics," 2003.
Future wireless standards offer a good               [12] IEC 60825-1. Safety of laser products
opportunity for the wider adoption of OW. In         part 1: British Standards Institution, 2001.
particular, as 4G networks will be highly            [13] S. H. Khoo, E. B. Zyambo, G. Faulkner,
heterogeneous, OW based air interfaces can           D. C. O'Brien, D. J. Edwards, M. Ghisoni,
be incorporated to terminals in addition to the      and J. Bengtsson, "Eyesafe optical link using
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components to implement them.                        Biosca, R. Mohedano, M. Labrador, F.
                                                     Munoz, K. Hirohashi, and M. Sakai, "Eye-
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                                                                                     Page 21 (22)
Appendix: standards                                  ISO CALM TC 204
                                                     This supports communications over 100m
IrDA                                                 (and may support longer distances) with
                                                     closing speeds between vehicles of 200km/h.
   The IrDA (Infrared Data Association) is a
                                                     Data rates of up to 100Mb/s and beyond are
worldwide       organization    that    develops
                                                     specified.
standards for point-to-point very short-range
optical communications, with a sub-meter
range. This optical wireless interfaces can be
found nowadays in a variety of portable
equipment, like mobile phones, laptop
computers and personal digital assistants,
among others. In its basic version this
standard defines a very low-cost, low-power
with a maximum operating range of 1 m and
data rates of up to 4 Mb/s. High speed IrDA
versions providing up to 16 Mb/s are also
available. Various interest groups are
working on financial messaging (IrFm),
transport (IrTM), and higher data rates
(IrBurst) and (UFIR).
   It is noted that the IrdA optical air interface
supports no mobility as it usually requires
manual alignment between transmitter and
receiver. The standard defines the physical
layer as well as different protocols used by
upper layers. An overview of IrDA can be
found in [81].

The IR Physical Layer of the IEEE
802.11 Standard
   The IEEE 802.11 standard, broadly known
for wireless LAN radio based air interfaces,
also includes an infrared based physical
layer. This air interface was developed in
parallel with the basic radio air interface and
it exploits diffuse transmission to provide
point-to-multipoint indoor connectivity. This
WLAN IR physical layer was originally
designed to support 1 and 2 Mb/s with
inexpensive optical transceivers. The main
application originally foreseen for the IR
802.11 was in establishing optical ad-hoc
networks. An introduction to the IR 802.11
standard can be found in [82]. Unfortunately
industry never was enough attracted to this
standard with the consequence that no
commercial products complying with this
standard were ever launched.


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