Optical wireless communication networks for first- and last-mile broadband access [Invited]

Optical wireless communication networks forfirst- and last-mile broadband access [Invited]

Qingchong Liu

Department of Electrical and Systems Engineering, Oakland University, Rochester, Michigan 48309

[email protected]

Chunming Qiao

Department of Computer Science and Engineering, The State University of New York,Buffalo, New York 14260-2000

[email protected]

Gregory Mitchell

Airin Technologies, Inc., Bethesda, Maryland 20814

Stuart Stanton

Aerospace Corporation, 15049 Conference Center Drive, Suite 600, Chantilly, Virginia 20151

RECEIVED 5 JULY 2005; REVISED 23 SEPTEMBER 2005;ACCEPTED 26 SEPTEMBER 2005; PUBLISHED 6 DECEMBER 2005

We review the progress in optical wireless communication networks for first- andlast-mile broadband access. The link budget is discussed. Modulation and codingare studied to deal with the atmospheric turbulence channel. Topics relatedto efficient topology design, survivable routing, and dynamic reconfigurationalgorithms are discussed. It is shown that optical wireless communicationnetworks have great potential to provide enough bandwidth for first-milebroadband applications. However, major challenges need to be overcome inpointing and acquisition, coding, and network design to make optical wirelessnetworks more popular with customers. © 2005 Optical Society of America

OCIS codes: 060.4250, 010.7060.

1. Introduction

In the past few decades, the bandwidth of a single link in backbone networks has beenincreased by almost 1,000 times thanks to fiber-optic wavelength division multiplexing(WDM) technology. Today, a fiber-optic backbone network can provide enough capacityto support a few gigabits per second of bandwidth for every office and home. However, fornine out of ten U.S. businesses with more than 100 employees, the first (or last) mile thatseparates the office building from the nearest fiber-optic backbone remains the bottleneck[1].

Although broadband communication systems based on DSL or cable modem technol-ogy have helped many homes and offices obtain so-called broadband access, the bandwidthdelivered to an end user is still limited [2]. More specifically, DSL, cable, or existing rfwireless systems using a carrier frequency lower than a millimeter wave cannot deliverhigh data rates such as that specified by IEEE, 802.3z Gbit ethernet. In the foreseeablefuture, high data rates over 1 Gbit/s can only be provided by use of lasers or millimeterwaves. However, the device technology for the latter is much less mature than the devicetechnology for lasers.

© 2005 Optical Society of AmericaJON 8036 December 2005 / Vol. 4, No. 12 / JOURNAL OF OPTICAL NETWORKING 807

Broadband access networks based on lasers can transmit through fibers by use of vari-ous forms of passive optical networks (PONs) or through the atmosphere (or free space) byuse of so-called optical wireless communication (OWC) systems. In this paper, we focuson OWC systems.

The OWC industry has experienced healthy growth in the past decade. In the early1990s, a group of scientists started a company in San Diego that provided equipment forfiber extension in the telephony network. Since then, notwithstanding the recent economicdownturns, the OWC industry has continued to evolve to include many new companies.This growth can be attributed to the following three major advantages of OWC over othercompeting technologies.

First, as a wireless technology, it has a niche when compared to competing wireline(including fiber-optic) technologies. In particular, the cost of OWC is about 10% that offiber communication. Also, it takes only a few hours to install an OWC link in the firstor last mile, about the same time as to set up a rf link over the same distance, whereas itmay take months to lay down a fiber. Note that, contrary to a popular myth that alignment isdifficult, past experiences have shown that it only takes two trained (and properly equipped)technicians each about 2 h to align an OWC link between two cellular towers 4 km apart.In fact, extensive studies conducted have shown that even an OWC link between buildingson the shore and ships at sea can have a stable alignment [3]. Another study conductedin Tokyo also concluded that the sway of a 60 floor building is negligibly small. Similarobservations were made for a 61 km link in Germany [4].

Second, an OWC link has longer reach and better quality than a wireless millimeter-wave link. More specifically, although an OWC link can easily cover a distance of morethan a kilometer, a millimeter-wave system cannot do so without the aid of repeaters [5].Additionally, a wireless millimeter-wave link is vulnerable to rain. An OWC link in con-trast can be affected by dense fog. In most places, it is much less common to have fogthan rain. Moreover, experiments have shown that, even in London, a city famous for itsfoggy weather, the chance of having fog that breaks down a class I eye-safe OWC link at155 Mbits/s over 1 km is less than 0.1% [5] (whereas the burst loss rate in existing rf wire-less systems is about 1%). It has been shown that for most cities in the U.S., an OWC linkcan achieve 99% link availability over more than a mile at 1.25 Gbits/s [6]. Note that thelasers used in that study required high power (more than 1 W) and therefore are expensiveand not eye safe. A key challenge is thus to develop low-cost, low-power, and eye-safesolutions (e.g., using a class I eye-safe laser) to provide reliable OWC communication forfirst- and last-mile applications through novel networking and link or physical layer tech-nologies.

Third, a laser focuses all the energy of a signal into one narrow spot of about 1 mrad(which translates to a circle of about 1.6 m diameter over a 1 mile link), whereas a rf-wavesignal spreads itself much wider in the air. This makes an OWC system much more securethan a rf wireless system because it is hard to intercept a focused laser signal in the air.Furthermore, such an interception can also be easily detected. A rf signal in contrast canbe easily intercepted, and worse, such an interception can hardly be detected. For the samereason, an OWC link needs a lower power and is thus more environmentally friendly (as itcreates no interference). For example, the lead automotive company in the U.S. allows theuse of OWC but bans the use of rf for their building-to-building communications.

Currently, thousands of OWC point-to-point links are deployed as fiber extensions.Looking forward, the major technical hurdles that must be overcome for OWC to be widelydeployed in first- and last-mile situations is the low availability and reliability of an OWClink due to heavy or dense fog, the relatively high cost at high data rates, and the difficultywith transceiver alignment. So far, it has been suggested that a millimeter-wave link beused as a backup to an OWC link to improve the availability and reliability, but such an


approach usually doubles the cost, even though the backup link cannot provide the samehigh data rate as the OWC link. We believe that, in addition to innovative link and phys-ical layer techniques, building and operating a properly designed OWC network can notonly mitigate the negative effects of heavy or dense fog but can also reduce the per-link(connection) cost.

This paper discusses physical and link layers and networking techniques to improve thequality of an OWC link. Achieving survivability is also studied, as are the issues of pointingacquisition and tracking. The interested reader is referred to the papers by Kedar and Arnon[7–9] for more detail on the causes, effects, and mitigation of optical misalignment.

The rest of this paper is organized as follows. Section 2 discusses the link budget foran OWC link. Section 3 describes modulation and coding schemes for overcoming thescintillation caused by atmospheric turbulence. Section 4 describes networking research is-sues, including lost-cost topology design, survivable routing, and dynamic reconfigurationalgorithms. Section 5 concludes the study.

2. Link Budget

A large number of publications, for example, Refs. [10] and [11], represent a mature levelof understanding of the nature of the atmosphere in its role as an optical medium for anOWC link. It is useful to review the essential challenges and distinctions from the fiber linkor equivalent short rf links to set the foundation for making OWC architecture trades. Thetypical OWC link budget usually sums the decibel losses due to a series of mechanismsin coupling a source beam to a collimated free-space beam to a receiver aperture and ulti-mately to a photodetector defined with a required photon count per bit at a desired bit errorrate (BER) and data rate. At a top level, written in a manner that defines the required sourcepower, such a budget appears as

Ps = Lt +La +Lr +Lc +Pd , (1)

where each item is in decibels, and

Ps is the source power to be budgeted;

Lt is the lumped efficiency term for optical losses to produce a collimated beam;

La and Lr are the lumped transmission terms for simple atmospheric losses (absorp-tion, scatter, fog) and line-of-sight loss, respectively;

Lc is the lumped coupling efficiency term at the receiver, including overfill loss,optical loss, receiver mispointing, and coupling effects associated with scintillation;

Pd is the required detector power (derived from photons/bit).

Here we simplify the important simultaneous consideration of signal-to-noise ratio(SNR) management that supports the BER at the detector. This would include design fea-tures such as suppression of ambient light noise through spectral and spatial filtering andoriginal SNR or modulation depth behaviors of the source. There is no dispersion effectin air to be concerned with and no nonlinear optic effects in air at practical power levelsand beam sizes. In this equation the losses L are positive decibel numbers. Several of thesebudget elements are random variables, with various probabilities and correlations. Thesebehaviors factor heavily into the network topography and individual link-element designconsiderations. The formation of a link budget must use values for each of these itemsbased on probabilistic descriptions (e.g., total included probability of 99.9%) consistentwith a simultaneous link availability budget.


Although the atmosphere has many wide transmission windows from the visible to thefar infrared, it is reasonable to assume that an operating wavelength such as 1550 nm willbe chosen for OWC to leverage mature component technologies from the fiber domain,at least to include sources and receivers. This assumption is consistent with several tradesone might otherwise consider. First of all, the obvious reason for using optical communi-cation is the high carrier frequency and potential bandwidth compared to rf. With WDMtechniques, the potential capacity of one fiber over about a 30 nm wavelength span near1550 nm is of the order of 1 THz. No demand for last-mile bandwidth is likely to comeanywhere near this, and thus there is no need to shorten the wavelength (increase the car-rier frequency) even though transmission and dispersion considerations in clear air wouldcertainly allow it. Even in the absence of WDM, a link bandwidth of 2.5 would easily meetlikely needs for large numbers of consumers. Second, the small free-space beam diver-gence, which is a major advantage in potential link-power reception efficiency comparedto rf, is only marginally improved as one shortens the wavelength. Since the divergencescales linearly with inverse wavelength, and the short distances of interest are unlikely torequire expensive, large, and precise optical telescopes (unlike optical links from ground tospace), this advantage is already adequate at 1550 nm versus ∼ 1 cm wavelengths. Anotherconsideration is the absence of multipath and near-field scattering effects in OWC, againdue to the short wavelength compared to rf, which would not be significantly better forwavelengths between 1550 nm and blue light. Finally, it should be noted that a modest dis-advantage in the strength of atmospheric scintillation [11] occurs as wavelengths go fromthe near infrared to blue, so the near infrared is a practical wavelength regime.

It has been shown in many references, for example, Ref. [12], that transmitter and re-ceiver telescope apertures of the order of several inches are adequate and practical for linkdistances from hundreds of to a few thousand meters. A potential disadvantage not seenwith rf is the need for a subwavelength optical figure, but this is not a practical problem foroptics of this size; however, it easily could be a problem for much longer links. Hence, froma cost and practical size perspective, the optical telescope for each home is fundamentallysimilar to a consumer’s small astronomical telescope. An additional challenge, of course, isthe pointing accuracy and stability, allowing the advantage of the low-divergence beam tobe useful. Overfill of a receiving aperture by the transmission beam footprint is a commonpractice to minimize the complexity of the pointing solution, such that jitter of the order of10% of the full width at half-maximum spot (e.g., of a Gaussian distribution) essentiallycauses no power variation effect. Various available commercial solutions that uniquely bal-ance the link budget trades in pointing, overfill, and source power are the subject of manypapers and may or may not include active stabilization and tracking schemes.

Of the complex stochastic phenomena in the link budget, there is some degree of sep-aration into two main categories, namely, scintillation and everything else. Scintillation[10, 11] is due to variations in the index of refraction of clear air turbulence causing a co-herent beam to become distorted such that time-varying interference effects appear on thedetector. It is the same effect as a twinkling star observed by eye or a small telescope, whichis caused by turbulent mixing of air cells of the order of a few centimeters that have smalltemperature differences. In effect, the air volume along the beam contains a time-varyingdistribution of long focal length air lenses. The index of refraction of air differs from unityin the fourth decimal place, but the cumulative index change effects of parts-per-million-scale variations are significant compared to the wavelength of light, thus causing dynamicinterference effects.

Other effects, such as birds and dense fog, will actually reduce the transmission tozero for periods of hundreds of milliseconds (birds) to minutes or hours (fog). Dense fogis a fundamental problem for visible and near-infrared wavelengths because of the sizeand density of the water droplets and the ensuing multiple-scattering diffuser effect that


cannot realistically be overcome by laser power. Furthermore, the issue of fog in particularis challenging because the relatively short distances of interest may be shorter than thecorrelation length for the presence of fog. That makes it difficult to choose reasonablealternate routes using available OWC network links from point A to point B in fog if pointC is likely to be in the same fog.

Although the occurrence of dense fog may be uncommon in many locations and tol-erably common in others, one often has to examine the phenomenon as the main limit tonetwork availability. Given the maturity of technologies that go into the individual link bud-get, the major design effort will be the examination of the network topology and manage-ment techniques responsive to the availability expectations of the user and the availabilitybreakdown into all system elements. In a location where dense fog is rare and occurs onlyin patches (e.g., near a pond), well-designed primary and alternate beam routes among arelatively small number of links may meet the requirements (e.g., a reliable link across thepond). In other cases, a topological solution may not be practical for the weather patternsand correlation lengths pertaining to fog in a geographic region to be served by an OWCnetwork, requiring a hybrid (e.g., rf backup) solution for times when OWC is simply notusable. This may be far more cost-effective and rational for a low-price service market thanfinding extraordinary or expensive techniques to force the singular OWC solution. At a lowservice price, a user may simply tolerate lack of service, as we are used to with wirelesslocal area networks (WLANs). At a different price, a user who demands total availabilityof a network with high average capacity and some small part of the time not operating atthe full data rate might be well served by a hybrid network.

3. Modulation and Coding

An optical beam through the atmosphere experiences scintillation, attenuation, and scatte-ring [10, 11]. The atmospheric turbulence causes wavefront distortion, random vibration ofpolarization, loss of spatial coherence, reduced mixing efficiency in the receiver, and de-graded detection performance [13, 14]. For optical communication systems employing in-tensity modulation, the atmospheric turbulence leads to scintillation in the received signal.The scintillation is a random process and is the largest challenge to optical communica-tion systems through the atmosphere. The attenuation can be caused by inclement weatherincluding dense fog, rain, and snow. In most areas, the probability of having severe attenua-tion to break an optical link can be ignored for access applications. The degradation causedby scattering is so small that it can also be ignored for access applications.

Scintillation of optical signal is always present on a sunny day. In Ref. [15], fieldmeasurements were performed with a link of 2.4 km in San Diego. It was found that thescintillation was of the order of 2–4 dB at 7:10 PST and 17:30 PST and reached 29 dB at13:20 PST. Continuous observations from 6 PST to 21 PST on 25 February 1997 [15], asunny and warm day, showed that the scintillation was the worst around 13:30 PST and wasvery bad for several hours. The scintillation is directly related to the difference in air andground temperature [15]. Observations on an overcast day and a rainy day [15] showed thatthe trends of the scintillation curves were about the same as on a sunny day. Similar resultswere observed by Ochs and Lawrence in experiments through 250 , 500 , and 750 m [16].Therefore, optical wireless systems have to be designed to provide robust services in thepresence of scintillation. This is a big challenge to the physical layer, including modulationand coding, synchronization, signal design, and acquisition.

Traditional optical communication systems through atmosphere have employed eitheron–off keying (OOK) or pulse position modulation (PPM). OOK systems are popular incommercial applications because abundant parts are mature and available from fiber com-munication. PPM has been employed by the Jet Propulsion Laboratory for ground-to-spaceoptical communication in which a high-power laser is employed [17]. PPM can be regarded


as a constrained version of OOK if one thinks of PPM as a precoding followed by OOK[17]. The precoding can give some gain.

In moderate cloud and fog conditions, the additional effect of temporal broadening isimposed on the optical signal (in addition to the expected power loss). This impairs detectonefficiency and induced intersymbol interference (ISI) brought on by multipath propagationof photons in the optically thick clouds and fog. Consequently, the communication sys-tem’s BER and throughput are further degraded by these conditions. Research has shownthat when OOK modulation is used, these effects can be mitigated by employing decisionfeedback equalizer structures in the receiver architecture [18].

The rest of this section focuses on the progress in modulation and coding for opticalcommunication through a turbulent atmosphere. To understand the effect of scintillation onthe BER, we start with OWC systems employing OOK. Then we review the progress madein optical communication through a turbulent atmosphere using either PPM or subcarrierphase-shift keying (PSK) intensity modulation.

3.A. On–Off Keying

The baseband electrical signal in the transmitter of an optical communication system em-ploying OOK can be written as

x(u, t) =∞

∑k=−∞

akg(t− kT ) , (2)

where ak ∈ {−1,1} is the kth symbol of the random data, T is time, and g(t) is the shapingpulse with g(t) = 1 for t ∈ [0,T ] and g(t) = 0 for t /∈ [0,T ]. The intensity of the transmittedlaser beam can be written as

It (u, t) = 1+∞

∑k=−∞

akg(t− kT ) . (3)

This laser signal goes through the atmosphere, which randomly changes the phase andamplitude of the laser signal. At the receiver, the signal is picked up by a telescope, focusingthe laser beam onto a photodetector. The photodetector output signal can be written as

Ir (u, t) = K

[A(u, t)+

∞

∑k=−∞

A(u, t)akg(t− kT )

]+n(u, t) , (4)

where K > 0 is a constant for the attenuation in free space, the photoelectric conversionefficiency of the photodetector A(u, t) is a random process caused by the atmospheric tur-bulence, and n(u, t) is the average white Guassian noise (AWGN).

The random process A(u, t) contributes to the scintillation of the received signal. In Ref.[19], a 61 cm cross section of a laser beam at 694.3 and 632.8 nm was recorded at rangesfrom 200 to 250 m. It was found that the log-amplitude variance increased for ranges upto about 700 m, at which distance saturation occurs and the variance does not increase withthe range anymore. For stronger turbulence, it decreased slightly with link length [16, 20].The power spectral density of a laser beam propagating through a turbulent atmospherehas been well measured and documented. In Ref. [21], the power spectral density of thereceived laser beam was measured for near-ground horizontal turbulent paths in the therange from1 to 2.5 km. The bandwidth of the measured power spectral density is less than1 kHz. Similar results were reported through extensive experiments by Russian scientists[22–24] in snow and rain and by Quinn and Alyassini in 1982 [25]. In Ref. [26], measureddata showed that the spectral density at 2 kHz was at least 25 dB below the peak power of


the optical signal. In summary, the scintillation is a baseband random process with a narrowbandwidth.

When the atmospheric turbulence is not strong, the received intensity has the lognormaldistribution [12, 16, 25, 27–30] for an unmodulated laser beam. The theory was developedby Tartarski [27] in 1961 and De Wolf [28] in 1969. Impressive experiments by Hohn [29]in 1966 through two links each of 4.5 and 14.5 km gave the earliest report on the distributionof the received intensity for an unmodulated laser beam. It was shown that the lognormaldistribution was of limited accuracy. Experiments by Fried et al. [12] in 1967, Ochs andLawrence [16] and Ochs et al. [26] in 1969, and Quinn and Alyassini [25] in 1982 showedthe received intensity had lognormal distribution. Although this distribution is commonlyused, it is worthwhile to consider some other models that have been developed. In Ref. [31]a heuristic model of irradiance fluctuations for a propagating optical wave in a weakly inho-mogeneous medium is developed. In Ref. [32] the same authors develop general models forpredicting power fluctuations over finite-size collecting apertures for small-scale irradiancefluctuations modulated by large-scale irradiance fluctuations of the wave. In Ref. [33] theauthors developed a two-parameter model distribution based on a doubly stochastic the-ory of scintillation. In this model, small-scale irradiance fluctuations were modulated bylarge-scale irradiance fluctuations of the propagating wave, both governed by independentgamma distributions.

Let Ra (τ) be the autocorrelation function of A(u, t). Let Ra (0) = 1 so that the averagepower of the converted electrical signal can be normalized. The distribution of a sampleA(u, t = t0) at any time instance t = t0 is determined by the parameter of the fading levelσ. Since the random process A(u, t) is stationary, a random variable with lognormal dis-tribution is used in modeling the channel when the turbulence is not strong [34–37]. Therandom process

A(u, t) = k exp [X (u, t)] (5)

is a lognormal random process, where k > 0 and X (u, t) is a stationary Gaussian randomprocess with zero mean and the autocorrelation function Rx (τ) with Rx (0) = σ2. The sam-ple A(u, t = t0) has the probability density function

f (a) =1√

2πσaexp

[− (ln a−m)2

2σ2

], (6)

where m = ln k. The nth moment of the sample A(u, t = t0) is

E [An (u, t = t0)] = kn exp(n2

σ2/2). (7)

When strong atmospheric turbulence is present, the scintillation random process has theK distribution [24, 38–40]. In Ref. [24], data for experiments in rain through two linkseach of 2.5 and 5 km were reported. It was found that the received laser intensity can beapproximated by the K distribution:

f (a) =2

IΓ(y)y(y+1)/2z(y−1)/2Ky−1 (2

√zy) , (8)

where z = a/I, I is the average of the received laser intensity, y = 2/(β2−1

), β is the

scintillation index [24], Γ(x) is the gamma function, and Ky (x) is the modified Besselfunction. The theory for the K distribution was developed in Refs. [38] and [39] usingrandom walk.

Fried et al. [12] in 1967 performed thorough measurements of an unmodulated laserbeam propagating over an 8 km path near the ground in winter, spring, and summer. Thecollection aperture ranged from 1 mm to 1 m in diameter. It was shown that the variance


of the received laser intensity decreased smoothly for diameters from 1 mm to 10 cm. Fordiameters between 10 cm to 1 m, the variance showed no decrease. In other words, havinga receiver aperture greater than 10 cm cannot help reduce the scintillation or cannot helpsee better in astronomy [41]. Churnside et al. [42] studied the aperture size and bandwidthrequirements for measuring optical signal scintillation in the atmosphere. Two links of1.2 and 2.4 km were employed. The aperture size was determined by the active area of thephotodiode in the receiver. The smallest active area was a square of 0.2 mm on a side. Threecircular active areas were employed for comparison, with diameters of 0.5, 1.0, and 2.5 mm.It was shown that the variance of the received laser intensity decreased significantly whenthe detector aperture increased. The detector aperture near the wave coherence length andthe bandwidth near the ratio of the transverse wind velocity to the wave coherence lengthwere recommended for good measurements [42].

More experimental results have been achieved in recent years. In Refs. [43–45], anoptical ground-to-ground direct-detection transmission experiment with a 980 nm laser at1 W was performed from the mountain Wallberg in the German Alps down to Oberpfaf-fenhofen (west of Munich) over 61 km. It was found that the beam path suffered stronglyfrom optical turbulence especially at the near-ground part along the last kilometers be-fore the receiver [44]. Under strong turbulence conditions the secondary transmitter with4 m lateral offset to the first provided statistically independent speckle patterns at the re-ceiver and improved system performance dramatically. Data transmission tests at bit ratesof 100 Mbits/s were performed. A two-transmitter configuration with a transmission powerof 1 W per laser and a sensitive avalanche photodiode (APD) receiver front-end pluggedinto a 75 mm receiving telescope was employed. In the presence of severe scintillation,BERs below 10−4 were observed with synchronization losses of the data and clock recov-ery. Tests at 155 Mbits/s (OC-3) and 270 Mbits/s (SMPTE 259 M) were not successful dueto high atmospheric attenuation. In Ref. [46], communication experiments were performedbetween a laser communication ground terminal and an optical payload on board a geosta-tionary satellite 38,000 km away. The downlink achieved a BER of the order of 10−10 inthe presence of atmospheric turbulence. The uplink achieved a BER of 2.5×10−5 becausethe turbulent layer near the Earth’s surface affects the uplink signal more than it does thedownlink signal. Long-term statistics of the optical signal were well recorded and agreedwith theory, which means that the stationary stochastic process can be applied to both staticlink analysis and dynamic link performance analysis.

The atmospheric turbulence often raises the BER of OWC with OOK so high that itbreaks the link [15, 47]. In Ref. [47], it was shown that an OOK system needs a SNR of65 dB to achieve a BER of 10−4, which means a high cost. An analytical expression of thebit error floor [47] was given in Ref. [48] for OOK optical communication systems in thepresence of atmospheric turbulence:

P(T ) = (1− p)Q(

ln 2− ln Tσ

−σ

), (9)

where p is the probability of sending the information bit 0, T is the decision threshold, and

Q(x) =∫

∞

x

1√2π

exp(−t2/2

)dt. (10)

Many methods in optics have been proposed to improve system performance in the pres-ence of atmospheric turbulence. Diversity receivers have been considered since the 1970s.Early work included the aperture integrator, the channel-matched filter receiver, and thephase-compensated receiver. The early work was well described in Ref. [14] and refer-ences therein. Kim et al. measured the reduction in signal scintillation when the number of


transmitters increased from 1 to 16 for 1.2 and 10.4 km links with a receiving telescope of 8inches aperture [50]. It was shown for the 1.2 km link that four transmitters can apparentlyreduce the variance of the received laser light better than two transmitters or one transmit-ter. Eight or 16 transmitters made the variance a little smaller than did 4 transmitters. Forthe 10.4 km link, multiple transmitters reduced the variance a little. The authors concludedthat 16 transmitters can handle 7 dB of fade margin. Haas and Shapiro [51] showed thatin the high-noise regime increasing the number of transmitters can help increase capacity.They also found that for a system with two transmitter and two receiver apertures in mild tomoderate fading, the use of an optimal receiver provides only 1%–10% capacity improve-ment over that obtained with a simple photon bucket receiver. The achievable informationrate for an OOK system with turbo code was studied in Ref. [52]. An intensity estimatorfor an OOK system was given in Ref. [53].

A novel coherent OWC system and method based on the grating writing and read-ing of a multiple-quantum-well (MQW) device was presented in Ref. [54]. The MQW,a new generation of photoelectric device developed in the 1990s, is one of the attractivefeatures of this method including high sensitivity and fast response. Compared with theconventional coherent detection method, the proposed method eliminates the complicatedintermediate-frequency tracking system. Experimental results show that the method can re-press the wavefront distortion introduced in transmission through the turbulent atmosphere.

Primmerman et al. performed an experiment over a 5.5 km horizontal path with a 241-channel adaptive optics system [55]. The experimental results showed a significant degra-dation in correction as the scintillation increased. The theory of adaptive optics in lasercommunication can be found in Refs. [56] and [57] and references therein. Lloyd–Hartgave an introduction to adaptive optics in Ref. [41].

In Ref. [34], maximum-likelihood sequence detection and pilot-symbol-assisted de-tection were applied to a 500 m building-to-building link at 675 nm and 3 kbits/s. Thetransmitted power was 0.95 mW and the receiver aperture was 8 cm. It was shown that2.4 dB gain was achieved by maximum-likelihood sequence detection and 1.9 dB gain wasachieved by pilot-symbol-assisted detection. These results were achieved at small scintilla-tion. The maximum-likelihood sequence detection in Ref. [34] needs 23 dB in the electricalsignal-to-noise power ratio to achieve a BER of 10−4.

Zhu and Kahn [35] proposed a single-step Markov chain model for the atmosphericturbulence channel. They derived two suboptimal maximum-likelihood sequence detectionalgorithms by use of per-survivor processing. Simulations for low atmospheric turbulenceshowed that these algorithms can give good gain. The detection theory was provided inRef. [36].

Zhu and Kahn [37] derived an upper bound for the pairwise code-word error probabilityin an OOK system in the presence of weak turbulence. The bound was applied to find anupper bound of the BER for block code, convolutional code, and turbo code.

Henniger et al. [58] simulated the performance of Bose–Chaudhuri–Hocquenghem(BCH) codes with (511,259,30), (255,131,18), and (127,64,10) using the statistics of opti-cal signals collected in [43–45]. It was shown that at a BER of 10−6 the coding gain was1.8 dB for the 61 km link and 2 dB for the 1.5 km link. When an interleaver with a depthof 200 kbits was employed, the coding and interleaving gain was 3.5 dB for the 61 km linkand 6.5 dB for the 1.5 km link. The difference was explained by the duration of fade beingmuch shorter in the 1.5 km link than in the 61 km link [58]. It was concluded that codeswith high redundancy are desired to keep the interleaver depth from being too high.

3.B. Pulse Position Modulation

In PPM, each of k > 1 information bits d = (d1 . . . ,dk), di ∈ {0,1}, is transmitted as apulse s = (0, . . . ,0,sm = 1,0, . . . ,0), where m = ∑

kj=1d j2 j−1. Let T be the pulse time. The


transmitted signal can be written as

s(t) ={

1 if mT ≤ t ≤ (m+1)T0 else . (11)

If one plots the transmitted optical signal versus time, it can be seen that the time is choppedinto slots. There is only one pulse in every M = 2k slots, whereas the rest of the M – 1 slotsare 0s. The vector s is a code word, and the map from the k > 1 information bits to the setof code words is 1–1 onto.

The duty cycle of a PPM system is 2−k, which is usually low. The low duty cyclegives sufficient time to turn the laser on or off, which makes it ideal for high-power andlow-data-rate applications such as deep-space communication. A PPM system transmitsk2−k bits per slot, which gives a flexible trade-off between photon efficiency and bandwidthefficiency [17].

Chan [59] analyzed the performance of a coded PPM optical communication system inthe presence of lognormal fading. It was found that coding gains increase with turbulencestrength, whereas the link margin required to combat fading increases too. It was shownthat for moderate turbulence strengths, the coding gain can offset the increases in signalenergy required due to fading to a large extent.

Prati and Gagliardi [60] studied block pulse encoding over multiple PPM frames.Maximum-likelihood decoding was developed for both shot-noise-limited and thermal-noise-limited optical receiver models. It was shown that optimal block decoders involveboth linear and quadratic operations, and block encoding over multiple frames can improvedetectability. Hamkins and Moision [17] compared the performance of Reed–Solomon(RS) codes and convolutional codes concatenated with PPM through the Poisson chan-nel. It was shown that, when iteratively decoded, concatenated convolutional codes areapproximately 0.5 dB from capacity over a wide range of signal levels, about 2.5 dB betterthan RS codes [17]. Moision and Hamkins [61] illustrated 0.5˘1.0 dB from capacity via theiterative decoding of a serial concatenation of a short convolutional code and coded PPMthrough a bit interleaver. It was proposed to compute and store only a subset of channellikelihoods. For a system with M = 256 PPM, a 4,096 bit interleaver, and eight iterations,0.1 dB degradation was observed when 1/64 of the likelihoods were kept for the AWGNchannel. Hamkins and Srinivasan [62] considered binary turbo codes with high-order PPMand an APD. It was shown that turbo codes outperform RS codes. Hamkins and Ceniceros[63] determined the capacity for an optical channel employing PPM and an APD. The de-tector output was characterized by a Webb-plus-Gaussian distribution. The capacity wasgiven as a function of the PPM order, slot width, laser dead time, average number of theincident signal and background photons received, and APD parameters. The results werebased on the current technology available. It was shown that for 256 PPM with a rate of 7/8coding, RS codes can handle all but the last 3.5 dB of the background levels that capacitypromises can be handled while operating at a BER of 10−6.

In Ref. [64], an Advanced Photonix model 118-70-74-641 thermoelectrically cooledAPD module and a near-infrared enhanced Perkin-Elmer 30659G APD were evaluated at532 and 1064 nm. The response of the APDs to 256 PPM laser pulses were recorded andstored for postanalysis. Probability density functions were constructed from the signal andnoise and compared with those obtained through an analytical model using the Webb-plus-Gaussian statistics. The BER was measured and compared with theoretical results. It wasshown that the 532 nm APD data matched with theory, and discrepancies were evidentfor the 1064 nm APD. A slot synchronization loop was tested on the laboratory-generateddetector output data and showed little to no loss when tracking-loop-generated timing wasemployed to calculate the BER. In Ref. [65], several types of practical slot synchronizerwere considered. A basic design involving analog correlators and slot gating was presented


with performance. Digital synchronizers were considered.Srinivasan and Vilnrotter discussed the statistical modeling of the APD electron output

[66]. The maximum-likelihood decision statistic for detecting PPM signals was derived. Itwas shown that for Webb-distributed output electrons, the maximum-likelihood rule is tochoose the PPM word in the slot of the maximum electron count. Hamkins et al.[63] derivedthe capacity of PPM systems on a general soft-output, memoryless channel and evaluatedthe capacity for optical channels including AWGN, Webb, and Webb plus Gaussian.

Impressive progress in adaptive detector arrays has been made by Vilnrotter et al. inRef. [67]. Also in Ref. [67], an optimal adaptive array receiver for free-space optical com-munication was studied. Kolmogorov phase screen simulations were employed to generatefocal-plane distributions of the received optical fields in the presence of turbulence. It wasshown that 5 dB can be gained over conventional single-detector photon-counting receiverswhen observing turbulent optical fields. In Ref. [68], it was shown that for ground-basedreception the number of array elements can be increased without incurring performancedegradation if the array telescope diameters exceed the coherence-length of the atmo-sphere. Maximum-likelihood detection of turbulence-degraded signal fields was developedfor PPM.

Hardware architecture for PPM can be found in Refs. [69–71] for low-cost implemen-taion.

3.C. Subcarrier Intensity Modulation

Subcarrier intensity modulation is a promising method for optical communication throughan atmospheric turbulence channel. In a subcarrier PSK intensity-modulated system as inFig. 1, a traditional PSK modulator is employed. The PSK modulator output drives theoptical modulator to modulate the intensity of the laser to be transmitted. At the receiver,a telescope focuses the received laser beam onto a photon detector. The photon detectoroutput signal is preamplified, filtered by a bandpass filter, and demodulated by a PSK de-modulator.

informationsource

PSKmapping

p(t)

p(t)

X

coswt

X

sinwt+

PSK Modulator

opticalmodulator

laser

(a) Modulator.

photodetectorpreamplifirer bandpass

filter

X

X

coswt

sinwt

matchedfilter

matchedfilter

sampling

decision

(b) Receiver. Fig. 1 Block diagram for the modulator and receiver in a subcarrier PSK intensity

modulated system. In [64], experimental results were reported for a link of 1.8 km using subcarrier DPSK with 360 MHz as the IF subcarrier frequency and a laser at 830 nm and 10 mW transmission power for 155.52 Mbps. It was shown that subcarrier PSK outperforms OOK. Eq. (6) in [64] gave the formula to compute the BER. Eq. (18)-(20) gave the integrals to compute BER for M-ary PSK subcarrier intensity modulated optical communications systems through the atmospheric turbulence. In [41], the uncoded BER was analyzed for both OOK and subcarrier PSK intensity modulated systems. The gain of subcarrier PSK intensity modulated system compared with a compatible OOK system was investigated in the nature of the stochastic process of the atmospheric turbulence. It was shown that more than 15 dB gain can be achieved by employing subcarrier BPSK intensity modulation compared with OOK at the intermediate turbulence level 2.0=σ and BER= 410− . In [42], the coded BER performance was studied for both OOK systems and subcarrier PSK intensity modulated systems. An upper bound was derived for BER in systems employing convolutional codes. It was shown that a subcarrier PSK intensity modulated system with the rate ½ convolutional code and the constraint length 3 gains more than 9 dB at 4.0=σ and BER= 510− compared with a compatible OOK system with the same channel coding. To achieve BER= 610− at 4.0=σ , the former system needs SNR=10 dB. Results in [42] agreed well with the results in [51] that the coding gain is larger in optical

Fig. 1. Block diagram for the modulator and receiver in a subcarrier PSK intensity-modulated system.

In Ref. [72], experimental results were reported for a link of 1.8 km using subcarrierdifferential phase-shift keying (DPSK) with 360 MHz as the IF subcarrier frequency and


a laser at 830 nm and 10 mW transmission power for 155.52 Mbits/s. It was shown thatsubcarrier PSK outperforms OOK. Equation (6) in Ref. [72] gave the formula to computethe BER. Equations (18), (19), (20) gave the integrals to compute the BER for M-ary PSKsubcarrier intensity-modulated optical communication systems through atmospheric turbu-lence.

In Ref. [48], the uncoded BER was analyzed for both OOK and a subcarrier PSKintensity-modulated systems. The gain of subcarrier PSK intensity-modulated system com-pared with a compatible OOK system was investigated in the nature of the stochastic pro-cess of the atmospheric turbulence. It was shown that more than 15 dB gain can be achievedby employing subcarrier binary phase-shift keying (BPSK) intensity modulation comparedwith OOK at an intermediate turbulence level of σ = 0.2 and a BER of 10−4. In Ref.[49], the coded BER performance was studied for both OOK systems and subcarrier PSKintensity-modulated systems. An upper bound was derived for the BER in systems employ-ing convolutional codes. It was shown that a subcarrier PSK intensity-modulated systemwith a rate of 1/2 convolutional code and a constraint length of 3 gains more than 9 dBat σ = 0.4 and a BER of 10−5 compared with a compatible OOK system with the samechannel coding. To achieve a BER of 10−6 at σ = 0.4, the former system needs a SNR of10 dB. Results in Ref. [49] agree well with the results in Ref. [59] that the coding gain islarger in optical communication systems through an atmospheric turbulence channel thanthe coding gain in an AWGN channel.

For subcarrier PSK intensity modulation, the output optical intensity of the transmittercan be written as

s(t) = 1+α [si (t)cos(2π fct)+ sq (t)sin(2π fct)] , (12)

where α ∈ (0,1] is the modulation index, fc is the intermediate frequency,

si (t) =∞

∑k=−∞

cos ϕk p(t− kTs) (13)

is the in-phase signal, ϕk ∈ {0,2π/M, . . . ,(M−1)2π/M} is the phase for the kth symbolrepresenting log2 M bits, p(t) is the shaping pulse, and

sq (t) =∞

∑k=−∞

sin ϕk p(t− kTs) . (14)

The inequality s2i (t)+ s2

q (t)≤ 1 holds to avoid nonlinearity. The received optical intensityin the receiver is

P(t) = A(u, t){

1+α [si (t)cos(2π fct)+ sq (t)sin(2π fct)]}

, (15)

where A(u, t) is the scintillation random process. The photon detector output signal can bewritten as

i(t) = a(u, t)+αa(u, t) [si (t)cos 2π fct + sq (t)sin 2π fct]+n(t) , (16)

where n(t) is the AWGN. It can be seen that the first term in Eq. (16) is the basebandscintillation random process, which does not carry any information. Filtering the photondetector output signal with a filter centered at the carrier frequency fc with the signal band-width plus the bandwidth of the scintillation process allows the filter output signal to bewritten as

i1 (t) = αa(u, t) [si (t)cos 2π fct + sq (t)sin 2π fct]+n(t) . (17)


Downconverting this signal into the baseband by mixing it with local oscillators ofcos 2π fct and sin 2π fct, one has the in-phase mixer output signal as

ri (t) = αA(u, t)si (t)+ni (t) (18)

and the quadrature phase-mixer output signal as

rq (t) = αA(u, t)sq (t)+nq (t) , (19)

where ni (t) and nq (t) are Gaussian noise processes with the power σ2g.

For OWC systems employing subcarrier BPSK through a turbulence channel, the prob-ability of bit error is [48]

Pb =1

2πσσgexp(−σ

2/2)∫ ∞

0x−2 exp

(−ln2 x/2σ

2)Q(

αxσg

)dx. (20)

When the ration of signal power to thermal noise power is large, this error probabilityapproaches zero. This result is not achievable by practical OOK systems [47] and is theadvantage of subcarrier PSK intensity-modulated systems.

For OWC systems employing subcarrier quadrature phase shift keying (QPSK), theBER can be written as [48]

Pb =1

2√

2πσσgexp(−σ

2/2)∫ ∞

0x−2 exp

(−ln2 x/2σ

2)Q

(αx√2σg

)dx. (21)

The symbol error rate for subcarrier intensity-modulated M ≥ 8 PSK systems is

Ps = 1−∫

π/M

−π/M

∫∞

0

12πσ

exp

[− r2

2σ2g−(ln r +σ2

)2

2σ2

]

×{

1√2π

+r cos θ

σg

[1−Q

(r cos θ

σg

)]exp

(r2 cos2 θ

2σ2g

)drdθ .

(22)

In Eqs. (21) and (22), the symbol energy is normalized to unity.A unidirectional laser communication link using quadrature amplitude modulation over

a path length up to 2.4 km was reported in Ref. [73]. The data rate was 160 Mbits/s with16-QAM and 64-QAM. A subcarrier frequency of 140 MHz was employed.

4. Networking Layer Techniques

OWC for broadband first- or last-mile applications has recently received much attention (asis evident from some of the papers published in the past few years alone [74–83]). However,past research on OWC has focused mostly on the point-to-point links and issues related tothe link and physical layers, with little on OWC networking research.

In this section, topics related to efficient topology design, survivable routing, and dy-namic reconfiguration algorithms are discussed. Some earlier work on topology control ofOWC networks was presented in Ref. [84].

In Ref. [83], the problem of how to adapt the backbone network to changing traf-fic demands (without considering survivability requirements was formulated as a multi-commodity flow and weight-matching problem with the objective being to maximize thenetwork throughput, while heuristic algorithms to accommodate dynamic traffic were de-scribed in Ref. [85]. Finally, Yang [86] proposed an approach to support the so-called high-availability class of traffic using one topology consisting of short OWC links only and thelow-availability class of traffic using another topology consisting of long OWC links.


4.A. Motivations

First- and last-mile access networks employing fiber cables are based on the star, tree,bus, or ring topology for distributing data. These topologies may also be used for OWCnetworks. Nevertheless, the flexibility of OWC links also makes it possible to build anOWC network with mesh topology. Since an OWC link can provide more than 1 Gbit/s(e.g., 2.5 Gbit/s) bandwidth over a long distance, e.g., 1 mile or longer, such an OWCnetwork consisting of these high bandwidth links will offer many exciting opportunitiesfor new applications. For example, one may establish a community-owned OWC networkin downtown (or other areas where laying down fibers would be too expensive) to connectmajor government buildings, hospitals, libraries, public schools, universities, and businesswith distributed servers and storage space to support peer-to-peer-like applications. AnOWC network using subcarrier PSK intensity modulation can also interconnect multipleWLANs (802.11) seamlessly, i.e., without having to demodulate the rf signal for signaltransmission.

Although much work has been done in designing wireline networks, the majority of theexisting techniques cannot be directly applied to OWC networks efficiently. For instance,in designing the topology of a fiber-optic network, the set of nodes that can and must beconnected is the same (and given). In addition, most of the research work has assumed agiven physical topology, and the task is to design a virtual (or overlay) topology using lightpaths (or wavelength-routed paths).

In designing the topology of an OWC network, one is given a set of buildings (houses)that can be connected. Although this set must include the buildings that originate or termi-nate the traffic to be carried by the OWC network being designed, other buildings that mayor may not be included in the topology must also be given. This is because, when usingOWC, each building is eligible for connection to only a subset of the buildings that have aline of sight and are located within a specified maximum transmission range. Thus a partof the topology design task is to determine which other buildings to include.

In addition, in designing OWC networks, the capital cost of the transceivers needed de-pends heavily on the desired maximum transmission range and minimum data rate. Evenwithin the maximum transmission range, the actual data rate that can be achieved dependson the transmission power (which may affect the lifetime of the transceivers) and the trans-mission distance.

Although the above-mentioned trade-off between transmission power and data rate alsoexists in other wireless networks including cellular and packet radio, the topology designproblem in those two types of network has a completely different set of objectives andconstraints, one of which is to limit the interference generated by the rf transmissions. Incontrast, with OWC, a transmitted signal will reach only one receiver and thus there is nointerference among multiple transmissions.

Meanwhile, the so-called topology control in wireless (mobile) ad hoc networks focuseson the trade-offs between energy consumption, interference, and connectivity but naturallydoes not consider the same problem of where to place transceivers at all. Accordingly, theexisting work on topology designs of rf wireless networks cannot be applied directly to thedesign of OWC networks either.

Perhaps one of the most unique characteristics of an OWC network that can be exploredis its ability to dynamically create an alternate link for low cost by simply redirecting thelaser beam. More specifically, as illustrated in Fig. 2, it is possible to share one trans-mitter on building A (or one receiver on building B) by preinstalling a primary and a sec-ondary transmitting (or receiving) telescope whose cost is low compared to the transceivers.Whereas the primary transmitting (or receiving) telescope is aligned with another building(say C) that is not shown, the secondary transmitting and receiving telescopes on buildings


A and B, respectively, are aligned with each another. By setting the 1-by-2 switch on build-ing A and the 2-by-1 switch (which could be a 2-by-1 passive coupler instead) on buildingB, we can replace the links from A to C and C to B with the link from A to B instantly.This is useful if the transceivers on building C fail. It is also useful when the links fromA to C and from C to B are used under normal conditions, but since they are much longerthan the link from A to B, they will no longer work well under dense fog (whereas the linkfrom A to B still can as long as their separation is under 400 m, for example). The use of apassive splitter or multicast-capable switch between the transmitter and multiple telescopescan also facilitate efficient load-balanced routing and multicasting.

Fig. 2 Beam-redirection routing in optical wireless communication networks. It is noted that in a wireline network, simply switching the transmitted signals to a different output is not sufficient as a physical link must be established between the two nodes before either of them can communicate directly. In a RF wireless network, on the other hand, directional (or smart) antenna may be used to form a (main) beam in one direction. However, the transmitted signal will interfere with the receivers in that direction, especially those geographically close to the intended receiver. In addition, due to the additional (smaller/weaker) beams formed in the process, even those receivers that are in different directions will be interfered. Accordingly, allowing the transmission direction to dynamically change may complicate the design as the amount of interference becomes very unpredictable. Furthermore, beam forming is usually done in software (using signal processing techniques) and as such, it is much less constrained with respect to which direction to choose, albeit the complexity and the cost of the transceivers becomes an issue. 4.2 Challenges and Opportunities As mentioned earlier, a main challenge to overcome is the degradation of an OWC link due to turbulence and dense fog. In the existing OWC networks, links longer than 400 m may become unusable in the presence of strong turbulence and dense fog. In addition, multiple links may be affected at the same time. As a result, the network may become partitioned, and some applications may experience several packet losses and delay, while some sessions/connections might also be terminated. The above challenge can be addressed by integrating OWC networks with complementary networking technologies based on either RF technologies or fiber-optic cables. On one hand, an OWC network can serve as fall-back resource when other types of connections simply fail (due to e.g., a fiber cable cut or rain which affects an RF link) or become overloaded. On the other hand, wireline networks such as Fast Ethernet, Gigabit Ethernet or fiber-optic metropolitan area networks (MANs) or RF networks can also be used to carry the overflow traffic from an OWC network in case it suffers from congestion or temporary/permanent links/node failures. The above challenges can also be addressed at several additional fronts including survivable topology designs, routing and re-routing algorithms, dynamic network reconfiguration, and cross-optimization of transport and application layer protocols. The following subsections describe related problems and approaches in more details. 4.3 Basic Off-line Topology Design and Optimization In topology design, it is typically assumed that the traffic matrix is given, and each element in this matrix specifies either the average amount of packet traffic or the required bandwidth between

Building B

Receiver Telescope Receiver Transmitter Telescope

Building A

Transmitter

Fig. 2. Beam-redirection routing in OWC networks.

It is noted that, in a wireline network, simply switching the transmitted signals to adifferent output is not sufficient because a physical link must be established between thetwo nodes before either of them can communicate directly. In a rf wireless network, onthe other hand, directional (or smart) antennas may be used to form a (main) beam in onedirection. However, the transmitted signal will interfere with the receivers in that direction,especially those geographically close to the intended receiver. In addition, due to the ad-ditional (smaller and weaker) beams formed in the process, even those receivers that arein different directions will be interfered. Accordingly, allowing the transmission directionto dynamically change may complicate the design as the amount of interference becomesunpredictable. Furthermore, beam forming is usually done in software (by use of signalprocessing techniques), and, as such, it is much less constrained with respect to whichdirection to choose, albeit the complexity and the cost of the transceivers become an issue.

4.B. Challenges and Opportunities

As mentioned earlier, a main challenge to overcome is the degradation of an OWC linkdue to turbulence and dense fog. In existing OWC networks, links longer than 400 m maybecome unusable in the presence of strong turbulence and dense fog. In addition, multiplelinks may be affected at the same time. As a result, the network may become partitioned,some applications may experience several packet losses and delay, and some sessions orconnections might also be terminated.

The above challenge can be addressed by integrating OWC networks with complemen-tary networking technologies based on either rf technologies or fiber-optic cables. On theone hand, an OWC network can serve as a fall-back resource when other types of connec-tion simply fail (due to, e.g., a fiber cable cut or rain, which affects a rf link) or becomeoverloaded. On the other hand, wireline networks, such as fast ethernet, gigabit ethernet,or fiber-optic metropolitan area networks (MANs) or rf networks can also be used to carrythe overflow traffic from an OWC network in case it suffers from congestion or failure oftemporary or permanent links or nodes.

The above challenges can also be addressed on several additional fronts including sur-vivable topology designs, routing and rerouting algorithms, dynamic network reconfigu-ration, and cross optimization of transport and application layer protocols. The followingsubsections describe related problems and approaches in more detail.


4.C. Basic Off-Line Topology Design and Optimization

In topology design, it is typically assumed that the traffic matrix is given and each ele-ment in this matrix specifies either the average amount of packet traffic or the requiredbandwidth between two source and destination buildings (or houses). In addition, a set ofbuildings (houses) that can be used to install transceivers, which must include the sourceand destination buildings but may also include other buildings as well, are known. A fewof these buildings may also be connected with each other via wired point-to-point gigabitethernet links or rf links or may have access to a MAN.

One of the main objectives is to design a topology for an OWC network that can min-imize the total delay encountered by the given packet traffic under constraints such as amaximum of K pairs of transceivers that can be used at each building. This delay includesthe transmission, queuing, and processing delay at the source or any intermediate nodes(when more packets arrive than what can be sent out by the node) as well as propagationdelay. Note that to connect each building to at least two other buildings with full-duplexOWC links, K must be no less than 2 (although this does not guarantee two node-disjointpaths between two buildings).

Another important design objective is to satisfy the given traffic demand in terms ofbandwidth requirements with a minimum total cost (including both capital and operatingcosts) associated with the transceivers and building space (e.g., installation and space leas-ing). Although the first objective makes sense in a packet-switched OWC network, the latterobjective is applicable primarily to a circuit-switched OWC network where the links oper-ate based on time-division multiplexing, WDM, or a combination of the two. It should benoted that the work in Ref. [83] dealt with the second objective only. In addition, it studieda few heuristic algorithms through simulations. Much work on analyzing, formulating, orsolving the optimization problems remains.

4.D. Redundancy Provisioning and On-Line Routing Issues

In addition to the above design objectives, one may also consider other possible variationswith the focus on providing spare resources or routing flexibility. For example, one suchobjective is to provide resilience to single or multiple node failures (due to the failure ofthe equipment or power supply) by offering node-disjoint routes between any pair of sourceand destination buildings.

Such a survivability requirement may be met by establishing either multiple (saym ≥ K ≥ 2) point-to-point links from one building to several other buildings, a 1-to-mmulticast link by taking advantage of the light-splitting capability, or a 1-to-m switchableand reconfigurable link. Whereas the first approach requires multiple lasers, one for eachbuilding, the second and third approaches both require only one laser and multiple tele-scopes. The major difference between the second and third approaches is that the formerrequires a 1-by-m multicast-capable switch or splitter (in which case, a higher-power laseror amplifier may also be needed), and the latter requires a 1-by-m switch and hence is lessexpensive as well as less powerful than the former. Similarly, neither the second nor thethird approach can send different data to different buildings at the same time as in the firstapproach, which is the most expensive. Clearly, there exists cost-performance trade-offs inusing one of these three approaches, and a combination of the three may also be used atdifferent parts of the network.

To guarantee at least low-bit-rate connectivity between some or all source and destina-tion buildings even in the presence of dense fog, one may connect these buildings in sucha way that the maximum distance between every two adjacent buildings does not exceed,say 250 m, for example (below this distance, an OWC link can still operate in dense fog,albeit at a lower bit rate such as 10 Mbits/s).


Since it may not always be possible to find a suitable building or buildings to connecta pair of source and destination buildings that are farther than 400 m, it may be necessaryto connect some buildings to those that have wired connections (or rf links, which are notaffected by dense fog) to be resilient to dense fog. Establishing or using such connectionswill incur some costs. Nevertheless, this method may be more cost-effective than providingall the redundancy needed using OWC links alone.

Once the topology design phase is completed, a key question to be studied then becomeshow to route traffic in the network to minimize delay and, in case a link or node failure oc-curs or an adverse weather condition affects the effective bit rate or bandwidth of certainlinks, how to reroute the traffic effectively. Note that under normal traffic and OWC net-work conditions, a packet may only go through a few OWC links, each offering a dedicatedhigh bandwidth (e.g., up to OC-48) to the two adjacent buildings. However, routing thepacket through a wired backbone that connects the two buildings may incur a longer delay.This is because, most likely, the packet has to go through two local area networks (LANs)at the source and destination buildings (in addition to a MAN), with each LAN having alower effective data rate. Although some of these issues may have already been consideredin the off-line topology design phase, the real traffic is likely to differ from that predicted bythe traffic matrix. Hence, one needs to study on-line routing algorithms that can minimizepacket loss (due to buffer overflow) in a packet-switched OWC network or the blockingprobability of requests for bandwidth-guaranteed connections in a circuit-switched OWCnetwork when the real traffic exceeds that predicted by the traffic matrix. Similarly, routingalgorithms that can minimize packet delay or maximize throughput while ensuring fairnessneed to be studied. In these studies, new routing capabilities such as load-balanced rout-ing, multipath routing, and multicasting enabled by hardware-supported beam steering andsplitting should be taken into consideration.

4.E. Topology Update and Reconfiguration

Whereas dynamic routing may be sufficient to accommodate temporary changes in trafficand link or node condition, topology update and reconfiguration may be needed for not-so-transient changes. There are two possible approaches to modifying the existing topology.One is enhancing the current topology with new links or nodes to accommodate additionaltraffic among the given set of source and destination buildings or including new source ordestination buildings. Here, the problem is how to arrive at a new optimal (or near-optimal)topology without having the freedom of changing any of the existing connectivity. Thesecond possible approach is to reconfigure the existing topology to cope with sustainedlink or node failures that cannot be easily repaired due to, for example, the demolition ofa building used by the original topology or the loss of line of sight between two originallyconnected buildings between which a new building has been constructed. Here, the uniquecharacteristic of an OWC network mentioned earlier, which enables one to dynamicallycreate an alternate link by simply redirecting the laser beam, may be explored. Althoughsome of the cost-performance trade-off issues are quite similar to what have already beenaddressed in the previous subsections, an open issue is how to trigger the reconfigurationand coordinate the transmitter and receiver pair at different buildings using appropriatesignaling protocols.

5. Concluding Remarks

This paper has addressed unique challenges as well as opportunities in using OWC to pro-vide broadband first- and last-mile access. Specifically, OWC’s advantages over and differ-ences from fiber-optic and rf access techniques have been discussed. Research issues havebeen reviewed for the link or physical layer including pointing and acquisition, modula-


tion, coding, and networking layers such as off-line topology design, survivable routing,and dynamic topology reconfiguration algorithms.

OOK is not good for OWC because its performance is severely degraded even in mildturbulence. Implementation and performance of high-data-rate PPM is yet to be understood.Subcarrier PSK intensity-modulated OWC systems are of great potential for applicationswhere a high data rate, low power, and low cost are needed. Realistic coding methodsneed to be developed for PPM systems and subcarrier PSK intensity modulation systems.Practical pointing and acquisition methods are demanded to reduce OWC system cost.

There are several additional issues worth investigation. For example, with respect tointegration of OWC networks with fiber-optic networks, it is possible to use inexpensiveoff-the-shelf optical add–drop multiplexers and amplifiers to route the signals through anintermediate node (building) transparently without any optical–electronic–optical conver-sion. Using standard tunable lasers, one may realize the vision of having WDM in space[88]. Furthermore, there has been a lot of interest in evaluating and enhancing transmissioncontrol protocol (TCP) performance in high-speed fiber-optic networks and error-pronewireless networks since the earlier flavors of TCP were optimized for links with a low BER(e.g., < 10× 10−9) and medium bandwidth (up to 100 Mbits/s). Nevertheless, new TCPflavors are needed for OWC networks to take advantage of high bandwidth (up to OC-48)while coping with the medium to high BER (e.g., < 10× 10−3) of OWC links, especiallyduring midday when the atmospheric turbulence is strong [15].

Acknowledgments

This work was supported in part by the National Science Foundation under grants CNS-0435341 and CNS-0435155.

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