Reintroducing Free-Space Optical Technology to Community Wireless

Mustafa et al. Reintroducing FSO Technology to CWNs

Proceedings of the Nineteenth Americas Conference on Information Systems, Chicago, Illinois, August 15-17, 2013.

Reintroducing Free-Space Optical Technology to Community Wireless Networks

Luka Mustafa Department of Electronic & Electrical Engineering

University College London London, WC1E 7JE

[email protected]

Benn Thomsen Department of Electronic & Electrical Engineering

University College London London, WC1E 7JE

[email protected]

ABSTRACT

This paper explores the use of Free-Space Optical technology in community and consumer networks and develops a gigabit Free-Space Optical system using WDM bidirectional SFP fiber optical modules, demonstrating a new cost-effective design approach for high-capacity, non-interfering point-to-point technology with a range of up to 100 m. A low-cost free-space collimation and alignment system is designed and experimentally evaluated in a prototype system, successfully demonstrating operation over 125 m with a 6 dB power margin. The application of this system is considered for three types of Community Wireless Networks, addressing deployment scenarios, spectral availability regulations and safety concerns.

KEYWORDS

Free-Space Optical, community wireless networks, BOSA, laser system, Ethernet, low-cost

INTRODUCTION

Community Wireless Networks (CWNs) and consumer networking in general are governed by low-cost Wi-Fi devices used as single Access points in households, as well mesh nodes in country-wide community networks. Their decreasing cost and increasing capacity allow community networks to be established at a very low-cost. They are however limited by a fundamental problem - the lack of license-free RF spectrum for high-bandwidth communications. Wi-Fi networks mostly use 2.4 GHz and 5 GHz frequency bands, in urban areas this, few hundred MHz wide, spectrum is shared amongst a large number of devices, creating a very high noise floor effectively limiting the range to a few tens of meters. Some countries in development, such as for example Kyrgyz Republic in Asia, have no public spectrum allocation for Wi-Fi use outdoors, thus effectively prohibiting the growth of CWNs.

Free-Space Optical (FSO) systems, employing collimated light to convey information through free space for point-to-point networking, have a great potential to mitigate these problems and limitations. The light spectrum is not as strictly regulated as RF spectrum, its use mainly limited by eye-safety regulations. Light sources can be very well controlled and focused, enabling the transmission of a modulated beam through free-space to a very small area and are thus subjected to extremely low spectrum congestion due to neighboring devices. At the time, there are several FSO networking systems available on the market, however all of them enterprise or carrier grade and prohibitively expensive for the consumer and CWN market.

A low-cost FSO technology that can compete with Wi-Fi in capacity, simplicity of use and cost is developed in this paper with the main goal to enable high-capacity point-to-point wireless networking between structures, bridging the building-to-building gap of about 100 m at 1 Gbps without digging up the road or installing overhead wires. Its application in different types of CWNs is discussed along with the reliability considerations. FREE-SPACE OPTICAL TECHNOLOGY IN CWNS

The Reasonable Optical Near Joint Access (RONJA) system designed in 2001 (Kulhav, 2006) and originating from Czech Republic is possibly the only documented open-source FSO networking project thus far. Several hundred of RONJA units with 10 Mbps duplex capacity, 1.4 km range, and material cost of about hundred euros were deployed in Czech Republic and around Europe at the time. A commercialized version of it - Crusader increased the capacity to 100 Mbps with the range of 900 m but at about ten times the cost (Soderberg, 2010). The complexity and the time required to construct a RONJA device or the cost of the commercialized version prohibited its wide-spread use, before being out-performed and out-priced by Wi-Fi systems, never gaining a significant position in consumer networking nor CWNs.



LOW-COST NETWORKING SYSTEMS

CWNs have very limited financing as they often exist on voluntary basis in a form of an association, merely as an initiative such as community network wlan slovenija or as non-profit organizations in large CWNs. The lack of large investments for constructing the network infrastructure calls for an alternative approach to using enterprise and carrier grade networking equipment that is not only prohibitively expensive but as well requires trained professionals to be set up.

The majority of CWNs thus use low-cost networking equipment with easy to use open-source firmware enabling reliable installation (Tsarmpopoulos, Kalavros, Lalis, 2005) and network deployment by untrained professionals. For example sub -$25 Wi-Fi devices accelerated the growth of wlan slovenija network since early 2011, in about two years becoming the most significant type of equipment used in more than half of the 600 node strong network, deployed in cities and rural areas.

As briefly shown above, two goals must be satisfied by networking equipment to suitable for use in CWNs: low-cost to enable the equipment to be obtained in the first place and simplicity of use allows non-skilled individuals to reliably set up networks.

Low-cost goal is achieved mostly by mass-production as the engineering and manufacturing fixed costs are reduced with the quantity, as well as by providing the minimum required functionality without over-engineering. FSO systems available on the market today are mostly carrier grade, due to the specialized market produced in small quantities and employing a full custom design, while aiming for 99.999% channel availability and thus requiring high link power margin (Kim & Korevaar, 2001). A market overview at the time of writing indicated most-affordable FSO systems on the market start approximately at $1000 per unit for 100 Mbps over 100 m (Polewall, 2013). At present FSO technology is unsuitable for use in CWSs due to very high cost. A simple, sufficiently reliable and low-cost system for consumer use is required.

This paper develops a new approach to designing a FSO system by using mass-produced electro-optical transceiver for wired fiber networks and reducing the channel availability to about 99%. This in turn significantly reduces the system cost. VALET SYSTEM DESIGN

Single mode fibre wavelength multiplexed systems requiring a single fibre for bidirectional communication are widely employed for consumer and last-mile networking, generally in the SFP module form (SFF Committee, 2010). Yoshida, Tsujimura, & Kurashima (2008) have designed a FSO system for collimating and coupling to the SMF that connects to the SFP module, successfully replacing a section of a fibre link with FSO by designing an active opto-mechanical system for alignment purposes. A Trans-Window FSO system by Tsujimura, Yoshida, Kurashima, & Mikawa (2008) employs this method, stating reliable operation up to 20 m, but requiring high precision opto-mechanical systems increase the system complexity, cost and additionally require opto-electrical transceivers for communication. The VALET system designs and experimentally evaluates a collimation system applied directly to the SFP transceiver without the optical fibre, thus reducing system complexity.

The system is designed to operate at wavelengths in the range of 1300nm -1600nm which coincides with the low-loss region of SMF and as well contains several low-loss free-space optical windows (Soni & Malhotra, 2011). The use of this wavelength range also increases the maximum eye safe power compared to shorter wavelengths. CHANNEL AVAILABILITY

The free-space communication channel affects the propagation of light and RF signals, subjecting both to weather effects. RF microwave propagation is mainly attenuated by heavy rain and high density of water droplets. Light attenuation is mainly affected by scattering, and thus mainly affected by fog and heavy snow. Several papers have considered in-depth these effects (Ghassemlooy & Popoola, 2010), determining the attenuation at 1500nm range ranging from 2dB/km on a clear day to 200dB/km in heavy fog with low visibility.

Kim & Korevaar (2001) aggregated the information on visibility in major North American cities and comparing it with the visibility loss model (Plank, Czaputa, Leitgeb, Muhammad, Djaja, Hillbrand, Mandl, Schonhuber, 2011), showing that greater than 90.9% channel availability (2.5 miles visibility) with attenuation 2dB/km and greater than 99% channel availability (1/4 miles visibility) with attenuation 34 dB/km are observed annually. This implies that an additional margin of only 3.4 dB per 100m over clear day attenuation (0.2 dB / 100 m) is required to provide 99% channel availability in the studied case. Medium-range consumer grade FSO systems can be thus designed with significantly lower link margins then carrier grade designs, the later required to cope with up to 20 dB per 100 m attenuation.

Commonly available mass-produced low-cost SFP modules designed for use with up to 20 km fibre links typically provide a 20 dB link power margin and are thus suitable for FSO use. In this work we design and develop the necessary free space



optics to couple to the Bi-Directional Optical Subassembly (BOSA) laser and photodetector module within the SFP and assess the required focal point positioning accuracy and the geometrical losses of this system to achieve operation over a free-space link of 100m.. BOSA RECEPTACLE ANALYSIS

Bi-directional WDM SFP modules commonly support LC or SC fibre connectors for SMF. The modules contain receptacles with a guide tube and a ferrule for aligning the fibre. The LC receptacle mechanical design clips the beam, whereas the SC does not, making it suitable for this application. The divergence of the output of the SC BOSA module is essentially similar to that of a SMF. Figure 1 shows the measured beam divergence compared to the calculated SMF output beam divergence.

Using this divergence we can then select an appropriate lens focal length and diameter. Figure 2 shows only a single lens is required in a design employing a BOSA module for bidirectional communication, this is in contrast to the single wavelength commercial systems that require two lenses, further decreasing the system cost.

COLLIMATING THE BEAM

The bidirectional design employs a single lens coupled to the BOSA module for receiving and transmitting different infrared wavelengths (1300 and 1500 nm). The use of a single lens for both the transmitter and the receiver further reduces the cost and simplifies the mechanical design. A suitable lens is chosen to match its angle of view to SMF beam divergence of 8.048, ensuring efficient coupling to the BOSA module. A greater lens diameter decreases the laser beam power density allowing

Figure 1: Fibre receptacle beam divergence experimental results and SMF model reference.

Figure 2: BOSA module structure and an external collimating lens.



for greater total power as per the eye safety requirements and more efficiently couples a diverging beam to the receiver BOSA module.

The typical 20 km bidirectional SFP module has greater than 20 dB link margin, thus the lens diameter is chosen to be sufficient large to allow for 100 m range accounting only for normal atmospheric and geometrical losses (Bloom, 2002), estimating the achievable collimated beam divergence angle of 0.5 mrad. Simulated geometrical and atmospheric losses as a function of transmission distance are shown in Figure 3, for a range of lens diameters. The minimum lens diameter satisfying the link margin specification, of 20 dB, is 25 mm with a focal length of 100 mm.

According to EN 60825-1 at 1550 nm for an exposure time of 10 s with beam divergence angle less than 1.5 mrad, the Maximum Permissible Exposure (MPE) is 1000 W/m. This implies a maximum transmitter power of 27 dBm for a 25 mm diameter collimating lens, which is several orders of magnitude greater than the 0 dBm transmit power of the SFP module used here. The minimum eye-safe lens diameter at 0 dBm power is 0.56 mm.

An uncoated plano-convex 25 mm diameter lens with 100 mm focal length has been coupled to SC SFP module in an experimental setup with the SFP module on a 3DOF linear translational stage. The lens was manually placed normally to the optical axis of the SFP module without angular compensation and illuminated with a collimated beam produced by an identical setup with a second SFP module. The received power in the BOSA module has been measured using SFP power monitoring feature according using SFP Digital Monitoring Mode (DDM). For a low-cost system the sensitivity to misalignment is important. Figure 4 shows alignment sensitivity of the lens position, using the orientation as indicated in Figure 2. Displacement normal to the optical axis results in received power decrease of 1 dB per 0.021 mm and of 1 dB per 0.70 mm along the optical axis, both observed form the maximum. The adjustment or manufacturing tolerance to positioning the collimating lens is set to 10 um for normal to optical axis position and 500 um for position along the optical axis.

Figure 3: Distance loss model for a range of common lens diameters, assuming 0.5 mrad beam divergence.



POINTING MECHANISM

The SFP module and collimating lens is plugged into an Ethernet Media Converter to provide data connectivity. This unit is then mounted on a manual 3DOF rotational stage and a tripod for FSO tests at an arbitrary distance. Two identical test units were constructed with an additional wireless monitoring system allowing for remote measurement of received power and thus easier alignment, the complete test system shown in Figure 5. Note that the Bi-Di SFP modules come in matched pairs i.e. 1500/1300 Tx/Rx and 1300/1500 Tx/Rx. A visual aiming mechanism is employed for initial alignment, before maximising the received power using the wireless monitoring system by adjusting the position with translational stages. A visible laser pointer used as the aiming system is mounted on top of the SFP module on a separate 3DOF stage, used to calibrate it to the IR beam, by gradually increasing the separation of test units, adjusting the alignment for maximal received power using the wireless RX power monitoring system and recalibrating the visual aid.

The collimated output of the SFP module at our disposal had very uneven power distribution, containing several propagation modes unsupported by SMF. As a result a test FSO link of 1m performed 5.2 dB better than a wired SC-SC fibre link of the same length, identifying an efficiency advantage of FSO over SMF in the case of poor-quality BOSA outputs. At a distance of 10 m the loss is significantly lower than in a FSO system coupling to the SMF using a 10 mm lens (Yoshida & Tanaka, 2008).

Figure 4: Received power as a function of linear displacement in 3DOF.

Figure 5: VALET system diagram



FSO EXPERIMENTAL RESULTS

The FSO link has been established for various distance using the prototype shown in Figure 7a, results shown in Figure 6 by the loss model projections overlaid with experimentally measured attenuation at distances up to 125 m. Experimental measurements have been conducted for maximum power at a particular distance as well as using fixed focus with 0.5 mrad beam divergence half-angle. Results indicate that very high collimation can be achieved; however establishing the link becomes prohibitively difficult without very precise alignment mechanism at greater distances. The fixed beam divergence allows for sufficient (6 dB) link margin at a distance of 100 m (62 mm spot size), while allowing for greater inaccuracies in aiming. This requires the aiming mechanism to be adjusted with at least 20 mm (at 100 m) accuracy or 0.0129. The constructed prototype provides sufficient link margin for a stable short term connection at 125 m on a clear day, however further experiments are required to study the effects of the pointing drift and long term disturbances, as well as weather effects.

VALET SYSTEM COST

The designed system using SFP modules as electro-optical transceivers in conjunction with a media converter, optical and mechanical components satisfied the distance and capacity requirements, however most importantly the prototype as shown in Figure 7b has been constructed with components commonly available with the estimated unit cost of about $400. While such a proof-of-concept prototype does not indicate the final cost, the conclusion can be made that an integrated and mass-production ready system based on this prototype can achieve at least comparable if not significantly lower cost. All the

Figure 6: Maximal received power as a function of distance on a plot predicting loss due to beam divergence and atmospheric losses for 25 mm lens.

Figure 7a: VALET system prototype used for tests up to 125m.

Figure 7b: VALET system prototype 3D model



established manufacturing tolerances for mechanical construction of the collimation system are achievable with common manufacturing procedures.

VALET system has the low-cost potential, however the mechanical aspects still need to considered and tested to insure a simple yet reliable installation method. Low-cost system design is achieved by sacrificing the system power margin, thus enabling SFP modules to be used, which will reduce channel availability in adverse weather conditions, however this can be compensated for with the existing CWN infrastructure or by the best-effort principle, accepting the connection unavailability for a shorter weather dependent period of time. REINTRODUCING FSO TO CWNS

FSO systems will have the potential to become a promising technology for CWNs if they become cost effective and minimise the effect of weather, structural and natural effects that contributed to decline in their use (Dulik, Bliznak, Jasek, 2012). Alternatively the FSO technologies have the potential to develop into a highest capacity wireless solution, thus increasing the benefits of their usage beyond associated drawbacks. The VALET system is taking the first step towards this goal, designed to fill the market gap between the price of low-cost 300 Mbps 802.11n products in 2.4 GHz and 5 GHz bands and the capacity of carrier grade FSO and RF products.

Capacity of 1 Gbps using a wireless technology has just recently become available at reasonably low but still rather inaccessible price or several thousand US dollars with range up to 13 km (Ubiquiti Networks, 2012). While some CWNs with formally organised structures and backed by large communities may be able to raise sufficient funding and invest in this equipment, the majority of either newly established or informally organised CWNs will be economically excluded and thus limited to low-cost solutions. One can argue the price range accepted as low-cost, however for the purpose of this paper it is assumed to be about $150 or about the price of one 802.11n unit with 300 Mbps capacity rated for outdoor use and thus the target price of VALET unit. As the proof-of-concept prototype is about three times higher, further development and cost optimization is required.

Here we consider the use case of the VALET system in three representative CWNs, exploring its potential impact and developing an application scenario based on user profiles, limitations of competing technologies and complimentary use of existing network infrastructure. BIG CWNs - FunkFeuer, FreiFunk, GUIFI, AWMN

Several CWNs in Europe have existed for more than a decade now and have grown with the demand for internet connectivity to enormous sizes, supporting several thousands of users and containing an equally great amount of wireless connections, with a significantly limited capacity of 54 Mbps due to 802.11g technology prevailing, often in mesh configuration. The organised growth of the network and funds for maintaining and expanding the backbone infrastructure has boasted the upgrade to 802.11n in hand with the increasing availability of the 802.11n technologies produced mostly by Ubiquiti Networks Ltd. and MikroTik, providing capacity up to 300 Mbps with quite substantial range of several kilometres, but greatly reduced by the WiFi spectrum congestion and interference in urban areas (Mahanti, Carlsson, Williamson, Arlitt, 2010). The lack of capacity of wireless technologies is driving the community to search for alternatives, the most viable being wired fibre networking (Stamatopoulos, 2012). Albeit possible and in use where the land is owned by CWN members and no special permissions are required or by collaborating with local authorities and acquiring permissions for construction or usage of existing infrastructure, the volunteer based involvement rarely produces such results.

Free-space optical systems are very similar in the deployment procedure to wireless technologies, thus the significant effort in structural changes is avoided. FSO products on todays market are carrier and enterprise grade and accordingly priced in the range of tens of thousands of euros, resulting in a very high return of cost rare CWNs can afford. The VALET system is designed to be several orders of magnitude cheaper, allowing mass deployment and is very suitable to branch out the capacity from main network nodes, thus increasing the network throughput. Figure 8 illustrates a rather typical CWN network structure that can be observed in the vicinity of a backbone node, connecting several local nodes within a few blocks. Assuming the backbone node has a high capacity or is connected by FTTH, all local clients are competing for the spectrum, with each connection receiving only a fraction of the backbone node capacity. Applying VALET system in a combination with wired networking to the same situation, significantly branching out the backbone capacity, will result in a number of very local wireless networks carrying the capacity to the end user.

The adaptation of a FSO system in existing networks is a gradual process, where existing wireless connections along high-capacity FSO links provide a backup, thus allowing for slightly lower channel availability. VALET system is expected to find



place in large CWNs to increase the network capacity in dense urban environments by providing its own capacity and reducing Wi-Fi spectrum utilisation, in turn increasing its capacity as well.

SMALLER CWNs wlan slovenija The majority of CWNs in Europe are of small size in various stages of growth as a result of increasing commercial broadband penetration. The abundance of broadband capacity for end users has for example sparked the development of wlan slovenija - a community open wireless network of Slovenia, where only informally organised individuals choose to freely share the abundance of their consumer broadband connection, but at the same time establish a wireless mesh network amongst them if so possible. The latter is a general observation in urban areas, where the same technology is used to branch out the capacity to rural areas. The networks growth is not centrally managed, with funds available for maintaining the core network infrastructure collected via donations, thus expensive networking equipment of any kind rarely used.

VALET systems matches purchasing power of individuals, becoming the first wireless technology of high-capacity available to them and are likely to use it for establishing the connections with neighbouring nodes with high capacity. Service providers invest in their infrastructure mostly in high population density areas, providing FTTH access to blocks of flats and other tall building, whereas neighbouring private homes do not get the same service, here VALET can be used to branch out to neighbouring units. EMERGING CWNs

Countries in development mostly benefit from CWNs since they provide at least basic internet access before commercial or state infrastructure penetrates the majority of the population. Due to socio-economic obstacles only the simple to use, low maintenance cost effective technologies find place in these environments (Abdelaal & Ali, 2007). FSO technologies of today may appear to be unsuitable in such case due to their primary use in high-capacity backbone infrastructure; however it is very important to consider regulatory limitations that render consumer grade networking equipment useless.

An example of very strict regulatory limitation is the Kyrgyz Republic in Central Asia, where ISM 2.4 GHz band is limited to indoor use at the time of writing and community networking is practically inexistent due to the lack of suitable technologies. FSO systems the only wireless technology that can be immediately used for community networking, provided it is affordable to individuals. VALET system has the potential to meet this criterion and boost the broadband penetration several fold.

Collaboration with service providers in the observed situation can establish the CWN run last-mile approach, employing building-to-building network structure, using them to create a several hop network branching from local exchanges. Figure 9 represents a last-mile scenario, assuming buildings have wired network connections installed. The initial capacity of the local exchange is shared amongst everyone, but the network capacity allows dynamic allocation of it, thus more efficiently using service provider resources. One must consider the security of such infrastructure, as the data passes physical locations of third-parties. These concerns are to be addressed as in all CWNs and are not discussed here.

Figure 8a: Typical CWN wireless branching from a common node.

Figure 8b: VALET high-capacity branching with wireless sub-branches.



FSO POLICY AND REGULATIONS

Unlike RF systems, FSO is according to the International Telecommunication Union (ITU) recommendation ITU-R F.2016 unlikely to be subjected to spectrum congestion due to high directivity of the light beam. Local interference from other systems is possible but can be dealt with on local levels. Main limitation of FSO systems is their compatibility with eye-safety standards, subject to local regulations. IEC 60825-12 standard compliance is required for all FSO products, if intended for use in uncontrolled environment, such as consumer applications, they must be compliant with Class 1 or Class 1M for laser safety. While output power of both RF and FSO systems are limited, however the latter have no bandwidth limitations thus allowing for very high capacity links to be set up. DISCUSSION

VALET system takes a small step in addressing the gap for a low-cost FSO in the consumer networking market by designing a simple and affordable FSO system that can be potentially be implemented in CWNs. The custom designed electro-optical transceiver employed by most carrier grade FSO systems to achieve 99.999% reliability has been replaced by mass-produced SFP modules originally designed for wired fiber communications, along with and optical collimation and alignment assembly using off-the-shelf components and a low-cost media converter. The cost of such direct approach is still too high for wide-spread consumer use, thus further development of a more integrated solution for the collimation and alignment assembly is required to meet the target affordable cost.

Reducing the link margin of the system and channel availability to a level that is acceptable to the consumer allows for significant cost saving, however, it does make the design less suitable for locations with severe weather conditions. The high-capacity potential of the technology may be sufficient to outweigh unavailability is such situations. Combining this FSO design with the existing RF infrastructure in CWNs further enhances the robustness of the network whilst providing increased capacity and access speeds. Here the high-capacity FSO links significantly speed up the network, while RF provides a slower alternative route to increase reliability. As most CWNs employ dynamic routing protocols no major changes need to be introduced to these networks when FSO systems are used. CONCLUSION

This paper develops a FSO system addressing a technology gap on the market by exploiting low-cost SFP modules for free-space applications. 1 Gbps communication has been demonstrated over the distance of 125 m at a much lower component cost than any commercially available system. Use cases for three different types of CWNs are presented, showing the potential benefit of using a low-cost FSO system in such networks. Further experimental evaluation of the designed system in a real-world application is required to evaluate the effects of aiming instability and weather as well as a development of a low-cost and accurate aiming mechanism suitable for mass-production. Development is in progress and the finished system design will be freely available and crowd-funded production considered.

ACKNOWLEDGEMENTS

Valuable contribution to use-case scenarios of VALET was made in participant discussions on The International Summit for Community Wireless Networks 2012.

Figure 9: Last-mile network using VALET in urban areas, buildings with internal wired networks.



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Reintroducing Free-Space Optical Technology to Community Wireless