RONJA

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Ronja device installed on a balcony. The transmitter pipe (right) emits red light. The retroreflector triangle (left) is used for an initial aiming. The white steel holder allows precise adjustment of direction.

Single high-brightness LED with a cheap loupe lens creates a bright narrow beam that can stream DVD-quality video over neighbourhoods. A few steps aside and the narrow beam becomes invisible.

Three bolts preloaded with pink rubber blocks facilitate fine adjustment of the optical head direction with a gear ratio 1:300. The bolt on the right side is a part of a rough adjustment mechanism which allows pointing the optical head in virtually any direction.

RONJA (Reasonable Optical Near Joint Access) is a Free Space Optics device originating in the Czech Republic, which almost anyone can build. It transmits data wirelessly using beams of light. Ronja can be used to replace a segment of LAN, and one can enjoy all activities as if they were directly connected via 10Mbps full duplex Ethernet - playing games, connecting to the Internet, streaming audio/video, or sharing files.

The range of the basic configuration is 1.4km. The device consists of a receiver and transmitter pipe (optical head) mounted on a sturdy adjustable holder. Two coaxial cables, like those used for TV antennas, are used to connect the rooftop installation with a protocol translator installed in the house near the PC or switch. The range can be extended to 1.9km by doubling or tripling the transmitter pipe.

Building instructions, blueprints, and schematics are published under the GNU Free Documentation Licence. Only free software tools are used in the development. The author calls this level of freedom User Controlled Technology. Ronja is a project of Twibright Labs.

1 Building by inexperienced
2 Models
3 Limitations
4 Innovations
- 4.1 Technical
  - 4.1.1 Preamplifier stage
  - 4.1.2 Nebulus infrared LED driver
- 4.2 Organizational
5 External links

[edit] Building by inexperienced

The building instructions are written with an inexperienced builder in mind. Basic operations like drilling, soldering etc, are explained. Several techniques - drilling templates, detailed checks after soldering, testing procedures - are employed to minimize errors at critical places and help to speed up work. Printed Circuit Boards are downloadable ready for manufacture, with instructions for the fabhouse. People with no previous experience with building electronics have reported on the mailing list that the device ran on the first try.

Around 141 installations worldwide have been registered into a gallery.

[edit] Models

Ronja Tetrapolis: Range of 1.4km, red visible light. Connect with RJ45 connector into a network card or switch.
Ronja 10M Metropolis: Range of 1.4km, red visible light. Connects to Attachment Unit Interface.
Ronja Inferno: Range of 1.25km, invisible infrared light.
Ronja Benchpress: A measurement device for developers for physical measurement of lens/LED combination gain and calculation of range from that

[edit] Limitations

The link stops working in dense fog. It cannot operate without direct visibility. Star topology, like in a Wireless LAN, is not possible.

[edit] Innovations

[edit] Technical

[edit] Preamplifier stage

The usual approach in FSO preamplifiers is to employ a transimpedance amplifier. A transimpedance amplifier is a very sensitive broadband high-speed device featuring a feedback loop. This fact means the layout is plagued with stability problems and special compensation of PIN diode capacitance must be performed, therefore this doesn't allow selection of a wide range of cheap PIN photodiodes with varying capacitances.

Ronja however uses a feedbackless design where the PIN has a high working resistance (100 kiloohms) which together with the total input capacitance (roughly 7 pF, 5 pF PIN and 2 pF input MOSFET cascode) makes the device operate with a passband on a 6 dB/oct slope of low pass formed by PIN working resistance and total input capacitance. The signal is then immediately amplified to remove the danger of contamination by signal noise, and then a compensation of the 6 dB/oct slope is done by derivator element on the programming pins of a NE592 video amplifier. A surprisingly flat characteristic is obtained. If the PIN diode is equipped with 3 kΩ working resistor to operate in flat band mode, the range is reduced to about 30% due to thermal noise from the 3 kΩ resistor.

[edit] Nebulus infrared LED driver

The HSDL4220 infrared LED is originally unsuitable for 10 Mbit/s operation. It has a bandwidth of 9 MHz, where 10 Mbit/s Manchester-modulated systems need bandwidth of around 16 MHz. Operation in a usual circuit with current drive would lead to substantial signal corruption and range reduction. Therefore Twibright Labs developed a special driving technique consisting of driving the LED directly with 15-fold 74AC04 gate output in parallel without any current limitation. As the voltage to keep the nominal LED average current (100mA) varies with temperature and other component characteristic, an AC-bypassed current sense resistor is put in series with the LED. A feedback loop measures voltage on this resistor and keeps it at a preset level by varying supply voltage of the 74AC04 gates. Therefore the 74AC04 is operating as a structured power CMOS switch completely in analog mode.

This way the LED junction is flooded and cleared of carriers as quickly as possible, basically by short circuit discharge. This pushes the speed of the LED to maximum, which makes the output optical signal fast enough so that the range/power ratio is the same as with the faster red HPWT-BD00-F4000 LED. The side effects of this brutal driving technique are: 1) the LED overshoots at the beginning of longer (5 MHz/1 MHz) impulses to about 2x brightness. This was measured to have no adverse effect on range. 2) A blocking ceramic capacitor bank backing up the 74AC04 switching array is crucial for correct operation, because charging and discharging the LED is done by short circuit. Underdimensioning this bank causes the leading and trailing edges of the optical output to grow longer.

[edit] Organizational

The whole toolchain is strictly built upon free tools and the source files are provided for free under the GPL. This allows anyone to enter the development, start manufacture or invest into the technology without entry costs. Such costs normally can include software licence costs, time investment into resolution of compatibility issues between proprietary applications, or costs of intellectual property licence negotiations. The decision to conceive the project this way was inspired by observed organizational efficiency of Free Software.

In Christmas 2001, Ronja became the world's first 10 Mbit/s Free Space Optics device with free sources.

Examples of tools used in development: