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The objective of this paper is to analyse and present the latest results obtained for free-space optics (FSO) within the EU COST Action IC-0802 and within the European Space Agency (ESA) contract. First, the FSO technology is briefly discussed and some performance evaluation criteria for FSO are provided. Some optical signal propagation experiments through the atmosphere (including the recent investigations in airborne and satellite application for FSO) are also shown. In the main part, considerations on suitability of different optical wavelengths are brought into question. The wavelength selection is dependent on the atmospheric effects and on the availability of receiver and transmitter components. Discussion on the available receiver(s) and transmitter(s) includes the focus on advantages and mainly the costs of the different systems. In the final part, we examine the latest practical results (carried out within the COST Action IC-0802) on modelling of the FSO channel under fog conditions and other atmospheric effects. Additionally, recent results, showing major performance improvement, based on the hybrid system and specific modulation and coding schemes are presented. Keywords: Free-space optics, broadband wireless, network architectures, last mile access, reliability and availability. INTRODUCTIONFSO communication systems consist of an optical transmitter which is a laser source or a LED, a modulator and a telescope. The receiver consists of a detector, a decoder and a telescope to collect the optical signal. The optical signal propagates through the free space which acts as the link channel. Interest in FSO continues to grow mainly for two reasons: first identification as an attractive alternative or a complementary to existing microwave (millimetre wave (MMW)) and the radio frequency (RF) communication links, and secondly being a broadband wireless solution for the "Last Mile" connectivity in metropolitan networks, point-to-point and point-tomultipoint link configurations. Last couple of years have witnessed a growing demand for higher data rates and wider bandwidths from the end user to manipulate multimedia information. This development will continue in the next couple of decades being a challenge for the future Next Generation Networks. So the end-user will need higher data rates and access to the fully available bandwidth within the backbone delivered to the home. Currently FSO is being researched for applications involving ground-to-ground (short and long distance terrestrial links), satellite uplink/downlink, inter-satellite, deep space probes to ground, and ground-to-air/air-toground terminal (UAV, HAP etc.) [1]. This has resulted in some successful experiments such as SILEX (a link between Artemis and SPOT-4). The prime advantages of FSO are: higher data rates exceeding easily 100 bit/s using wavelength division multiplexing (WDM) techniques, security aspects, EMC/EMI immunity and frequency regulation issues. Additionally, small terminal size, light weight, minimal aperture sizes and low ...

The fully integrated 800 μm. diameter avalanche photodiode optical receiver is implemented in 0.35 μm BiCMOS technology without any process modifications. The integrated receiver reaches sensitivities of-33 dBm at 1 Gbit/s and-29.3 dBm at 2 Gbit/s. The reached sensitivities are well within the state-of-the-art of integrated avalanche photodiode receivers and can even be compared to a hybrid avalanche photodiode receiver comprised of high-performing commercial components. The performance of the designed receiver was verified in visible light communication experiments. The receiver could reach up to 16.5 m at 2 Gbit/s and 27 m at 1 Gbit/s of error free transmission distance using a 675-nm point laser source as transmitter. The common indoor illuminance levels up to 500 lux could be tolerated when pointed directly towards the receiver. High sensitivity and high speed make this integrated receiver suitable for future optical wireless communication systems, where due to its integrated nature the manufacturing cost can be lowered, and at the same time the design is compact in size and easy to assemble and scale. Furthermore, no optics is used in front of the receiver due to its large area resulting in a wide field of view.

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