KEYWORDS: Free space optics, Antennas, Dense wavelength division multiplexing, Radio over Fiber, Modulation, Atmospheric optics, Radio optics, Televisions, Optical amplifiers, Single mode fibers
In this paper we present the design concept plus experimental results and evaluation of a newly developed
advanced DWDM Radio-on-Free-Space Optical (RoFSO) communication system capable of simultaneous transmission
of multiple RF signals. The RoFSO system is evaluated based on the performance metric parameters
defined for the various RF signals comprising of different wireless services including terrestrial digital broadcasting
signals, cellular 3GPP W-CDMA signals, IEEE 802.11 WLAN based signals etc being transmitted over the
RoFSO link. The performance metric parameters being considered include standard optical received power, CNR
and BER characteristics, W-CDMA signal transmission metric parameters like Adjacent Channel Leakage Ratio
(ACLR) and Error Vector Magnitude (EVM), modulation error ratio (MER) for digital terrestrial television
broadcasting signals as well as spectrum mask and EVM for IEEE 802.11 Wireless LAN signal transmission.
This paper firstly derives a new model of RoFSO (Radio on Free Space Optics) channel considering fluctuation speed of
the scintillation and the probability density function (p.d.f) of the random and time-correlated optical intensity disturbed
by scintillation. The relationships among frequency characteristic of RoFSO channel, variance of received optical
intensity and atmospheric conditions (temperature and insolation) are statistically analyzed by the proposed model. By
using the proposed model and the relationships, the throughput performance of WLAN (802.11a) over RoFSO link is
evaluated with computer simulations. For two cases of without-shadowing and with-shadowing in the radio propagation
channel, evaluations of degradation due to scintillation are conducted. As a result, it is found that the degradation due to
scintillation is much smaller than that due to shadowing in the throughput performance for WLAN.
We present initial results on research and development of an optical antenna module suitable for Radio-on Free-Space
Optics (RoFSO) links. This new optical communication system is envisaged to be an effective means of realizing a
ubiquitous society and therefore eliminating the digital divide. The RoFSO system is a trial system applying Radio on
Fiber (RoF) technology for transmission through free space. Based on the results of research of next generation high-speed
free-space optical communication system conducted in the past two years at Waseda University, we have
developed an optical antenna module with efficient laser receiving characteristics as well as simple adjustment. The
tracking system adopts two phases including rough tracking by the beacon light at 0.85 μm wavelength and fine tracking
using communication light at 1.55 μm wavelength to improve compensation precision for the atmospheric turbulence at
the time of beam propagation. We present results on the evaluation of performance characteristics (static characteristics)
of the separate functions for RoFSO antenna module we have developed and confirmed the coupling efficiency and fine
tracking characteristics which were set as goals at the beginning.
Radio on Free-Space Optics (RoFSO) communication systems have attracted a considerable attention for a variety of
applications where optical fibers are not feasible, especially in rural areas, to provide ubiquitous wireless services
quickly and more effectively. RoFSO links can be used to transmit signals like cellular W-CDMA, terrestrial digital TV
or WLAN signals. In spite of its potential, such links are highly dependent on the deployment environment
characteristics in particular the weather conditions. Severity and duration of the atmospheric effects have direct impact
on the availability of the links as well as on the quality of RF signal transmitted over it. Thus, the necessity of
investigating the effects of various weather conditions on RF signal transmission using FSO links.
In collaboration with several institutions, we are currently developing an advanced Dense Wavelength Division
Multiplexing (DWDM) RoFSO antenna capable of transporting multiple RF signals. As preliminary work, we are
conducting experiments on a 1 km link using an off-the-shelf Radio Frequency - FSO (RF-FSO) antenna, with the
objective of obtaining and characterizing performance related parameters of RF-FSO transmission in operational
environment. As an example, we examine the influence of atmospheric turbulence on the transmission quality of W-CDMA
signal. Among the performance metric of interest is the Adjacent Channel Leakage Power Ratio (ACLR) which
will be measured, analyzed and correlated with the weather conditions. An atmospheric fluctuation model for estimating
the communication quality of RF signal transmission on FSO links is being developed. Also the obtained results will be
used for the deployment environment characterization as well as baseline for the design and performance evaluation of
new advanced DWDM RoFSO communication systems we are currently developing.
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