Atmospheric absorption, scattering, and turbulence are impairments in practical high-speed free-space laser
communications. These atmospheric effects can be mitigated by choosing the proper transmission wavelength. It is
well known that the MWIR (~3.8 μm) has many low-absorption spectral lines suitable for low-loss propagation. Also,
MWIR can be more robust to turbulence in the weak-turbulence regime. Since high-speed laser transceivers are not
available in the MWIR, a 3.8-μm signal can be generated and detected using a 1.55-μm telecom transceiver via
wavelength conversion. Free-space transmission of optical homodyne RZ-QPSK through a turbulent channel at 3.8 μm
has been investigated. A pair of Ti:PPLN-based nonlinear wavelength converters were used to down- and up-convert
from 1.55 to 3.8 and back to 1.55 μm at the transmitter and at the homodyne receiver, respectively. The converted RZQPSK
signal was transmitted through a tabletop wind tunnel that produces a weak turbulent path. Comparison of 1.55
and 3.8 μm transmission through the wind tunnel shows that under weak-turbulence 3.8 μm transmission is more robust
than 1.55 μm. Under the same turbulence condition, the scintillation index measured at 3.8 μm is consistently lower
than that at 1.55 μm. Extrapolated scintillation indexes for 3.8 and 1.55 μm using the Rytov variance (~ λ-7/6 ) and
independent measurement at 632.8 nm are consistent with the RZ-QPSK scintillation data for 3.8 and 1.55 μm. Under
the most severe turbulence condition, the average bit-error-rate of 3.8-μm transmission is better than that of 1.55-μm
giving an estimated receiver sensitivity improvement of at least 6 dB.
Delay-diversity transmission employing two orthogonal polarizations in turbulent media has been shown to be effective
in combating fading. The technique employs simultaneous transmission of two orthogonal polarizations carrying the
same information but delayed relative to each other by an amount (Td) equal to or greater than the turbulence correlation
time (τc). At the receiver, the polarization signals are detected separately with Td compensated on the delayed
polarization, resynchronizing the two signals, and they are then combined. Because Td is comparable to τc, scintillation
suffered by the two resynchronized polarization signals is essentially independent in a statistical sense. As a result, the
average bit-error-rate (BER) of the polarization-combined signal is less than the average BER of either one of the
polarization signals. The signal-to-noise ratio (SNR) is effectively improved. We describe here an experiment and test
results of fading mitigation via polarization delay-diversity reception in homodyne optical RZ-QPSK transmission at
1.55 μm through a tabletop wind tunnel and phase plates. The wind tunnel and the phase plates produce, respectively,
weak and strong turbulence with scintillation indexes less than and greater than one. The turbulence correlation time for
the wind tunnel is less than 1 ms. The homodyne detected optical RZ-QPSK signals in the two orthogonal polarizations
were captured and stored for off-line processing. Test results show that the computed BER of the polarizationcombined
signal is lower than the BER of either polarization signals. As a result, an equivalent signal-to-noise ratio
improvement of at least 2 dB was deduced.
Coherent homodyne detection using local laser oscillator is an important technique for applications requiring high receiver sensitivity. Conventional homodyne detection uses a continuous-wave (CW) local laser oscillator (LO) in which the only adjustable parameter is its average power. A pulsed LO with repetition rate same as the data symbol rate is proposed which provides new degree of freedom in receiver design, namely, its pulse shape and duty cycle. It is shown that pulsed LO is overall effective in enhancing receiver sensitivity compared with CW LO. We have investigated and compared the performance of coherent detection of 12.5 Gb/s binary phase-shift-keyed signals using integrated LiNbO3 optical 90° hybrid with pulsed and CW LO for different receiver bandwidths. Our results showed that pulsed LO provide at least 2 dB in sensitivity improvement. We also observed that pulsed LO is effective in reshaping broadened signal pulses. Our simulation results agreed well with experiment and predicted that for a given signal and receiver bandwidth there is an optimal LO pulse width that gives maximum eye opening. Our simulation results also showed that pulsed LO is potentially effective in reducing penalty of pulse smearing as a result of beam steering impairment in free-space laser communications.
KEYWORDS: Distributed computing, Databases, Local area networks, Network architectures, Standards development, Operating systems, Nomenclature, Data transmission, Data modeling, Computer architecture
Deployment of distributed applications in the wireless domain lack equivalent tools, methodologies, architectures, and network management that exist in LAN based applications. A wireless distributed computing environment (KeyWareTM) based on intelligent agents within a multiple client multiple server scheme was developed to resolve this problem. KeyWare renders concurrent application services to wireline and wireless client nodes encapsulated in multiple paradigms such as message delivery, database access, e-mail, and file transfer. These services and paradigms are optimized to cope with temporal and spatial radio coverage, high latency, limited throughput and transmission costs. A unified network management paradigm for both wireless and wireline facilitates seamless extensions of LAN- based management tools to include wireless nodes. A set of object oriented tools and methodologies enables direct asynchronous invocation of agent-based services supplemented by tool-sets matched to supported KeyWare paradigms. The open architecture embodiment of KeyWare enables a wide selection of client node computing platforms, operating systems, transport protocols, radio modems and infrastructures while maintaining application portability.
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