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In this study the performance of the differentially coherent detected signal based FSO communication system is investigated considering the effect of cloud caused Inter Symbol Interference ISI. To mitigate the effect of fading, the differential coherent detection technique is employed. BER performance of the system has been analyzed under the presence and absence of ISI caused by cloud. A key motivation for employing differentially coherent detection is that BER performance has been improved significantly in binary DPSK than non-coherent OOK which has been observed in simulation.
Different graphical analysis by varying carrier wavelength of the system has been investigated in this study. FSO communications is a complementary technology to the radio frequency RF technology and optical fiber networks. Other advantages of FSO consist of THz license-free band width, secure transmission, smaller transceiver architecture, low development and installation cost and immunity to electromagnetic interference.
However, the performance of FSO system is limited by varying atmospheric conditions. One such a scenario is the turbulence induced fading experienced by the received optical signal, error control coding introduces vast processing coherent detection of binary ask signal in awgn channels and efficiency degradation.
Increments in the aperture size to an optimum value results in improved signal -to -noise ratio SNR coherent detection of binary ask signal in awgn channels.
Beyond this very little improvement in the SNR performance can be achieved. An important factor on the selection of modulation technique for FSO systems is the receiver sensitivity as there is always a trade -off between the receiver sensitivity and complexity.
Though amplitude shift keying ASK is the simplest and widely reported, it does not offer immunity to the turbulence induced fading. Differential phase shift keying DPSK with coherent phase -diversity system offers the best sensitivity in optical fiber systems.
However, there is an additional power penalty caused by the frequency offset because of delayed and not-delayed bits not being in phase. Furthermore, there is a further power penalty due to the phase noise of the semiconductor lasers sources. The inter symbol interference ISI due to multipath propagation is considered because of cloud. Doppler distortion causes inter -carrier interference which prevents the use of differentially coherent detection in OFDM systems.
Doppler Effect is not considered for current situation. On-Off keying is the simplest form of amplitude shift keying ASK modulation that represents digital data as the presence or absence of a carrier wave.
In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero. However, channel estimation errors degrade the performance of coherent detection, rendering it equal or even inferior when the channel variation is non -negligible .
Clouds cause temporal widening and attenuation of optical pulse power as a part of optical communication channel. In all practical casespart of the optical channel passes through the earth's atmosphere that contains cloud s. One important distortion effect imposed by the atmosphere is the signal temporal broadening. This produces inter symbol interference which limit s maximum transmission band width .
Using transmission with a wider temporal frequency band width will cause significant degradation in received signal quality because of the narrow information band width per mitted by clouds. Usually, in open optical communication severe bandwidth coherent detection of binary ask signal in awgn channels occur s particularly when cloud s are.
In order to imp rove performance, adaptive method s may be used according to atmospheric conditions. A theoretical model is presented. It is followed by calculations of the electro-optical properties of the cloud s. This includes solving the Mie equations of scattering and absorption coefficients and the scattering phase function for the poly dispersion case.
All calculations were carried out at three different wavelengths in the visible and near infrared IR spectral range, i. These wavelengths are those under consideration for optical satellite communication [ 6,7 ]. Longer wavelengths in the IR atmospheric windows exhibit very high absorption. Mid -IR wavelengths exhibit much more scattering than shorter ones because of the size distribution of cloud particulates.
Mathematical models were developed for temporal impulse response at the three wavelengths listed above for the visible and near IR. Optical radiation propagating through clouds experiences temporal distortions. A function that describes well the temporal impulse response is the double gamma function which is shown below:. The value of the gamma constants are shown in following table for different wavelengths :.
Differential coherent detection offers the simplest way of achieving carrier synchronization with phase - shift keying PSKand, thus, represents an attractive solution for coherent detection of binary ask signal in awgn channels where error in signal is caused by the channel itself.
However, differentially coherent detection is based the premise that there is no inter symbol interference ISI in the received signal. When a frequency selective multipath channel introduces ISI, differentially coherent detection must be combined with equalization . In fact, when carrier phase noise effect s are not severe Coherent detection performs better than non - coherent detection. In no n coherent detection, a receiver computes decision variables based o n a measurement of signal energy.
I n differentially coherent detection, a receiver computes decision variables based on a measurement of differential phase between the symbol of interest and one or more reference symbol s. In differential phase -shift keying DPSKthe phase reference is provided by the previous symbol .
In this article, a maximum likelihood approach to partially coherent detection is taken. There has been a considerable improvement in performance by introducing a maximum likelihood sequence estimation MLSE type algorithm. The base band representation of transmitted signal has the following form:. The received sequence r can be expressed as.
For present case the signal is assumed to be added with a white. Gaussian noise and the length of data sequence is 2N sent at a time. For partially coherent detection, the receiver contains a tracking loop which provides an estimation o f unknown channel phase.
Now such values of has to be chosen so that the value of becomes maximum. Where, is a particular sequence of transmitted signal phase. The first term of equation 10 is representing the component associated with differential detection and the second term is associated with ideal coherent detection.
Bit error rate of the received signal is determined after propagation through cloud. It is observed from the transfer coherent detection of binary ask signal in awgn channels of cloud that high attenuation of transmitted signal occurs while it passes through cloudy environment.
Therefore there is higher probability of error due coherent detection of binary ask signal in awgn channels inter symbol interference which is occurred by pulse broadening in cloud. The inherent non -linear frequency response of cumulus cloud causing successive symbols to blur together. The presence of ISI in the system introduces error s in the decision device at the receiver output. It is found that for three different wavelengths 0.
I n figure 2 the I SI effect of cloud on bit error r ate performance is depicted. There is significant reduction of signal power and quality while it goes through cloud. Figure 2 plots the received signal with and without the presence of ISI effect in channel. Though having this high amplification the cloud caused ISI contained signal shows worse performance than the signal which is not affected by cloud caused ISI.
Analysis shows that differential encoding. However, In this paper cloud exists as physical channel between the transmitter and receiver.
Hence in this case the differential schemes can yield a better error -rate than the ordinary schemes which rely on precise phase information. In our wo coherent detection of binary ask signal in awgn channels, usi ng a ma ximum li keli hood approach a deci sion me tric i s determin ed to fi nd out t he optimum condition for di ffere nt ial detec ti on of BP SK si gna l.
It is evide nt from anal ysis t ha t there is si gnal power and si gnal qualit y bot h de grades a lot for cloud e ffect. Howe ver wit h the i ncr ea se of received data seque nce le ngt h a nd combi nation of adaptive algorit hm wi th di ffere ntiall y coherent de tected signal better BE R p erfo rma nce can be ac hieved tha n convent ional OO K detec ti on.