Coherent detection of binary ask signal in awgn channels
The base band representation of transmitted signal has the following form:. 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. Beyond this very little improvement in the SNR performance can be achieved. Gaussian noise and the length of data sequence is 2N sent at a time. The received sequence r can be expressed as.
This includes solving the Mie equations of scattering and absorption coefficients and the scattering phase function for the poly dispersion case. Where, is a particular sequence of transmitted signal phase. Now such values of has to be chosen so that the value of becomes maximum. Though amplitude shift keying ASK is the simplest and widely reported, it does not offer immunity to the turbulence induced fading.
Analysis shows that differential encoding. Differential coherent detection offers the simplest way of achieving carrier synchronization with phase - shift keying PSKand, thus, represents an attractive solution for systems where error in signal is caused by the channel itself. Beyond this very little improvement in the SNR performance can be achieved.
Doppler distortion causes inter -carrier interference which prevents the use of differentially coherent detection in OFDM systems. Mathematical models were developed for temporal impulse response at the three wavelengths listed above for the visible and near IR. Usually, in open optical communication severe bandwidth limitation occur s particularly when cloud s are. However, the performance of FSO system is limited by varying atmospheric conditions.
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. Beyond this very little improvement in the SNR performance can be achieved. These wavelengths are those under consideration for optical satellite communication [ 6,7 ]. It is found that for three different wavelengths 0. 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.
Clouds cause temporal widening and attenuation of optical pulse power as a part of optical communication channel. When a frequency selective multipath channel introduces ISI, differentially coherent detection must be combined with equalization . In all practical casespart of the optical channel passes through the earth's atmosphere that contains cloud s. Bit error rate of the received signal is determined after propagation through cloud. However, there is an additional power penalty caused by the frequency offset because of delayed and not-delayed bits not being in phase.
Increments in the aperture size to an optimum value results in improved signal -to -noise ratio SNR performance. It is observed from the transfer function of cloud that high attenuation of transmitted signal occurs while it passes through cloudy environment. In differential phase -shift keying DPSKthe phase reference is provided by the previous symbol .
Therefore there is higher probability of error due to inter symbol interference which is occurred by pulse broadening in cloud. Mid -IR wavelengths exhibit much more scattering than shorter ones because of the size distribution of cloud particulates. The presence of ISI in the system introduces error s in the decision device at the receiver output.
However, channel estimation errors degrade the performance of coherent detection, rendering it equal or even inferior when the channel variation is non -negligible . For present case the signal is assumed to be added with a white. The received sequence r can be expressed as.