KEYWORDS: Free space optics, Error analysis, Receivers, Statistical analysis, Signal to noise ratio, Scintillation, Turbulence, Interference (communication), Signal detection, Modulation
Incoherent receivers of Free Space Optical (FSO) signals have no knowledge of instantaneous channel state. Thus,
the receiver requires some information about the noise and fading statistics for a maximum likelihood (ML)-based
optimal detection. Using pilot-aided symbols, we develop a simple multi slot averaging (MSA) estimation technique to
approximate the values of parameters required at the incoherent detector. No channel state information (CSI) is available
at the receiver side and this work will not be also trying to estimate it. But the estimation of noise and fading statistics
will be practically investigated. We evaluate the bit error rate (BER) performance of FSO links with MSA estimation
over both Gaussian and lognormal atmospheric turbulence fading (scintillation) channels. Numerical simulation will be
completed to evaluate the estimation error of the MSA estimator. We will see that at signal to noise ratio (SNR)=13dB,
the performance loss of the Gaussian estimator improves from 3dB to 0.4dB when we increase the number of pilot
symbols from 16 to 64. This paper also presents the hardware design of the estimator using Xilinx system generator.
KEYWORDS: Free space optics, Signal to noise ratio, Receivers, Telecommunications, Computer simulations, Transmitters, Signal detection, Control systems, Scintillation, Modulation
In hybrid FSO/RF (Free Space Optics/Radio Frequency) systems, the FSO link is the primary link while the RF link
is the secondary (backup) link. In current hybrid systems, once the FSO signal to noise ratio (SNR) decreases above a
preset threshold, the system switches from FSO to RF to maintain the communication connection. But this scenario does
not provide maximum utilization of the available bandwidth in terms of spectrum efficiency. This paper proposes an
adaptive FSO bit rate algorithm to maintain communication using the FSO link as long as its bit rate is greater than that
can be offered by the RF, thus requiring no switching to the secondary link. The system switches to the secondary link if
and only if the FSO bit rate while maintaining the system bit error rate (BER) drops below the bit rate that can be
achieved by the RF link. This paper shows the dependence of FSO bit error rate on channel bit rate using analysis and
computer simulation. Furthermore, computer simulation is carried out and the results confirm the algorithm's
effectiveness on maintaining higher bit rate communication connection using FSO link. Hence, the overall system
performance achieves better throughput.
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