There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In this letter, we introduce a detection technique based on differential signaling scheme for outdoor free space optical communications. This method requires no channel state information (CSI) and does not suffer from the computational load compared with the conventional receivers, where adjusting dynamically the detection threshold level (either based on CSI knowledge, or by using pilots) leads to increased computational time and reduced link throughput. This letter shows that the performance of the proposed technique only depends on the correlation of propagating optical beams. Especially under highly correlated-channels condition the fluctuation of the detection threshold level in the receiver is significantly small. We also show experimentally that under weak turbulence regime the variance of detection threshold level reduces for the correlated channel.

/pdf/fso-detection-using-differential-signaling-in-outdoor-4z34psndus.pdf

FSO Detection Using Differential Signaling in Outdoor Correlated-Channels Condition

As wireless communication systems become ever-more important and pervasive parts of our everyday life; system capacity and quality of service issues are becoming more critical. In order to increase the system capacity and improve the quality of service, it is necessary that we pay closer attention to bandwidth and power efficiency issues. In this paper, the bandwidth and power efficiency issues in Free Space Optics (FSO) transmissions are addressed under Pulse Position Modulation (L-PPM) and Pulse Amplitude Modulation (M-PAM) schemes, and their performance in terms of power and bandwidth efficiencies and the Bit Error Rate (BER) versus Signal-to-Noise Ratio (SNR) are compared analytically. The comparative study of the L-PPM and MPAM schemes is discussed, and showed that for similar SNR, L-PPM scheme offered improved performance. For FSO communication systems, although the power efficiency is inferior to L-PPM scheme, On-Off Keying (OOK) modulation scheme is more commonly used due to its efficient bandwidth usage, but M-PAM is the bandwidth efficient modulation scheme in this research for more than “2” bits of information can be sent, while L-PPM is the power efficient modulation scheme for more number of bits can be sent, and it may be able to improve performance by increasing the number of bits in L-PPM scheme. General Terms Optical Communications, Modulation schemes, Bit Error Rate (BER), Signal to Noise Ratio (SNR), Bandwidth Efficiency, Power Efficiency.

/pdf/performance-comparison-between-ook-ppm-and-pam-modulation-356isr72fn.pdf

Performance Comparison between OOK, PPM and PAM Modulation Schemes for Free Space Optical (FSO) Communication Systems: Analytical Study

We describe a lab setup for analyzing impairments of terahertz (THz) and infrared (IR) free space links caused by local refraction index changes in the signal’s propagation paths that could be induced by turbulence, particles, humidity, etc. A THz signal comprising a 2.5  Gb/s data load modulated on a carrier at 625 GHz, is launched through a weather emulating chamber, detected, and its performance analyzed. An IR beam at 1.5 um wavelength carrying the same data load is superposed with the THz beam, propagating through the same weather conditions and also performance analyzed. We modulate the IR channel with a usual non-return-to-zero (NRZ) format but use duobinary coding for driving our THz source, which enables signaling at high data rate and higher output power. As both beams pass through the same channel perturbations and as their degradations are recorded simultaneously we can simultaneously compare the weather impact on both. We investigate scintillation and fog attenuation effects for the THz and IR signals by measuring bit error rates (BER), signal power, and phase front distortions.

https://web.njit.edu/~sirenko/PapersNJIT/josaa-29-2-179.pdf

Experimental comparison of performance degradation from terahertz and infrared wireless links in fog

SIMO optical wireless links with nonzero boresight pointing errors over M modeled turbulence channels

This paper experimentally investigates and compares the performance of the free space optics system employing three different modulation schemes, on-off keying (OOK) with non-return-to-zero (NRZ) and return-to-zero (RZ) and the binary phase shift keying (BPSK) operating under the turbulent atmosphere. The received average signal is measured and used to characterize the strength of the turbulence. The experiment is performed with a temperature gradient of 4 degrees at a wind velocity of 4 m/s. The temperature gradient within the controlled channel results in turbulence of a log irradiance variance of 0.002, which is classified as a very weak turbulence. The received signal eye diagram and power histograms are presented and analyzed for performance evaluation of the selected modulation schemes in the weak turbulence model.

/pdf/experimental-investigation-of-the-performance-of-different-4gebwpqzif.pdf

Experimental investigation of the performance of different modulation techniques under controlled FSO turbulence channel

The paper reports experimental results for the performance of a free space optical (FSO) communication link employing different modulation schemes under the influence of the atmospheric scintillation. A dedicated experimental atmospheric simulation chamber has been developed where weak and medium turbulence can be generated and its effect on the FSO link is investigated. The experimental data obtained is compared to the theoretical prediction. The paper also depicts the effect on the data transmission performance depending on the position of the turbulence source within the chamber.

/pdf/experimental-study-of-bit-error-rate-of-free-space-optics-4knnwhap94.pdf

Experimental study of bit error rate of free space optics communications in laboratory controlled turbulence

A number of modulation schemes have been proposed and thoroughly studied for the successful operation of the optical wireless communication (OWC). Each modulation scheme has its advantages and disadvantages for the particular channel conditions. In this paper the comparative studies of on- off keying (OOK) return-to-zero (RZ) and non-return-to-zero (NRZ) modulation techniques have been carried out experimentally under controlled turbulence channel for free space optics (FSO) communication link. Weak turbulence effect is generated within a controlled chamber of 5.5m length. The experiment is performed with a temperature gradient of �¤& at wind velocity of 1m/s. A laser source is modulated using the selective modulation schemes and optical signal is allowed to propagate through the turbulence simulated chamber. The received signal is analyzed to measure the true effect of turbulence on the optical beam carrying information. The eye- diagram and signal distributions are analyzed for comparative studies. This research work is carried out under the EUCOST ACTION IC0802 project.

/pdf/experimental-investigation-of-the-performance-of-ook-nrz-and-2cn202q3mt.pdf

S. Ansari

Papers

Experimental investigation of the performance of different modulation techniques under controlled FSO turbulence channel

Experimental study of bit error rate of free space optics communications in laboratory controlled turbulence

Experimental Investigation of the Performance of OOK-NRZ and RZ Modulation Techniques under Controlled Turbulence Channel in FSO Systems