Due to the uneven distribution of the underwater laser transmission channels, the transmission beam will be affected by water absorption and scattering effects, which induce inter-symbol interference and the degradation of link performance. Based on the binomial Henyey–Greenstein scattering phase function and considering the forward and backward effects, the pulse response expression of the non-line-of-sight underwater laser channel is derived. The effectiveness of the underwater photon transmission channel model established is simulated and verified by the Monte Carlo method. The fitting effects in the coastal waters between the theoretical calculation value of channel impulse response and simulation are better. Under the same simulation parameters, the pulse-broadening factor in coastal waters is higher than that in harbor waters, and the time-domain broadening effect of laser pulse transmission in harbor waters is large. When the asymmetry factor is large, the influence of front scattering on pulse broadening is greater.
KEYWORDS: Modulation, Signal to noise ratio, Atmospheric optics, Optical engineering, Monte Carlo methods, Signal detection, Laser applications, Error analysis, Signal processing, Receivers
Optical spatial modulation (OSM), a recent optical multiple-input multiple-output transmission scheme in wireless optical communication, has aroused great research interests. OSM scheme requires accurate channel state information (CSI) during transmitter and receiver sides, but it is difficult to accurately estimate CSI in OSM scheme. Therefore, a scheme named differential optical spatial modulation scheme with pulse position modulation (DOSM-PPM) based on the space-time mapping block of orthogonal design is proposed in order to completely avoid CSI estimation. The bit error rate (BER) performance of DOSM-PPM is analyzed under log-normal turbulent channel model and simulated using Monte Carlo method. The results show that the transmission rate of DOSM-PPM is improved effectively. Meantime, the proposed DOSM-PPM scheme outperforms DOSM scheme with spatial pulse amplitude modulation by 1 dB at 1 × 10 − 3 BER with the same spectrum efficiency.
The ergodic channel capacity of MIMO WOC systems with combined effects is investigated under correlated channel. The results indicate that spatial correlation has the most serious influence on ergodic channel capacity, followed by the pointing error and finally the atmospheric turbulence.
Conventional space-time trellis code cannot be directly applied to the intensity modulation/direct detection atmospheric optical transmission systems due to the complex and negative signal problem. An asymmetrically clipped space-time trellis code scheme is proposed, which combines the Hermitian symmetry of the inverse fast Fourier transform and the N/4 points to N points signal mapping to make the processed bipolar signal sequence satisfies the odd harmonic symmetry. The conversion of complex signal to positive real signal can then be achieved by clipping the bipolar signal sequence. Monte Carlo method is utilized to verify the feasibility of the proposed scheme. The results show that the proposed method can effectively resist the atmosphere scintillation.
A novel double-clad As2Se3 chalcogenide photonic crystal fiber is proposed and the slow light via stimulated Brillouin scattering is theoretically investigated. The Brillouin gain spectrum by taking into account the high-order acoustic modes is analyzed. The simulated results indicate that the slow light can be tuned by varying the air filling fraction in the inner cladding. The time delay upto 1120ns can be achieved with 1-m-long fiber when pumped with 10mW. But these features of slow light are less affected with the change of the air filling fraction in the outer cladding.
Intermodal Brillouin frequency shift and Brillouin gain spectrum in few-mode fibers are investigated by full vectorial finite element method, and the influences of pump power on the time delay and pulse broadening factor are also simulated. The simulation results show that Brillouin gain of intermodal stimulated Brillouin scattering varies with different modes pairs. Time delay increases with increasing of pump power. Pulse broadening factors decrease with the input signal pulse width but increase with the input pump power. Optimized results show that time delay of LP01 - LP01mode pair is 213.2ns, and the corresponding pulse broadening factors is 1.126.
The spatial correlation extensively exists in the multiple-input multiple-output (MIMO) free space optical (FSO) communication systems due to the channel fading and the antenna space limitation. Wilkinson’s method was utilized to investigate the impact of spatial correlation on the MIMO FSO communication system employing multipulse pulse-position modulation. Simulation results show that the existence of spatial correlation reduces the ergodic channel capacity, and the reception diversity is more competent to resist this kind of performance degradation.
Nowadays, information technology is becoming more and more important to improve the productivity of agriculture,
especially for real time environment monitoring. However, the traditional method of environmental data collection is
unable to provide real-time and highly accurate data of the monitored region to meet the requirements of precision
agriculture. As wireless sensor networks(WSNs) has profound impacts on many fields due to its promising capability, in
this paper, a WSN-based environment monitoring system is proposed. A prototype of the system that utilizes GAINSJ
nodes based on Zigbee communication protocol has been implemented, and its packet error rate in different conditions
was evaluated. Based on the proposed system architecture and technologies, the real time data can be measured,
transmitted and stored in high accuracy. Moreover, the system was applied in upland grassland in Yushu, Qinghai
province, and compared the results with the data acquired by local weather station. The system evaluation and
experimental results show the effectiveness and reliability of the system in measuring the variations of temperature and
humidity data within monitored region.
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