A MPR optimization algorithm for FSO communication system with star topology

The free space optical (FSO) communication system has attracted many researchers from different countries, owning to its advantages such as high security, high speed and anti-interference. Among all kinds of the channels of the FSO communication system, the atmosphere channel is very difficult to deal with for two typical disadvantages at least. The one is the scintillation of the optical carrier intensity caused by the atmosphere turbulence and the other is the multipath effect by the optical scattering. A lot of studies have shown that the MIMO (Multiple Input Multiple Output) technology can overcome the scintillation of the optical carrier through the atmosphere effectively. So the background of this paper is a MIMO system which includes multiple optical transmitting antennas and multiple optical receiving antennas. A number of particles such as hazes, water droplets and aerosols exit in the atmosphere widely. When optical carrier meets these particles, the scattering phenomenon is inevitable, which leads to the multipath effect. As a result, a optical pulse transmitted by the optical transmitter becomes wider, to some extent, when it gets to the optical receiver due to the multipath effect. If the information transmission rate is quite low, there is less relationship between the multipath effect and the bit error rate (BER) of the communication system. Once the information transmission rate increases to a high level, the multipath effect will produce the problem called inter symbol inference (ISI) seriously and the bit error rate will increase severely. In order to take the advantage of the FSO communication system, the inter symbol inference problem must be solved. So it is necessary to use the channel equalization technology. This paper aims at deciding a equalizer and designing suitable equalization algorithm for a MIMO free space optical communication system to overcome the serious problem of bit error rate. The reliability and the efficiency of communication are two important indexes. For a MIMO communication system, there are two typical equalization methods. The first method, every receiving antenna has an independent equalizer without the information derived from the other receiving antennas. The second, the information derived from all of the receiving antennas mixes with each other, according to some definite rules, which is called space-time equalization. The former is discussed in this paper. The equalization algorithm concludes training mode and non training mode. The training mode needs training codes transmitted by the transmitter during the whole communication process and this mode reduces the communication efficiency more or less. In order to improve the communication efficiency, the blind equalization algorithm, a non training mode, is used to solve the parameter of the equalizer. In this paper, firstly, the atmosphere channel is described focusing on the scintillation and multipath effect of the optical carrier. Then, the structure of a equalizer of MIMO free space optical communication system is introduced. In the next part of this paper, the principle of the blind equalization algorithm is introduced. In addition, the simulation results are showed. In the end of this paper, the conclusions and the future work are discussed.

Relay-assisted cooperative free space optical (FSO) communication system is a powerful technique, which offers advantages like fading mitigation and significant gain in spatial diversity due to shorter transmission hops. Even so, it can be hampered by jamming activities which in turn degrade the overall system performance. In this paper, the effect of relay jamming in the decode-and-forward (DF) protocol based cooperative FSO communication system, is introduced and studied thoroughly. The probability density function (pdf) of a random variable containing mixture of the negative exponential and Gaussian random variables is derived. The error performance of the considered FSO system is analysed over this new kind of additive mixture noise for different probabilities of jamming. By utilizing the derived pdf, a closed-form expression of the average bit error rate (BER) of the DF FSO relaying is obtained. A comparison of the BER performance of maximum-likelihood (ML) detector and a derived threshold based detector is also provided. It is observed that the derived threshold based detector offers almost similar performance as that of the ML detector. A thorough numerical study is also provided by considering different pointing error parameters of jammer's and user's channels.

Free space optical (FSO) communication system is a groundbreaking technology in the field of communication systems. Non-orthogonal multiple access (NOMA), on the other hand, currently being used in fifth-generation (5G) wireless systems, is a powerful technique for establishing communication for multiple users using the same time and frequency resources. In this paper, the performances of NOMA and orthogonal multiple access (OMA) in an FSO-based communication system are studied. In the FSO communication system, two base station units are considered to be connected to a central unit for FSO backhauling using an uplink-fixed NOMA scheme. Bit error rate (BER), ergodic capacity, and energy efficiency (EE) performances of the NOMA-based FSO system are studied and compared with an OMA-based FSO system. It is found that the NOMA-based system provides approximately 10% of the ergodic capacity gain and increases EE by 37%-60% for a given BER compared to the OMA-based system.

Performance of free space optical communication systems over exponentiated Weibull atmospheric turbulence channel for PPM and its derivatives

This paper proposes an ultra-high capacity free space optical communication (FSO) system, employing optical dual polarization quadrature phase shift keying (DP-QPSK) modulation format. It is a 16-channel dense wavelength division multiplexing (DWDM) system with 120 Gbps/channel bit rate. The FSO system is analyzed with three optical advanced modulation schemes; such as 4-quadrature amplitude modulation (4-QAM), quadrature phase shift keying (QPSK) and DP-QPSK. The main focus of the projected system is to improve the FSO setup performance in terms of eye diagram, bit error rate (BER), high received power and Q-factor. The center channel wavelength used for the proposed system is 1550 nm. In the present wireless communication world, the proposed system will be highly helpful as an ultrahigh capacity wireless system.

In a free space optical communication (FSOC) system, atmospheric turbulence will increase the bit error ratio (BER) and impair FSOC link reliability. Since computational temporal ghost imaging (CTGI) has anti-interference, we present an FSOC system over atmospheric turbulence based on CTGI. The simulation results show that the BER performance of CTGI is better than on-off keying under different atmospheric turbulence regimes. To improve the performance of the CTGI scheme, the influence of the number of transmission samples and code length is analyzed. It is shown that BER performance improves with the increment of the number of samples, while code length has no impact. This scheme provides an idea for reliable communication over atmospheric turbulence and an important reference for improving wireless optical communication in an extreme environment.

In this study, Mie and Rayleigh scattering in free space optics (FSO) communication systems were investigated in terms of the atmospheric attenuation. Because of the movement of the Earth, the communication distance and surrounding gas densities are inconsistent in each region. This change leads to atmospheric attenuation and then data losses and inefficient communication in FSO occur. Therefore, the density change and distance must be calculated in each communication once the data is transmitted. In the literature, it was observed that the atmospheric attenuation is regarding some FSO communication parameters such as transmission distance, visibility, and scatter particle size distribution, the number of particles per unit volume, scatter cross-sectional area, and wavelength. Besides, in real-time communication, it is necessary to update FSO parameters simultaneously. However, this updating process for all parameter takes a long time to adapt to a new position. This paper proposes the design of the experiment method (Doe) to determine the severity of the FSO parameters. And Taguchi’s Doe method allows analyzing of FSO communication system parameters to avoid long calculation time. Results show that the proposed method helps in understanding the priorities of the parameters in FSO and reducing the updating time.

In this paper, we propose a simultaneous fine tracking and signal detection subsystem based on QD (Quadrant Detector) for FSO (Free Space Optical communication) system in 1550nm without beacon. Then we test the sensitivity, resolution, SNR characteristics and the influence the size of light spot on QD by building an indoor experiment. We select 1550nm waveband which suffers relatively less from background radicalization and atmospheric channel as the waveband of transmitter and receiver, also our selection catered for the trend of FSO system. We use the same QD for both fine tracking and signal detection for FSO in our design. Besides we use the scheme without beacon beam, for which we reduce the use of a beacon laser source, so we can obtain weight reduction and size decrease which is very important for FSO system.

There are many room temperature applications to include free space optics (FSO) communication system combining quantum cascade lasers sources where HgCdTe long-wave (8-12 micrometer) infrared radiation (LWIR) detector reaching ultrafast response time 109 cmHz1/2/W. Since commercially available FSO could operate separately in SWIR, MWIR and LWIR range - the dual band detectors should be implemented into FSO. This paper shows theoretical performance of the dual band back-to-back MWIR and LWIR HgCdTe detector operating at 300 K pointing out the MWIR active layer influence on LWIR operating regime.

A free-space optical (FSO) communication system that does not implement compensation strategies for the effects of beam wander may experience deep long-term channel fades or even severe communication outage when the laser spot wanders away from the receiver aperture due to the effects of atmospheric turbulence. The impacts of laser beam diverging angles on channel fading probability density function (pdf) and on bit-error-rate (BER) performance of both uncoded and coded FSO communication systems are studied for different receiver aperture sizes and propagation distances. The optimal diverging angles are determined according to the average uncoded and coded BER for both uncoded and coded system, respectively. For a fast-tracked FSO communication system, a focused beam always provides better channel fading, coded BER, and uncoded BER than beams with larger diverging angles. For an untracked system, smaller beam diverging angels have better BER performance and lower channel fading for short distances when beam wander is small compared with the receiver aperture size, and larger beam diverging angles are preferred for longer distances when beam wander becomes significant. The optimal beam diverging angles of the coded FSO communication systems are smaller than those of the uncoded systems.

The beam motion is one of the main causes for major power loss which severely degrades the performance of the Free Space Optical Communication (FSOC) system. Designing a suitable controller to correct the beam motion to increase the beam stability at a point becomes significant. This paper presents an investigation on the performance of two types of controller designed for aiming a laser beam to be at a particular spot under dynamic disturbances generated at the transmitter. The experimental nonlinear input---output data mapping is used as the principal components for the controller design. The first design is based on the Taguchi method while the second is the artificial neural network--cognitive method. These controllers process the beam pointing (beam location) information from an opto-electronic position detector and then generate the necessary outputs to steer the beam with fast steering mirror. Conceptually, the controller design problem is addressed with looking the position error information to decide the level of nonlinear static output map for generating the correction control information. The pipelined-parallel architecture of both controllers are developed in a field programmable gate array (FPGA) and installed at the receiver station. Evidence of the suitability and the effectiveness of the proposed controller in terms of prediction exactness, response to impulse, scintillation, Q-factor and bit error rate are provided through experimental results obtained from the 155 Mbps FSOC data link set-up established for the horizontal range of 0.5 km at an altitude of 15.25 m.

In free space optical communication (FSOC) systems, channel fading caused by atmospheric turbulence degrades the system performance seriously. However, channel coding combined with diversity techniques can be exploited to mitigate channel fading. In this paper, based on the experimental study of the channel fading effects, we propose to use turbo product code (TPC) as the channel coding scheme, which features good resistance to burst errors and no error floor. However, only channel coding cannot cope with burst errors caused by channel fading, interleaving is also used. We investigate the efficiency of interleaving for different interleaving depths, and then the optimum interleaving depth for TPC is also determined. Finally, an experimental study of TPC with interleaving is demonstrated, and we show that TPC with interleaving can significantly mitigate channel fading in FSOC systems.

Clouds are key driver in the performance of free space optical communication (FSOC) systems. Clouds are composed of liquid water and/or ice crystals and depending on the physical thickness can produce atmospheric fades easily exceeding 10 dB. In these more common cases, impacts on FSOC systems may be severe. On the other hand, there are times when cloud fades may be as low as 1 or 2 dB as a result of thin, ice crystal based cirrus clouds. In these cases, the impacts on FSOC communication collectors may be limited. The ability to characterize the distribution and frequency of clouds are critical in order to understand and predict atmospheric impacts. A cloud detection system has been developed and applied to produce high resolution climatologies in order to investigate these impacts. The cloud detection system uses geostationary, multi-spectral satellite imagery at horizontal resolutions up to one kilometer and temporal resolutions up to fifteen minutes. Multispectral imagery from the visible wavelengths through the longwave infrared is used to produce individual cloud tests which are combined to produce a composite cloud analysis. The result represents a high spatial and temporal resolution climatology that can be used to derive accurate Cloud Free Line of Sight (CFLOS) statistics in order to quantify atmospheric effects on optical communication systems. The Lasercom Network Optimization Tool (LNOT) is used along with a mission CONOPS and the cloud database to find configuration of geographically diverse ground sites which provide a high availability system.

In this paper, we propose and demonstrate a free space optical (FSO) communication system with 10 Gbit s−1 on-off keying traffic, which is integrated in a passive optical network (PON) for broadcasting wireless data. After a transmission distance of 25 km of single-mode fiber and a free space transmission length of 6 m, the observed optical sensitivity of the FSO signal can reach −35.2 dBm within the forward error correction level at the bit error ratio of 3.8 × 10−3. Here, according to the obtained results of experiment and simulation, the FSO signal can support 2048 remote optical wireless units (OWUs) for wireless access via the PON network in free space transmission lengths of 0 to 160 m. Moreover, the proposed FSO-PON system also can support 1024, 512 and 256 remote OWUs in free space transmission lengths of 250, 350 and 500 m, respectively.

In free space optical (FSO) communication systems, atmospheric turbulence is the major cause of bit error rate (BER) performance degradation. The error performance of the system can be significantly improved with the help of spatial diversity by employing multiple transmit or receive apertures. However, correlation of channels under turbulence in the case of small separation between transmit apertures can substantially reduce the benefits of spatial diversity. Still, a thorough analysis of the system performance in terms of diversity order and coding gain for the FSO multiple-input single-output (MISO) system under turbulence influenced correlated channels has not been studied when the repetition coding is used to achieve improved BER performance. In this paper, we provide unique results of the BER performance analysis of the considered correlated FSO-MISO system over Gamma–Gamma distributed turbulence channel. Using the joint moment generating function of received signal-to-noise ratio (SNR), a novel generalized approximate BER expression is derived, followed by convergence test of the power series based BER expression using the Cauchy's ratio test. Then, an asymptotic analysis at high SNR is performed to obtain a novel closed-form expression for BER of FSO-MISO system. We also derive expressions for coding gain, diversity gain, and coding gain loss due to correlation in channels. Although the effect of correlation in channels on the BER performance of the system is analyzed under different scenarios, while it is observed that it does not affect the diversity order; it significantly degrades the coding gain of the considered system.