Recently, free space optical communication has attracted a considerable amount of interest as an alternative medium to
radio for wireless transmission. It has many advantages such as a huge unregulated bandwidth, high data rates, and
resistance to multi-path fading relative security against eavesdropping. The work presented in this thesis focuses on
modulation schemes. Concerns of eye safety and power consumption limit the average transmitter power, which suggests
that modulation techniques possessing a high peak-to-mean power ratio are favorable. This is generally achieved by
trading off power efficiency against bandwidth efficiency. A novel dual-header pulse interval modulation (DH-PIM) for
optical wireless communications is proposed in this paper. The symbol structure, the spectral properties, and the
expression for bandwidth requirement and optical power requirement of DH-PIM in the absence of and in the presence
of multi-path dispersion is presented in this paper.
In the decade of information, with the increment of the data rate and capacity in communication, free space optical
communication (FSO) has been paid much attention by many countries since it integrates the advantages of fiber
communication and other traditional wireless communication technologies, such as large capacity, low cost, convenient
deployment and flexibility. This paper concerns with the research on the signal tracking technology of FSO. Considering
that suppressing vibration is a difficulty of FSO, so suppressing methods are discussed in the paper. As a base, the ATP
(Acquisition, Tracking, Pointing) technology of FSO communication system is analyzed in this paper. And a new signal
tracking system for FSO communication is described. The main structure, integral parts and detailed work process of this
signal tracking system are introduced in detail.
The 3G network security was a problem in the practice. The particularity of the 3G core network determined that the new firewall technology must be adopt in order to protect the core network. The development of the network processor (NP) technology offered a high-performance realization platform for core network firewall. On the basis analyzing the shelter method of the attack to each interface in the 3G core network, a scheme of 3G core network firewall design was put forward. It was on the network processor hardware platform. How to use IPSEC protocol to protect the GPRS tunnel protocol (GTP) and implement on the NP was discussed in detail. The aggregated bit vector algorithm was applied on NP for packet classification. In the end result table of firewall performance test with SMART BIT instrument was provided.
The exponential growth in optical link speed has stressed the performance of routers and firewall. Consequently, a new breed of microprocessors, called Network Processors (NP), are designed and fabricated specifically to effectively process packets on firewalls and routers. Packet classification is a major function in network processors to fit requirements of next-generation Internet. The paper presents a hardware-based packet classification algorithm for NP. The innovative aspect of the proposed algorithm is to use the NP's parallel character well enough. The algorithm use hash algorithm to make six-dimension classification be three-dimension classification. Compressing the size of classification fields are used for hash input while TCAM performs the lookup of the hash value and source IP. The memory of the algorithm is the total amount of memory needed to store the rules. The search speed of our algorithm is neither sensitive to the size of the rule table. It is only relate to the collision of the hash function. The performance of the proposed algorithm test result demonstrates that the proposed scheme is a sound approach that can be implemented on NP efficiently.
KEYWORDS: Telecommunications, Free space optical communications, Monte Carlo methods, Systems modeling, Atmospheric turbulence, Mathematical modeling, Turbulence, Receivers, Transmitters, Error analysis
Free space optical communication systems represent one of the most promising approaches for addressing the emerging broadband access market, it can provide high bandwidth with no physical contact, but are hampered by signal fading effects due to particulate scattering caused by atmospheric turbulence. In this paper, we propose a new channel model of MIMO free space optical communication system. The physics meaning of this model is very clear, and its format is very simple. Mathematic results show that MIMO is a very effective way for intensity fluctuation reduction induced by turbulence, thus reduce the bit-error-rate of the system.
KEYWORDS: Free space optics, Global Positioning System, Receivers, Telecommunications, Transmitters, Signal processing, Automatic alignment, Free space optical communications, Laser systems engineering, Satellites
Normally free-space laser communication system uses the narrow transmission laser beam. It is important to align the laser beams between two laser communication terminals. We developed the automatic alignment system used in the free-space laser communication system. The alignment system adopts GPS (Global Positioning System), the GPS makes the alignment automatic for initial alignment, to use the signal power received or the signal of CCD camera achieves the fine alignment. The principles and configuration frame of the automatic alignment system are discussed in detail.
The uncooled IRFPA thermal imaging systems, which are designed to operate at room temperature, will make the sensors less expensive, lighter, and more reliable. The sensor of this infrared FPA imaging system is the 320x240 LW IRCOMS uncooled integrated microbolometer detector. The time sequence required by the detector is described. The timing control circuit of the camera is designed using Altera EPLD.
The characteristics of a RF-excited all-metal CO2 portable laser with 1.0 m infrared optical fiber are described. The laser excited by 45.38 MHz can give 12 W- output power with square-pulse waveform (4 - 15 KHz) at the end of IR-fiber's coupler. The power-density is about 8 KW/cm2. Also using CW-CO2 laser the IR-fiber (0.2 db/m) has transmitted 102W (30 KW/cm2) laser power in the laboratory conditions.
The effective attenuation coefficient of underwater target signals and that of backscattered signals are considered same in the present discuss. By the experiments and simulations of underwater target lidar signals, it is found that they are different. A new Monte Carlo method is introduced in the paper, the scattering phase function is approximately by a distorted Henyey-Greenstein (H-G) function, and the expression of the scattering angle (theta) is obtained from H-G function. The calculation efficiency of this method is improved five times than the traditional Monte Carlo method. Various sizes of the target are simulated in the paper. From the calculation results it is concluded that the effective attenuation coefficient of underwater target signals is larger than that of backscattered signals.
Monte Carlo method is a classical method to simulate the backscattered lidar signals, but its disadvantage is that the calculation efficiency is very low. Based on the traditional semianalytic Monte Carlo method, the new method use a distorted Henyey-Greenstein (H-G) function to approximate the scattering phase function, obtain the expression of the scattering angle (theta) from H-G function, and compte photoelectric transform according to the characteristic of photomultiple. From the calculation efficiency is improved greatly.
The underwater laser communication system is an ideal but difficult programme. The influences of the water depth and channel characteristics on the received signals of underwater laser communication are studied in the paper, it is the reason that causes the degression of communication quality. In the last a programme of adaptive gain control of PMT is presented, and the composed principle frame and the test results are given.
In this paper, we first demonstrate that the signal received from the laser underwater target detection system may be chaotic through phase space reconstruction, correlation dimension analysis and Lyapunov exponent calculation. Then the result of the correlation dimension analysis is used to construct a neural network predictor which is considered as an approximation of the basic dynamics of the received signal. Finally we introduce a chaos-based detection method and apply it to detect the underwater target. The performance of this new method is superior to that of the conventional method.
The information format in the experimental underwater laser system is discussed. Two-pulse modulation for the laser is proposed. This method clearly reduces the modulated laser pulse's vibration range which is caused by the random change of Q switch's trigger of the laser. Thus the reliability of the communication is increased. The hardware and software flow chart of the modulator are shown in this paper.
This paper analyzed the signal conditioning received by the Underwater Object Laser Detection system and designed a kind of Photomultiplier Gain Control system to compress the signal's dynamic range. Finally, some experimental results are presented and analyzed.
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