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.
Optical fiber offers many advantages over coaxial cable for the transmission of radio frequency (rf) signals in antenna-remoting applications, as well as cellular networks and cable television (CATV) signal distribution networks. Optical fiber shows significantly less loss, can support signals demanding much higher bandwidth, is immune to electromagnetic interference (EMI), and enables considerable size and weight savings when compared to coaxial cable. Free-space optics (FSO) communications is a technology that uses modulated optical beams to transmit information line of sight through the atmosphere. FSO can be deployed faster and cheaper when compared with optical fiber. Recently, FSO has been investigated by the telecommunications industry and research centers to transport digital signals for civilian "last mile" applications and military applications. We demonstrate the successful transport of modulated rf analog signals over an FSO link and compare key performance measures against a fiber optic link configured in an identical manner. Results of measurements of optical power, transmission response, reflection response, group delay that defines phase distortion, carrier-to-noise ratio (CNR), and dynamic range that defines nonlinear distortion are presented. Results from this comparative study indicate that FSO for rf applications is a suitable replacement for fiber optic transmission links over short distances.
KEYWORDS: Free space optics, Signal to noise ratio, Optical amplifiers, Analog electronics, Transmitters, Fiber amplifiers, Modulation, Receivers, Signal attenuation, Atmospheric optics
Free space optics (FSO) is a technology that uses modulated optical beams to transmit information line-of-sight through the atmosphere. To date, the primary focus of FSO research and development has been toward the transmission of digital signals, primarily for "last mile" applications. This paper reports the use of FSO to transport modulated radio frequency (RF) analog signals, together with an investigation of key performance measures. Results indicate minimal RF signal distortion when transmitted over FSO. The advantages of using FSO include increased security and insusceptibility to electromagnetic interference (EMI).
The role of the Advanced Air Transportation Technologies program undertaken at the NASA Glenn Research Centers has been focused mainly on the improvement of air transportation safety, with particular emphasis on air transportation communication systems in on-board aircraft. The conventional solutions for digital optical communications systems specifically designed for local/metro area networks are, unfortunately, not capable of transporting the microwave and millimeter RF signals used in avionics systems. Optical networks capable of transporting RF signals are substantially different from the standard digital optical communications systems. The objective of this paper is to identify a number of different communication link architectures for RF/fiber optic transmission using a single backbone fiber for carrying VHF and UHF RF signals in the aircraft.
To support these architectures, two approaches derived from both hybrid RF-optical and all-optical processing methodologies are discussed with single and multiple antennas for explicitly transporting VHF and UHF signals, while the relative merits and demerits of each architecture are also addressed. Furthermore, the experimental results of wavelength division multiplexing (WDM) link architecture from our test-bed platform, configured for aircraft environment to support simultaneous transmission of multiple RF signals over a single optical fiber, exhibit no appreciable signal degradation at wavelengths of both 1330 and 1550 nm, respectively. Our measurements of signal to noise ratio carried out for the transmission of FM and AM analog modulated signals at these wavelengths indicate that WDM is a fiber optic technology which is potentially suitable for avionics applications.
Integrated Services (IntServ) and Differentiated Services (DiffServ) are two of the current approaches to provide Quality of Service (QoS) guarantees in the next generation Internet. IntServ aims at providing
guarantees to end applications (individual connections) which gives rise to scalability issues in the core of the network. On the contrary, DiffServ is designed to provide QoS to aggregates, and does not suffer from
scalability. It is therefore, believed that the combination of IntServ at the edge and DiffServ at the core will be able to provide QoS guarantees to end applications. Although there have been several proposals on how to perform mapping of services between IntServ and DiffServ, there hasn't been any study to quantitatively show the level of QoS that can be achieved when the two networks are connected. The of this paper is to quantitatively demonstrate the QoS guarantees that can be obtained by end applications when IntServ is run over DiffServ. We have used goodput, drop ratio and non-conformant ratio of packets from the different services and the queue size of DiffServ router to determine the QoS obtained by packets belonging to different traffic classes.
KEYWORDS: Networks, Internet, Atrial fibrillation, Data communications, Navigation systems, Signal processing, Data modeling, Associative arrays, Error control coding, Integration
Aeronautical Telecommunication Network (ATN) has been developed by the International Civil Aviation Organization to integrate Air-Ground and Ground-Ground data communication for aeronautical applications into a single network serving Air Traffic Control and Aeronautical Operational Communications. To carry time critical information required for aeronautical applications, ATN provides different Quality of Services (QoS) to applications. ATN has been designed as a standalone network with its own protocols which requires building an expensive separate network for ATN. However, the cost of building ATN can be reduced if it can run over a public network such as the Internet. Although the current Internet does not provide QoS, Internet Engineering Task Force (IETF) is standardizing the Differentiated Services (DiffServ) network to provide differential QoS to users of next generation data networks. The objective of this paper is to investigate the possibility of providing QoS to ATN applications when it runs over the DiffServ network in the next generation Internet. Our results show that the QoS requirements of ATN applications can be successfully provided when they run over a DiffServ backbone in the next generation Internet.
ATM is a connection oriented network while IP is based on a connectionless protocol. Connecting the two networks requires protocol conversion. When IP packets arrive at the interconnection between the two networks, signalling is used to open a switched virtual circuit on the ATM side. In this paper, we look at the management of the ATM connection with a view to reducing the cost of the connection. We develop and compare the cost functions of three connections management policies: the delayed vacation policy, the control operating policy and the permanent virtual circuit. Furthermore, we have developed the conditions under which the control operating policy (COP) will be more cost effective than the other two policies. We conclude that there always exists some condition under which the COP will be better than the other two policies.
KEYWORDS: Control systems, Internet, Feedback control, Networks, Control systems design, Asynchronous transfer mode, Virtual colonoscopy, Phase modulation, Statistical analysis, Multiplexing
The design of a traffic management mechanism for intranets connected to the Internet via an available bit rate access- link is presented. Selection of control parameters for this mechanism for optimum performance is shown through analysis. An estimate for packet loss probability at the access- gateway is derived for random fluctuation of available bit rate of the access-link. Some implementation strategies of this mechanism in the standard intranet protocol stack are also suggested.
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