Downlink Path Research Articles

Influence of different propagation paths on the propagation of laser in atmospheric turbulence

The analytical expressions for the average intensity, root mean square (RMS) beam width and angular spread of Gaussian Schell-model (GSM) beams propagating under slant atmospheric turbulence are derived, and they are used to study the influence of different propagation paths on the propagation of laser beams in atmospheric turbulence. It is shown that under the same condition, the influence of atmospheric turbulence along a downlink path on the GSM beam propagation is the smallest among the three paths. Therefore, the downlink propagation is more beneficial to the beam propagation through atmospheric turbulence compared with the uplink propagation and horizontal propagation.

MODELING OF ADS-B DATA TRANSMISSION VIA SATELLITE

For modelling transmission of ADS-B messages via low-orbit satellite constellation Iridium, the original model of a communication channel “Aircraft-to-Satellite-to-Ground Station” was built using MATLAB Simulink. The model comprises “Aircraft Uplink Transmitter” (Bernoulli Random Binary Generator, Convolutional Encoder, BPSK Baseband Modulator, High Power Amplifier with a memoryless nonlinearity, Transmitter Dish Antenna Gain), “Uplink Path” (Free Space Path Loss, Phase/Frequency Offset), “Satellite Transponder” (Receiver Dish Antenna Gain, Satellite Receiver System Temperature, Complex Baseband Amplifier, Phase Noise, Transmitter Dish Antenna Gain), “Downlink Path” (Free Space Path Loss, Phase/Frequency Offset), “Ground Station Downlink Receiver” (Receiver Dish Antenna Gain, Ground Receiver System Temperature, Viterbi Decoder), “Error Rate Calculation” block and “Display”. The modelling was realized without and with convolutional coding (r = 3/4, K = 7) at different noise temperatures and free space losses. Dependencies of a Bit Error Rate on free space path losses, antenna's diameter, phase/frequency off-sets, satellite transponder linear gain, aircraft and satellite transponder high power amplifier back-off level, and phase noise were received and analysed.

Extending Reading Range of Commercial RFID Readers

In this paper, multisine excitation signals are used to extend the reading range of commercial RF identification readers. To do so, a commercial reader is equipped with an external multisine front-end that implements previous mathematical proposals. A mathematical description is presented in order to show the ability of multisine signals to communicate data, with minimal changes in the downlink path, while no changes are required in the conventional tag architecture. Moreover, and most important, if a proper multisine design is performed, a conventional reader receiver is still able to demodulate and decode the backscattered multisine signal from the tag, without any hardware change. Thus, guidelines are presented for multisine design, including multisine nature, central tone positioning, tone separation, and bandwidth requirements. In order to evaluate the reading range improvement, when compared with the conventional single carrier approach with the same average power, two experiments are conducted: in the first one, an oscilloscope is used to measure the tag response and to decide whether the tag does or does not respond. In the second measurement scenario, the downlink path is implemented by the reader combined with the front-end and the uplink is implemented solely by the reader. In this case, the decision on successful tag response is taken when the reader reads the tag identification. The first measurement scenario has pointed out for a maximum reading range improvement of near 43% for an eight-tone multisine signal with 2-MHz tone separation. In the second scenario, a more realistic one, a reading range improvement of almost 25% has been obtained for a 8 + 1 tones multisine.

Effect of clouds on Ku-band satellite link in tropical island countries – preliminary report

Dependence of radio wave attenuation by cloud water content has been investigated by analyzing simultaneous records of the strength of Ku-band satellite downlink and 'insolation'. Preliminary analysis suggests that with increasing cloud coverage in the satellite downlink path, the cloud induced attenuation also increases. The cloud attenuation showed a logarithmic dependence on reduction in insolation. In Fiji, a tropical island country, the maximum attenuation of Ku-band signals by cloud was ~ 11%.

Demonstration of bi-directional LED visible light communication using TDD traffic with mitigation of reflection interference

In this work, we experimentally demonstrate a bi-directional transmission link using light emitting diode (LED) visible light communication (VLC) for both downlink and uplink paths. Time-division-duplex (TDD) is proposed and demonstrated to significantly eliminate the reflection interference in VLC. A free space bi-directional transmission of 2 m using simple on-off keying (OOK) modulation with bit error rate (BER) of < 10(-5) in both directions are achieved. Moreover, the influence of reflection interference is analyzed, showing the proposed scheme can significantly eliminate the reflection interference.

Comparative study of partially coherent vortex beam propagations through atmospheric turbulence along a uplink path and a downlink path

Based on the extended Huygens-Fresnel principle, the propagations of a partially coherent vortex beam through atmospheric turbulence along an uplink path and a downlink path are studied, where the Gaussian Schell-model (GSM) vortex beam is taken as a typical example of partially coherent vortex beam. The analytical expressions for the average intensity, rms width and cross-spectral density function of GSM vortex beam propagating through atmospheric turbulence along a slant path are derived and used to study the influence of atmospheric turbulence along an uplink path and a downlink path on GSM vortex beam propagation and on coherence vortex. It is shown that under the same conditions the influence of atmospheric turbulence along a downlink path on GSM vortex beam propagation is smaller than that along a uplink path, and the conservation distance of the topological charge of GSM vortex beam along a downlink path is longer than that along a uplink path. The results are explained in physics.

Efficient Power Loading in MIMO-OFDM Based Cognitive Radio Networks

Due to their spectrum shaping capability and high adaptability, the multi-carrier systems are considered as prime candidates for use in the Cognitive Radio (CR) networks. In these structures, the deactivation of the primary users’ (PUs) bands makes the secondary users (SUs) use a limited number of sub-carriers, so the total capacity of CR networks is reduced. However, multiple transmit antennas can be applied to OFDM- based CR to compensate for such a low capacity. The present study investigates the problem of power allo-cation in MIMO-OFDM based CR Networks. The objective is maximizing the total capacity of CR systems in the downlink path under both interference on the PUs’ bands and maximum power of the CR transmitter constraints. It is shown that the optimal solution has high computational complexity, and therefore, an effi-cient sub-optimal algorithm is suggested for this application. As simulation results show, the suggested algo-rithm in the present paper is more enhanced and efficient than the previous algorithms.

The effects of tilt corrected residual aberration on the detection probability of single-photon propagation in a slant path turbulent atmosphere

Based on the assumption of a pulse laser beam with an initial Gaussian temporal shape and a collimated fundamental-model Gaussian beam, the Rytov approximation and Kolmogorov spectrum model for the index-of-refraction fluctuation of atmosphere, the effect of turbulence on the probability density, acquisition transmittance probability, transmittance probability density, acquisition probability of single-photon propagation in atmospheric communication channel with z-tilt and centroid-tilt aberration corrected are studied theoretically. The probability density, acquisition transmittance probability, transmittance probability density and acquisition probability models for single-photon propagation in uplink path and downlink path are derived. Our results shown that the detection probability and the acquisition transmittance probability of the single-photon are obvious increase, when the beams are propagation in the z-tilt corrected communication channel.

Improved Radio-Over-Fiber Transponder With Multistage Automatic Gain Control

In this paper, improved transponders with multistage automatic gain control (AGC) for radio-over-fiber-based distributed antenna systems are presented. Error vector magnitude performance measurements for both uplink and downlink paths are carried out to verify the feasibility of applying this system for IEEE 802.11g signal transmission. The developed conventional radio-over-fiber system shows its capability of supporting the wireless local area network signal transmission for an input dynamic range of approximately 14 dB. The improved radio-over-fiber systems with a single- and two-stage AGC are also demonstrated to have a better input dynamic range of 27 dB and more than 37 dB, respectively. The error vector magnitude for the case with a two-stage AGC is much better than that with a single-stage AGC.

Personal and Mobile Satellite DMB Services in Korea

A new mobile multimedia broadcasting service using a geostationary satellite was launched on May 2005 in Korea. The Satellite Digital Multimedia Broadcasting (S-DMB) system provides high quality audio, video and data broadcasting services to handheld and mobile receivers. S-DMB is a personal digital media which can provide bidirectional and interactive services via the convergent platform of broadcasting and communication. At present, TU Media, the only S-DMB service provider in Korea, provides 15 TV channels, 19 audio channels, and 3 data channels. The number of subscribers is about 890,000 by Nov. 2006. Thousands of low power gap fillers (repeaters) have been installed in the shadowed or blocked reception areas in the metropolitan Seoul. This paper presents the system requirements, standards and specifications, architecture, and service status of S-DMB in Korea. Comparisons with other satellite broadcasting systems and the services are also presented. Link budget analysis on the direct path down-link (satellite to receivers) and relay path feeder link (gap fillers to receivers) are carried out in order to ensure the satisfactory QoS

Performance Evaluation of Electroabsorption Modulators as Optical Transceivers in Radio-Over-Fiber Systems By FDTD Method

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, the limitations of the performance of electroabsorption modulators (EAMs) as optical transceivers in passive picocell systems by using the finite-difference time-domain (FDTD) method are theoretically investigated. EAMs with no bias voltage are modeled by the FDTD method with consideration of the interaction between electromagnetic fields and optical powers in optical waveguides. Transmission and responsivity of EAMs according to various optical input powers and chip lengths are calculated. Then, power margins of the downlink and uplink paths are estimated to define the service ranges of the passive picocell systems. In addition, EAMs with biased voltage are characterized for comparison. The results suggest that the service range of the passive picocell system using a no-biased EAM can be extended up to 20 m with a chip length of 400 <formula formulatype="inline"><tex>$\mu\hbox{m}$</tex></formula> and an optical input power larger than 10 dBm. </para>

Measurement of Ku-band rain attenuation using several VSATs in Kyushu Island, Japan

Rain attenuation at three different locations about 100 km from each other within Kyushu Island, Japan, are simultaneously measured in the Ku-band by utilizing JCSAT-1B communication satellite and several very small aperture terminals (VSATs). Two points are highlighted. The first one is the relation between rain attenuation and rain intensity. The relation is shown at uplink, downlink, and total link paths and is compared with the estimated International Telecommunication Union Radio Communication Sector (ITU-R). The second one is the rain attenuation distribution for a small percentage of time for a year for the total link path. These experimental results suggest the necessitity of detailed study on rain attenuation at local points and of presentation of a method for predicting bit errors, when using effectively Ka-band satellite systems with multispot beams.

Scintillation model for a satellite communication link at large zenith angles

A scintillation model is developed for uplink-downlink optical communication channels applicable in moderate to strong fluctuation conditions that may arise under large zenith angles between transmitter and receiver. The model developed here is an extension of a recently published theory that treats irradiance fluctuations along a horizontal path as a modulation of small-scale scintillation by large-scale scintilla- tion. For a downlink path the scintillation index is modeled like that of an infinite plane wave, and for an uplink path we consider a spherical wave model. In both cases the scintillation index agrees with conventional weak-fluctuation-theory results out to zenith angles of 45 to 60 deg. The covariance function of irradiance fluctuations is also developed under the same conditions as assumed for the scintillation index. On a downlink path under small zenith angles the implied correlation length is propor- tional to the Fresnel-zone scale. For zenith angles exceeding 85 deg, the downlink correlation length varies directly with the spatial coherence ra- dius weighted by a factor that depends on changes in C n the refractive index structural parameter with altitude. © 2000 Society of Photo-Optical Instru- mentation Engineers. (S0091-3286(00)02412-0)

Radio-Optical System Design and Transmission Experiments for a Mobile Broadband Communications System at 60 GHz

The paper presents the system design concept for a mobile broadband communications system at 60 GHz. A dense WDM system is used for the optical backbone employing novel add/drop multiplexers. For simplicity, there is no signal processing and frequency generation at the base station and a master oscillator signal are used for millimetric carrier generation. The master oscillator signal is modulated onto the optical carriers by a novel concept of simultaneous external modulation, simplifying system design significantly. Transmission experiments over a complete downlink path using uncoded 2-FSK have been carried out and a BER of 10-9 at 50 MBit/s has been obtained. Furthermore, 512-carrier DQPSK-OFDM transmission at 45.8 MBit/s using 35 MHz bandwidth at 60 GHz has been demonstrated.

Active Imaging through Cirrus Clouds

The presence of clouds of ice particles in the uplink and downlink path of an illumination beam can severely impede the performance of an active imaging system. Depending on the optical depth of the cloud, i.e., its density and depth, the beam can be completely scattered and extinguished, or the beam can pass through the cloud with some fraction attenuated, scattered, and depolarized. In particular, subvisual cirrus clouds, i.e., high, thin cirrus clouds that cannot be observed from the ground, can affect the properties and alignment of both uplink and downlink beams. This paper discusses the potential for active imaging in the presence of cirrus clouds. We document field data results from an active imaging experiment conducted several years ago, which the authors believe to show the effects of cirrus clouds on an active imaging system. To verify these conclusions, we include the results of a simulation of the interaction of a coherent illumination scheme with a cirrus cloud.

Multiple-access capacity in mobile user satellite systems

The channel capacity of a satellite direct sequence CDMA system is analyzed including the effects of faded user interference, overlapping antenna beams, imperfect equalization of the antenna pattern across an antenna cell, and diversity reception. Simplified models are used to describe the impact of these effects on the channel capacity of single and multiple cofrequency CDMA systems. In a comparison of the uplink and downlink paths, the uplink of the CDMA system is shown to limit the channel capacity because the downlink can utilize code-orthogonality and coherent demodulation. In a multiple system comparison between band-shared CDMA and band-segmented FDMA/TDMA technologies, FDMA/TDMA is shown to provide about the same capacity for uniformly distributed traffic conditions over many cells and dramatically better capacity when traffic is concentrated in one cell. Due to the peak nature of telephony, this result supports the use of band-segmented systems in mobile user satellite systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Digital data transmission over nonlinear satellite channels

The M-ary PSK system over nonlinear satellite channels, in the presence of an arbitrary number of interferers and additive Gaussian noise on the uplink and downlink paths, is investigated. The effect of intersymbol interference due to the transmitter filter is also included. The bit error probabilities are obtained using the Gauss quadrature formula and a two-dimensional moment technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Prediction of slant path rain propagation statistics using dual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5 degrees elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z/sub H/) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions. >

Measurement and modelling of amplitude and phase scintillations in an Earth–space path

This paper gives a complete summary of the theory and experimental results obtained by Portsmouth Polytechnic on the subject of amplitude and phase scintillations on a down-link path through the atmosphere during the Orbital Test Satellite campaign of research at 11.786 GHz and special and emphasis is given to engineering applications. The paper introduces first a brief summary of relevant theoretical studies, inclusive of antenna aperture smoothing, which facilitates the understanding of the scintillation process and the use of the experimental results in system design and planning. The results include chart recording observations, spectra of the scintillations and correlation with meteorological conditions together with results of cumulative distributions of scintillation fade, fade rate and intensity. The subject of propagation-induced phase noise is also dealt with and includes the theoretical estimates of the phase noise due to turbulence and precipitation as well as an outline of the experimental techniques used to measure the f.m. noise of the beacons.

Minimum-shift keyed modem implementations for high data rates

IGITAL communications links capable of conveying data at hundreds of megabits per second (Mb/s) are becoming increasingly important in various applications. An example is the use of time division multiple access (TDMA) transmission through a satellite relay wherein several data channels are routed through single uplink and downlink paths (antenna beams). Separation of the channels from each other is accomplished through interleaving and deinterleaving in time, with possible rerouting taking place in the satellite. When carrying traffic from large metropolitan areas, the required data rates can exceed 100 Mb/s. One such implementation, currently under study by NASA,' involves data transmission at rates of up to 550 Mb/s. Such applications require the use of modulation schemes that use the available bandwidth efficiently. Furthermore, because power is at a premium on board a satellite, the utmost in power efficiency is desired of the chosen modulation scheme. This implies the use of a constant-envelope modulation scheme since amplifiers, such as traveling wavetube amplifiers, are most efficient when operated near saturation. Three popular constant-envelope modulation schemes for efficient transmission of digital data are quadriphase-shift keying (QPSK), offset (or staggered) quadriphase-shift keying (OQPSK), and minimum-shift keying (MSK). Two excellent recent articles [1,2] in IEEE Communications Magazine have discussed their general properties and attributes as well as those of other modulation schemes with constant (or nearly constant) envelopes. Other recent papers [3-61 have presented analytical results and computer