Vijitha Weerackody

Free-Space Optical Communications for Next-generation Military Networks

There is a continuing need for increased capacity for military applications, especially in network-centric operational concepts that promote the use of information as fundamental for gaining superiority on the battlefield. As an example, the access to, and distribution of, sensor data is a major tenet of network-centric warfare and yet radio frequency (RF) links will struggle to provide the needed capacity. Free-space optical communications (FSOC) has the potential to meet these emerging military needs by offering dramatic increases in capacity. However, there are many technical challenges al multiple layers of the communications protocol stack. This article describes these challenges and discusses some mitigation approaches to provide a path to realizing this capability on the battlefield

Mobile small aperture satellite terminals for military communications

The United States Army is currently developing a satellite-based network-centric waveform. Mobile terminals that are small and compact are highly desirable for the military. This article gives an overview of the technical challenges and performance issues when mobile small aperture satellite terminals are connected to a network- centric communication system. The specific issues addressed are regulatory limits on off-axis emissions, performance of small aperture terminals in the presence of adjacent satellite interference, and the effects of motion induced antenna pointing errors on the satellite communication system.

Adaptive Coding and Modulation for Satellite Communication Links in the Presence of Channel Estimation Errors

Adaptive coding and modulation (ACM) schemes are very useful in satellite communication systems because they provide variable coding rates and modulation levels that can be used under different channel gain conditions. The effectiveness of the ACM scheme depends on the accuracy of the channel estimates. Because channel estimation plays a key role in ACM schemes, we present an estimation technique for the channel SNR. Furthermore, because channel SNR estimates are unreliable, we modify the ACM scheme to incorporate channel estimation errors. It is shown that this scheme significantly improves the performance of the ACM scheme.

Interference Analysis for a Network of Time-Multiplexed Small Aperture Satellite Terminals

Small aperture satellite terminals have larger beamwidths, so they may result in higher interference levels to victim satellite links. Before large-scale deployment of these terminals, it is necessary to quantify the interference from these terminals. A detailed analysis is presented of interference generated from a network of small aperture satellite terminals that may operate with antenna pointing errors and use a time-division multiple access (TDMA) protocol. The analysis presented here makes use of existing applicable ITU-R recommendations, and to limit interference from a network of these terminals, both short- and long-term interference criteria are established.

A statistical approach to specifying the off-axis EIRP spectral density in on-the-move satellite communications

The mobile very small aperture terminal is a new satellite terminal with applications in both military and commercial sectors. In some terrains, antenna pointing errors of these terminals may exceed satisfactory limits. In these terrains, since these errors may increase interference to neighboring satellites, it may be necessary to reduce the effective isotropic radiated power (EIRP) spectral density transmitted to satisfy the strict regulatory off-axis emission requirements. In this paper antenna pointing errors are considered as random variables and analysis carried out to characterize the probability that the EIRP spectral density exceeds its regulatory mask. This analysis can be used to specify a new mask for the off-axis EIRP spectral density for these terminals. The objective of this work is to develop a new ITU-R recommendation for mobile very small aperture terminals and this constitutes some of the contributions made to the ITU-R Working Party 4A Group.

Wavelength Correlation in Free Space Optical Communication Systems

Free space optical (FSO) communication systems are attractive for the military because such systems are capable of transmitting extremely high data rates over very long point-to-point links. However, in the presence of atmospheric turbulence the FSO channel will be distorted and this limits the effective data rate that can be transmitted over the link. These distortions can affect the signal in the spatial, time and wavelength (frequency) domains. It is well known that channel coding and diversity techniques can be employed to mitigate the channel distortions and improve the performance of the communication link. For these techniques to be effectively utilized the FSO channel properties have to be characterized. In this paper we will examine the correlation of the FSO channel across wavelengths under the weak turbulence propagation model. Specifically, we will determine the wavelength correlation of the FSO signal for the satellite-to-ground and ground-to-satellite links with coherent and direct detection schemes

Sensitivity of Interference to Locations of Vehicle-Mounted Earth Stations

Adjacent satellite interference from a satellite terminal depends on its location. Vehicle-mounted Earth stations (VMES) are mobile terminals and in many applications the locations of the VMES may be available only as approximate values. In this paper we investigate the effect on the adjacent satellite interference when the locations of the VMES are given by approximate values.

On a method to establish satellite links for dynamic bandwidth allocation

This paper examines the bandwidth and power allocation problem for a multi-frequency time division multiple access (MF-TDMA) based satellite communication system and presents a method to optimize the use of bandwidth and power at the satellite transponder. The constraints that are examined are: power and bandwidth limitations at the satellite transponder, terminal EIRP spectral density mask determined by the regulatory and standards bodies, and a minimum data rate that should be supported by all terminals. A simple Ku-band bent-pipe repeater satellite and a Ka-band wideband Gapfiller satellite are considered. In the former case the transponder gain is fixed across all frequency bands; in the latter case discrete, variable gains across the frequency bands are accommodated. We consider the solution to the above resource allocation problem for a typical distribution of the terminals. Practical methods of realizing this solution are also addressed

Criteria to Limit Interference Resulting from Antenna Pointing Errors

Vehicle-Mounted Earth Station (VMES) terminals are new terminals and interference from these terminals is time varying. Recommendations (Recs.) ITU-R S.1857 and S.2029 were developed to assess and limit the time-varying interference from these terminals. In this paper we consider the effects of interference due to motion-induced antenna pointing errors using the traditional worst-case interference criterion and the criterion established in Rec. ITU-R S.1857. It is shown that in order to satisfy the worst-case interference criterion the power level of the terminal has to be reduced by a large amount, for example 6 dB. On the other hand, it is shown that according to the methodologies developed in Rec. ITU-R S.1857 these same interference levels can be accommodated at the victim receiver with a negligible power penalty at the VMES terminal.

EIRP Variations due to Uplink Power Control in Small-Aperture Satellite Communication Links

Power control techniques that compensate for rain fading in the uplink have been used extensively in the past. In the case of small-aperture terminals, because they are usually transmitting at their maximum allowed equivalent isotropically radiated power (EIRP) spectral density level, uplink power control may increase interference to adjacent satellites. Therefore, uplink power control requires special consideration when it is applied to small-aperture terminals. In this paper we examine several different techniques available for uplink power control in the context of resulting variations in the EIRP of the terminal. Specifically, we provide techniques for estimating the rain fades, carry out a statistical analysis of these estimates, and analyze the variations of the EIRP resulting from uplink power control.