| On the move satellite communications is a major new capability that is being sought by the Army to support new operation concepts. Because of their size and weight, mobile very small aperture terminals (MVSAT) are attractive in military on the move systems. These terminals will have wider beamwidths and, therefore, will increase interference to and from other satellite systems. When the MVSAT is in the transmit mode, in order to limit interference to other systems, regulatory and standards bodies have established strict limits on its off-axis power emissions and the antenna gain pattern. The power transmitted by a wide beamwidth MVSAT in the direction of its boresight will be severely limited by these off-axis emission regulations. This transmit power limitation will result in a lower data rate for a given bandwidth.
Rugged terrains are common in military applications and communications on the move in such conditions require accurate and robust antenna tracking systems. Because of platform motion, practical antenna tracking systems used in communications on the move applications typically have small antenna pointing errors. Since the motion of the platform is random these motion induced antenna pointing errors can be characterized by a random process. In the presence of such motion induced antenna pointing errors the effective isotropic radiated power (EIRP) from the terminal fluctuates randomly and may exceed the strict limits established by the regulatory and standards bodies. In these cases, in order to limit the probability of the terminal EIRP exceeding the regulatory limit, the transmit power from the antenna has to be reduced. Previously, in contributions to ITU-R WP4A meetings, we characterized the motion induced antenna pointing errors using a symmetric Levy distribution and demonstrated that the EIRP spectral density mask established by the ITU will be exceeded even for very small variations of the antenna pointing error. In order to facilitate communications on the move systems, in these contributions we suggested a modification to the above EIRP spectral density mask. In this paper we will examine the performance of the system subject to this modified EIRP spectral density mask. Note that without this modification the MVSAT will not be able to transmit data at an acceptable rate since, for any reasonable boresight EIRP level, there is always a non-zero probability of exceeding the existing EIRP spectral density mask.
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