Amt Experiment Results

The Advanced Communications Technology Satellite (ACTS) Mobile Terminal (AMT) experiments have provided a terminal technology testbed for the evaluation of Kand Ka-band mobile satellite communications (satcom). Such a system could prove to be highly beneficial for many different commerical and government mobile satcom users. Combining ACTS’ highly concentrated spotbeams with the smaller, higher-gain Kaband antenna technology, results in a system design that can support a much higher throughput capacity than today’s commercial configurations. To date, experiments in such diverse areas as emergency medical applications, enhanced Personal Communication Services (PCS), disaster recovery assistance, military applications, and general voice and data se,rvices have already been evaluated. Other applications that will be evaluated over the next year include telemedicine, ISDN, and television network return feed, Baseline AMT performance results will be presented, including Bit Error Rate (BER) curves and mobile propagation data characterizing the Kand Kaband mobile satcom channel, In addition, observations from many of the applicationspecific experiments will also be provided. \ INTRODUCTION Throughout the eighties NASA, through JPL, has been involved in the development and demonstration of system concepts and high risk technologies to enable the introduction of commercial mobile satellite services (MSS). This initial effort occurred at L-band (1.5 GHz), and currently commercial L-band MSS are available through a host of U.S. and international companies. It is expected that the present allocation for L-band MSS will become saturated by the turn of the century. In view of this, and the already existing non-MSS frequency allocations at other bands (C-, X-, and Ku-bands for example), NASA and JPL have focused on Kand Ka-bands for further expansion of MSS. Kand Ka-bands have outstanding potential for higher data rate communications and more highly diversified MSS for a number of reasons. Unlike L-band, Kand Kabands have a significant amount of bandwidth (500 MHz at each Kand Ka-bands) already allocated for MSS services. Moreover, these higher frequencies can supporl antenna designs that while physically smaller than their L-band counterparts, can provide higher gain, often 10 dB or more, Kand Ka-bands, therefore, are excellent candidates for the pursuit of higher capacity services for commercial users (i.e., compressed video). However, satellite communication system design at these higher frequencies poses significant technical challenges including: a young technology with Iossy RF components, significant rain attenuation effects, pc)tentially large frequency uncertainties, and large Doppler shifts due to vehicular motion. NASA has provided a platform for the initial evaluation and exploitation of Kand Kabands through their development of the ACTS. JPL’s goal, through the use of ACTS and the development of the AMT, is to overcome these technical challenges with a system architecture and components that will exploit the potential benefits of such a migration from L-band. The final phase of this effort has been and continues to be the transfer of such technologies to interested groups within U.S. industry. By directly involving US. industry in these experiments, NASA hopes to expedite the commercialization of this technology. The remainder of this paper provides a brief description of the ground terminal “ equipment and it’s baseline performance in a series of internal JPL experiments. Furthermore, an explanation of the various experiments with US. industry that have been conducted or are in the process of being planned will be presented. Finally, the .’ experiment results to date will be provided. . AMT DESCRIPTION ,,$ ) The complete technical details and architecture of this terminal can be found in. [1]. ~ The AMT can be broken down into two broad divisions, namely, the baseband and ‘‘ microwave processors. The baseband processor consists of a speech codec, a modem, and a terminal controller (TC). AISO included as part of this setuPl stfictly for ‘”! experimental purposes, is a Data Acquisition System (DAS). The elements of the microwave processor are: the IF Converter (IFC), the RF Converter (RFC), the antenna controller, and the antenna. The TC is the “brain” of the terminal. It contains the algorithms that translate the satcom protocol into operational procedures and interfaces to all of the other terminal subsystems. The TC is also responsible for providing the user with a system monitoring capability, and a variety of test functions during experimentation, such as bit stream generation and bit error rate (BER) calculations. Two different modems have been used as part of the AMT. The baseline AMT modem, that was designed in-house, implements a simple yet robust DPSK scheme with rate 1/2 convolutional coding and interleaving. The performance specification for this modem is for a BER of 10-3 at an E~NO of 7 dB in AWGN. Further capabilities have been built into this modem to compensate for frequency offsets of up to 10 kHz with an additional performance degradation of only 0.5 dB. This modem is operational at 2,4, 4.8, and 9.6 kbps. The second modem that has been utilized as part of this setup is a commercially developed satcom modem that includes such features as coherent .’ BPSK with convolutional coding, concatenated coding interleaving. The performance specification for this modem is E~NO of 5 dB in AWGN. This modem is operational at data kbps to 2.048 Mbps. (Reed-Solomon), and for a BER of 10-6 at an rates ranging from 9,6 The vehicle antenna is the critical K-/Ka-band technology item in the microwave processor. The design of this antenna called for a “passive” elliptical reflector-type antenna to be used in conjunction with a separate high powered amplifier. Complete with a spherical radome, it stands approximately 5 inches in height, and is approximately 8 inches in diameter at its base. This antenna is fully tracking in azimuth, while manually positioned in elevation to one of five distinct settings.l Combined with a 10 W lWTA, this antenna system provides at least 32 dBW transmit EIRP on boresight. The 3 dB beamwidth is 12° in azimuth and 18° in elevation, Receive specifications for this antenna have been set at -5 dB/°K, once again on boresight. The antenna pointing system enables the antenna to track the satellite for all practical land-mobile vehicle maneuvers. The antenna is mated to a simple, yet robust, mechanical steering system. A scheme wherein the antenna is smoothly dithered about its boresight by about a degree at a rate of 2 Hz is used. The pilot signal strength is measured through this dithering process, and is used to compliment the inertial rate sensor’s information. This information allows the antenna to track the satellite while experiencing a shadowing event of up to 10 seconds in duration. Preceding (or following) the antenna, the RFC converts an IF signal around 3.373 GHz to (from) 30 (20) GHz for transmit (receive) purposes. The IFC translates signals between 3.373 GHz and a lower 70 MHz IF at the output (input) of (to) the modem. A key function of the IFC is pilot tracking and Doppler compensation (for the return communications link). AMT BASELINE TEST RESULTS BER Results The initial baseline AMT tests collected fall into two categories: (1) terminal performance characterization (utilizing the baseline AMT modem) and (2) mobile satcom Kand Ka-band propagation characterization. Complete details of these tests can be found in [2], The single best method for determining the terminal’s performance were accomplished through a series of stationary BER tests. The test setup for the baseline system performance is provided in Figure 1. A set of baseline BER tests were performed from the fixed terminal (FT) to the mobile terminal (MT). PN sequence data was transmitted from the FT’s Terminal Controller (TC) and a final BER was determined at the MT’s counterpart, The EJNO value was determined by the EjNO ‘ These five settings allow for complete elevation coverage for the continental United States. ‘box prior to and after the actual transmission of the PN sequence, once again using an unmodulated data signal. These two values were then averaged for the test run. This test was performed for all three of the lower operational data rates of the AMT (2.4/4 .8/9,6 kbps). Four different EJNO values were recorded for each data rate corresponding to final BER values ranging between approximately 10-5 and 10“’. Several test conditions were established at the beginning and maintained throughout these tests to ensure consistent results. They are as follows: (1) the van was stationary, (2) the van’s engine was turned off to minimize external noise sources (3) the power used in operating the terminal was supplied by the AC generator, (4) the mobile terminal’s antenna pointing was accomplished in the manual mode to minimize any sources of error due to pointing (dithering), and (5) no pilot signal was transmitted to minimize the effects of intermodulation on-board the satellite. The results of these BER tests for 9.6 kbps, 4.8 kbps, and 2.4 kbps are provided in Figures 2, 3, and 4, respectively. For comparison purposes, the pre-satellite test results, which utilized a satellite simulator, have also been included. These are listed as Xlator in the figures. Both of these results agree to within experimental error (0.25 dB or less) with each other. For terminal operation at a 9,6 kbps data rate, an E~NO of 6.8 dB is required to achieve a BER of 103. For the 4.8 and 2,4 kbps cases, this performance level is achieved for an E~NO level of 6.7 and 8.5 dB, respectively. The significantly larger E~NO requirement for the 2.4 kbps case can be attributed to higher sensitivity to system frequency offsets at this data rate. Propagation Test Results The objectives of the mobile propagation experiments were to measure and analyze the fading characteristics of the Kand Ka-band channel. The analysis involved examining multipath, shqdowing and blockage effects. Field tests were conducted in various environments; results presented here include 1 ) rural freeway runs free of obstructions except for occasional overpasses and 2) shadowed suburban runs with occasional obstructions from buildings, utility poles, and trees. Additional information on the propagation test results can be found in [3], Data from the first category, that is rural freeway runs, was collected on Interstate 210 between California State Highway 2 and California State Highway 118. This is 15 mile east-west span. A map of this route is provided in Figure 5, A representative time series of the pilot power transmitted by the fixed station and received the AMT is shown in Figure 6. The statistics of the shadowing/fading are summarized by a histogram of the cumulative distribution of the pilot power received at the AMT. The histogram of the run shown in Figure 6 is provided in Figure 7. It can be seen that the 1 YO fade level for the 20 GHz channel is 1 dB. This is typical for a clear line-of-sight (LOS) channel. 2 Residual frequency offsets in this particular system setup have been calculated to be several kHz in a worst case scenario. .