D - 7 3 8 2 N 92 - 10119 Chapter 3 Antenna Beam Coverage Concepts

The strawman PASS design calls for the use of a CONUS beam for transmission between the supplier and the satellite and for fixed beams for transmission between the basic personal terminal (BPT) and the satellite. The satellite uses a 3m main reflector for transmission at 20 GHz and a 2m main reflector for reception at 30 GHz. The beamwidth of the reflector is 0.35°. To cover CONUS 142 fixed beams are needed. A sample fixed beam coverage plan for CONUS is shown in Figure 3.1. In the strawman design, suppliers transmit to the users on a 100 Kbps TDMA carrier for the low rate channel and on a 300 Kbps TDMA carrier for the high rate channel. The uplink frequency of this TDMA carrier is chosen according to the coverage area or beam location in which the user is located. Users with Basic Personal Terminals (BPTs) transmit a SCPC signal at 4.8 Kbps in the frequency band assigned to their coverage area. The time and duration of supplier transmissions are determined by the network management center (NMC) as are the specific transmit frequencies for the users [1]. The decision to employ spot beams to link users to the satellite was motivated by several factors. Major differences in beam characteristics are listed in Table 3.1; the entries are based on various satellite descriptions [2,3,4,5]. The use of spot beams, whether fixed, switched or scanning, allows users to operate with satellites having higher EIRP and G/T than possible with CONUS beams. Operation with small terminals is therefore possible. Spot beams also allow spectral reuse through assignment of the same frequency to geographically separated beams. Connecting users via spot beams rather than with a CONUS beam brings about two problems: difficulty in providing a broadcast channel and the necessity for the satellite transponder to be designed specifically to connect users in various spot beams. The importance of the latter is reduced in the strawman design by the use of CONUS and spot beams, the drawback then being that user to user communication requires a two hop satellite link. For PASS, the advantages of spot beam use primarily the ability to work with small terminals outweigh the disadvantages and motivate their use over designs employing only