Bandwidth and modulation techniques under study by NASA

As NASA programs demand higher data rates, the need for more bandwidth-efficient high-rate modulation schemes grows. This is of special interest to NASA if they are to reduce future users' required communication bandwidth and minimize spacecraft contact time to the ground stations. Currently, the Tracking and Data Relay Satellite System (TDRSS) KSA channel operates with Quadrature Phase Shift Keying and a rate 1/2 convolutional code. One of the strongest candidates for increasing-data throughput without increasing the required bandwidth is Octal Phase Shift Keying (8PSK). Eight phase Trellis Coded Modulation using a rate 2/3 convolutional code, for example, will double the throughput without any change in the bandwidth requirements when compared to the current system. Other coding schemes can be employed for higher throughput or coding gain depending on the program requirements. In order to ensure that NASA can support high data rate programs through TDRSS, the error-rate performance of 8PSK operated through the telecommunication link at 450 Mega-bits per second (Mbps) was measured by the New Mexico State University (NMSU). Octal phase-shift keying was chosen because it has proven to be the strongest candidate for upcoming systems, exhibiting a higher information rate than QPSK for the same transition bandwidth. Channel measurements were made at the White Sands Ground Terminal (WSGT) and are summarized.