A high capacity third-generation mobile satellite system design

This paper describes a CDM/CDMA based approach for a 3‐rd generation multi‐beam satellite mobile and personal communications system using Highly Elliptical Orbit (HEO) satellites. The forward link uses a synchronous CDM intra‐beam/asynchronous CDM inter‐beam approach, while the return link uses a quasi‐synchronous CDMA intra‐beam/asynchronous CDMA inter‐beam approach. Using probabilistic analysis, these approaches are shown to provide higher forward and return link capacities than equivalently synchronized FDM/FDMA approaches. Forward link transmissions are modulated using QPSK. Return link transmissions are modulated using either ϕ/4‐QPSK or precompensated frequency modulation (PFM4). PFM4 is intended for use with a saturated amplifier in low‐cost/power‐efficient, e.g. hand‐held, terminals, and can be demodulated using a ϕ/4‐QPSK receiver. All forward and return link transmissions are coded using a rate 1/2, constraint length 9, convolutional code. The paper summarizes several key tradeoffs and the resulting system design.