The FRAMES multiple access (FMA) in order to meet the UMTS requirements and thereby also the IMT2000, has proposed a harmonized platform based in two access techniques, i.e. time division multiple access (TDMA) and code division multiple access (CDMA). Thus, the MA concept has two modes: FMA1, wideband TDMA with and without spreading, and FMA2, wideband CDMA. The FMA1 included several options such as spreading (CDMA features) and orthogonal frequency division multiplexing (OFDM) modulation. This paper presents the performance of this latter option in order to outline the flexibility of the FMA1 and the advantages of the OFDM modulation when used instead of the single carrier setting following the classical TDMA approach. Link level results illustrate that the E/sub h//N/sub 0/, values are comparable or even better than the ones obtained following traditional modulation techniques.

The multipath fading in mobile satellite systems plagues the propagation medium by imposing random amplitude and phase variations on the transmission signal. However, the relative effect of an undesired random variation is reduced by the presence of a strong line-of-sight (l.o.s) component. In order to analyze the data transmission performance of the satellite-to-ground mobile channel, a Rician fading model is used. For this model the received multipath signal consists of the sum of a direct or (l.o.s) component and a fading component due to reflections from objects adjacent to the transmission path. Generally, when a data packet encounters a fade, it most likely contains errors and it will have to be retransmitted. Thus, the size of the packet is crucial to determine the performance of the channel. The present study finds for the first time the optimum packet size by applying an analytical model of the dynamics of the Rician channel. The performance criterion is based on the rate of information transfer through the network, the vehicle speed and the modulation fade margin.>

It is shown how Rician fading limits the performance of the HDLC protocol. It is observed that for accurate reception, a packet must be transmitted entirely during an interfade interval. This means that packets ought to be sufficiently short so that most of them fall in the interfade time intervals. This leads to a high transmission efficiency. Consequently, the HDLC protocol is more efficient with shorter packets. However, a smaller packet size increases the overhead fraction of the packet and thereby reduces the effective throughput. In addition, the HDLC peak efficiency and optimum packet size increase with increasing fade margin and decrease with increasing vehicle velocity. Among other effects, the bit rate of the land-mobile satellite system channel is the most critical for optimum packet size and HDLC efficiency. Thus, a low bit rate implies long packet duration for a moderate packet size in bytes.>