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Aiming at future multimedia land mobile-satellite services (LMSS) consisting of a large number of nongeostationary earth-orbit satellites, we present an LMSS propagation channel model for assessing the effect of a satellite diversity scheme so that high service availability and high signal quality are assured. We classify general fading environments for LMSS into three states. By taking the occurrence probability of each state into account, a new fading channel model is developed. The validity of the model is identified by comparing its predicted values in terms of cumulative distribution function (CDF) with measured data available so far. Then, based on this model, we calculate the satellite diversity effect assuming that the area is illuminated simultaneously by at least two satellites moving in low-earth orbits (LEO) over urban and suburban environments. In addition, state transition characteristics based on a Markov model are presented. Index Terms - LEO, LMSS, propagation model, satellite diversity.

A very simple but general scheme for calculating irreducible bit error rate (BER) (namely, BER floor) due to intersymbol interference in frequency-selective Nakagami-Rice fading environments has been developed. The scheme, which we call the equivalent transmission-path model, plays a role in connecting wave propagation with digital transmission characteristics in a general manner. Through computer simulations assuming various types of delay profiles, we first identify key parameters of Nakagami-Rice fading dominating principally the occurrence of irreducible errors, and we develop a simple method for the calculation of irreducible BER utilizing the nature of the key parameters. Then we examine the accuracy of the scheme for various types of phase-shift keying (PSK) transmission systems. Finally, based on the scheme, we show calculation examples of BER floor characteristics in line-of-sight fading environments. Index Terms - BER floor, BER map, intersymbol interference, key parameters, Nakagami-Rice fading.

The equivalent transmission-path model, which was proposed by the authors, has a function to assess bit error rate (BER) due to intersymbol interference (ISI) in a Nakagami-Rice fading environment with a simple formula. In this paper, improvement of usability is pursued taking the similarity of BER map shape in the delay difference between two waves into account. Moreover, an extended method predicting overall BER and frame error rate (FER) affected by both thermal noise and ISI is proposed, and accuracy is evaluated by computer simulation. Finally, a self-consistent flowchart showing how to calculate overall BER and FER in Nakagami-Rice fading environments is presented. Index Terms--Error floor, intersymbol interference, Nakagami-Rice fading, propagation modeling.

This paper proposes a diversity-receiving array antenna scheme achieved by digital beamforming (DBF) for the purpose of self-beam steering toward the unknown direction-of-arrival (DOA) of a modulated signal in a mobile satellite communications environment. This scheme uses a multibeam former with a spatial fast Fourier transform (FFT) for the received baseband signals to coarsely acquire the attenuated signal. It also uses a self-phasing processor (SPP) applied to the multibeam outputs for precise acquisition and tracking. In the SPP, maximal-ratio combining is achieved with simple digital numerical operations in the complex baseband, allowing for rapidity and stability in beamforming. As a result, the array antenna steers its receiving beam toward the DOA automatically without the need for nonlinear optimization methods. Moreover, the beam is steered toward the DOA of multipath signals according to their signal levels, leading to the optimal cophase recombination of all correlated signals, which improves the quality of the total receiving signal. This paper discusses the scheme, and its performance is evaluated by computational simulation and experiments conducted in a radio anechoic chamber. Index Terms - Digital beamforming, field-programmable gate array, maximal ratio combining, mobile satellite communication.