Several candidate crosslink communications architectures are described for a packet-switched, low-altitude, multiple satellite system (MSS). One architecture represents a frequency-division approach, in which a cellular concept is combined with dynamic time-domain allocation within a cell. A second architecture uses an unsynchronized space-time division approach consisting of directional antennas and random accessing. A third architecture uses toroiclal antennas with unsynchronized random accessing. The recommended architecture uses a synchronized space-time division approach, consisting of directional antennas and contention-based pair-wise scheduling. This architecture, called PRS (Pseudo-random Scheduling) is found to have significantly reduced power and bandwidth requirements relative to the other approaches.

This study is concerned with the problem of monitoring the performance of communication links, as indicated by bit error rate, with the objective of detecting as rapidly as possible failures or extreme degradation caused by any source. The monitoring method is based on the use of an artificial threshold for the decision variable and the generation of a measurable "pseudoerror." The study relates pseudoerror probability to true-error probability for two models-one using an analytical-numerical approach and the other using a Monte Carlo simulation. Variation of the parameters in the models adequately approximates the first-order effects of many types of link failure. The results indicate that a monitor indication based on pseudoerror is feasible and has many desirable properties.

The problem of eliminating intersymbol interference is studied from the viewpoint of channel-state estimation. The channel state vector, at the beginning of the present message baud, contains the information required to eliminate the effects of past channel inputs. A receiver is proposed that makes a maximum-likelihood estimate of the channel state vector conditioned upon all previous digits having been received correctly. The channel-state estimate is used to control the motion of a variable decision threshold. Correct adjustment of the threshold removes the effects of past symbols on the present decision. The receiver performance is studied via certain approximation techniques used in the study of threshold learning systems. In particular, the performance of one- and two-pole channels is evaluated. In these cases, the receiver is shown to effectively eliminate intersymbol interference and perform as well as a matched-filter receiver that uses only the signal energy received during one message interval.