The Lincoln Experimental Terminal

Abstract

LEXINGTON MASSACHUSETTSThis report is a description of selected portions of the system emphasizing the most novel features. The first chapter gives a general description of LET-1 as a whole, and the second an over-all description of the signal processing system. Chapter 3 discusses the equipmental realization of the signal processing system and Chapter 4 the utilization of a general-purpose computer as an element of the signal processing system. Chapters 5 and 6 treat two specific subsystems: the sequential decoder and the vocoder. ABSTRACTThe Lincoln Experimental Terminal (LET) is a complete, self-contained air-transportable ground terminal for testing and demonstrating evolving space communications techniques in a realistic environment. Its present equipment complement permits efficient, highly reliable, multiplexed digital communication of voice and record traffic with a variety of channels, including the moon and active satellites. Its modulation system, using a 16-symbol alphabet frequency-hopped over a 20-MHz band, together with efficient coding, provides multiple access to a wide-band satellite.A. INTRODUCTION*For a number of years the M.I.T. Lincoln Laboratory has worked on various techniques applicable to the solution of space communications problems. In addition to microwave technology and components such as cooled X-band parametric amplifiers and rapidly switchable frequency synthesizers, these techniques have included: modulation and demodulation for dispersive channels such as the troposphere. Moon and the West Ford belt; digitized narrowband speech processing with emphasis on speaker recognizability; and practical realization of coding and decoding schemes which are economically competitive with the more conventional means of achieving greater information rates on a given channel.Some results of this work have been combined to produce an experimental air transportable terminal, called the Lincoln Experimental Terminal (LET), which has a number of desirable and unique features, particularly from a military communications viewpoint. The terminal will work efficiently on both coherent and time-varying dispersive channels; it provides good quality digital speech with speaker recognizability in a reasonably narrow band; and it permits multiple access of a broad-band satellite by spectrum spreading, without the severe synchronization problems commonly associated with the use of a pseudonoise carrier for this purpose.* Numbered references for Chapter 1 are given on page 17. B. GENERAL DESCRIPTION OF TERMINALThree terminals have been constructed in the LET program. The first of these is selfcontained in two trailers and is shown in its road configuration in Fig. 1-1. One trailer, the socalled electronics vehicle, is a modified low-bed commercial van which contains the signal processing equipment, a communications and antenna control console, a prime power generator and its fuel, an air conditioner, and storage for the antenna panels. The second trailer, which we call the antenna vehicle, contains the t...

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