Modern array processor and digital signal processing technologies have been applied to develop a real-time, software-based system that simulates high-frequency (HF) radio links. This system is called an HF Channel Simulator. In this paper we describe the design features and performance of one such simulator, based on the Floating Point Systems AP-120B array processor. The simulator characteristics are a 4 kHz signal bandwidth and a great flexibility in the selection of link parameters available to the user. All distortions are at least 50 dB below the desired signal output level.

The evaluation of FM radios designed primarily for analog voice traffic, but used for data transmission, requires operation over a wide range of data rates and modulation formats. A versatile full-duplex baseband modem has been built which interfaces with tactical VHF radios and operates from 75 bits/s to 16 kbits/s. The choice of modulation formats, modem implementation, and some performance results are presented in this paper.

A digital data modem which can operate at 1.5, 3.1, 6.3, 9.4, or 12.6 Mbit/s has been developed under the Megabit Digital Troposcatter Subsystem (MDTS) program for operation over Defense Communications System troposcatter links. An adaptive decisionfeedback equalizer (DFE) is used in the demodulator to eliminate intersymbol interference, combine diversity channels, and augument the existing diversity configuration with implicit diversity. This paper presents the results of a field test of the MDTS modem at the Rome Air Development Center troposcatter test facility during the winter of 1976. The field test, which encompassed generally weak signal and strong multipath conditions, yielded excellent performance results at all data rates. The field tests established the capability of the MDTS modem to efficiently communicate digital data over DCS troposcatter links at data rates up to 12.6 Mbit/s.