Transmission Facilities for General Purpose Wideband Services on Analog Carrier Systems

Abstract

Three new general purpose wideband data facilities provide for the transmission of synchronous and nonsynchronous, 2-level data at bit rates ranging from 19.2 to 250 kb/s. These facilities make use of half-group, group, and supergroup bandwidth telephone facilities. The major components involved are the type 303 wideband data station, the exchange area transmission systems (local loop, N-Carrier, T1 Carrier), and the long-haul transmission systems [L-type multiplex (LMX) terminals, and coaxial cable and microwave radio circuits]. The group band system is representative and is discussed in some detail. The analog carrier systems were designed principally to handle voice frequency transmission and they present special problems when they are adapted to accommodate wideband signals. For example, up to eight LMX group connector filters may be encountered in a 4000 mile circuit and each connector introduces over 150μs of delay distortion in the transmission band. Special delay equalizers are added to each connector which reduce the distortion to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm 3\mu</tex> s in the band 64 to 104 kHz. The modulation schemes utilized in the various wideband modems were carefully tailored to the respective carrier systems. The LMX modem, used as an example in this paper, utilizes vestigial sideband, suppressed carrier, amplitude modulation. A carrier frequency pilot is added to the transmitted signal to facilitate carrier recovery in the receiver. The pilot is separated from the wideband signal in the receiver by a crystal bandpass filter. The filtered pilot is used for two functions; it is used to control an AGC circuit and hence the level of the received signal, and it is used in a multiple loop APC circuit to coherently demodulate the wideband signal. The APC circuit is new and promises improved performance over the conventional phase-locked oscillator when used in a coherent detector. It is capable of maintaining smaller phase errors in the presence of phase perturbations and, furthermore, the recovered carrier is always at exactly the correct frequency and hence the circuit is incapable of breaking lock.