NEXTNOIS-a programmable noise generating system for testing wire-based loop transmission systems

Since the early 1980s, full-duplex transmission systems have been developed to transmit higher data rates over the local wire-based loop plant. The performance of these systems must be tested against a set of requirements. Many of these performance tests include a procedure for measuring the system´s near-end crosstalk (NEXT) noise margin. This procedure always requires a NEXT noise source to be connected onto the line to inject NEXT noise into the receiver of the transmission system. The paper presents the design of a prototype IBM PC/XT-based highspeed programmable digital NEXT noise injection system (NEXTNOIS). NEXTNOIS can accurately produce NEXT noise from a multitude of different full-duplex transmission systems, including data/voice systems, DDS, Basic Rate ISDN, HDSL, T1 lines, and future systems operating at higher speeds. The system is flexible; the power spectral density of the noise as well as the noise level can be changed under computer control. If the output of NEXTNOIS is followed by a modulator, it can generate NEXT and far-end crosstalk (FEXT) for bandpass systems such as FSK and QAM. The system can also be used to generate other types of noise, such as white and colored noise (up to 1.6 MHz), and possibly impulse noise