Severe Nonlinear Distortion Research Articles

Simple nonlinear equalisers for binary baseband signals

Some very recent work on equalisers has led to a simple development of a conventional nonlinear (decision-feedback) equaliser, that involves only changes to the equalisation process itself and can, under certain conditions, give a useful improvement in tolerance to additive white Gaussian noise. The technique is particularly effective when binary data symbols are transmitted, at a high signal/noise ratio, and there is severe amplitude distortion in the received data signal. The paper is concerned with a further development of the technique, suitable for applications where a binary data signal is transmitted at a high rate over a linear baseband channel and where the channel introduces severe amplitude distortion. The channel may be either minimum phase or linear phase, or else it may approximate to some compromise between these two. It is shown that, under the given conditions, the linear feedforward transversal filter that forms the first part of a conventional nonlinear equaliser can often be replaced by a simple two-tap feedforward transversal filter, with either an improvement in performance or, at least, no serious degradation in performance. The paper describes both the new equaliser and a further development of this for the case where the received data signal is sampled at twice the data symbol rate. Finally, results of computer-simulation tests are presented, comparing the tolerances to additive white Gaussian noise of the two new equalisers with those of more conventional detectors, for models of various channels. Three quite different applications have been considered, one of which involves the transmission of digital data at 140 Mbit/s over a length of coaxial cable.

Non-linear equalizers having an improved tolerance to noise

The paper describes a simple development of a conventional non-linear equalizer which involves only changes to the equalization process itself and can, under certain conditions, give a useful improvement in tolerance to additive white Gaussian noise. The technique is particularly effective when binary data-symbols are transmitted and there is severe amplitude distortion in the received signal. The optimum design of the modified equalizer together with an interesting sub-optimum arrangement, for the application of greatest practical interest, are derived first and then approximate estimates of their tolerances to additive white Gaussian noise. Finally, results are presented of computer-simulation tests, with binary signals transmitted over various channels in he presence of additive white Gaussian noise, and the results are compared with the corresponding theoretical predictions.

Fractionally-Spaced Equalization: An Improved Digital Transversal Equalizer

Here we describe and demonstrate, via analysis and simulation, the performance improvement of voice-grade modems which use a Fractionally-Spaced Equalizer (FSE) instead of a conventional synchronous equalizer. The reason for this superior performance is that the fse adoptively realizes the optimum linear receiver; consequently it can effectively compensate for more severe delay distortion than the conventional adaptive equalizer, which suffers from aliasing effects. An additional advantage of the FSE is that data transmission can begin with an arbitrary sampling phase, since the equalizer synthesizes the correct delay during adaptation. We show that an FSE combined with a decision feedback section, which can mitigate the effect of severe amplitude distortion, can compensate for a wide range of linear distortion. At 9.6 kbit/s, the FSE provides a 2 to 3 dB gain in output signal-to-noise ratio, relative to the synchronous equalizer, over worst-case private-line channels. This translates to a theoretical improvement of approximately two orders of magnitude in bit error rate.

Aerosol−induced thermal blooming

Experiments have been carried out to study the nonlinear distortion of a laser beam due to absorption by aerosol particles in the air. A 15−W 10.6−μ beam was passed through a 1.5−m cell containing a carbon aerosol in air at STP, and various measurements of the distorted beam profile were made. The data show that aerosol absorption can lead to severe nonlinear distortion and that, for the regime in which the beam transit time is long compared to the interparticle thermal diffusion time, this distortion is qualitatively similar to that produced by molecular absorption, but has a magnitude somewhat below that predicted by conventional thermal blooming theory.