Out-of-band and adjacent-channel interference reduction by analog nonlinear filters

Abstract

In a perfect world, we would have 'brick wall' filters, no-distortion amplifiers and mixers, and well-coordinated spectrum operations. The real world, however, is prone to various types of unintentional and intentional interference of technogenic (man-made) origin that can disrupt critical communication systems. In this paper, we introduce a methodology for mitigating technogenic interference in communication channels by analog nonlinear filters, with an emphasis on the mitigation of out-of-band and adjacent-channel interference. Interference induced in a communications receiver by external transmitters can be viewed as wide-band non-Gaussian noise affecting a narrower-band signal of interest. This noise may contain a strong component within the receiver passband, which may dominate over the thermal noise. While the total wide-band interference seen by the receiver may or may not be impulsive, we demonstrate that the interfering component due to power emitted by the transmitter into the receiver channel is likely to appear impulsive under a wide range of conditions. We give an example of mechanisms of impulsive interference in digital communication systems resulting from the nonsmooth nature of any physically realizable modulation scheme for transmission of a digital (discontinuous) message. We show that impulsive interference can be effectively mitigated by nonlinear differential limiters (NDLs). An NDL can be configured to behave linearly when the input signal does not contain outliers. When outliers are encountered, the nonlinear response of the NDL limits the magnitude of the respective outliers in the output signal. The signal quality is improved in excess of that achievable by the respective linear filter, increasing the capacity of a communications channel. The behavior of an NDL, and its degree of nonlinearity, is controlled by a single parameter in a manner that enables significantly better overall suppression of the noise-containing impulsive components compared to the respective linear filter. Adaptive configurations of NDLs are similarly controlled by a single parameter and are suitable for improving quality of nonstationary signals under time-varying noise conditions. NDLs are designed to be fully compatible with existing linear devices and systems and to be used as an enhancement, or as a low-cost alternative, to the state-of-art interference mitigation methods.

Nonlinear rank-based analog loop filters in delta-sigma analog-to-digital converters for mitigation of technogenic interference

“…Note that for a sufficiently large α this filter would exhibit nonlinear behavior only intermittently, in response to noise outliers, while otherwise acting as a 1st order linear lowpass filter. Further note that for µ = α/τ 0 equation (5) corresponds to the Canonical Differential Limiter described in [9], [10], [24], [32], and in the limit α → 0 it corresponds to the Median Tracking Filter described in Section VI-A.…”

“…Further, the temporal and/or amplitude structure (and thus the distributions) of non-Gaussian signals are generally modifiable by linear filtering, and non-Gaussian interference can often be converted from sub-Gaussian into super-Gaussian, and vice versa, by linear filtering [9], [10], [32, e.g.]. Thus the ability of the proposed ∆Σ ADCs with analog nonlinear loop filters to mitigate impulsive (super-Gaussian) noise can translate into mitigation of non-Gaussian noise and interference in general, including sub-Gaussian noise.…”

“…This physical proximity, combined with a wide range of possible transmit and receive powers, creates a variety of challenging interference scenarios. Existing empirical evidence [8], [28], [29] and its theoretical support [6], [7], [10] show that such interference often manifests itself as impulsive noise, which in some instances may dominate over the thermal noise [5], [8], [28].…”

“…There are two main problems with this approach. First, in the process of analog-to-digital conversion the signal bandwidth is reduced, and an initially impulsive broadband noise will appear less impulsive [7]- [10], [32]. Thus its removal by digital filters may be much harder to achieve.…”

Practical Implementation of Adaptive Analog Nonlinear Filtering for Impulsive Noise Mitigation

“…This classical approach has two main problems. First, the signal bandwidth decreases in the process of analog-to-digital conversion and an initially impulsive broadband noise will appear less impulsive making it challenging to remove outliers via digital filters [18]- [19].…”