Impulsive interference in communication channels and its mitigation by SPART and other nonlinear filters

Nikitin et al. EURASIP Journal on Advances in Signal Processing 2012, 2012:79 http://asp.eurasipjournals.com/content/2012/1/79 RESEARCH Open Access Impulsive interference in communication channels and its mitigation by SPART and other nonlinear filters Alexei V Nikitin1,2*, Marc Epard2, John B Lancaster1,2, Robert L Lutes2 and Eric A Shumaker2 Abstract A strong digital communication transmitter in close physical proximity to a receiver of a weak signal can noticeably interfere with the latter even when the respective channels are tens or hundreds of megahertz apart. When time domain observations are made in the signal chain of the receiver between the first mixer and the baseband, this interference is likely to appear impulsive. The impulsive nature of this interference provides an opportunity to reduce its power by nonlinear filtering, improving the quality of the receiver channel. This article describes the mitigation, by a particular nonlinear filter, of the impulsive out-of-band (OOB) interference induced in High Speed Downlink Packet Access (HSDPA) by WiFi transmissions, protocols which coexist in many 3G smartphones and mobile hotspots. Our measurements show a decrease in the maximum error-free bit rate of a 1.95 GHz HSDPA receiver caused by the impulsive interference from an OOB 2.4 GHz WiFi transmission, sometimes down to a small fraction of the rate observed in the absence of the interference. We apply a nonlinear SPART filter to recover a noticeable portion of the lost rate and maintain an error-free connection under much higher levels of the WiFi interference than a receiver that does not contain such a filter. These measurements support our wider investigation of OOB interference resulting from digital modulation, which appears impulsive in a receiver, and its mitigation by nonlinear filters. Keywords: electromagnetic interference, impulsive noise, interchannel interference, nonlinear filtering, out-of-band interference, SPART filter 1 Introduction and motivation It is becoming more and more common that multiple digital communication devices coexist and concurrently operate in close physical proximity. A typical example would be a smartphone equipped with WiFi, Bluetooth, and GPS, and capable to operate at various data protocols and in multiple frequency bands. This physical proximity, combined with a wide range of possible transmit and receive powers, creates a variety of challenging interference scenarios. Plenty of empirical evidence indicates that such interference often manifests itself as impulsive noise [1,2], which in some instances dominates over the thermal noise [1,3]. A particular source of impulsive noise in digital communication systems is interchannel interference [4,5]. * Correspondence: 1 Avatekh Inc., 900 Masachusetts Street, Suite 409, Lawrence, KS 66044, USA Full list of author information is available at the end of the article For example, a strong close transmitter (say, WiFi) can noticeably interfere with a receiver of a weak signal (say, GPS) even when the separation of their frequency bands exceeds the respective nominal bandwidths of the channels by orders of magnitude. When time domain observations of such far-OOB interference are made at the receiver frequency, in a relatively wide bandwidth to avoid excessive broadening of the transients, this interference is likely to appear impulsive. Understanding the mechanism of this interference and its impulsive nature is important for its effective mitigation. As shown in [4,5], the fundamental origin of the interchannel interference lies in the unavoidable non-smoothness in the modulation of the interfering transmitter, which leads to discontinuities in the higher order derivatives of the modulating signal. This non-smoothness is exacerbated by the non-idealities in the hardware © 2012 Nikitin et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nikitin et al. EURASIP Journal on Advances in Signal Processing 2012, 2012:79 http://asp.eurasipjournals.com/content/2012/1/79 implementation, and by the coupling of other interfering signals from the adjacent circuitry. As outlined in [4-7], certain analog nonlinear filters deployed early in the signal chain of the receiver can improve the signal-to-noise ratio (SNR) and increase the data rates of a communication channel (e.g., GPS or WCDMA) in the presence of impulsive interchannel interference, for example, from WiFi transmissions. Although analog by definition, these filters can also be implemented digitally, for example, in FPGA or software, and, unlike other typical nonlinear filters such as median filters, they require little memory and computational resources and can operate in real time even at very high sampling rates. In this article we present the measurements, under various experimental conditions, of the impact of a 2.412 GHz 802.11g (Channel 1) WiFi transmitter on the HSDPA protocol [8] running at 1.95 GHz in the UMTS-FDD band II downlink (used in the United States by AT&T Mobility), and quantify the improvement in the channel quality provided by a particular nonlinear filter, ‘SPART’, described in [6]. a This continues our wider investigation into the impulsive nature of the OOB interference from different digital modulation protocols, and the mitigation of impulsive noise by nonlinear filters. 1.1 Main result The main experimental result of this study is summarized in Figure 1. The figure shows the data throughput Page 2 of 29 of HSDPA at various signal levels with and without strong WiFi interference of constant power, and with and without the SPART filter deployed in the receiver signal chain. In the figure, the black line shows the HSDPA throughput without the WiFi interference, and the insertion of the SPART filter in the signal chain of the receiver does not affect the data throughput. WiFi interference lowers the throughput and/or causes the loss of connection (red line). A SPART filter in the signal chain of the receiver recovers a portion of the lost data rate, and/or restores the lost connection (solid blue line). The error bars indicate the standard error of the measurements. For reference, dashed blue lines indicate 25, 50, and 100% improvement in the data rate, respectively. One can see that, when the SPART filter is used during the WiFi interference, the connection (positive throughput) is maintained for lower HSDPA signal levels. This effectively increases the link budget, as indicated by the horizontal black arrow line in the figure. The result presented in Figure 1 supports the following two statements: (i) a strong WiFi transmitter can noticeably interfere with an HSDPA receiver even though their operating bands are almost half a GHz apart, and (ii) becaus (...truncated)