SIViP DOI 10.1007/s11760-009-0111-x ORIGINAL PAPER New adaptive complex IIR filters and their application in OFDM systems Zlatka Nikolova · Vladimir Poulkov · Georgi Iliev · Karen Egiazarian Received: 12 September 2008 / Revised: 31 January 2009 / Accepted: 1 February 2009 © Springer-Verlag London Limited 2009 Abstract In this article, very low sensitivity variable com- plex filter (VCF) sections are developed. They have two important advantages: extremely low passband sensitivity and independent tuning of the bandwidth and the central fre- quency over a wide frequency range. The first advantage pro- vides resistance to quantization effects, while the second one gives a better digital signal processing quality and extends the area of possible applications of the developed filters. Based on the proposed VCF sections an adaptive complex system is designed which demonstrates very fast convergence and low computational complexity. This system is applied for narrowband interference cancellation in multiband orthogo- nal frequency division multiplexing receivers and wideband noise cancellation in OFDM receivers. It is experimentally verified that a better signal-to-noise ratio and signal-to-inter- ference ratio can be achieved using the proposed adaptive complex filtering scheme. Keywords Complex digital IIR filters · Adaptive complex filtering · Narrowband interference · OFDM systems Z. Nikolova (B ) · V. Poulkov · G. Iliev Department of Communication Networks, Faculty of Telecommunications, Technical University of Sofia, 8 Kliment Ohridski, 1000 Sofia, Bulgaria e-mail: zvv@tu-sofia.bg V. Poulkov e-mail: vkp@tu-sofia.bg G. Iliev e-mail: gli@tu-sofia.bg K. Egiazarian Department of Signal Processing, Tampere University of Technology, P.O. Box 527, 33101 Tampere, Finland e-mail: karen.egiazarian@tut.fi 1 Introduction The issue of interference/noise avoidance, suppression or cancellation is of primary importance for many telecom- munication applications and has been studied extensively in the recent years. Wideband wireless communication systems are very sensitive to narrowband interference (NBI), which can even block the system performance [1]. NBI suppres- sion in quadrature phase shift keying (QPSK) spread-spec- trum communication systems is discussed in [2]. Discrete multi-tone (DMT) modulation systems, such as DMT VDSL, are very sensitive to radio frequency interference (RFI) and RFI suppression has been discussed in many works, such as [3–5]. Reference [6] studies algorithms for the RFI can- cellation in VDSL systems. Besides DMT modulation, the orthogonal frequency division multiplexing (OFDM) is other leading technology for many broadband communication sys- tems, such as MB-OFDM ultra wideband systems. Because of NBI, signal-to-interference ratio (SIR) dropping can seri- ously degrade the characteristics of these systems [7]. The problem of detection, tracking and suppression/elim- ination of harmonic and narrowband signals mixed with broadband noise have been discussed in a number of publica- tions. In [8], a parameter estimation of chirp signals embed- ded in noise is made. Related problems are discussed in [9] with respect to CDMA and TDMA systems. The problem of interference is encountered in various kinds of broadband telecommunication systems, but the methods for interfer- ence suppression proposed so far can be generalized into two approaches. The first approach concerns various frequency excision methods, while the second relates to so-called cancellation techniques. These techniques aim to eliminate or reduce interference in the received signal by the use of adaptive notch filtering-based methods or NBI identifi- cation [10]. 123