Blind I/Q Signal Separation-Based Solutions for Receiver Signal Processing
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Blind I/Q Signal Separation-Based Solutions for Receiver Signal Processing Mikko Valkama Institute of Communications Engineering, Tampere University of Technology, P.O. Box 553, 33101 Tampere, Finland Email: [email protected]
Markku Renfors Institute of Communications Engineering, Tampere University of Technology, P.O. Box 553, 33101 Tampere, Finland Email: [email protected]
Visa Koivunen Signal Processing Laboratory, Helsinki University of Technology, P.O. Box 3000, 02015 HUT, Finland Email: [email protected] Received 5 February 2004; Revised 14 October 2004 This paper introduces some novel digital signal processing (DSP)-based approaches to some of the most fundamental tasks of radio receivers, namely, channel equalization, carrier synchronization, and I/Q mismatch compensation. The leading principle is to show that all these problems can be solved blindly (i.e., without training signals) by forcing the I and Q components of the observed data as independent as possible. Blind signal separation (BSS) is then introduced as an eļ¬cient tool to carry out these tasks, and simulation examples are used to illustrate the performance of the proposed approaches. The main application area of the presented carrier synchronization and I/Q mismatch compensation techniques is in direct-conversion type receivers, while the proposed channel equalization principles basically apply to any radio architecture. Keywords and phrases: radio communications, complex-valued (I/Q) signals and systems, I/Q mismatch, carrier synchronization and tracking, channel equalization, blind signal separation.
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INTRODUCTION
In order to increase the receiver flexibility while also emphasizing the receiver integrability and other implementationrelated aspects, the design of radio receivers is no longer dominated by the traditional superheterodyne architecture. Instead, alternative receiver structures, like the directconversion [1, 2] and low-IF [1, 3, 4] architectures, are receiving more and more interest. The analog front-end of these types of receivers is partially based on complex or I/Q signal processing [5, 6, 7]. More specifically, the frequency translation from radio frequencies (RF) closer to baseband is carried out using I/Q downconversion. Since, in theory, the I/Q downconversion corresponds to a pure frequency translation, the fundamental image signal problem is basically avoided during the downconversion. In this manner, the This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
requirements for RF image rejection filtering are greatly relaxed in practice [1, 2, 3, 4]. The assumption of infinite image signal rejection during the downconversion is strictly valid only if amplitudes and phases of the analog front-end I and Q branches are perfectly matched [7]. In practice, this is not the case. Some mismatch or imbalance will always exist due to imperfections of practical
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