Blind Separation of Two Users Based on User Delays and Optimal Pulse-Shape Design
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Research Article Blind Separation of Two Users Based on User Delays and Optimal Pulse-Shape Design Xin Liu,1 Athina P. Petropulu,1 H. Vincent Poor,2 and Visa Koivunen3 1 Electrical
and Computer Engineering Department, Drexel University, 19104-2875 PA, USA of Engineering and Applied Science, Princeton University, Princeton, NJ 08544, USA 3 Signal Processing Laboratory, Aalto University, 00076 Aalto, Finland 2 School
Correspondence should be addressed to Xin Liu, [email protected] Received 4 December 2009; Accepted 9 June 2010 Academic Editor: Yan Xin Copyright © 2010 Xin Liu et al. 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. A wireless network is considered, in which two spatially distributed users transmit narrow-band signals simultaneously over the same channel using the same power. User separation is achieved by oversampling the received signal and formulating a virtual multiple-input multiple-output (MIMO) system based on the resulting polyphase components. Because of oversampling, high correlations can occur between the columns of the virtual MIMO system matrix which can be detrimental to user separation. A novel pulse-shape waveform design is proposed that results in low correlation between the columns of the system matrix, while it exploits all available bandwidth as dictated by a spectral mask. It is also shown that the use of successive interference cancelation in combination with blind source separation further improves the separation performance.
1. Introduction We consider the problem of multiuser separation in wireless networks via approaches that do not use scheduling. This problem is of interest, for example, when traffic is generated in a bursty fashion, in which case fixed bandwidth allocation would result in poor bandwidth utilization. Lack of scheduling results in collisions, that is, users overlapping in time and/or frequency. To separate the colliding users, one could enable multiuser separation via receive antenna diversity, or code diversity, as in code-division multiple-access (CDMA) systems. However, the former requires expensive hardware since multiple transceiver front ends involve significant cost. Further, the use of multiple antennas might not be possible on small-size terminals or devices. CDMA systems require bandwidth expansion, which requires greater spectral resources, and also introduces frequency-selective fading. In the following, we narrow our field of interest to randomaccess systems that for the aforementioned reasons cannot exploit antenna diversity, and that are inexpensive in terms of bandwidth. In such systems, the use of different power levels by the users can enable user separation by exploiting the capture effect [1], or successive interference cancellation
(SIC) [2]. Different power levels can result from different distances between the users and the destination, or could be intentionally assigned to use
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