Strong Interference Alignment
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Strong Interference Alignment Zainalabedin Samadi1 · Vahid Tabataba Vakili1 · Farzan Haddadi1 Accepted: 29 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Interference alignment (IA) adjusts signaling scheme such that all interfering signals are squeezed in interference subspace, and obtains the maximum degrees of freedom in an interference channel. However, IA mostly achieves its performance via infinite extension of the channel, which is a major challenge in practical systems. In this paper, we schedule part of interference to be strong and achieve perfect IA within limited number of channel extensions. A single-hop 3 user single antenna interference channel (IFC) is considered and it is shown that one of the interfering signal streams needs to be strong so that perfect IA is feasible. Practical implementation of the proposed scheme is discussed in detail for the case of two extensions of the 3 user single antenna IFC. Keywords Interference alignment · Interference channel · Degrees of freedom · Strong interference
1 Introduction Interference is one of the fundamental aspects that makes wireless multi-user communication challenging, and has attracted great research attention for decades. Some of the traditional methods of interference management can be summarized as treating the weak interference as noise [1–3], or decoding the strong interference. Carleial [4], discovered in 1975 that interference may not reduce the capacity. That is, if both of the two receivers encounter very strong interference, the receiver can first decode the message of the interference link and then subtract it before decoding its own message. However, the general condition for strong interference in a K > 2 user IFC is unknown. The problem has been solved for some special cases such as symmetric IFC. Lattice-based codes have been used to characterize a “very strong” regime [5], the generalized degrees-of-freedom [6], and the approximate sum capacity [7], for symmetric K user IFCs. There are cases where strength of interference is comparable to the desired signal. Orthogonal methods, such as time (frequency) division multiple access schemes, handle the interferences by providing each user with exclusive access to a fraction of wireless resources. Considering the entire bandwidth as a cake, these schemes cut the cake equally * Zainalabedin Samadi [email protected] 1
School of Electrical Engineering, Iran University of Science and Technology, Tehran, Iran
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between the users. Therefore, if there are K users in the channel, each user gets roughly 1/K of the channel. Although each user can utilize a part of wireless resources in the orthogonal access schemes free of interference, the performance of this “cake-cutting” approach is far from the capacity of interference networks. IA is an emerging technique in interference management [8–11], which is a degrees of freedom (DoFs) optimal approach. In a multiuser channel, precoding matrices are cooperati
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