Degrees of freedom of rank-deficient interference channels

Abstract

Interference management is essential for improving the system performance in multiuser communications, and it becomes more important as the number of users (K) increases. Key features of interference management can be obtained by exploring the interference channels (ICs). Since the exact capacity of the IC is still unknown in general even for the two user case, approximate characterization by finding the optimal degrees of freedom (DoF) of IC has been investigated for many cases recently. Especially when K 3, a novel interference management scheme called interference alignment (IA) has been shown to achieve the optimal DoF of full-rank ICs recently. However, for rank-deficient ICs, the optimal DoF and suitable schemes are still unknown. In this thesis, we study the DoF of rank-deficient ICs. We propose novel interference management schemes suitable for rank-deficient channels that can improve the DoF substantially. Furthermore, to achieve a higher DoF, we introduce polarimetric antenna arrays, relays, and feedback for rank-deficient ICs and investigate their effects on the DoF. Specifically, we first focus on the line-of-sight (LoS) channel and analyze an achievable DoF of the K-user IC with polarization. An array of polarimetric antennas, each composed of three orthogonal electric dipoles and three orthogonal magnetic dipoles in which all six elements are co-located, is used in this case. For a K-user LoS IC with single polarization, the maximum achievable total DoF is only K regardless of the number of transmit and receive antennas at each node due to the key-hole effect. If polarimetric antennas are used at each node, a trivial upper bound on the DoF is now 2K. We show the optimal DoF of 2K is achievable if M>K+1/6 polarimetric antennas are used at each node for any K. In addition, we obtain minimal antenna configurations that achieve this upper bound for K < 5. Second, we analyze the achievable total DoF of the time-varying K-user rank-deficient (M,N,D) I...