Degrees of freedom of M xc3x97 N SISO X channel with synergistic alternating CSIT In this paper, we investigate degrees of freedom (DoF) of the M xc3x97 N single input single output (SISO) X channel with alternating channel state information at the transmitter (CSIT). It is known that sum DoF of 2user SISO X channel with synergistic alternating CSIT is the same as the sum DoF of 2user (M  N  2) SISO X channel with perfect CSIT 8. As an extension of the 2user case, achievable schemes of the M xc3x97 N SISO X channel with synergistic alternating CSIT are proposed. It is shown that the proposed M xc3x97 N X channel schemes with synergistic alternating CSIT achieve the sum DoF 2M(M  1), which is better than the weighted sum DoF of the conventional schemes for M xe2x89xa4 N. The DoF of the proposed scheme with M  N  K is strictly better than the best known DoF for the Kuser X channel with delayed CSIT.

Towards an Extremal Network Theory  Robust GDoF Gain of Transmitter Cooperation over TIN Significant progress has been made recently in Generalized Degrees of Freedom (GDoF) characterizations of wireless interference channels (IC) and broadcast channels (BC) under the assumption of finite precision channel state information at the transmitters (CSIT), especially for smaller or highly symmetric network settings. A critical barrier in extending these results to larger and asymmetric networks is the inherent combinatorial complexity of such networks. Motivated by other fields such as extremal combinatorics and extremal graph theory, we explore the possibility of an extremal network theory, i.e., a study of extremal networks within particular regimes of interest. As our test application, we study the GDoF benefits of transmitter cooperation in a K user IC over the simple scheme of power control and treating interference as Gaussian noise (TIN) for three regimes of interest  a TIN regime where TIN was shown to be GDoF optimal for the K user interference channel, a CTIN regime where the GDoF region achievable by TIN is convex without timesharing, and an SLS regime where a simple layered superposition (SLS) scheme is shown to be optimal in the K user MISO BC, albeit only for Kleq 3. As our first result, we show that under finite precision CSIT, TIN is GDoF optimal for the K user IC throughout the CTIN regime. Furthermore, under finite precision CSIT, appealing to extremal network theory we obtain the following results. In both TIN and CTIN regimes, we show that the extremal GDoF gain from transmitter cooperation over TIN is bounded regardless of the number of users: the gain is exactly a factor of 32 in the TIN regime, and 21K in the CTIN regime, for arbitrary number of users Ku003e1. However, in the SLS regime, the gain is Theta(log_2(K)), i.e., it scales logarithmically with the number of users.