On the Degrees of Freedom of SISO X-networks with Synergistic Alternating Channel State Information at transmitters

TL;DR

This paper investigates the degrees of freedom in SISO X-networks with synergistic alternating channel state information, demonstrating that specific CSI distributions can achieve optimal or improved DoF bounds through innovative two-phase transmission schemes.

Contribution

It introduces a novel approach using synergistic alternating CSI with a specific distribution to achieve tight bounds on DoF, matching or surpassing previous feedback-based results.

Findings

Achieves a 4/3 DoF for two-user X-channel with equal CSI state fractions.

Establishes a lower bound of 2K/(K+1) DoF for K-user X-networks under the same CSI distribution.

Shows that synergistic alternating CSI with a specific distribution is equivalent to partial output feedback.

Abstract

In this paper, we consider the two-user single-input single-output (SISO) X-channel and $K$-user SISO X-network in fast fading environment. It is assumed that the transmitters have access to synergistic alternating channel state information (CSI). Specifically, the CSIT alternates between three states, namely, perfect, delayed and no-CSIT, in a certain manner to enable these states to work together cooperatively. These states are associated with fractions of time denoted by $\lambda_P$, $\lambda_D$ \text{and} $\lambda_N$, respectively. For the two-user $X$-channel, simple upper bound is developed to prove the tightness of the achievability result of $4/3$ DoF under a certain distribution of the availability of three CSIT states for $\Lambda(\lambda_P=1/3, \lambda_D= 1/3, \lambda_N=1/3)$. For the $K$-user $X$-network, it is shown that the sum Degrees of freedom (DoF) is at least $2K/(K + 1)$, using two-phase transmission schemes over finite symbols channel extension and under the same distribution of the availability of $\Lambda(\lambda_P=1/3, \lambda_D= 1/3, \lambda_N=1/3)$.This achievability result, can be considered as a tight lower bound, coincides with the best lower bound known for the same network but with partial output feedback in stead of alternating CSIT. Hence, we show that the role of synergistic alternating CSIT with distribution $\Lambda(1/3,1/3,1/3)$ is equivalent to the partial output feedback. Also, this lower bound is strictly better than the best lower bound known for the case of delayed CSI assumption for all values of $K$. All the proposed transmission schemes are based on two phases transmission strategy, namely, interference creation and interference resurrection, which exploit the synergy of instantaneous CSI and delay CSIT to retrospectively align interference in the subsequent channel uses.