Interference Aligned Space-Time Transmission with Diversity for the $2 \times 2$ X-Network

TL;DR

This paper introduces a new transmission scheme for the 2x2 MIMO X-Network that achieves optimal degrees of freedom using only local channel information and guarantees high diversity gain for any number of antennas.

Contribution

The paper presents an explicit scheme that attains the sum-DoF of 4M/3 for arbitrary M using only local CSIT, unlike previous schemes requiring global CSIT.

Findings

Achieves the sum-DoF of 4M/3 for any M.

Guarantees a diversity gain of M+1 with finite constellations.

Requires only local CSIT, simplifying implementation.

Abstract

The sum degrees of freedom (DoF) of the two-transmitter, two-receiver multiple-input multiple-output (MIMO) X-Network ($2 \times 2$ MIMO X-Network) with $M$ antennas at each node is known to be $\frac{4M}{3}$. Transmission schemes which couple local channel-state-information-at-the-transmitter (CSIT) based precoding with space-time block coding to achieve the sum-DoF of this network are known specifically for $M=2,4$. These schemes have been proven to guarantee a diversity gain of $M$ when a finite-sized input constellation is employed. In this paper, an explicit transmission scheme that achieves the $\frac{4M}{3}$ sum-DoF of the $2 \times 2$ X-Network for arbitrary $M$ is presented. The proposed scheme needs only local CSIT unlike the Jafar-Shamai scheme which requires the availability of global CSIT in order to achieve the $\frac{4M}{3}$ sum-DoF. Further, it is shown analytically that the proposed scheme guarantees a diversity gain of $M+1$ when finite-sized input constellations are employed.