User Arrival in MIMO Interference Alignment Networks

TL;DR

This paper investigates how secondary users can access a MIMO interference channel with active users employing interference alignment, determining antenna requirements and precoding strategies to maximize secondary rates without harming active users.

Contribution

It derives the minimum number of antennas needed for secondary users to access the channel without affecting active users' sum rate and proposes precoding methods for different antenna scenarios.

Findings

Secondary users can achieve interference alignment with enough antennas.

Secondary users can maximize their sum rate without degrading active users.

Large rate losses at active users can be avoided with proper precoding.

Abstract

In this paper we analyze a constant multiple-input multiple-output interference channel where a set of active users are cooperating through interference alignment while a set of secondary users desire access to the channel. We derive the minimum number of secondary transmit antennas required so that a secondary user can use the channel without affecting the sum rate of the active users, under a zero-forcing equalization assumption. When the secondary users have enough antennas, we derive several secondary user precoders that approximately maximize the secondary users' sum rate without changing the sum rate of the active users. When the secondary users do not have enough antennas, we perform numerical optimization to find secondary user precoders that cause minimum degradation to the sum rate of the active users. Through simulations, we confirm that i) with enough antennas at the secondary users, gains equivalent to the case of all the users cooperating through interference alignment is obtainable, and ii) when the secondary users do not have enough antennas, large rate losses at the active users can be avoided.