On the Coexistence of Macrocell Spatial Multiplexing and Cognitive Femtocells

TL;DR

This paper explores a coexistence scheme for macrocell and cognitive femtocell networks using multiuser MIMO, interference alignment, and power control to optimize throughput and reduce interference.

Contribution

It proposes a novel interference alignment scheme for macrocell-femtocell coexistence leveraging uplink/downlink duality and distributed power control algorithms.

Findings

Significant throughput gains with macrocell user grouping.

Effective interference management via power control.

Enhanced macro/femto coexistence performance.

Abstract

We study a two-tier macrocell/femtocell system where the macrocell base station is equipped with multiple antennas and makes use of multiuser MIMO (spatial multiplexing), and the femtocells are "cognitive". In particular, we assume that the femtocells are aware of the locations of scheduled macrocell users on every time-frequency slot, so that they can make decisions on their transmission opportunities accordingly. Femtocell base stations are also equipped with multiple antennas. We propose a scheme where the macrocell downlink (macro- DL) is aligned with the femtocells uplink (femto-UL) and, Vice Versa, the macrocell uplink (macro-UL) is aligned with the femtocells downlink femto-DL). Using a simple "interference temperature" power control in the macro-DL/femto-UL direction, and exploiting uplink/downlink duality and the Yates, Foschini and Miljanic distributed power control algorithm in the macro- UL/femto-DL direction, we can achieve an extremely attractive macro/femto throughput tradeoff region in both directions. We investigate the impact of multiuser MIMO spatial multiplexing in the macrocell under the proposed scheme, and find that large gains are achievable by letting the macrocell schedule groups of co-located users, such that the number of femtocells affected by the interference temperature power constraint is small.