Opportunistic Interference Mitigation Achieves Optimal Degrees-of-Freedom in Wireless Multi-cell Uplink Networks

TL;DR

This paper proposes opportunistic interference mitigation protocols for multi-cell uplink networks, achieving optimal degrees-of-freedom by user scheduling strategies that minimize interference, with performance depending on user scaling relative to SNR.

Contribution

The paper introduces two novel OIM protocols, OIN and OIA, demonstrating their DoF optimality and analyzing the trade-offs between DoFs and user scaling requirements.

Findings

OIN achieves KM M DoFs with N scaling as SNR^{(K-1)M}

OIA achieves KS S DoFs with N scaling faster than SNR^{(K-1)S}

OIM protocols are shown to be DoF optimal under certain conditions

Abstract

We introduce an opportunistic interference mitigation (OIM) protocol, where a user scheduling strategy is utilized in $K$-cell uplink networks with time-invariant channel coefficients and base stations (BSs) having $M$ antennas. Each BS opportunistically selects a set of users who generate the minimum interference to the other BSs. Two OIM protocols are shown according to the number $S$ of simultaneously transmitting users per cell: opportunistic interference nulling (OIN) and opportunistic interference alignment (OIA). Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As our main result, it is shown that $KM$ DoFs are achievable under the OIN protocol with $M$ selected users per cell, if the total number $N$ of users in a cell scales at least as $\text{SNR}^{(K-1)M}$. Similarly, it turns out that the OIA scheme with $S$($<M$) selected users achieves $KS$ DoFs, if $N$ scales faster than $\text{SNR}^{(K-1)S}$. These results indicate that there exists a trade-off between the achievable DoFs and the minimum required $N$. By deriving the corresponding upper bound on the DoFs, it is shown that the OIN scheme is DoF optimal. Finally, numerical evaluation, a two-step scheduling method, and the extension to multi-carrier scenarios are shown.