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

We introduce an opportunistic interference mitigation (OIM) protocol for cellular networks, where a user scheduling strategy is utilized in $K$-cell uplink environments with time-invariant channel coefficients and base stations (BSs) having $M$ receive antennas. In the OIM scheme, 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: an opportunistic interference nulling (OIN) and an 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 $\text{SNR}^{(K-1)M}$, where SNR denotes the received signal-to-noise ratio. Similarly, it turns out that the OIA scheme with $S$ selected users (smaller than $M$) 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 number $N$ of users per cell, based on the two proposed schemes. Furthermore, by deriving the corresponding upper bound on the DoFs, it is shown that the OIN scheme is DoF-optimal. Finally, numerical evaluation is performed; a two-step scheduling strategy, in which a logarithmic gain can further be obtained, is shown; and our achievability result is extended to multi-carrier systems. Note that the proposed OIM schemes do not require the global channel state information and time/frequency expansion, thereby resulting in easier implementation.