Benefits of Local Cooperation in Sectorized Cellular Networks Under a Complexity Constraint

The paper presents upper and lower bounds on the per-sector degrees of freedom (DoF) of a sectorized hexagonal cellular model when neighboring base stations (BSs) can cooperate during at most Δ interaction rounds over rate-limited backhaul links. The lower bound is based on practically implementable beamforming and adapts the way BSs cooperate to the sectorization of the cells. It improves over the naive approach that ignores this sectorization in terms of the sum-rate, both at finite signal-to-noise ratio (SNR) and in the high-SNR limit. For moderate SNR, the new scheme improves also over an opportunistic cooperation strategy where each message is decoded based on the signals received at the three adjacent sectors with the best SNR. The upper bound is information-theoretic and holds for all possible coding schemes, including for example interference alignment with unlimited symbol extensions whose practical implementation currently seems out of reach. Lower and upper bounds show that the complexity constraint, imposed by limiting the number of interaction rounds Δ, indeed limits the largest achievable sum-rate and DoF. In particular, irrespective of the backhaul capacity μ, the per-sector DoF cannot exceed a threshold which depends on Δ.