DoF in Sectored Cellular Systems with BS Cooperation Under a Complexity Constraint

The paper presents upper and lower bounds on the per-user degrees of freedom (DoF) of a sectored hexagonal cellular model where neighboring basestations (BS) can cooperate during at most κ interaction rounds over backhaul links of capacities μ = μ_DoF·[1/2]log P, with P denoting the transmit power at each mobile user. The lower bound is based on practically implementable beamforming and adapts the way BSs cooperate to the sector structure of the cells. It improves over the naive approach that ignores this sector structure. The upper bound is information-theoretic and holds for all possible coding schemes, including for example ergodic interference alignment 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 κ indeed limits the largest achievable DoF. In particular, irrespective of the backhaul capacity μ, the per-user DoF cannot exceed a threshold which depends on κ.