Degrees of freedom (DoF) is studied in the downlink of a heterogenouswireless network modeled as a two-layered interference network. The first layerof the interference network is the backhaul layer between macro base stations(MB) and small cell base stations (SB), which is modeled as a Wyner type linearnetwork. The second layer is the transmission layer between SBs and mobileterminals (MTs), which is modeled as a linear Wyner $L_T$ network, i.e., eachMT is connected to $L_T+1$ SBs. The SBs are assumed to be half-duplex, thusrestricting the per user degrees of freedom (puDoF) in the system to $1/2$. ThepuDoF can be further restricted by the number of antennas at the MB. For $L_T\in \{1,2\}$, the optimal puDoF can be achieved by using simple interferenceavoidance schemes. The increase in the connectivity of transmission layerbeyond $L_T=2$ limits the achievable puDoF using only zero-forcing schemes toless than 1/2, even in the presence of large number of antennas at each MB butthe optimal puDoF can be achieved by making each message available at multipleSBs. This is done by sending an appropriate linear combination to the SB tozero-force interference at the intended user. The maximum per user DoF of 1/2can be achieved in the linear network with sufficient number of antennas usingonly interference avoidance schemes. These results are also extended to a morerealistic hexagonal cellular model as well.
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