Design of a Heterogeneous Cellular Network with a Wireless Backhaul

The downlink of a two-layered heterogeneous network is studied with macro basestations (MBs), small-cell basestations (SBs) that act as half-duplex analog relays, and mobile terminals (MTs). The first layer is a wireless backhaul layer between MBs and SBs, and the second is the transmission layer between SBs and MTs. The layers use the same time/frequency resources for communication, limiting the maximum per user degrees of freedom (puDoF) to half, due to the half-duplex nature of the SBs. For linear network models, it is established that the optimal puDoF can be achieved by cooperation with an appropriate number of antennas that depends on the connectivity of the network. The proposed zero-forcing schemes achieve cooperation without overloading the backhaul, through each MB sending an appropriate linear combination of MTs' message signals to the SBs in the backhaul layer. The achievable schemes exploit the half-duplexity of the SBs, and schedule the SBs and MTs to be active in different time-slots to manage interference. These results are then extended to a more realistic hexagonal cellular network and it is shown that the optimal puDoF of half can be approached using only zero-forcing schemes.