The downlink of a two-layered heterogeneous hexagonal cellular network is studied with macro base stations (MB), small cell base stations (SB) that act as half duplex analog relays, and mobile terminals (MT). The first layer is a point-to-multipoint wireless backhaul between macro base stations and small cell base stations, and the second layer is the transmission layer between SBs and MTs. The wireless backhaul layer and the transmission layer use the same time/frequency resources for communication. The degrees of freedom (DoF) metric is used to characterize the capacity of the network at high signal to noise ratio (SNR). The maximum achievable per user DoF in the system is equal to half, due to the half-duplex nature of the SBs. The proposed schemes are simple zero forcing schemes that employ joint processing and achieve cooperation without overloading the wireless backhaul. This is achieved by sending an appropriate linear combination from the MBs that zero force (ZF) interference at the MTs directly. The achievable schemes exploit the half duplexity of the SBs in the system and schedule the SBs and MTs to be active in different time-slots in a smart manner to reduce interference. The optimal per user DoF of half can be approached in the hexagonal sectored cellular network using only zero forcing schemes, without the use of interference alignment.
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