Expanding Coverage Range and Control Channel Capacity of Co-Channel LTE Small Cells by Using PDCCH Orthogonalization

In heterogeneous cellular networks (HetNets) for mobile communications, small cells are deployed within the coverage range of primary macro cells to provide for a localized capacity boost in traffic hotspots. The sharing of communication channels among the macro-cell and the small-cell tiers is spectrally efficient but causes failures of control signaling in LTE networks due to unmitigated co-channel interference. Consequently, the small-cell coverage range and control channel capacity is reduced. The present study explores to what extent the control channel bottleneck in inter-tier co-existence can be eliminated by using orthogonalized scheduling of the user-specific physical downlink control channels (PDCCH). Implementation-wise, the optimization of PDCCH scheduling is closely related to the management of the cell radio network temporary identifiers (C-RNTIs) as these determine the PDCCH resource allocation. Simulations of a 3GPP-compliant HetNet in a Dublin scenario show that the small-cell size can be doubled for a better macro-cell traffic offload by trading the number of active PDCCHs for a higher small-cell expansion bias. Alternatively, the small-cell PDCCH capacity can be tripled for high-load applications such as Voice over LTE by using macro-cell PDCCH muting.