Two-tier femtocell networks-- comprising a conventional macrocellular networkplus embedded femtocell hotspots-- offer an economically viable solution toachieving high cellular user capacity and improved coverage. With universalfrequency reuse and DS-CDMA transmission however, the ensuing cross-tiercochannel interference (CCI) causes unacceptable outage probability. This paperdevelops an uplink capacity analysis and interference avoidance strategy insuch a two-tier CDMA network. We evaluate a network-wide area spectralefficiency metric called the \emph{operating contour (OC)} defined as thefeasible combinations of the average number of active macrocell users andfemtocell base stations (BS) per cell-site that satisfy a target outageconstraint. The capacity analysis provides an accurate characterization of theuplink outage probability, accounting for power control, path-loss andshadowing effects. Considering worst case CCI at a corner femtocell, resultsreveal that interference avoidance through a time-hopped CDMA physical layerand sectorized antennas allows about a 7x higher femtocell density, relative toa split spectrum two-tier network with omnidirectional femtocell antennas. Afemtocell exclusion region and a tier selection based handoff policy offersmodest improvements in the OCs. These results provide guidelines for the designof robust shared spectrum two-tier networks.
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