In a two tier cellular network -comprised of a central macrocell underlaid with shorter range femtocell hotspots -cross-tier interference limits overall capacity with universal frequency reuse. To quantify near-far effects with universal frequency reuse, this paper derives a fundamental relation providing the largest feasible cellular Signal-to-Interference-Plus-Noise Ratio (SINR), given any set of feasible femtocell SINRs. We provide a link budget analysis which enables simple and accurate performance insights in a two-tier network. A distributed utility-based SINR adaptation at femtocells is proposed in order to alleviate cross-tier interference at the macrocell from cochannel femtocells. The Foschini-Miljanic (FM) algorithm is a special case of the adaptation. Each femtocell maximizes their individual utility consisting of a SINR based reward less an incurred cost (interference to the macrocell). Numerical results show greater than 30% improvement in mean femtocell SINRs relative to FM. In the event that cross-tier interference prevents a cellular user from obtaining its SINR target, an algorithm is proposed that reduces transmission powers of the strongest femtocell interferers. The algorithm ensures that a cellular user achieves its SINR target even with 100 femtocells/cell-site, and requires a worst case SINR reduction of only 16% at femtocells. These results motivate design of power control schemes requiring minimal network overhead in two-tier networks with shared spectrum.
-We analyze a low power, wideband CDMA system with multiple antennas. The joint "slope region" -of transmission rates at minimum energy per bit -is derived as a function of proportion between vanishing rates for multiple users. Vertices of the slope region may be achieved by a matched filter linear interface, followed by successive interference cancellation. We introduce and evaluate "robust slope region," the largest region which is inside every other slope region and is therefore independent from the relative proportion between transmission rates. Furthermore, we find the "robust slope," maximum slope which can always be guaranteed to all users.
Abstract-In this paper, we introduce the model for a Gaussian soft handover channel (SHC), which adds a new dimension of flexibility to the well known interference channel (IC). We provide a unified framework for computing achievable rate regions for the soft handover channel in both the uplink and downlink cases. This achievable rate region for SHC is given by the convex hull of the union of certain multiple access, interference, broadcast and Z−channels. Some properties of the achievable region are studied. Specifically, we show the following key results: i) In an uplink SHC, there are channel conditions under which decoding at a single receiver based, for example, on a maximum SNR condition, does not achieve the entire boundary of the achievable region. ii) In a downlink SHC, multiple base stations should transmit independent information to all users to achieve the boundary points of the achievable region and iii) When a mobile communicates with multiple base stations, the ratio of uplink rates with different base stations could be different from the ratio of the downlink rates with those base stations. A simple outer bound on SHC capacity based on the capacity of MIMO systems is also given.
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