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**1 - 2**of**2**### On Uplink-Downlink Duality for Cellular IA

"... In our previous work [1], [2] we considered the uplink of a hexagonal cellular network topology and showed that linear “one-shot ” interference alignment (IA) schemes are able to achieve the optimal degrees of freedom (DoFs) per user, under a decoded-message passing framework that allows base-statio ..."

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In our previous work [1], [2] we considered the uplink of a hexagonal cellular network topology and showed that linear “one-shot ” interference alignment (IA) schemes are able to achieve the optimal degrees of freedom (DoFs) per user, under a decoded-message passing framework that allows base-stations to exchange their own decoded messages over local backhaul links. In this work, we provide the dual framework for the downlink of cellular networks with the same backhaul architecture, and show that for every “one-shot ” IA scheme that can achieve d DoFs per user in the uplink, there exists a dual “one-shot ” IA scheme that can achieve the same DoFs in the downlink. To enable “Cellular IA ” for the downlink, base-stations will now use the same local backhaul links to exchange quantized versions of the dirty-paper precoded signals instead of user messages. I.

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"... Interference alignment promises that, in Gaussian interference channels, each link can support half of a degree of freedom (DoF) per pair of transmit-receive antennas. However, in general, this result requires to precode the data bearing signals over a signal space of asymptotically large diversity, ..."

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Interference alignment promises that, in Gaussian interference channels, each link can support half of a degree of freedom (DoF) per pair of transmit-receive antennas. However, in general, this result requires to precode the data bearing signals over a signal space of asymptotically large diversity, e.g., over an infinite number of dimensions for time-frequency varying fading channels, or over an infinite number of rationally independent signal levels, in the case of time-frequency invariant channels. In this work we consider a wireless cellular system scenario where the promised optimal DoFs are achieved with linear precoding in one-shot (i.e., over a single time-frequency slot). We focus on the uplink of a symmetric cellular system, where each cell is split into three sectors with orthogonal intra-sector multiple access. In our model, interference is “local”, i.e., it is due to transmitters in neighboring cells only. We consider a message-passing backhaul network architecture, in which nearby sectors can exchange already decoded messages and propose an alignment solution that can achieve the optimal DoFs. To avoid signaling schemes relying on the strength of interference, we further introduce the notion of topologically robust schemes, which are able to guarantee a minimum rate (or DoFs) irrespectively of the strength of the interfering links. Towards this end, we design an alignment scheme which is topologically robust and still achieves the same optimum DoFs.