In this paper, the spatial interference alignment (IA) is investigated in the downlink (DL) of a relay-assisted
multi-cell multi-user network. The transmission from the base station (BS) to the mobile station (MS) takes place in
two phases with the help of the selected relay station (RS). The closed-form IA algorithms are employed in both the
BS to RS and RS to MS links. The performance of this cooperative scheme is analyzed in terms of the sum rate and
the sum degrees of freedom (DoF) for the amplify-and-forward (AF) and decode-and-forward (DF) relaying schemes.
The simulation results of the sum rate show that the DF scheme significantly outperforms the noncooperative minleakage
and max-SINR iterative algorithms over the entire range of SNR, and the AF scheme performs very close to
the min-leakage algorithm. Moreover, the DF scheme outperforms the noncooperative closed-form IA algorithm in
the low and medium-SNR regimes.

Keywords: Cellular network, degrees of freedom, interference alignment, relay, sum rate

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