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High SNR Broadcast Channel Differential Capacity

Castanheira, D. ; Gameiro, A.

High SNR Broadcast Channel Differential Capacity, Proc ICT Mobile and Wireless Communications Summit, Santander, Spain, Vol. *, pp. 1 - 8, June, 2009.

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Abstract
n a distributed antenna system (DAS) the joint processing of a high number of transmit antennas is beneficial in terms of power/rate offset but it also introduces more complexity and overall costs to the network. Following the law of diminishing returns it is expected that has we connect the users to more and more antennas these gains will be increasingly smaller. We study these gains for a DAS, when one more transmit antenna is added to the system, in terms of the ergodic channel capacity. We analyse both the case of more transmit than aggregate receive antennas and the case of more aggregate receive than transmit antennas. For the first case we compare dirty paper coding (DPC), to zero-forcing (ZF) and block-diagonalization (BD) and provide a closed form expression for the corresponding gains, for a co-located transmit antennas scenario. For the general scenario, of a DAS, a numerical analysis is provided showing that the users’ positions that attain the highest capacity increase values are very close to the system symmetry lines, indicating that system symmetry plays an important role in attaining most of the gains provided by the users’ connection to more antennas. When the number of aggregate receive antennas is higher than the number of transmit antennas we only analyse DPC and consider that all users are co-located and all transmit antennas are co-located. For this case we show that the most important gain is the multiplexing gain.