  In ultra-dense networks (UDNs), distributed radio access points (RAPs) are
configured into small virtual cells around mobile users for fair and
high-throughput services. In this correspondence, we evaluate the performance
of user-centric joint transmission (JT) in a UDN with a number of virtual
cells. In contrast to existing cooperation schemes, which assume constant RAP
transmit power, we consider a total transmit power constraint for each user,
and assume that the total power is optimally allocated to the RAPs in each
virtual cell using maximum ratio transmission (MRT). Based on stochastic
geometry models of the RAP and user locations, we resolve the correlation of
transmit powers introduced by MRT and derive the average user throughput.
Numerical results show that user-centric JT with MRT provides a high
signal-to-noise ratio (SNR) without generating severe interference to other
co-channel users. Moreover, we show that MRT precoding, while requiring
channel-state-information (CSI), is essential for the success of JT.
