  Base station (BS) cooperative transmission can improve the spectrum
efficiency of cellular systems, whereas using which the channels will become
asymmetry. In this paper, we study the impact of the asymmetry on the
performance of channel estimation and precoding in downlink BS cooperative
multiple-antenna multiple-carrier systems. We first present three linear
estimators which jointly estimate the channel coefficients from users in
different cells with minimum mean square error, robust design and least square
criterion, and then study the impact of uplink channel asymmetry on their
performance. It is shown that when the large scale channel information is
exploited for channel estimation, using non-orthogonal training sequences among
users in different cells leads to minor performance loss. Next, we analyze the
impact of downlink channel asymmetry on the performance of precoding with
channel estimation errors. Our analysis shows that although the estimation
errors of weak cross links are large, the resulting rate loss is minor because
their contributions are weighted by the receive signal to noise ratio. The
simulation results verify our analysis and show that the rate loss per user is
almost constant no matter where the user is located, when the channel
estimators exploiting the large scale fading gains.
