Coordinated Multipoint (CoMP) transmission provides high theoretic gains
in spectral efficiency with coherent Joint Transmission (JT) to multiple users.
However, this requires accurate Channel State Information at the Transmitter (CSIT)
and also user groups with spatially compatible users.
The aim of this paper is to use measured channels to investigate if
significant CoMP gains can still be obtained with channel estimation errors.
This turns out to be the case, but requires the combination of several techniques.
We here focus on coherent downlink JT CoMP to multiple users within a
cluster of cooperating base stations. The use of Kalman predictors is investigated to
estimate the complex channel gains at the moment of transmission.
It is shown that this can provide sufficient CSIT quality for JT CoMP even for
long (>20 ms) system delays at 2.66 GHz at pedestrian velocities or, for lower
delays, at 500 MHz, at vehicular velocities.
A user grouping and resource allocation scheme that provides appropriate groups
for CoMP is also suggested. It provides performance close to that obtained
by exhaustive search at very low complexity, low feedback cost and very
low backhaul cost. Finally, a robust linear precoder that takes channel
uncertainties into account when designing the precoding matrix is considered.
We show that, in challenging scenarios, this provides large gains compared
with zero-forcing precoding.
Evaluations of these design elements are based
on measured channels with realistic noise and intercluster interference assumptions.
These show that high JT CoMP gains can be expected, on average over large sets
of user positions, when the above techniques are combined - especially in
severely intracluster interference limited scenarios.
