  Most multiuser precoding techniques require accurate transmitter channel
state information (CSIT) to maintain orthogonality between the users. Such
techniques have proven quite fragile in time-varying channels because the CSIT
is inherently imperfect due to estimation and feedback delay, as well
quantization noise. An alternative approach recently proposed by Maddah-Ali and
Tse (MAT) allows for significant multiplexing gain in the multi-input
single-output (MISO) broadcast channel (BC) even with transmit CSIT that is
completely stale, i.e. uncorrelated with the current channel state. With $K$
users, their scheme claims to lose only a $\log(K)$ factor relative to the full
$K$ degrees of freedom (DoF) attainable in the MISO BC with perfect CSIT for
large $K$. However, their result does not consider the cost of the feedback,
which is potentially very large in high mobility (short channel coherence
time). In this paper, we more closely examine the MAT scheme and compare its
DoF gain to single user transmission (which always achieves 1 DoF) and partial
CSIT linear precoding (which achieves up to $K$). In particular, assuming the
channel coherence time is $N$ symbol periods and the feedback delay is $N_{\rm
fd}$ we show that when $N < (1+o(1)) K \log K$ (short coherence time), single
user transmission performs best, whereas for $N> (1+o(1)) (N_{\rm fd}+ K / \log
K)(1-\log^{-1}K)^{-1}$ (long coherence time), zero-forcing precoding
outperforms the other two. The MAT scheme is optimal for intermediate coherence
times, which for practical parameter choices is indeed quite a large and
significant range, even accounting for the feedback cost.
