<s>
Zero-forcing	O
(	O
or	O
null-steering	O
)	O
precoding	B-Protocol
is	O
a	O
method	O
of	O
spatial	O
signal	O
processing	O
by	O
which	O
a	O
multiple	O
antenna	O
transmitter	O
can	O
null	O
the	O
multiuser	O
interference	O
in	O
a	O
multi-user	O
MIMO	O
wireless	O
communication	O
system	O
.	O
</s>
<s>
where	O
is	O
the	O
vector	O
of	O
transmitted	O
symbols	O
,	O
is	O
the	O
noise	O
signal	O
,	O
is	O
the	O
channel	O
vector	O
and	O
is	O
some	O
linear	O
precoding	B-Protocol
vector	O
.	O
</s>
<s>
is	O
the	O
precoding	B-Protocol
matrix	O
,	O
</s>
<s>
If	O
the	O
transmitter	O
knows	O
the	O
downlink	O
channel	O
state	O
information	O
(	O
CSI	O
)	O
perfectly	O
,	O
ZF-precoding	O
can	O
achieve	O
almost	O
the	O
system	O
capacity	O
when	O
the	O
number	O
of	O
users	O
is	O
large	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
with	O
limited	O
channel	O
state	O
information	O
at	O
the	O
transmitter	O
(	O
CSIT	O
)	O
the	O
performance	O
of	O
ZF-precoding	O
decreases	O
depending	O
on	O
the	O
accuracy	O
of	O
CSIT	O
.	O
</s>
<s>
ZF-precoding	O
requires	O
the	O
significant	O
feedback	O
overhead	O
with	O
respect	O
to	O
signal-to-noise-ratio	O
(	O
SNR	O
)	O
so	O
as	O
to	O
achieve	O
the	O
full	O
multiplexing	O
gain	O
.	O
</s>
