Service manual
WLAN TX Measurements
R&S
®
FSV-K91/91n/91ac/91p
28Operating Manual 1176.7649.02 ─ 04
(10) it is clear that first the channel transfer function H
k
must be removed. This is ach-
ieved by dividing the known coarse channel estimate Ĥ
(LS)
k
calculated from the LS.
Usually an error free estimation of the data symbols can be assumed.
In the next block a better channel estimate Ĥ
(PL)
k
of the data and pilot sub-carriers is
calculated by using all nof_symbols symbols of the payload (PL). This can be accom-
plished at this point because the phase is compensated and the data symbols are
known. The long observation interval of nof_symbols symbols (compared to the short
interval of 2 symbols for the estimation of Ĥ
(LS)
k
) leads to a nearly error-free channel
estimate.
In the following equalizer block Ĥ
(LS)
k
is compensated by the channel estimate. The
resulting channel-compensated sequence is described by γ
δ,k
''
. The user may either
choose the coarse channel estimate Ĥ
(LS)
k
(from the long symbol) or the nearly error-
free channel estimate Ĥ
(PL)
k
(from the payload) for equalization. If the improved esti-
mate Ĥ
(LS)
k
is used, a 2 dB reduction of the subsequent EVM measurement can be
expected.
According to the IEEE 802.11a measurement standard [6], the coarse channel estima-
tion Ĥ
(LS)
k
(from the long symbol) has to be used for equalization. Therefore the default
setting of the R&S FSV-K91/91n is equalization from the coarse channel estimate
derived from the long symbol.
In the last block the measurement variables are calculated. The most important varia-
ble is the error vector magnitude of the sub-carrier "k" of the current packet:
packetsnof
counter
counterEVM
packetsnof
EVM
_
1
2
)(
_
1
(14) (3 - 6)
Furthermore, the packet error vector magnitude is derived by averaging the squared
EVM
k
versus k:
26
)0(26
2
52
1
kk
k
EVMEVM
(15) (3 - 7)
Finally, the average error vector magnitude is calculated by averaging the packet EVM
of all nof_symbols detected packets:
symbolsnof
l
klklk
aKr
symbolsnof
EVM
_
1
2
,mod
''
,
_
1
(16) (3 - 8)
This parameter is equivalent to the so-called "RMS average of all errors": Error
RMS
of
the IEEE 802.11a measurement commandment (see [6], ).
Signal Processing of the IEEE 802.11a Application