WiMAX, WiBro Measurements (Options K92/K93) R&S FSL
1300.2519.12 2.148 E-11
compensated from the pilots. Therefore the "symbol wise phase tracking'' (Tracking Phase) is activated
a
s the default setting of the R&S FSL–K92/K93.
Furthermore the timing drift in equation (29) is given by
lkNNphase
skl
=
2
)timing(
,
(32)
with
being the relative clock deviation of the reference oscillator. Normally a symbol–wise timing jitter
is negligible and thus not modeled in equation (32). There may be situations where the timing drift has
to be taken into account. This is illustrated by an example: In accordance to [6] the allowed clock
deviation of the DUT is up to
=
max
8 ppm. Furthermore the maximal length of a frame
2420_ =symbolsnof symbols
6
is assumed. From equations (29) and (32), it results that the phase drift
of the highest sub–carrier
100=k in the last symbol symbolsnofl _= is to–do degrees. Even in the
noise–free case, this would lead to symbol errors. The example shows that it is actually necessary to
estimate and compensate the clock deviation, which is accomplished in the next block.
Referring to the IEEE 802.16–2004 measurement standard [6], the timing drift
)timing(
,
kl
phase is not part of
the requirements. Therefore the "time tracking'' (Tracking Time) is not activated as the default setting of
the R&S FSL–K92/K93.
The time tracking option should rather be seen as a powerful analyzing option.
In addition the tracking of the gain
l
g in equation (29) is supported for each symbol in relation to the
reference gain
1
g
at the time instant of the Short Preamble (SP). At this time the coarse channel
transfer function
)(
ˆ
SP
k
H is calculated. This makes sense since the sequence
k
l
r
,
' is compensated by the
coarse channel transfer function
)(
ˆ
SP
k
H before estimating the symbols. Consequently a potential change
of the gain at the symbol
l (caused, for example, by the increase of the DUT amplifier temperature)
may lead to symbol errors especially for a large symbol alphabet
of the MQAM transmission. In this
case the estimation and the subsequent compensation of the gain are useful.
Referring to the IEEE 802.16–2004 measurement standard [6], the compensation of the gain
l
g is not
part of the requirements. Therefore the "gain tracking'' (Tracking Gain) is not activated as the default
setting of the R&S FSL–K92/K93.
The unknown deviations of gain, frequency and time are calculated by an optimum maximum likelihood
procedure, which works well even at low signal to noise ratios with the Cramer Rao Bound being
reached. After estimation of these parameters, the received signal is fully compensated for the decision
of the ideal reference signal
kl
a
,
ˆ
and compensated according to the user settings to get the
measurement signal
kl
r
,
' . Then the measurement signal is equalized by the inverse channel transfer
function. According to the chosen setting, either the preamble estimation of the channel transfer
function or a data aided estimation using the ideal reference signal is used. According to the IEEE
802.16–2004 measurement standard [6], the coarse channel estimation
)(
ˆ
SP
k
H (from the short
preamble) has to be used for equalization. Therefore the default setting of the R&S FSL–K92/K93 is
equalization from the coarse channel estimate derived from the short preamble.
In the last block the measurement variables
are calculated. The most important variable is the error
vector magnitude
2
,mod,
_
1
_
1
''
klkl
Symbolsnof
l
Symbolsnof
k
aKrEVM =
%
=
(33)
of the sub–carrier
k of the current packet. Furthermore the packet error vector magnitude
6 Assuming the maximal System Sampling Rate Fs = 32MHz.
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