Service manual
WLAN TX Measurements
R&S
®
FSV-K91/91n/91ac/91p
26Operating Manual 1176.7649.02 ─ 04
The reason for the phase jitter dγ
l
in Equation (11) may be different. The nonlinear
part of the phase jitter may be caused by the phase noise of the DUT oscillator.
Another reason for nonlinear phase jitter may be the increase of the DUT amplifier
temperature at the beginning of the burst. Note that besides the nonlinear part the
phase jitter, dγ
l
also contains a constant part. This constant part is caused by the fre-
quency deviation Δ f
rest
not yet compensated. To understand this, keep in mind that the
measurement of the phase starts at the first symbol l = 1 of the payload. In contrast the
channel frequency response H
k
in Equation (10) represents the channel at the long
symbol of the preamble. Consequently, the frequency deviation Δ f
rest
not yet compen-
sated produces a phase drift between the long symbol and the first symbol of the pay-
load. Therefore, this phase drift appears as a constant value ("DC value") in dϒ
l
.
Referring to the IEEE 802.11a measurement standard Chapter 17.3.9.7 "Transmit
modulation accuracy test'' [6], the common phase drift phase
l
(common)
must be estima-
ted and compensated from the pilots. Therefore this "symbol-wise phase tracking''
(Tracking Phase) is activated as the default setting of the R&S FSV-K91/91n.
Furthermore, the timing drift in Equation (10) is given by:
lkNNphase
skl
/2
)timing(
,
Equation (12) (3 - 3)
with ξ: the relative clock deviation of the reference oscillator
Normally, a symbol-wise timing jitter is negligible and thus not modeled in Equation
(12). However, 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 devi-
ation of the DUT is up to ξ
max
= 20 ppm. Furthermore, a long packet with 400 symbols
is assumed. The result of Equation (10) and Equation (12), is that the phase drift of the
highest sub-carrier k = 26 in the last symbol l = nof_symbols is 93 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 accom-
plished in the next block.
Referring to the IEEE 802.11a measurement standard [6], the timing drift phase
l,k
(timing)
is not part of the requirements. Therefore the "time tracking" (Tracking Time) is not
activated as the default setting of the R&S FSV-K91/91n. The time tracking option
should rather be seen as a powerful analyzing option.
In addition, the tracking of the gain g
l
in Equation (10) is supported for each symbol in
relation to the reference gain g = 1 at the time instant of the long symbol (LS). At this
time the coarse channel transfer function Ĥ
(LS)
k
is calculated.
This makes sense since the sequence r
'
l,k
is compensated by the coarse channel trans-
fer function Ĥ
(LS)
k
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 M of the
MQAM transmission. In this case the estimation and the subsequent compensation of
the gain are useful.
Signal Processing of the IEEE 802.11a Application