WLAN TX Measurements (Option K91) R&S FSL
1300.2519.12 2.130 E-11
frequency
c
ompensation
F
IR
0
F
FT
estimation
o
f
gain, frequency, time
user defined
c
ompensation
payload
w
indow
packet search:
1
.coarse timing
2.fine timing
t
iming
L
S
p
ilot
table
16.4 MHz
estimate
data symbols
c
hannel
estimation
measurement
o
f
parameters
pilots + data
f
s
3
=20MHz
f
e
-j
I
F
·kT
S
2
!
H
F
IR
( f )
4
"
r(i)
r
l,k
a
l
,k
r'
l,k
data
r
''
l,k
1
f
s
1
= 81.6 MHz
r
I
F
(t)
~
~
~
Resampler
f
s
2
=
80 MHz
ADC
full
compensation
N
= 64
f
coarse
H
k
(
LS
)
g
l
H
k
(PL)
f
r
es
t
,
d
#
l
l
H
k
H
k
a
l
,k
Fig. 2-87 Signal processing of the IEEE 802.11a application
In the lower part of the figure the subsequent digital signal processing is shown. In the first block the
packet search
is performed. This block detects the Long Symbol (LS) and recovers the timing. The
coarse timing
is detected first. This search is implem ented in the time domain. The algorithm is based on
cyclic repetition within the LS after
64
N samples. Numerous treatises exist on this subject, e.g. [1] to
[3]. Furthermore a coarse estimate
coarse
f
ˆ
1
o f t h e R x –T x f r eq ue n c y o f fs e t f is derived from the metric in
[6]. This can easily be understood because the phase of )()(
*
Nirir + is determined by the frequency
offset. As the frequency deviation
f can exceed half a bin (distance between neighbor sub–carriers)
the preceding Short Symbol (SS) is also analyzed in order to detect the ambiguity.
After the coarse timing calculation the time estimate is improved by the fine
timing calculation. This is
achieved by first estimating the coarse frequency response
)LS(
ˆ
k
H , with ]26,26[=k denoting the channel
ind e x o f th e o c c up ie d s u b –c ar ri er s. Firs t the F FT o f th e L S i s ca lc ulate d . Af t e r t h e F FT c a lc u lat i o n t h e k no wn
symbol information of the LS sub–carriers is removed by dividing by the symbols. The result is a coarse
estimate
k
H
ˆ
of the channel transfer function. In the next step the complex channel impulse response is
computed by an IFFT. Next the energy of the windowed impulse response (the window size is equal to
the guard period) is calculated for every trial time. Afterwards the trail time of the maximum energy is
detected. This trial time is used to adjust the timing.
Now the position of the LS is known and the starting point of the useful part of the first payload symbol
can be derived. In the next block this calculated time instant is used to position the payload window
.
Only the payload part is windowed. This is sufficient because the payload is the only subject of the
subsequent measurements.
In the next block the windowed sequence is compensated
by the coarse frequency estimate
coarse
f
ˆ
. This is
necessary because otherwise inter channel interference (ICI) would occur in the frequency domain.
The transition to the frequency domain is achieved by an FFT
of length 64. The FFT is performed
symbol–wise for every of the
symbolsnof _ symbols of the payload. The calculated FFTs are described
by
kl
r
,
with
the symbol index ]_,1[ symbolsnofl = and
the channel index
]32,31[ =k .
1
The hat generally describes an estimate. Example: x
is the estimate of x.
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