Service manual

WLAN TX Measurements
R&S
®
FSV-K91/91n/91ac/91p
22Operating Manual 1176.7649.02 ─ 04
●
chapter 3.3.1, "Understanding Signal Processing of the IEEE 802.11a Application",
on page 22
●
chapter 3.3.2, "Literature to the IEEE 802.11a Application", on page 29
Abbreviations
a
l,k
symbol at symbol l of subcarrier k
EVM
k
error vector magnitude of subcarrier k
EVM error vector magnitude of current packet
g signal gain
Δf
frequency deviation between TX and RX
l symbol index l = [1, nof_Symbols]
nof_symbols number of symbols of payload
H
k
channel transfer function of subcarrier k
k channel index k = [–31,32]
K
mod
modulation-dependent normalization factor
ξ relative clock error of reference oscillator
r
l,k
subcarrier of symbol l
3.3.1 Understanding Signal Processing of the IEEE 802.11a Application
A diagram of the interesting blocks is shown in figure 3-1. First the RF signal is down
converted to the IF frequency f
IF
= 96 MHz. The resulting IF signal r
IF
(t) is shown on the
left-hand side of the figure. After bandpass filtering, the signal is sampled by an Analog
to Digital Converter (ADC) at a sampling rate of f
s1
= 128 MHz. This digital sequence is
resampled. Thus the sampling rate of the down sampled sequence r(i) is the Nyquist
rate of f
s3
= 20 MHz. Up to this point the digital part is implemented in an ASIC.
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 implemen-
ted in the time domain. The algorithm is based on cyclic repetition within the LS after N
= 64 samples. Numerous treatises exist on this subject, e.g. [1] to [3].
Furthermore a coarse estimate Δ
coarse
of the Rx-Tx frequency offset Δf is derived from
the metric in [6]. (The hat generally indicates an estimate, e.g. is the estimate of x.)
This can easily be understood because the phase of r(i) Δ r* (i + N) is determined by
the frequency offset. As the frequency deviation Δf can exceed half a bin (distance
between neighboring 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 cal-
culation. This is achieved by first estimating the coarse frequency response Ĥ
(LS)
k
, with
k = [–26, 26] denoting the channel index of the occupied sub-carriers.
Signal Processing of the IEEE 802.11a Application