WiMAX, WiBro Measurements (Options K92/K93) R&S FSL
1300.2519.12 2.156 E-11
Synchronization
The first of the synchronization tasks is to detect areas of sufficient power within the captured I/Q data
stream. The subframe detection block determines the beginning and end of each subframe and
coarsely estimates both timing and carrier frequency offset. The fine timing block prior to the FFT allows
a timing improvement using a level–based search for the beginning and end of the coarsely estimated
channel impulse response. In the DL the coarse estimate of the CIR can be directly obtained from the
preamble. Other than that the UL consists only of payload information with scattered pilots in the
subcarrier–symbol plane, thus several OFDM symbols have to be observed to get a reliable estimate of
the CIR. Since the OFDM symbols need to be phase synchronized prior to the channel estimation, the
blue blocks in Fig. 2-93 have to be carried out twice. In the first iteration the timing estimate
coarse
ˆ
i
is
used to position the window of the FFT. Having found the pilot–based estimate of the CIR, the fine
timing estimate
fine
ˆ
i
is used in the second iteration.
After the time to frequency transformation by an FFT of length
FFT
N , the tracking estimation block is
used to estimate the following:
• relative sampling frequency offset
• residual carrier frequency offset
res
f
• common phase error
l
1
• gain
l
g
Corresponding to [3] and [4], the uncompensated samples
kl
r
,
can be expressed as
{
kl
lTfNNlkNN
klkllkl
nHagr
l
,
CFO res.
2j
SFO
2j
C
PE
j
,,,
r
esFFTsFFTs
eee +=
1
44 344 2144 344 21
0
(36)
with
• data symbol
kl
a
,
on subcarrier k at OFDM symbol l
• channel transfer function
kl
H
,
• number of Nyquist samples
s
N within the symbol time
s
T
• useful symbol time
gs
TTT =
•
independent and Gaussian distributed noise sample
kl
n
,
Within one OFDM symbol both the CPE and the residual CFO respectively cause the same phase
rotation for each subcarrier, while the rotation due to the SFO linearly depends on the subcarrier index.
A linear phase increase in symbol direction can be observed for the residual CFO as well as the SFO.
The results of the tracking estimation block are used to compensate the samples
kl
r
,
. While a full
compensation is performed in the reference path, the signal impairments that are of interest to the user
are left uncompensated in the measurement path.
Channel Estimation / Equalization
According to Fig. 2-93, there are two coarse and one fine channel estimation blocks. Which of the two
coarse estimation blocks is used depends on the link direction. For DL subframes the coarse channel
estimation is based on the preamble and directly follows the coarse frequency compensation block. The
pilot–based estimation for UL subframes is tapped behind the full compensation block of the reference
path. Both of the coarse estimation blocks use available training symbols to determine initial estimates
kl
H
,
ˆ
of the channel transfer function at fixed positions in the subcarrier–symbol plane. Based on these
nodes, the missing CTF values are obtained by interpolation in both time and frequency direction. The
coarse estimation results are used for the above mentioned fine timing and to equalize the samples
kl
r
,
of the reference path prior to symbol decision. Based on the decided data symbols, a fine channel
estimation is performed and then used to equalize the partially compensated samples of the
measurement path.
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