R&S
®
CMW 500 WiMAX Applications
WiMAX TX Measurements
Operating Manual 1202.3986.32 – 03 375
estimation is fed into the ACF stage. The phase at the maximum of the auto correlation
function is a measure for the fine frequency offset f
fine
.
The fine frequency offset is used to correct the time-domain samples prior to the Fast
Fourier Transform block (FFT).
Channel and Symbol Estimation
The channel estimation block determines the channel transfer function H
lk
for all
symbols and subcarriers. The channel transfer function is needed to calculate the
spectral flatness.
Channel estimation for the uplink (UL) signal is based on the transmitted pilot symbols.
The received signal is tapped behind the FFT block. An UL subchannel is composed of
"tiles" with four pilot subcarriers and eight data subcarriers as shown below (see Frame
Settings and Burst Selection). The channel transfer function is first estimated for the
known pilot symbols (training symbols). Based on these values, the channel transfer
function for data symbols is obtained by linear interpolation in both time (symbol) and
frequency (sub-carrier) direction. The average channel transfer function H
m
for tile no.
m can then be calculated as the average of the interpolated values over the second
(central) line, corresponding to the second symbol in the tile. This average channel
transfer function is calculated tile by tile.
The R&S CMW 500 is able to decode the three different modulation schemes used in
the OFDMA UL signal: QPSK, 16-QAM and 64-QAM. Decoding involves an
assignment of the estimated symbol points â
lk
to the ideal constellation points a
lk
. The
assignment is based on the decision lines shown in the figures below. The hard-
decided symbols a
lk
are used as a reference for further calculations.
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