R&S FSV-K70, Руководство пользователя

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Brief Description of Vector Signal Analysis
R&S
®
FSV-K70
47 Operating Manual 1176.7578.02 ─ 09
(i.e. modulation scheme and transmit filter). The IQ pattern search can also be referred
to as the I/Q waveform. An I/Q pattern is considered detected if the correlation metric,
i.e. the correlation value between the ideal IQ pattern and capture buffer, exceeds a
specified "I/Q Correlation Threshold" (see
"I/Q Correlation Threshold" on page 161.)
If the burst search is switched on, the I/Q pattern search only searches the I/Q pattern
in bursts previously detected by the burst search. Furthermore, it only finds the first I/Q
pattern within each burst. If the burst search is switched off, the I/Q pattern search
searches for the I/Q pattern in the entire capture buffer.
2.4.3 Demodulation and Symbol Decisions
This stage operates on the Result Range and aims to make the correct symbol deci-
sions. The algorithm is illustrated in
Figure 2-45 using the example of a QPSK modula-
tion. After timing and scaling recovery, a frequency offset and phase offset estimator is
employed.
After this coarse synchronization, the R&S FSV-K70 option makes symbol decisions,
i.e. recovers which symbols were transmitted by the device under test (DUT).
Typically, the employed estimators are "non-data-aided" (NDA) estimators. This means
that they operate on an unknown data sequence. Since the local oscillators (LO) of the
transmitter (device under test) and the receiver (R&S
FSVA/FSV) are normally not cou-
pled, their phase offset with respect to each other is unknown. The unknown transmis-
sion delay between DUT and R&S
FSVA/FSV adds a further unknown phase offset.
Due to this unknown phase offset, the result of the demodulation can be ambiguous
with respect to the absolute phase position because of the rotational symmetry of e.g.
a PSK constellation. For example, in the case of non-differential QPSK modulation, the
measurement signal, the reference signal and the decided IQ symbols may have a
constant phase offset of {0,
π
/2,
π
, or 3
π
/2}. This offset can only be detected and elimi-
nated if a pattern was successfully detected at symbol level (see also Chapter 2.4.4,
"Pattern Symbol Check" , on page 49).
If modulation types are used where the information is represented by the phase transi-
tion, e.g. differential PSK or MSK, the absolute phase position is not an issue. Thus,
the ambiguity of the starting phase does not have an influence on the symbol deci-
sions.
If the measurement signal contains a known pattern, it is also possible to use a "data-
aided" (DA) estimator at this stage. This means that the estimator operates on a known
data sequence, i.e. the pattern. If the signal contains a pattern, it is possible to choose
between the above-described non-data-aided estimator and the data-aided estimator
with the setting "
Coarse Synchronization : Pattern". If the data-aided estimator is
employed, the phase ambiguitiy can be resolved at this stage.
Demodulation Overview