Service manual

WLAN TX Measurements

R&S

FSV-K91/91n/91ac/91p

37Operating Manual 1176.7649.02 ─ 04

In contrast the I/Q Offset, Gain Imbalance and Quadrature Offset results are calculated

for the transmit antenna signals. Ie they are corresponding to the physical channel.

Phy

= H

eff

-1

Ie in order to determine the physical channel from the effective channel,

the precoding matrix Q (spatial mapping) must be invertible. Note that “transmit

antenna signals” means the ideal transmit signal so that the estimated channel transfer

functions include cross talk (between the antennas) introduced by the DUT, the con-

nection between DUT and Analyzer and the Analyzer itself (whereas we regard the

cross terms contributed by the cable connection and the Analyzer hardware as to be

negligible).

Furthermore the spectral results (channel flatness and group delay) are available for

the effective and the physical channel, i.e. based on streams or Tx antennas. Note that

the physical channel is not in any case derivable from the initially estimated effective

channel (but only if the precoding matrix is invertible) and thus the physical channel

results are only available under specific conditions. In contrast the effective channel

results are always available. This can be explained by the fact, that the channel esti-

mation is done on the HT-LTF fields of the preamble (see figure below), which are

transmitted by one individual (known) preamble symbol per each space time stream

and therefore allowing to estimate the channel map between Rx Antennas and space

time streams in the first step:

Fig. 3-5: Possible results and Channel Representation (effective / physical)

The so estimated effective channel (using HT-LTF fields as described above) can then

be transformed into the physical channel (map between Rx and Tx Antenna signals) by

applying the inverse mapping matrix Q. Now it is clear, that the physical channel can

only be calculated if Q can be inverted. For example this is not the case if the signal

contains less space time streams than Tx antennas.

3.6.1

Trigger Synchronization Using an R&S

FS-Z11 Trigger Unit

For simultaneous MIMO measurements, it is important to analyze the Tx antenna sig-

nals sent at the same instant of time from the Device Under Test (DUT). The R&S

FS-

Z11 Trigger Unit can ensure that all analyzers start capturing I/Q data at the same

time.

IEEE 802.11n/ac MIMO Measurements