MIMO Decoding (LTE)
3GPP MIMO Decoding
3GPP MIMO Decoding, No Decoding
determines how much of the transmit chain is decoded by the demodulator. The selection of this parameter directly affects what values are shown on the IQ Meas trace and all other traces that depend on the data (error vector traces).
applies only to channels that have undergone Tx Diversity or Spatial Multiplexing precoding.
3GPP MIMO Decoding
When is selected, the data points shown on the trace are equivalent to the data points before precoding was applied in the transmit chain. In other words, the demodulator will undo MIMO precoding and show the results on . Although the data points are mapped onto "subcarriers" when being shown on the layer traces, the data points do not have a one-to-one correspondence to the transmitted subcarrier that they are mapped onto. For instance, when there is a frequency null that affects a subcarrier, there will be several (depending on the precoding) data points in that are affected. Another way of looking at this is that each transmitted subcarrier contains information from multiple data points after precoding is performed (this does not apply to channels and signals which do not undergo precoding).
For exact information on how the different channels and signals are mapped to Layer traces, see the Layer Traces and Resource Element Distribution topic.
Transmit Diversity
For physical-layer channels that undergo transmit diversity, the demodulator will undo transmit diversity precoding, undo codeword-to-layer mapping, and show the resulting codeword data points in their respective resource elements, copied on all layer traces (except for PDSCH). That is, constellation points on layer traces for transmit diversity-precoded channels will be the same for all layer traces.
When a signal uses Tx Diversity, the amount of data transmitted is not increased, but the reliability of the signal is increased by transmitting multiple copies of the data.
In two Tx Antenna mode, each C-RS antenna port transmission carries enough information to determine all the data.
In four Tx Antenna mode, each C-RS antenna port transmission only carries enough information to determine half the data. Any data that cannot be determined from the detected antenna ports will be considered part of Non-Alloc signals and shown as blanks on the Symbol Table (unless the Non-Alloc parameter is selected; then the data will be shown as gray zeros).
Spatial Multiplexing
For channels that undergo spatial multiplexing, the demodulator will only undo Spatial Multiplexing precoding and show the layer data points in their respective resource elements on the appropriate layer traces.
For precoded channels, subcarrier points on the layer traces do not have a one-to-one correspondence to on-air subcarriers. Rather, each subcarrier point is actually the demodulated value of a codeword data point that was present prior to the codeword-to-layer mapping at the transmitter.
No Decoding
When is selected, no decoding or cross-channel equalization will be performed on the measured IQ data. This means that, for LTE signals that have been precoded (multi-antenna signals), subcarrier points shown on measured IQ traces (IQ Meas and IQ Meas Time) will actually be an addition of multiple modulation points, resulting in non-standard constellations.
For example, in a two antenna port signal, there will be subcarrier points that are an addition of two QPSK points. The resulting diagram will be a 9QAM constellation. These are effectively the points that were transmitted on the OFDM subcarriers.
Reference C-RS/Rx path equalization will still be performed when Equalizer Training is enabled (set to or ).
Only one VSA measurement channel is analyzed. The signal from the reference measurement channel will be equalized using C-RS from the reference C-RS/Rx path. The points on layer traces will correspond to actual subcarriers, and each set of layer traces will be identical (Layer n trace = Layer m trace).
The selection is useful for the case that you have four antenna signals, and you want to isolate channel effects from transmit chain effects (filters, mixers, etc.). You could connect each transmit port directly to your measurement instrument with identical cables. That way, any observed anomalies would come primarily from the RF transmit chain.
When is selected, EVM results will not be relevant since the ideal symbol points (shown on the IQ Ref and IQ Ref Time), which are used to compute EVM, would still be standard constellation points and hence may not match the non-standard constellation points of IQ Meas arising due to .
See Also
