MIMO Decoding (LTE)

Default: 3GPP MIMO Multiple Input, Multiple Output: A physical layer (PHY) configuration in which both transmitter and receiver use multiple antennas. Decoding

Range: 3GPP MIMO Decoding, No Decoding

MIMO 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 IQ Meas data (error vector traces).

MIMO Decoding applies only to channels that have undergone Tx Diversity or Spatial Multiplexing precoding.

3GPP MIMO Decoding

When 3GPP MIMO Decoding is selected, the data points shown on the IQ Meas 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 IQ Meas. 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 IQ Meas 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 Physical Downlink Shared Channel). 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 Cell-specific 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 No Decoding is selected, no decoding or cross-channel equalization will be performed on the measured IQ data. This means that, for LTE Long Term Evolution 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 Quadrature phase shift keying points. The resulting diagram will be a 9QAM constellation. These are effectively the points that were transmitted on the OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. subcarriers.

Reference C-RS/Rx path equalization will still be performed when Equalizer Training is enabled (set to RS or RS+Data).

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 No Decoding 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 Radio Frequency: A generic term for radio-based technologies, operating between the Low Frequency range (30k Hz) and the Extra High Frequency range (300 GHz). transmit chain.

When No Decoding is selected, EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. 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 No Decoding.