IQ Meas (LTE)

In downlink mode, IQ Meas shows the IQ values for channels/signals in the selected layer. When the LTE Long Term Evolution signal contains only one layer, IQ Meas shows the values of the subcarriers from 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. symbol FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum..

When analyzing channels transmitted on C-RS Cell-specific RS antenna ports with transmit diversity or spatial multiplexing, this trace shows the channel data values after precoding has been removed. When precoding is removed, the subcarrier notation does not apply to channel data values since the values do not have a one-to-one correspondence with on-air subcarriers. See the MIMO Decoding topic for more information.

For downlink signals, PDSCH Physical Downlink Shared Channel IQ data used in calculating this trace comes from the layer chosen when selecting this trace. Other LTE channels/signals are included in Layer traces for convenience. For more details, see the Layer Traces and Resource Element Distribution topic.

For uplink signals, the IQ data comes directly from the OFDM symbol FFT, except for PUSCH Physical Uplink Shared Channel. PUSCH subcarriers are despread before being used in calculations.

To view the on-air subcarrier values for signals with multiple C-RS antenna ports, set the MIMO Decoding parameter to No Decoding. When this is done, only the reference measurement channel will be analyzed, and each layers' traces will be identical to the other layers' traces.

In uplink mode, when PUSCH is included in analysis, IQ Meas shows PUSCH IQ values after despreading (IFFT Inverse Fast Fourier Transform), overlaid on the the other channels/signals' reference IQ values, which come directly from the output of the symbol FFT. See the SC-FDMA Symbol Points section below for more information.

To view frequency-domain data points for PUSCH, use the IQ Freq Meas or IQ Freq Ref trace.

Trace information

Only the points of channels/signals selected in Composite Include are shown on this trace. The color of a point indicates which channel/signal the point belongs to. The colors of the channel/signals are shown in the Frame Summary trace.

In the constellation (Const) format, reference constellation circles/crosses are drawn where the symbol points are expected to fall only when Power Boost Normalize is selected.

With the IQ Meas constellation view, you can get a feel for how close the constellation points are to the reference. And, you can turn off various corrections such as Equalizer Training and EVM Minimization to see how well, and in what way, they are correcting the signal.

With the exception of PUSCH described above, the data in IQ Meas (which comes from the data shown in the Time trace) is a 2x2 matrix with frequency along one dimension and time along the other. In addition, each one of the points in the matrix is a complex value; therefore there are 4 total dimensions. The choice of trace format determines which two dimensions will be on the x-y plane, and which dimensions will be overlapped, averaged, or ignored. The relevant trace formats and their corresponding view of the data are described below.

Constellation, IQ - The I-Q plane is mapped to the x-y plane and each point contains both a subcarrier and a symbol-time reference. In other words, each point plotted on the complex plane came from a complex value transmitted on a specific subcarrier at a certain time.
LogMag, LinMag, Real, Imag, Wrapped Phase, Unwrapped Phase - Subcarriers are plotted along the x-axis. All the modulation symbols transmitted on a subcarrier have been plotted above the corresponding subcarrier tick on the x-axis, in the specified format (whether it be dB magnitude or the real value of the symbol point, etc.).

SC-FDMA symbol points

SC-FDMA Single Carrier - Frequency Division Multiple Access modulation is performed by the following process:

The data is

modulated onto a single carrier using QPSK Quadrature phase shift keying, QAM16, or QAM64
transformed to the frequency domain by an FFT
mapped to the desired OFDM subcarriers
transformed back into the time domain and transmitted

IQ Meas shows the IQ symbol data for PUSCH in the time domain (step 1) before being spread in frequency by steps 2-4.

See the uplink modulation section of the LTE Physical Layer Overview for more information about SC-FDMA Frequency Division Multiple Access: Method of allowing multiple users to share the radio frequency spectrum by assigning each active user an individual frequency channel. In this practice, users are dynamically allocated a group of frequencies so that the apparent availability is greater than the number of channels. modulation.