Introduction

This example characterizes the impact of LTE interference in the RF reception of an NR waveform. To evaluate the impact of the interference, the example performs these measurements:

The impact of the receiver RF impairments such as in-phase and quadrature (IQ) imbalance, phase noise, and power amplifier (PA) nonlinearities are also considered.

The example works on a subframe by subframe basis. For each subframe, the workflow consists of these steps:

This example uses a Simulink model to perform these operations. Baseband signal processing (steps 1, 2, and 6) uses MATLAB® Function blocks, whereas the RF receiver modeling (steps 3 and 4) uses RF Blockset. This model supports Normal and Accelerator simulation modes.

Simulink Model Structure

The model contains three main parts:

modelName = 'NewRadioRFReceiverWithLTEInterferenceModel';
open_system(modelName);

Baseband Generation

The NR-TM Transmission block transmits standard-compliant 5G NR test model (NR-TM) waveforms for frequency range 1 (FR1) and frequency range 2 (FR2) [ 1 ]. For the NR-TM waveform generation, you can specify the NR-TM name, the channel bandwidth, the subcarrier spacing (SCS), the duplexing mode, the cell identity, and the TS 38.141 version using the Waveform Parameters block.

Similarly, the LTE-TM Transmission block transmits standard-compliant LTE-TM waveforms [ 2 ]. You can also specify the TM name, the channel bandwidth, the duplexing mode and the cell identity. This model resamples, if necessary, the LTE waveform to match the sampling rate of the NR waveform. The Waveform Parameter block does not accept an LTE bandwidth wider than the NR bandwidth.

Additionally, the Waveform Parameters block provides the option to enable or disable the ACLR test. When the ACLR measurement is enabled, the waveform is oversampled to visualize the spectral regrowth. The Waveform Parameters block also includes a parameter called Interferer Gain which allows control the linear gain of the LTE interference. To cancel the transmission of the LTE interference, set the Interferer Gain parameter to 0. The Interferer Gain block is connected between the Baseband Generation and the RF Reception stages.

For more information on how to generate NR-TMs and LTE-TMs, see 5G NR-TM and FRC Waveform Generation and LTE Downlink Test Model (E-TM) Waveform Generation (LTE Toolbox), respectively.

As the example works on a subframe by subframe basis, the NR-TM and LTE-TM Transmission blocks generate one subframe at a time. Transmitting ten subframes, corresponding to one frame in the case of FDD duplexing mode, lasts 10 ms. If the simulation time is longer than 10 ms, both blocks transmit the same frame cyclically. The Subframe Counter block stores the number of the currently transmitted subframe. If the simulation time is longer than a frame period, the Subframe Counter block resets to 0.

RF Reception

The RF Receiver block is based on a superheterodyne receiver architecture. This architecture applies passband filtering and amplification and downconverts the received waveform to an intermediate frequency. The RF components of this superheterodyne receiver are:

set_param(modelName,'Open','off');
set_param([modelName '/RF Receiver'],'Open','on');

To send both waveforms to the RF Receiver, insert a Vector Concatenate block between the Baseband Reception and the RF Reception stages. The Vector Concatenate block concatenates both waveforms horizontally, one column per waveform. Then, the Inport block inside the RF Receiver converts the two concatenated complex baseband waveforms into RF signals considering the center frequencies chosen in the Carrier frequencies parameter of this block (every frequency selected in Carrier frequencies is assigned to a different concatenated waveform). The Outport block converts the RF signal back into complex baseband.

Due to the fact that the RF Receiver accepts a maximum of 1024 samples per subframe, the Input Buffer, before the RF Receiver block, reduces the number of samples sent to the RF Receiver. In the current configuration, the Input Buffer sends 1024 samples at a time.

Before sending the samples onto the Decode Subframe block, the Output Buffer (after the RF Receiver) buffers all samples within a subframe and the ADC block digitizes the signal. You can modify the ADC block parameters using its mask.

The Delay block accounts for buffer-induced delays. As the duration of the delay is equivalent to the transmission of a subframe, the Decode Subframe block does not demodulate the first information received during a subframe period.

You can configure the RF Receiver components using the RF Receiver block mask.

The RF Receiver block exhibits typical impairments, including:

NR Baseband Reception and Measurements

The Decode Subframe block performs OFDM demodulation of the received subframe, channel estimation, and equalization to recover and plot the PDSCH symbols in the Constellation Diagram. This block also averages the EVM over time and frequency and plots these values:

According to TS 38.141-1 [ 1 ], not all PDSCH symbols are considered for the EVM evaluation. Using the RNTI, the helper function hListTargetPDSCHs selects the target PDSCH symbols to analyze.

The Spectrum Analyzer block provides frequency-domain measurements such as ACLR (referred to as ACPR), occupied bandwidth, channel power, and CCDF. To visualize the spectral regrowth, the ACLR test oversamples the waveform. The oversampling factor depends on the waveform configuration and should be set such that the generated signal is capable of representing first and second adjacent channels. The ACLR evaluation follows the specifications in TS 38.141-1 [ 1 ].

Model Performance

To characterize the impact of the LTE interference on the NR reception you can compare the EVM and ACLR results for two different cases: 1) there is no interference, for example only the NR waveform; and 2) there is interference, you transmit both waveforms, NR and LTE.

set_param([modelName '/Waveform Parameters'],'InterfererGain','0');
sim(modelName);

--- Starting simulation ---
 Transmitting subframe 0 ...
 Transmitting subframe 1 ...

--- End of simulation ---

According to TS 38.104 [ 3 ], the minimum required ACLR for conducted measurements is 45 dB and the maximum required EVM when the constellation is 256-QAM is 3.5%. As the ACLR values, around 53 dB, are higher than 45 dB, and the overall EVM, around 0.8%, is lower than 3.5%, both measurements fall within the requirements.

set_param([modelName '/Waveform Parameters'],'InterfererGain','1');
sim(modelName);
slmsgviewer.DeleteInstance();

--- Starting simulation ---
 Transmitting subframe 0 ...
 Transmitting subframe 1 ...

--- End of simulation ---

Compared to the previous case, the constellation diagram is more distorted and the spectral regrowth is higher. In terms of the measurements, the ACLR values, around 48 dB, and the overall EVM, around 2%, still fall within the requirements of TS 38.104 [ 3 ].

The RF Receiver is configured to work with the default values of the Waveform Parameters block and with the NR and LTE carriers centered at 2190 MHz and 2120 MHz, respectively. These carriers are within the NR operating band n65 [ 4 ] and the E-UTRA operating band 1 [ 5 ]. If you change the carrier frequency or the values in the Waveform Parameters block, you may need to update the parameters of the RF Receiver components as these parameters have been selected to work for the default configuration of the example. For more information, see the Summary and Further Exploration section of this example.

Summary and Further Exploration

This example demonstrates how to model and test the reception of an NR waveform when coexisting with an LTE waveform. The RF receiver consists of bandpass filters, amplifiers and an IQ demodulator. To evaluate the impact of the LTE interference, the example modifies the gain of the LTE waveform and performs ACLR and EVM measurements. You can explore the impact of altering the RF impairments as well. For example:

Additionally, you can check the occupied bandwidth, the channel power, and the CCDF measurements by using the Spectrum Analyzer block.

If you change the carrier frequencies or the values in the Waveform Parameters block, you may need to update the parameters of the RF Receiver components as these parameters have been selected to work for the default configuration of the example. For instance, a change in the carrier frequencies requires revising the bandwidth of the filters. If you select an NR bandwidth wider than 20MHz, you may need to update the Impulse response duration and Phase noise frequency offset (Hz) parameters of the IQ Modulator block. The phase noise offset determines the lower limit of the impulse response duration. If the phase noise frequency offset resolution is too high for a given impulse response duration, a warning message appears, specifying the minimum duration suitable for the required resolution. For more information, see IQ Modulator (RF Blockset).

This example could be the basis for testing the coexistence between NR-TM and LTE-TM waveforms for different RF configurations. You can try replacing the RF Receiver block by another RF subsystem of your choice and configuring the model accordingly.

References