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Modeling and Testing an NR RF Receiver with LTE Interference

The example shows how to characterize the impact of radio frequency (RF) impairments in the RF reception of a new radio (NR) waveform when coexisting with a long-term evolution (LTE) interference. The baseband waveforms are generated using 5G Toolbox™ and LTE Toolbox™, and the RF receiver is modeled using RF Blockset™.

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:

  • Error vector magnitude (EVM): vector difference at a given time between the ideal (transmitted) signal and the measured (received) signal.

  • Adjacent channel leakage ratio (ACLR): measure of the amount of power leaking into adjacent channels. It is defined as the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency.

  • Occupied bandwidth: bandwidth that contains 99% of the total integrated power of the signal, centered on the assigned channel frequency.

  • Channel power: filtered mean power centered on the assigned channel frequency.

  • Complementary cumulative distribution function (CCDF): probability of a signal's instantaneous power to be a level specified above its average power.

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:

  1. Generate the baseband NR waveform using 5G Toolbox.

  2. Generate the baseband LTE waveform (interference) using LTE Toolbox.

  3. Upconvert both waveforms to their carrier frequencies using RF Blockset.

  4. Use an RF receiver to downconvert the waveform centered at the NR carrier to baseband frequency.

  5. Calculate the ACLR/ACPR, occupied bandwidth, channel power, and CCDF using the Spectrum Analyzer block.

  6. Demodulate the NR baseband waveform to measure EVM.

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:

  • Baseband Generation: generates the baseband NR and LTE waveforms.

  • RF Reception: the upconverted waveforms pass through an RF receiver which downconverts the waveform centered at the NR carrier.

  • NR Baseband Reception and Measurements: performs the RF measurements and demodulates the baseband waveform to calculate the EVM.

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:

  • Bandpass filters

  • Low noise and power amplifiers

  • IQ demodulator consisting of mixers, phase shifter, and local oscillator

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:

  • I/Q imbalance as a result of gain or phase mismatches between the parallel sections of the receiver chain dealing with the IQ signal paths.

  • Phase noise as a secondary effect directly related to the thermal noise within the active devices of the oscillator.

  • PA nonlinearities due to DC power limitation when the amplifier works in saturation region.

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:

  • EVM per OFDM symbol: EVM averaged over each OFDM symbol.

  • EVM per slot: EVM averaged over the allocated PDSCH symbols within a slot.

  • EVM per subcarrier: EVM averaged over the allocated PDSCH symbols within a subcarrier.

  • Overall EVM: EVM averaged over all the allocated PDSCH symbols transmitted.

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.

  • Without LTE interference (Interferer gain = 0). To eliminate the LTE interference, set the Interferer gain parameter of the Waveform Parameters block to 0. To simulate a whole frame, run the simulation long enough to capture 10 subframes (10 ms). During simulation, the model displays the EVM and ACLR measurements and the constellation diagram. These are the results when transmitting 2 subframes.

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.

  • With LTE interference (Interferer gain = 1). To activate the LTE interference, set the Interferer gain parameter of the Waveform Parameters block to any available value different from 0. For example, choose value 1.

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:

  • Increase I/Q imbalance by using the I/Q gain mismatch (dB) and I/Q phase mismatch (Deg) parameters on the IQ Demodulator tab of the RF Receiver block.

  • Increase the phase noise by using Phase noise offset (Hz) and Phase noise level (dBc/Hz) parameters on the IQ Demodulator tab of the RF Receiver block.

  • Reduce the input back-off of the two amplifiers (PA I and PA Q) increasing the Gain (dB) parameter on the LNA tab of the RF Receiver block.

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

  1. 3GPP TS 38.141-1. "NR; Base Station (BS) conformance testing Part 1: Conducted conformance testing." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

  2. 3GPP TS 36.141 "E-UTRA; Base Station (BS) conformance testing" 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

  3. 3GPP TS 38.104. "NR; Base Station (BS) radio transmission and reception." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

  4. 3GPP TS 38.101-1. "NR; User Equipment (UE) radio transmission and reception." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

  5. 3GPP TS 36.101. "E-UTRA; User Equipment (UE) radio transmission and reception." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

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