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Modeling and Testing an 802.11ax RF Receiver with 5G Interference

The example shows how to characterize the impact of RF impairments, such as phase noise and power amplifier (PA) nonlinearities, in the RF reception of an IEEE® 802.11ax™ waveform coexisting with an adjacent 5G or 802.11ax interferer. The example generates the baseband waveforms by using WLAN Toolbox™ and 5G Toolbox™ and models the RF receiver by using RF Blockset™. The example does not require 5G Toolbox if it models an 802.11ax interferer.

Introduction

This example characterizes the impact of receiver RF impairments, such as phase noise and power amplifier (PA) nonlinearities, and the impact of an adjacent 5G [ 1 ] or 802.11ax [ 2 ] interferer in the RF reception of an 802.11ax waveform. To evaluate the impact of the interference, the example performs these measurements:

  • Error vector magnitude (EVM): vector difference between the ideal (transmitted) signal and the measured (received) signal

  • Adjacent channel rejection (ACR): power difference between the desired signal and an interfering signal in the adjacent channel

  • Packet error rate (PER): number of packets containing errors divided by the total number of received packets

The example works on a packet-by-packet basis. For each desired HE packet, the workflow consists of these steps:

  1. Generate the baseband 802.11ax waveform (desired) by using WLAN Toolbox.

  2. Generate the baseband 5G waveform (interferer) by using 5G Toolbox. You can generate an 802.11ax interferer by using WLAN Toolbox instead. Alternatively, you can remove the interference.

  3. Oversample and filter the waveforms by using a Finite Impulse Response (FIR) Interpolation block.

  4. Measure and display the ACR by calculating the power difference between both waveforms.

  5. Aggregate both waveforms by using the Frequency Shift and Aggregation block.

  6. Convert the baseband waveform into an RF signal by using the RF Receiver block. The block uses an RF carrier frequency to carry the baseband information in RF Blockset. You can select the RF carrier frequency of your choice.

  7. Downconvert the waveform to an intermediate frequency by using an RF superheterodyne receiver. You can accurately model the impairments introduced by an actual RF receiver by using the RF components available in RF Blockset. You can also explore the impact of altering the RF impairments or replace the RF superheterodyne receiver with another RF subsystem of your choice.

  8. Downsample and filter the waveform by using an FIR Decimation block.

  9. Extract the data symbols and measure the EVM by demodulating the baseband waveform.

  10. Calculate the PER by extracting the received bits and comparing them to the transmitted bits.

This example performs these operations by using a Simulink® model. The Simulink model carries out the baseband signal processing (steps 1, 2, 9 and 10) by using MATLAB® Function blocks and performs the RF receiver modeling (steps 6 and 7) by using RF Blockset. This model supports Normal and Accelerator simulation modes.

Simulink Model Structure

The model contains three main parts:

  • Baseband Waveform Generation: generates the baseband 802.11ax and 5G waveforms

  • RF Reception: downconverts the waveform to an intermediate frequency by modeling the effect of RF components

  • Baseband Waveform Reception: calculates EVM and PER

modelName = 'HERFReceiverNRInterfererModel';
open_system(modelName);

Baseband Waveform Generation

The HE Waveform block generates standard-compliant high-efficiency single-user (HE SU) waveforms [ 2 ]. For the waveform generation, set transmission and configuration parameters by specifying options in the HE Waveform block.

The HE Waveform block contains two tabs:

  • HE SU Format: configure the transmission parameters selected in this section by using a wlanHESUConfig object.

  • Generator Configuration: generate each packet, which contains random data, with the wlanWaveformGenerator function for the specified HE SU configuration and parameters.

Similarly, the NR Interferer block transmits standard-compliant 5G NR waveforms for frequency range 1 (FR1) [ 1 ]. For the NR waveform generation, you can specify the channel bandwidth, modulation, subcarrier spacing (SCS), and cell identity in the NR Interferer block. The NR Interferer block transmits a full band and uniform PDSCH. The model resamples the NR waveform so that the sampling rate of the NR waveform matches the sampling rate of the 802.11ax waveform.

Alternatively, you can model an 802.11ax interferer instead of a 5G interferer by selecting Choice_HE in the Variant Source block. The model shifts the HE interferer so that it is not synchronized with the desired HE waveform.

Control the power of both waveforms by setting the HE Gain and Interferer Gain blocks. To cancel the transmission of the interferer, set the Gain parameter of the Interferer Gain block to 0.

After generating the waveforms, a Vector Concatenate block concatenates both waveforms horizontally, one column per waveform. Then, an FIR Interpolation block oversamples and filters the waveforms to show the effect of the nonlinear impairments on the adjacent channels. To capture at least third order and fifth order nonlinearities, oversample the combined bandwidth (both waveforms) around 5 times. As the combined bandwidth is 40MHz by default (20 MHz each waveform and a 20 MHz spacing between them), set an Oversampling factor of 10 to provide a sample rate of 200 MHz, which is 5 times the combined bandwidth. You can set the Oversampling factor in the Multirate Parameters block, which provides an interface to easily configure the parameters of the FIR Interpolation and Decimation blocks.

Once the waveforms have been oversampled, the Frequency Shift and Aggregation block frequency shifts and aggregates them. To measure the ACR, the center frequency of the adjacent channel shall be placed 20, 40, 80, or 160 MHz away from the center frequency of the desired signal [ 2 ]. By default, the example centers the HE waveform at baseband (0 Hz) and sets the spacing between the HE and interfering waveforms to 20 MHz. You can adjust the center frequencies by specifying the Desired output center frequencies (Hz) parameter in the Frequency Shift and Aggregation block. The ACR measurement is displayed in the ACR (dB) block.

Specify Simulation Time

The Packet transmission time ( $\mu s$ ) parameter in the HE Waveform block calculates the time required to transmit each HE packet. Hence, the Stop Time value in the Simulink model must be equal to or higher than the value specified in Packet transmission time ( $\mu s$ ) to obtain the EVM results and constellation diagram of at least one packet. As the filters in the FIR Interpolation and Decimation blocks introduce a delay, you can use the Idle time (s) parameter in the HE Packet block to compensate for the delay.

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:

  • RF and IF bandpass filters

  • Low-noise and IF amplifiers

  • Demodulator consisting of mixers, phase shifter, and local oscillator

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

The Inport block inside the RF Receiver converts the complex baseband waveform into the RF domain. You can vary the center frequency of this RF signal by modifying the Carrier frequency parameter of this block. By default, the Carrier frequency parameter corresponds to the center frequency of the desired HE waveform and the carrier frequency of the NR waveform is located 20 MHz from the HE carrier. The Outport block converts the RF signal back to complex baseband.

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

The RF Receiver block exhibits typical impairments, including:

  • Phase noise as an effect directly related to the thermal noise within the active devices of the oscillator

  • Amplifier nonlinearities due to DC power limitation when the amplifiers work in the saturation region

Use an Input Buffer block before the RF Receiver block to send fewer samples at a time to the RF Receiver block. For simplicity, the Input Buffer in the current configuration sends one sample at a time, resulting in the RF Receiver block being sample-based.

As the current RF Receiver block configuration sends one sample at a time, the Output Buffer block (after the RF Receiver block) collects all samples within the baseband HE waveform before sending the samples to the HE Demodulation and EVM Calculation block. At the output of the RF Receiver block, the FIR Decimation block downsamples the waveform back to its original sampling rate. Additionally, the ADC block digitizes the signal. You can modify the ADC block parameters using its mask.

Baseband Waveform Reception

The HE Demodulation and EVM Calculation block recovers and plots the HE-Data symbols in the Constellation Diagram block by performing frequency and packet offset corrections, channel estimation, pilot phase tracking, OFDM demodulation, and equalization. This block performs these EVM measurements:

  • EVM per subcarrier (dB): EVM averaged over the allocated HE-Data symbols within a subcarrier

  • EVM per OFDM symbol (dB)

  • Overall EVM (dB and %): EVM averaged over all transmitted HE-Data symbols

This block also decodes each packet to recover the transmitted bits. The example compares the recovered bits to those transmitted for each packet to determine the packet error rate for the simulation duration by using the PER Calculation block.

The ACR measurement is displayed in the ACR (dB) block. You can also measure the ACR by calculating the power difference between the Channel Power levels of each waveform in the Spectrum Analyzer Input block. To check the Channel Power levels of each waveform, set this configuration in the Spectrum Analyzer Input block:

  • The Span (Hz): must be the bandwidth of the waveform to measure. By default, the example sets this value to 20 MHz, which is the bandwidth of both waveforms, the desired HE and the interferer.

  • The CF (Hz): must be 0 for the desired HE waveform or the spacing between both waveforms (defined in the Frequency Shift and Aggregation block) for the interferer. By default, the example sets this value to 0 Hz to measure the channel power of the desired waveform.

To measure ACR according to IEEE P802.11ax/D7.0, set the desired waveform power 3 dB above the rate-dependent sensitivity specified in Table 27-51 (-71 dBm for the default configuration) and adjust the power level of the interferer waveform to achieve a 10% PER for a PSDU length of 4096 octets.

Model Performance

To characterize the impact of the NR interference on the HE reception you can compare the EVM for two different cases: 1) without interference, for example, transmit only the HE waveform; and 2) with interference, for example, transmit both HE and NR waveforms. You can also measure the ACR in the second case.

  • Without NR interference (NR gain = 0). To eliminate the NR interference, set the Gain parameter of the Interferer Gain block to 0. To calculate the EVM and plot the constellation diagram, run the simulation long enough to capture one packet (Stop Time equal to 85.5 microseconds for the default configuration).

set_param([modelName '/Interferer Gain'],'Gain','0');
sim(modelName);

When you disable the interference, the overall EVM is around -20 dB.

  • With NR interference (NR gain = -37.72 dB). To activate the NR interference, set the Gain parameter of the Interferer Gain block to any value greater than 0. For example, in order to measure the ACR when the PER is approximately 10% for a PSDU length of 4096 octets [ 2 ], choose a gain value of around -37.72 dB and increase the APEP length. If you want to measure the PER for several packet transmissions, for example 100 packets, multiply the current Stop Time value by 100. By default, the example transmits one packet and sets the APEP length to 50 bytes.

set_param([modelName '/Interferer Gain'],'Gain','db2mag(-37.72)');
sim(modelName);

Compared to the case without interference, the constellation diagram is more distorted and the overall EVM is around -17 dB.

The ACR is around 28 dB. You can also measure the ACR when the interferer is an HE waveform. In this case, to measure the ACR when the PER is approximately 10% for a PSDU length of 4096 octets [ 2 ], set the Gain value of the Interferer block to around -72.4 dB.

Summary and Further Exploration

This example demonstrates how to model and test the reception of an HE waveform coexisting with an NR waveform or another HE waveform. The RF receiver consists of bandpass filters, amplifiers, and a demodulator. To evaluate the impact of the NR interference, the example modifies the gain of the NR waveform and performs EVM, PER, and ACR measurements. You can explore the impact of altering the RF impairments. For example:

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

  • Decrease the LO to RF isolation by using the LO to RF isolation (dB): parameter on the Demodulator tab of the RF Receiver block.

This example configures the RF Receiver block to work with the default values of the HE Waveform and NR Interferer blocks and with the HE and NR carriers centered at 5950 MHz and 5970 MHz, respectively. These carriers are within the IEEE 802.11 HE frequency bands (between 1 GHz and 7.125 GHz [ 2 ]) and the NR operating band n96 [ 3 ]. If you change the carrier frequencies or the waveform configurations, you may need to update the parameters of the RF Receiver block as these parameters have been selected to work for the default configuration of the example. For instance, a change in the HE carrier frequency requires revising the bandwidth of the filters. Modifying the waveform bandwidth may require updating the Impulse response duration and Phase noise frequency offset (Hz) parameters of the Demodulator 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 Demodulator (RF Blockset).

This example could be the basis for testing the coexistence between HE and NR or HE waveforms for different RF configurations. You can replace the RF Receiver block with another RF subsystem of your choice and configure the model accordingly.

Bibliography

  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. IEEE P802.11ax™/D7.0 Draft Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications - Amendment 6: Enhancements for High Efficiency WLAN.

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