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Modeling and Testing an 802.11ax RF Transmitter

This example shows how to characterize the impact of radio frequency (RF) impairments in an 802.11ax transmitter. The example generates a baseband IEEE® 802.11ax™ waveform by using WLAN Toolbox™ and models the RF transmitter by using RF Blockset™.

Introduction

This example characterizes the impact of RF impairments such as in-phase and quadrature (IQ) imbalance, phase noise, and power amplifier (PA) nonlinearities in the transmission of an 802.11ax waveform [ 1 ]. To evaluate the impact of these impairments, 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

  • Spectral mask: test that ensures a transmission in one channel does not cause substantial interference in adjacent channels

  • 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 that the signal's instantaneous power is at a specified level above its average power

  • Peak-to-average power ratio (PAPR): relation between the peak power of the signal and its average power

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

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

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

  3. Import the baseband waveform as an RF signal into the RF Transmitter block implemented by using RF Blockset. The model uses an RF intermediate frequency to carry the baseband information in RF Blockset.

  4. Upconvert the waveform to the carrier frequency by using an RF transmitter. You can accurately model the impairments introduced by an actual RF transmitter by using the RF components available in RF Blockset.

  5. Calculate the spectral mask, occupied bandwidth, channel power, CCDF and PAPR by using the Spectrum Analyzer block.

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

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

This example performs these operations by using a Simulink® model. The Simulink model carries out the baseband signal processing (steps 1, 2, 6 and 7) by using MATLAB® Function blocks and performs the RF transmitter modeling (steps 3 and 4) 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 waveforms

  • RF Transmission: upconverts the waveform to the carrier frequency

  • Baseband Waveform Reception: performs the RF measurements and calculates EVM by demodulating the baseband waveform

modelName = 'HERFTransmitterModel';
open_system(modelName);

Baseband Waveform Generation

The HE Packet block generates standard-compliant high-efficiency single-user (HE SU) waveforms [ 1 ]. For the waveform generation, set transmission and configuration parameters by specifying options in the HE Packet block.

The HE Packet 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.

After generating each packet, the FIR Interpolation block oversamples and filters the waveform to allow you to see the effect of the high-power amplifier (HPA) on the out-of-band spectral emissions. At the output of the RF Transmitter block, the FIR Decimation block downsamples the waveform back to its original sampling rate. The Multirate Parameters block provides an interface to easily configure the parameters of the FIR Interpolation and Decimation blocks.

Specifying Simulation Time

The Packet transmission time ( $\mu s$ ) parameter in the HE Packet block calculates the time required to transmit each 802.11ax packet. Hence, the Stop Time value in the Simulink model must be equal to or higher than the value depicted 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 Transmission

The RF Transmitter block is based on a superheterodyne transmitter architecture. This architecture upconverts the waveform to the carrier frequency and applies passband filtering and amplification. The RF components of this superheterodyne transmitter are:

  • IQ modulator consisting of mixers, a phase shifter, and a local oscillator

  • Bandpass filter

  • Power amplifier

In addition to these components, this RF Transmitter block also includes a variable gain amplifier (VGA) to control the input back-off (IBO) level of the HPA.

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

The Inport block inside the RF Transmitter 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 (the default frequency, which is considered to be an intermediate frequency in this example, is 70 MHz). The Outport block converts the RF signal back into complex baseband.

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

The RF Transmitter block exhibits typical impairments, including:

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

  • Phase noise as an 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 the saturation region

You can use an Input Buffer block before the RF Transmitter block to reduce the number of samples sent to the RF Transmitter block. For simplicity, the Input Buffer in the current configuration sends one sample at a time, resulting in the RF Transmitter block being sample-based.

Adapt the power level of the baseband waveform to the RF configuration by adding a Gain Control block after the Input Buffer block.

As the current RF Transmitter block configuration sends one sample at a time, the Output Buffer block (after the RF Transmitter block) collects all samples within the baseband HE waveform before sending the samples onto the Demodulation and EVM calculation block.

Baseband Waveform Reception

The 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 also 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

The default configuration of the Spectrum Analyzer block depicts the spectral mask according to IEEE P802.11ax/D7.0 Section 27.3.19.1 [ 1 ]. The Spectrum Analyzer block also provides additional measurements such as occupied bandwidth, channel power, CCDF and PAPR. The Power Meter block measures the RF waveform channel power, which is displayed in the Output Power (dBm) block.

Effect of Power Amplifier Nonlinearities

To characterize the impact of HPA nonlinearities in the EVM evaluation, you can measure the amplitude-to-amplitude modulation (AM/AM) of the HPA. The AM/AM refers to the output power levels in terms of the input power levels. The helper function hePlotHPACurve displays the AM/AM characteristic of the HPA selected for this model.

hePlotHPACurve();
figHPA = gcf;

P1dB is the power at 1 dB compression point and is normally used as a reference when selecting the IBO level of the HPA. You can see the HPA impact on the RF Transmitter block by analyzing the EVM results for different operating points of the HPA. For example, compare the case when IBO = 11 dB, corresponding to HPA operating in the linear region, with the case when IBO = 3 dB, corresponding to HPA operating in saturation. The gain of the VGA controls the IBO level. To keep a VGA linear behavior using the default parameters, select gain values lower than 15 dB.

  • Linear HPA (IBO = 11 dB). To operate at an IBO level of 11 dB, set the Available power gain parameter of the VGA block to 5 dB. To calculate the EVM and plot the constellation diagram, run the simulation long enough to capture one packet (Stop Time equal to 304.4 us for the default configuration).

set_param([modelName '/RF Transmitter'],'vgaGain','5');
sim(modelName);

According to IEEE P802.11ax/D7.0 Table 27-49 [ 1 ], the allowed relative constellation error (EVM) in an HE SU PPDU when the Dual carrier modulation parameter is disabled and the Modulation/coding is equal to 3 (16-QAM, 1/2) is -16 dB. As the overall EVM, around -41 dB, is lower than -16 dB, this architecture falls within the requirements of IEEE P802.11ax/D7.0 [ 1 ].

  • Nonlinear HPA (IBO = 3 dB). To operate at an IBO level of 3 dB, set the Available power gain parameter of the VGA block to 13 dB.

set_param([modelName '/RF Transmitter'],'vgaGain','13');
sim(modelName);
slmsgviewer.DeleteInstance();

% Restore to default parameters
set_param([modelName '/RF Transmitter'],'vgaGain','5');

Compared to the previous case, the constellation diagram is more distorted. In terms of measurements, the overall EVM, around -28 dB, is still lower than -16 dB, so it also falls within the requirements of IEEE P802.11ax/D7.0 [ 1 ].

Summary and Further Exploration

This example demonstrates how to model and test the transmission of an 802.11ax waveform. The RF Transmitter block consists of a bandpass filter, amplifiers and an IQ modulator. The example highlights the effect of HPA nonlinearities on the performance of the RF Transmitter block. You can explore the impact of altering other 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 Modulator tab of the RF Transmitter block.

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

Additionally, you can check the occupied bandwidth and the CCDF and PAPR measurements by using the Spectrum Analyzer window: select the Channel Measurements button on the toolbar for the occupied bandwidth and the CCDF Measurements button for the CCDF and PAPR.

The RF Transmitter block is configured to work with the default values of the HE Packet block and with the RF carrier centered at 5950 MHz. This carrier is within the IEEE 802.11 HE STA frequency bands (between 1 GHz and 7.125 GHz [ 1 ]). If you change the carrier frequency or the values in the HE Packet block, you may need to update the parameters of the RF Transmitter block components, as these parameters have been selected to work for the default configuration of the example. For instance, a change in the 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 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 HE waveforms for different RF configurations. You can replace the RF Transmitter block by another RF subsystem of your choice and configuring the model accordingly.

References

  1. 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.