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Targeting the HDL Optimized 802.11 WLAN Beacon Frame Receiver Using Analog Devices AD9361/AD9364

This example shows how to run an HDL-optimized Beacon frame receiver model with AD9361/AD9364 SDR hardware and how to prototype it on the FPGA of the SDR platform using the HDL Coder™ workflow advisor. For a full description of targeting, see Targeting Overview in the documentation.

Refer to the Getting Started documentation for details on configuring your host computer to work with the Support Package for Xilinx® Zynq-Based Radio. Additionally, targeting requires HDL Coder and Xilinx Vivado®, which must be on the system path. You can add Vivado to the path when you call setupTools. See the targeting requirements for more details.

Required products:

  • Simulink

  • Communications System Toolbox

  • HDL Coder

  • Communications System Toolbox Support Package for Zynq-Based Radio

Introduction

The IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example shows a model that receives beacon frames in an 802.11 wireless local area network (WLAN) as described in [ 1]. This example shows a hardware friendly version of the model and how to use it with SDR hardware, as well as the steps taken to prototype it on the FPGA.

There are two methods of running this example, based on the targeting/retargeted pair of models:

  1. Run the zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364SL targeting model directly. See the Running the Example: Simulated section.

  2. Generate a targeted bitstream and use the retargeted model zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364RetargetSL. See the Running the Example: Targeted section.

Zynq-Radio WLAN Beacon Receiver

The hardware friendly version of the IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example is placed within an enabled subsystem 802.11 WLAN Beacon Frame Receiver, which ensures that only valid samples coming from the SDR hardware will be processed.

The HDL-optimized subsystem is called HDLRx and this is the DUT that will be implemented in the FPGA fabric. The contents of the subsystem are displayed below.

The HDLRx subsystem, in partnership with the Unpack FPGA Outputs subsystem of the model, ensures that the start signal can be packed into the 16 bit complex value required by the hardware.

The FPGA to Host subsystem packages the start flag and complex fixed-point data by replacing the least significant bit of the 'Symbols' output with the Boolean value representing the start flag. The Unpack FPGA Outputs subsystem is then used to recover the start flag and complex data representing the recovered symbols for further processing in Simulink.

HDL Modifications to Beacon Frame Receiver Example

The IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example consists of three main components: front end, receiver controller, and detector. The front end and the receiver controller operate at a high rate in the receiver, so they have been optimized for HDL code generation in this example. The following sections describe the details of these modifications.

To generate efficient HDL code the following modifications to the model are required:

  • The IP Core Generation: Requires that the input and output interfaces of the HDL subsystem have two 16-bit data signals and a valid signal. These interface signals should be at the highest sample rate of the subsystem.

  • Streaming Input and Output: The HDL optimized beacon frame receiver processes data one sample at a time. The real-world signal is streamed into the receiver front-end. The streaming output of the receiver controller is buffered and passed to the detector, which operates on the data on a per-frame basis.

  • Delay Balancing: To ensure functional integrity is preserved to the original model delay balancing has been used on the data valid line.

  • Fixed-point: The receiver front-end and the controller logic operate in fixed-point mode.

  • HDL optimized architecture: Several blocks have been redesigned to use hardware efficient algorithms and architectures.

Receiver Front-End

The receiver front-end design is modeled in the subsystem HDLRx Front End. The receiver front-end is composed of a matched-filter, AGC, and coarse frequency compensation.

Modifications were made to the coarse frequency estimation algorithm [ 2 ] implemented in the original beacon receiver model:

  • The auto correlation operation has been replaced by a simple smooth filter.

  • The angle function has been implemented using the Complex to Magnitude-Angle HDL Optimized block. This block computes the phase using the hardware friendly CORDIC algorithm. To learn more about the Complex to Magnitude-Angle HDL Optimized block, refer to the DSP System Toolbox documentation.

  • The detected phase offset is sent to NCO HDL Optimized block to generate a complex exponential signal that is used to correct the phase offset in the original signal.

The NCO HDL Optimized block provides hardware friendly options, maps the lookup table into a ROM, and provides a lookup table compression option to significantly reduce the lookup table size. To learn more about HDL support for HDL Optimized NCO block, refer to the documentation.

Receiver Controller

The receiver controller finds the correlation between the phase offset corrected signal and synchronization signal. When the peak of the correlation is detected, the signal is delayed based on the peak position before despreading. Because of the large beacon frame size (2816 samples), implementing the correlator using either an FFT or a matched filter is not efficient in hardware. In addition, finding the maximum of a 2816-element vector is not hardware friendly. The correlation and despreading algorithm has been redesigned in this example. Despreading is performed before the correlation, aiming to reduce the filter size from 2816 to 128. Because the start of the beacon signal is unknown when performing the despreading early, 22 channels have been designed in the Despread_Matched Filter module, with each channel offsetting the adjacent channel by one sample. The maximum of the outputs of 22 filters is computed and the despread results from the channel that produces the maximum are selected to send to the Detector.

The downsample operation before the 22-channel matched filter in the Despread_Matched Filter subsystem shown below allows the possibility of sharing resources across the 22 channels of the FIR matched filters, as a way to balance the hardware speed and resource usage. To enable the sharing of resources, the ChannelSharing HDL property for the Discrete FIR Filter has been turned on. this reuses the FIR Filter hardware across all 22 channels. To see this property, right-click on the Discrete FIR Filter block, choose HDL Code, then HDL Block Properties. From the command line you can use hdlget_param to get, hdlset_param to set and hdldispblkparams to display the HDL properties of a block.

The despreaded signal is buffered in frames of 128 symbols before being processed in the detector. In the original Beacon Frame Receiver example, the PLCP information was fed back to the receiver controller to decide the length of payload to be collected. In this example, in order to simplify the interface between the hardware and software components and not require real-time communication of information between the two, the maximal length of the payload is collected in the hardware and sent to the software.

Setup

Before running the example, ensure you have performed the following steps:

1. Configure your host computer to work with the Support Package for Xilinx Zynq-Based Radio. See Getting Started for help.

2. Make sure you have a suitable source to receive the 802.11 Beacons from. This can be done in multiple ways:

  • Grab the signals off the air from a local access point. You will need to find out which channel it will be transmitting from. The most common channels in use are 1, 6 and 11.

  • Use a wireless router and optionally set the beacon interval to a convenient value (usually 50 or 100ms). Having a lower beacon interval time will allow more beacons to be captured in a burst and help configure the model.

Running the Example: Simulated

To run the example directly, open zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364SL, configure the model parameters and start the simulation.

The Model Parameters block can be used to modify:

  • Radio parameters - the channel number, RF gain and center frequency offset.

  • Receiver parameters - AGC max gain and step size, correlation threshold. (These are used to tune the HDL-optimized receiver subsystem and will affect the hardware implementation).

Note that since the model is optimized for hardware, it will run significantly slower than its floating point, frame-based counterparts. Due to this, the example uses burst mode to receive chunks of contiguous data (otherwise the SDR would be passing samples through at a much faster rate than the Simulink model can process them, resulting in lost samples). Also, to increase simulation performance, the synchronization scope has been commented out by default.

The SDR hardware is set to capture 103ms worth of data per burst (2816 samples per frame x 805 frames per burst / 22Msps). This should be enough to receive at least one beacon frame on each burst in a standard setting (most access points will be transmitting a beacon every 100ms with the beacon lasting 3ms, however beacon intervals can vary).

To obtain the scope outputs seen below, a wireless router was configured to transmit with a beacon interval of 50ms.

As seen from the synchronization scope, there were 2 distinct instances 50ms apart where the correlation threshold was reached. The SFD Synchronization scope outputs are only on for the duration of packet detection which is turned on by the 'start' flag from the HDLRx Controller. To read more about the scope outputs, visit the IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example.

The decoded packet service type can be seen in the MAC Header information tab of the MPDU GUI as shown below.

The receiver also decodes the SSID, seen in the Information Elements tab.

Running the Example: Targeted

The zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364SL model can be used to generate a targeted SD card image for the SDR hardware that includes the HDLRx subsystem as a DUT. The DUT is automatically placed in the SDR receiver chain that is implemented on the FPGA.

To generate the new bitstream, perform the following steps:

  1. Make sure you have added Vivado to the system path. See related workflow setup.

  2. Right click on the HDLRx subsystem, select Block Parameters (Subsystem), and ensure that the treat as atomic unit checkbox is selected.

  3. Open the HDL workflow advisor by right-clicking on the HDLRx subsystem and selecting HDL Code -> HDL Workflow Advisor.

  4. In step 1.1, at Target workflow, select IP Core Generation. At Target platform, select the appropriate option for your radio hardware (e.g. 'ZC706 and FMCOMMS2/3/4', 'Zedboard and FMCOMMS2/3/4' or 'PicoZed SDR'). The Synthesis tool field should automatically set to Xilinx Vivado if you added Vivado to the path. Click Apply, then click Run This Task.

  5. In step 1.2, ensure that Reference Design is set to Receive path. Set the synthesis frequency to a lower rate (30MHz would be appropriate). Click Apply, then click Run This Task.

  6. In step 1.3, ensure Processor/FPGA synchronization is set to Free running, and that each port is mapped to the appropriate interface as shown in the figure below.

  1. The default values for step 2 and step 3 can be used.

  2. Run the Workflow Advisor to step 4.1. A quick way to run all the preceding steps is to right click on 4.1 Create Project in the workflow window and click Run to selected task.

  3. In step 4.2, ensure that the Skip This Task checkbox is selected, and then click Run This Task.

  4. In step 4.3, ensure that the Run build process externally checkbox is selected, then click Run This Task.

If no errors are found, the FPGA programming file generation process starts in an external command shell. You can monitor the implementation progress in the external shell. A message indicating successful completion of file generation is displayed in the shell upon completion.

At this point there are two methods of downloading the generated bitstream to the board. The first method (which requires the Embedded Coder product) is to return to the HDL Workflow Advisor, select Download from the drop-down menu in step 4.4, and click Run This Task. When using this method, note that the bitstream is not persistent across power cycles.

If you want the generated FPGA image to load each time you turn on the board, call the downloadImage method as shown below (assuming the default 'hdl_prj' folder was used in step 1.1). This method does not require Embedded Coder.

>> dev = sdrdev('PicoZed SDR');
>> downloadImage(dev, 'FPGAImage','hdl_prj\vivado_ip_prj\vivado_prj.runs\impl_1\system_wrapper.bit');

The FPGA implementation of the beacon receiver can be verified using the companion retargeted model zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364RetargetSL.

This model can be built from the original zynqRadioWLAN80211BeaconRxFPGAAD9361AD9364SL model by removing the Data Type Conversion and HDLRx blocks from the subsystem. The functionality of these blocks will be implemented on the FPGA.

Running this model, with the SD Card image downloaded, you should notice the model performing much more quickly, as now most of the baseband processing is being performed on the FPGA. Also, since the HDL-optimized subsystem is sending back downsampled, more tightly packed information in the same data burst, there will be many more beacons demodulated and decoded per burst as shown by the SFD Synchronization scope below.

Note that the scope no longer represents 103ms of recorded data from the RF card. The new time interval of captured and processed data will be 22 times longer at 2266.9ms (2816 samples per frame x 805 frames per burst / 1Msps).

Another important fact to consider is that the HDLRx subsystem parameters for AGC and Synchronization will now be hard coded on the FPGA fabric, thus it is advisable to first run and tune the simulation model before attempting to target the HDL-optimized subsystem.

List of Example Helper Files

This example uses the following helper files:

References

  1. IEEE Std 802.11-2007: IEEE 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, IEEE, New York, NY, USA, 1999-2007.

  2. M. Luise and R. Reggiannini, "Carrier frequency recovery in all-digital modems for burst-mode transmissions," IEEE Trans. Communications, pp. 1169-1178, Feb.-March-Apr. 1995.

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