MATLAB Examples

IEEE 802.11b WLAN - Beacon Frame Receiver Using Analog Devices AD9361/AD9364

This example shows reception of beacon frames in an 802.11b wireless local area network (WLAN) as described in [ 1 ]. The example utilizes SDR hardware to receive radio signals and transfer them to Simulink® for processing. For more information refer to IEEE® 802.11 WLAN - Beacon Frame and IEEE 802.11 WLAN - Beacon Frame with Captured Data examples.

Refer to the Getting Started documentation for details on configuring your host computer to work with the Support Package for Xilinx® Zynq-Based Radio.

Contents

Structure of the Example

The model has three main parts:

  • Model Parameters block, where you can adjust several receiver parameters
  • 802.11b receiver, which comprises a receiver front end, receiver controller, and detector
  • Results, where you view several signals and the received information

The following sections describe modifications made to the model presented in IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example to make it work with the SDR hardware.

This 802.11b WLAN example includes all the receiver signal processing in an enabled subsystem. Connecting the DataLength output of the SDR receiver block to the enable input of the subsystem ensures that the receiver only processes valid data.

Running the Example

You can observe several signals in the scopes. The MPDU (MAC Protocol Data Unit) GUI figure shows the PLCP (Physical Layer Convergence Procedure) and MPDU CRC status and also the content of correctly decoded MPDU packets.

Comment out the scatter plots in Receiver/Receiver Controller if not needed (right mouse click, Comment out) to increase the simulation speed.

Several parameters that influence the ability to receive the beacon are in the Model Parameters block:

  • If the received signal is too weak or too strong, you might notice some garbled message output. In that case, you can change the gain of the SDR receiver block for better reception via RF board gain.
  • You can determine the responsiveness of the automatic gain control using AGC loop gain and Maximum AGC gain.
  • You can adjust for slight center frequency mismatches between the transmitter and receiver using RF center frequency offset.
  • You can increase the signal magnitude going into the downstream processing using Receiver frontend gain.

It is also worth noting that this example pushes the gigabit Ethernet link to near its limit. If you run the example and the scopes don't show any data being received at all, try running a less demanding example first. This will help identify if the problem is performance related or a hardware setup issue. The QPSK Transmitter and QPSK Receiver examples are a good starting point.

SDR Receiver

802.11b uses 1e6 symbols per second for beacon signaling. Since the standard [ 1 ] calls for a spreading factor of 11, the chip rate is 11e6 chips per second. The receiver needs at least two samples per chip, which is 22e6 samples per second. The Baseband sample rate in the SDR Receiver block can be set as 22e6 samples per second, which is the required rate.

Running this receiver simulation requires more time than processing the same data in real-time, especially when using the visualization scopes. To help alleviate this time requirement, the SDR receiver block uses burst mode processing. Burst mode processing enables you to utilize the visualization capabilities of Simulink, while processing real data without the need of capturing and saving it.

In burst mode, the block stores a contiguous burst of samples. The number of samples is determined by the values specified in the Frame length and Number of frames in burst parameters. Each Simulink time step, the SDR receiver block sends a frame of samples to the Receiver subsystem. Most Wi-Fi® routers use a beacon interval of 100 Time Units (TU), which is 102.4 msec and the beacon packet lasts approximately 3 msec. Therefore, the receiver requires at least 106 msec of data to receive one beacon packet.

Exploring the Example

You can try different channel numbers from the Model Parameters block mask. The most widely used channels are 6 and 11.

This example allows you to modify several receiver parameters through the Model Parameters block mask dialog to optimize the receiver performance. If you notice that the AGC (Automatic Gain Control) gain reaches its maximum gain even when your signal is present at the receiver input, increase the maximum gain of the AGC. If the AGC is slow to respond to changes in the input signal amplitude, increase the AGC loop gain. Observe the AGC behavior in the AGC scope.

If your signal results in smaller peaks in the Synchronization Scope, which do not turn the receiver on, reduce the synchronization threshold.

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.

Appendix

This example uses the following helper functions: