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IEEE 802.11 WLAN - Beacon Frame Receiver with SDR Hardware

This example shows reception of beacon frames in an 802.11 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 FrameIEEE 802.11 WLAN - Beacon Frame and IEEE 802.11 WLAN - Beacon Frame with Captured DataIEEE 802.11 WLAN - Beacon Frame with Captured Data examples.

To run this model, you must have an SDR board with an appropriate receiver daughterboard that supports the 2.4 GHz ISM band (for example, ADI FMCOMMS). Refer to the SDR docthe SDR doc to learn how to configure your host computer to work with an SDR board.

Structure of the Example

The model has three main parts:

  • Model Parameters block, where you can adjust several receiver parameters

  • 802.11 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 DataIEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data example to make it work with the SDR hardware.

This 802.11 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.

Load FGPA Image

The model communicates with the SDR board using the SDR receiver block. First, we must download the FPGA image to the board. Set the motherboard name and the RF board name in the Model Parameters block. Then double-click the Download bitstream block to load the appropriate image that ships with the product via a JTAG cable. See the documentation for alternative methods to download an FPGA programming file.

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.

SDR Receiver

802.11 uses 1e6 symbols per second for beacon signaling. Since the standard [ 1 ] calls for a spreading factor for 11, the chip rate is 11e6 chips per second. The receiver needs at least two samples per chip and the maximum data rate the SDR hardware can pass to the host computer is 25e6 samples per second. Therefore, this example uses a decimation factor of four.

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 you specify for the number of frames in burst parameter and the frame length parameter. 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 length of approximately 3 msec. Therefore, the receiver requires at least 106 msec of data to receive one beacon packet.

Receiver

Since the chip rate is 11e6 chips per second, the receiver utilizes an FIR Rate Conversion block that resamples the signals to 22e6 samples per second. This approach provides an oversampling factor of two.

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.

Selected Bibliography

  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.

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