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IEEE 802.11 WLAN - Beacon Frame Receiver with Captured Data

This example shows reception of beacon frames in an 802.11 wireless local area network (WLAN) as described in [ 1 ]. For more information refer to the IEEE 802.11 WLAN - Beacon Frame example.

Structure of the Example

The model has three main parts:

  • Model Parameters block, where you can select one of several captured signals and some receiver parameters,

  • 802.11 receiver, which comprises a receiver front end, receiver controller, and detector,

  • Results, where you can view several signals and received information.

The IEEE 802.11 WLAN - Beacon Frame example shows transmission and reception of beacon frames in an 802.11 based wireless local area network (WLAN). The following section describes modifications made to this example to make it work with the captured real world signals.

Receiver Front-End

The receiver front-end consists of a gain stage, matched filtering, and coarse frequency compensation. The captured signals usually have a smaller amplitude than simulated signals. Therefore, the model includes a constant gain to reduce the dynamic range requirements of the AGC.

The captured signals contain frequency offsets of up to 40 kHz on the captured signals. The synchronization algorithm, which utilizes the correlation properties of the scrambled SYNC signal, is sensitive to such high offsets. We include a coarse frequency compensator to reduce the frequency offset so the receiver can detect received SYNC signals.

The frequency compensation subsystem removes the DBPSK modulation by taking the second power of the received complex signals [ 2 ]. It then decimates the signal and applies the frequency estimation algorithm described in [ 3 ]. The subsystem uses the Phase/Frequency Offset block of Communications System Toolbox™ to correct the phase offset. You can further optimize this subsystem by including the multiplication by -1 inside the frequency estimation algorithm. This subsystem shows the multiplication explicitly for clarity.

Fine Packet Synchronization

The IEEE 802.11 WLAN - Beacon Frame example model is fully synchronized. In other words, the scrambler and the descrambler have the same phase at all times. This situation makes sure that the peak correlation value always points to the end of the SYNC signal. However, in a real system, the transmitter and receiver may have different scrambler phases. As a result, the peak correlation value does not always point to the end of the SYNC signal and has an ambiguity equal to the length of the scrambling sequence. The receiver uses the (Start Frame Delimiter) SFD field, which is a known bit sequence of length 16, to finely synchronize the received packet bits. Packet parser utilizes the Fine Packet Synchronization block to determine the beginning of the (Physical Layer Convergence Procedure) PLCP header.

This approach also eliminates the need to send PLCP and (MAC Protocol Data Unit) MPDU indices from the receiver controller to the detector. The detector can decide which bits are for PLCP and which bits are for MPDU based on the fine packet synchronization.

Results and Displays

When you run the simulation, it displays several scopes. The first scope is the (Automatic Gain Control) AGC scope. It shows the input to the AGC subsystem, the gain applied to the input signal, and the output signal. You can select the AGC step size and the maximum AGC gain from the Model Parameters block mask.

The second scope is the estimated frequency offset scope. You can see that when there is no valid signal the estimate is not stable. Once a valid signal is received, the estimate becomes stable.

Warning: The specified simulation stop time (0.008) is not an integer multiple
of the fixed step size (0.000128).  Changing the stop time to (0.007936).  You
can disable this diagnostic by either specifying a stop time that is an integer
multiple of the fixed step size, or setting 'Automatic solver parameter
selection' diagnostic to 'none' in the Diagnostics tab of the Configuration
Parameters dialog 

The third scope is the synchronization scope, where you can view the output of the SYNC correlator the model uses to detect and coarsely synchronize to the received packets. This scope also shows when the correlator triggers a packet detection and the receiver turns on. When the receiver controller receives all the payload bits, it turns the receiver off. If the PLCP CRC does not check, then the controller turns the receiver immediately.

The fourth scope is the SFD synchronization scope, where you can view the output of the SFD correlator that is used to finely synchronize with the received packet. Since the SFD sequence is 16 bits, an output of 16 means an SFD signal is found.

Warning: The specified simulation stop time (0.008) is not an integer multiple
of the fixed step size (0.000128).  Changing the stop time to (0.007936).  You
can disable this diagnostic by either specifying a stop time that is an integer
multiple of the fixed step size, or setting 'Automatic solver parameter
selection' diagnostic to 'none' in the Diagnostics tab of the Configuration
Parameters dialog 

The next two scopes are the received chips and symbol scatter plots. Note that received symbols are despread symbols. The received symbols scope is active only when the receiver is on as indicated by the Synchronization scope's lower axes. The received symbols rotate due to the coarse frequency estimation but the DBPSK demodulator can handle this residual frequency offset.

Warning: The specified simulation stop time (0.004707) is not an integer
multiple of the fixed step size (0.000128).  Changing the stop time to
(0.004608).  You can disable this diagnostic by either specifying a stop time
that is an integer multiple of the fixed step size, or setting 'Automatic solver
parameter selection' diagnostic to 'none' in the Diagnostics tab of the
Configuration Parameters dialog 

The last output is the MPDU GUI, where you can see all the information contained in the MPDU packet together with the PLCP and MPDU packet CRC check results. This GUI appears after the system detects the first 802.11 packet. The tabs show the contents of the MAC header, frame body, and the information elements, including the (Service Set Identifier) SSID .

Exploring the Example

You can try four different signals captured from the 2.4 GHz band in an office environment using both channel 5 and 9 of the Wi-Fi band.

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 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 step size. 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.

  2. A.J. Viterbi and A.M. Viterbi, "Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission," IEEE Trans. Information Theory, pp. 543-551, July 1983.

  3. 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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