Communications System Toolbox
This model shows an end-to-end baseband model of the physical layer of a wireless local area network (WLAN) according to the IEEE® 802.11a standard. The model supports all mandatory and optional data rates: 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. The model also illustrates adaptive modulation and coding over a dispersive multipath fading channel, whereby the simulation varies the data rate dynamically. Note that the model uses an artificially high channel fading rate to make the data rate change more quickly and thus make the visualization more animated and instructive.
Structure of the Example
The model contains components that model the essential features of the WLAN 802.11a standard. The top row of blocks contains the transmitter components while the bottom row contains the receiver components.
The communication system in this example performs these tasks:
1. Generation of random data at a bit rate that varies during the simulation. The varying data rate is accomplished by enabling a source block periodically for a duration that depends on the desired data rate.
2. Coding, interleaving, and modulation using one of several schemes specified in the standard.
To examine these operations, select the
Modulator Bank block and choose Look Under Mask from the window's Edit menu. Then select any of the modulator blocks in the subsystem and choose Look Under Mask from the window's Edit menu.
In particular, each modulator block in the bank performs these tasks:
Convolutional coding and puncturing using code rates of 1/2, 2/3, and 3/4
BPSK, QPSK, 16-QAM, and 64-QAM modulation
3. OFDM (orthogonal frequency division multiplexing) transmission using 52 data subcarriers, 4 pilots, 64-point FFTs, and a 16-sample cyclic prefix.
4. PLCP (physical layer convergence protocol) preamble modeled as four long training sequences.
5. Dispersive multipath fading channel. You can configure channel properties using the dialog box of the
Multipath Channel block.
6. Receiver equalization.
7. Viterbi decoding.
Simplifications and Assumptions. For simplicity, this example
Fixes the number of data symbols in each packet and omits pad bits
Operates continuously from frame to frame and thus omits tail bits that would have been used for resetting the decoder state
Fixes the transmit power level, instead varying the average SNR of the channel
Assumes idealized timing/frequency acquisition
Also, the example does not model these aspects of the IEEE 802.11a standard:
MAC/PHY interface and PLCP header (TXVECTOR/RXVECTOR)
Data scrambling, which is unnecessary in this example because the data is random
Short training sequences (for automatic gain control, diversity, timing/frequency acquisition)
Color Legend. The model uses colors at the top level of the hierarchy to help you distinguish blocks that play different roles.
Exploring the Example
A configuration block called Model Parameters enables you to set parameters such as the composition of each OFDM frame, and traceback depth for the Viterbi decoder.
One parameter of particular interest for the adaptive modulation and coding in this example is the Low-SNR thresholds parameter. This is a seven-element vector that indicates how the simulation should choose a data rate based on the SNR estimate. The model has eight modes, each associated with a particular modulation scheme and convolutional code. The seven thresholds are the boundaries between eight adjacent regions that correspond to the eight modes. Ideally, the simulation should use the highest-throughput mode that achieves a desired packet error rate. Determining appropriate thresholds often involves running the simulation multiple times, varying the values of the Low-SNR thresholds parameter.
Results and Displays
To view data graphically, open the display window by double-clicking the Signal Visualization icon. The plots within the display window show
A portion of the random binary data, meant to help you visualize the varying data rate.
Scatter plots of the received signal before and after equalization. From the plot of the equalized signal, you can tell which modulation type the system is currently using, because the plot resembles a signal constellation of 2, 4, 16, or 64 points.
The power spectrum of the received signal before and after equalization, in dB. The dynamics of the signal's spectrum before equalization depend on the Fading mode parameter in the
Multipath Channel block.
The estimate of the SNR based on the error vector magnitude.
The bit rate of the transmission.
The bit error rate per packet. For most packets, the BER is zero. Because this plot uses a logarithmic scale for the vertical axis, BER values of zero do not appear in the plot.
The following blocks display numerical results:
PER block shows the packet error rate as a percentage.
SNR block at the top level of the model shows an estimate of the SNR based on the error vector magnitude. The
SNR block in the Multipath Channel subsystem shows the SNR based on the received signal power.
Bit Rate block shows which of the bit rates specified in the standard is currently in use.
 IEEE 802.11 Task Group a, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-speed Physical Layer in the 5 GHz Band, 1999.
 O'Hara, Bob, and Al Petrick, The IEEE 802.11 Handbook: A Designer's Companion, New York, IEEE Press, 1999.