Communications System Toolbox
This demo represents an end-to-end baseband model of the physical layer of a wireless metropolitan area network (WMAN), according to the IEEE® 802.16-2004 standard [ 1 ]. More specifically, it models the OFDM-based physical layer, called WirelessMAN-OFDM, supporting all of the mandatory coding and modulation options. It also illustrates Space-Time Block Coding (STBC), an optional transmit diversity scheme specified for use on the downlink. Finally, it illustrates the use of digital pre-distortion, a technique for extending the linear range of a nonlinear amplifier.
This demo showcases the main components of the WMAN 802.16-2004 OFDM physical layer using two models: one with STBC and one without. Both models have all the mandatory coding and modulation options.
The tasks performed in the communication system models include:
Both models also use an adaptive-rate control scheme based on SNR estimates at the receiver to vary the data rate dynamically based on the channel conditions. The models use the standard-specified set of seven rates for OFDM-PHY, each corresponding to a specific modulation and RS-CC code rate as denoted by rate_ID (see the following table as per [ 1 ]).
The STBC link model uses a MISO fading channel to model a two transmitter, one receiver (2x1) system. The demo assumes that the fading parameters are identical for the two links. The Space-Time Diversity Combiner block uses the channel estimates for each link and combines the received signals as per [ 3 ]. The combining operation performs simple linear processing using the orthogonal signalling employed by the encoder.
Furthermore, both models include blocks for measuring and displaying the bit error rate after FEC, the channel SNR and the rate_ID. Spectrum Scope blocks display the spectra of both the OFDM transmitter output and the faded AWGN channel output. Also, a Scatter Plot scope displays the AM/AM and AM/PM characteristics of the signal at the output of the memoryless nonlinearity. Finally, a Scatter Plot scope displays the received signal, helping you to visualize channel impairments and modulation adaptation as the simulation runs.
The subsystems and blocks in the models are color-coded to make viewing easier. The communication system operations are in blue, control systems and signals are in orange, and the performance evaluation, displays and plots are in yellow.
Simplifications and Assumptions
For simplicity, the models in this demo:
For both models, the Model Parameters configuration block allows you to choose and specify system parameters, such as channel bandwidth, number of OFDM symbols per burst and the cyclic prefix factor. Varying these parameter values allows you to experiment with the different WiMAX profiles as defined by the WiMAX Forum [ 5 ], and gauge the system performance for each.
You can vary the state of the nonlinearity and the digital pre-distortion via the Amplifier nonlinearity and Digital pre-distortion parameters. A Saleh model implements the nonlinearity, with three different backoff options. The digital pre-distortion function fits polynomials to the empirically determined AM/AM and AM/PM characteristics of the nonlinearity, then creates a lookup table by which to pre-distort the signal. Since the nonlinearity induces a gain compression on the input signal, the pre-distortion applies a "gain expansion" on the signal, such that the composite gain is linear. However, the pre-distortion is effective only over the input amplitude range up to the peak of the AM/AM nonlinearity. You should see that the pre-distortion is only marginally effective when the input signal amplitude drives the amplifier heavily into saturation.
Another parameter of interest is the Low SNR thresholds for rate control parameter, as it directly affects the adaptive-rate control in both models. This parameter is a six-element vector representing the boundaries between the adjoining seven SNR ranges that correspond to the seven rates. Ideally, the simulation should use the highest throughput mode that achieves the desired bit error rate.
Another area of variability includes the channel blocks in both models. The models allow you to vary the fading parameters [ 2 ] and the AWGN variance (in SNR mode) added to the signal. As a result, you can examine how well the receiver performs with different fading characteristics (choosing the appropriate K factor, maximum Doppler shift, number of paths, path gains) and generate BER curves for varying SNR values.
When you simulate either of the two models, windows come up automatically to display:
Use the spectrum plots to verify the channel bandwidth in use and the subcarrier spacing. Use the scatter plots to gauge which modulation type is in use, as the plot resembles a signal constellation of 2, 4, 16, or 64 points under good channel conditions.
Use the AM/AM and AM/PM plots to determine how well the digital pre-distortion function compensates for the degradation induced by the memoryless nonlinearity. Ideally, the AM/AM curve should be linear, and the AM/PM characteristic should be horizontal.
The following blocks display numerical results:
Two versions of the demo model files are available in the Communications System Toolbox product:
OFDM PHY downlink with STBC: commwman80216dstbc.mdl
OFDM PHY downlink without STBC: commwman80216d.mdl