This model shows 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.
Structure of the Example
This example 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:
Generation of random bit data that models a downlink burst consisting of an integer number of OFDM symbols.
Forward Error Correction (FEC), consisting of a Reed-Solomon (RS) outer code concatenated with a rate-compatible inner convolutional code (CC).
Modulation, using one of the BPSK, QPSK, 16-QAM or 64-QAM constellations specified.
Space-Time Block Coding using an Alamouti code [ 3 ]. This implementation uses the OSTBC Encoder and Combiner blocks in the Communications System Toolbox™.
Orthogonal Frequency Division Multiplexed (OFDM) transmission using 256-point FFTs and a variable cyclic prefix length. Among the 256 subcarriers, 192 are for data, 8 are for pilots, 28 are left guard band, 27 are right guard band and 1 is DC null.
A single OFDM symbol length preamble that is used as the burst preamble. For the STBC model, both antennas transmit the single symbol preamble.
An optional memoryless nonlinearity that can be driven at several backoff levels.
An optional digital pre-distortion capability that corrects for the nonlinearity.
A Multiple-Input-Single-Output (MISO) fading channel with AWGN for the STBC model. You can choose a non-fading, flat-fading, or dispersive multipath fading channel for the non-STBC model.
OFDM receiver followed by channel estimation using the inserted preambles. For the STBC model, this implies diversity combining as per [ 3 ].
Hard-decision demodulation followed by deinterleaving, Viterbi decoding, and Reed-Solomon decoding.
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 example 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 Analyzer blocks display the spectra of both the OFDM transmitter output and the faded AWGN channel output. Also, two Constellation Diagram blocks display the AM/AM and AM/PM characteristics of the signal at the output of the memoryless nonlinearity. Finally, a Constellation Diagram block 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 example:
Set the number of OFDM symbols to be constant for all data bursts generated. As a result, for any given profile, the frame duration in Simulink remains the same. Within the downlink frame, the example models only the downlink burst. It does not model the long preamble and the FCH burst.
Do not model the randomization specified as a part of the channel coding as the data is randomly generated. The library file has blocks which cover this functionality.
Assume perfect synchronization between the transmitter and receiver. As a consequence, they only use a short preamble for every downlink burst.
Estimate the channel at the receiver using only the inserted preambles and not the pilot subcarriers. This assumes that the channel is not changing very rapidly (or is constant for the number of OFDM symbols in a burst).
Exploring the Models
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.
Results and Displays
When you simulate either of the two models, windows come up automatically to display:
a spectrum plot of the transmitted signal,
an AM/AM plot at the nonlinearity output,
an AM/PM plot at the nonlinearity output,
a spectrum plot of the signal at the channel output, and
a constellation diagram of the received signal prior to demodulation.
Use the spectrum plots to verify the channel bandwidth in use and the subcarrier spacing. Use the constellation diagrams 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:
The Bit Error Rate Display block shows the bit error rate, number of errors and the total number of bits processed.
The Est. SNR (dB) display block at the top level shows an estimate of the SNR based on error vector magnitude. The SNR block in the Channel subsystem shows the SNR based on received signal power.
The RateID display block shows the
rate_ID that corresponds to the specific modulation RS-CC rate currently in use.
Available Example Models
Two versions of the example model files are available in the Communications System Toolbox product:
IEEE Standard 802.16-2004, "Part 16: Air interface for fixed broadband wireless access systems," October 2004. http://ieee802.org/16/published.html
IEEE 802.16 Broadband Wireless Access Working Group, "Channel models for fixed wireless applications," IEEE 802.16a-03/01, 2003-06-27.
S. M. Alamouti, "A simple transmit diversity technique for wireless communications," IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8, Oct. 1998, pp. 1451-1458.
Carl Eklund, et.al., "WirelessMAN: Inside the IEEE 802.16 Standard for Wireless Metropolitan Area Networks," IEEE Press, 2006.
J. G. Andrews, A. Ghosh and R. Muhamed, "Fundamentals of WiMAX: Understanding Broadband Wireless Networking," Prentice Hall, 2007.