Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Figure 4-6: OFDM receiver VPI/MATLAB components
The resulting I and Q channels are then quantised and passed to a MATLAB simulator for 
further processing using the CoSim interface described in Figure 4-7.
Figure 4-7: MATLAB interface for OFDM receiver
The CoSim interface in the receiver operates in a similar manner to the transmitter interface, 
except this time the I and Q channels are fed separately to MATLAB with the input matrix size 
equal to the total number of symbols as defined in eq. 4.1. 

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Chapter 4 FDM Simulation Platform with Wireless Compatibility 
76 
TotalNumSymbols 
BitRate x TimeWindow
BitsPerSymbol
4.1
The transmission evaluation of the OFDM symbols over the multi-path channel is then 
performed using the MATLAB function described in Appendix A. The application of ISI, to 
model the effect of different cycle prefixes, is achieved by introducing random OFDM symbols 
(
SymbolTx = [NextSymbol NextSymbol Frame])
to the transmitted symbols before filtering. 
Finally, to obtain the original QAM symbols, FFT processing and cyclic removal are then 
performed by what is identified as the 
receiver
function. This also includes the 
equalization
function used to compensate for multi-path channel distortion of the resulting QAM symbols 
prior to EVM and SER estimations performed in VPI.
4.3.1 High Power Amplifier (HPA) 
Equally important, the additive distortion of the high power amplifier (HPA) on the transmitted 
WiMAX signals had to be precisely modelled in VPI for accurate network operation. This is 
displayed in Figure 4-8.
A limiter block is used for representing nonlinear transfer function characteristics exhibiting 
amplitude clipping and consequently distorting transmitted signal to noise ratio. This is then 
filtered to avoid excessive leakage of unwanted components to adjacent channels. The required 
transmitted EVM, as defined by the standard [4], could be adjusted based on setting the 
maximum and minimum amplitudes of the limiter block.