Integrated Wireless-pon access Network Architectures Milos Milosavljevic


Figure 4-3: VPI/MATLAB components of the OFDM transmitter



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Figure 4-3: VPI/MATLAB components of the OFDM transmitter
Inside the OFDM coder, symbol mapping, IFFT and cyclic prefix are performed using 
MATLAB and the resulting output is passed to VPI for further processing (i.e. SER and EVM 
estimation). The MATLAB interface configuration is given in Figure 4-4. 
Figure 4-4: MATLAB interface for OFDM transmitter


Chapter 4 FDM Simulation Platform with Wireless Compatibility 
74 
In order to transfer the correct number of samples to the CoSim interface the PRBS sequence 
from VPI is initially packed into a matrix form containing Nbits=BitRate x TimeWindow 
number of samples, where the TimeWindow parameter represents the duration of simulation 
[9]. Consequently, the total number of symbols simulated (i.e. Nsymbols=N/BitsPerSymbol) is 
determined by this parameter.
The input bit stream is then passed to MATLAB and symbol mapping is performed by the 
cod
function described in Appendix A. The Appendix A provides detail simulation coding and 
corresponding functions for the OFDM transmitter and receiver.
Furthermore, this is followed with the conversion of frequency domain symbols to time domain 
by performing 256 point IFFT. The 1/4 cyclic prefix, using matrix concatenation, is also added. 
Once all symbols have been formed, the output matrix variable is passed to the VPI for digital 
to analog conversion and signal shaping. The digital to analog conversion is achieved by 
initially normalising the symbol amplitudes, followed by quantisation as shown in Figure 4-5 
[9]. The maximum number of quantisation levels considered in the simulations, as stated before, 
is 14 [4]. 
Figure 4-5: Quantisation process


Chapter 4 FDM Simulation Platform with Wireless Compatibility 
75 
Finally, square root-raised-cosine filters with a roll of factor of 0.25 [4], shown previously in 
Figure 4-3, were used for the I and Q channels to shape the signals and minimise inter symbol 
interference (ISI) prior to RF up-conversion. The bandwidth of these filters is equivalent to the 
OFDM symbol duration defined as its bit rate divided by the number of bits per symbol. 
For OFDM reception, the incoming signal is first down-converted and subsequently applied to 
an ISI shaping filter, using the same parameters with the one used at the transmitter. The 
receiver design is shown in Figure 4-6.

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