Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Chapter 4 FDM Simulation Platform with Wireless Compatibility 
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At each ONU/BS, the received optical signal is detected by an avalanche photodetector (APD) 
followed by RF subcarrier down-conversion to result to the transmitted WiMAX channels. To 
account for 1:16 splitter losses, a 14 dB optical attenuator is used after the fibre. 
Performance evaluation measures included EVM estimations at each ONU/BS1 remote antenna 
input as well as spurious free dynamic range (SFDR) graphs for various FDM subcarriers 
spacings. The 3rd order IMD products for direct and external modulation are modelled by either 
increasing the modulation index of the DFB laser or the modulation power inside the MZM RF 
input respectively.
4.5.1 EVM and SFDR with DML
As shown in Figure 4-10 (left), direct modulation of the five frequency shifted WiMAX 
channels, has produced an EVM versus laser modulation index figure higher than -31 dB, as 
required by the WiMAX standard for 64-QAM modulation [4]. This characteristic was drawn 
for various subcarrier spacing including 50, 100 and 300 MHz. In particularly the -31 dB EVM 
has been achieve at modulation indexes in the range of 0.8-0.9 for all subcarrier spacing.
Figure 4-10: EVM (left) and SFDR (right) estimation for DML

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Chapter 4 FDM Simulation Platform with Wireless Compatibility 
83 
In addition, the SFDR plot has displayed maximum dynamic range of 19 dB, with respect to the 
central FDM subcarrier at 3.5 GHz, for 50 MHz and 100 MHz subcarrier spacing at 0.8 
modulation index, as shown in Figure 4-10 (right). As the channel spacing is increased from 
100 MHz to 300 MHz, the SFDR is reduced as expected due to the higher level of 3
rd
order 
IMD products, interacting with the desired channels. However, even at the highest assumed 
channel spacing of 300 MHz, an EVM of -31 dB could still be achieved. Since direct 
modulation (DML) could limit the maximum number of allowed FDM subcarriers, due to 
SFDR, a chirpless option, produced by externally modulating the DFB lasers, was also 
investigated.