Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Chapter 4 FDM Simulation Platform with Wireless Compatibility 
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4.5 FDM Subcarrier Spacing Evaluation
Taking into consideration the developed models, in order to exhibit the network feasibility, and 
constitute the FDM architecture as a viable solution for the transparent transmission of WiMAX 
over PONs, transmission measurements were initially focused in downstream. Since IMDs, 
resulting from nonlinearities in the optical link, could degrade the performance of FDM 
subcarriers, the application of direct and external laser modulation was first examined. Simple 
frequency up and down-conversion of the WiMAX channels and their transmission over the 
fibre was evaluated. In upstream, TDMA operation is assumed (as detailed in chapter 5) 
therefore not requiring the application of FDM.
Five distinct WiMAX channels at 3.5 GHz are generated in the OLT, with frequency reuse of 
one for each base station, and shifted to address five ONU/BSs. The spacing of the five FDM 
subcarriers is varied in order to measure IMDs at an ONU/BS. To avoid the interference with 
G/EPON baseband spectrum the first subcarrier in the FDM window was set at 4 GHz.
The IEEE802.16d WiMAX channels, modelled in MATLAB, comprised of 64-QAM, 256-
OFDM modulation with maximum data rate of 70 Mbit/s per channel and relative constellation 
error (RCE) of -38 dB [4].
The combined FDM subcarriers were initially used to directly modulate commercially available 
distributed feedback (DFB) lasers at 1490 nm [16] and then applied over an AWG, supplying 
the fibre with +3 dBm power [16]. To implement an alternative scenario MZM was used to 
modulate the distributed feedback (DFB) lasers externally, with the FDM subcarriers applied at 
the modulator’s RF input. The laser output power was set to account for various optical 
component losses, resulting in +3 dBm power launched into the fibre.

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