Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Chapter 3 Optical and Wireless PHY Integration
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Figure 3-8: Generating RF signal by direct modulation (left) and external modulation (right) [32] 
One drawback of the RF and IM-DD techniques is that it is difficult to be utilised for high 
frequency mm-wave applications due to the fibre chromatic dispersion and limited bandwidth 
of electrical components. However, the RoF approach described in this thesis is based on 
transmission of microwave frequencies and is therefore not limited to the aforementioned 
impairments. 
3.4.1 Effects of Fibre Chromatic Dispersion on IM-DD Link 
In IM the resulting optical spectrum will consist of two sidebands located at the wireless carrier 
frequencies on either side of the optical carrier, exhibiting an optical double-sideband (ODSB) 
modulation format. When the optically modulated signal propagates along a dispersive fibre, its 
two sidebands will experience a different amount of phase shift relative to the optical carrier. 
Upon detection at the system receiver, the square-law process generates two beat components at 
the wireless frequency. To that extent, the received RF power of the wireless signal varies 
depending on the relative phase difference between the two beat components. This variation is 
dependent on the fibre dispersion parameter, the transmission distance and also the wireless 
carrier frequency used [33]. 
The effect of chromatic dispersion on the analog wireless signal has been demonstrated, using 
VPI, by the development of a simple P2P system based on external optical modulation, as 
shown in Figure 3-9. An electrical impulse is used to excite the fibre at all frequencies. The 

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Chapter 3 Optical and Wireless PHY Integration
52 
thermal and shot noise figures of the photo-diode are excluded from this analysis in order to 
determine only the effect of the chromatic dispersion.