Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Figure 3-6: Antenna at the pole of a building facade [25]
 
 
 

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Chapter 3 Optical and Wireless PHY Integration
49 
3.4 Simulation of Radio Signal Transmission over Fibre 
RoF signal transport over optical fibre links has been extensively studied in the past as a 
possible technology for simplifying the architecture of remote BSs [27-29]. While the fibre has 
enormous bandwidth to support high capacity transmission, current and future wireless 
applications will still use a wide range of frequencies [27].
This raises more challenges to the 
implementation of RoF systems in a cost-effective manner since in an analog photonic link the 
wireless signals experience a number of inevitable signal impairments, especially at higher 
frequency bands, as illustrated in Figure 3-7 [30, 31]. 
Figure 3-7: Typical impairments with RoF links [30] 
These impairments arise as a result of the nonlinear characteristics of the optoelectronic 
conversion process at the transmitter and receiver which leads to increased inter-modulation 
distortion and reduced dynamic range. Studies have shown that nonlinearities in the optical 
front-end limit the overall system dynamic range [27]. To minimize nonlinear distortion arising 
from optoelectronic conversion, the amplitude of the drive signals has to be greatly reduced 
which subsequently leads to low conversion efficiency and reduced link gain.

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Chapter 3 Optical and Wireless PHY Integration
50 
Considering the frequency of the RF signal is fed into the RoF link at the head-end, RoF 
implementation may be classified into three categories, namely RF-over-fibre, IF-over-fibre, 
and baseband-over-Fibre [32]. The RF over fibre approach involves the transmission of the 
actual RF signal over the fibre. However, in case of IF or baseband over fibre the desired 
microwave signal is generated at the remote antenna units through up-conversion with a local 
oscillator (LO), which is either provided separately at the remote antenna units, or is transported 
remotely. Therefore, the complexity of the base station will depend on the transmission method 
used with the RF over fibre approach providing for the highest simplicity. In that direction, 
simplicity of implementation is supported in the current network architecture by FDM to reduce 
component cost and signal processing requirements at remote radio heads. 
To generate and transport microwave signals over a fibre, a variety of schemes can be applied.
The most popular and cost effective solution is based on IM-DD [32]. In this approach, the 
intensity of the light is directly modulated with the analog signal, using direct detection at the 
photo-detector to recover the original signal.
IM could be either performed by directly modulating a laser current or using an external 
modulator, such as a Mach-Zehnder modulator (MZM), to externally modulate the intensity of a 
continuous wave laser. The two options are displayed in the Figure 3-8 [32]. After transmission 
through the fibre and direct detection on a photodiode, the photocurrent is a replica of the 
modulating RF signal. 

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