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