Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Chapter 6 Multi-Wavelength WiMAX-PONs with Overlapping Cells 
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6.2 Network architecture design 
The revised network architecture is shown in Figure 6-1. The OLT and ONU/BSs are modified 
to accommodate multi-wavelength transmission over a passive splitter in the remote node. Any 
wavelength in the selected operating spectrum could be partially or exclusively assigned to 
different ONU/BSs, providing in the latter service levels similar to WDM-PONs without 
requiring any modifications in the ODN. Therefore, this approach will provide smooth 
migration path from TDM-PONs to WDM-PONs.
For simplicity, the centre frequency of the tuneable optical filters in ONU/BSs can be adjusted 
from the OLT by means of a controller circuit. Unused AWG input ports, as shown in Figure 6-
1, could be exploited to assign a unique wavelength to each ONU/BS or to support multiple 
wireless-enabled PONs from a single OLT, considered as a prerequisite for NG-PONs [8]. 
Figure 6-1: Multi-wavelength WiMAX-PON
On each wavelength, multiple microwave WiMAX channels are arranged in a FDM window to 
address individual ONU/BSs. The same FDM window could be carried on multiple 

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Chapter 6 Multi-Wavelength WiMAX-PONs with Overlapping Cells 
118 
wavelengths relaxing significantly the bandwidth requirements of the optical and electrical 
devices in the network. In addition, the maximum radio frequency on a single wavelength is 
relatively low therefore no dispersion compensation is required.
Figure 6-1 displays the formation of overlapping cells/sectors, e.g. between ONU/BS2 and 
ONU/BS9, operating as expected at different frequency channels. Since different ONU/BSs are 
operating on different radio channels no interference would be expected between overlapping 
wireless users and thus maintaining their network capacity. The transmission distances formed 
between ONU/BSs in Figure 6-1, allow for higher spectral efficiency compared to traditional 
WiMAX deployment while could be potentially extended with the application of relay 
techniques [9]. 
The use of low-cost long-wavelength VCSEL arrays [5, 10] at ONU/BSs demonstrate colorless 
terminations upstream with simple coupling optics. In contrast to RSOAs [11], VCSEL arrays 
do not require wavelengths to be used in upstream to have been first transmitted simultaneously 
with downstream data. Although RSOAs could be possibly investigated [11], interference 
among multiple delayed versions of the upstream signal generated due to Rayleigh 
backscattering could potentially degrade performance. Following the use of VCSEL arrays, 
upstream wavelength selection can be managed by a means similar to tuning the ONU/BS 
filters. TDMA, where each ONU/BS is transmitting at specific time slot on a single wavelength, 
could still be applied upstream [12]. 

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