Chapter 5 Interoperability of xPON and WiMAX
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manage resources across multiple ONU/BSs from the OLT according to traffic penetration and
requirement in bandwidth. This chapter presents a complete transparent optical/wireless access
network architecture based on RoF and FDM.
Figure 5-1: WiMAX over xPON architectural platform
The network architecture, depicted in Figure 5-1 [1], consists of a standard GPON with novel
wireless-enabled OLT and ONUs. As outlined in the previous chapter, the ONU/BSs are
relatively simple in their design since they are only responsible for down-conversion and
filtering of the received downstream radio channels. Each WiMAX transmitter at the OLT is
serving a single radio cell at an ONU/BS that is divided into small parts called sectors,
operating on the same or different frequencies, in order to make the cell more efficient in terms
of reduced co-channel interference and increased capacity. In some current deployment
scenarios all sectors are operating on the same frequency to reduce the cost of BSs, however in
the novel proposed architecture frequency reuse patterns of higher order could be considered
due to the deployment cost reduction enabled by the centralised processing in the OLT.
Chapter 5 Interoperability of xPON and WiMAX
90
A different RF subcarrier was dedicated to each ONU/BS in downstream to enable concurrent
transmission of multiple WiMAX channels to each ONU antenna. Within the OLT, standard
microwave OFDM WiMAX symbols [2], generated by Transmitter (Tx) for BS
n
, for different
sectors in each ONU/BS were up-converted to the predetermined RF subcarriers spectrum
before they were combined with the packets for wired users and subsequently modulated onto
the optical carrier for 20 km fibre downstream transmission.
Figure 5-2 illustrates how this technique addresses each individual ONU/BS. The microwave
RF wireless channels are shifted in frequency using a predetermined LO and BPFs in the OLT
prior to being combined and modulated onto an optical carrier. At an ONU/BS a single LO is
only required operating on the same frequency for the specific ONU/BS to downshift the
WiMAX channels. Multiple band pass filters (BPFs) are needed to select each channel prior to
transmission over the air. This approach would significantly simplify the BS design, compared
to traditional base-band or IF FDM over fibre approaches [3]. This is expected since only a
single LO and multiple electrical filters required. The LO leakage and higher order IMDs of the
non-linear mixer inside the OLT and ONU/BSs will not degrade the system performance since
in the deployed scenario the unwanted products fall outside the band of interest and are filtered
by the BPF.
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