Integrated Wireless-pon access Network Architectures Milos Milosavljevic

Chapter 3 Optical and Wireless PHY Integration
44 
have found application in WDM-PONs where they are typically installed at ONUs for 
colourless transmission [20, 21] since they are not limited by Rayleigh backscattering and small 
bandwidth typically encountered with RSOA devices [22]. Significantly, the use of VCSELs 
has been also proposed in RoF architectures [23, 24].
Figure 3-2: Triple-Format radio-over-fiber experimental set-up with long-wavelength VCSEL [24] 
As seen in Figure 3-2, successful transmission of UWB, WiFi and WiMAX has been achieved 
with long wavelength VCSELs in a point-to-point network architecture. A VCSEL bias current 
of just 7 mA has been demonstrated requiring low cost drive circuits. However, these 
investigations were performed over short fibre distances that are not common in typical 
deployment scenarios.
Although several research paths are set to investigate optical/wireless integration scenarios, they 
collectively do not deploy techniques such as RoF with FDM in a sense of providing 
bandwidth-on demand and redundancy. As a result they do not investigate extended features of 
these networks that could be gained by the integration, automatically distinguishing this project 
from these initiatives. 
 

---

Chapter 3 Optical and Wireless PHY Integration
45 
3.2 Layer-1 Integration
Figure 3-3 represents a common PHY structure widely applicable to all emerging broadband 
wireless standards including WiMAX and LTE [25]. This illustration could be utilised to 
demonstrate possible integration points with the xPON.
The WiMAX is widely described in detailed in the remaining chapters of this thesis. The LTE is 
not consider further apart from only at the very end as part of conclusion as an emerging 
technology. Nevertheless, it should be mentioned at this point that LTE should also be 
supported since in comparison to WiMAX, although uses same modulation format downstream, 
it provides for higher spectral efficiency, lower transmit time interval and it exploits current 3G 
network infrastructure. 
Following the forward error correction (FEC) block, user data is mapped to complex QAM 
symbols and structured according to the logical mapping mechanisms. Up to this point the 
packets to be transmitted to the diverse mobile users are still separated. Four possible splitting 
points (a-d) can be identified. These can vary from transmitting solely the user payload (as it is 
done today) and perform the complete baseband processing chain within the respective BS (a), 
up to finalizing the user specific processing within the central office (d) and solely performing 
framing and multicarrier modulation within the wireless nodes.
Once the user data is mapped to its logical bins the multicarrier frames are assembled. Here the 
physical assignment of user data is performed in (e). Additionally control signals are generated 
and added (e.g. maps and preamble for WiMAX, control channels and reference signals for 
LTE). Once the multicarrier symbols are assembled they are fed to the inverse-FFT (IFFT) and 
the cyclic prefix (CP) is added. After parallel to serial conversion the baseband processing is 
completed.

---