“Today, a Passive Optical Network (PON) uses one fibber and one transceiver at the central data station per street or district. This saves money, and because a large number of users connected deployment of PONs also significantly reduces space and power requirements. In theory, every connection to individual home in a PON may be equally long. However, the situation is often less optimal: some users may be very close to the central data station of the network, while others are quite far away. There might even be an extra power splitter between them and the main PON. This leads to differences in signal quality: the closer the user, the higher the signal quality. Since providers want to guarantee a minimum data rate for everybody, current networks are over-dimensioned, leading to non-used overcapacity.”

 

Robbert came up with the idea of applying ADSL/VDSL modulation techniques, normally associated with copper, to fibre as a way of solving this challenge. “Copper networks have more challenging channel conditions than fibre. That’s why adaptive modulation techniques have been mandatory in copper networks to achieve an acceptable performance. However, current fibre standards still allow up to roughly 15 dB of difference in received signal quality - a relatively high difference. At the same time, standards allow for only one single method of communication, leading to possible network inefficiencies.”

 

“We acquired a data set describing channel qualities of a deployed network. This showed us the standards allow for this variance, and the examined deployed networks also actually showed this variance. If we could optimize this factor, it would improve the networks. The data allowed us to make accurate predictions on the real-world influence of our system and offered a solid basis for our lab tests.” The team did need to overcome some technical challenges. As current fibre access networks only utilize On-Off Keying (the laser is either on or off), the implementation of this new system would require upgraded electronics to support higher order modulation formats (there are multiple possible amplitudes to transmit). “The big benefit of our solution is that we can reuse the current optics - a relatively high portion of device costs. Our approach allows more efficient utilization of already deployed equipment. Now, the worst-case scenario no longer determines the performance of the entire network, but the performance is tailored to the actual network deployment. This offers users higher data rates and reduces the cost per bit, thereby reducing costs for network operators.”

 

Robbert believes that if the growing connectivity demands of society and business aren't met, serious congestion will occur. This makes dynamic bandwidth allocation in the PON necessary. “There is enough bandwidth in the ground, but this is configured in a static way. We need to come up with a solution that allows greater flexibility in allocating bandwidth to wherever it is needed. For example, during working hours we need to bring more bandwidth to industrial areas and after working hours to residential areas. The same strategy should be applied to accommodating hotspots wherever a concentration of people occurs at a certain time. Another aspect that needs to be addressed is opening the PON to more operators. This can be done by virtualization of PONs, allowing other service providers to enter the same infrastructure to reduce operational costs. With 5G wireless networks on the way, optical front- and backhauling for a large number of base stations needs to be fitted with PONs.”

 

Robbert van der Linden’s research was part of the FlexCom project within the NWO-TTW Perspectief program MEMPHIS II, and carried out in cooperation with fibre broadband equipment supplier Genexis. Robbert and his team are now looking into options for incorporating the new techniques in chips. “We’re also looking into developing a method for virtualizing PONs. Control and management protocols could steer the hardware and also guarantee access for other players to the PON infrastructure.”