FEATURE MOBILE FRONTHAUL
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point in the network. ‘When operators purchase services from a third party it’s very useful to have a demarcation point. WDM systems by their design happen to have this convenient point,’ said Lively. Te grey optics from the RRU can plug into the add-drop, which may be at the bottom of the tower, to form the connection into WDM once plugged into the line side.
Converging the network Te growth in mobile data will not just push the data rates skywards, but may change the way that access networks are built as operators need to connect up many more antenna than before. ‘Five years from now we’re going to be
looking at early stage deployment of 5G, whatever that is, and going to be looking at next-generation optical access,’ said Lively. ‘Lots of people are talking about getting greater efficiency by carrying multiple services on the same network. Tis is being talked about quite a bit in conferences. If you have an access network that can do gigabit then it’s quite conceivable that it could do fronthaul.’ Te upcoming version of passive optical networks (PON), NGPON2, will be able to
Fronthaul links need to meet strict latency and jitter requirements
so-called ‘grey optics’ at either 850 or 1310nm wavelength. Speeds range from 1 Gb/s to 10 Gb/s, with 3, 6, and 10 Gb/s being the most common today, but the product mix is already shiſting towards the higher end of the range, said Lively. Tere is also limited demand for 12.5Gb/s
but nothing beyond that, and 12.5Gb/s demand is unlikely to grow quickly, he adds. Tat is because the individual RRHs are not increasing in capacity. Instead, mobile operators add capacity by adding more antenna, each of which requires its own RRH and fronthaul connection. Bandwidth demand is satisfied mainly by the proliferation of links rather than by increasing the speed of the individual links, Lively said. ‘One of the interesting aspects is that if you
look at how it might evolve, the initial deployments were mostly point-to-point links using grey optics, but there have been a few instances of operators using WDM optics,’ he
24 FIBRE SYSTEMS Issue 6 • Winter 2015
noted. Te South Korean operator SK Telecom used WDM fronthaul when it built out its LTE network, and now has tens of thousands of small cells connected this way. WDM has an obvious benefit in situations
where optical fibre has to be leased on the mast, or cables aren’t available in sufficient numbers. In the case where you lease the fibre and pay by the strand it becomes very important. Coarse WDM would provide up to 16 channels, but DWDM would provide many more. ‘We have up to 15 remote radio heads on a
single site but we don’t have 15 fibres up the pole so we need some kind of multiplexing,’ noted Anna Pizzinat, senior research engineer at Orange Labs R&D. Te French operator isn’t yet deploying this technology, she adds; it is waiting for a low-cost DWDM technology to be developed. Another scenario where operators might choose WDM is to provide a demarcation
support fronthaul speeds and distances easily. However, fronthaul links need to meet strict latency and jitter requirements in order to synchronize the transmissions across the mobile network. Te approach that has been chosen for NGPON2, which is based on both time- and wavelength-division multiplexed passive optical networking (TWDM-PON – see article, page 26), may face some challenges in this regard. As a result, the Full Service Access Network
(FSAN) forum has set up a Mobile Fronthaul Study Group, with Orange taking the lead. Te Study Group will collect mobile fronthaul requirements from operators and evaluate the technical challenges before presenting its recommendations on how best to implement the technology. One potential method would be to assign one of the available wavelengths defined for
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