## 1 Introduction  
Passive optical network (PON) research and technology have matured over the last three decades and have established PONs as a cornerstone of today's deep fiber access solutions [1]. Emerging next-generation PON (NG-PON) technologies come in two flavors: (i) NG-PON1 technologies for an evolutionary growth of existent Gigabit-class PONs with co-existence requirements on the same optical distribution network (ODN); and (ii) disruptive NG-PON2 technologies for a revolutionary upgrade of current PONs without any co-existence requirements with existent Gigabit-class PONs on the same ODN [2]. NG-PON solutions include a number of performance-enhancing technologies, most notably, time division multiplexing (TDM) XG-PON to support (a)symmetric 10 Gb/s or even higher-rate TDM PONs, (dense) wavelength division multiplexing ((D)WDM) PON as an option to realize an overlay of multiple XG-PONs on the ODN, reach extenders to enable long-reach PONs, as well as code division multiplexing (CDM) and orthogonal frequency division multiplexing (OFDM) PONs. Figure 1 shows the anticipated NG-PON roadmap and migration from widely deployed ITU-T G.984 GPON and IEEE 802.3 ah EPON to near-term NG-PON1 and mid- to long-term NG-PON2 broadband access solutions, as envisioned and widely agreed upon back in 2009. Beside resolving the notorious cost and complexity issues of cost-sensitive access networks, the primary design goal for future NG-PON1&2 broadband access networks was the provisioning of an ever increasing capacity over time, as illustrated in Figure 1.  
Clearly, NG-PON capacity upgrades are needed to support increasingly video-dominated traffic, handle the explosion of mobile data traffic by means of offloading, and also stimulate the creation of new services and applications. On the other hand, it is vital to the business needs of next-generation optical access (NGOA) network operators to recognize when \"enough is enough.\" To better understand the consequences of blind capacity upgrade strategies beyond the actual need, it may be helpful to go  
Figure 1: Next-generation passive optical network (NG-PON) roadmap as of 2009 [2]. back in history of optical networks. During the Internet bubble, the expected bandwidth requirements were hugely overestimated and way too many optical networks were built, flooding the market with unneeded capacity. As a consequence, prices for dark fiber became so low that customers, e.g., banks and corporations with large data transfer needs, started to buy up low-cost dark fibers and run their own optical links and networks. Similarly, the prices for monthly leases of optical fiber conections decreased significantly. For instance, the prices for monthly leases on 10 Gb/s links between Miami and New York City fell from around $75,000 in 2005 to below $30,000 at the end of 2007, and prices for 10 Gb/s connections between New York and London fell by 80% from 2002 to 2007 [3]. To make things look even dimmer for network operators (but not necessarily customers), it is worthwhile to note that about 80%-90% of the world's installed fiber is unlit, _i.e._, is not used, and only 18% of the world submarine fiber is lit [4]. In the PON community, researchers from both industry and academia have begun to contemplate on what the future may hold for NG-PON1&2 and beyond. Recently, at the 2013 OFC/NFOEC conference, a workshop on \"Post NG-PON2: Is it More About Capacity or Something Else?\" was held to find out whether it is reasonable to ever increase the system bandwidth or rather explore service and application as well as business and operation related aspects, which motivate access technology to move into a substantially different direction in the long rung than continued capacity upgrades. Clearly, NG-PONs are expected to play an important role in the support of coordinated multipoint (CoMP) coordination schemes among base stations (BSs) in 4G LTE/WiMAX networks.