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Verizon has already launched two distinct LTE networks since it first turned on 4G in 2010, but now it’s started paving the way for the third. Unlike the first two, however, this new network won’t tap virgin airwaves. Instead Verizon has started cannibalizing its old CDMA EV-DO systems for PCS spectrum, marking the beginning of what will likely be a very slow death for 3G.

Gigaom’s favorite network spotter Milan Milanovic discovered signs of [company]Verizon[/company]’s new LTE network in Manhattan when he connected to it with a Nexus 5 (a device Verizon traditionally hasn’t supported because it doesn’t work over its primary 4G network) and a Galaxy S4. The 1980MHz/1990MHz chunk of frequencies has traditionally been part Verizon’s 3G EV-DO network, or it was until last month when Milanovic noticed it was turned off (Milanovic happens to be kind of guy who totes around an industrial spectrum analyzer.)

This week, though, Milanovic noticed that those empty airwaves had once again jumped back to life, but with LTE instead of CDMA signals. Milanovic said that he’s found Verizon LTE in the PCS band at cell sites all over Manhattan, but so far nothing in Brooklyn and Queens. The transmit power of the network is still very low and the internet speeds he’s getting are still very slow, he said, indicating that Verizon is still in the early stages of testing. There have also been reports on network-tracking site SG4U of LTE popping up on Verizon’s PCS band in Cleveland.

I pinged Verizon, and spokeswoman Debra Lewis confirmed that Verizon is indeed testing LTE on the PCS band, though she said Verizon wouldn’t go into any specifics on locations or timing for a commercial launch. Lewis also made the point that this should hardly come as surprise since Verizon has said it would begin repurposing a portion of its 3G bandwidth for LTE in 2015. In fact, as early as 2011, Verizon CTO Tony Melone told me Verizon would likely shut down 3G completely as all of Verizon’s data traffic moved over to 4G networks.

That day is still a long time coming, though. About 80 percent of Verizon’s mobile data traffic now rides over LTE, but some 40 million (41 percent) of the total devices on Verizon’s networks only have 2G and 3G radios. That means for the foreseeable future, Verizon will have to keep a modicum of EV-DO capacity online at every cell site to support those devices. That’s what we’re starting to see in NYC at least: Verizon appears to have shut off half of its upper-band 3G capacity across Manhattan.

As for 2G, it will be around even longer than 3G since it’s still Verizon’s primary voice network, but eventually Verizon will begin the bulk of its voice traffic onto its new voice-over-LTE service. What we’re witnessing is the very beginning of a long, slow march toward death for Verizon 3G and the gradual transformation of Verizon into a carrier providing all its voice and data services over a single network technology.