What’s Slowing Down Verizon’s LTE Speeds?

Verizon (s vz) continues to say it will launch its fourth-generation, super-fast Long Term Evolution wireless network to cover 100 million people by the end of this year, but it’s also been clear that it expects its LTE network speeds to be just 5-12 Mbps. So how does LTE — a technology that can deliver a theoretical 150 Mbps — get whittled down to less than a tenth of its top speed?
Thanks to the recent attention on the potential shortage of wireless spectrum for mobile broadband, and the hope that mobile broadband could act as a decent substitute for wired broadband, it’s worth figuring out how spectrum, cell towers and subscribers all factor into the speeds a wireless carrier can offer. Peter Rysavy, a wireless analyst, offers a great explanation using Verizon as an example. Note that in a Network World article, Tony Melone, executive VP and CTO of Verizon Wireless, confirmed that the carrier will use all of its 700 MHz spectrum for LTE service. Here’s what Rysavy has to say on that:

“Looking forward to advanced technologies such as LTE, capacity will higher, but it will still be extremely limited compared to wireline capacity. Verizon Wireless’ LTE network will operate in the 700 MHz band using 10 MHz radio channels. With a spectral efficiency of 1.5 bps/Hz, this delivers sector throughput of 15 Mbps.
Meanwhile, there are about 1000 subscribers in the US for every cell site, which makes for an average of 333 subscribers per sector. If 10% of them were using the LTE data service, that would mean 33 users for the 15 Mbps data channel. Now, compare this with a subscriber of a wireline high?speed Internet service of 50 Mbps that is dedicated, and not shared, as shown in Figure 2 (below). The point is not that the wireless network cannot deliver extremely useful and valuable services, since it can, but rather that wireless capacity is inherently limited compared to wireline capacity.”

Basically, Verizon can cram only a certain number of bits into each hertz — a function of how LTE allocates bits and a physics constraint. Multiply that number of bits times Verizon’s 10 MHz channels, which are dictated by regulatory policy as well as what Verizon paid for during the spectrum auction, and you have the top speed. Deliver those 15 Mbps channels to your cell sites (determined by Verizon’s investment decisions as well as community regulations that govern where towers are allowed) and divide by the number of devices sucking bits.
It’s complex math getting the infrastructure in place, but add in the infinite variables of iPads, smartphones, M2M sensors and even human traffic patterns and you have a situations in which delivering fast mobile broadband looks nothing short of miraculous.
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