fiIn effect, T-Mobile and Sprint are sharing spectrum and that increases capacity. Large spectrum blocks are less likely to be congested, partly a result of queueing theory. Google Fi uses both Sprint & T-Mobile - and lots of Wi-Fi - choosing whichever has capacity when the phone needs it. With Sprint itself, when capacity is used up the data doesn't get through, even when there is spare capacity on T-Mobile. There's no way in practice to hand over the connection. The same, of course, applies to T-Mobile and every other network. 

Until now. When Sprint is congested but T-Mobile isn't, the connection automatically switches to the T-Mobile network and gets through. The two networks have different user bases, peak areas, and physical networks. The peaks are often at different times.

There's no reason Sprint and T-Mobile couldn't do the same with all their customers. The result would be the same as adding spectrum: with cooperation and sharing, you can handle more traffic.

The improvement will vary enormously by time and location. Occasionally, the peaks will coincide and there will be no gain. As far as I know, no one has ever modeled this effect on a real network. The ~20% is a wild guess.

Gabrielle Gauthey of Alcatel taught me this during a discussion of ideal spectrum policy for Kenya. A single 100 MHz LTE-A network has far more capacity than five 20 MHz networks. The first improvement comes from eliminating guard bands. The second is efficiency from more efficient queueing with the larger number of customers.

The most interesting advantage is that there's no practical way to give each network an ideal allocation. In the American context, at a busy moment in a particular area, AT&T might be at peak, Verizon at 85%, Sprint and T-Mobile much less crowded. Despite AT&T hitting peak, there's wasted capacity on the other networks.

My best guess is the single network would yield 30-70% more capacity and it could be even more. In addition, building one network is cheaper than building two, four, or six. Competition is great but the cost is very high. In Africa, India and Indonesia, there are very few landlines. Broadband will be almost entirely wireless, which for at least the next decade will have far less capacity. In Kenya's case, politics have held things up. Rwanda is moving ahead. African spectrum policy in many ways is more advanced than the U.S. or Europe. The Russian Minister tried to do the same. China's three telcos are joined in a tower company to avoid duplication. It's logical for them to eventually share spectrum. If you don't have competition, you need effective regulation, always hard to control. But in many ways we are heading to a "post-competition era." 

Verizon and the other U.S. telcos could similarly share networks, as could the Europeans.


An alternate approach to understanding the potential of network sharing comes from the important research on cognitive radio. Cognitive radio ultimately will be able to dramatically imcrease capacity and reliability. Sharing like this is a first step.

Breaking Spectrum Gridlock With Cognitive Radios: An Information Theoretic Perspective

Goldsmith, A. ; et. al

Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowledge about the activity, channels, codebooks, and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay, or interweave the cognitive radios' signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.

The Google service will probably prove revolutionary because it's designed for Wi-Fi first and contemporary efficiency. Google has been quietly hiring many of the best wireless engineers for several years and they have a slew of innovations soon to come. Here's how they are describing what they are doing.

Our three focus areas include: 

Helping you get the highest-quality connection
Project Fi aims to put you on the best network wherever you go. As you move around, the best network for you might be a Wi-Fi hotspot or a specific 4G LTE network. We developed new technology that gives you better coverage by intelligently connecting you to the fastest available network at your location whether it's Wi-Fi or one of our two partner LTE networks. As you go about your day, Project Fi automatically connects you to more than a million free, open Wi-Fi hotspots we've verified as fast and reliable. Once you're connected, we help secure your data through encryption. When you're not on Wi-Fi, we move you between whichever of our partner networks is delivering the fastest speed, so you get 4G LTE in more places. Learn more about our network of networks. 

Enabling easy communication across networks and devices
Project Fi works to get technology out of the way so you can communicate through whichever network type and device you're using. Wherever you're connected to Wi-Fi—whether that's at home, your favorite coffee shop or your Batcave—you can talk and text like you normally do. If you leave an area of Wi-Fi coverage, your call will seamlessly transition from Wi-Fi to cell networks so your conversation doesn’t skip a beat. We also want to help phone numbers adapt to a multi-screen world. With Project Fi, your phone number lives in the cloud, so you can talk and text with your number on just about any phone, tablet or laptop. So the next time you misplace your phone, you can stay connected using another screen. Check out how it works.

Making the service experience as simple as possible
Project Fi takes a fresh approach to how you pay for wireless, manage your service, and get in touch when you need help. We offer one simple plan at one price with 24/7 support. Here's how it works: for $20 a month you get all the basics (talk, text, Wi-Fi tethering, and international coverage in 120+ countries), and then it's a flat $10 per GB for cellular data while in the U.S. and abroad. 1GB is $10/month, 2GB is $20/month, 3GB is $30/month, and so on. Since it's hard to predict your data usage, you'll get credit for the full value of your unused data. Let's say you go with 3GB for $30 and only use 1.4GB one month. You'll get $16 back, so you only pay for what you use. Get all the details about our plan.

Be part of the project from the start
We're beginning Project Fi's Early Access Program to invite people to sign up for the service. Project Fi will be available on the Nexus 6, which we developed with Motorola and is the first smartphone that supports the hardware and software to work with our service. If you live where we have coverage in the U.S., request an invite at to get started. 

We look forward to connecting!