Spectrum: Multiplication Beats Addition
Martin Cooper recalls the days of mobile radio-telephones before cellular service:
You’d have one station in a city and you could conduct in that city 12 phone calls at one time. During the busy hour, the probability of connecting, of getting a dial tone, was about 10%. Of course, the reason was a city with 12 channels could support perhaps 50 people with reasonable service. They put 1,000 people on it. So the service was abominable.
The solution had been developing for a long time before Cooper made the first cellular call in 1973. Back in 1947, engineers at Bell Labs came up with a scheme for using relatively low-powered transmitters to serve hexagonal cells. With some care and cleverness in assigning channels, the same spectrum could be reused, provided the cells were far enough apart. Over time, AT&T developed the technology that allowed a call to stay connected as a mobile phone moved from one cell to another and Motorola created the mobile handsets. An industry and a new way of life was born.
The sort of subdivision that made the cell phone possible will also enable a vast expansions of the amount of data that wireless networks can carry without a commensurate increase in wireless spectrum. Get ready for heterogeneous networks, or hetnets, that will use a variety of techniques to chop up spectrum and space into smaller chunks that will allow for greater reuse.Get ready for heterogeneous networks, or hetnets, that will use a variety of technologies to boost data capacity.
Wi-Fi handoff will be a key part of the hetnet. It’s being used that way today, albeit in a somewhat random and uncoordinated way. Nearly all Wi-Fi-capable mobile devices are designed to switch to Wi-Fi for data whenever it is available. One big problem, is that the device has only a vague idea of what available means. This works fine when I come home and my devices automatically connect to the network, whose password is in memory. My iPhone connects automatically to AT&T hotspots and my iPad does the same for Verizon.
Many other networks, however, require a login. Sometimes it’s a password that you can enter and it will be remembered from then on. Sometimes its a popup page that just wants you to agree to terms and conditions. And sometimes it’s a page that require a username, a password, and often a credit card number for payment. While these methods vary int he annoyance they cause, all are a serious impediment to a seamless handoff. Even worse, is that your device will try to use a Wi-Fi network to which you haven’t connected, either because you lack a password or don’t care to pay. Sometimes you have to manually turn Wi-Fi off to get your phone or tablet to work properly.
Change is coming, through a technology known as Passpoint or Access Point 2.0. This will allow truly seamless handoffs between cellular and Wi-Fi (and perhaps, in the future, white space) networks, which the device itself providing authentication. The standards are nearing final ratification, Once that happens, says Doug Lodder, vice president for business development of hotpot provider Boingo, “the carriers will run it through their labs and will negotiate roaming agreements. It’s starting to roll out, but we won’t see widespread availability until 2014.”
Small cells. Traditional cell antennas, mounted on towers or other structures, typically serve a radius of from several kilometers to several hundred meters, depending mostly on the height of the tower. Small cells, also known as microcells, picocells, and femtocells, serve ranges from a couple hundred meters down to a few tens of meters. Home femtocells are designed to provide connectivity to otherwise unserved places and connect to the network through a residential broadband connection. But other small cells are a fully managed part of a cellular network, intended to multiply the use of spectrum by chopping areas into very small cells.
You can’t just plop small cells down in areas already covered by standard cell service, at least not using the same frequencies. The Federal Communications commission is proposing that the shared 3500megahertz band be dedicated to small-cell use. Higher frequency signals have shorter range and less ability to penetrate obstructions than the 700 to 2100 megahertz signals typically used for wireless data, making them well suited to small cells.
Small cells, and a related technology known as distributed antenna systems (DAS) have the advantage of making it much easier to provide good coverage inside buildings. As Cooper says, “It’s kind of an anomaly that if you think about it, most of our cellular conversations are in buildings and in offices, because that’s where we spend most of our time. But all the stations that provide services, almost all of them are outside. It’s kind of backwards.” Whereas small cells use multiple miniature access points, not unlike a Wi-Fi network, DAS splits the signal of a single base station among multiple antennas, each serving a small region. “You have smaller pipes, but fewer people attached to each pipe,” says Boingo’s Lodder. A single DAS array can also carry signals for several cellular networks.
Smart antennas. Cellular communication is a broadcast service. A single cell antenna covers, typically a 120° sector of its cell. But smart antenna technology makes it possible to focus that beam and steer the signal to a recipient, allowing closer reuse of spectrum. There has been a lot of research on smart antennas, but limited deployment in the field. A versions, called multi-input, multi-output (MIMO) is used with Wi-Fi and LTE, but the purpose has been more to extend range than to increase spectrum reuse. Smart antennas are one more tool in the engineering toolbox that can allow us to move a lot more data on the spectrum we have.
More wireless data spectrum is always welcome and the growth of demand for bandwidth probably cannot be met entirely within existing spectrum allocations, But new spectrum is getting harder and harder to find and the politics of prying it loose are exhausting and not terribly productive. Our best hope for meeting demand is to do more with what we have. And, fortunately, there is a great deal more that can be done.