700-MHz LTE

I have been watching the news reports recently and some reporters and analysts have finally picked up on the fact that having an LTE device in the 700-MHz band does not ensure that it will work on both AT&T and Verizon Wireless, nor the other 700-MHz license holders’ networks. Few people really got that until recently, but since I have been working on the Public Safety portion of the 700-MHz band I have known about this for some time.

The reason most smartphones and perhaps even dongles and tablets won’t be able to make use of LTE on both AT&T and Verizon networks has nothing to do with the LTE standard. Both companies are deploying LTE that meets the 3GPP specifications although there may be some differences in the options implemented by these companies, theoretically a device should be able to be used on either network.

The reason this won’t happen, at least in the near term, is not a marketing decision nor is it an anti-competitive decision, but a decision based on construction costs for the devices. The 700-MHz band spans spectrum from 698 Megahertz (old TV Channel 52) to 806 MHz (old TV Channel 69). This is a lot of spectrum (108 MHz to be exact) and while chipsets can and do cover the entire spectrum, other parts of a phone (radio) do not.

As can be seen by the chart below, the 700-MHz band is actually two bands, or to confuse us further is it really three bands as defined by the 3GPP standards organization. These bands are referred to as band class 13, which is the upper portion of the band; band class 14, which is basically the Public Safety and D Block spectrum; and band class 17, which is the lower portion of the spectrum. Verizon’s C Block is located within band 13 and most of AT&T’s spectrum is located within band 17. Or to simplify it, AT&T’s spectrum is mostly in the lower portion of the band and Verizon’s spectrum is in the upper portion of the band.

(Courtesy of the FCC)

The rub is that while chipsets that provide the basic foundation of the devices are capable of providing access to the entire U.S. 700-MHz band, presently, the software built above it is built for only one portion of the band. Further, the filters and duplexers that are also a part of the phone, as well as the antenna system, at present can only support one portion of the band or the other. What is a duplexer? It is a device that permits full duplex operation of the transmitter and receiver using a single antenna. It is a series of filters that has to be tuned carefully to mix both transmit and receive signals on a common antenna. For this reason, it cannot be as broad as the entire band. Therefore, in order to provide a device (smartphone) that will cover both bands, two sets of filters and duplexers will be required. It is possible that dongles might be first to market with both bands in them since adding more components to a device causes some other problems.

A smartphone that is designed to work on 2G and 3G systems as well as Bluetooth and Wi-Fi, and that has a GPS receiver in it, is already chock full of components and antennas. Adding one more is being done but adding two more becomes an issue. If you want the device to be a world device capable of operating on all of the U.S. bands plus those in other parts of the world, the guts of a device become even more complex. If you then want to build the 700-MHz portion of the device with MIMO on LTE (two antennas to improve the range and throughput), we are really giving the design engineers a series of tough challenges.

Another glitch in all of this is that some network operators will also be using the AWS-1 spectrum for LTE in the future so in addition to 700 MHz, 850 MHz, and 1900 MHz, the devices will have to be capable of operating on the AWS-1 spectrum. As the FCC makes even more spectrum available for broadband services, the number of bands a standard device has to be capable of will only increase.

All of these engineering considerations must be taken into account as well as the cost of the parts. I have heard estimates that the add-on cost of building in the extra components to cover both portions of the 700-MHz band could add around $8 to the total cost of a phone. If you are a network operator that subsidizes the cost of a phone this means you will have to absorb this $8 bill of material increase just so your customer can leave your network and move to your competitor’s network.

Software-defined chipsets are making it easier to build more radio frequencies and more over-the-air technologies into a single device but the antennas and associated circuitry are the issue. Sometimes the same antenna can be used for multiple purposes as there are electrical relationships between lengths or they can be made to work okay on multiple frequencies, for example, Wi-Fi and Bluetooth can easily share the same antenna because they are in the same band. However, if both are to work at the same time, some additional and tricky engineering has to be employed to make that happen.

Now to add to all of this, while it is obvious that LTE is fast becoming the world standard for fourth-generation broadband services and at some point voice services, at present, LTE is being deployed on more than fourteen different portions of the spectrum worldwide. This makes it virtually impossible for a U.S. wireless device to become a world phone for LTE as well as to be able to operate across 2G, 3G, and U.S. LTE networks. The very smart engineers who develop these phones are already putting 25 pounds of components into a small 15-ounce package. There is some good news, perhaps. Several companies are working on active antenna technology, or antennas that can change their characteristics and operate on multiple bands. Part of the solution may include these new antenna technologies.

In the meantime, we have what we have. U.S. LTE devices will operate on 2G and 3G systems and one of the two LTE 700-MHz bands. There are some who believe that the FCC should mandate that every LTE device in the United States must be able to be used on both the lower and upper portions of the 700-MHz LTE band. I don’t believe this should be mandated by law since the challenges in providing such a device are both cost and engineering related. Mandating across-the-board 700-MHz devices might mean losing fallback capabilities on one of the other bands as well as an increased cost to network operators, which in turn will have to pass these costs along to the customer.

Once again, those who are looking at all this are not people who understand the nuances of radio and antenna technologies. The industry will sort this out at some point in the future, just as we have before. In the meantime, the operators should be permitted to make their own decisions, after all, how is the LTE issue any different from today’s 3G issue? An iPhone or other device on AT&T’s chosen 3G technology is not compatible with the 3G technology used by Verizon. So switching networks today means acquiring a new device. Wireless technology is very complex and those considering making the rules should take the time to understand the trade-offs and engineering issues involved. Some things are simply not possible even if you pass a law mandating them. There are laws of physics and while our engineers have been good at bending them a little, we cannot break these laws even if some mandate says we must.

Andrew M. Seybold