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But when this was presented to some of the commissioners, their response was that this would preclude the use of WiMAX and favor LTE. This statement amazes me, yes, amazes me

Damn the Interference; Full AWS-3 Ahead!

Thursday, December 11, 2008

Chairman Martin of the Federal Communications Commission (FCC) is determined to put the AWS-3 block of spectrum up for auction. M2Z is pushing hard for this spectrum and has agreed, as would be required of any winning bidder, to cover 95% of the U.S. population and to provide 25% of its bandwidth free of charge to anyone wanting to make use of wireless broadband at a speed of 786 Kbps.


I have a number of issues with this plan, the first being that the FCC and M2Z have an unrealistic idea of the true cost to build and operate the network, even if there is an advertising model to support it. (So far, no one has been very successful with generating advertising income in the wireless world.)


However, the biggest single problem with this plan is that the AWS-3 band, as planned, permits the use of the entire 25 MHz of spectrum for both transmitting from the cell sites to the devices and for the devices to transmit back to the cell sites. This is known as Time Division Duplex or TDD, and it requires the cell site and device transmitters to share the same spectrum with the cell transmitter “talking” to the mobile device and then “listening” for a response. This gives customers the allusion that they are using a system that permits simultaneous two-way communications, which is called “duplex” and is common in all commercial wireless systems. Yes, Clearwire is using this type of system, but not adjacent to spectrum using the standard method for deploying wireless voice and data systems.


Why does this matter? It matters because part of the AWS-1 spectrum is right next to the AWS-3 band, and AWS-1 is based on technology employed by commercial wireless networks within the United States and every commercial voice and data network around the world. This type of full-duplex operation technology is referred to as Frequency Division Duplex or FDD, and it means that systems use two different portions of the spectrum: one portion over which cells sites transmit and the other, lower portion over which devices in the field transmit back to the cell site. This system was set up from the very beginning of cellular phone service in order to prevent interference between network operators.


Operators may use spectrum next to other operators’ spectrum, but all of the cell sites transmit in the higher spectrum while they listen for mobile devices in the lower spectrum. There is no more efficient way to minimize interference between network operators, and this system has been working well for more than 25 years. Now, however, the FCC believes it can mix Time Division Duplex (TDD) and Frequency Division Duplex (FDD) on adjacent spectrum.


As a result, any company that has bid on and obtained a licensed for the AWS-1 FDD spectrum will be faced with having its spectrum subject to interference from the AWS-3 spectrum licensee. The FCC Commissioners are planning to approve the creation of the AWS-3 spectrum block and set an auction date in its December 18, 2008, open meeting. It has had engineers run tests, but in very controlled conditions, and has been convinced by those seeking the AWS-3 spectrum that there will not be any interference to existing AWS-1 spectrum holders.


But no one can be sure; other tests prove there will be interference. The net result is that AWS-1 spectrum license holders will not be protected from interference while all of the cellular spectrum license holders on other cellular bands are. If the FCC passes the AWS-3 Notice of Proposed Rulemaking, we will almost certainly see a number of problems arise due to interference.


Comments on this issue were filed not only by one or two of the companies that will be affected by this interference, but by many others including Motorola and Nokia. Those that won the AWS-1 spectrum at auction and are now building networks on it include T-Mobile, Metro PCS, Verizon Wireless, AT&T Wireless, a number of cable companies, and many more. All of these companies filed comments with the FCC as prescribed by law, but it appears as though the FCC Commissioners have chosen to believe those with little knowledge of radio communications who have assured them there will be no interference.


In addition to the Interference issues, others have concerns about why the auction is to be encumbered with free Internet access, and why there seems to be a favored bidder. The U.S. Department of Commerce on December 10, 2008, sent a letter to Chairman Martin questioning the AWS-3 auction and its terms and conditions, and the Wall Street Journal ran an op-ed piece that drew some interesting conclusions about why the AWS-3 auction was being held.


But Back to the Interference Issue


Interference, especially to the portion of the spectrum used to transmit signals from cell towers to devices, will have the greatest impact on the receivers in handheld devices, which cannot be as well protected as radios at cell sites where there is room to use filters and other methods of limiting interference. These small receivers will see the interference as noise that will block calls and lower data rates of the wireless broadband connection and customers will believe it is the network operator’s fault that the system is not working properly. They will not have any idea that the problem is being created by another network operator adjacent to the AWS-1 spectrum of their own network.


This is the same problem Nextel faced after it deployed its networkagain, in spectrum where other networks were not properly aligned. In that case, the interference caused by the Nextel network was to police, fire, paramedic, and other land mobile radio operators. It took the FCC years to even acknowledge there was a problem and more years to decide how to fix it. Nextel is still in the midst of what is called “rebanding,” which should solve many of the problems, but the first responders using adjacent spectrum had to suffer the results of interference for many years and the fix will have cost billions of dollars when it is finally completed in 2012 or later.


Spectrum is a finite resourcewe cannot make any more if it. We have to use what we have wisely, and that means we have to manage interference in order to maximize the efficiency of the spectrum we do have. Many of those filing comments made many of the same points I have, and many of them used sound engineering methods to explain why there will be interference, but to no avail. Those determined to move this “wireless Internet” project forward seem to believe we can ignore the laws of physics, or at least circumvent them with some new types of technology. The truth is that these are laws of nature that cannot be broken, and those within the radio community have the knowledge and experience to understand this. They also understand that the FCC is planning to move ahead with this plan regardless.


Most interesting to me is that T-Mobile, which is relying on AWS-1 spectrum (which it paid a lot of money for at the auction ) for its 3G network because it did not have enough prior to the AWS-1 auction, and is well into building out its network, not only have filed comments objecting to the interference issue, but they also filed a plan that would still permit the AWS-3 wireless broadband network to be built but in a way that would minimize the interference issues.


What it is proposing makes a lot of sense because it allows M2Z or whoever wins the AWS-3  spectrum at auction to obtain a license and build out its network while protecting AWS-1 license holders. I have long supported companies that don’t simply complain about a problem but work actively to find alternatives that will solve the problemin this case, still allowing wireless broadband on the AWS-3 spectrum. It should also be noted that anyone who believes T-Mobile’s1 objections are simply to stifle another broadband wireless market entrant should consider what T-Mobile has invested in this effort. It invested a lot of engineering manpower and dollars developing a solution that would both protect its own spectrum and accommodate auctioning and licensing of AWS-3 spectrum. Further, T-Mobile’s solution would actually increase the capabilities of AWS-3 spectrum holders when it comes to providing high-speed wireless data services.


T-Mobile’s recommendation is really very simple. The AWS-3 spectrum would be made up of what is known as the AWS-3 spectrum (not paired) and half of what is known as the J Block, (sometimes called AWS-2) which is currently paired in the same manner as typical cellular spectrum. Under the FCC’s plan, the bottom portion of the J Block sits fallow and is not really good for anything. However, with T-Mobile’s plan, the J Block becomes paired again and the entire AWS-3/upper J Block is used for the downlink while the lower portion of the J Block is used for the uplink, solving the problem and probably adding value to the AWS-3 spectrum as well.


But when this was presented to some of the commissioners, their response was that this would preclude the use of WiMAX and favor LTE. This statement amazes me, yes, amazes me, for two reasons. First, WiMAX has both TDD and FDD on its roadmap and in fact, several WiMAX companies are already shipping FDD equipment. Further, the Third Notice of Proposed Rulemaking for the 700-MHz D Block/First Responder spectrum includes three auctions: one for a nationwide license with the winner choosing the technology, one for 58 regional licenses using LTE, and one for 58 regional licenses using WiMAX. Because the D Block uses paired spectrum, deployment would require WiMAX FDD, not TDD, yet the commissioners think requiring TDD on the AWS-3 spectrum would preclude WiMAX—amazing!


Okay, those with a technical background reading this will probably say, wait a minute, that is a total of 30 MHz of spectrum for the downlink and only 5 MHz for the uplink and that is unbalanced. They are correct, but the data shows it is not that much of a mismatch. Let’s look at some numbers.



As Proposed TDD

Asymmetric FDD


AWS-3 + J Block Downlink

AWS-3 +J Block Up/Downlink

Duplex Approach



Total Spectrum (MHz)



Guard Band (MHZ)2



Usable Spectrum (MHz)



Downlink (DL) Bandwidth (MHz)



DL Spectrum Efficiency (bps/Hz)



Downlink Capacity (Mbps)



UL Bandwidth (MHz)



UL Spectral Efficiency (bps/Hz)



UL Capacity (Mbps)



Total Capacity (Mbps)



Overall Efficiency (bps/Hz)



Capacity Advantage





There is a lot more documentation available, but I think this is a clear case for choosing a better option than mixing TDD and FDD on adjacent spectrum. The T-Mobile solution actually provides M2Z or whoever wins the AWS-3 spectrum auction with more capacity and faster download data rates, and it solves interference problems for T-Mobile, AT&T Wireless, Verizon, the cable companies, and a host of other AWS-1 spectrum licensees. It is a shame that the FCC commissioners are more concerned about their legacy than they are about keeping our spectrum viable. I will guarantee you that this legacy is one of several that they won’t be so proud to put on their resumes in the future.


Andrew M. Seybold



1 T-Mobile USA is not a client of Andrew Seybold, Inc.

2 Guard Band will be required to help prevent interference and is already employed in Europe to help mitigate the problem

3 This equates to 5 MHz below and 5 MHz above the spectrum (AWS-1 is below and Mobile Satellite Services are above)

4 Based on a time asymmetry of 3.0

5 This chart courtesy of T-Mobile and it is part of its FCC public filings.

COMMENTS: This is an archived post. Commenting is no longer available.

Paul Zawada - 12/13/2008 16:51:01

Andy, Andy, Andy....

The folks at the FCC know that interference is only a metaphor. Modern radio technology doesn't have these problems. (Just check out .) Don't you know that the only reason that we have interference is because the old spectrum policies encourage people to buy only "dumb" receivers? If the FCC allows the winner of AWS-3 spectrum to do whatever the hell they want, the users of adjacent spectrum will finally have to demand modern technology that no longer relies on this oppressive metaphor. Finally, the FCC is listening to expert computer scientists instead of people with real world RF backgrounds!

If there does turn out to be a problem, the FCC will simply issue new rules to improve everyone's communications. If it hadn't been for their prior actions, the FCC would have had no opportunity to "Improve Public Safety Communications in the 800 MHz Band."


Andrew Seybold - 12/13/2008 19:32:43

Paul--of course you are correct, the radio systems which made use of 25 and 30 KHz channels (and still do) and whose receivers were designed to be "tuned" to a single channel really worked very well, now however, the "Modern radio technology" you refer to includes broadband services where receivers are expected to be able to receive 5 MHz of spectrum at once, anywhere in a band that might be 30 MHz or so wide.
Of course the RF engineers are not the right ones to build such devices, they should be built by computer folks who can use bits and bits to make sure that the receiver only hears what it is supposed to hear, even if the noise level is higher than the tranmist signal.
Today's RF engineers are designing handsets which include many different transmitters and even more receivers, and the antennas to transmit and receive these radio signal to the best of their ability.
I realize that "computers" can do all of this only better, so I guess that you are correct the entire AWS-3 and TV White Space rules are about getting the RF engineers who work with real world RF issues and interference to just turn the job over to computer engineers who have told all of us repeatedly that they can do a better job of utilizing the spectrum.
Thanks for the reminder, and I guess I will keep my landline phone for a while longer.