To the Editor,
I just read the May 3 issue of RCR and wanted to say thanks for the mention and to congratulate you on a good discussion of TDD versus FDD. I especially appreciate your candid discussion of the issues with the benefits of TDD clearly listed.
I am concerned, however, that some of the information the FDD sources gave was unclear and not in perspective. So, I offer the following in order to put the comments in the proper light.
FDD historically was used for fixed wireless, point-to-point systems because the communication signals were analog and the more robust and flexible digital scheme, TDD, could not be used since it requires the signals to be digitized and buffered. So I would posit that TDD is yet another step in the digitalization of information and communications systems for increased flexibility and cost reduction in our society. This, then, is not unlike the introduction of the T1 in the 1960s over analog multiplexing, digital PCs in the late ’70s over existing analog ones, digital music CDs in the ’80s over analog record albums, and digital PCS in the 1990s over analog cellular, etc.
Recently Nokia, California Microwave, and several other companies have endorsed TDD as a next generation approach for fixed wireless access.
FDD is proven only in its historical use for point-to-point, backhaul communications going back to the 1920s. For the point-to-multipoint, BWA industry that is the current focus, no technology has yet been proven. In fact, the industry in general has suffered through approximately six years of active trials without a major deployment. The main reason is the high cost and low functionality of the use of legacy approaches such as FDD from point-to-point, backhaul systems to solve the next generation challenge of point-to-multipoint, access. TDD, with its lower cost radio architecture and higher revenue potential, is just the type of technology that the BWA industry has been needing to get going after all these years of false starts.
Many TDD systems are commercially available today. In fact the founders of Ensemble come from two companies, PCSI and CTP Systems, that were responsible for the deployment of more than 100,000 TDD base stations in less than 18 months in Japan. And we are applying that knowledge in bringing TDD to the BWA marketplace.
When the FDD vendors referred to the fact that most of the spectrum allocations were done with FDD in mind, they were again focusing on legacy point-to-point systems, not the point-to-multipoint systems that BWA license holders demand currently. And I would go further to say that most of the point-to-multipoint allocations in fact are not done with FDD in mind. Recent allocations, such as the LMCS bands in Canada, the LMDS bands in the U.S., MWS in Europe, LMDS in Australia and New Zealand and various others around the world, were done without restriction as to FDD or TDD, leaving it up to the carriers to decide how to use their spectrum. Block allocations in BWA licenses which most point to multipoint bands are these days, lack the guard band needed for FDD to work and so in fact are allocated specifically without FDD in mind.
Finally, all of the studies that we have seen about broadband access to small and medium businesses, the focus of the BWA industry, conclude traffic is not symmetrical, but rather is asymmetrical, and worse the asymmetry varies based upon particular applications and ultimately time of day.
Further, what the traffic may average out to during some period of time is not the proper focus, rather it is the instantaneous bandwidth needs of users.
And I would say that the increase in e-commerce and other upstream traffic streams cited by the FDD vendors that will make the average traffic needs more symmetrical actually will make the variable asymmetry needs of end users worsen due to the introduction of yet another asymmetrical service into the access pipe.
The awareness of the benefits of TDD are growing which is evidenced by the fact that both the European standards body, ETSI, and the American standards body, 802.16, have both agreed to support both legacy FDD and new TDD standards for BWA.
Sincerely,
Carlton O’Neal
Vice President-Marketing and Sales
Ensemble Communications Inc.
San Diego, CA
