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Frequency division duplex (FDD) systems keep a customer's upstream and downstream transmissions from interfering with each other by transmitting them on frequencies which are sufficiently separated so that an electronic filter can keep the upstream and downstream signals separate. 

Time division duplex (TDD) systems avoid such interference by never having an upstream transmission occur at the same moment as a downstream transmission.  Transmission time is divided into very short "slots," and the system assigns permission for slots to be filled with upstream and downstream data at different times.  The allocation of slots can be changed according to the nature of demand, so that, say, the system might assign 90% of slots for downstream use at one moment, and only 75% to that purpose shortly thereafter if a number of customers simultaneously seek to upload large files.  In theory, this flexibility makes TDD systems more efficient than FDD systems. 

Because of the reliance upon time slots,  proper timing of TDD systems is crucial.   All parts of the system need to use the same precise "clock." Timing and assignment of slots is all the more crucial in multi-cell TDD systems as a means of controlling self-interference. 

For the purposes of an ITFS licensee, the potential benefit of a TDD system is that no frequency separation of upstream and downstream transmissions is necessary, which makes it easier for a single four-channel ITFS system to be able to operate independently.   Because of the rules governing adjacent-channel interference, however, it is likely that one would have to de-interleave an ITFS licensee's channels to form a contiguous block in order to gain such independence. 

Copyright 1999 John B. Schwartz

Acknowledgment

I would like to thank those in the field who took the time to comment on and suggest revisions to this article, and, in particular, Dane E. Ericksen, P.E., of Hammett & Edison, Inc., Consulting Engineers, a firm located near San Francisco, California. 

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John B. Schwartz
P.O. Box 6060                                   Telephone   303-442-2707
Boulder, CO  80306                              FAX         303-442-6472

schwartz@usa.net

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