|
Thursday, April 5, 2007
Andrew M Seybold
At our recent Wireless Data University (renamed Andrew Seybold University, School of Wireless Mobility), I talked about WiMAX, where it fits and what spectrum it will be deployed on. I stated that it would not be deployed on the new AWS spectrum that was just auctioned, nor on the 700-MHz spectrum that will be auctioned later this year or early next year, because WiMAX is a TDD (Time Division Duplexing) technology and the AWS and 700-MHz bands are reserved for FDD (Frequency Division Duplexing) technologies.
I was questioned by an attendee about this statement, but fortunately for me, Dr Mo Shakouri from Alverion, a WiMAX equipment vendor with a lot of product already in the marketplace, was a speaker and after his discussion of WiMAX and where it fits, I asked him the question about TDD and FDD and WiMAX. His answer was that today, WiMAX is a TDD-only technology and that it would be at least two years before there was an FDD flavor.
So what does all of this mean? What is the difference and why does it matter? Let's start at the beginning with two-way radio systems that were developed in two ways. The first was when the base station and mobile radio transmitted on the same channel. This is known as simplex operation (or, in first responder speak, these channels are known as tactical channels). This type of radio system worked, but range was limited, so a system was developed where the base station transmits on one channel and the mobile unit on a second channel. The mobile unit's transmission is then received by the base station and rebroadcast so all of the other units can hear both sides of the conversation. A later refinement of this technique was developed and it is known as trunking.
There are many advantages to having the base station transmit on one channel and the mobile units on another, and when the first cellular licenses were granted, this is the way the 850-MHz band was allocated. Both A and B block operators transmit in the upper portion of the band and the mobile units transmit in the lower portion. One reason for this is to minimize interference between operators. Since all of the high-power base (cell site) transmitters transmit on the same portion of the band and the receivers at the cell site receive on a different portion, the operators are less likely to cause each other interference.
This becomes even more important when you look at the PCS (1900-MHz) spectrum because now there are more than two license holders in most cities and four or five different operators can be sharing the same band. PCS channels were also allocated for one portion of the band for base station transmitters and one portion for mobile transmitters. The AWS and 700-MHz bands have also been allocated in this way.
|
