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Old 22-Jan-2010, 6:59 PM   #2
andy.s.lee
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Join Date: Dec 2009
Posts: 54
Part 2

We've shown a very basic equal-power two transmitter example to keep things simple and to help with visualization, but in reality, there are going to be different transmitter powers, heights, terrain obstructions, and multiple transmitters (perhaps much more than 2) that are going to complicate things.

For example, if we just make one small change where we now look at an uneven power example, we get something that looks like this:


What we see here is that the worst-case interference area (where the two signals approach equal power) has shifted to the right because the transmitter on the left is much stronger than the transmitter on the right. The balance point is now closer to the weaker transmitter.

However, we can also see that the equalizer "corrections" are still taking place at the half-way point between the transmitters (there's a slight tweaking of the coverage map colors in the middle). That is because all along we have been assuming the both transmitters are perfectly synchronized and are broadcasting their signals at identical moments in time. Since most equalizers have their best performance when the echo delay offsets are near to 0 (see equalizer profile chart above), that puts the equalizer's best coverage in the places that are equidistant from both transmitters. Since the equal-distance point is not the same as the equal-power point, the "sweet spot" for the equalizer is not providing help where it is needed the most.

So now what? Well, as it turns out, we don't really have to restrict our transmitters to emitting their symbols at exactly the same time. As part of the transmitter setup, it is possible to programmatically set each transmitter to add or subtract a small timing offset when referencing their local synchronization reference (GPS-derived clock). By artificially advancing or delaying the relative time of transmission from each site, it is possible to adjust things such that the equalizer is at its peak performance at the locations where DTS mutual interference is predicted to be at its worst. To demonstrate this effect, the following animation shows how the equalizer "sweet spot" can be moved with different timing offsets between the transmitters (the image goes back and forth with relative timing offset between -200 and +200 microseconds):


If done correctly, the DTS network can be optimized by controlling power levels and timing offsets so that the probability of coverage gaps is minimized. Of course, if the number of transmitters increases, or if the terrain / environment is creating unusual signal overlap regions, then more care must be taken during the network planning stages to achieve the best results.



###Under Construction### Please stay tuned for more...
Attached Images
File Type: jpg 2Tx-Uneven.jpg (11.0 KB, 2294 views)
File Type: gif 2Tx-Equalizer-Delay-Animation.gif (343.2 KB, 3325 views)

Last edited by andy.s.lee; 28-Jan-2010 at 4:45 PM.
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